Astrobites Archives - Page 27 of 46

Astrobites RSS

5 May 2020

An Alternative to Planet 9: Maybe There Is Nothing Special

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.

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.

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.

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

28 April 2020

Double-Peak and Destroy: Accretion in a Tidal Disruption Event Reveals Itself

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: Prompt Accretion Disk Formation in an X-Ray Faint Tidal Disruption Event
Authors: Tiara Hung et al.
First Author’s Institution: University of California, Santa Cruz
Status: Submitted to ApJ

To Catch an Accretion Disk

The universe reveals a variety of ways in which stars can die. We observe stars imploding, erupting, and merging, yet the tidal disruption event (TDE) is one of the most tumultuous spectacles of stellar destruction we have discovered so far. This transient phenomenon begins with a star orbiting near a supermassive black hole (SMBH) in the galaxy center. Oblivious to its impending doom, the star’s trajectory pushes it too close to the SMBH’s sphere of gravitational influence and tidal forces begin to shred the stellar structure. The woeful star is now a fly in a supermassive spider’s web: the star will be ripped apart, spaghettified stellar gas coming to form an accretion disk. This then results in a violent eruption of radiation as bits of star fall into the central black hole (Figure 1).

Figure 1: Artist’s interpretation of a tidal disruption event (TDE). Here a star is shredded by a supermassive black hole, forming an accretion disk that then emits bright optical radiation. The type of hydrogen emission from a TDE is dependent on whether we observe accretion disk (A) face-on or (B) edge-on. [Adapted from NASA/CXC/M. Weiss]

While we’ve detected nearly a hundred TDEs, the nature of how the star is disrupted and comes to form an accretion disk around a SMBH is still very much an open question. Theoretical predictions spanning the past two decades suggest that this infall of gas from the disrupted star can, however, be uniquely recognized in spectroscopic observations. For example, as shown in Figure 1 (A), a smoking-gun indication of accreting material would be to spot a double-peaked H-alpha emission line that arises from excited hydrogen being consumed by the SMBH. And now this exact signature was observed!

Theoretical Predictions Confirmed

In an exciting leap for the study of TDEs, the authors of today’s paper present the first confident detection of a newly formed accretion disk around a SMBH. The discovered explosion is a TDE called Astronomical Transient (AT) 2018hyz, which was observed spectroscopically by the team for over 300 days after the explosion was detected. In Figure 2 we see that by Day 51 the SMBH’s stellar consumption has revealed itself in the form of “horned” hydrogen emission line profiles.

Figure 2: Spectroscopic observations of TDE 2018hyz for over 300 days after the explosion was detected. By Day 51, we can see the infamous double-peaked line profiles emerge in H-alpha (marked in grey). These are directly linked with an SMBH accreting material from a disrupted star. [Hung et al. 2020]

This exquisite display of accretion around a SMBH allowed the authors to precisely model the TDE’s physical parameters such as the velocity, orientation, inclination, and eccentricity of the stellar gas being accreted. By running a 10-parameter grid search, the authors fit the peaked H-alpha emission in AT 2018hyz’s spectra with a multi-component model shown in Figure 3. Specifically, their modeling revealed that TDE 2018hyz was observed at a large enough inclination angle to allow for the detection of this double-peaked line profile, a direct signature of a visible accretion disk. The confirmation that TDE spectra are influenced by the angle at which we view the accretion disk will be extremely applicable to future TDE observations. This discovery has demonstrated that any TDE without double-peaked features was most likely observed with only the edge of the accretion disk visible to us.

Figure 3: Combined, multi-component model of the TDE shown in red. The dashed blue/green lines arise from the accretion disk while the dotted orange line is from the outward ejection of material after the star is ripped apart. [Hung et al. 2020]

The most exciting thing about confidently detecting an accretion disk is that it is now possible to distinguish between individual components of the TDE as a whole. For example, an accretion disk model cannot completely fit the H-alpha profile in Figure 3. The authors show that you also need a Gaussian line profile that physically represents a turbulent outflow of gas following the disruption of the star. While subtle, this newfound ability to separate the pieces of star plummeting into a SMBH from the gas that is violently ejected outwards will be instrumental in painting an accurate picture of how these brilliant bursts of radiation occur.

It should be noted that other teams have published journal articles on this same TDE, e.g., Short et al. 2020 and Gomez et al. 2020.

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.

21 April 2020

How to Grow a Giant 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: MOSEL Survey: Tracking the Growth of Massive Galaxies at 2 < z < 4 using Kinematics and the IllustrisTNG Simulation
Authors: Anshu Gupta et al.
First Author’s Institution: University of New South Wales, Australia
Status: Published in ApJ

How, exactly, galaxies form is still very much an open question in astrophysics. It’s not like we can watch a galaxy evolve — most are about 12 billion years old, and even the youngest we’ve discovered is about 500,000 million years old.

There are two ways to work around this problem. The first is a simple matter of looking back into time. Light takes a finite amount of time to travel to us, and so the farther away we look, the older that light is. That means that the farther a galaxy is, the younger we see it. Instead of watching a single galaxy evolve over time, we can compare farther (“younger”) galaxies to closer (“older”) galaxies, and interpolate what may have happened to cause any changes.

The second way to work around our observational conundrum is to watch galaxies evolve in simulation space. The authors of today’s paper used IllustrisTNG100, part of a suite of large cosmological simulations of galaxy evolution. The cover image above shows a subset of luminous matter in the TNG100 simulation.

Observed Mass, Movement and Star Formation History

The kinematic properties (how things are moving) of star-forming galaxies is strongly linked to how they gained their mass. Today’s authors compared the velocity dispersion of observed “younger” galaxies at redshift z = 3.0–3.8 to “older” galaxies from previous studies of redshift z ~ 2 and found that their most massive galaxies had smaller velocity dispersions than massive “older” galaxies.

Figure 1: Velocity dispersion as a function of mass, shown on a log–log scale. The authors’ “younger” galaxies are shown as gold stars. Other shapes represent previous studies of “older” galaxies at z ~ 2. The more massive galaxies in the authors’ sample are represented by larger stars and have smaller velocity dispersions than “older” galaxies of the same mass (shown in the red circle). [Adapted from Gupta et al. 2020]

By looking at the spectra of these galaxies, the authors could also extract their star formation histories. Basically, this looks at how old current stars are to extract the star formation rate over time. The top panels of Figure 2 show the authors’ results (keep in mind, time reads as newer on the left and older on the right). The bottom two panels show results from previous studies of galaxies at z ~ 2. While the less massive galaxies in the authors’ survey (top left panel) show the same pattern of increasing star formation rate, the most massive galaxies on the right have relatively flat star formation histories. This is in contrast to massive galaxies at z ~ 2, which show an increasing star formation rate over time.

Figure 2: Star formation histories for four different populations of galaxies. The x-axis is galaxy time before observation and the y-axis is star formation rate. The top two panels are the galaxies from the authors’ survey at z ~ 3 and the bottom two are from a previous survey at z ~ 2. The less massive galaxies are on the left and the more massive galaxies are on the right. The shaded areas indicate errors and the red arrows point toward trends. [Gupta et al. 2020]

Both the odd star formation histories and velocity dispersions point to something happening between z = 3 and z = 2 that changed massive galaxies. To determine what that might be, the authors turn to simulations.

Into the Simulation

The IllustrisTNG100 simulation starts with a distribution of mass at a redshift of z = 127 and runs until present day, z = 0. As it runs, the random fluctuations in density at z = 127 turn into galaxies, which grow, form stars and merge. The authors wanted to look at how these galaxies acquired their stars over time.

There are basically two ways that a galaxy can gain stars: either by forming them from gas belonging to the galaxy (in situ) or by accreting the stars from other, mostly smaller, galaxies (ex situ). Figure 3 shows the fraction of stellar mass in the simulation that was accreted ex situ, rather than formed in the galaxy. It shows that for the most massive galaxies (in red), the fraction of ex situ stellar mass increases between z = 3 (pink dotted line) and z = 2 (black dotted line). Meanwhile, the ex situ stellar mass fraction remained largely constant for less massive galaxies (blue).

$ex situ stellar mass fraction$

Figure 3: The fraction of a galaxy’s stellar mass that was obtained from other galaxies, rather than formed in situ. Massive galaxies are shown in red, while less massive galaxies are shown in blue. The salmon and blue shaded regions are, respectively the error for the more massive and least massive galaxies. The black and pink dotted lines indicate, respectively, z = 2 and z = 3. [Gupta et al. 2020]

Uniting Simulations and Observations

The authors speculate that this increase in ex situ stellar mass fraction seen in simulations may be responsible for the increase in velocity dispersion seen in observed massive galaxies between z = 3 and z = 2. Turbulence and gravitational instabilities driven by accretion of stars and gas would increase the randomness of velocities (i.e. the velocity dispersion).

This could also explain the difference in star formation history between massive galaxies at z = 2 and z = 3 (Figure 2). Gas is necessary for the formation of stars and if the galaxies at z = 2 have been able to gain gas from accretion, they would be able to increase their star formation rate, as seen in the bottom right panel of Figure 2. In contrast, a smaller ex situ stellar mass fraction for z = 3 galaxies indicates that there has been less accretion and less opportunity to gain new gas and thus form new stars, leading to the flat star formation trend seen in the top right panel of Figure 2.

Essentially, the younger galaxies at z = 3 have had less time to merge with other galaxies, leading to smaller velocity dispersions and less star formation.

The authors note that their conclusions are limited by many factors, including a small sample size. However, these are promising results and show how much can be gained by comparing observations and simulations.

About the author, Bryanne McDonough:

Graduate student at Boston University where I am currently studying the distribution of dark matter and satellites around galaxies using data from the Illustris simulations. My primary research interests are galactic and extragalactic astrophysics using computational methods. Outside of grad school I enjoy reading and crafting.

14 April 2020

How It’s Made, Fast Radio Burst Edition

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: Spectropolarimetric analysis of FRB 181112 at microsecond resolution: Implications for Fast Radio Burst emission mechanism
Authors: Hyerin Cho et al.
First Author’s Institution: Gwangju Institute of Science and Technology, Korea
Status: Published in ApJL

Fast radio bursts (FRBs) are probably the fastest growing and most interesting field in radio astronomy right now. These extragalactic, incredibly energetic bursts last just a few milliseconds and come in two flavors, singular and repeating. Recently the number of known FRBs has exploded, as the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope has discovered about 20 repeating FRBs (and also redetected the famous FRB 121102) and over 700 single bursts (hinted at here). However, despite the huge growth in the known FRB population, we still don’t know what the source(s) of these bursts is (are). Today’s paper looks at possible explanations for the properties of one FRB in particular to try to figure out what its source might be.

Your Friendly Neighborhood FRB

A number of previous astrobites have discussed the basics of FRBs (here, here, and here for example) but the FRB that the authors of this paper focus on is FRB 181112. FRB 181112 was found with the Australian Square Kilometer Array Pathfinder (ASKAP) and localized to a host galaxy about 2.7 Gpc away from us even though it has not been observed to repeat. That’s over a hundred times farther away than the closest galaxy cluster, the Virgo Cluster! One quality of FRB 181112 that makes it particularly interesting to study is that the way ASKAP records data allows the authors to study the polarization of the radio emission. Polarization of light is a measure of how much the electromagnetic wave (here the radio emission) rotates due to any magnetic fields it propagates through. The two types of polarization are linear polarization (Q for vertical/horizontal, or V for ±45°), which occurs if the electromagnetic wave rotates in a plane, and circular (either left- or right-handed depending on the rotation direction) if the light rotates on a circular path. By looking at the polarization of FRB 181112, shown in Figure 1, the authors can determine the strength of the magnetic field it traveled through.

Figure 1: a) The full polarization profile of FRB 181112 showing four profile components. The black line, I, is the sum of all the polarizations of light, or the total intensity of the burst. The red line, Q, is the profile using only (linearly) horizontally or vertically polarized light; the green line, U, is using only the (linearly) ±45° polarized light; and the blue line, V, is the profile using only circularly polarized light. Negative values describe the direction of the polarization. b) The polarization position angle of the zoomed in profiles from panel (a) seen in panel (c). Variation here suggests the emission is coming from different places in the source. d) A three second time series of the data where the FRB is clearly visible at about 1.8 seconds. [Cho et al. 2020]

In addition to polarization, the dispersion measure (DM), or difference in time of arrival of the FRB at the telescope between the highest and lowest radio emission frequencies due to its journey through the interstellar medium (ISM), can provide information about the properties of the environment(s) the burst travels through. Each of the four components of FRB 181112 (visible in panel (a) of Figure 1 in three different polarizations, Q, U, and V, as well as total intensity, I) are shown in the bottom row of Figure 2, and each component has a slightly different DM. By looking at how the DM changes, the authors can not only look at different emission processes that could lead these apparent changes, but can also measure how scattered the radio emission of FRB 181112 might be due to the ISM. The intensity of the emission as a function of time and radio frequency for each of the four polarization profiles (I , Q, U, and V) are shown in the top row of Figure 2. The four different components that make up FRB 181112 are shown in total intensity, I, in the bottom row of Figure 2.

Figure 2: Top row: Intensity of the radio emission of each of the four polarization profiles, I, Q, U, and V (described in Figure 1) as a function of time and radio frequency. Bottom row: Close up of the four different pulse components of the total intensity polarization profile, I, of FRB 181112 as a function of time and radio frequency. All components have been assumed to have a DM of 589.265 pc cm^-3 , and a slight slope in the intensity as a function of time and frequency can be seen in pulse 4, indicating it may have a slightly different DM. [Cho et al. 2020]

Properties of FRB 181112

Figure 3: Degree of polarization of FRB 181112. The black line (P/I) shows the total polarization, the red line (L/I) shows the linear polarization, and the blue line (V/I) shows the circular polarization. The red and black lines show a large amount of polarization constant in time, while the blue line shows the circular polarization changes over the pulse. [Cho et al. 2020]

The authors first find that FRB 181112 is highly polarized (see Figures 1 and 3), and while the degree of both the total (P/I) and linear (L/I) polarization is constant across all four components of the pulse, the degree of circular (V/I) polarization varies, as shown in Figure 3. This indicates that the FRB must have either traveled through a relativistic plasma, a cold plasma in the ISM that is moving at relativistic speeds, or that the emission was already highly polarized at the time it was emitted, meaning the source of FRB 181112 would have to be highly magnetized. However if the source of the polarization is due to the plasma in the ISM, the expected polarization would be almost completely linear (Q or U), whereas we observe significant circular polarization (V).

The authors next analyzed the four different components shown in the bottom row of Figure 2 for variations in DM and find there are some small, but significant differences between each component. These differences could be due to some unmodeled structure in the ISM, again possibly a relativistic plasma, but is unlikely since the burst lasts for only 2 milliseconds. The authors also suggest these differences in DM could be due to gravitational lensing, the radio light being bent around a massive object. This would mean different components travel through different paths in the ISM, accounting for the different DMs and four different components. However, gravitational lensing cannot explain the high degree of polarization seen in FRB 181112.

The Million Dollar Question

So how was FRB 181112 made? What caused the polarization and differences in DM? Well, the authors can’t say anything for certain. They suggest that the most likely model is a relativistic plasma close to the source of the emission, which has polarization properties similar to known magnetars (highly magnetized neutron stars known to emit radio bursts), but none of their models can fully explain all of the different properties of FRB 181112. The source of FRB 181112 remains a mystery for now, but with the huge number of FRBs now being detected, the answer may lie just around the corner.

About the author, Brent Shapiro-Albert:

I’m a fourth year graduate student at West Virginia University studying various aspects of pulsars. I’m a member of the NANOGrav collaboration which uses pulsar timing arrays to detect gravitational waves. In particular I study how the interstellar medium affects the pulsar emission. Other than research I enjoy reading, hiking, and video games.

7 April 2020

You’ve Got a Friend in Me: A Hot Jupiter with a Unique Companion

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: TESS spots a hot Jupiter with an inner-transiting Neptune
Authors: Chelsea X. Huang, et al.
First Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJL

For centuries, humankind has wondered if other planets exist outside of our own solar system, or if we are in fact unique. The first recorded attempts to observe other planets date to around the 19^th century — although exoplanets have been speculated since the 16^th century — but we did not have the technology to make the detailed measurements required to detect planets around other stars until the last few decades. The first detected exoplanet, 51 Pegasi b, was discovered in 1995, and since then we have learned that exoplanets are actually more of the rule than the exception. Some of the most common exoplanets that we are able to detect are called hot Jupiters — large gas giants like our Jupiter, but so close to their host stars that their orbital periods are on the order of 10 days or less — and mini Neptunes, similar in composition to our Neptune, but smaller.

The radial velocity curve of TOI-1130 c. The radial velocity method is based on the slight circular or elliptical movements of a star due to the gravitational effects of its planet(s), and their resulting Doppler shifts. The orange line indicates the best fit of the curve, while the blue error bars account for systematic and astrophysical unknowns. [Adapted from Huang et al. 2020]

In this paper, the authors discuss a unique system called TOI-1130, which contains both a hot Jupiter and a mini Neptune. The hot Jupiter, TOI-1130 c, has been confirmed by radial velocity measurements (see Figure 1) and is roughly 0.974 M_Jup with an orbital period of 8.4 days. Less is known about the mini Neptune, TOI-1130 b, since there are no radial velocity detections of it, but the authors are able to put an upper limit of 40 times the mass of the Earth on its mass. They do this by fitting the radial velocity data based on the assumption that there are two planets and determining what the largest mass for the Neptune could be based on the known mass of the hot Jupiter.

But why is this system unique? TOI-1130 one of only three known systems in which a hot Jupiter-type exoplanet has another planet within its orbit around the host star; the other two are WASP-47 and Kepler-730. It is thought to be a strange occurrence both because of the small sample size, and because current migration models indicate the hot Jupiter would kick smaller planets out of its way as it settled into its current orbit, like a schoolyard bully.

Despite the prevalence of hot Jupiters, the way they are formed is still a hot research topic, and systems such as these three could help shed more light on the formation problem. The three main theories for hot Jupiter formation mentioned in this paper are:

Migration: the hot Jupiter formed further out in the protoplanetary disk and migrated inward due to various potential processes
In situ formation: the hot Jupiter formed where it is now, very close to its host star
Planet-planet scattering: planets that pass close to each other gravitationally interact and push each other onto new, different orbits

Artist’s impression of TESS observing planets orbiting a dwarf star. [NASA Goddard SFC]

In systems such as TOI-1130, the first theory, migration, is likely ruled out, due to the aforementioned lack of bullying of the mini Neptunes. This indicates that different formation mechanisms could be at work for different hot Jupiters (physics must keep things interesting, so we don’t get bored). There are several current and upcoming instruments capable of large exoplanet surveys, like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope (JWST, if it ever gets off the ground), and the Wide Field Infrared Survey Telescope (WFIRST). If a larger sample of these systems could be discovered with instruments such as these we could likely learn more about the formation mechanisms of hot Jupiters, as well as their lesser-known cousins, warm Jupiters (10 days < P_orb < 100 days). Additionally, since the hot Jupiter in TOI-1130 has a longer orbital period than those of WASP-47 or Kepler-130, the authors believe that learning more about it could shed light on the differences between the formations of short-period and long-period giant exoplanets specifically, since its period is close to the 10-day limit for planets are considered to be hot Jupiters. TOI-1130 also has the benefit of being a much brighter host star than either WASP-47 or Kepler-730, which makes it easier to observe changes to the stellar shape and spectra caused by the exoplanets. By learning more about these strange systems, we can hopefully get a better idea of how these and other planetary systems form and what sort of systems we can expect to find in the future!

About the author, Ali Crisp:

I’m a second year grad student at Louisiana State University. I study both hot Jupiter exoplanets and binary star systems in the bulge of the Milky Way. I am originally from Tennessee and attended undergrad at Christian Brothers University, where I studied physics and history. In my “free time,” I enjoy cooking, hiking, and photography.

31 March 2020

Interstellar Travel with Sailing (Space) Ships

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: Propulsion of Spacecrafts to Relativistic Speeds Using Natural Astrophysical Sources
Authors: Manasvi Lingam and Abraham Loeb
First Author’s Institution: Florida Institute of Technology and Harvard University
Status: Accepted to ApJ

Travelling to distant stars is one of humanity’s long-term aspirations. However, interstellar travel is a challenging endeavour due to the vast distances between objects in the universe. For example, our closest stellar neighbour, Proxima Centauri, is over four light-years away. The journey there would take over 73,000 years with the Voyager 1 spacecraft — too long for a human crew. In general, travelling to other stars is only possible at relativistic speeds.

A critical factor limiting the velocity of traditional rockets is their need to carry fuel. Faster speeds require more fuel, but the weight of the fuel also slows the rocket down. Accordingly, more fuel is needed to accelerate! This dilemma, also called the tyranny of the rocket equation, implies that interstellar travel is difficult with conventional fuel-powered rockets.

The authors of today’s paper explore a different approach to space travel — sailing spaceships. Similar to sailing boats, these spaceships do not carry any fuel. Instead, they employ sails, which are accelerated by external forces, such as photon pressure and electrostatics. One project making use of such sails is Breakthrough Starshot, which intends to use a laser array to propel a spacecraft. In addition to laser arrays, astrophysical sources can also provide “wind” for the sails. Today’s paper investigates the dynamics of these sources and how they might thrust a spacecraft to relativistic speeds!

Sailing Ships in Space: How Do They Work?

A spacecraft can use one of two types of sails: light or electric sails. Light sails are large, thin sheets of reflective material. Photons bounce off this reflective material, thereby transferring momentum to the sail and speeding up the spaceship. You can view an artist’s impression of what a light sail might look like in Figure 1, above.

Electric sails work (and appear) quite differently from light sails. Instead of a large sheet, they consist of invisible electric fields. Figure 2 shows an artist’s impression of ESTCube-1, a 2013 experimental spacecraft to test electric sails. The space probe includes multiple electrically charged tethers, between which an electric field builds up. When charged particles arrive at this electric field, the field deflects them and captures a bit of their momentum.

Figure 2: Artist’s rendering of ESTCube-1, which was the first satellite launched with an electric sail in 2013. [Taavi Torim]

These effects might seem tiny, as every single photon or charged particle carries little momentum. Yet, if enough photons or particles bombard a sail, they can accelerate it to relativistic velocities! Accordingly, the sails require a luminous photon or particle source to accelerate to high speeds.

Which Astrophysical Sources Can We Use?

The authors studied the Sun, massive stars, supernovae (SNe), and active galactic nuclei (AGNs) as potential photon sources for light sails. The luminosity of these sources determines the maximal velocity a light sail can reach, the so-called ‘terminal velocity’ (see Figure 3). While the Sun is too faint to allow for relativistic velocities, light from massive stars can accelerate a spacecraft to several percent of the speed of light. With SNe and AGNs, even higher velocities are possible. These relativistic terminal velocities could enable interstellar space travel!

Figure 3: Luminosity (in units of solar luminosity, L_☉) of sources needed for spacecraft with light sails to reach a certain terminal velocity (in units of the speed of light c). Marked in red are the peak luminosities of possible sources: active galactic nuclei (AGNs), supernovae (SNe), massive stars and the Sun. With SNe and AGNs, relativistic velocities (10–70% of the speed of light) can be reached. [Adapted from Lingam & Loeb 2020]

However, accelerating a spacecraft to high terminal velocities takes some time. Therefore, the authors studied the acceleration time to get to 5%, 10%, and 20% of the speed of light, depending on the source luminosity (Figure 4). For sources dimmer than 10⁷ L_☉, it takes several thousand years to reach relativistic terminal velocities. Comparatively, brighter sources only require a few months to reach 10% of the speed of light. This acceleration time is problematic for SNe because their peak brightness usually lasts shorter. AGNs, though, operate on timescales of 10–100 million years, so an acceleration over a few months is no problem.

Figure 4: The time needed to reach a certain final velocity v_F for a light-sail-powered spacecraft, depending on the source luminosity. Marked in red are typical luminosities of massive stars, supernovae (SNe) and active galactic nuclei (AGNs). Terminal velocities of 10% of the speed of light and more can be reached with SNe and AGNs after a few months of acceleration. [Adapted from Lingam & Loeb 2020]

What About Electric Sails?

Electric sails might be even more promising than light sails. They need charged particles, so possible astrophysical sources are stellar winds, AGN outflows, blazar jets, or pulsar wind nebulae. Since these phenomena are quite complex, it is harder to give general predictions for electric sails than for light sails. Nonetheless, the authors found that a spaceship can reach highly relativistic velocities (up to 90% of the speed of light) with blazar jets and pulsar wind nebulae. So, electric sails could provide more efficient propulsion for interstellar sailing ships.

So When Will We Have Sailing Space Ships?

Sadly, it will be a while until interstellar space travel with sailing ships is a reality. Although the authors showed that light and electric sails are compelling propulsion techniques for interstellar journeys, two caveats remain. First, the paper neglects engineering limitations, such as the space probe’s stability during the trip. Interstellar spaceships have to endure more extreme conditions than spacecraft inside the solar system, so we need to overcome various technical hurdles before building them. For example, dust in the interstellar medium can severely damage a spacecraft traveling at relativistic speeds, and a potential crew also needs to be shielded from ionizing radiation. Second, constructing a completely new kind of spacecraft will be expensive! Currently, space agencies are mainly concerned with exploring our own solar system and satellites within Earth’s orbit. Missions to stars outside of our solar system will require additional funding.

However, the paper points out another exciting possibility — advanced extraterrestrial civilizations may have already implemented interstellar travel with sailing ships! We could find signs of them by detecting the reflections from their spaceship’s sails or by measuring their radio signals near sources. Light and electric sails might accelerate not only space travel but also our search for intelligent extraterrestrial life!

About the author, Laila Linke:

I am a second year PhD Student at the University of Bonn, where I am exploring the relationship between galaxies and dark matter using gravitational lensing. Previously, I also worked at Heidelberg University on detecting galaxy clusters and theoretically predicting their abundance. In my spare time I enjoy hiking, reading fantasy novels and spreading my love of physics and astronomy through scientific outreach!

24 March 2020

Good Luck Sorting This Hat

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 Strikingly Metal-rich Halo of the Sombrero Galaxy
Authors: Roger E. Cohen, et al.
First Author’s Institution: Space Telescope Science Institute
Status: Published in ApJ

Is This Galaxy an Elliptical (Gryffindor) or Ordinary Spiral (Slytherin)?

The Sombrero galaxy, famous for its hat-like shape, has been observed many times. However, it maintains a certain level of mystery: much like the sorting hat struggled to sort Harry Potter into a Hogwarts house, the Sombrero galaxy is difficult to sort into a galaxy classification. According to Hubble’s galaxy classification system, galaxies fit into four main categories: ellipticals, ordinary spirals, barred spirals, and irregulars. We have a fantastic edge-on view of the Sombrero galaxy, which allows us to image both its disk and hazy bulge, as seen in the cover image above. Because of its disk structure and lack of developed spiral arms, many astronomers classify the Sombrero galaxy as an early-type spiral. However, there is evidence that the size of the Sombrero’s halo (its extended sphere of stars) and its number of globular clusters are more similar to values found in elliptical galaxies. This leads us to believe that the Sombrero galaxy may have two parent components that merged: a spiral disk galaxy and an elliptical galaxy, and therefore simultaneously belongs to two different Hogwarts houses.

First-Years, This Way…

In order to determine how the Sombrero galaxy formed, the authors of today’s paper used Hubble Space Telescope (HST) images to analyze the halo of the galaxy. Their fields of view for the two images were 16 and 33 kiloparsecs (kpc) above the center of the galaxy, which is pretty far from the brightest component that we usually recognize as the Sombrero.

The goal of these images was to analyze the metallicity distribution function of the galaxy, or how much the metal content of stars changes as you move further away from the Sombrero’s center. Metallicity is measured by the quantity [Z/H], which takes the log of the ratio of metals (Z) to hydrogen (H) and compares it to what we see in the Sun. A metallicity value of 0 means that a star has the same metal content as the Sun. Values above zero are very metal-rich, and metallicity drops as you move towards negative values.

In general, galaxies with massive halos and a steeper fall-off in their metal content as you move away from the center tend to have fewer parent galaxies (galaxies that merged together to form a new baby galaxy). Therefore, a metallicity distribution function can tell us about the number of parent galaxies and the formation history of the Sombrero galaxy. We can also use a metallicity distribution function to help classify the Sombrero galaxy by comparing its peak metallicity to values found in other known elliptical and spiral disk galaxies.

Scarlet and Gold or Green and Silver?

Calculating the metallicity of stars requires photometry of the stars. Basically, we take an image of a star and count how many photons we receive in each wavelength band. In general, metal-poor stars appear bluer, and metal-rich stars appear redder. However, other effects can change their colors as well, such as the age of stars (bluer = newer!) and the dust along our line of sight. To remove these effects, today’s authors use a basic dust map to correct for the foreground dust, as well as assume a uniformly old age of 12 billion years. Once they do this, they can use models to fit the metallicity of the stars in their sample.

Figure 1: The metallicity distribution function for both fields of view (16 kpc in black, 33 kpc in red). Shaded gray areas and red error bars represent the respective uncertainties. A majority of the stars are metal-rich (have [Z/H] values near zero). [Cohen et al. 2020]

Not Slytherin, Not Slytherin, Not Slytherin

Today’s authors find that the metallicity within the Sombrero’s halo decreases as you move away from the center of the galaxy, but is dominated overall by metal-rich stars, as seen in Figure 1. Using their calculated peak metallicities, they compare the Sombrero galaxy to other galaxy metallicity measurements in Figure 2. They find that the halos of disk galaxies are more metal-poor on average (they have lower [Z/H] values), and that because of its high peak metallicity, the Sombrero galaxy fits better with the population of elliptical galaxies.

Figure 2: Comparison of the Sombrero galaxy (red pentagons) to other galaxies in peak metallicity and visible magnitude. Elliptical galaxies are on the left and disk galaxies are on the right. The dashed line on the left represents the best fit to elliptical galaxies, and the line in the right panel is the same but extrapolated to a different magnitude scale. [Cohen et al. 2020]

Using the HST images, today’s authors were also able to model the stellar mass of the Sombrero’s halo and compare it to the amount of mass the galaxy has accreted, or stolen from other galaxies. The number density of stars within the images allowed the authors to calculate a total halo mass with a few basic assumptions about the age of the galaxy. Separately, the authors used their metallicity values and a known correlation with accretion mass to calculate the Sombrero galaxy’s accretion mass. They found the accretion mass to be very similar to the total halo mass, which tells us that the Sombrero likely accreted its entire halo in a single major merger event several billion years ago!

In addition to this massive merger event, the Sombrero has other properties that defy the norm. Looking back to Figure 1, the Sombrero has a complete lack of low-metallicity stars, and it also has a higher average metallicity than any galaxy halo known to date! It is possible that a population of low-metallicity stars exists further out in the halo, but we would need images even more distant from the center of the galaxy in order to find them.

Just like Hogwarts houses, the galaxy classification system isn’t exactly black and white. However, unlike Harry Potter, we can’t just decide that we want the Sombrero to fit into one of our established categories. Despite looking very much like a disk from our edge-on view, the Sombrero galaxy possesses a host of characteristics that we associate with elliptical galaxies, likely because it formed as the result of a galaxy merger. As such, the Sombrero provides a unique overall picture of what galaxies may look like after they interact. The Sombrero galaxy clearly demonstrates the variety that exists in our universe and the contributions that instruments like HST make to our understanding of astronomy!

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.

17 March 2020

An Iced Cosmic-Ray Macchiato

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: Bottom-up Acceleration of Ultra-High-Energy Cosmic Rays in the Jets of Active Galactic Nuclei
Authors: Rostom Mbarek and Damiano Caprioli
First Author’s Institution: University of Chicago
Status: Published in ApJ

Our universe is littered with particles of unbelievably high energy, called cosmic rays. The most extreme of these particles carry the same amount of energy as a professional tennis serve, like the Oh-My-God Particle detected nearly 30 years ago. The catch: we don’t know exactly what processes can pack so much energy into a single particle. The authors of today’s article discuss how these particles might gain their energy in a way analogous to your morning trip to Dunkin’™.

Cosmic Rays at a Glance

Cosmic rays are atomic nuclei that have been accelerated to high energies in astrophysical environments, such as supernova remnants or active galactic nuclei. Although they might seem like a great tool in the multi-messenger astronomy toolbox, astronomy with cosmic rays is no simple task, as these particles get deflected by extragalactic magnetic fields.

Cosmic rays (red) consist of individual protons and nuclei of heavier elements. They are deflected by magnetic fields along their cosmological odysseys and can’t be used to point back to the place of their origin. [IceCube Neutrino Observatory]

Despite efforts to pinpoint the origins of cosmic rays, especially those of the highest energies, we’ve come up empty-handed (check out these bites for previous studies: Galactic cosmic rays, cosmic-ray anisotropy).

Even though we can’t measure where they come from, we do know their energies, and a variety of cosmic-ray experiments detect millions of these particles every year. Many of them are thousands to millions of times more energetic than the particles in the largest terrestrial particle accelerator, the Large Hadron Collider, but we don’t know how the highest energy cosmic rays get their energy.

Cosmic-Ray Acceleration: Old News

Many theories of cosmic-ray acceleration tend to revolve around the idea of Fermi acceleration. In this scenario, objects such as supernova remnants can create shocks, consisting of material moving together with supersonic speeds, and these shocks can accelerate particles to high energies. As a shock wave propagates, particles bounce back and forth across the shock boundary. Over time, successive bounces across the shock front lead to a net transfer of energy to the particles.

While Fermi acceleration does a good job of explaining cosmic rays with moderate energies and has been a staple of models for decades, it has a few pitfalls, and many argue that it can’t provide the whole story for cosmic-ray acceleration at the highest energies.

A Cosmic Cup o’ Joe

The authors of today’s paper propose a new way of looking at cosmic-ray acceleration: the espresso mechanism. Why espresso? Because instead of gradually gaining energy over time, particles gain their energy from a single shot.

In the “espresso mechanism”, particles gain tremendous amounts of energy from entering a jet for a short period of time. Here, a particle with initial momentum and energy p_i, E_i enters a jet with characteristic Lorentz factor Γ and leaves the jet with an energy equal to roughly Γ²E_i. [Caprioli et al. 2018]

Consider an object with a jet, such as an active galaxy. If a low-energy cosmic ray enters the jet (or steam), then it can be shot down the barrel of the jet and get kicked out at much higher energy. In many cases, particle energies can increase by a factor Γ², where Γ is the Lorentz factor (this reflects how fast the jet is moving). For some jets, this means particles can exit nearly 1,000 times as energetic as they were when they entered the jet.

In realistically modeled jets, material tends to clump in some regions, and these regions of overdensity (color scale in figure) cause the jet to locally move faster or slower. [Mbarek & Caprioli 2019]

While this espresso scheme sounds great in principle, many previous calculations have relied on spherical cow treatments of jets, when in reality they are remarkably dynamic and complex structures.

That’s where the authors of today’s paper come into play. These authors take a simple treatment of the espresso mechanism and complexify it by performing a full magnetohydrodynamic (MHD) simulation of ultrarelativistic jets. This takes factors like small-scale fluctuations of jet speed and jet density into account, to give a more accurate picture of the dynamics of jets.

By simulating the full structure of jets, the authors find that complex environments don’t weaken the promises of espresso acceleration. In fact, the very imperfections that manifest in realistic jets can help with particle acceleration. What’s more, jet perturbations allow particles to receive double or even triple shots of energy.

Throughout the paper, the authors describe the emergent spectra of espresso-accelerated cosmic rays. In doing this, they find that espresso acceleration is consistent with current measurements of ultra-high-energy cosmic rays in terms of energy, chemical composition, and spatial distributions, an accomplishment which no other model of cosmic-ray acceleration can boast.

Sample particle trajectories (black curves) are overlaid on top of slices of the jet, with jet velocity represented by the color in the top panels. Bottom panels show the amount of energy gained along the particle paths, showing that particles can leave jets with much more energy than they entered with. [Mbarek & Caprioli 2019]

In light of all of this, it’s probably safe to say that the future of cosmic-ray science will be very caffeinated.

About the author, Alex Pizzuto:

Alex is a PhD candidate at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. His work focuses on developing methods to locate the Universe’s most extreme cosmic accelerators by searching for the neutrinos that come from them. Alex is also passionate about local science outreach events in Madison, and enjoys hiking, cooking, and playing music when he is not debugging his code.

10 March 2020

A New Approach to Tilting Uranus

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: Tilting Ice Giants with a Spin-Orbit Resonance
Authors: Zeeve Rogoszinski and Douglas Hamilton
First Author’s Institution: University of Maryland
Status: Published in ApJ

Songs about the solar system were a significant part of my childhood. These songs taught one fact about each of the nine planets (these were the pre-Pluto’s-demotion days) and yes, I find myself singing them in the shower every now and then. The fact about Uranus is always the same: “Uranus spins on its side.” Not only is this bizarre and memorable to a child, it happens to be true. Uranus has an obliquity (tilt) of 98º, making its axis of rotation closer to the ecliptic plane than any other planet. And yet, nobody knows how it got that way.

Problems with the Current Theory

Uranus has an obliquity of 98°. [NASA]

The conventional wisdom for many years has been that one or more giant impacts must have turned Uranus onto its side when it was very young and giant impacts were common. The authors of today’s paper outline four potential problems with this theory.

If Uranus were impacted many times but Neptune were not, one would expect their rotation rates to differ significantly. The reason is that some of the impacts might have sped up or slowed down Uranus’ rotation. Instead, a day on Uranus and Neptune only differs by 6% (17.2 hours vs. 16.2 hours, respectively).
Giant impacts would disrupt the satellites orbiting Uranus. If this were true, the authors argue, the total mass of Uranus’ moons should be lower than we observe; instead, Uranus has the “appropriate” amount of moon-mass.
It’s extremely hard to design one impactor large enough to tilt Uranus. That doesn’t mean multiple impacts are out of the question, but it is a challenging scenario to model.
Giant impacts would have heated Uranus so much that a lot of the interior ice would have sublimated to gas and been ejected into space. If this occurred, Uranus’ satellites would be mostly ice, but they are actually mostly rock and only a little ice.

The authors propose a different way to tilt Uranus: spin–orbit resonance caused by a massive circumplanetary disk. To determine if this is possible, they simulated a young Uranus and Neptune evolving, each with a large orbiting disk of dust and gas.

What Is Spin–Orbit Resonance?

Figure 1. Animation showing precession of the Earth’s spin axis. [Robert Simmon/NASA]

Resonance is when two periods are integer multiples of each other. Think of pushing a kid on a swing. If you push with the right frequency, the swing will go high; if you push at other times, it won’t be as much fun. An example of orbital resonance is the system of Neptune and Pluto. Pluto orbits the Sun twice for every three times Neptune orbits, so Pluto’s orbit is reasonably stable. Spin–orbit resonance is a type of secular resonance, meaning the precession of Uranus’ spin axis resonates with Uranus’ orbit. Orbital precession means that the orbit changes orientation over time, causing the pole to point in a different direction, as shown in the animation in Figure 1.

Normally, precession happens way too slowly to resonate with orbits. For instance, Earth takes 26,000 years to precess once, but its orbital period is only one year. In this paper, the authors impart Uranus with spin–orbit resonance by increasing its precession rate — a consequence of having a circumplanetary disk.

Figure 2. The first model with a constant disk (bottom) showing changing obliquity (top) over one million years. Obliquity changes are sensitive to the exact lifetime of the disk. The middle plot shows how close the system is to perfect resonance (dashed line). [Rogoszinski & Hamilton 2020]

Growing a Tilted Ice Giant

As the ice giants formed, they each had a circumplanetary disk. Disks are relatively short-lived and only hang around for about 1 million years before the material either falls into the planet or forms a moon. That means Uranus only has one million years to turn on its side before the disk dissipates and the tilt is permanent.

In order to determine if different types of disks are capable of tilting Uranus or Neptune, the authors built computer models that capture the physical interactions of the disk and the planet. Three of the most relevant models are summarized here.

Model 1 is the simplest. It has a disk with constant mass for 1 million years before dissipating instantly. The result is shown in Figure 2, where the obliquity (top panel) can be seen changing from 0º up to about 65º and then back to 0º. The authors find that a constant disk could cause Uranus to tilt, but it could just as easily undo the tilt, making it too unpredictable.

Figure 3. The results of the second model, in which the disk slowly dissipates over one million years (bottom). Uranus’ obliquity increases and then settles around 45º (top). [Rogoszinski & Hamilton 2020]

Model 2 contains a more realistic disk that slowly dissipates over one million years, as shown in Figure 3 (bottom panel). The obliquity (top panel) reaches a maximum of about 55º and then settles to about 45º. A dissipating disk does a better job of keeping Uranus tilted, but 45º is not even half of the tilt seen today.

Model 3 is the same as model 2, but Uranus grows in mass as material from the disk falls onto it, shown in Figure 4. Here the obliquity (top panel) reaches a maximum of about 70º and settles to 60º once the disk is gone. This model gets closest to the current value of 98º, but is still not tilty enough.

One Last Tilt

Figure 4. The result of the final model, which contains a slowly dissipating disk (second from bottom) and material falling onto Uranus. After one million years, the obliquity of Uranus settles at around 60º (top). [Rogoszinski & Hamilton 2020]

The authors find that model 3 is the most realistic because it accurately captures orbital physics while plausibly tilting Uranus up to a maximum of 70º. They conclude that this supports the hypothesis that spin-orbit resonance could be responsible for Uranus’ high obliquity. They add that it would still take a massive impact, most likely, to push Uranus from 70º up to 98º, but a single impact is significantly more likely than a series of giant impacts.

The authors created many more spin-orbit models than shown here in their attempt to explain the evolution of Uranus and Neptune. As encouraging as it is that some of these models reproduce observed effects, understanding a process that occurred nearly five billion years ago, with only extremely limited observational data, will always be challenging.

About the author, Will Saunders:

I am a second year Ph.D. student at Boston University, where I study planetary atmospheres. I work with Prof. Paul Withers at BU and Dr. Mike Person at MIT using stellar occultations to measure waves and climatic changes in the atmosphere of Mars. I received my Bachelors in Physics from the University of Pennsylvania. I am also excited about co-hosting the new podcast astro[sound]bites. Check us out on Apple Podcasts, Google Play, Soundcloud, and Spotify. In my free time I enjoy traveling, visiting museums, and tasting new wines.

3 March 2020

You Were Cool, Betelgeuse

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: Betelgeuse Just Isn’t That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse
Authors: Emily M. Levesque and Philip Massey
First Author’s Institution: University of Washington, Seattle
Status: Accepted to ApJL

Betelgeuse, shown here in an infrared image from the Herschel Space Observatory, is a luminous red supergiant star located only 650 light-years away from the Sun. [ESA/Herschel/PACS/L. Decin et al.]

Veteran star Betelgeuse has been getting a lot of attention in recent months. This is because this nearby red supergiant (RSG) has been looking fainter than usual. Given that Betelgeuse is (typically) the brightest RSG in the night sky, professional and amateur astronomers alike have been able to track changes in its brightness over time, plotting its light curve.

The brightness of a star is often measured in specific wavelength bands because starlight is not monochromatic. The V band is defined at a wavelength of ~ 550 nm, which closely corresponds to wavelengths seen by the human eye.

On December 7th 2019, a team of astronomers observed that Betelgeuse had a brightness of V = 1.12 magnitude, compared to its typical value of V ~ 0.2–0.3 magnitude. This registered its faintest value in 50+ years (for historical reasons, magnitude increases as brightness decreases). Over time, the decrease in brightness continued, reaching its lowest value so far on January 30th (V = 1.61 magnitude). Around this time, it was also suggested that Betelgeuse’s dimming was slowing down.

Starry Eyed or Starry Died?

Many in the scientific community and general public have interpreted Betelgeuse’s erratic behaviour as an indication of an imminent supernova, which would be visible with the naked eye and likely to last for many days. While this is an exciting interpretation, it is also the least likely explanation. It is instead argued by many that the dimming could be due to the composition of Betelgeuse itself. Specifically, variations on the surface of Betelgeuse could lower its apparent temperature temporarily, which, according to laws of blackbody emission, would push some of its emitted light into longer wavelengths that wouldn’t be observed in the V band.

Keep Your Cool

The authors of today’s paper investigate how cool Betelgeuse actually is, and whether it is enough to explain its dimming in the night sky. They start by performing optical spectrophotometry on the RSG on February 15th 2020 using the spectrograph on the 4.3-meter Lowell Discovery Telescope in Arizona. The term “spectrophotometry” simply means measuring the spectrum of the star, where its flux is scaled according to its wavelength.

Artist’s illustration of one of the most massive star clusters within the Milky Way. The center of the cluster contains 14 red supergiant stars. [NASA, ESA and A. Schaller (for STScI)]

The authors estimate the apparent temperature of a sample of 74 galactic RSGs including Betelgeuse. They use a well-known method of measuring the strength of absorption lines due to titanium-oxide (TiO) in the star and comparing them to predicted values from stellar atmosphere models.

They compare the February 2020 Betelgeuse spectrum to a previous observation taken in March 2004. During this time, Betelgeuse had V ~ 0.5 magnitude, roughly 1.1 magnitude brighter than its brightness on 2020 Feb 15. Its corresponding apparent temperature was 3,650 K in 2004, compared with 3,600 K in 2020. The authors note that while the apparent magnitude (or flux) in 2020 is considerably lower than 2004, the overall shape of the spectrum is similar (Figure 1). The appearance of stronger TiO bands correlates with a lower apparent temperature. They go on to compare the temperature of Betelgeuse with other known RSGs in their sample, concluding that Betelgeuse is slightly cooler in 2020 compared to 2004. It is important to note that these measured temperatures each have a margin of error of ~25 K, which can exaggerate or diminish the actual temperature difference considerably.

The authors find that the temperature of Betelgeuse has not decreased proportionally to its dimming. If surface convection effects were responsible, a difference of larger than 50 K would be expected. This means a temporary “cold” period on the surface of Betelgeuse is likely not the primary cause of Betelgeuse’s loss of brightness, and some other effect must be at play.

Dust in Time

The authors also measure the amount of dust content along the line of sight to Betelgeuse, as this poses another possible explanation for its dimming. Circumstellar dust created from mass loss in Betelgeuse could potentially cause it to obscure its own light and therefore appear fainter. They find that there is no apparent change in the amount of dust between 2004 and 2020, but they nevertheless acknowledge that this conclusion assumes a particular model for dust absorption.

Observations have instead demonstrated that dust produced by RSG mass loss has a much larger grain size than expected by the typical dust model. Circumstellar dust composed of larger grains would produce an extinction effect that is more “grey”, absorbing light across the optical spectrum rather than preferentially absorbing bluer wavelengths. This large-grain dust could therefore cause dimming consistent with the change in V-band magnitude of Betelgeuse.

As of February 22nd, it was noted in this Astronomer’s Telegram that Betelgeuse has officially stopped dimming and has, in fact, begun to gradually brighten again. Various multi-wavelength observations, particularly in the ultraviolet and infrared range, are needed to shed some light on the stellar processes taking place in Betelgeuse which might be responsible for this exciting period of activity, while astronomers will keep their eyes peeled for any future episodes like this. Even though Betelgeuse might not be in the spotlight anymore, I will go on the record to say I always thought it was cool.

About the author, Sunayana Bhargava:

I’m a 3rd year PhD student at the University of Sussex, looking at X-ray observations of galaxy clusters to learn more about dark matter and large-scale structure.

Previous 1 … 25 26 27 28 29 … 46 Next