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MU69

One of the joys of getting new data from astronomy missions is the opportunity to test predictions. NASA’s New Horizons space probe is now beaming us data from its flyby of 2014 MU69 — and there’s a lot to learn!

A Distant Target

Pluto and Charon

This composite image with enhanced colors shows New Horizons observations of Pluto (foreground) and Charon (background). These Kuiper-belt bodies have very few craters … will MU69 be similar? [NASA/JHUAPL/SwRI]

After New Horizons passed by Pluto and its moon Charon in 2015 — sending back spectacular images of these two distant solar-system bodies — it set its sights on a new target: 2014 MU69 (nicknamed Ultima Thule). MU69 is a small object located in the Kuiper belt, the disk beyond Neptune consisting of small bodies and remnants from when our solar system formed.

On 1 January 2019, New Horizons flew by MU69, passing within 3,500 km — three times closer than the probe approached Pluto! Now, the spacecraft is slowly sending us its stored, high-resolution images, and we’re getting a progressively more detailed look at this odd object.

What predictions can we hope to test as data continues to trickle in from New Horizons? One intriguing question was asked long before we had any views of this body: How cratered is MU69’s surface?

predicted crater density

Authors’ predicted crater density above a given crater diameter vs. the crater diameter, with curves showing different times. The horizontal dashed line corresponds to 1 crater/MU69 surface. The vertical dotted line marks the smallest craters we’ll be able to detect with New Horizons images. These results suggest that after 4 billion years, only a few dozen craters of varying sizes will have formed. [Greenstreet et al. 2019]

What’s the Damage?

Months ago, a team of scientists led by Sarah Greenstreet (B612 Asteroid Institute and University of Washington) conducted a study in which they made predictions for the crater count they expected to find on MU69’s surface. Greenstreet and collaborators used observations of Pluto and Charon’s surfaces and models of known Kuiper-belt populations to explore the bombardment of MU69 over the solar system’s life span and calculate the number of craters of different sizes its surface should host.

The authors’ results were intriguing: they found that, despite getting bombarded for 4+ billion years, MU69 should be marred by very few craters. Greenstreet and collaborators estimate that MU69 should have only ~25–50 craters larger than ~200 m in size, which is the smallest size we’re likely be able to see with the full-resolution New Horizons images.

Awaiting Confirmation

Since the publication of Greenstreet and collaborators’ paper, we now have the first images of MU69 to compare to — and, as you can see for yourself in the cover image, its surface seems remarkably smooth. We’ll have to wait for the high-resolution images to finish downlinking to be sure (higher-resolution images are expected near the end of this month, and the full data are expected by September 2020), but so far, our views of MU69 seem to support the authors’ predictions.

What implications would this confirmation have? First, this would mean that MU69 is a sample of the primordial solar system: it now much like it was when it first formed ~4 billion years ago. This is in stark contrast to, for instance, the main-belt asteroids, which have undergone considerable evolution since they formed.

New Horizons

Artist’s impression of the New Horizons mission flying past Pluto and Charon. [NASA/JPL]

In addition, MU69’s low crater count may imply that there are far fewer small bodies in the Kuiper belt than we had originally expected. This scarcity would have far-reaching implications for what solar-system-formation models are possible, potentially changing our understanding of how our planets were built.

Keep sending along your data, New Horizons! We can’t wait to see what we’ll learn.

Citation

“Crater Density Predictions for New Horizons Flyby Target 2014 MU69,” Sarah Greenstreet et al 2019 ApJL 872 L5. doi:10.3847/2041-8213/ab01db

Protoplanetary disk

Astronomers still don’t fully understand how planets form, especially ultra-dense, iron-rich planets like Mercury. How do trillions of tiny dust grains clump together to make pebbles, planetesimals, and eventually the cores of rocky planets?

Arrested (Dust) Development

Protoplanetary disk

Protoplanetary disks are composed of gas and dust, as this artist’s impression shows. How the tenuous gas–dust blend forms planets is still an open question. [NASA/JPL-Caltech]

When iron-rich dust grains in a protoplanetary disk collide, they stick together to form porous dust aggregates. Further collisions increase the size of aggregates, but only to a point; the growth of dust grains is limited by something called the bouncing barrier. The bouncing barrier kicks in when the once-fluffy dust aggregates become so compacted by collisions that incoming grains bounce off rather than stick. This limits the size of dust aggregates to just a couple of millimeters.

If the bouncing barrier stops dust grains from getting progressively larger, how do planets ever manage to form? One possibility is that the presence of an external magnetic field enhances the intrinsic magnetization of the iron in dust grains, which increases the attractive forces between them, making them “stickier.” If magnetic fields can help dust aggregates grow to centimeter size, other growth mechanisms like the streaming instability — a clumping mechanism caused by gas drag in a disk — can take over from there.

From simulations and observations, we expect the inner regions of protoplanetary disks to have strong magnetic fields — somewhere between 1 and 100 mT. How strongly do magnetic fields of this scale affect the growth of iron-rich dust aggregates?

Kruss & Wurm 2018 Fig. 8

Evolution of the chain length as the magnetic field is turned on and off. [Kruss & Wurm 2018]

Levitation and Magnetization

Maximilian Kruss and Gerhard Wurm (University of Duisburg-Essen, Germany) used laboratory experiments to explore the effects of magnetic fields on the bouncing barrier. Kruss and Wurm used a heat lamp to levitate micron-scale iron–silicate dust grains above a glass lens, which allows them to move and collide freely.

With no external magnetic field, the bouncing barrier stops the grains from growing larger than 2 mm. The authors find that for dust composed of equal amounts of iron and silicate, the bouncing barrier begins to shift when the external magnetic field strength reaches 2.2 mT. The size of the aggregates increases with increasing magnetic field strength, reaching 6 mm in length when the maximum field strength for the experimental setup (7 mT) is applied.

Kruss & Wurm 2018 Fig. 10

The minimum magnetic field necessary to shift the bouncing barrier as a function of iron mass fraction. Grains with an iron mass fraction below 0.33 didn’t form grains, while those with an iron mass fraction above 0.63 were too dense to be levitated. [Kruss & Wurm 2018]

The Missing Link?

The magnetic field strength necessary to shift the bouncing barrier could be even lower; if dust grains in protoplanetary disks are more iron-rich than those used in this experiment, the required field strength could be below 1 mT — well within the expected range for protoplanetary disks.

It isn’t yet known what proportion of dust grains can be expected to be enriched in iron, but this experiment clearly shows that the presence of an external magnetic field encourages iron-rich dust grains to grow larger.

If magnetic fields can suppress the bouncing barrier so that the grains grow large enough for the streaming instability to take over, we may be able to explain how planet formation gets going on the very smallest scales, especially for iron-rich planets like Mercury.

Citation

“Seeding the Formation of Mercurys: An Iron-sensitive Bouncing Barrier in Disk Magnetic Fields,” Maximilian Kruss and Gerhard Wurm 2018 ApJ 869 45. doi:10.3847/1538-4357/aaec78

FSR 1758 zoom

You might think that we’d already discovered all the large clusters of stars orbiting our galaxy. Surprisingly, there are still detections to be made — such as the recently discovered cluster FSR 1758. But is this large group of stars an enormous globular cluster? Or a newly detected dwarf galaxy?

A New Cluster

FSR 1758

A wide-field view (2.5° x 2.5°; top) and a zoomed-in view (18’ x 18’; bottom) centered on FSR 1758 from the DECaPS survey. [Barbá et al. 2019]

FSR 1758 was first discovered last year, hiding in the extremely dense bulge at the center of our galaxy. Objects in the galactic bulge are very difficult to detect: due to the high density of surrounding stars and dust, not much of the light of bulge objects makes it to us.

Based on the limited observations we initially had of FSR 1758 — and because clusters residing in the galactic bulge are typically expected to be of low mass — it was assumed that this object was a globular cluster: a spherical collection of stars that all formed around the same time from the same cloud and are bound by their mutual gravity.

But could the difficulty peering into the galactic bulge mean that we’re missing important details? Another theory posits that the part of FSR 1758 we’re seeing is instead the nucleus of a faint dwarf galaxy orbiting the Milky Way; we simply can’t see the fainter outer reaches of the galaxy.

So which is it: globular cluster or dwarf galaxy? A team of scientists led by Rodolfo Barbá (University of La Serena, Chile) have gathered observations to find out.

Revealing Details

Barbá and collaborators use three different sets of observations to explore FSR 1758: optical data from Gaia’s DR2 and the DECam Plane Survey, and near-infrared data from the VISTA Variables in the Via Lactea Extended Survey. From these data, the authors determine the cluster’s position and distance, as well as its size, metallicity, absolute magnitude, and proper motion.

common PM stars

Spatial distribution of stars that have common proper motion, suggesting that they belong to FSR 2758. Beyond the visible custer of stars at the center of the group, FSR 1758 appears to have a possible larger extended structure, suggesting it may be the nucleus of a dwarf galaxy. [Barbá et al. 2019]

FSR 1758 has a number of intriguing properties. If it’s a globular cluster, it’s one of the largest around our galaxy — and the part that we see is probably just the metaphorical tip of the iceberg, as much of its population is likely hidden by contamination and reddening due to its location in the galactic bulge. Furthermore, FSR 1758’s properties don’t fit known relationships for globular clusters, such as the correlation between size and metallicity.

Lastly, the authors find additional asymmetrically distributed stars further out in the field with motions and colors indicating that they also belong to FSR 1758. These suggest that the cluster may be more extended than originally thought and might have tidal tails. These signs support a picture in which FSR 1758 is the nucleus of a dwarf galaxy — which the authors tentatively name the Scorpius dwarf galaxy.

Though we still don’t have a definitive answer about FSR 1758’s nature, we can hope that future spectral data for its stars will settle the debate. And in the meantime, it’s good to be reminded that our galaxy is still hiding some surprising discoveries.

Citation

“A Sequoia in the Garden: FSR 1758—Dwarf Galaxy or Giant Globular Cluster?,” Rodolfo H. Barbá et al 2019 ApJL 870 L24. doi:10.3847/2041-8213/aaf811

K2-288Bb

In today’s era of big data, we often rely on computers to do sorting, searching, and analyzing. Sometimes, however, there’s just no substitute for the human eye and brain, which comes pre-loaded with excellent pattern-detection capabilities. This is where citizen science come in.

Managing Heaps of Data

raw K2 photometry

Top: raw K2 photometry of K2-288. Spacecraft systematics must first be corrected before we have a chance of searching for planet transit signals! Bottom: the light curve after it has been corrected for systematics and detrended. Previously, systematics corrections had trimmed a transit off the front end of these observations, preventing automatic detection of the planet. Those data are included here, however. [Feinstein et al. 2019]

With the advent of major transiting-planet surveys like Kepler, its successor mission K2, and now the Transiting Exoplanet Survey Satellite (TESS), we have more light curves to sort through than ever before. Combined, these missions will produce light curves for hundreds of thousands — if not millions — of stars! Reasonably, scientists have turned to computers to search these light curves for the faint signs of transiting exoplanet candidates.

In the K2 mission, scientists used an automated search pipeline to check the light curves of tens of thousands of stars for transit candidates. Wisely, however, the K2 team didn’t dismiss the inconclusive light curves after this first automated pass.

Those curves with candidate transits that didn’t quite meet the algorithm’s thresholds were then passed to a citizen-science project known as Exoplanet Explorers, a Zooniverse spinoff. There, citizen scientists had the opportunity to examine individual candidate transit events and phase-folded light curves, hunting for features consistent with a transiting planet that the automated search may have missed.

Exoplanet Explorers vetting diagnostics

The vetting diagnostics for K2-288 presented to citizen scientists on Exoplanet Explorers: a stack of individual transits (left), the full K2 light curve folded onto the candidate transit period (right top), and the zoom on the transit in the period-folded light curve (right bottom). [Feinstein et al. 2019]

Successful Discovery

Sixteen citizen scientists flagged the star K2-288 as showing signs of hosting a transiting planet. Upon further inspection, it was evident that this candidate had been missed by the automated search because the original data had been trimmed in order to remove systematics. This trimming had dropped one of the planet’s three observed transits; with only two possible transits visible, the automated search didn’t flag K2-288.

With access to reprocessed data, however, the Exoplanet Explorers citizen scientists identified K2-288 as a candidate of interest and passed it along to the waiting team of scientists. Led by Adina Feinstein (Tufts University and University of Chicago), that team then conducted follow-up observations of K2-288 to confirm the presence of a planet and determine its properties.

An Unusual World

Feinstein and collaborators report that the confirmed planet, K2-288Bb, is a small (~1.9 Earth radii) planet that lies in or near the habitable zone of its host — a host that is one of two M-dwarf stars in a binary system.

K2-288

An example follow-up image of K2-288, taken with Keck/NIRC2 adaptive optics. The two stars of the binary system are evident. [Adapted from Feinstein et al. 2019]

K2-288 is an interesting system for a number of reasons. First, the unique architecture of the system — a relatively wide-orbit planet around one star of a moderate-separation binary — provides us with an opportunity to learn about the complex dynamics that allowed it to form and evolve.

Additionally, K2-288Bb’s estimated radius places it in an observed gap in planet radius distribution. Its unusual size suggests that its atmosphere may be evolving, causing it to transition either from gaseous mini-Neptune to rocky super-Earth or the other way around.

More observations of this intriguing system will help us to better understand what we’re seeing. In the meantime, we can be glad for citizen scientists helping us to spot what automated techniques might miss!

Citation

“K2-288Bb: A Small Temperate Planet in a Low-mass Binary System Discovered by Citizen Scientists,” Adina D. Feinstein et al 2019 AJ 157 40. doi:10.3847/1538-3881/aafa70

SMBH binary

You might think that a passing star getting ripped apart by a supermassive black hole sounds like more than enough drama. But a new study takes this picture a step further, exploring what happens when a stellar binary interacts with a pair of supermassive black holes.

Stellar Destruction

tidal disruption event

Illustration of a tidal disruption event, in which a star is torn apart by a black hole’s gravitational forces and its material falls onto the black hole. [NASA/CXC/M. Weiss]

First suggested in the 1970s, the theory of tidal disruption events (TDEs) has since been supported by the discovery of many dozens of observed candidates. These spectacular eruptions often arise from previously dark regions, and they’re thought to indicate the accretion of debris after a star is torn apart by a lurking supermassive black hole.

But this simple model can’t adequately explain all of the disruption-like signals we’ve observed. Could more complex interactions be at play too? Two clues support this possibility:

  1. A large fraction of stars exist in binary pairs.
  2. Supermassive black holes can also form binaries, when their host galaxies merge.

With this in mind, a team of scientists led by Eric Coughlin (an Einstein Postdoctoral Fellow at UC Berkeley at the time) has explored a more complicated tidal disruption scenario: that in which a stellar binary interacts with a supermassive black-hole binary. 

Simulating Paired Encounters

double tidal disruption delays

In some of the authors’ simulated encounters, both stars are tidally disrupted, but with a delay between the two disruptions. This plot shows the distribution of times (measured in number of orbits of the black holes) for the delay between the two disruptions. [Adapted from Coughlin et al. 2018]

By performing hundreds of thousands of simulations of the gravitational interactions between a stellar binary and a supermassive black-hole binary, Coughlin and collaborators conclude that there are a number of possible outcomes.

Most encounters result either in the entire intact stellar binary being ejected from the system, or in the two stars being ejected one after the other, after the stellar binary is broken up. But several more interesting outcomes are also possible:

  • Hills capture, in which one star is ejected and the other is captured into orbit around one of the black holes.
  • Single and double TDEs, in which either one or both stars are torn apart and their material accretes onto the black-hole binary.
  • Stellar mergers, in which the two stars lose angular momentum and merge with each other as a result of interacting with and being ejected by the black-hole binary.

Telltale Signals

Coughlin and collaborators point out that these exotic possibilities are interesting because they create distinctive signals — some of which are consistent with signals that we’ve observed, and some of which we can hope to look for in the future.

hypervelocity star

Artist’s impression of a hypervelocity star escaping a galaxy. [ESO]

A double TDE, for instance, could nicely account for the very bright, double-peaked transient known as ASASSN-15lh. The accelerated inspiral of a stellar binary — after having been flung from its galaxy by the supermassive black-hole binary — could account for some calcium-rich transient signals we’ve spotted. And two members of a stellar binary, individually ejected from a galaxy, may later be detectable as hypervelocity stars that have similar spectroscopic properties despite being thousands of light-years apart.

The intriguingly broad range of outcomes that result from the meeting of stellar and black-hole binaries demonstrates that these possibilities are worth exploring further. It would seem that in some cases, this extra drama may be just what we’ve been missing.

Bonus

Check out the video below, which shows just one of the authors’ many simulation results. In this case, the interactions between the stellar and black-hole pairs breaks up the stellar binary. One of the stars is ejected from the system, and the other is captured around one of the black holes.

Citation

“Stellar Binaries Incident on Supermassive Black Hole Binaries: Implications for Double Tidal Disruption Events, Calcium-rich Transients, and Hypervelocity Stars,” Eric R. Coughlin et al 2018 ApJL 863 L24. doi:10.3847/2041-8213/aad7bd

Transiting Exoplanet Survey Satellite (TESS)

What’s the news coming from NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS)? Launched in April 2018, TESS is expected to discover tens of thousands of exoplanets orbiting the nearest and brightest stars. Now that observations are underway, what exciting discoveries have been made? Read on for an update from just a few of the latest TESS studies published in AAS journals.

Around a Far Sun-like Star

Huang et al. 2018 Fig. 2

Raw and corrected TESS light curves showing the five transits of π Men c. The bottom panel shows the folded light curve. [Huang et al. 2018]

Within the first six months of TESS’s launch, a team led by Chelsea Huang (MIT) reported the first official discovery of a planet by TESS — a super-Earth orbiting the Sun-like star π Men.

At a distance of about 59 light-years, π Men is dimly visible to the naked eye near the south celestial pole. Over a decade ago, the Anglo-Australian Planet Search discovered a giant planet — π Men b — orbiting the star every 5.7 years. What TESS’s high-cadence observations revealed was a second, smaller planet orbiting the star every 6.27 days. By combining TESS photometry with precise radial-velocity measurements, Huang and collaborators determined the parameters of the planetary system, including the mass and radius of the newly discovered π Men c.

With a radius twice that of Earth and a mass nearly five times greater, π Men c isn’t an entirely “rocky” planet like the terrestrial planets in our solar system. Instead, it probably has a hydrogen-helium or water-methane envelope. We hope to learn more from future observations with the James Webb Space Telescope, Gaia, or even TESS itself!

Vanderspek et al. 2019 Fig. 4

Physical parameters of LHS 3844 b compared to other known exoplanets. Click to enlarge. [Vanderspek et al. 2019]

Another First for TESS

TESS won’t only find planets around Sun-like stars. Thanks to its redder observing bandpass (600–1000 nm, as opposed to Kepler’s 420–900 nm), TESS is especially sensitive to planets orbiting the small, reddish stars called M dwarfs. Because M dwarfs are so small and cool, planets in their habitable zones complete orbits in days rather than years, making them great observing targets.

A team led by Roland Vanderspek (MIT) analyzed data from the first month of TESS’s science operations, leading to the discovery of the first M-dwarf-orbiting planet detected by TESS. The planet, dubbed LHS 3844 b, has a radius 32% larger than Earth’s and orbits its small parent star at a distance of just 0.006 AU — swinging around LHS 3844 in just 11 hours. Although it’s unclear whether or not LHS 3844 b will have an atmosphere — it orbits so close to its host star that any atmosphere may have been torn away by stellar winds long ago — it’s definitely worth investigating the first (of many!) M-dwarf planets discovered by TESS.

Seeking Confirmation

hot Jupiter

Artist’s impression of a hot-Jupiter exoplanet. [NASA/ESA/G. Bacon (STScI)]

As always with new detections, astronomers are rushing to confirm TESS’s discoveries with other telescopes and instruments. HD 202772A b is the first TESS hot Jupiter to have been confirmed by follow-up observations. In a recent study led by Songhu Wang (Yale University), the authors detail radial-velocity measurements made with the CHIRON, HARPS, and TRES spectrographs that confirm the planetary nature of HD 202772A b. This inflated gas giant is orbiting a quickly evolving star that’s one of the brightest and most massive stars known to host a hot Jupiter. As a result, HD 202772A b is one of the most strongly irradiated hot Jupiters currently known.

Light Curves All Around

Want a chance to explore some TESS data on your own? Ryan Oelkers and Keivan Stassun (Vanderbilt University) have extracted and made available light curves from all the stars in TESS Sector 1, which is home to all three of the stars discussed above. Their website provides both raw and cleaned (systematic trends removed) light curves for each star, as well as information about each target (mass, luminosity class, magnitude, etc.).

As more TESS data rolls in, Oelkers and Stassun plan to update their website with the latest light curves for each observing sector. Happy planet hunting!

Citation

“TESS Discovery of a Transiting Super-Earth in the pi Mensae System,” Chelsea X. Huang et al. 2018 ApJL 868 L39. doi:10.3847/2041-8213/aaef91

“TESS Discovery of an Ultra-short-period Planet Around the Nearby M Dwarf LHS 3844,” Roland Vanderspek et al. 2019 ApJL 871 L24. doi:10.3847/2041-8213/aafb7a

“HD 202772A b: A Transiting Hot Jupiter around a Bright, Mildly Evolved Star in a Visual Binary Discovered by TESS,” Songhu Wang et al 2019 AJ 157 51. doi:10.3847/1538-3881/aaf1b7

“Light Curves for All Stars Observed in TESS Full-frame Images: Sector 1 and Beyond,” Ryan J. Oelkers and Keivan G. Stassun 2019, Res. Notes AAS 3 1. doi:10.3847/2515-5172/aafc34

early quasar

The collapse of enormous stars in our early universe may have given birth to the first supermassive black holes. But will we be able to find these early, giant stars to test this theory?

Considering Giant Seeds

pop iii stars

Artist’s impression of the first stars in the universe. [NASA/WMAP Science Team]

An unsolved mystery of our universe is how the very first quasars — powerful, accreting supermassive black holes — formed. Based on our observations, these earliest quasars had less than a billion years to grow to a billion solar masses. So what seeded these monsters, jump-starting them on their way to this tremendous size?

One theory is that they resulted from the direct collapse of enormous early stars, known as supermassive primordial stars. These giant stars are a little different from typical population III stars, the first generation of stars theorized to form from the metal-poor gas of our early universe.

While ordinary pop III stars cap out at around 1,000 solar masses, supermassive primordial stars form in primordial halos that are — by one means or another — prevented from collapsing into stars until after they reach masses of 107-108 solar masses. According to models, these halos then undergo sudden cooling and catastrophic collapse, resulting in rapidly accreting stars that are much larger than typical pop III stars.

accretion disk

An accretion disk at 0.625 Myr in the authors’ simulations of forming supermassive primordial stars. [Surace et al. 2018]

Hide and Seek

The theory of supermassive primordial stars suggests that these giants could undergo gravitational instabilities and directly collapse into black holes, seeding the earliest quasars in our universe. So how do we test this picture?

The first step is to find observational evidence of supermassive primordial stars! Unfortunately, there’s a catch: these stars would generally evolve as cool, red hypergiants, possibly shrouded by the dense flows of gas accreting onto them. Do we stand a chance of being able to spot these giants with upcoming technology? Or will they remain hidden to us?

In a recent study led by Marco Surace (University of Portsmouth, UK), a team of scientists have modeled supermassive primordial stars to answer these questions.

Exploring Visibility

NIR AB magnitudes

Near-infared AB magnitudes for two primordial stars formed in the authors’ simulations (solid and dashed lines), shown in the JWST NIRCam bands (different colors). JWST NIRCam’s predicted detection limits are at ~31.5. [Surace et al. 2018]

Surace and collaborators used simulations of a rapidly cooled halo to create their supermassive primordial stars. They then calculated what the spectra of these stars would look like to telescopes near Earth, such as the upcoming James Webb Space Telescope (JWST), Euclid, and Wide-Field Infrared Space Telescope (WFIRST).

The authors find that the accretion envelopes surrounding these stars may be helpful rather than detrimental: rather than obscuring the stars, this gas can enhance the stars’ visibility by reprocessing the short-wavelength radiation from the star into photons that we can detect with our near-infrared telescopes.

Surace and collaborators show that some of these supermassive primordial stars will, indeed, be visible to JWST out to redshifts of z ~ 20, and they may even be detectable by the wide-field Euclid and WFIRST missions if some of these stars are modestly gravitationally lensed by foreground objects. This is a promising result, leaving us with some hope that the next generation of telescopes will help us to finally address the mystery of how the first quasars formed in our universe.

Citation

“On the Detection of Supermassive Primordial Stars,” Marco Surace et al 2018 ApJL 869 L39. doi:10.3847/2041-8213/aaf80d

Kelt-9b

What’s the atmosphere like on the hottest planet we’ve ever discovered? A new study suggests this toasty world may also be cloudless.

Hotter and Hotter

More than two decades ago, the discovery of the first hot Jupiters — gas giant exoplanets that orbit extremely close to their host stars — signaled that other solar systems may host very different planets than our own. Since then, we’ve discovered a whole zoo of unusual planets — including ever hotter Jupiters.

Kelt-9b orbit

Still from an animation of the ultra-hot Jupiter Kelt-9b orbiting its host. [NASA/JPL-Caltech]

Thus far, Kelt-9b holds the record. This scalding giant orbits once every 1.49 days while its 10,000-K host blasts it with ultraviolet radiation. Its dayside temperature (Kelt-9b is tidally locked, so the same side always faces its host) is somewhere between 4,000–4,600 K — which is hotter than many stars.

Under these extreme conditions, what is Kelt-9b’s atmosphere like? Is it similar to its slightly cooler hot-Jupiter cousins? Or do the extreme temperatures lead to unusual features? To learn more, our best bet is to watch this planet disappear.

Vanishing Light

secondary eclipse

Schematic of the primary and secondary eclipses that occur when a transiting exoplanet orbits its host. [S. Seager]

Transiting exoplanets are detected when they pass in front of the disk of their host star, blocking some of the star’s light. But these planets have an additional opportunity for study: the period known as the secondary eclipse, when the planet disappears behind its host. This eclipse can appear in observations for two reasons:

  1. The thermal light emitted by the planet is blocked out when it ducks behind its host.
  2. The light the planet reflects from its host star is blocked out as the planet transits behind the star.

In a new study, a team of scientists led by Matthew Hooton (Queen’s University Belfast, UK) has explored Kelt-9b’s secondary eclipse in near-UV wavelengths to learn more about this extremely hot Jupiter.

Constraints from a Missing Dip

Intriguingly, Hooton and collaborators found no evidence for a dip in the UV light curve when Kelt-9b passed behind its host. This non-detection reveals quite a bit of information, allowing us to place limits on the planet’s dayside temperature and the amount of UV light that it reflects.

UV light curve for Kelt-9b

Ultraviolet light curve for Kelt-9b as it transits behind its host. Unbinned data is shown in gray, binned data in black. The best-fit eclipse model, shown in red, indicates that no eclipse is evident in the data. [Hooton et al. 2018]

From these limits, we can infer that Kelt-9b’s atmosphere is likely to be largely cloud-free — otherwise more light would scatter back toward us. These observations are consistent with theoretical models of ultra-hot-Jupiter atmospheres that suggest that their temperatures are too high for condensates to form.

Looking to the Future

The authors’ study marks another important capability. Previously, all studies exploring reflected light of transiting exoplanets were conducted with space-based telescopes like Hubble or Kepler. But Hooton and collaborators made their observations of Kelt-9b with the ground-based 2.5-m Isaac Newton Telescope at the Roque de los Muchachos Observatory in the Canary Islands.

Since the only suitable space-based near-UV coverage right now is provided by Hubble — which won’t be around forever — it’s an important outcome that a ground-based telescope has proven capable of making these measurements. Hooton and collaborators’ work demonstrates that even when Hubble is no longer an option, we will still be able to obtain valuable observations of bizarre planets like Kelt-9b.

Citation

“A Ground-based Near-ultraviolet Secondary Eclipse Observation of KELT-9b,” Matthew J. Hooton et al 2018 ApJL 869 L25. doi:10.3847/2041-8213/aaf6a9

molecules in space

What do methylidyne, cyanamide, vinyl alcohol, and rugbyballene all have in common? They’re all molecules that have been detected in space — and they’re all included in a recent census of our universe’s chemical makeup.

molecule detections over time

Cumulative number of known interstellar molecules over time. Commissioning dates of major contributing facilities are noted with arrows. [McGuire 2018]

Looking For Complexity

Since the first detection of methylidyne (CH) in the interstellar medium in the 1930s, scientists have been on the lookout for the many molecules — groups of two or more atoms held together by chemical bonds — they know must exist beyond our own planet.

Observations of molecules can help us to understand the chemical evolution of the interstellar medium, the formation of planets, and the physical conditions and processes of the universe around us. But molecules produce complex spectral features that are difficult to correctly attribute, making definitive observations of specific molecules challenging — which means that we’re still only just beginning to understand the chemical composition of our universe.

In a recent publication, scientist Brett McGuire (Hubble Fellow of the National Radio Astronomy Observatory, Harvard-Smithsonian Center for Astrophysics) provides an overall summary of observed interstellar, circumstellar, extragalactic, protoplanetary-disk, and exoplanetary molecules. This publication marks the first “living paper” published in AAS journals — a paper that will continue to be updated over years to come as our observations amass and our understanding of the universe around us grows.

Location, Location, Location

molecule sources

Percentage of known molecules that were detected for the first time in carbon stars, dark clouds, LOS clouds, and SFRs. [McGuire 2018]

McGuire’s census, which includes observations from dozens of facilities across the electromagnetic spectrum, identifies the molecules that have been discovered in various locations.

  1. Interstellar and circumstellar molecules
    All of the molecules that we’ve detected beyond Earth have been spotted in the interstellar or circumstellar medium in our galaxy. In total, 204 different molecules have been identified, ranging in size from two atoms (like methylidyne) to 70 atoms (like rugbyballene, C70).
  2. Extragalactic molecules
    67 of the known interstellar and circumstellar molecules (33%) have also been detected in observations of external galaxies.
  3. Protoplanetary-disk molecules
    Only 36 of the known interstellar and circumstellar molecules have been found in protoplanetary disks, in part due to the harsh physical environment around young stars and the challenge of maintaining gas-phase molecules under these conditions.
  4. Exoplanetary molecules
    Just five molecules — CO, TiO, H2O, CO2, and CH4 — have been found in exoplanetary atmospheres thus far.
atoms that make up molecules

Periodic table of the elements color-coded by number of detected species containing each element. [McGuire 2018]

Analyzing Detections

What can we learn from this census? It’s interesting to note that the entirety of the known molecular inventory is constructed from just 16 of the 118 known elements. As for where they form, more than 90% of the detected molecules were made in a carbon star, a dark cloud, a diffuse/translucent/dense cloud that lies between us and a background source, or a star-forming region.

McGuire points out that our detection of new molecules has progressed at a fairly constant rate since the 1960s. Nonetheless, there are many prospects for future advances — such as the upcoming James Webb Space Telescope’s ability to study exoplanet atmospheres in greater detail. Be sure to check back with this living paper in the future to see how our knowledge of the universe’s chemistry changes!

Citation

“2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules,” Brett A. McGuire 2018 ApJS 239 17. doi:10.3847/1538-4365/aae5d2

Blue stragglers in NGC 6362

As stars age, they gradually lose angular momentum and spin more slowly. This process occurs so predictably for normal, solar-type stars that we can treat them as cosmic clocks using a technique called gyrochronology. But could the same strategy be applied to an unusual type of main-sequence star called blue stragglers?

M55 color-magnitude diagram

The blue stragglers in globular cluster M55 are easily identified in a color-magnitude diagram (cyan circle). [Adapted from B.J. Mochejska, J. Kaluzny (CAMK), 1-m Swope Telescope]

Stars That Linger

Based on their mass and age, we would expect blue-straggler stars to have exhausted their core hydrogen and evolved off the main sequence already. Instead, these oddball objects have managed to loiter long past their time by gaining mass — either by siphoning it from a binary companion star or by consuming another star altogether through a collision.

Blue stragglers are easy to pick out in a star cluster, where they are bluer and brighter than the main-sequence turnoff point on a color–magnitude diagram. Post-mass-transfer stars like blue stragglers also exist outside of clusters, where they can be identified by abnormal chemical abundances or the presence of a white-dwarf companion.

To better understand post-mass-transfer stars like blue stragglers, we would like to know how long ago they accreted mass from their companions. We know that these stars experience a jump in spin rate immediately after mass accretion — but what happens after that point? Do they undergo predictable spin-down like normal, solar-type stars, allowing us to use gyrochronology to determine their post-mass-transfer ages?

Going for a Spin

To explore this question, a team led by Emily Leiner (Northwestern University) studied the rotation-rate slowdown of blue-straggler and other post-mass-transfer stars. Leiner and collaborators compiled a sample of post-mass-transfer binaries of varying ages by selecting stars with spectral types F, G, and K with white-dwarf companions in close orbits. Here, age doesn’t refer to time since the star formed, but rather time since the mass transfer took place.

The very young systems were selected by direct detection of the white-dwarf companion in the extreme ultraviolet. In older systems, the white-dwarf companion is too cool to be visible but can be detected by gravitational microlensing.

Leiner and collaborators combined the age estimates from white-dwarf cooling models with rotation periods derived from photometric or spectral measurements. The authors found that the stars spin faster after the mass transfer, then steadily slow down after about 100 million years since the mass transfer have passed.

Leiner et al. 2018 Fig. 1

Ages and rotation periods for this sample of post-mass-transfer systems. The purple and gold lines are single-star models, while the red and cyan lines are collisional-product models. Click to enlarge. [Leiner et al. 2018]

A Model for Spin-down

To understand the physics of post-mass-transfer star spin-down, the authors compared the observed spin-down to models for single solar-type stars and stellar collision products. They found that the models for the stellar collision products showed distinctly different behavior; the collision products maintained their rapid rotation rates far longer than the single stars or post-mass-transfer stars.

Leiner and collaborators attributed this to the possibility that the collision products don’t form normal stellar magnetic fields and can’t lose angular momentum through magnetic braking the way single main-sequence stars do.

On the other hand, the models for spin-down of single solar-type stars matched the blue-straggler observations well. This suggests that blue stragglers and other post-mass-transfer stars have a promising future as gyrochronometers!

Citation

“Observations of Spin-down in Post-mass-transfer Stars and the Possibility for Blue Straggler Gyrochronology,” Emily Leiner, Robert D. Mathieu, Natalie M. Gosnell, and Alison Sills 2018 ApJL 869 L29. doi:10.3847/2041-8213/aaf4ed

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