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8 March 2019

Reversing Winds on Hot Jupiters

Exoplanets HAT-P-7b and CoRoT-2b have an unusual quirk: instead of having eastward equatorial winds, like the majority of hot Jupiters, these two hot Jupiters have westward winds. A new study explores whether magnetic fields cause this odd reversal.

Blowing the Wrong Way

Artist’s impression of HAT-P-7b, an inflated hot Jupiter. [NASA, ESA, and G. Bacon (STScI)]

You might think that the hottest — and therefore brightest — part of a tidally locked hot Jupiter should be the part that directly faces its nearby host star. Surprisingly, our observations of hot Jupiters have generally revealed an offset for the peak brightness that’s slightly east of the point directly facing the host. These observations suggest that hot Jupiters host strong eastward-blowing winds near their equators that can displace their hottest point.

Two planets break this rule, however: HAT-P-7b and CoRoT-2b. Observations of both of these hot Jupiters instead reveal hotspots that lie west of the point facing the host. Astronomers have generally interpreted this to imply that these two planets have westward-blowing equatorial winds — but why?

There are a number of proposed explanations for this odd apparent reversal:

The planet may not be tidally locked as expected; if it rotates on its axis slightly slower than it orbits its host, this could drive westward winds.
The apparent offset hotspot location could be an illusion caused by asymmetric cloud distribution.
Interactions of the planet’s magnetic field with its atmosphere could modify its wind pattern.

In a new study led by Alexander Hindle (Newcastle University, UK), a team of scientists explores the feasibility of this third option.

Magnetic Waves

Plot of the geopotential, which traces temperature, in the authors’ simulations, with (bottom) and without (top) the presence of magnetic fields. The hotspot (marked with a white cross) displaces to the east for the hydrodynamic case and to the west for the magnetohydrodynamic case. [Hindle et al. 2019]

Hindle and collaborators use both analytic models and simulations to show what happens in the atmosphere of a planet with a strong magnetic field. They explore a layer of atmosphere that can behave like shallow water, developing planetary-scale waves. Without a magnetic field, these waves will naturally travel eastward. But in the presence of a strong toroidal magnetic field, the wave shears as it travels, resulting in westward-tilting eddies. This drives the winds to switch direction to the west.

The authors next calculate the minimum magnetic field strength needed to create this equatorial wind reversal for planets with the properties of HAT-P-7b and CoRoT-2b. They find that an inflated hot Jupiter like HAT-P-7b would need a field strength above just 6 Gauss (for comparison, the Earth’s magnetic field is ~1 G). Estimated field strengths for inflated hot Jupiters lie in the 50–100 G range, so attributing HAT-P-7b’s wind reversal to magnetic fields is well within reason.

For an ordinary hot Jupiter like CoRoT-2b, however, a field strength of 3,000 G is needed. The maximum expected field strength for a hot Jupiter like CoRoT-2b is 250 G, which isn’t sufficient to drive the reversal. Hindle and collaborators conclude that a different mechanism is likely at work on this planet.

More observations of hot Jupiters in the future — as well as three-dimensional simulations — will help us to further understand the wind behavior in the atmospheres of these toasty planets.

Citation

“Shallow-water Magnetohydrodynamics for Westward Hotspots on Hot Jupiters,” A. W. Hindle et al 2019 ApJL 872 L27. doi:10.3847/2041-8213/ab05dd

6 March 2019

First Disrupted Star for a New Survey

What happens when a black hole makes a meal out of a passing star? So far, we’ve only detected a few dozen candidate tidal disruption events to help us answer this question — but now a new player is in the observing game.

Snacks for Black Holes

When a star passes within the tidal radius of a supermassive black hole, things don’t end well for the star. After the unfortunate object is torn apart by gravitational forces, some of the resulting debris accretes onto the black hole, causing a multi-wavelength flare.

To date, we’ve observed this flare emission from several dozen candidate tidal disruption events (TDEs), but many of them were only noticed significantly after the moment of disruption, when the flare emission is already ramping back down again. We also have only a handful of detections of TDEs across multiple wavelengths.

ZTF installed on the 1.2-meter Samuel Oschin Telescope at Palomar Observatory in California. [Caltech Optical Observatories]

In short, TDE observations thus far — though tantalizing — aren’t yet enough to help us complete the picture of what happens when a star is torn apart by a supermassive black hole. Clearly, the next step is to gather many more such observations! Luckily, a new tool has recently come online that will help us do exactly that: the Zwicky Transient Facility (ZTF).

A New Player

ZTF is a wide-field optical survey that hunts for transient objects in our night sky. ZTF images image the entire northern sky once every three nights, and the plane of the Milky Way twice a night. By scanning the same regions frequently, the survey can detect and monitor rapidly changing objects — like a suddenly rising tidal disruption flare.

ZTF began its first major public observing survey in mid-March 2018. In the weeks before that, ZTF was still in its commissioning phase, testing the camera and the alert pipeline. It was in this time that the survey detected its first tidal disruption event candidate, AT2018zr.

ZTF optical and Swift ultraviolet and optical light curves for AT2018zr. The data capture both the sudden rise and gradual decay of the flare. [van Velzen et al. 2019]

Early View of Destruction

The transient AT2018zr triggered a ZTF alert on 6 March 2018. In the weeks that followed, it was observed by additional telescopes across a number of wavelength bands. In a new publication led by Sjoert van Velzen (University of Maryland and New York University), team members detailed the ZTF and multi-wavelength follow-up observations of AT2018zr.

By reprocessing earlier ZTF image frames, van Veltzen and collaborators found that ZTF had actually captured this tidal disruption event starting in early February, 50 days before the peak of the flare light curve. These detailed optical observations, combined with the broadband follow-up, provide an unusually complete view of this flare.

Harbingers of Data to Come

The host of AT2018zr, as observed by the Sloan Digital Sky Survey before the TDE occurred. [SDSS]

With many more events like AT2018zr, we can hope to build a large sample of flares that will finally shed light on TDE processes. ZTF is conveniently poised to start producing those observations; estimates suggest that, now that ZTF is fully operational, it will produce ~30 TDE detections per year.

What’s more, ZTF is providing researchers with a chance to test clever analysis techniques in advance of an even larger flood of data: the upcoming Large Synoptic Survey Telescope (LSST) is expected to detect ~1,000 TDEs per year! While only one event, AT2018zr is likely something more — the beginning of a new era for TDE observations.

Citation

“The First Tidal Disruption Flare in ZTF: From Photometric Selection to Multi-wavelength Characterization,” Sjoert van Velzen et al 2019 ApJ 872 198. doi:10.3847/1538-4357/aafe0c

4 March 2019

Inflating a Super-Puff Planet

Super-puffs — fluffy planets observed to have abnormally low densities — are a problem. According to theoretical models, they shouldn’t exist — and yet we’ve already detected half a dozen of them with Kepler alone. A new study explores what theory might be getting wrong.

A Fluffy Puzzle

Among the assortment of odd and unexpected exoplanets we’ve discovered through years of observing transits, super-puffs stand out as a peculiar puzzle. Super-puffs are planets with masses lower than that of Neptune (< 5 Earth masses), but with sizes equivalent to those of gas-giant planets (transit radii > 5 Earth radii).

Photoevaporation by a planet’s host can boil off the planet’s atmosphere. This process works all the more quickly if the planet doesn’t have strong surface gravity to hold on to its atmosphere. [MPIA]

This doesn’t inherently seem problematic, until we consider our understanding of planet evolution. Given a super-puff’s low density, its atmosphere should be tenuous at best; without strong surface gravity to contain it, the atmosphere should get boiled off by the planet’s host star within perhaps 1,000 years. The super-puffs we see are much older than this, and yet they still retain their extended atmospheres.

Piling on further intrigue, transmission spectra of super-puffs show no evident spectral lines. Lines can be washed out by the presence of aerosols — clouds and hazes made up of dust or liquid droplets — but we wouldn’t expect these heavier particles to be able to stay lofted high enough in a fluffy planet’s atmosphere to flatten out its lines.

In a new study, scientists Lile Wang (Flatiron Institute and Princeton University Observatory) and Fei Dai (MIT and Princeton University Observatory) suggest a revamped theoretical model that solves both of these problems.

Spectral strengths of the water feature vs. planet mass. Planets with masses lower than ~10 Earth masses don’t have strong enough gravity to avoid having their spectral features washed out by dusty outflows in the planet atmosphere. [Wang & Dai 2019]

Solutions in Flow

Wang and Dai’s model has a significant change from previous pictures: their proposed atmosphere is not static. Instead, they suggest super-puffs have atmospheres that contain outward flows, continually carrying very small dust grains to high altitudes.

By populating the upper reaches of the atmosphere with dust, these outflows increase the overall opacity of the atmosphere, which prevents it from boiling off quickly. And the dust carried to the upper atmosphere indeed washes out the spectral lines, providing an explanation for the flat spectra we observe.

The authors model one prominent super-puff, Kepler 51b, and show that reasonable outflow rates (a loss of just 10^-10 Earth masses of atmosphere per year) can carry dust grains of ~10 Å in size to high altitudes. They show that this process inflates the observed transit radius of the planet to the ~7 Earth radii we see, and it also flattens the planet’s transmission spectra.

Spotting Signs of Dust

The Spitzer Space Telescope could be used to extend the spectral wavelength coverage of planets of interest. Planets with dusty atmospheres would look larger in optical than in infrared wavelengths. [NASA/JPL-Caltech]

How might we verify that a planet has dusty outflows in its atmosphere? Wang and Dai point out that the apparent radius of such a planet will be wavelength-dependent: their model planet, for instance, would appear 10–20% larger at wavelengths of 0.5 µm than at wavelengths of 1 µm.

This phenomenon has already been observed for several exoplanets, and the authors suggest that we could extend the wavelength coverage for transmission spectra to identify signatures of dusty atmospheres in other planets. Should dusty atmospheres prove common among young, low-mass exoplanets, we clearly will need to reformulate how we think about these bodies.

Citation

“Dusty Outflows in Planetary Atmospheres: Understanding “Super-Puffs” and Transmission Spectra of Sub-Neptunes,” Lile Wang and Fei Dai 2019 ApJL 873 L1. doi:10.3847/2041-8213/ab0653

1 March 2019

Solving a Stellar Abundance Problem (with a Little Help from Our Oceans)

When solving mysteries about distant astronomical objects, sometimes it pays to take inspiration from sources closer to home. In today’s example, strange fluid behavior in the Earth’s oceans — combined with a healthy helping of magnetic fields — may provide the answer to a long-standing puzzle about the changing composition of red-giant stars.

Simulated salt fingers in fluids with decreasing Rayleigh numbers. The Rayleigh number determines whether heat in a system is transferred primarily through diffusion or convection. [Fariarehman]

A Possibility for Instability

Red giants undergo a process called dredge-up, during which their outer convective envelopes bring fusion products up to the surface, altering the chemical abundances there. After the dredge-up, surface abundances aren’t expected to change — yet observations show that they continue to evolve long after the dredge-up is complete. What drives this unexpected late-stage mixing in red giants?

One solution involves an instability called fingering convection. Fingering convection occurs in fluids with vertical gradients in temperature and chemical composition — a setup we see everywhere from the interiors of stars to Earth’s oceans. When the equilibrium of such a fluid is perturbed, the temperature diffuses more quickly than the chemical composition as the system seeks to reestablish equilibrium, triggering a runaway effect.

What does this look like in practice? Take the ocean as an example. The density of saltwater is determined by temperature and salt content, and warm saltwater often lies atop denser, colder water that is less salty. When a bubble of warmer water is pushed into the colder water beneath it, it cools quickly, but the salt is slow to diffuse outward. The cold, salty water is now denser than the water surrounding it, causing it to sink deeper. As this process continues, salt-rich “fingers” dive downward, eventually depositing the saltier water deep in the ocean.

The density of the material in stellar interiors depends on temperature, which diffuses rapidly, and chemical composition, which diffuses slowly — the perfect setup for fingering convection.

Vertical velocity of fluid parcels for three values of the Lorentz force coefficient, H_B, which increases as the square of the magnetic field strength. Click to enlarge. [Harrington & Garaud 2019]

Chemical Mixing

Past modeling has shown that fingering convection does help mix chemical species in red-giant stars, but two orders of magnitude too slowly to explain observations. However, these simulations didn’t consider what effect magnetic fields — which are certainly present in the interiors of these stars — have on convection. What happens when we throw magnetic fields into the mix?

Peter Harrington and Pascale Garaud (University of California, Santa Cruz) used numerical models to explore the effect of magnetic fields on the rate of convection in stellar interiors. In their simulations, the authors apply a vertical background magnetic field of varying strength and randomly impose small perturbations in the temperature and composition. The perturbations grow as the instability takes hold, forming narrow fingers aligned with the magnetic field.

Evolution of the compositional Nusselt number (a measure of the strength of the vertical compositional transport) over time. Simulations with higher magnetic field strengths saturate more rapidly and reach higher rates of vertical transport. [Harrington & Garaud 2019]

Implications for Convection

The authors find that including magnetic fields in their simulations increases the rate of convection, with stronger magnetic fields leading to more rapid convection. For a purely vertical magnetic field of 0.03 Tesla (reasonable for stellar interiors), the convection rate increases by two orders of magnitude — enough to resolve the disagreement between theory and observations.

Magnetized fingering convection should affect more than just red giants; the authors note that main-sequence stars and white dwarfs should also exhibit this behavior, which needs to be accounted for when interpreting observed surface abundances.

Citation

“Enhanced Mixing in Magnetized Fingering Convection, and Implications for Red Giant Branch Stars,” Peter Z. Harrington & Pascale Garaud 2019 ApJL 870 L5. doi:10.3847/2041-8213/aaf812

27 February 2019

Clues from Hubble a Year After a Neutron-Star Merger

When two neutron stars merged in August of 2017, telescopes around the world watched the fireworks that came next. But it’s not just the seconds and minutes after merger that can teach us about what happened! Hubble observations of the afterglow a year later are now providing new clues.

A Wealth of Observations

Late-time Hubble observations of GW170817 spanning 170 days to 358 days after the merger. [Adapted from Lamb et al. 2019]

The discovery of GW170817 — the first gravitational-wave detection of a binary-neutron-star merger with an observed electromagnetic counterpart — finally promised a close look at what happens when two compact objects merge. Following the merger, we gathered observations of the accompanying signals: a weak short-duration gamma-ray burst, a kilonova powered by radioactive decay, and a long-lived afterglow traced in radio, X-ray, and optical wavelengths.

Two leading questions we hoped GW170817 would answer were how matter is expelled during the sudden explosion of energy when neutron stars collide, and what the environment around the colliding stars is like. Now, a team of scientists led by Gavin Lamb (University of Leicester) has used Hubble observations from a year post-merger to address these questions.

Model light curves fit to various observations of GW170817’s afterglow, spanning a year after merger. [Lamb et al. 2019]

Structuring a Jet

Two leading theories for the observed outflow from GW170817 are a jet-dominated model, in which a fast-moving jet punched through the surrounding material and carried away mass and energy, or a cocoon-dominated model, in which a jet tried to escape, but was instead choked by a surrounding cocoon of slow-moving material.

The authors’ analysis of the afterglow observed by Hubble indicates that a structured jet did, in fact, appear — an outcome that supports previous radio observations also favoring the jet-dominated model.

To better understand the structure of the jet, Lamb and collaborators model GW170817’s outflow in two ways:

a two-component jet that consists of a very narrow (only ~5° wide), incredibly fast-moving core (flowing at nearly the speed of light!) surrounded by a slower-moving, slightly wider (~15°) cocoon of swept-up material; or
a single-component jet with a smooth, Gaussian distribution of energy and no cocoon.

The authors find that both models well fit the afterglow observed by Hubble, though the single-component Gaussian model provides a slightly better fit at late times. Continued observations may help to distinguish between the models.

A Cluster Home?

Hubble image of the lenticular galaxy NGC 4993, host to GW170817 (shown over time in the zoomed insets]. Could GW170817 have been hosted in a star cluster within this galaxy? [NASA and ESA]

In addition to exploring the jet, the authors place constraints on the host environment for GW170817.

It’s possible that neutron-star mergers may occur in star clusters, as the high density of stars in clusters could lead to binary pairs. We’ve identified the host galaxy for GW170817, but we can’t directly tell whether the merger occurred within a star cluster in the galaxy.

Using the Hubble observations, however, the authors show that the brightness of any underlying host cluster would have to be extremely low — so low, in fact, that it would be dimmer than 99% of Local-Group globular clusters.

We can add these constraints to the pile of intriguing clues we continue to accumulate as we study this landmark event!

Citation

“The Optical Afterglow of GW170817 at One Year Post-Merger,” G. P. Lamb et al 2019 ApJL 870 L15. doi:10.3847/2041-8213/aaf96b

22 February 2019

A Hazy Day Around TRAPPIST-1?

The multi-planet system around the star TRAPPIST-1 is an excellent target for probing exoplanet atmospheres. A new study explores whether the skies of these exoplanets are likely cloudy or clear.

It’s All Unclear

Much like a spherical cow, a clear hydrogen atmosphere is a simple, clean, easy-to-work-with model. And much like real-life, lumpy, leggy cows, most exoplanet atmospheres are probably more complicated than the simple model. In particular, atmospheric aerosols muddy things up. These particles come in two forms: clouds, condensations of solid or liquid particles, and hazes, solid suspended particles that result from photochemical reactions in the atmosphere.

Atmospheric aerosols have pesky side effects for observations — like washing out spectral features, preventing us from easily learning about an exoplanet’s composition. But they also have intriguing benefits — like protecting hypothetical life on those planets’ surfaces from the high-energy radiation of their host stars. For this reason, understanding aerosol content in exoplanetary atmospheres is an important component of learning about distant worlds.

Artist’s illustration of the TRAPPIST-1 planetary system. [NASA/JPL-Caltech]

Observing the TRAPPIST-1 Family

Unfortunately, this is also a challenging process! We learn about atmospheres through transmission spectroscopy, in which we examine spectral lines in the light that filters through a planet’s atmosphere as it transits its host. The James Webb Space Telescope (JWST) will do a better job of making observations like these once it launches — but in the meantime, we’re learning as much as we can with Hubble.

Recent Hubble observations of the TRAPPIST-1 family of exoplanets — a system of seven planets, many of which lie in their host’s habitable zone — revealed some muted spectral features from a few of their atmospheres; from these, we’ve tried to build an understanding of their properties. Now, a new study led by Sarah Moran (Johns Hopkins University) has used the latest TRAPPIST-1 mass constraints and some recent laboratory astrophysics results to update this picture.

Setting Limits

By comparing new models to the Hubble spectra for TRAPPIST-1 planets d, e, f, and g, Moran and collaborators explore the possible clouds and hazes these four planets could host. The authors vary different components of their models independently, placing limits on the planet atmospheres’ haze scattering cross sections, their metallicities, and the heights of their possible cloud decks.

Different models (colored lines) for four TRAPPIST-1 planet atmospheres, with varying metallicities and cloud-deck heights. The black data points show the Hubble observations. Click to enlarge. [Moran et al. 2018]

The authors then take a unique step: they compare their results to recent laboratory astrophysics experiments studying haze formation under a range of planetary temperatures and atmospheric compositions. By comparing their model limits to the laboratory experiment results, Moran and collaborators are able to make sure that their limits are physically realistic.

Future Answers

So what do Moran and collaborators find? We still don’t know exactly what the atmospheres of the TRAPPIST planets look like, but the authors’ limits suggest that planets d, e, and f could have volatile-rich atmospheres that didn’t form at the same time as the planet. For TRAPPIST-1 g, we can’t yet rule out the spherical-cow picture of a clear hydrogen-rich atmosphere.

This isn’t the end of the story though: the authors show that increased-precision observations will help break many degeneracies in their models. As soon as JWST is on the job, we can hope for more answers!

Citation

“Limits on Clouds and Hazes for the TRAPPIST-1 Planets,” Sarah E. Moran et al 2018 AJ 156 252. doi:10.3847/1538-3881/aae83a

20 February 2019

Holding Together a Speeding Cloud

High-velocity clouds observed in our galaxy’s halo pose a conundrum: given their tenuous nature and large speeds, why haven’t they been ripped apart? New observations of one such cloud now provide a possible answer.

The Sombrero galaxy, M104, provides an excellent example of a galaxy and its halo — the region that extends above and below the galaxy’s disk. High-velocity clouds have been detected speeding through our own galaxy’s halo. [ESA/C. Carreau]

Plunging Gas

The halo of our galaxy isn’t only host to stars. So-called “high-velocity clouds” — massive collections of gas moving at more than 150,000 mph — zip through the halo, plunging toward and through the galactic disk.

Where does this speedy gas come from? How do the clouds evolve as they pass through the halo? And what protects them from having their gas stripped in the process? There are still many questions about high-velocity clouds that future observations may help us to answer. One cloud in particular makes an ideal target for further exploration: the Smith cloud.

A Useful Target

The Smith cloud consists of at least a million solar masses of gas and lies in the southern sky. It’s shaped as a bright knot with diffuse emission trailing behind it, suggesting this cloud is traveling toward the disk of the galaxy.

From simulations of cloud infall, we expect that any cloud that travels more than ~33,000 light-years through the galactic halo would be stripped of its neutral gas by the hot interstellar medium. Surprisingly, though the Smith cloud has traveled more than that distance, it retains its gas — which means that something must be protecting it. But what?

The relative nearness of the Smith cloud and its large size make it a convenient target to search for the answer. The cloud spans an enormous angular diameter of 10–12 degrees, or about 20 times the diameter of the Moon! By looking through this diffuse cloud at objects behind it, we can learn more about its properties — and in particular, about its magnetic field.

Dragging a Magnetic Field

Rotation measures (RMs) — measurements of how much the cloud caused the background source’s polarization to rotate — for distant radio sources near or behind the Smith cloud. Previous data is shown in cyan and magenta; the authors’ new data is shown in blue and red. Blue and cyan indicate negative RMs (the magnetic field points away from the observer); red and magenta indicate positive RMs (magnetic field points toward the observer). [Betti et al. 2019]

Led by student Sarah Betti (Haverford College; University of Massachusetts), a team of scientists obtained Jansky Very Large Array observations of 1,105 distant radio sources behind and next to the Smith cloud. By measuring how the polarizations of these sources rotate as a result of passing through the magnetic field of the cloud, Betti and collaborators were able to map out the strength and geometry of the cloud’s field.

The Smith cloud’s magnetic field, the authors find, appears to be draped over the ionized gas and compressed at the head of the cloud. This geometry is consistent with a picture in which the cloud has swept up the ambient field as it plunges toward the plane of the galaxy, compressing it ahead of the cloud and dragging it along with it.

A Powerful Shield

Can this scenario explain the surprising persistence of the cloud? Perhaps! Past studies have shown that such magnetic field accumulation could be strong enough to shield a cloud’s neutral gas from the hot interstellar medium, protecting it from being stripped as the cloud passes through the halo.

Now that we have detailed observations of the Smith cloud’s magnetic field, careful future modeling can provide tests of whether the field strength is enough to explain how the cloud has survived its travels.

Citation

“Constraining the Magnetic Field of the Smith High-velocity Cloud Using Faraday Rotation,” S. K. Betti et al 2019 ApJ 871 215. doi:10.3847/1538-4357/aaf886

18 February 2019

Insights from MU69’s (Lack of) Craters

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

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?

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.

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

15 February 2019

Can Magnetic Fields Help Planetesimals Form?

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 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?

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.

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

13 February 2019

Dwarf Galaxy or Giant Globular Cluster?

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

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.

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

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