Astrobites RSS

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: Tycho’s supernova: the view from Gaia
Authors: Pilar Ruiz-Lapuente, Jonay I. Gónzalez Hernández, Mercè Romero-Gómez, et al.
First Author’s Institution: Institute of Fundamental Physics, Spanish National Research Council
Status: Submitted to ApJ

In the last 2000 years, only 8 supernovae have occurred within our galaxy that were bright enough to be recorded by humans. Among these is SN 1572, which was first spotted in the year 1572. It was observed around the world, but is perhaps most famously associated with the Danish astronomer Tycho Brahe, who wrote a small book about it titled De Nova Stella. As an aside, the title of that book is where the modern-day terms ‘nova’ and hence ‘supernova’ come from. The appearance of a new star in the sky helped to challenge the old Aristotelian understanding that the heavens were unchanging. Even today, there is a lot that we can learn from this supernova.

At the site where SN 1572 occurred, we see today a supernova remnant — a cloud of gas that was thrown off by the supernova (see Figure 1). In fact, the gas shell is still visibly expanding, as you can see in this video. By studying light emitted by the supernova and reflected from surrounding material, researchers in 2008 were able to tell that SN 1572 was a type Ia supernova. Supernovae of this type are used to measure the distances to far-away galaxies, because of their unique feature that each explosion has almost the same luminosity. We know that Type Ia supernovae are caused by exploding white dwarfs, but we don’t fully understand what triggers the explosion. There are two important models: either the white dwarf collides with another white dwarf, or it grows in mass by pulling in material from a companion star.

In the second of those two models, the companion star should survive the explosion and be flung away at a relatively high speed. Apart from having a high velocity, it would look just like a normal star near the supernova remnant. Therefore, by looking for a star near the supernova remnant that might be a surviving companion, researchers can hope to tell which of the two models triggered the explosion in SN 1572.

Just Add Gaia

Figure 2: All of the stars studied for today’s paper, lettered according to how far away they are from the SN 1572 remnant. The remnant is at the centre of the image. [Ruiz-Lapuente et al. 2018]

The team behind today’s paper set out to do just this using data from Gaia. Regular readers will no doubt have heard of Gaia by now, but a brief refresher: Gaia is a satellite that measures the positions of stars, the movements of stars due to their own innate velocity (the “proper motion” of a star), and the apparent movements of the stars created by the Earth’s movement around the Sun (the “parallax” of a star, which effectively tells you the distance of the star from the Earth). Combining these gives you information on each star’s position in 3D, and its velocity in 2D — the third component, its radial velocity (towards and away from the Earth), can be found from spectroscopy.

Today’s authors studied all the stars close to the SN 1572 remnant using the Gaia data, plus spectroscopy for stars where it was available. Figure 2 shows which stars were studied. Their aim was to determine whether any star appears to be an ejected companion to the supernova. They considered various criteria, most importantly the distance of each star from the supernova remnant, the velocity of each star, and the direction in which each star is moving. Figure 3 shows the distance of each star from the Earth compared to the distance of the supernova remnant from the Earth.

Figure 3: The distance of each of the stars studied from Earth (x-axis) and the proper motion of each star (y-axis). Vertical lines show the range of distances estimated for the SN 1572 remnant. Stars where the distance is consistent with the SN 1572 remnant are shown in blue, while those that disagree are shown in black dashes or dots. [Ruiz-Lapuente et al. 2018]

Considering these various criteria, the authors selected star G (see the labels in Figure 2) as the best candidate for being an ejected companion. This agrees with most of the previous studies, and with the Gaia data is now more certain. Star G is quite close to the supernova remnant, is moving away, and is moving at a higher speed than most of the stars in the area. It is moving at a speed which is quite unusual given its young age, as young stars tend to belong to the slower-moving ‘thin disk’ of the galaxy. In most ways star G looks much like any star, which is generally what we’d expect, so we can’t say for sure whether it is an ejected companion or just an innocent passer-by of a star.

The authors also consider another possibility for an ejected companion: what if the companion that was ejected was another white dwarf? A theory called D6 (dynamically driven double-degenerate double-detonation) predicts that this should be possible, and white dwarfs that look like they’ve been ejected have already been found. Ejected white dwarfs can end up moving a lot faster, because their lower masses mean they get a bigger kick from the supernova. Today’s authors performed a search for high-speed white dwarfs in the area around the supernova remnant. They found none. The result is not necessarily conclusive, given that such a white dwarf would be faint and there’s still a chance that Gaia would have missed it.

Overall, it seems that star G is more likely than any other star to have been the companion to SN 1572. Whether it truly was the companion remains to be seen, however — if it was not, then it becomes likely that SN 1572 formed by the collision of two white dwarfs which were completely obliterated, leaving no star to be ejected. Further study of star G will hopefully be able to tell us for sure, giving us an exciting new piece of information on how type Ia supernovae work. Even a 450-year-old supernova still has something to tell us!

About the author, Matthew Green:

I am a PhD student at the University of Warwick. I work with white dwarf binary systems, and in particular with AM CVn-type binaries. In my spare time I enjoy writing of all kinds, as well as playing music, board games and rock climbing. For more things written by me, take a look at my website.

SN 2011fe

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: Red vs Blue: Early observations of thermonuclear supernovae reveal two distinct populations?
Authors: Maximilian Stritzinger, Benjamin Shappee, Anthony Piro, et al.
First Author’s Institution: Aarhus University, Denmark
Status: Submitted to ApJ

Type Ia (pronounced “one-A”) supernovae are powerful explosions caused by a stellar remnant known as a white dwarf undergoing runaway nuclear fusion so violent that it blows the star apart. They are an important part of astronomy, as they can help astronomers estimate distances to far-away galaxies. (In fact, observations of Type Ia supernovae led to the 2011 Nobel Prize in physics for the discovery of the acceleration of the expansion of the universe.)

Type Ia supernovae are useful because we can use them as standardizable candles — objects whose inherent luminosity we can figure out based on various properties such as how long it takes for them to fade after brightening. Once we know their luminosity (how much light they actually emit) we can calculate their distance by measuring their brightness (how much light we measure from them here on Earth) and applying the inverse-square law. Actually figuring out their luminosity requires some care, however. We think that Type Ia supernovae can happen in at least two different ways (see this bite from 2012 for an explanation), and astronomers using them for distance measurements need to make various empirical corrections in order to do so.

Figure 1: This plot shows the light curves of the thirteen supernovae examined in the paper. The red and blue colors show the “Red” and “Blue” populations and how their light curves diverge within the first four days since exploding. All light curves have been normalized to correct for reddening and time-dilation. [Stritzinger et al. 2018]

Today’s paper offers another potential factor to consider, by looking at thirteen supernovae that were discovered very early after the initial explosion (within a few days, sooner after the explosion than most supernovae are discovered). The authors compared the light curves of the supernovae after normalizing for reddening caused by intervening dust and time-dilation caused by the expansion of the universe and found what appear to be two distinct populations. Based on their colors (in the astronomical sense of measuring the difference in brightness of their light between two standard filters) the two populations were named “Red” and “Blue.” Interestingly, after about five days the light curves of both populations were mostly indistinguishable; the difference was only seen prior to that time.

The authors present four possible explanations for why these two distinct populations may exist: interaction with a stellar companion, the presence of radioactive nickel-56 in the outer layers of the star*, interactions with the circumstellar medium (gas and dust around the star), and simple differences between the composition or opacity of the progenitor white dwarfs. They note, however, that none of these explanations completely explain all the evidence, so we still have more to learn about these intriguing explosions.

Part of the problem is the small number of objects to work with. Although there are hundreds of Type Ia supernovae known, most of them aren’t caught soon enough after the initial explosion to see the dichotomy found in today’s paper, as it only shows up very early on. This situation should be remedied in the future, however, as more — and more sensitive — automated surveys come online that will enable us to find more supernovae sooner after their initial explosions.

*It bears noting that much of the light in the later stages of a Type Ia supernova’s lightcurve comes from the decay of large amounts of radioactive nickel-56 (and iron-56 and cobalt-56) created during the explosion. However, this nickel-56 would have been created from material deep in the interior of the white dwarf. The proposed explanation involves nickel-56 existing in the outer layers of the white dwarf very soon after the start of the explosion, perhaps from being dredged up from the center due to strong mixing or from being created by a double-degenerate merger where two white dwarfs collide and explode.

supernovae

Examples of supernovae in the authors’ early red vs. blue sample, with the supernovae positions indicated by stars. [Stritzinger et al. 2018]

About the author, Daniel Berke:

I’m a first-year grad student at Swinburne University of Technology in Melbourne, where I search for variation in the fine-structure constant on the galactic scale. When I’m not at uni I enjoy a variety of creative enterprises including photography, blogging, and video editing, or just relaxing with a good video game or some classical music.

Taurus molecular cloud

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 Greater Taurus–Auriga Ecosystem. I. There is a Distributed Older Population
Authors: A.L. Kraus, G.J. Herzig, A.C. Rizzuto, et al.
First Author’s Institution: University of Texas at Austin
Status: Published in ApJ

Introduction

If only our eyes were sensitive enough, the night sky would be ablaze with nearby astronomical structures. The Andromeda Galaxy would stretch across the sky, spanning about six times the angular width of the full Moon. The Magellanic Clouds would be fireballs tens of Moons across. And the reddened molecular filaments of the nearby star-forming region Taurus-Auriga, glittering with young stars, would stretch over an area about 30 Moons across.

Taurus-Auriga is almost too close to us for its own good. It’s the nearest substantial star-forming region, lying only 145 pc (about 470 light-years) away. Much of it is extremely young, perhaps less than 5 Myr old. Its age is invaluable for studying star formation, but its size across the sky makes it a tricky business to make a full accounting of its members. But this accounting is unavoidable if we want to establish, among other things, the timescales of star formation or molecular-cloud dissipation, and the system’s initial mass function (IMF).

Young circumstellar disks re-emit stellar light in the infrared, because the disks behave roughly like a blackbody at a temperature cooler than the host star. This “infrared excess” makes young stars with disks relatively easy to identify with infrared photometry. Indeed, the Wide-field Infrared Survey Explorer (WISE) mission has gone a long way to painting in the population of Taurus-Auriga stars that possess disks. But disk-less stellar members of Taurus are much more poorly quantified. Which stars separated by entire degrees are associated?

In fact, previous attempts suggest that the Taurus IMF is a bit weird, because it’s heavy on the low-mass end. Is this indicative of deeper physics of the star formation process? Or are we just being hoodwinked and bamboozled by a masquerade of stellar interlopers? How are we to tell apart Taurus-Auriga’s serious card-carrying members from the masked ball of harlequins and mountebanks? At 30 Moons across, it’s quite a carnival.

diskless stars in Taurus

Fig. 1: Left: The author’s sample of candidate disk-less stars in Taurus. Likely members are red, likely nonmembers blue. Right: All likely members that have disks (blue) and don’t (green). The greyscale indicates dust extinction. Note the stars that have disks tend to concentrate around the dusty filaments. Note also that the full Moon is about the size of one of the zeros on the axes. [Kraus et al. 2017]

Ferreting Out the Interlopers

The authors of today’s paper began by dredging the literature for a long list of all possible diskless Taurus members, as suggested by previous studies (Fig. 1, left). They then applied multiple criteria for deciding whether they are Taurus members or not. Here’s a table of them:

Criterion Reasoning Possible complications
Kinematics Young stellar siblings will tend to move in a co-moving crowd Stellar activity or binary systems give misleading radial velocities
HR diagram position Co-eval stars will reside on isochrones There may be a prolonged period of star formation, or other field star contaminants may be passing through
H-alpha emission Young stars will emit H-alpha due to accretion and chromosphere activity Older stars that rotate rapidly may emit H-alpha
Lithium abundances Lithium is quickly burned away in late-K and M stars, so its presence is a marker of youth Older members that have already burned it away are indistinguishable from many non-members, and F-G stars keep their lithium longer
Surface gravity Surface gravity via sodium doublet lines indicate stars are still contracting

 

Interlopers may slip through any one of these criteria, but by applying all of them, this possibility is minimized. In addition to conducting literature searches, the authors acquired values for some of these criteria by taking spectra of some of candidates with a 2.2-m telescope on Mauna Kea.

The criterion of kinematics was the last to be applied. The authors’ reasoning was that, if members’ projected radial velocities and proper motions — Taurus-Auriga is so close to us and is so extended that the motions of widely separated members will exhibit perspective effects — translated into absolute velocities that are close to the average velocity of Taurus around the center of the Milky Way, then they can be considered members.

Taurus stars as disk fraction

Fig. 2: A representation of the Taurus stars in terms of disk fraction (color; see color key on the right) and density of stars (color brightness). From this you can see that there are dense (i.e., bright) agglomerations of stars with and without disks, surrounded by an extended low-density (i.e., dark) region of mostly disk-less stars. Compare this plot to Fig. 1. [Kraus et al. 2017]

Disky Stars Tend to Clump

In the end, they found their sample to contain 218 “confirmed or likely” members, and 160 stellar charlatans (Fig. 1, right). Interestingly, between angular scales of 0.03 and 3 degrees, the level of spatial clustering is the same among disk-hosting and disk-less stars. But disky stars tend to cluster more at separation scales of <0.03 degrees, and disk-less stars are more common at scales of >10 degrees. This means that, in addition to the disk-hosting stars that tend to clump in the molecular clouds, there is a widely-distributed pall of other member stars (Fig. 2). Based on their disk-hosting fractions, it appears that the more widely-distributed ones could be ~10 Myr old — still very young for a star, but older than the disky stars. In fact, three-quarters of Taurus stars appear to be disk-less.

Have disk-less stars blown off their surrounding gas, or have they been dynamically sprayed out of denser regions? The authors cannot answer this, but they point out that the precise parallaxes and proper motions from Gaia may help. In any case, it appears that star formation in Taurus-Auriga took place over an extended timescale. How much does the low-mass-heavy IMF of Taurus-Auriga change? The answer is … not a whole lot. But the authors also note that there may yet be more members out there, including early-type stars. In fact, more member candidates were found while this paper was going through peer review.

The authors finish with some arguments based on the spatial distribution of the kinematics to suggest that the star formation of Tauris-Auriga may just be a component part of a still grander star formation history over an even larger swathe of the sky. Be on the lookout for Gaia papers that will help us piece more of this history together.

About the author, Eckhart Spalding:

I am a graduate student at the University of Arizona, where I am part of the LBT Interferometer group. I went to college in Illinois, was a secondary-school physics and math teacher in Kenya’s Maasailand for two years, and got an M.S. in Physics from the University of Kentucky. My out-of-office interests include the outdoors, reading, and unicycling.

protoplanetary disk

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: Outer Solar System Possibly Shaped by a Stellar Fly-By
Authors: Susanne Pfalzner, Asmita Bhandare, Kirsten Vincke, & Pedro Lacerda
First Author’s Institution: Max Planck Institute for Radioastronomy, Germany
Status: Accepted to ApJ

A star is born from the gravitational collapse of a cloud of gas and dust. Yet not all of the material ends up in the star, and instead forms a flat protoplanetary disk that surrounds the new star. Over time, the materials in this disk coalesce to form planets, moons, asteroids, and most other objects you might expect to find near a typical star.

Since protoplanetary disks are flat, the expectation is that all of the planets and objects orbiting a star that formed out of a protoplanetary disk should orbit on a single plane. So when we find stars with planets that orbit at multiple different inclinations, this raises questions. A recent astrobite discussed such a case, where an exoplanet was observed orbiting on a completely different plane than the other exoplanets in that same system. But we needn’t look that far to find deviations like this — our very own solar system exhibits several features that don’t line up, so to speak.

Inclinations and Eccentricities and Truncations, Oh My!

For the first 30 AU around the Sun (until right around where Neptune orbits) things are relatively “well-behaved”: most planets’ orbital inclinations only differ from each other by 1–2 degrees, and no planet has an inclination of more than 8 degrees. But beyond Neptune, in the outer solar system, orbital inclinations are considerably higher. Pluto, recently demoted from planet to dwarf planet, is one example; its orbital inclination is more than 17 degrees. The same trend exists for orbital eccentricities, which tend to be significantly larger for objects beyond 30 AU compared to those inside the 30 AU cutoff.

A similar pattern also exists in our solar system’s surface density profile, which can be obtained by smoothing out the cumulative mass of solar system objects (planets, moons, asteroids, etc.) to approximate what the Sun’s protoplanetary disk might have looked like. The surface density profile gradually declines until ~30–35 AU, where it drops abruptly by a factor of nearly 1,000 (a phenomenon often referred to as disk truncation). Coincidence? Perhaps not.

Figure 2. The orbits of select objects in our solar system. Neptune’s orbit and the orbits of the planets interior to Neptune (not labeled in the figure) are constrained to a thin plane, but the orbital inclinations of the dwarf planets beyond Neptune (Pluto, Makemake, and Eris are shown here) are significantly larger. [journalofcosmology.com].

Astronomers seek to develop theories that can help explain these peculiar features in our solar system. One proposed explanation claims that the existence of a yet undiscovered faraway planet (sometimes called Planet 9) could cause these effects on the outer solar system. A second possible explanation, involving a supernova going off near the solar system in its early days, was covered in another recent astrobite.

Today’s paper offers yet another explanation — what if a star flew by our Sun early on, stealing a bunch of the outer material from the Sun’s protoplanetary disk with it, and throwing what was left into inclined and eccentric orbits?

A Stellar Thief

To test this so-called stellar-encounter theory, the authors simulated the Sun surrounded by a smooth, flat disk composed of test particles, and then checked what happened after this system encountered another star (a.k.a. “the perturber”). They tested a broad set of initial conditions by varying the perturber’s mass, how close it came to the Sun, and the inclination of its trajectory with respect to the disk of test particles. In all cases they started with a disk that extended much further than our modern-day solar system does, and selected only simulations in which the fly-by truncated the disk at 30–35 AU, similar to where the solar system’s density drops off. They next checked which of the remaining simulations reproduced other features of our solar system, and in particular, which were left with a sparse population of objects at inclined and eccentric orbits.

Figure 3. Three simulations from today’s paper, showing the trajectory of the perturbing star in black. Going from left to right, the perturber masses are 0.5, 1, and 5 solar masses. The top row of panels depicts the average positions of the particles that remain after the fly-by, colored by how eccentric their orbits have become (note the black circle around 30 AU, the radius within which almost all particles are blue, meaning they all have very low eccentricities). The bottom row of panels indicates the initial positions (before the fly-by) of the different eccentricity populations shown in the top row, with the grey regions signifying particles that became unbound due to the perturber [Pfalzner et a. 2018].

The authors report a good fit to the observed properties of our solar system for a broad range of initial conditions, with the best fits coming from perturbers with masses ranging from 0.5–1 solar masses. The simulation that most closely resembled our solar system was obtained from an encounter with a star half as massive as the Sun passing 100 AU away from the Sun.

In addition to this success, there are two further noteworthy triumphs of the fly-by theory. First, it only takes a single fly-by to reproduce the features the authors originally set out to explain. And second, beyond solving the questions that were originally posed, the simulations also provided natural explanations to several additional unexplained features of our solar system, including the mass ratio between Neptune and Uranus, and the existence of two distinct populations of Kuiper Belt objects.

Convinced? Well, you shouldn’t be, at least not yet. As the authors point out, showing that even only one event is enough to reproduce the observed effects is meaningless until we can quantify how likely it is for an event like that to happen in the first place.

Sunny With a Chance of Fly-Bys?

For our solar system, fly-bys have been extremely rare in the last couple of billions of years. But the good news is that stars like the Sun are typically born in large groups of stars called open clusters. Due to the higher density of stars in an open-cluster environment, the chance for fly-bys goes up significantly as we go further back in time. But just how likely are they?

To check this, the authors produced simulations of open clusters of the type in which the Sun could have formed, and checked for the likelihood of fly-bys like the ones they studied above. In Figure 4 below, they show that about 5–10 million years after the Sun’s birth, there is a probability of around 0.075% for a stellar encounter per million years. Aggregated over the first billion years of the Sun’s life, the chance of experiencing an encounter reaches 20–30% — not that unlikely at all!

Figure 4. The frequency of fly-bys that lead to a truncation of the disk around 30 AU, as a function of the age of the Sun. The dotted line at 2 million years signifies the gas expulsion phase of an open cluster, after which stellar encounters become considerably less frequent. Note that fly-bys before 5 million years should be disregarded because they do not allow enough time for the outer solar-system objects to form [Pfalzner et a. 2018].

Though today’s authors are still far from proving that a stellar fly-by caused the peculiar features of the outer solar system, they demonstrated that it is at the very least a reasonable theory — one that can reproduce many observational facts, and one that is relatively realistic.

So what comes next? The fly-by simulations produced a few additional predictions, mostly involving the detailed properties of orbits in the outer solar system, which the authors are currently attempting to confirm against observational data. Ultimately, precise observations of more objects beyond Neptune hold the key to either strengthening or disproving the theory.

About the author, Tomer Yavetz:

I am a first year graduate student at Columbia University, where I currently focus on galactic dynamics and collisions between large galaxies and smaller objects like dwarf galaxies or globular clusters. I grew up in Israel, received my bachelor’s degree from Princeton University, and spent three years solving earthly problems as a business management consultant, before deciding to return to stargazing. When I’m not busy thinking about colliding galaxies and writing horoscopes, you can usually find me either cooking, eating, or exploring new running routes through the concrete jungle that is NYC.

WISE stars

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: Wide-field Infrared Survey Explorer (WISE) Catalog of Periodic Variable
Authors: Xiaodian Chen, Shu Wang, Licai Deng, Richard de Grijs, and Ming Yang
First Author’s Institution: Key Laboratory for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences
Status: Accepted to ApJS

Figure 1. An artist’s conception of WISE. The spacecraft “is about the height and weight of a big polar bear, only wider“. [NASA/JPL-Caltech]

Despite being such temperamental objects, variable stars can be reliable tools for probing the universe. By measuring the period of a variable star, we can learn about its composition, age, and brightness, broadly speaking. Applying that knowledge to large populations of variable stars can tell us about the structure of the Milky Way and our nearby universe (shoutout to Henrietta Swan Leavitt and the Cepheid period-luminosity relation) as well as stellar evolution.

Given how useful they are, it stands to reason that creating a large, reliable sample of variable stars would be a great help for astronomers. The Wide-field Infrared Survey Explorer (WISE) is an infrared space telescope whose purpose is to image “the entire sky in the infrared” (see this Astrobite for an early overview of WISE’s goals and accomplishments). Between its initial mission (2009–2011) and its revival mission NEOWISE (2013–present), WISE has examined all sorts of objects, from asteroids to distant galaxies. In this paper, the authors searched for variable stars in the trove of WISE data to create a catalog of variable stars spanning the Milky Way.

CLASSIFIED (Just Kidding)

Between WISE and NEOWISE, about five years of data were available to the authors. They worked primarily with data taken in the W1 and W2 bands, since data could be taken in those bands even after WISE’s cryostat had failed. The authors used the Lomb-Scargle periodogram (see this blog post by Jake VanderPlas for more on that) to identify any periodicity, and excluded long-period or unstable objects to get a total of 50,296 variables.

Figure 2. Example light curves of an eclipsing binary (left) and an RR Lyrae variable (right). The y-axis is the object magnitude in the W1 band and x-axis is the light curve phase. [Chen et al. 2018]

To sort the variables into different classes, the authors used the objects’ periods, colors (difference in an object’s magnitude when observed in different bands), and light-curve shape. The different variable classes considered were Type ab and c RR Lyrae, classical Cepheids, Type II Cepheids, (all types of intrinsic variables, which change in brightness as they expand and contract) and EW- and EA-eclipsing binaries (extrinsic variables, which change in brightness because the starlight is blocked by something in the system, like the other star in a binary pair). 

Light curves were particularly useful when it came to classifying the variables. In particular, extrinsic variables could be separated from intrinsic variables using their fairly symmetric shapes (see Figure 2). Light-curve amplitudes were also used to distinguish between the different types of variables.

Cataloging Differences

Figure 3. Map of Milky Way showing the location of the variables in the catalog. Most of the variables are concentrated near the galactic plane. [Chen et al. 2018]

After the classification process, the authors compared their objects to those in existing catalogs. They first checked if any of their objects were SIMBAD objects, then referenced the Catalina, OGLE, and ASAS catalogs to see if they had recovered any variables. They found that while they had rediscovered 15,527 objects, they had also turned up 34,769 new variables! Most of them were eclipsing binaries, which composed roughly 78% of the new objects, followed by RR Lyrae.

A particularly promising result from the WISE data was that the periods of recovered Catalina variables were within 0.001 days of their measured periods in the optical. What this implies is that periods measured from infrared data are as usable as those measured in the optical. This is good to know — especially since most of the new variables lie within 20 degrees of the galactic plane, an area that cannot be probed with optical telescopes. The rest of the objects are located close to the equatorial poles (see Figure 3).

The result of the authors’ analysis is a catalog of 50,282 periodic variables with 17,000 variable candidates (follow-up needed!). Given that each variable has upwards of 100 detections, this catalog is very robust, and the variables listed within span a region of the Milky Way that hasn’t before been studied with these sorts of objects. When all is said and done, this paper details a resource that will be extremely helpful for studying galactic structure, stellar evolution, and more.

About the author, Tarini Konchady:

I’m a first year graduate student at Texas A&M University. Currently I’m looking for variable stars to better calibrate the distance ladder. I’m also looking for somewhere to hide my excess yarn (I’m told I may have a problem).

Illustration of the Gaia spacecraft in front of the 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: Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2
Authors: Travis A. Berger, Daniel Huber, Eric Gaidos, Jennifer L. van Sanders
First Author’s Institution: Institute for Astronomy, University of Hawaii
Status: Submitted to the AAS Journals

One of the most important mantras in studying exoplanets is this: you only know your planet as well as you know your star. This is because almost 95% of exoplanets have been detected indirectly, relying on stellar measurements to infer the existence of a planet. Using the transit method, we search for small dips in star light as the planet passes in front of the star. From this dip, we can approximate the size of the planet, assuming we know the size of the star. For radial-velocity detections, we measure the “wobble” in star light as the planet tugs on its host star. From this wobble we can measure the approximate mass of the planet — but again this is assuming we know the mass of the star. Because we rely so heavily on the stellar characteristics to infer planetary properties, we must characterize and classify these stars to the best of our ability.

But determining the masses, sizes, and even temperatures of stars is a challenge. Stars only provide one observable — their light. It is only when we combine their spectrum with models that we are able to deduce their other physical properties. Yet, this is also difficult as we require their intrinsic luminosity, without effects of distance. A dim close object may appear brighter than a distant bright one. Therefore, to determine a star’s actual brightness we need to know its distance. Without a precise distance measurement, we cannot accurately obtain its intrinsic luminosity, and without this information, we cannot properly constrain any of its characteristics and may even potentially misclassify the star!

With the recent Gaia data release of new parallaxes, we now have accurate distance measurements to over a billion stars. In other words, we are now able to better characterize these billion stars, which is an essential step towards better understanding exoplanets. That is the goal of this paper. The authors use Gaia distances to improve the stellar radii measurements of nearly all of the stars observed by Kepler. From these updated radii, the authors are also able to better constrain the sizes of thousands of confirmed and potential exoplanets.

Stars Before Planets

Figure 1: A comparison of stellar radii before Gaia (Kepler DR25 Catalog) and after Gaia. The red line represents a 1:1 relation between the two data sets. It appears that after Gaia, many stars have larger radii than previously reported. [Berger et al. 2018]

Of the ~190,000 stars observed by Kepler, the authors improved the radii measurements of >180,000 of them. With the Gaia data, the authors achieved a precision 4–5 times better than previous measurements. They also discovered that while the radii for many stars remained the same after applying the Gaia correction, there was still significant scatter, which is shown in Figure 1. The color in this figure correlated with the number of stars corresponding to that measurement. A general 1:1 trend is plotted in red, indicating the line for which there is no change in stellar radii before and after Gaia. For some of the Kepler stars, the Gaia data produced larger radii that what was initially reported, notably for stars that were initially reported at 1 solar radius.

Figure 2 plots a Hertzsprung-Russell (HR) Diagram for all updated Kepler stars. The black points represent main-sequence stars (like our Sun), while the green points plot the subgiants, and the red points plot the giants or evolved stars in the data set. Kepler’s main task was to find Earth-like planets around Sun-like stars, which is why there is a large number of stars between 5,500 and 6,500K. Initially these stars were assumed to be on the main sequence, though the authors now show that quite a few are actually subgiants, or stars that are beginning to evolve off the main sequence. Regardless, 65% of stars targeted by Kepler are main sequence stars, while 23% are subgiants and 12% are giants. The authors note that the re-classification of stars may lead to future studies on stellar and planetary evolution, as several of these subgiants host their own planetary systems.

Figure 2: HR Diagram for the stars observed by Kepler. Instead of plotting luminosity as a function of temperature, which is standard for HR diagrams, the authors chose to compare temperature and stellar radius. The stellar radius can act as a proxy because it is proportional to the square root of the luminosity. [Berger et al. 2018]

And Now For the Planets

But what about those stars that actually host planetary systems? Figure 3 plots the HR diagram of only stars with detected planets or planet candidates. Red points are the confirmed stellar hosts, while the black points are the stars that have a planet candidates but follow-up is required to confirm. Again, there is still a pile-up of stars around 6,000K as seen in Figure 2. Interestingly, not many planets are found around larger main-sequence stars, but this is biased as signals are smaller around large stars. There also appears to be a large number of planetary candidates around giant stars. It is difficult to confirm these planets because giant stars are active and can have large signals that mask planet signals, creating false positives. These stars are also larger than main-sequence stars, which creates shallower transits.

Figure 3: A subset of stars from Figure 2 that host planets or unconfirmed planets. Confirmed planets appear to prefer main-sequence and subgiant stars, though this is likely to be a detection bias. [Berger et al. 2018]

Figure 4: A comparison of planetary radii from pre-Gaia (DR25) and post-Gaia. Most planets follow the 1:1 correlation with some scatter, though there appears to be a preference for larger planets after Gaia. This is due to the larger stellar radii found with Gaia and displayed in Figure 1. [Berger et al. 2018]

When comparing the planetary radii before and after Gaia, the results show less scatter than with the stars (Figure 4). This is promising, though there is still some scatter in the data, especially among the planetary candidates. There is also some skew towards larger planets after Gaia, a consequence of finding larger stellar radii. The authors also find that the more precise stellar radii do provide better constraints on the planetary radii, though this varies from planet to planet.

Exoplanetary Science With Gaia

With this paper, the authors demonstrate the exciting potential of Gaia for future exoplanetary studies. By better understanding the stellar hosts, we can continue to constrain the characteristics of exoplanets. These will prove essential with the upcoming exoplanet missions, including the just-launched TESS mission. If we have any hope of performing accurate statistical studies of planets or even searching for potential signs of habitability, we must first focus our attention on the stars. After all, we can only observe a star’s light, and look at what we have accomplished with it. All we needed to know was where that light was coming from!

About the author, Jessica Roberts:

I am a graduate student at the University of Colorado, Boulder, where I study extra-solar planets. My research is currently focused on understanding the atmospheres of the extremely low-mass low-density super-puffs. Out of the office, you will probably find me running, cross-stitching, or playing with my dog.

planet transits

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 Compact Multi-Planet System With A Significantly Misaligned Ultra Short Period Planet
Authors: Joseph E. Rodriguez et al.
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Submitted to the AAS Journals

Our solar system is flat: all the planets orbit in the same plane as the Sun’s equator, at 90 degrees to its pole, with low orbital inclination relative to each other. While distant exoplanets can orbit at any inclination relative to the plane of the sky, the exoplanets that we detect via transits nearly always have inclination values very close to 90° — i.e, they orbit perpendicular to the plane of the sky. This is what allows us to see them passing in front of their star.

However, the authors of today’s paper have discovered a transiting exoplanet with an inclination of 76.5°. This is possible because EPIC248435473 b is an ultra short period planet (USP) with a period of just 0.66 days, meaning from our viewpoint the planet still transits in front of its star in spite of its larger inclination angle. What makes this discovery more interesting is that the authors have found up to five other transiting planets in this system, all aligned with inclinations 88-90°, making EPIC248435473 b the odd one out.

Discovering and Modeling the Planets

Figure 1: The full K2 light curve of EPIC248435473 from Campaign 14. Top: light curve corrected for systematics. Bottom: flattened light curve with best fit model from EXOFASTv2 showing modeled transits of its planets. [Rodriguez et al. 2018]

Three planet candidates (later named b, d, e) around EPIC248435473 were picked up automatically by the pipeline run on the K2 data (see Figure 1). The authors identified three additional candidates from visual inspection of the light curves. The phase-folded light curves of the six potential planets can be seen in Figure 2. Only four of these planet candidates could be statistically validated, as noisier light curves were extracted to exclude a close background star, making the weakest two candidates no longer reliable. These two remaining planet candidates will require more data to confirm.

Figure 2: Phase folded light curves for validated planets: b,c,d,e and candidate planets .02 and .06. All the repeating transit data from K2 is folded onto each planet’s period, combining into one transit to show the best period and quality of fit of the EXOFASTv2 model (shown by the red line). [Rodriguez et al. 2018]

The authors then simultaneously modelled the SED, radial velocities (RVs), and flattened K2 photometry for the system of four validated and two candidate planets using a software package called EXOFASTv2. The stellar radius and mass were inferred from the spectroscopy. Simultaneous modeling refined the system properties, including the planet masses and inclinations, to fit all the available data.

Modeling planet b was made tricker as it has a grazing transit — only part of the planet passes in front of the stellar disk — resulting in the V shape transit in Figure 2. In this case, the transit data cannot constrain the planetary radius well. As an approximation, the planetary mass found using RVs can be used to estimate the planet radius. Unfortunately the RV data is not precise enough to constrain the mass (see Figure 3), so only an upper limit is found. Using Chen and Kipping’s exoplanet mass–radius relation results in a wide possible radius distribution between 1–10 Earth radii, shown in Figure 4, with the radius most likely about 3 Earth radii.

Figure 3: Radial-velocity data folded on planet b’s period. RV data is not precise enough to measure the planet mass, as RV error bars are quite large compared to the RV signal. Therefore only an upper limit on mass can be found. [Rodriguez et al. 2018]

Figure 4: Probability of planet b having different radii. Transit depth sets a lower limit of 1 Earth radius while an upper mass limit comes from the non-detection of RV signal which is converted to a radius. [Rodriguez et al. 2018]

A Misaligned Planet in a Multi-Planet System

Explaining how EPIC248435473 b entered its short misaligned orbit is an open question. Theorists do not believe it could have formed so close to its star; they argue it must instead have moved in from further away in the system.

Proto-USP planets have been proposed to start in periods of 5–10 days before being gravitationally tugged into eccentric and inclined orbits by other planets. These orbits later circularize at the point closest to the star, but the planets stay inclined. However, this planet’s radius of 3 Earth radii, if confirmed, is larger than other USP planets and implies a dense and water-rich atmosphere that must have formed much further away from the star to resist the strong photoevaporation.

Additional complications arise as EPIC248435473 b is in a compact system. Generally, the planets moved into ultrashort period orbits by the mechanism described above would only have companions with periods of 10 days or more. Planet c and candidate .02 are closer, with periods of 6 and 7 days, so they must have migrated inwards after the USP planet reached its present location.

Further observations to constrain the mass and radius of EPIC248435473 b, as well as study of its atmosphere, may help narrow down its formation history. The planet bulk density and atmospheric constraints on water could help restrict its origin within the disk and determine when it reached its current location. This information should help explain how this strange system came to exist.

About the author, Emma Foxell:

I am a PhD student at the University of Warwick. My project involves searching for transiting exoplanets around bright stars using telescopes on the ground. Outside of astronomy, I enjoy rock climbing and hiking.

AB Doradus

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 Young Ultramassive White Dwarf in the AB Doradus Moving Group
Author: Jonathan Gagné, Gilles Fontaine, Amélie Simon, & Jacqueline K. Faherty
First Author’s Institution: Carnegie Institution of Washington DTM
Status: Published in ApJL

Figure 1: Artist’s impression of a white dwarf. [All About Space/Imagine Publishing]

In today’s astrobite, we’re talking about a paper on the AB Doradus co-moving group of stars. The paper shows that a white dwarf named GD 50 is part of the AB Dor group; we’ll be talking about how the authors arrived at this result, and what this can tell us about the history and stellar evolution of the group.

A co-moving group of stars is just what it says on the tin: a group of stars that are all moving in about the same direction at about the same speed. Such a group is normally around a couple of dozen stars. They’re generally not too far away from each other in space as well, but they’re not nearly as tightly bunched up as clusters are; the space inside a co-moving group can contain many stars that aren’t part of the group. For instance, the Ursa Major moving group includes most of the brightest stars in the Big Dipper, and it also extends to include some stars at the far end of the sky in Triangulum Australe, but it does not include the Sun — even though the Sun is between the two. Stars in a co-moving group are generally thought to have formed all at once, perhaps as part of an open cluster that has broken apart. This shared history can make co-moving groups useful for constraining models of stellar evolution.

One of the closest co-moving groups is the AB Doradus moving group, a group of about 30 stars centred about 20 parsecs away from the Earth. A possible member of the group for some time has been a white dwarf called GD 50, but there have never been precise enough data on the star to know for sure. If true, GD 50 would be the only white dwarf in the group. This makes it interesting for stellar evolution purposes, as it would provide a new window onto the stellar history of the group. White dwarf models have their own set of strengths and weaknesses that differ from those for main sequence stars; for instance, the age of a white dwarf can generally be determined much more precisely than a main sequence star.

Testing Membership

Figure 2: The measured velocity of GD 50 (shown as a red star) compared with other members of the AB Dor group of stars. U, V and W refer to velocity components in each of three different directions (see text). Because the spectroscopic radial velocity of GD50 is much less certain than other velocity measurements, the yellow dashed line here shows the direction in which GD 50 would move if its radial velocity were to change, and the spread of purple dots show randomly-generated models spread around the best-fit value to give an idea of the uncertainty. The star called AB Dor, after which the co-moving group is named, is highlighted in green. [Gagné et al. 2018]

Today’s authors set out to test GD 50’s membership of the AB Dor group using new data from Gaia‘s second data release. Regular readers might be growing sick of reading about Gaia by now, but for those who aren’t familiar with the idea, Gaia is a satellite that has measured the distance to over a billion stars by the parallax method

, as well as measuring the proper motions (motion across the sky) of those stars. By combining these with a spectroscopic measurement of the star’s velocity towards or away from the Earth, we can pin the star down quite well in both position and velocity space.

In order to test whether GD 50 really belongs to the AB Dor group, the authors took the position and velocity of the star in three dimensions (away from the Galactic centre, around the Galactic centre, and up-and-down relative to the Galactic plane) to give six properties. For each property, they modelled the group as having a Gaussian distribution, and they found the probability of a group member having the property measured for GD 50. They compared this to the probability of the star being just any old member of the galactic field by modelling the local galactic field as a more complex combination of 10 Gaussians. Overall, they found a 99.7% likelihood that the star is a part of the AB Dor group (see Fig 2).

Figure 3: Histogram of initial stellar masses in the AB Dor group. The right-most bin includes only one object, which is the main sequence star that evolved into GD 50. The red dashed line shows a model distribution for inital stellar masses. For the 4 highest-mass bins the model agrees well. The lowest-mass bin has fewer stars than the model predicts. This is probably due to the selection effect against low-mass, faint stars — ie, these stars exist but we haven’t detected them yet. Source: Figure 3 in today’s paper.

The History of GD 50

So what do we know about GD 50, and what can this tell us about the AB Dor group? Firstly, we know what kind of main sequence star GD 50 evolved from. Stars lose most of their mass as they evolve into white dwarfs, but this process is pretty well understood. GD 50 is a pretty heavy white dwarf, with a mass 1.28 times the mass of the Sun. This means that, as a main-sequence star, it must have come in at about 7.8 times the mass of the Sun. This would make it the only star in the AB Dor group with this much mass, but given models of initial mass functions for clusters, we would expect the group to contain around one star this massive (see Fig 3).

We can also tell the age of GD 50 quite well based on models of stellar evolution and white-dwarf cooling — the first tells you how long a star of 7.8 solar masses would live, and the second tells you how long it must have been a white dwarf in order to match GD 50’s temperature. Today’s authors find a total age for GD 50 of 117 +/- 22 million years. Previous estimates for the age of the AB Dor group gave around 130-200 million years, so the estimate in today’s paper is a touch shorter and quite a lot more tightly constrained. This puts the AB Dor group among the youngest known nearby co-moving groups.

Overall, the identification of GD 50 as part of the AB Dor group allows us to put some constraints on the history of that group, and gives us a way to check previous measurements of properties like the group’s age. The data from Gaia is only beginning to make its impact, and we can hope that, in the future, more results like this will be found and help us to build towards a better understanding of the histories of these stellar groups.

About the author, Matthew Green:

I am a PhD student at the University of Warwick. I work with white dwarf binary systems, and in particular with AM CVn-type binaries. In my spare time I enjoy writing of all kinds, as well as playing music, board games and rock climbing. For more things written by me, take a look at my website.

Gaia

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: Off the beaten path: Gaia reveals GD-1 stars outside of the main stream
Author: Adrian M. Price-Whelan, Ana Bonaca
First Author’s Institution: Princeton University
Status: Submitted to ApJL

The Gaia space telescope is revolutionizing our understanding of the Milky Way. This European satellite (Figure 1) is carefully tracking the positions of over a billion stars over five years, providing us with an evolving map of stellar locations and velocities. Just a couple months ago the second Gaia data catalog was released, including brand new information about the motions of many times more stars than in previous datasets to accuracies never before achieved, launching a scramble to see what exciting surprises this new data would reveal about our galaxy. (For more examples of exciting Gaia science see these Astrobites.)

This new data is a powerful tool for studying the stellar halo of our galaxy — the outer region of the Milky Way system that consists of a spherical cloud of stars extending beyond the central spiral, into the outer reaches of the Milky Way’s dark matter halo. This halo of stars, and all of the systems it contains, are powerful probes of the local dark-matter distribution and contain clues as to the history of the formation and evolution of our galaxy. Today’s paper considers one particular type of system in the stellar halo — stellar streams. These groups of stars, stretched into arcs across the sky, are the remnants of small galaxies and clusters of stars that have been torn apart as they orbit around the Milky Way. Previous Astrobites have considered how stellar streams are essential tools for studying galaxy formation and the nature of dark matter, and today’s paper illustrates how powerful these streams can be with the help of Gaia.

Figure 2. A stellar stream wrapped around a galaxy. [Jon Lomberg in collaboration with David Martinez-Delgado for the Stellar Tidal Stream Survey]

The authors of today’s paper look at one stream in particular, GD-1. This nearby, long stream is on an orbit that makes it ideal for constraining the properties of dark matter in the Milky Way. It also provides an excellent example for how Gaia velocities can enhance our understanding of these exciting systems.

So, how do the velocities from Gaia help in the study of stellar streams? First of all, they help to trace out the path that the stellar streams follow as they orbit around the Milky Way. This improves our understanding of the formation and evolution of the stellar streams, and thereby tightens our constraints on the local distribution of dark matter. Second of all, and perhaps less obviously, they can also help with picking out the stars that belong to these streams, clearing up our view of the systems. These objects are far out in the stellar halo of the galaxy, beyond the extent of the central disk (Figure 2), which means that their velocities are distinct from the coherently rotating disk of the galaxy. By selecting only a small range of velocities we can throw away many of the contaminating foreground stars.

Figure 3 shows this process for the stellar stream GD-1. The right-hand panel of Figure 3 shows velocities along (x-axis) and perpendicular to (y-axis) the length of the stream in the plane of the sky. The little clump of stars highlighted in orange corresponds to the stellar stream and nicely stands out from the larger clump corresponding to all of the other Milky Way stars in the region. As expected, the stream stars aren’t moving much perpendicular to the stream (close to 0 along the y-axis), but are mostly moving along the length of the stream, which traces out their orbit around the galaxy. The left panel shows the positions of the stars selected within the orange box, and the stellar stream clearly stands out!

Figure 3. Left: Positions of stars with velocities near GD-1. The thin stream stands out clearly. Right: Velocities along (x-axis) and perpendicular to (y-axis) the stream on the sky. The orange box outlines a clump of stars corresponding to GD-1. [Price-Whelan & Bonaca 2018]

This is already quite impressive, but our view of this stellar stream really opens up when the Gaia astrometry (the measurement of precise positions and velocities) is combined with photometry (the measurement of light) from other experiments! In Figure 4, the authors have matched the Gaia data with data from Pan-STARRS to consider the brightness of these stars as a function of color (right panel). These properties are related to the age and chemical composition of stars, allowing the authors to select out particular stellar populations. The left-hand panel of Figure 4 shows how clearly the stream stands out when these two types of data are used in tandem.

Figure 4. Left: Positions of stars likely to belong to GD-1 based on astrometry and photometry. This is the most clear view of GD-1 to date. The gray dashed line indicates the portion of the stream that was previously undetected, before Gaia. Other interesting features with unusually high (blobs, spurs) or low (gaps) numbers of stars are labeled along the stream. Right: Color (x-axis) and brightness (y-axis) of stars around GD-1. The orange selected region corresponds to a stellar population with a similar age and chemical composition to the stellar stream. [Price-Whelan & Bonaca 2018]

Now, what exactly have we gained from this clear view of GD-1? First, this is the most complete view of the stream to date. A large segment of the stream is visible that had previously been hidden among the foreground stars (left of the gray line in Figure 4), extending the length of the stream by 20 degrees across the sky.

Second, variations in density along the stream are immediately apparent. Figure 4 points out high-density blobs and low-density gaps, which potentially indicate very exciting science! The blob of stars may be the progenitor — the original cluster of stars that is gradually being torn apart to form the stream. The location of the progenitor is an essential piece of information for accurately modeling the formation and the orbit of this stellar stream, which can be used to study the large-scale distribution of dark matter in our galaxy. The low-density gaps are perhaps even more exciting. As discussed in previous Astrobites, small invisible clumps of dark matter can punch holes through stellar streams, leaving behind gaps. Further work will reveal the nature of these intriguing density features along GD-1, perhaps leading to new insights into the nature of dark matter.

This paper illustrates the power of combining Gaia velocities with photometry to study stellar streams, with the excellent example of GD-1. This procedure can also be applied to a variety of systems in the galactic halo, like dwarf galaxies and globular clusters, to expand our understanding of the Milky Way halo as a whole, ultimately informing theories on the nature of dark matter and the evolution of the Milky Way. This exciting new era of Gaia has only begun — stay tuned to see what other unexpected science this new view of our galaxy will reveal.

About the author, Nora Shipp:

I am a 2nd year grad student at the University of Chicago. I work on combining simulations and observations to learn about the Milky Way and dark matter.

Rho Ophiuchi

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: Compact Dusty Clouds and Efficient H2 Formation in Diffuse ISM
Author: A. V. Ivlev, A. Burkert, A. Vasyunin, and P. Caselli
First Author’s Institution: Max-Planck-Institut for Extraterrestrial Physics, Germany
Status: Accepted to ApJ

An Element of Epic Proportions

Figure 1: A filament of the Taurus Molecular Cloud, which contains both (1) stars that are newly formed and (2) stars that have yet to form. [ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2. Acknowledgment: Davide De Martin.]

The simplest known element in the universe, hydrogen, is also the most important. Hydrogen (aka “H”) plays a crucial role for many of the awesome astrophysical phenomena that have happened (and will happen!) across the universe’s history. This element fuses in the cores of stars, lights up ionized regions in supernova remnants, and serves as a building block for other elements in the periodic table — just to name a few of its many talents.

But that’s not all this epic element can do!  Its molecular form, H2, is the primary ingredient for molecular clouds, like the one shown in Figure 1. These molecular clouds are made up of both gas and dust grains. They are often found within spiral galaxies (like our own Milky Way), and they’re interesting in part because they’re the only known sites where glorious star formation occurs.

To better understand molecular clouds and their star formation, scientists have long studied the timescales for these clouds to form within the less dense, diffuse interstellar medium (a collective term for all of the matter and radiation between star systems) of space.

Classic calculations of these timescales have often assumed that the diffuse interstellar medium (aka, ISM) is pretty homogeneous, even when we zoom in. In other words, they assumed that the gas and dust in the diffuse ISM are spread out uniformly, so that every bit of the diffuse ISM looks the same as every other bit. But observations from the last 50 years or so indicate that the diffuse ISM is actually very not homogeneous, with lumps and bumps even across small (relatively speaking) solar-system-sized regions.

There have been numerous studies on what could be causing this non-uniformity at such small scales. But today’s authors look at a possible mechanism that is particularly significant for H2: the formation of tightly-packed dusty clouds and equilibrium gaseous clumps in the diffuse ISM. We’ll dive into what these clouds and clumps are made of in the next section.

Forming Clouds and Clumps

The authors discuss two main bits of physics that could form these aptly-named dusty clouds and gaseous clumps:

Major Physics #1: Attractive shadowing forces between dust grains in the diffuse ISM. These mysterious-sounding forces refer to the collective interactions that can happen between dust grains within a gas. In the diffuse interstellar medium, these forces are attractive: they pull the dust grains towards each other like little magnets, gathering them together into dusty clouds within the diffuse ISM: tightly-packed (aka, compact) dust grains with gas squeezed in between the grains.

Major Physics #2: Efficient thermal coupling between the dust and gas within these dusty clouds. The fancy term in italics here basically means that the gas within a dusty cloud adapts the same temperature as the dust within the dusty cloud. But the temperature of the dust within the cloud, Tdust, is lower than the temperature of the gas outside the cloud, Tgas. That means the gas within the cloud decreases in temperature from Tgas to Tdust; to do that (without breaking the laws of physics), the gas within the cloud also increases in density relative to the gas outside of the cloud. Then we end up with a gaseous clump within and just around the dusty cloud: gas squeezed into the space between the dust grains that is denser than the gas outside of the dusty cloud.

illustrations of the diffuse ISM

Figure 2: Illustrations of the diffuse ISM, if the diffuse ISM were actually homogeneous (left panel) and if the diffuse ISM formed compact dusty clouds (right panel). On the left, we see that the dust (the black dots) and the gas (the pale blue background) are pretty uniformly distributed across the medium. The temperature of the dust grains (Tdust) is smaller than the temperature of the gas (Tgas). On the right, we see dust (again the black dots) has gathered together to form dusty clouds, which are much denser than dust in the surrounding diffuse ISM. The gas has also gathered in gaseous clumps (shaded in dark blue) within and around the edges of the dusty clouds. Here the temperature of the gas within the dusty clouds is about equal to that of the dust within the dusty clouds (Tgas(cl)), and both temperatures are smaller than the temperature of the surrounding gas (still Tgas). [Ivlev et al. 2018]

By these two physics processes, we end up with diffuse ISM that is studded with compact dusty clouds/gaseous clumps, as illustrated in Figure 2.

It All Comes Together

With these dusty clouds embedded within, the diffuse ISM is definitely not homogeneous. This means that the chemistry happening within the diffuse ISM is also not homogeneous, and will unfold differently for the dusty clouds than it will for the surrounding diffuse ISM.

The authors derived how the rate of H2 formation would change due to these dusty clouds and equilibrium gaseous clumps. They found that the new rate of formation, compared to the rate if the diffuse ISM was actually homogeneous, increases by a factor ranging from about ~5–10 for typical gas and dust temperatures.

For the typical diffuse ISM values that the authors explored, this increased rate means that the transition from H to molecular H2 in the diffuse ISM could happen in just a few million years. The authors point out that this reduced timescale has wide astrophysical applications… but first and foremost, it affects how we expect giant molecular clouds, and thus their glorious star formation, to physically evolve. The faster transition from H to H2 could also affect other chemical species in the diffuse ISM, not just our epic element hydrogen. But we’ll need more investigation to see for sure!

About the author, Jamila Pegues:

Hi there! I’m a 2nd-year grad student at Harvard. I focus on the evolution of protoplanetary disks and extra-solar systems. I like using chemical/structural modeling and theory to explain what we see in observations. I’m also interested in artificial intelligence; I like trying to model processes of decision-making and utility with equations and algorithms. Outside of research, I enjoy running, cooking, reading stuff, and playing board/video games with friends. Fun Fact: I write trashy sci-fi novels! Stay tuned — maybe I’ll actually publish one someday!

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