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Super-Earth

Short-period super-Earths — planets larger than Earth, but smaller than gas giants — have been found orbiting ~60% of Sun-like stars around us. But how do these planets form? A new study proposes a mechanism for birthing these contradictory planets, as well as their rarer counterparts, “super-puffs”.

Two Formation Puzzles

Super-Earths are exoplanets with radii of 1–4 Earth radii and masses of 2–20 Earth masses. These numbers pose a puzzle: this mass range includes cores massive enough that they should trigger runaway accretion and result in the formation of gas giants. Yet super-Earths don’t accumulate that much gas: their atmospheres are only 1-10% the mass of their core. How do these planets manage to avoid runaway accretion?

Super-Earths may often have too little gas for their large core masses, but the flip side of this puzzle is that of super-puffs. Super-puffs are a rare class of short-period Kepler planets with the opposite problem: they have too much gas relative to their core mass. Super-puffs have large radii of 4–10 Earth radii, but small masses of 2–6 Earth masses. Their atmosphere-to-core ratios are > 20%.

atmosphere-to-core mass ratio

The final planet atmosphere-to-core mass ratio for models with different factors of gas depletion in the inner disk. The ratio is too high (like a gas giant) if the disk is not depleted (black curve), but it’s in the range of 1-10% if the disk is depleted by a factor of 100 to 10,000,000 (colored curves). [Lee&Chiang 2016]

In a recent study, two scientists at University of California Berkeley, Eve Lee and Eugene Chiang, investigated the possible scenarios that could lead to the formation of these two types of planets.

A Late Birth

Lee and Chiang found that super-Earths are able to form without running away and becoming Jupiter-like — if they aren’t born until late in the game! The inner regions of the gas disk surrounding the host star will gradually clear out over time. Planetary cores that assemble and accrete gas at the tail end of the inner disk’s lifetime — when most of the gas is already depleted — will be able to grow only small atmospheres. The limit is primarily set by the timescale: these planets have only 1 Myr to form, rather than 10 Myr, so they run out of time before running away.

The minimal gas in the inner disk in this scenario means there’s also little friction to cause migration of the planetary core. The authors demonstrate that this supports in situ formation of these super-Earths close to the host star.

Schematic of a transitional disk, in which the inner regions have been cleared of gas. [Catherine Espaillat]

Schematic of a transitional disk, in which the inner regions have been cleared of gas. [Catherine Espaillat]

Formation at a Distance

As for super-puffs, the authors’ calculations show that these planets can build their thick atmospheres further out in the disk (at distances beyond ~1 AU), where the nebular gas is colder and less dense. The rapid cooling of their dust-free atmospheres allows them to amass gas much more quickly. After building their atmospheres, the super-puffs then migrate inwards to their currently observed orbits < 0.1 AU from their host stars.

With these scenarios, the authors are able to reconcile the puzzles of both super-Earths and super-puffs with the model of planet formation via nebular accretion. Intriguingly, their proposed picture of super-Earths — forming in a gas-poor inner disk that’s fed by gas bleeding inward from a massive outer disk — is consistent with observations of transitional disks that have their inner regions cleared of gas. Future observations of such disks may help to confirm this formation model.

Citation

Eve J. Lee and Eugene Chiang 2016 ApJ 817 90. doi:10.3847/0004-637X/817/2/90

disk galaxies

Pure disk galaxies — thin disk galaxies that don’t have a central bulge — are a puzzling presence in our universe. How were these galaxies able to escape the effects that normally generate bulges? A new study has examined the properties of pure disk galaxies over the last 8 billion years in an effort to learn more.

Challenging the Model

pure disk galaxy

Example of a pure disk galaxy at z=0.86. Its brightness profile, in which surface brightness is plotted against semi-major axis, exhibits a pure exponential decay from the center of the galaxy to its edge. [Sachdeva&Saha 2016]

According to the commonly accepted picture of galaxy formation, galaxies grow hierarchically via major and/or minor mergers. These mergers ultimately scramble the galaxies’ preexisting disks, thicken the disk structures, and cause the formation of classical bulges at the disk centers.

The fact that we also observe pure disk galaxies without central bulges challenges this picture. If all galaxy formation is driven by the hierarchical model, then how have pure disk galaxies manage to escape the effects of merger activity?

In a new study, Sonali Sachdeva and Kanak Saha (Inter-University Centre for Astronomy and Astrophysics, India) examine the population of pure disk galaxies out to a redshift of z~1. Their goal is to better understand the properties of this strange category of galaxies throughout the last 8 billion years.

Disks Past and Present

Sachdeva and Saha examine the light profiles of ~570 galaxies from the Hubble Deep Field and the Sloan Digital Sky Survey. They categorize as pure disk galaxies those that can be described well by a single exponential function from the center of the galaxy out to its outer edges. Galaxies requiring an additional functional component to model the excess light in the center are classified as galaxies with bulges.

Using this categorization, the authors find that 94 of the 570 galaxies are pure disk galaxies. When they bins these galaxies into three redshift bins between z~1 and z~0, pure disk galaxies account for 15–18% of the total galaxies in each bin. This tells us that the fraction of pure disk galaxies hasn’t altered much in the last 8 billion years.

B/T ratio

Distribution of the bulge-to-total light ratio for the pure disk galaxies (blue) and the other disk galaxies (red) in the sample. [Sachdeva&Saha 2016]

Non-Merger Growth

Further examining the brightness profiles for these pure disk galaxies, Sachdeva and Saha find that both the average central surface brightness and the average scale length are the same across different redshift bins — which means that gas isn’t being fed into the interior parts of these galaxies over time. Yet in spite of this, the total stellar mass and the size of these galaxies grows substantially from z~1 to the present day — by 40% and 60%, respectively.

How could these galaxies be growing without changing their profiles (as would happen if their growth were caused by mergers)? The authors propose that these galaxies may be isolated and protected from mergers, and they grow through smooth accretion via cosmic filaments of cold gas onto their outskirts. Additional study of this unique category of galaxies may provide further insight into different mechanisms of galaxy evolution.

Citation

Sonali Sachdeva and Kanak Saha 2016 ApJ 820 L4. doi:10.3847/2041-8205/820/1/L4

pulsar timing array

Though the recent discovery of GW150914 is a thrilling success in the field of gravitational-wave astronomy, LIGO is only one tool the scientific community is using to hunt for these elusive signals. After 10 years of unsuccessful searching, how likely is it that pulsar-timing-array projects will make their own first detection soon?

GW frequencies

Frequency ranges for gravitational waves produced by different astrophysical sources. Pulsar timing arrays such as the EPTA and IPTA are used to detect low-frequency gravitational waves generated by the stochastic background and supermassive black hole binaries. [Christopher Moore, Robert Cole and Christopher Berry]

Supermassive Background

Ground-based laser interferometers like LIGO are ideal for probing ripples in space-time caused by the merger of stellar-mass black holes; these mergers cause chirps in the frequency range of tens to thousands of hertz. But how do we pick up the extremely low-frequency, nanohertz background signal caused by the orbits of pairs of supermassive black holes? For that, we need pulsar timing arrays.

Pulsar timing arrays are sets of pulsars whose signals are analyzed to look for correlations in the pulse arrival time. As the space-time between us and a pulsar is stretched and then compressed by a passing gravitational wave, the pulsar’s pulses should arrive a little late and then a little early. Comparing these timing residuals in an array of pulsars could theoretically allow for the detection of the gravitational waves causing them.

Globally, there are currently four pulsar timing array projects actively searching for this signal, with a fifth planned for the future. Now a team of scientists led by Stephen Taylor (NASA-JPL/Caltech) has estimated the likelihood that these projects will successfully detect gravitational waves in the future.

Probability for Success

detection probabilities

Expected detection probability of the gravitational-wave background as a function of observing time, for five different pulsar timing arrays. Optimistic and conservative assumptions are made for merger rates (blue and red lines, respectively) and environmental conditions (solid and dashed lines, respectively). [Taylor et al. 2016]

Taylor and collaborators statistically analyzed the detection probability for each of the projects as a function of their observing time, based on the projects’ estimated sensitivities and both conservative and optimistic assumptions about merger rates and environmental influences.

First the bad news: based on the authors’ estimates, small arrays — which contain only a few pulsars that each have minimal timing noise — will not be likely to detect gravitational waves within the next two decades. These arrays are more useful for setting upper limits on the amplitude of the gravitational-wave background.

On the other hand, large pulsar timing arrays have far more promising detection probabilities. These include the Parkes Pulsar Timing Array, the European Pulsar Timing Array, and NANOGrav — which each target tens of pulsars, with the intent to add more in the future — as well as the International Pulsar Timing Array, which combines the efforts of all three of these projects. There is an 80% chance that, within the next decade, these projects will successfully detect the gravitational-wave background created by orbiting supermassive black holes.

Based on this study, the outlook for these large arrays remains optimistic even in non-ideal conditions (such as if supermassive-black-hole merger rates are lower than we thought). So, though we may still have to wait a few years, the possibility of probing an otherwise inaccessible range of frequencies continues to make pulsar timing arrays a promising avenue of study for gravitational waves.

Citation

S. R. Taylor et al 2016 ApJ 819 L6. doi:10.3847/2041-8205/819/1/L6

Hot Jupiter

Weather variations in the atmosphere of a planet on a highly eccentric orbit are naturally expected to be extreme. Now, a study has directly measured the wild changes in the atmosphere of a highly eccentric hot Jupiter as it passes close to its host star.

HD 80606 system

Diagram of the HD 80606 system. The inset images labeled A–H show the temperature distribution of the planet at different stages as it swings around its star. [de Wit et al. 2016]

Eccentric Opportunity

For a hot Jupiter — a gas giant that orbits close to its host star — the exoplanet HD 80606 b exhibits a fairly unusual path. Rather than having a circularized orbit, HD 80606 b travels on an extremely elliptic 111-day orbit, with an eccentricity of e ~ 0.93. Since the amount of flux HD 80606 b receives from its host varies by a factor of ~850 over the course of its orbit, it stands to reason that this planet must have extreme weather swings!

Now a team of scientists led by Julien de Wit (Massachusetts Institute of Technology) has reanalyzed old observations of HD 80606 and obtained new ones using the Spitzer Space Telescope. The longer observing time and new data analysis techniques allowed the team to gain new insights into how the exoplanet’s atmosphere responds to changes in the stellar flux it receives during its orbit.

Extreme Variations

By measuring the infrared light coming from HD 80606, de Wit and collaborators modeled the planet’s temperature during 80 hours of its closest approach to its host star. This period of time included the ~20 hours in which most of the planet’s temperature change is expected to occur, as it approaches to a distance a mere 6 stellar radii from its host.

The authors find that the layer of the atmosphere probed by Spitzer heats rapidly from <500K to 1400K (that’s ~440°F to a scalding 2000+°F!) as the planet approaches periastron.The atmosphere then cools similarly quickly as the planet heads away from the star once more.

HD 80606 b light curve

Relative infrared brightness of HD 80606 b at 4.5 and 8 µm. The dip marks where the planet passes behind the star, as viewed from Earth. [de Wit et al. 2016]

Exploring an Atmospheric Layer

Based on the authors’ models, the layer of the planet’s atmosphere probed by Spitzer absorbs ~20% of the radiation incident from the host star. This atmospheric layer has a ~4-hour radiative timescale, much shorter than the ~93-hour rotation period the authors estimate for HD 80606 b — which means that the heat is not transported efficiently from the day side to the night side of the planet.

These measurements are the first of their kind for an exoplanet’s atmosphere, opening a new window into our understanding of hot Jupiters. Applying the methods used here to other eccentric planets should help us to better understand the formation mechanisms and atmospheres of these extreme planets.

Citation

Julien de Wit et al 2016 ApJ 820 L33. doi:10.3847/2041-8205/820/2/L33

Caterpillar halo

The Caterpillar Project is a beautiful series of high-resolution cosmological simulations. The goal of this project is to examine the evolution of dark-matter halos like the Milky Way’s, to learn about how galaxies like ours formed. This immense computational project is still in progress, but the Caterpillar team is already providing a look at some of its first results.

Lessons from Dark-Matter Halos

Why simulate the dark-matter halos of galaxies? Observationally, the formation history of our galaxy is encoded in “galactic fossil record” clues, like the tidal debris from disrupted satellite galaxies in the outer reaches of our galaxy, or chemical abundance patterns throughout our galactic disk and stellar halo.

But to interpret this information in a way that lets us learn about our galaxy’s history, we need to first test galaxy formation and evolution scenarios via cosmological simulations. Then we can compare the end result of these simulations to what we observe today.

This figure illustrates the difference that mass resolution makes. In the left panel, the mass resolution is 1.5*10^7 solar masses per particle. In the right panel, the mass resolution is 3*10^4 solar masses per particle [Griffen et al. 2016]

This figure illustrates the difference that mass resolution makes. In the left panel, the mass resolution is 1.5*10^7 solar masses per particle. In the right panel, the mass resolution is 3*10^4 solar masses per particle [Griffen et al. 2016]

A Computational Challenge

Due to how computationally expensive such simulations are, previous N-body simulations of the growth of Milky-Way-like halos have consisted of only one or a few halos each. But in order to establish a statistical understanding of how galaxy halos form — and find out whether the Milky Way’s halo is typical or unusual! — it is necessary to simulate a larger number of halos.

In addition, in order to accurately follow the formation and evolution of substructure within the dark-matter halos, these simulations must be able to resolve the smallest dwarf galaxies, which are around a million solar masses. This requires an extremely high mass resolution, which adds to the computational expense of the simulation.

First Outcomes

These are the challenges faced by the Caterpillar Project, detailed in a recently published paper led by Brendan Griffen (Massachusetts Institute of Technology). The Caterpillar Project was designed to simulate 70 Milky-Way-size halos (quadrupling the total number of halos that have been simulated in the past!) at a high mass resolution (10,000 solar masses per particle) and time resolution (5 Myr per snapshot). The project is extremely computationally intense, requiring 14 million CPU hours and 700 TB of data storage!

Halo mass evolution

Mass evolution of the first 24 Caterpillar halos (selected to be Milky-Way-size at z=0). The inset panel shows the mass evolution normalized by the halo mass at z=0, demonstrating the highly varied evolution these different halos undergo. [Griffen et al. 2016]

In this first study, the Griffen and collaborators show the end states for the first 24 halos of the project, evolved from a large redshift to today (z=0). They use these initial results to demonstrate the integrity of their data and the utility of their methods, which include new halo-finding techniques that recover more substructure within each halo.

The first results from the Caterpillar Project are already enough to show clear general trends, such as the highly variable paths the different halos take as they merge, accrete, and evolve, as well as how different their ends states can be. Statistically examining the evolution of these halos is an important next step in providing insight into the origin and evolution of the Milky Way, and helping us to understand how our galaxy differs from other galaxies of similar mass. Keep an eye out for future results from this project!

Bonus

Check out this video (make sure to watch in HD!) of how the first 24 Milky-Way-like halos from the Caterpillar simulations form. Seeing these halos evolve simultaneously is an awesome way to identify the similarities and differences between them.

Citation

Brendan F. Griffen et al 2016 ApJ 818 10. doi:10.3847/0004-637X/818/1/10

quasar

This past December, researchers all over the world watched an outburst from the enormous black hole in OJ 287 — an outburst that had been predicted years ago using the general theory of relativity.

Outbursts from Black-Hole Orbits

OJ 287 is one of the largest supermassive black holes known, weighing in at 18 billion solar masses. Located about 3.5 billion light-years away, this monster quasar is bright enough that it was first observed as early as the 1890s. What makes OJ 287 especially interesting, however, is that its light curve exhibits prominent outbursts roughly every 12 years.

OJ 287 orbit

Diagram illustrating the orbit of the secondary black hole (shown in blue) in OJ 287 from 2000 to 2023. We see outbursts (the yellow bubbles) every time the secondary black hole crosses the accretion disk (shown in red, in a side view) surrounding the primary (the black circle). [Valtonen et al. 2016]

What causes the outbursts? Astronomers think that there is a second supermassive black hole, ~100 times smaller, inspiraling as it orbits the central monster and set to merge within the next 10,000 years. In this model, the primary black hole of OJ 287 is surrounded by a hot accretion disk. As the secondary black hole orbits the primary, it regularly punches through this accretion disk, heating the material and causing the release of expanding bubbles of hot gas pulled from the disk. This gas then radiates thermally, causing the outbursts we see.

Attempts to model this scenario using Newtonian orbits all fail; the timing of the secondary black hole’s crossings through the accretion disk (as measured by when we see the outbursts) can only be explained by a model incorporating general-relativistic effects on the orbit. Careful observations and precise timing of these outbursts therefore provide an excellent test of general relativity.

Watching a Predicted Crossing

The model of OJ 287 predicted another disk crossing in December 2015, so professional and amateur astronomers around the world readied more than two dozen ground-based optical telescopes and the Swift/XRT satellite to observe OJ 287 in this time frame. The outburst occurred right on schedule, peaking on 5 December 2015, and the results of the observing campaign are now presented in a study led by Mauri Valtonen (University of Turku).

OJ 287 outburst

Optical photometry of OJ 287 from October to December 2015, showing the outburst that resulted from the secondary black hole crossing the disk. [Valtonen et al. 2016]

Because the secondary black hole’s orbit is affected by the spin of the primary black hole, Valtonen and collaborators were able to use the timing of the outburst to measure the spin of OJ 287’s primary black hole to remarkably high precision. They find that its Kerr parameter is 0.313 ± 0.01 — which means it’s spinning at about a third of the maximum rate allowed by general relativity.

The outburst timing also confirmed several general-relativistic properties of the system, including its loss of energy to gravitational waves. Remarkably, the energy lost as the secondary black hole punches through the accretion disk is still ten thousand times smaller than the amount of energy it loses through gravitational waves!

The observations from this outburst have provided important black-hole measurements and tests of general relativity — which are especially relevant in this new era of gravitational wave detections. And we may be able to perform still more tests on the secondary’s next pass through the disk, which should occur in 2019.

Bonus

Check out this awesome animation of the orbits in a system similar to OJ 287! The secondary’s orbit precesses around the primary due to general-relativistic effects. The sound you hear is an audio representation of the increasing frequency as the two black holes inspiral. You can find more information about this animation here. [Steve Drasco & Curt Cutler]

Citation

M. J. Valtonen et al 2016 ApJ 819 L37. doi:10.3847/2041-8205/819/2/L37

Neutron-star X-ray binary

In X-ray binaries that consist of a neutron star and a companion star, gas funnels from the companion into an accretion disk surrounding the neutron star, spiraling around until it is eventually accreted. How do the powerful magnetic fields threading through the neutron star affect this accretion disk? Recent observations provide evidence that they may push the accretion disk away from the neutron star’s surface.

Truncated Disks

Theoretical models have indicated that neutron star accretion disks may not extend all the way in to the surface of a neutron star, but may instead be truncated at a distance. This prediction has been difficult to test observationally, however, due to the challenge of measuring the location of the inner disk edge in neutron-star X-ray binaries.

In a new study, however, a team of scientists led by Ashley King (Einstein Fellow at Stanford University) has managed to measure the location of the inner edge of the disk in Aquila X-1, a neutron-star X-ray binary located 17,000 light-years away.

Fe K line feature

Iron line feature detected by Swift (red) and NuSTAR (black). The symmetry of the line is one of the indicators that the disk is located far from the neutron star; if the inner regions of the disk were close to the neutron star, severe relativistic effects would skew the line to be asymmetric. [King et al. 2016]

Measurements from Reflections

King and collaborators used observations made by NuSTAR and Swift/XRT — both X-ray space observatories — of Aquila X-1 during the peak of an X-ray outburst. By observing the reflection of Aquila X-1’s emission off of the inner regions of the accretion disk, the authors were able to estimate the location of the inner edge of the disk.

The authors find that this inner edge sits at ~15 gravitational radii. Since the neutron star’s surface is at ~5 gravitational radii, this means that the accretion disk is truncated far from the star’s surface. In spite of this truncation, material still manages to cross the gap and accrete onto the neutron star — as evidenced by X-ray flaring (almost certainly caused by accretion) that occurred during the authors’ observations.

Magnetic Effects

What could cause the truncation of the disk? The authors believe the most likely factor is pressure from the neutron star’s sizable magnetic field, pushing the inner edge of the disk out. They calculate that a field strength of roughly 5*108 Gauss (for comparison, a typical refrigerator magnet has a field strength of ~100 G!) would be necessary to hold the inner edge this far out. This is consistent with previous estimates for the field of the neutron star in Aquila X-1.

The authors point out that magnetic field lines could also explain how the neutron star is still accreting material despite the gap between it and its disk: gas could be channeled along field lines from the inner edge of the disk — which is roughly co-rotating with the neutron star — onto the neutron star poles.

The observations of Aquila X-1’s truncated disk are an important step toward confirming models of how neutron stars’ magnetic fields interact with their accretion disks in X-ray binaries.

Citation

Ashley L. King et al 2016 ApJ 819 L29. doi:10.3847/2041-8205/819/2/L29

Galactic center

New radio images of the center of the Milky Way are providing an unprecedented view of the structure and processes occurring in the Galactic center.

Sgr A structure

JVLA images of Sgr A at 5.5 GHz. The large-scale, bright ring structure is Sgr A East, a supernova remnant. The mini-spiral structure along the lower-right edge of the ring is Sgr A West, and Sgr A* is located near the center of the mini-spiral structure. Click for a closer look! [Zhao et al. 2016]

Improved Radio View

A recent study led by Jun-Hui Zhao (Harvard-Smithsonian Center for Astrophysics) presents new images of the Galactic center using the Jansky Very Large Array (JVLA) at 5.5 GHz. The images center on the radio-bright zone at the core of our galaxy, with the field of view covering the central 13’ of the Milky Way — equivalent to a physical size of ~100 light-years.

Due to recent hardware and software improvements in the VLA, these images are much deeper than any previously obtained of the Galactic center, reaching an unprecedented 100,000:1 dynamic range. Not only do these observations provide a detailed view of previously known structures within the Sagittarius A radio complex in the Milky Way’s heart, but they also reveal new features that can help us understand the processes that formed this bright complex.

Features in Sagittarius A

Sgr A consists of three main components nested within each other: the supernova remnant Sgr A East, the mini-spiral structure Sgr A West (located off-center within the Sgr A East structure), and the compact radio source Sgr A* (located near the center of the mini-spiral). Sgr A* is the supermassive black hole that resides at the very center of the Milky Way.

The newest JVLA images reveal numerous filamentary sources that trace out two radio lobes, oriented nearly perpendicular to the Galactic plane and ~50 light-years in size. These are smaller radio counterparts to the enormous (on the scale of 30,000 light-years!) gamma-ray Fermi bubbles that have been observed to extend from the Galactic center. The bipolar radio structures appear to be due to winds emanating from Sgr A* itself, from a central cluster of massive stars, or from a combination of the two.

Top: Superposition of the JVLA image of Sgr A (blue) and a molecular line image taken with the SMA (red) that shows Sgr A*’s circumnuclear disk. Bottom left: Molecular emission is shown in contours, and the Sigma Front is traced by blue lines. Bottom right: The authors’ geometrical model for the supernova explosion and resulting emission. [Adapted from Zhao et al. 2016]

Top: superposition of the JVLA image of Sgr A (blue) and a molecular line image (red) showing Sgr A*’s circumnuclear disk. Bottom left: molecular emission is shown in contours, and the Sigma Front is traced by blue lines. Bottom right: a geometrical model for the supernova explosion and resulting emission. [Zhao et al. 2016]

Supernova Structures

The outermost shape of Sgr A East — which looks like an elliptical ring — is thought to be an expanding spherical shell from a past supernova explosion, appearing as an ellipse because of our angle of view. In the newest JVLA images, Zhao and collaborators identify a new structure inside of the ring that they term the “Sigma Front”.

The authors argue that this emission front — which is shaped like the capital Greek letter sigma — may be the reflection of the supernova blast wave bouncing off of the dense, clumpy circumnuclear molecular disk around Sgr A* (which encircles the mini-spiral, but isn’t visible in radio wavelengths). Under this assumption, they use the Sigma Front to constrain the geometry of the supernova explosion.

These new JVLA images contain a wealth of information in their detail, and analysis is only just beginning. Further examination of these images will continue to help us learn about the activity at the heart of our galaxy.

Citation

Jun-Hui Zhao et al 2016 ApJ 817 171. doi:10.3847/0004-637X/817/2/171

Circumbinary planets

What happens to Tattooine-like planets that are instead in unstable orbits around their binary star system? A new study examines whether such planets will crash into a host star, get ejected from the system, or become captured into orbit around one of their hosts.

Orbit Around a Duo

At this point we have unambiguously detected multiple circumbinary planets, raising questions about these planets’ formation and evolution. Current models suggest that it is unlikely that circumbinary planets would be able to form in the perturbed environment close their host stars. Instead, it’s thought that the planets formed at a distance and then migrated inwards.

One danger such planets face when migrating is encountering ranges of radii where their orbits become unstable. Two scientists at the University of Chicago, Adam Sutherland and Daniel Fabrycky, have studied what happens when circumbinary planets migrate into such a region and develop unstable orbits.

Producing Rogue Planets

Collisions and ejections

Time for planets to either be ejected or collide with one of the two stars, as a function of the planets’ starting distance (in AU) from the binary barycenter. Colors represent different planetary eccentricities. [Sutherland & Fabrycky 2016]

Sutherland and Fabrycky used N-body simulations to determine the fates of planets orbiting around a star system consisting of two stars — a primary like our Sun and a secondary roughly a tenth of its size — that are separated by 1 AU.

The authors find that the most common fate for a circumbinary planet with an unstable orbit is ejection from the system; over 80% of unstable planets were ejected. This has interesting implications: if the formation of circumbinary planets is common, this mechanism could be filling the Milky Way with a population of free-floating, “rogue” planets that no longer are associated with their host star.

The next most common outcome for unstable planets is collision with one of their host stars (most often the secondary), resulting in accretion of the planet onto the star. Only rarely do unstable planets make it through the 10,000-yr integration without being removed from the system via ejection or collision.

Tidal Effects

As a final experiment, the authors also added the effects of tidal stripping, which occurs when the stars of the binary tear away some of the planets’ mass during close encounters. They found that this alters the orbit of the planets that have close encounters with one of the stars, making it slightly more likely that they can be captured around a star.

How can we test these models? When a star tidally strips a planet or accretes a planet in a collision, this process leaves its mark on the star in the form of stellar pollution. By comparing the amount of planetary material in the two stars of a binary, it may be possible to confirm the rates predicted here — thereby answering the question of what happens to unstable Tattooines.

Citation

Adam P. Sutherland and Daniel C. Fabrycky 2016 ApJ 818 6. doi:10.3847/0004-637X/818/1/6

VCC 1287

A series of recent deep-imaging surveys has revealed dozens of lurking ultra-diffuse galaxies (UDGs) in nearby galaxy clusters. A new study provides key information to help us understand the origins of these faint giants.

What are UDGs?

There are three main possibilities for how UDGs — galaxies with the sizes of giants, but luminosities no brighter than those of dwarfs — formed:

  1. They are “tidal dwarfs”, created in galactic collisions when streams of matter were pulled away from the parent galaxies and halos to form dwarfs.
  2. They are descended from “normal” galaxies and were then altered by tidal interactions with the galaxy cluster.
  3. They are ancient remnant systems — large galaxies whose gas was swept away, putting an early halt to star formation. The gas removal did not, however, affect their large dark matter halos, which permitted them to survive in the cluster environment.

The key to differentiating between these options is to obtain mass measurements for the UDGs — how large are their dark matter halos? In a recent study led by Michael Beasley (Institute of Astrophysics of the Canary Islands, University of La Laguna), a team of astronomers has determined a clever approach for measuring these galaxies’ masses: examine their globular clusters.

Masses from Globular Clusters

halo mass vs. stellar mass

VCC 1287’s mass measurements put it outside of the usual halo-mass vs. stellar-mass relationships for nearby galaxies: it has a significantly higher halo mass than is normal, given its stellar mass. [Adapted from Beasley et al. 2016]

Beasley and collaborators selected one UDG, VCC 1287, from the Virgo galaxy cluster, and they obtained spectra of the globular clusters around it using the OSIRIS spectrograph on the Great Canary Telescope. They then determined VCC 1287’s total halo mass in two ways: first by using the dynamics of the globular clusters, and then by relying on a relation between total globular cluster mass and halo mass.

The two masses they found are in good agreement with each other; both are around 80 billion solar masses. This is an unprecedented factor of 3,000 larger than the stellar mass for the galaxy (obtained from the galaxy’s luminosity) — which means that VCC 1287 has an unusually large dark matter halo given its stellar population.

Clues to Origins

This result makes it unlikely that VCC 1287 is a tidal-dwarf system, since these usually have dark-matter fractions of less than 10%. The authors also don’t believe it is a tidally stripped system, since no obvious tidal features were revealed in their imaging. Instead, they think the most probable scenario is that VCC 1287 is a massive dwarf galaxy that had its star formation quenched by gas starvation as it fell into the Virgo cluster long ago.

To learn whether VCC 1287 is typical of UDGs, the authors encourage finding additional UDG masses using the same techniques outlined in this study. Additional observations of the globular-cluster populations for UDGs will significantly help understand these unusual galaxies.

Citation

Michael A. Beasley et al 2016 ApJ 819 L20. doi:10.3847/2041-8205/819/2/L20

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