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brown dwarf

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Two intermediate-mass transiting brown dwarfs from the TESS mission
Authors: Theron W. Carmichael et al.
First Author’s Institution: Harvard University
Status: Accepted to AJ

Brown dwarfs are objects with masses 13–80 times the mass of Jupiter but roughly the same radius (0.7–1.4 Jupiter radii). The lower mass limit separates them from planets: unlike planets, brown dwarf cores are massive enough to fuse deuterium. On the other hand, if they get too massive (80 Jupiter masses) then their cores start to fuse hydrogen and they become a main sequence star.

Similarly to a planet, when a brown dwarf passes in front of its host star, it causes a dip in the star’s light curve. This allows us to detect brown dwarfs with missions like the Transiting Exoplanet Survey Satellite (TESS).

Brown Dwarf Desert

Because of their Jupiter-like radius and larger mass, brown dwarf transits should be as easy to detect as giant planets — yet there are only 23 known transiting brown dwarfs. The lack of known transiting brown dwarfs is known as the “brown dwarf desert”. The answer to this problem may lie in their formation mechanism — whether they form like stars or like planets. Accurate measurements of mass, radius and orbital parameters are necessary to understand the formation and evolution of brown dwarfs.

Today’s paper reports the discovery of two new transiting brown dwarfs with reliable measurements of mass, radius, and age. With ages greater than 3 Gyr, they are the oldest transiting brown dwarfs with well-constrained measurements.

Brown Dwarfs and Stellar Parameters

The authors observed two targets of interest (TOIs) with TESS and the Las Cumbres Observatory (LCOGT). Figure 1 shows the transit light curves for the two host stars: TOI-569 and TOI-1406.

brown dwarf transit light curves

Figure 1: Left: TESS and LCO light curve for TOI-569. Right: TESS and LCO lightcurve for TOI-1406. Both lightcurves include the EXOFASTv2 transit model in red. [Adapted from Carmichael et al. 2020]

Since the depth of the transit depends on the ratio of the radii of the brown dwarf and its host star, the authors are able to measure the radius of the brown dwarfs from these light curves. However, objects with a range of masses may have the same radius, so additional radial velocity (RV) measurements are needed in order to measure mass and verify that the transiting object is actually a brown dwarf.

Spectroscopic observations for TOI-569 were taken using the CHIRON echelle spectrograph, as well as the CORALIE and FEROS spectrographs. Spectra for TOI-1406 were obtained with CHIRON and the echelle spectrograph on the Australian National University (ANU) telescope. The resulting RV curves are shown in Figure 2.

radial velocity curves

Figure 2: Left: RV curve for TOI-569. Right: RV curve for TOI-1406. Both RV curves include the EXOFASTv2 model in red. [Adapted from Carmichael et al. 2020]

To obtain reliable masses and radii for the brown dwarfs, the authors simultaneously fit the light curves and RV curves using the MCMC fitting software EXOFASTv2. As an additional check, the authors use the pyaneti fitting software and find results consistent with the EXOFASTv2 models within 1 sigma. The measured masses and radii relative to Jupiter values are M = 64.1 MJ, R = 0.75 RJ for TOI-569b and M = 46.0 MJ, R = 0.86 RJ for TOI-1406b.

The authors also used EXOFASTv2 to fit the stellar parameters like age and mass using isochrone models. The brown dwarf is assumed to have the same age as the host star. TOI-569b and TOI-1406b have ages of 4.7 Gyr and 3.2 Gyr, respectively.

Mass–Radius Diagram

One way to test brown dwarf evolutionary models is using a diagram of mass vs. radius. Unlike a star, the deuterium-fusing core of a brown dwarf does not produce enough energy to fight gravitational collapse, causing the radius of the dwarf to decrease as it ages. This means that a brown dwarf of a certain age has an expected location in a mass–radius diagram. If you have an independent measurement of the brown dwarf age, then you can change the isochrone model parameters to match the observed position of the brown dwarf.

brown dwarf mass–radius diagram

Figure 3: Mass–radius diagram for the known transiting brown dwarfs. The different colored curves show substellar isochrone models for different ages. The newly discovered brown dwarfs are shown as the red and cyan points. [Carmichael et al. 2020]

Figure 3 above shows the mass–radius diagram for transiting brown dwarfs, focusing on those with accurate ages. The black points in Figure 3 have well-measured ages from star clusters. The addition of TOI-569b and TOI-1406b to the diagram allows the authors to test, for the first time, substellar isochrones for populations older than 2.5 Gyr.

TESS has so far allowed the discovery of 4 new transiting brown dwarfs and may find as many more in the future. However, many more discoveries are needed to fully understand the formation and evolution of brown dwarfs.

About the author, Gloria Fonseca Alvarez:

I’m a third year graduate student at the University of Connecticut. My current research focuses on the inner environments of supermassive black holes.

inner solar system

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Constraining the Formation of the Four Terrestrial Planets in the Solar System
Authors: Patryk Sofia Lykawka and Takashi Ito
First Author’s Institution: School of Interdisciplinary Social and Human Sciences, Kindai University, Japan
Status: Published in ApJ

Our solar system is really unique. As far as we can tell after our first ~decade of exoplanet hunting and planet formation observations and theory, it’s quite the special snowflake. In order to answer one of the biggest questions in astronomy — “How did the Earth form?” — we first need to ask how our solar system formed. Our moon, Jupiter and Saturn, the asteroid belt, and our Sun’s solar activity all provide hints as to how one can possibly form this incredibly habitable planet on which we live. Inspired by this, today’s bite answers the question: How did the terrestrial planets form? The authors argue that in order to fully understand the formation of one planet, we really should be thinking of its formation as one member of a system.

planet formation models

Figure 1: The embryo (large circle) and planetesimal (small circle) mass and radial distribution derived to represent three different planet formation models: A normal distribution (top), Empty Asteroid Belt Model (middle), and pebble accretion (bottom). [Adapted from Lykawka & Ito 2019]

Simulating the Formation of Our Terrestrial Planets

The authors explore terrestrial planet formation through the medium of N-body simulations. They start with a number of embryos and planetesimals in a simulated solar environment. The embryos and planetesimals represent the large and small rocky bodies that are left over after gas dissipation of a protoplanetary disk, which is a remnant of stellar formation. We believe that our rocky/terrestrial planets are born out of collisions of these objects. The authors allow their simulated rocks to collide and interact with each other for a series of timesteps that equate to 400 million years. By that time, all of the planets will have formed and a stable system should be established. The authors create different initial distributions of the embryos and planetesimals that are inspired by different planet formation models, for example the Grand Tack model, Empty Asteroid Belt, and pebble accretion. They explore the mass ratios of embryos to planetesimals — i.e. which type of pre-solar body will carry the most mass. Is it mostly in the smaller pebble sized objects, or in the boulder sized objects?

The authors also have different configurations of the gas giant planets, which we believe should have mostly formed by the time terrestrial planets started to form. Their location and eccentricities would affect the movements of bodies within 2 AU. The authors run 540 different combinations of these initial contentions and let the rocks go wild. In addition to the final mass and location of the planets that form, they also keep track of the water mass fraction, which is the fraction of mass that water makes up on each planet. Water is transported to these planets via collisions. We know generally the water mass fraction of Mars and Venus, we know the value pretty darn well for Earth, and we really have no idea for Mercury. Finally, the authors also keep track of the giant impacts that each planet experiences. We believe our Moon came from a giant impact that happened early in the Earth’s formation. So for a simulated solar system to be an analog of our system, the planets should have similar water mass fractions and the correct number and timing of giant impacts.

Do You Have What It Takes to Be a Planet in the Solar System?

Out of these 540 different solar systems, it’s time to find the systems that are most like our own solar system. There are a few criteria to narrow down the options. The authors define the region within 2 AU of the star (where 1 AU is the distance of the Earth to the Sun, today) as the planetary region, and outside of that is the asteroid belt. Are there at least three planets in the planetary region? Yes? Good. Moving on. Are there at least two that are of Venus/Earth mass? Yeah? Fantastic. Next, identify which planet-analog is Venus and which is Earth. We can now define the Mercury and Mars regions inside the orbit of Venus and outside the orbit of Earth, respectively. Are there planets in these regions that correspond to Mercury’s and Mars’s masses? Maybe there are multiple! If so, we determine which is going to be the planet analog by using its radial location from the Sun. The authors used the above checklist to begin to determine which planet systems were most like ours. In the end, they found only seventeen systems that were similar to our terrestrial planet system.

solar-system analogs

Figure 2: The 17 terrestrial systems that were deemed solar-system analogs. The simulation system 0 is our solar system. Green dots are Mercury, pink is Venus, blue is Earth, red is Mars, and black is the dwarf planet Ceres. The size of each dot corresponds to a mass, and the arrow represents inclination. [Lykawka & Ito 2019]

So what does all this tell us about how our solar system formed? Right away, it’s clear that it is difficult to form Mercury. Only 38 total Mercury analogs were formed, and most of them were still over two times the mass of Mercury. The disk models that formed the most Mercury-analogs typically had an “inner region,” which meant there were embryos and planetesimals located very close to the Sun at the beginning of their model run. They also saw that Mercury always formed too close to Venus.  The models did a pretty good job creating Venus and Earth — yay! However, they did not get enough water to Earth. In most of their simulations, Earth had a giant impact within 10 Myr, which is consistent with when we think the Moon was formed. Go Moon! Mars analogs typically formed with a larger mass than the real Mars, and often there were multiple Mars-like planets that formed nearby.

The Ingredients to Make a Solar System

After synthesizing their results, the authors came down to five crucial criteria that are necessary to form the terrestrial planets:

  1. Disks need to start out with a concentration of total disk mass between ~0.7–1.2 AU — it helps form Venus and Earth.
  2. Disks need an inner region ~0.3–0.4 AU in order to form my favorite planet: Mercury.
  3. Beyond 1.2 AU up to the asteroid belt (~2 AU), there needs to be significantly less mass. This aids in the production of dwarf planets in the asteroid belt and helps achieve the correct mass ratio between Venus and Earth.
  4. The embryos (large boulder-y rocks) need to carry most of the mass of the disk, as opposed to planetesimals.
  5. Jupiter and Saturn need to be on eccentric orbits. This keeps the Mars-analog planet from getting too massive and chucks out any planets that meander over to the asteroid belt.

This paper aids our understanding of terrestrial planet formation and tests how well current planet formation theories can recreate Mercury, Venus, Earth, and Mars analogs. Some aspects, like the excess water we have on Earth, the orbital spacing, and inclination of the planets are still a mystery, but the authors do take us one step closer to understanding the uniqueness of our solar system.

About the author, Jenny Calahan:

Hi! I am a second year graduate student at the University of Michigan. I study protoplanetary disk environments and astrochemistry, which set the stage for planet formation. Outside of astronomy, I love to sing (I’m a soprano I), I enjoy crafting, and I love to travel and explore new places. Check out my website: https://sites.google.com/umich.edu/jcalahan

BlackLivesMatter logo of an upheld, closed fist, superposed on a photograph of the Milky Way.

Editor’s Note:

The American Astronomical Society endorses and is participating in the grassroots efforts to #ShutDownSTEM, #ShutDownAcademia, and #Strike4BlackLives on Wednesday, 10 June. We encourage everyone in our community to make a lifelong commitment to action to eradicate anti-Black racism in the astronomical sciences — and in academia and research more generally — beginning today.

Instead of our usual highlight, today we are sharing a post recently published on Astrobites that contains suggestions of ways to support Black astronomers and make astronomy more anti-racist. If you find this post useful, you can look for future posts in the #BlackInAstro series on Astrobites. For further reading, the AAS also has a list of discipline-wide resources on diversity, equity, and inclusion.

A Spanish translation of this post can be found here at Astrobitos.
Una traducción al español de este artículo está aquí en Astrobitos.

The U.S. is rising in protest in the wake of the murder of George Floyd by the Minneapolis Police Department. The murders of George Floyd, Breonna Taylor, Tony McDade, and Ahmaud Arbery are the most recent in a long history of extrajudicial murders of Black people in the U.S. We at Astrobites stand in solidarity with the protestors, and against the systemic anti-Blackness that continues to enact violence on Black people in this country. We recognize that these same systems pervade academia and our field, and contribute to the inequities present in astronomy.

Why are we discussing these issues on an astronomy website? First, our scientific research is stronger when it comes from a community grounded in respect and diversity. But most importantly, we believe that the people in our community should be prioritized over our science. In order to do so, astronomy must be explicitly anti-racist and actively work to support Black students and researchers.

As authors for a publication widely read by astronomers and students, we at Astrobites have a responsibility to address issues of inequity. We acknowledge that we can be and should be doing more to amplify Black voices and build a better community for our Black colleagues. As one effort, we are starting a new series called #BlackInAstro, aimed to highlight the work of Black astronomers and the ways in which racism affects their experiences.

In this first post, we’ll share things that astronomers (particularly non-Black astronomers) can do, both during this time and to address racism and anti-Blackness in the long term in our field. We’ve also put together a list of suggested resources — compilations, readings, and podcasts — for folks working towards becoming more anti-racist.

What Can Astronomers Do?

We have compiled some ways that astronomers — particularly non-Black astronomers — can support Black folks in the field, both immediately in response to events like this week’s and in the longer term. Some of these are specific to our positions in academia, while many are general things that everyone can do, and that Black folks have been asking everyone to do for a long time. Whether you are an undergraduate, a graduate student, a postdoc, or a professor, you can help make the field of astronomy more anti-racist!

  1. Listen to Black people and amplify their voices. Work to understand what Black people are telling you, and work to listen without getting defensive. This can be difficult, particularly because as astronomers we’re often trained to defend our scientific results — but it’s important to listen, and to reflect on and learn from our mistakes.
  2. Check in with Black colleagues, friends, and students. This is a time of immense grief, pain and trauma for Black folks. Check in with the Black folks in your professional and personal circles; share your support and offer to do anything you can.
  3. Talk to non-Black people about what’s happening. The burden to address issues of racism and anti-Blackness often falls on Black folks, which is labor-intensive and can be traumatic. Non-Black people have a responsibility to start these conversations.
  4. Advocate and don’t gatekeep. Direct effort into advocating for Black students and folks in the field, both by supporting institutional efforts to improve equity and by stepping up for individual Black colleagues. On the flip side, do not engage in gatekeeping behaviors that make the field more exclusionary, such as those that continue to occur in introductory STEM courses and graduate admissions.
  5. Educate ourselves. Don’t expect Black people to do the additional work of educating everyone else. Take time to read about the history of the oppression of Black people and the ways anti-Blackness manifests today. Focus on writings by Black folks. In an academic setting, one way to do this is to start an equity-focused journal club. If this is newer to you, this glossary is a good place to start. A list of further suggested reading is at the bottom of this post.
  6. Support #BlackInSTEM efforts. Signal-boost Black students and researchers in STEM and share their work. This is critical both for supporting these individuals and for showing others that Black folks can succeed in the field; representation matters. For one great list of Black junior astronomers, check out this Twitter list compiled by Ashley Walker.
  7. Speak up and push leadership to speak up. We should expect leaders in our spaces — department chairs, PIs, project/telescope/survey directors — to speak up and express support for Black folks under their leadership. Silence plays into the myth that our workplaces and identities can be separated.
  8. Don’t expect productivity. During this time, all of us should be focusing our energy on supporting Black colleagues and anti-racism efforts. Black folks especially are processing, grieving, and fighting. Expecting academic productivity in these times of crisis ignores the experiences and needs of Black people.
  9. Don’t take up space. During times of crisis for Black folks, focusing on unrelated issues can seem minimizing. This is especially true for academic-related matters which Black folks may not have time and energy for right now.
  10. Donate. Many of us are in the privileged position of still receiving paychecks during this pandemic. Donate to Black justice organizations and to organizations that promote Black folks in STEM. Here are some suggestions:

As a reminder, many of these suggestions aren’t just action items for the immediate crisis! We must continually work to support Black peers and colleagues.

Suggested Reading Lists

Note: Consider buying from Black-owned bookstores! Here is a list of some in the US.

Comprehensive Guides to Anti-Racist Reading (and Watching)
Some Suggested Reading on Race and Anti-Racism in the US and UK

Stamped from the Beginning: The Definitive History of Racist Ideas in America
Ibram X. Kendi
Award winning history of how racist ideas were created, spread and rooted in American society. A staple of US anti-racism literature.

How To Be an Antiracist
Ibram X. Kendi
Award winning guide through antiracist ideas, and how to work to oppose them in society and ourselves.

So You Want To Talk About Race
Ijeoma Oluo
A contemporary overview of race in the United States, including police brutality, the BLM movement, intersectionality and microaggressions in day-to-day life.

Me and White Supremacy
Layla Saad
A guide on understanding your own privilege, examine unconscious biases, and how to combat racism.

Why I’m No Longer Talking To White People About Race
Reni Eddo-Lodge
An incredible resource detailing untaught black history, relations between class and race, and how White people can improve allyship. A must-read if you think the UK doesn’t have a race problem.

White Privilege: The Myth of a Post Racial Society
Kalwant Bhopal
An academic look at why black and minority ethnic communities continue to be marginalised in modern times, and how this is linked to government policy in the United Kingdom.

The History of White People
Nell Irvin Painter
A book describing the origin of the concept of ‘whiteness’, and the building of race as a social construct over centuries.

White Fragility: Why It’s So Hard for White People to Talk about Racism
Robin DiAngelo
Examines how the behaviors of White people in response to challenges of racism – including anger, defensiveness, and argumentation – protects racial inequality, and discusses ways to engage more constructively.

Books on Race in Academia and STEM

Superior: The Return of Race Science
Angela Saini
An in-depth, cross-disciplinary look at the presence of race science in modern society, while thoroughly debunking it.
(Note that in some regions of the world, Amazon is offering free versions of this book for a limited time.)

Algorithms of Oppression: How Search Engines Reinforce Racism
Safiya Noble
Challenges the idea that search engines like Google offer an equal playing field. Describes data discrimination as a real social problem, in which a biased set of search algorithms privilege whiteness and discriminate against people of color, specifically women of color.

Racism without Racists: Color-Blind Racism and the Persistence of Racial Inequality in the United States
Eduardo Silva-Bonilla
Discussion of color-blind racism and the evolution of new racial stratification.

The Immortal Life of Henrietta Lacks
Rebecca Skloot
Discusses really important questions of bioethics of the HeLa cells that are used in almost every lab in the US today, but note that it is not by a black author!

Presumed Incompetent: The Intersections of Race and Class for Women in Academia
Edited by Gabriella Gutiérrez y Muhs, Yolanda Flores Niemann, Carmen G. González, Angela P. Harris
Documents and analyses lived experiences of women and women-presenting academics of color. A collection of essays about the intersectionality of race and gender.

African Cultural Astronomy
Jarita C. Holbrook, Johnson O. Urama, and R. Thebe Medupe
A collection of essays describing the current status of archaeoastronomy and ethnoastronomy research in Africa.

Podcasts

Cite Black Women Collective Podcast
This bi-weekly podcast features reflections and conversations about the politics and praxis of acknowledging and centering Black women’s ideas and intellectual contributions inside and outside of the academy through citation.

The Nod
The Nod uplifts Black experiences in the U.S. and abroad. For example, in episodes over the last couple years, hosts Brittany Luse and Eric Eddings have told stories about everything from Josephine Baker’s “rainbow tribe” to an oral history of the song “Knuck If You Buck.” Where else have you heard or read those stories? Probably nowhere.

The Stoop
Similar to The Nod, The Stoop highlights Blackness by digging deeper into stories that we don’t hear enough about. Hosts Leila Day and Hana Baba discuss what it means to be Black and how we talk about our Black experiences through conversations between the two, as well as experts and Black people across the diaspora. A recent episode examined the word “hotep,” its meaning, and how its use has changed over the years.

Code Switch (on NPR)
Code-switching is the practice of shifting between languages or forms of expression in different contexts. Hosted by journalists of color, Code Switch explores race and how it impacts every part of society. The most recent episode, “A Decade of Watching Black People Die,” is particularly relevant right now.

About the Author, the Astrobites Collaboration

This post was written collectively by multiple members of the Astrobites team. Meet the authors of Astrobites.

Milky Way

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
Authors: Laura L. Watkins et al.
First Author’s Institution: University of Chicago
Status: Published in ApJ

We can’t put it on a digital scale, we can’t hang it on a balance and compare it against something else, so how does one measure the mass of our home galaxy? The authors of today’s paper use measurements of globular clusters in the halo of the galaxy taken from the Gaia satellite to estimate a mass for the Milky Way.

What Is Our Galaxy Made of and Why Should We Weigh It?

Our galaxy contains four major parts: the bulge, the disk (which contains the thin disk and the thick disk), the bar, and the halo (see Figure 1). The first three components are made up of baryons, particles that make up protons and neutrons and therefore most of the things around us. The halo, however, is dominated by dark matter, and the percentage of baryonic mass in the halo depends on how much dark matter there is. Dark matter is a mysterious substance that pervades the galaxy, interacting strongly with gravity and weakly with light. We know dark matter is there because of the rotation curve of the galaxy; if the mass was concentrated at the center, the velocity of the outer regions would be slower than the inner regions. In the case of the Milky Way, we see that the rotational velocity stays fairly constant all the way out, which points to some unseen matter being present (matter that we identify as dark matter). Because of its weak interactions with light, it can be really tough to measure the amount of dark matter, and thus how much it weighs. Overcoming this challenge to calculate a mass for the dark matter in our galaxy’s halo would be a big step in obtaining the mass of the Milky Way.

Measuring the mass of our galaxy is very useful for two reasons: first, because the mass of the galaxy and its distribution are linked to the formation and growth of our universe. Accurately determining the mass will help us understand where our galaxy sits on the scale of the cosmos. Second, it helps us learn about the dynamical history and future of the Local Group and the satellite population (specifically stellar streams).

Milky Way schematic

Figure 1: Left: where the Sun sits in the Milky Way, from a face-on perspective. Right: The different parts of the galaxy, from an edge-on perspective. [ESA]

How to Weigh a Galaxy

The estimate of the mass of a galaxy is dependent on many things, including which satellites are bound and how long they have been that way, the shape of the Milky Way, and the method used for analysis. Three techniques have been mainly used to measure the mass of the galaxy: the timing argument, abundance-matching studies, and dynamical methods. The timing argument measures the speed at which two galaxies are approaching each other and uses those dynamics to predict a mass. Abundance-matching studies uses the number of galaxies versus their circular velocity and the Tully-Fischer relation to obtain their luminosity, which can be used to estimate their mass. Finally, dynamical methods look at the velocity of tracer objects such as globular clusters; any mass distribution gives rise to a gravitational potential that causes objects to move, so by studying the motions of the objects, we can work backwards to recover the gravitational potential, and thus the mass. The authors of today’s paper use this dynamical method to measure the mass of the Milky Way.

Using Gaia to Map Motions

The team used data from the Gaia mission’s 2nd data release (DR2) to measure the proper motions of stars, or how they are moving across the sky. Gaia is a space-based instrument whose goal is to make a 3D map of the galaxy, and this data release contained measurements for billions of stars and 75 globular clusters. Gaia’s observations are so precise that it can measure a human hair’s width at 1,000 km, which is a resolution 1,000–2,000 times higher than that of the Hubble Space Telescope! (Check out this really cool video on Gaia to learn more about this amazing satellite.) Figure 2 shows just how many sources Gaia has measured. Out of the 75 globular cluster measurements released in DR2, the authors used 34 of them that spanned a range of distances from 2.0 to 21.1 kiloparsecs from the center of the galaxy — which allowed the authors to trace the Milky Way’s mass out to the outer halo.

Gaia data map

Figure 2: A map of the number of sources Gaia measures on a projection of the plane of the galaxy (centered on the galactic center). The lighter the color, the more sources. The two circles in the bottom right are two very small dwarf galaxies that orbit the Milky Way. This figure shows the billions of stars contained in DR2. [Brown et al. 2018]

In order to map the mass of the galaxy correctly, they need parameters like velocity anisotropy (which measures how the motions of stars vary in different directions), the density of the galaxy, and the potential of the galaxy. The team uses an NFW model, which is a model for how the density is distributed within the galaxy, to describe the potential of the galaxy. The authors then run simulations to determine the radius inside which particles are gravitationally bound to each other (the virial radius) and the mass contained inside the virial radius (the virial mass). By varying the virial parameters and sampling different models of the halo, the team was able to figure out the most probable mass of the galaxy. In addition, they use the velocities of the stars to map the circular velocity of the galaxy out to the radius of the farthest globular cluster. Figure 3 shows the potential of the different components of the galaxy and the results of varying the virial parameters of the halo.

galaxy potential vs distance

Figure 3: The potential of the galaxy versus distance. Each component of the galaxy is labeled. The authors vary the virial radius and concentration (which represents the density) of the halo, and the different values they sample over are shown by the shaded region around the halo curve. The combination of the components (i.e., the total potential of the galaxy) is the gray line. The authors map the potential of the entire galaxy, but the vertical dotted lines show the area in which they’re interested, which is the distance of the nearest and farthest globular cluster in their sample. The solid lines show the extent of the best-fitting power law to that region, and the dashed lines show the power-law fit outside the region of interest. [Watkins et al. 2019]

Evidence for an Intermediate Mass Milky Way

The authors find that the mass of the galaxy is 0.21 x 1012 solar masses, the circular velocity of the galaxy at the maximum radius they look at (21.1 kpc) is 206 km/s, and the virial radius is 1.28 x 1012 solar masses. This virial mass fits in most with intermediate values found by other studies. The circular velocity measurement the authors made indicates that the velocity is fairly constant in the outer regions, supporting the idea that dark matter is present in our galaxy. Some of the clusters the team used for measurements are on very radial or very tangential orbits, which could have been the result of galactic collisions. If they remove these clusters, the mass and velocity measurements are still within their error bars, showing that these estimates are robust even if there are substructures of globular clusters in the galaxy.

The amazing wealth of data from the Gaia mission has allowed the team to make one of the most precise estimates of the mass of the galaxy that has ever been achieved. As Gaia continues its mission over the next few years, it will obtain positions and velocities of even more clusters, paving the way for more robust studies of the mass of our galaxy.

About the author, Haley Wahl:

I’m a third year grad student at West Virginia University and my main research area is pulsars. I’m currently working with the NANOGrav collaboration (a collaboration which is part of a worldwide effort to detect gravitational waves with pulsars) on polarization calibration. In my set of 45 millisecond pulsars, I’m looking at how the rotation measure (how much the light from the star is rotated by the interstellar medium on its way to us) changes over time, which can tell us about the variation of the galactic magnetic field. I’m mainly interested in pulsar emission and the weird things we see pulsars do! In addition to doing research, I’m also a huge fan of running, baking, reading, watching movies, and I LOVE dogs!

molecules

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Case of H2C3O Isomers, Revisited: Solving the Mystery of the Missing Propadienone
Authors: Christopher N. Shingledecker et al.
First Author’s Institution: Center for Astrophysics Studies Max Plank Institute for Extraterrestrial Physics & Institute for Theoretical Chemistry at the University of Stuttgart
Status: Published in ApJ

Finding and Making Molecules

Looking for different chemicals in space is a lot like searching for Waldo in the infamous search and find series “Where’s Wally?” Only imagine that the search and find page is light years away from you and all you have is a flashlight.

Waldo

“Waldo” is famous for his red and white striped shirt, large circular glasses, cap, and blue jeans. Yet, he is difficult to find in the chaotic illustrations. Can you find Waldo in this search and find in space? [Where’s Wally in Out Space Activity Book]

As our knowledge and understanding of chemical evolution in space grows, astronomers are seeking the detection of more and more complex organic molecules (COMs). Molecules that could lead to the production of life (like prebiotic molecules that may eventually form DNA) and other larger COMs are rather difficult to detect, so we often use theoretical calculations to predict the evolution and abundance of these larger molecules.

Chemical models commonly use kinetics, how energy changes over as a reaction progresses, to determine the rate at which chemical reactions occur, and thus the rate at which more complex molecules form and how abundances vary over time. Kinetics tells us that chemical reactions typically have an energy barrier to get from reactants to products. However, space is so cold that there isn’t enough energy available to overcome energy barriers (imagine pushing a 500 pound boulder over the top of Mount Everest). So, we assume that only barrier-less reactions can occur in space. There is a noteworthy exception of ultra hot regions like HII regions, supernovae, and such, where temperatures are high enough to overcome reaction barriers.

reaction barrier

Most chemical reactions must overcome a reaction barrier to get from reactants to products, but most astronomical settings aren’t warm enough to provide the energy necessary to overcome these barriers. [Libretexts]

One of the most important aspects of theoretical research is matching observational data. If theoretical models using activation barriers and chemical kinetics are not able to match observations, then that usually indicates that there is a physical or chemical process that we don’t know about.

The Missing Molecule

In the last decade, one important molecule that has alluded astronomers is CH2CCO, or propadienone. CH2CCO is actually one of three different molecules that can be made from two hydrogen atoms, three carbon atoms, and one oxygen atom (H2C3O). These are known as structural isomers, meaning they’re made up of all the same atoms, but the atoms can be arranged differently to make different molecules.

Waldo isomers

The three molecules we can make from H2C3O. Each isomer is made up of the same components, just as the three “Waldo” cartoons above them. However, each H2C3O isomer is put together in a different order, similar to the “Waldo isomers.” Each Waldo is made up of the same colors, but the colors are arranged in different orders.
[H2C3O isomer structures: Hudson & Gerakines 2019; “Waldo”: Waldo Wiki]

Propadienone (CH2CCO) is the most stable isomer of H2C3O, meaning CH2CCO has the lowest ground state energy and the H2C3O atoms are “happiest” in the CH2CCO configuration. According the the minimum energy principle, which uses thermodynamics rather than kinetics to predict chemical evolution, CH2CCO should be the most abundant of the three isomers, since it is the most stable of the three. Despite observational efforts and archival data searches, no one has been able to detect CH2CCO in space even though the other two H2C3O isomers have been detected. As the minimum energy principle states that CH2CCO should be detectable as well, this disagreement between observations and theory challenged the minimum energy principle and questioned the validity of relying on kinetics for chemical models.

Where’s Waldo CH2CCO?

So, where is CH2CCO? As it turns out, we still haven’t detected it in space. However, today’s paper uses theoretical calculations to find “where” CH2CCO is hiding. The authors map reactions associated with the H2C3O isomers using density functional theory (DFT). DFT uses quantum mechanics and kinetics to determine the most stable structures of molecules and their associated energies. CH2CCO can react with two hydrogen atoms to form propenal (CH2CHCHO). The process of adding a single H atom, or a proton, is a common reaction known as hydrogen addition. CH2CCO undergoes two hydrogen additions to form CH2CHCHO, both of which were found to be barrier-less reactions.

reaction diagram

Left: Reaction diagram from today’s paper showing that adding a hydrogen to CH2CCO is a barrier-less reaction, and thus able to occur in space. Right: Hydrogen additions to CH2CCO to form CH2CHCHO. Each reaction adds a single H atom to the carbon chain. Note the black dots are single, unpaired electrons (radicals). [Shingledecker et al. 2019]

Interestingly enough, hydrogen addition to the second most stable H2C3O isomer, propynal (HCCCHO), is found to have a reaction barrier. Thus propynal is able to persist in molecular clouds, while CH2CCO is converted to CH2CHCHO. These findings are consistent with both previous experimentation and observations of the Sagittarius B2 molecular cloud, where the two less stable H2C3O isomers and CH2CHCHO were detected, but CH2CCO was not.

Today’s paper shows that the “missing” molecule propadienone (CH2CCO) was never actually missing; it was just masquerading as CH2CHCHO. This discovery is important, since it shows us that kinetic theory and observations of CH2CCO are actually in agreement, rather than disagreement. Additionally, today’s paper confirms the validity of using chemical kinetics and reaction barriers (or lack of barriers) to predict chemical evolution in astronomical settings.

Sometimes search and finds, like finding molecules in astronomical settings, can be difficult — but ultimately, finding the missing pieces helps us better understand our universe.

Now that we’ve found CH2CCO, did you find Waldo in the first figure?

About the author, Abygail Waggoner:

I am a second year chemistry graduate student at the University of Virginia and NSF graduate fellow. I study time variable chemistry in protoplanetary disks. When I’m not nerding out about space, I’m nerding out about fantasy by reading or playing games like dungeons and dragons.

high-redshift galaxy

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The MOSDEF survey: a stellar mass-SFR-metallicity relation exists at z∼2.3
Authors: Ryan L. Sanders et al.
First Author’s Institution: University of California, Davis
Status: Published in ApJ

Galaxy evolution is a complicated thing! Our current theory is that gas comes in, stars get made & explode, the surrounding interstellar medium (ISM) heats up and gets enriched with metals, and then gas goes out. These processes are happening in different stages all across the galaxy and can make simulating and observing galaxy evolution very difficult. Thankfully, through years of observation of local galaxies, we know that some galactic properties are correlated! For example, Tremonti et al. (2004) showed that there is a relation between stellar mass (M∗) and gas-phase oxygen abundance (12 + log(O/H) or Z) in the local universe (redshift z ~ 0). In 2008, Ellison et al. discovered that this M∗–Z relation also has a dependence on the star-formation rate (SFR), in the local universe. This local M∗–SFR–Z relation was shown later to be more correlated than the M∗–Z relation on its own!

The questions that then arise are: is there also evidence for a M∗–SFR–Z relation at high redshift? And if so, does it agree with the one at z ~ 0? Or does it evolve with redshift? Many studies have tried to answer these questions, but most were based on large samples with low signal-to-noise (S/N) or small samples with intermediate S/N and have relied on a single metallicity indicator. But a 2018 study using a new, deep survey has changed that.

What Did They Do?

Completed in May 2016, the MOSFIRE Deep Evolution Field (MOSDEF) survey was a 4-year program in which the MOSFIRE instrument on the 10-m Keck 1 telescope was used to get near-IR spectra of ~1,500 galaxies spanning redshifts 1.4 < z < 3.8. The authors of today’s article elected to use the ~700 galaxies observed in the 2.01–2.61 redshift range. After S/N cuts, the authors were left with a 260-galaxy sample with an average redshift of z ~ 2.3 (see Figure 1, left). To make a conclusion about the (possible) redshift evolution of the M∗–SFR–Z relation, the authors used a comparison sample of 208,529 star-forming galaxies at z ~ 0 from Andrews & Martini (2013).

redshift and star formation relations

Figure 1: Left: The redshift distribution of their sample. The median redshift is z = 2.29. Middle: The SFR–M∗ relation of the sample. Right: The sSFR–M∗ relation of the sample. Here sSFR is the “specific star-formation rate,” which is just SFR/M∗. In the right two sections, the red-dashed line shows the best fit to the z ~ 2.3 data. This best-fit relation will be used when calculating the M∗–SFR–Z relation. [Sanders et al. 2018]

From their near-IR spectra, the authors measured SFRs from the H-alpha luminosities, which were then reddening-corrected. This correction is needed because dust grains between us and the galaxies scatter blue light and cause the galaxies to appear redder than they really are. Using broad- and mid-band photometric fits, the authors were able to robustly determine galaxy masses. Finally, using six emission-line ratios (which they abbreviate as N2, O3N2, N2O2, O3, R32, & O32; see the paper for definitions), they were able to get several metallicity estimates for each galaxy. Here, metallicity refers to the gas-phase oxygen abundance, 12 + log(O/H).

What Did They Find?

The authors did detect a M∗–SFR–Z relation at z ~ 2.3! This is best shown in Figure 2. This relation was found using the metallicity estimates from O3N2, N2, and N2O2. The ratios for R32 and O3 are double-valued with metallicity (think of these like a parabola) and can’t be used empirically to discover a relation like this. They can, however, be used to support a finding; in this case, the results from R32 and O3 are consistent with those found from the other three. Results from O32 were inconsistent with their findings and the authors concluded that this was likely due to biases in the reddening correction. Another main goal of this project was to determine if the M∗–SFR–Z relation evolved with redshift — and the authors found that it did! At a given mass and SFR, the metallicity of the z ~ 2.3 sample is 0.1 dex less than the z ~ 0 sample, also shown in Figure 2. The authors speculate that this evolution may be caused by an increase in the mass-loading factor from z ~ 0 to z ~ 2 and by a decrease in the metallicity of infalling gas at z ~ 2.

metallicity–M* relations

Figure 2: Shown above are the Z–M∗ relations using O3N2, N2, and N2O2. Points are colored by star formation. Squares represent the z ~ 0 data set while stars represent the z ~ 2.3 set. The red-dashed line shows the best fit to the z ~ 2.3 data. We see a M∗–SFR–Z relation at z ~ 2.3 and at fixed mass and SFR, the z ~ 2.3 set has 0.1 dex smaller metallicity than the z ~ 0 set. [Sanders et al. 2018]

What’s Left to Discover?

The authors established that a M∗–SFR–Z relation exists at z ~ 2.3 and that this relation evolves with redshift. The existence of this relation implies that our understanding of galaxy evolution is right… at least up to z ~ 2.3. The next step is to investigate this relation at higher redshifts, but that is no trivial task. As shown through the use of five emission-line ratios, measuring metallicities at high redshift can be difficult and will take great care. Uncertainties and inconsistencies with reddening corrections and other calibrations can cause large uncertainties in the results, like the case with O32. Thankfully, the introduction of large telescopes (like JWSTand TMT) will allow us to lessen these uncertainties through their increased sensitivities.

About the author, Huei Sears:

Huei Sears (she/her/hers) is a second-year graduate student at Ohio University studying astrophysics! Her research is focused on Gamma-Ray Burst host galaxies & how they fit into the mass-metallicity relationship. Previously she was at Michigan State University searching for the elusive period of B[e]supergiant, S18. In addition to research, she cares a lot about science communication, and is always looking for ways to make science more accessible. In her free time, she enjoys going to the gym, baking a new recipe, listening to Taylor Swift, watching the X-Files, and spending time with her little sister.

Koala

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z = 0.27
Authors: Anna Y. Q. Ho et al.
First Author’s Institution: California Institute of Technology
Status: Accepted to ApJ

Furry Animals and Relativistic Transients

Astrophysicists love clever titles, and in transient astronomy, we can get some fun ones! Transient sky surveys discover thousands of new explosions every year, with each one receiving a name based on when it was discovered. For example, in 2018, a peculiar transient called Astronomical Transient (AT) 2018cow was discovered and aptly deemed “the Cow” after the last letters of its International Astronomical Union (IAU) name. Coincidently, the Cow happened to be so unique that it received worldwide acclaim as one of the most exciting discoveries of 2018!

Following its discovery, the Cow became the prototypical transient in a new class of explosions called Fast Blue Optical Transients (FBOTs), whose name is reflective of their unique observational signatures. After detection, FBOTs rise to peak brightness in only a few days (i.e. fast!) and have extremely hot temperatures, which makes them appear bluer in color than typical supernova explosions. However, astrophysicists are still puzzled at how objects like the Cow are formed: a black hole shreds a white dwarf? Or maybe a massive star implodes to form an accreting black hole or magnetar? Either way, FBOTs present a fresh mystery that can only be solved by finding and studying similar events!

Innocuous Bear or Violent Explosion?

The authors of today’s paper present the discovery of a fuzzy new FBOT, ZTF18abvkwla, which was nicknamed the “Koala” after the last four letters of its official transient name. Furry animals keep making their way into astrophysics! Unlike Earth-based koalas, this transient creature is anything but docile: observations spanning the electromagnetic spectrum revealed that the Koala was a luminous event whose turbulent explosion resulted in high temperatures and rapid ejection of stellar material. The Koala was first observed by the Zwicky Transient Factory and is located in a distant dwarf galaxy with a high star formation rate of 7 solar masses per year. The large number of new stars in the Koala’s host galaxy may indicate that this FBOT came from the explosion of a young massive star rather than from an older star system containing a white dwarf.

Koala and Cow light curves

Figure 1: Optical light curve of the Koala (points) and prototypical FBOT, the “Cow” (lines). The remarkable evolution of the Koala shows that it rose and fell in luminosity in only 5 days, but it had an energy output similar to a normal supernova explosion! [Ho et al. 2020]

As shown in Figure 1, the authors demonstrate that the Koala’s luminosity evolution (i.e., light curve) is very similar to the Cow’s: rising to peak brightness in only a few days, very blue colors and a rapid decline in magnitude. Consequently, studying how FBOT light curves evolve in time is a powerful tool in uncovering the origin of these mysterious explosions. For instance, FBOTs rise and fade too quickly (day timescales) to be powered by radioactive decay of heavy elements, which is the mechanism invoked for most supernovae and occurs on about week timescales.

To produce both a luminous and rapidly evolving light curve like the Koala’s, the author’s discuss the possibility that the explosion was powered by the collision of outflowing material with gas in the local environment. However, this gas must also be quite opaque to radiation since the spectrum (Fig. 2) does not show hydrogen or helium emission lines that typically occur from an explosion interacting with material surrounding the progenitor star. Contrary to the comfortable climates of Australia’s native bear, the Koala’s spectrum revealed that it was an incredibly hot explosion that reached temperatures > 40,000 Kelvin!

Koala optical spectrum

Figure 2: Optical spectrum of the Koala with host galaxy emission lines marked in black. The spectrum indicates that the Koala had a temperature >40,000 Kelvin. [Ho et al. 2020]

The final piece of the puzzle discussed in today’s paper is the detection of luminous radio emission coming from the Koala. Shown in Figure 3, the Koala has one of the highest radio luminosities of any FBOT-like event (black stars), which is an important clue in deciphering the mechanisms behind the explosion. Similar to the Cow, the authors conclude that the Koala’s observed radio emission arose from an outflow of material moving at almost half the speed of light. That’s 100 million times faster than a koala running on Earth! This turbulent ejection of material in the Koala may be linked to the formation of a central engine like a black hole or rapidly rotating neutron star. Such a compact object could accrete material from an exploded star and violently expel semi-relativistic gas that would be visible as radio emission.

transient radio luminosities

Figure 3: Radio luminosities of different transients: FBOTs (black), gamma ray bursts (orange), relativistic/normal supernovae (red/blue) and tidal disruption events (purple). Another study on new FBOT CSS161010 was published soon after today’s paper. The Koala’s high radio luminosity suggests an accelerated outflow of material from a central compact object. [Ho et al. 2020]

While not your average woodland critter, the Koala has exposed how little we know about where FBOTs come from and the physics behind their energetic explosions. Unfortunately, understanding their complex origins will be difficult, as the authors conclude that the revised rate of FBOT discovery is almost 3 orders of magnitude less than that of typical supernovae. Nonetheless, new advancements in sky surveys will increase the number of new FBOTs and, with any luck, they will have more animal-themed names!

About the author, Wynn Jacobson-Galan:

Hi there! My name is Wynn (he, him, his) and I am an NSF Graduate Research Fellow at Northwestern University where I work with Prof. Raffaella Margutti on supernova progenitor systems and transient astronomy. I am fascinated by the final moments of stellar evolution before a star dies and becomes the violent supernova explosions we observe across the universe everyday. Consequently, as a researcher, I am both a stellar physician and a mortician: I use observational astronomy to wind back the cosmic clock in order to understand how certain stars were “living” right before their explosive “death.” Outside of research, I enjoy reading (specifically 20th century literature) and skateboarding. Also, if I’m not playing music (trumpet & saxophone), I am usually trying to find fun live music in Chicago.

Magnetic Orion

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: HAWC+/SOFIA Multiwavelength Polarimetric Observations of OMC-1
Authors: David T. Chuss et al.
First Author’s Institution: Villanova University
Status: Published in ApJ

OMC-What?

The Orion Molecular Cloud 1 (OMC-1) is part of the Orion Nebula, and one of the most massive star-forming regions in the solar neighborhood. The gas and dust within OMC-1 act as a nursery for young stars, providing them with the necessary materials to develop. As such a close and large stellar nursery, OMC-1 is an easily accessible and important laboratory for studying the still-mysterious conditions that surround and encourage star formation. Today’s paper contributes to our understanding of star formation by determining OMC-1’s magnetic field and dust properties using polarimetry (more on this technique later!).

OMC-1 is a particularly interesting target for magnetic field and dust measurements because of the variation in structure across the cloud, which is shown in Figure 1. In front of OMC-1, there is an HII region ionized by a relatively young group of stars, the Trapezium cluster. The west side of OMC-1 hosts the Kleinman-Low (KL) Nebula and the Becklin-Neugebauer (BN) object. The KL Nebula is a clump of molecular gas and dust with a bunch of massive stars inside, of which the BN object is the brightest. In the infrared, the KL Nebula appears to be exploding because stellar winds from the massive stars heat up the surrounding gas. The southeast region of OMC-1 contains the Orion Bar, a photodissociation region that is cold, neutral, and creates the divide between HII and molecular gas. These features contribute to a complex magnetic field structure within OMC-1 that today’s authors map with polarimetry measurements.

OMC-1

Figure 1: The OMC-1 region, with overlaid magnetic field lines. Blue shows the KL Nebula and BN object, gray shows the HII region ionized by the Trapezium cluster, and purple shows the Orion bar. [NASA/SOFIA/D. Chuss et al. & ESO/M. McCaughrean et al.]

So What Is This Polarimetry I Speak Of?

We’ve all heard of polarized sunglasses, which block sunlight and reduce glare. Thinking of light as a wave, it travels in one direction and oscillates in the two planes perpendicular to that direction of travel. Polarized sunglasses block out one of these planes of vibration, and allow only half of the light to travel through the lenses.

Polarization measurements in astronomy work much the same way. For today’s paper, we are looking at the infrared light that is emitted from dust, but stars and other sources can emit polarized light too. For dust, the primary concepts of polarization remain the same as the case of blocking light with sunglasses. However, instead of blocking the light, dust actually emits light that has one plane of vibration brighter than the other from the start. To understand the reason behind this, assume that the dust has an egg shape. Because there is more surface area along the long axis of the egg than the short axis, we get more emission traveling in the direction of the long axis. This creates a net polarization of the signal: we get more infrared light in the direction that is parallel to the long axis of the dust. Lots of theories suggest that dust aligns its long axis perpendicular to the magnetic field, so by measuring the direction of the polarization, we can infer the direction of the magnetic field!

Flying High

Today’s authors used the HAWC+ instrument onboard NASA’s airborne observatory (the Stratospheric Observatory for Infrared Astronomy, or SOFIA)  to look at the infrared emission of the dust in OMC-1. They measured the total flux and polarization at four different wavelengths, and the results of their measurements can be seen in Figure 2. Interestingly, they found that at the smaller wavelengths (upper two panels), the magnetic field direction near the BN/KL objects, represented by the white star, is radically different than the surrounding region. The authors also discovered that the magnetic field direction in the Orion Bar differs significantly from elsewhere in OMC-1, and the magnetic field strength and dust temperature are highest near the BN/KL explosion location.

Polarimetry of OMC-1

Figure 2: Polarimetry measurements at 53, 89, 154, and 214 microns. The star symbol represents the location of the BN object, while the Orion Bar can be seen at the lower left. Colors represent total intensity, with red the highest and blue the lowest. Lines represent magnetic field direction. Click and zoom in to notice the change in the magnetic field with wavelength near the BN object! [Chuss et al. 2019]

Sweeping (Up the Dust) Conclusions

So why the change in magnetic field direction and strength across the OMC-1 region? The authors propose some interesting explanations. Remember how the KL nebula appears to be exploding from stellar winds? Well, it’s possible that this explosion has compressed the magnetic field opposite the material that it spits out, creating the distinctly different direction of the magnetic field that we see at shorter wavelengths. And the reason we don’t see the same compression at longer wavelengths? Longer wavelengths are emitted by the colder dust (Wein’s Law) that is likely to be outside of the explosion range! The authors also provide an explanation for the change in magnetic field direction that is present in the Orion Bar: the magnetic field of the bar may run parallel to its long side. When the vector of the magnetic field along the bar is added to the vector of the magnetic field in the surrounding region, it is likely to cancel itself out.

These insights on the magnetic field structure of OMC-1 demonstrate the power of polarimetry in astronomy, and the HAWC+ instrument on SOFIA will continue to make similar measurements more prevalent for molecular clouds. Because molecular clouds act as stellar nurseries, learning about their properties (like the direction and strength of their magnetic field) provides us with a better understanding of star formation processes.

About the author, Ashley Piccone:

I am a second year PhD student at the University of Wyoming, where I use polarimetry and spectroscopy to study the magnetic field and dust around bowshock nebulae. I love science communication and finding new ways to introduce people to astronomy and physics. In addition to stargazing at the clear Wyoming skies, I also enjoy backpacking, hiking, running and skiing.

Active Galactic Nucleus

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Large Population of Obscured AGN in Disguise as Low Luminosity AGN in Chandra Deep Field South
Authors: Erini Lambrides et al.
First Author’s Institution: Johns Hopkins University
Status: Submitted to ApJ

Light emanating from within a host galaxy travels a huge distance to reach an observer. Along its path, the photons can encounter obstacles that change their wavelength or diminish the total intensity of the light. Depending on the host galaxy’s orientation relative to Earth, said emission could even be impeded by material contained within the galaxy. This can make the identification and classification of the photon’s source much harder. Models predict that there are a huge number of active galactic nuclei (AGN) growing behind dense screens of gas and dust that surround their host galaxies. Deep X-ray surveys are thought to produce the most complete and unbiased surveys of the AGN population, but we have yet to observationally confirm the high predicted fraction of these obscured AGN. Today’s paper re-evaluated such a deep X-ray survey containing a large sample of AGN in the Chandra Deep Field South (CDFS) and investigated whether the lower luminosity sources are, in fact, bright sources hidden behind larger amounts of obscuring material than previously thought.

AGN model

Figure 1: Standard model of an active galactic nucleus. [Urry & Padovani 1995]

The original study contains approximately 500 X-ray selected AGN in the CDFS at redshifts above z = 0.5. For each detection, the original study calculated an estimate of the column density — a quantity describing the likely density of gas and dust around the galaxy that impedes photons travelling between the source and the observer. Using the column density of each AGN, today’s authors corrected the original observations and estimated an intrinsic X-ray luminosity. Assuming the original study calculated the column density correctly then this intrinsic X-ray luminosity should make all the AGN in the sample appear completely un-obscured. Alongside this, they gathered mid-IR (6 and 24 µm) and optical emission line data, which can be used to describe other aspects of the AGN’s behaviour.

Tales of the Un-Obscured

In an AGN, the observed X-ray emission is absorbed by material in the surrounding torus (Figure 1). This is then re-emitted at the 6 µm, mid-IR, wavelength by the torus. Assuming the original study correctly calculated the column density, we would expect these AGN to show a direct correlation between X-ray and mid-IR luminosity. However, there are a large number of AGN that have a much lower intrinsic X-ray luminosity for their torus luminosity, known as a ‘mid-IR excess’. In addition, 90% of these AGN are found in the faintest group of observations. Whilst this suggests that these faint AGN could be heavily obscured, there are a number of other possible explanations for why lower luminosity AGN have a mid-IR excess.

AGN luminosities

Figure 2: Comparing the intrinsic X-ray luminosity of an AGN against its torus luminosity. All of the AGN are coloured based on the group their observed flux falls into, with the faintest being blue ranging up to the most luminous in red. The dashed blue line highlights the region we expect an un-obscured AGN to lie within. [Lambrides et al. 2020]

One possibility is simply that the intrinsic X-ray luminosity is correct and we are observing an un-obscured AGN with genuinely lower emission than expected. In Figure 3, the authors compare the intrinsic luminosity to the [OIII] optical emission line luminosity — an independent measure of AGN activity — to see whether these AGN are intrinsically faint. Not only do the faint AGN (blue points) sit outside the region that suggests they are un-obscured, but they appear to be much more active than expected for their intrinsic X-ray luminosity and also occupy a similar region to obscured AGN from a previous study. So, these faint AGN don’t have an intrinsically lower activity, but they do behave similarly to obscured AGN.

AGN luminosity vs [OIII]

Figure 3: Comparing the intrinsic X-ray luminosity with the luminosity of the [OIII] optical emission line — an independent measure of AGN power. As with Figure 2, the AGN are coloured by their observed X-ray flux and we expect un-obscured AGN to lie within the dashed blue lines. There are also a number of AGN from a separate study: filled, grey points are un-obscured AGN and the hollow, grey points are obscured AGN. [Lambrides et al. 2020]

If the AGN aren’t less active then perhaps a non-AGN emission component, such as star formation, could be causing the mid-IR excess. 24 µm emission is commonly used to trace star formation as warm dust associated with high-mass star-forming regions emits strongly at this wavelength. However, 24 µm emission can also be associated with activity from un-obscured AGN. Figure 4 shows that whilst many of these AGN don’t sit within the un-obscured region, crucially none of them are anywhere near the star-formation-driven relationship. In particular, the faint AGN (highlighted with a red circle) occupy this in-between area where they don’t appear to behave like un-obscured AGN, nor are they all explained by star formation.

Intrinsic X-ray luminosity vs 24 µm luminosity

Figure 4: Comparing the intrinsic X-ray luminosity against the 24 µm luminosity. All of the AGN are coloured based on their redshift (as this determined the instrument recording 24 µm emission) but the lowest flux AGN (previously blue in Figures 2 & 3) are highlighted with red circles. Objects with 24 µm emission largely driven by the AGN are found in the grey region, whilst those driven by star formation would be found within the red region. [Lambrides et al. 2020]

Given that this mid-IR excess cannot be explained by a less active AGN or star-formation, the authors turn to their initial hypothesis: these faint AGN are more heavily obscured than the original study proposed. In Figure 5 the authors investigated how different levels of obscuration could help describe the observations. Most of the faint AGN (blue points) are consistent with column densities at least 10 times larger than calculated in the original study. Such an increase in obscuration would see a similarly sized increase in intrinsic X-ray luminosity and account for the mid-IR excess. Thus, the authors conclude that they have discovered a new sample of highly obscured AGN.

observed X-ray luminosity vs. torus luminosity

Figure 5: Similar to Figure 2, but instead comparing the observed X-ray luminosity with the torus luminosity. Overlain on these points are regions within which we would expect to find AGN of varying levels of obscuration. Un-obscured AGN are found within the red region, more heavily obscured AGN are found within the blue region. [Lambrides et al. 2020]

Today’s paper is important as it provides a new successful approach to probing a fainter luminosity population than previously investigated in the field. In addition, assuming that all the newly identified obscured AGN have an increased column density, then the authors find that estimations of the space density of obscured AGN increases between 40% – 50%, depending on redshift. Such an increase helps close gap between the observed and predicted densities of obscured AGN in the universe.

About the author, Keir Birchall:

Keir is a PhD student studying methods to identify AGN in various populations of galaxies to see what affects their incidence. When not doing science, he can be found behind the lens of a film camera or listening to the strangest music possible.

Planet 9

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Testing the Isotropy of the Dark Energy Survey’s Extreme Trans-Neptunian Objects
Authors: Pedro H. Bernardinelli, Gary M. Berstein, Masao Sako et. al
First Author’s Institution: University of Pennsylvania
Status: Submitted to The Planetary Science Journal (PSJ)

Out beyond the orbit of Neptune lie small solar system bodies called trans-Neptunian Objects (TNOs). They are rocky, icy, dirt balls that lie far beyond Neptune for the majority of their orbits, but their perihelia exist within the orbit of Neptune, or less than about 30 AU.

TNO orbits

Figure 1: The orbits of the seven trans-Neptunian objects discovered in the Dark Energy Survey. These are polar plots, so it’s similar to what we would see if we looked at the orbits of these objects from a (space)bird’s-eye view. The figure on the left shows the full extent of their orbits; green orbits have an aphelion (their furthest extent) greater than 250 AU, purple orbits have an aphelion between 150 and 250 AU. The dashed lines have a perihelion (closest approach to the Sun) less than 37 AU (so they get pretty close to Neptune), and the solid lines have a perihelion greater than 37 AU (so they are not strongly affected by Neptune’s gravity). The figure on the right is a zoom-in showing their perihelia compared to Neptune’s orbit in blue. [Bernardinelli et al. 2020]

Finding Anti-Symmetric Orbits

This paper uses data from the Dark Energy Survey (DES), which, while on the quest for dark energy signatures far beyond our solar system, has found some extreme trans-Neptunian objects (eTNOs, basically very distant TNOs). Based on the observed TNOs from DES, we can see that their orbits appear to be aligned. As you can see in Figure 1, they appear to lie on one side of the sky, having similar ecliptic longitudes. That’s weird, because things in space tend to be symmetrically distributed, or isotropic. So, shortly after astronomers saw these weirdly aligned orbits, an interesting hypothesis came about. Maybe there is a super-Earth located way beyond the orbit of Neptune that is pushing these TNOs onto these aligned orbits. That hypothesized planet was nicknamed Planet 9 and is still being hunted for after about 4 years of searching.

But What If We Just Aren’t Looking Hard Enough?

Before we hype up this underdog of a planet, we must ask ourselves, is Planet 9 really the most likely explanation for the TNO clustering? We have not observed the full TNO population. What if that population is actually isotropic, and we are just looking at a few members of that population that happen to be on one side of the sky? This is the question that today’s paper poses. Given how we observed these objects, and where we’ve pointed our telescopes, could the observed TNOs be just one part of an isotropic population? If so, then Planet 9 doesn’t need to exist.

This paper starts to explore that question by creating a simulated population of 40 million TNOs that are defined by certain orbital parameters. The longitude of the ascending node (ᘯ), argument of perihelion (⍵), and mean anomaly (M, which is approximately the angular distance of object from its pericenter), are all given random values, i.e., they are distributed isotropically, so there is no preferred value. The eccentricity, inclination, and aphelion are kept within certain values taken from the seven observed TNOs from the Dark Energy Survey.

orbital parameter schematic

Figure 2: Schematic showing the different orbital parameters used to characterize TNOs in this study. This study created a simulated population of TNOs that had ᘯ, ⍵, and the mean anomaly distributed randomly, but kept orbital parameters such as inclination, eccentricity, and aphelion consistent with observed TNOs. [Arpad Horvath]

They then look to pare down their simulated population to only include eTNOs that could have been observed with DES. That includes finding the eTNOs that are bright enough, have traveled a significant distance across the sky, and could have been seen for multiple days. If a TNO passes all of those tests, then it is deemed to be observable. This observable population is their final sample, with which they can then run some statistical tests. They want to compare the orbital parameters of this simulated sample to the TNOs that we have observed.

histograms of orbital parameters

Figure 3: Histograms showing the highest likelihood values for three orbital parameters that describe TNOs. There were four different eTNO cases explored based on differing definitions of what an eTNO is. The green and purple lines are the eTNOs that the Dark Energy Survey has discovered. [Bernardinelli et al. 2020]

This study ran two statistical tests to compare the two samples: Kuiper’s test and a likelihood test. From Kuiper’s test, they calculate a p-value; a higher p-value means that the two populations are similar. They run the test to compare the ᘯ,  ⍵, and ᘯ + ⍵ values found in the simulated population to that which was observed. In the likelihood test, they compare the observed orbital angles for each sample to produce an f-value, which can be converted to a % likelihood. A low percent means that it is very unlikely that the observed sample comes from an isotropic population and a high percent means that it is very likely.

They also had four different observed samples, each of which is a subset of the 7 observed eTNOs, and each case was motivated by a different definition what an eTNO is. Each case appears to be aligned to some degree. Case 1 – 4 went from lenient definitions to strict definitions describing eTNOs. Case 1 included all seven TNO objects, while Case 4 only included those that had an aphelion beyond 250 AU and a perihelion greater than 37 AU — which includes only three objects that appear to be strongly aligned. The most extreme TNOs are going to be least affected by Neptune, and most affected by Planet 9, if it exists.

They then ran their statistical tests on these four cases. They found that when they include all seven eTNOs, it agrees well with coming from an isotropic population — which means that Planet 9 is not necessary to explain the TNO orbits. When they go down to the most strict definition of eTNOs, the p-values tend to drop. This means it becomes less likely the eTNOs that we see come from an isotropic population, however the p-values are not low enough to completely rule that out.

So Are the Authors Trying To Kill Planet 9?

In the end, this paper was able to recreate the orbits of known TNOs using an isotropically distributed TNO population. This means that Planet 9 doesn’t need to exist. However, their results change when they run the same statistical test against 3 out of the 7 TNOs. In this case, it’s harder to show that these orbits come from a randomly distributed population, leaving some hope for the Planet 9 enthusiasts. When working with such a small sample size (seven objects!) it’s hard to come to a confident conclusion. The authors look forward to more years of DES so that more eTNOs can be discovered, improving our understanding of TNOs and the mysterious Planet 9.

About the author, Jenny Calahan:

Hi! I am a second year graduate student at the University of Michigan. I study protoplanetary disk environments and astrochemistry, which set the stage for planet formation. Outside of astronomy, I love to sing (I’m a soprano I), I enjoy crafting, and I love to travel and explore new places. Check out my website: https://sites.google.com/umich.edu/jcalahan

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