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COROT-7b

Matching theory to observation often requires creative detective work. In a new study, scientists have used a clever test to reveal clues about the birth of speedy, Earth-sized planets.

Former Hot Jupiters?

hot Jupiter

Artist’s impression of a hot Jupiter with an evaporating atmosphere. [NASA/Ames/JPL-Caltech]

Among the many different types of exoplanets we’ve observed, one unusual category is that of ultra-short-period planets. These roughly Earth-sized planets speed around their host stars at incredible rates, with periods of less than a day.

How do planets in this odd category form? One popular theory is that they were previously hot Jupiters, especially massive gas giants orbiting very close to their host stars. The close orbit caused the planets’ atmospheres to be stripped away, leaving behind only their dense cores.

In a new study, a team of astronomers led by Joshua Winn (Princeton University) has found a clever way to test this theory.

star sample

Planetary radius vs. orbital period for the authors’ three statistical samples (colored markers) and the broader sample of stars in the California Kepler Survey. [Winn et al. 2017]

Testing Metallicities

Stars hosting hot Jupiters have an interesting quirk: they typically have metallicities that are significantly higher than an average planet-hosting star. It is speculated that this is because planets are born from the same materials as their host stars, and hot Jupiters require the presence of more metals to be able to form.

Regardless of the cause of this trend, if ultra-short-period planets are in fact the solid cores of former hot Jupiters, then the two categories of planets should have hosts with the same metallicity distributions. The ultra-short-period-planet hosts should therefore also be weighted to higher metallicities than average planet-hosting stars.

To test this, the authors make spectroscopic measurements and gather data for a sample of stellar hosts split into three categories:

  1. 64 ultra-short-period planets (orbital period shorter than a day)
  2. 23 hot Jupiters (larger than 4 times Earth’s radius and orbital period shorter than 10 days)
  3. 243 small hot planets (smaller than 4 times Earth’s radius and orbital period between 1 and 10 days)

They then compare the metallicity distributions of these three groups.

Back to the Drawing Board

metallicity distributions

Metallicity distributions of the three statistical samples. The hot-Jupiter hosts (orange) have different distribution than the others; it is weighted more toward higher metallicities. [Winn et al. 2017]

Winn and collaborators find that hosts of ultra-short-period planets do not have the same metallicity distribution as hot-Jupiter hosts; the metallicities of hot-Jupiter hosts are significantly higher. The metallicity distributions for hosts of ultra-short-period planets and hosts of small hot planets were statistically indistinguishable, however.

These results strongly suggest that the majority of ultra-short-period planets are not the cores of former hot Jupiters. Alternative options include the possibility that they are the cores of smaller planets, such as sub-Neptunes, or that they are the short-period extension of the distribution of close-in, small rocky planets that formed by core accretion.

This narrowing of the options for the formation of ultra-short-period planets is certainly intriguing. We can hope to further explore possibilities in the future after the Transiting Exoplanet Survey Satellites (TESS) comes online next year; TESS is expected to discover many more ultra-short-period planets that are too faint for Kepler to detect.

Citation

Joshua N. Winn et al 2017 AJ 154 60. doi:10.3847/1538-3881/aa7b7c

stellar orbits

When a normally dormant supermassive black hole burps out a brief flare, it’s assumed that a star was torn apart and fell into the black hole. But a new study suggests that some of these flares might have a slightly different cause.

Not a Disruption?

tidal disruption event

Artist’s impression of a tidal disruption event, in which a star has been pulled apart and its gas feeds the supermassive black hole. [NASA/JPL-Caltech]

When a star swings a little too close by a supermassive black hole, the black hole’s gravity can pull the star apart, completely disrupting it. The resulting gas can then accrete onto the black hole, feeding it and causing it to flare. The predicted frequency of these tidal disruption events and their expected light curves don’t perfectly match all our observations of flaring black holes, however.

This discrepancy has led two scientists from the Columbia Astrophysics Laboratory, Brian Metzger and Nicholas Stone, to wonder if we can explain flares from supermassive black holes in another way. Could a different event masquerade as a tidal disruption?

orbit and radius evolution

Evolution of a star’s semimajor axis (top panel) and radius (bottom panel) as a function of time since Roche-lobe overflow began onto a million-solar-mass black hole. Curves show stars of different masses. [Metzger & Stone 2017]

Inspirals and Outspirals

In the dense nuclear star cluster surrounding a supermassive black hole, various interactions can send stars on new paths that take them close to the black hole. In many of these interactions, the stars will end up on plunging orbits, often resulting in tidal disruption. But sometimes stars can approach the black hole on tightly bound orbits with lower eccentricities.

A main-sequence star on such a path, in what is known as an “extreme mass ratio inspiral (EMRI)”, slowly approaches the black hole over a period of millions of years, eventually overflowing its Roche lobe and losing mass. The radius of the star inflates, driving more mass loss and halting the star’s inward progress. The star then reverses course and migrates outward again as a brown dwarf.

Metzger and Stone demonstrate that the timescale for this process is shorter than the time delay expected between successive EMRIs. The likelihood is high, they show, that two consecutive EMRIs would collide while one is inspiraling and the other is outspiraling.

Results of a Collision

EMRI orbits

Schematic diagram (not to scale) showing how two circular EMRI orbits can intersect as the main-sequence star migrates inward (blue) and the brown dwarf very slowly migrates outward (red). [Metzger & Stone 2017]

Because both stars are deep in the black hole’s gravitational well, they collide with enormous relative velocities (~10% the speed of light!). If this collision is head-on, one or both stars will be completely destroyed. The resulting gas then accretes onto the black hole, producing a flare very similar to a classical tidal disruption event.

If the stars instead meet on a grazing collision, Metzger and Stone show that this liberates gas from at least one of the stars. The gas forms an accretion disk around the black hole, causing a transient flare similar to some of the harder-to-explain flares we’ve observed that don’t quite fit our models for tidal disruption events.

In this latter scenario, the stars survive to encounter each other again, decades to millennia later. These grazing collisions between the pair can continue to produce quasi-periodic flares for thousands of years or longer.

Metzger and Stone argue that EMRI collisions have the potential to explain some of the flares from supermassive black holes that we had previously attributed to tidal disruption events. More detailed modeling will allow us to explore this idea further in the future.

Citation

Brian D. Metzger and Nicholas C. Stone 2017 ApJ 844 75. doi:10.3847/1538-4357/aa7a16

upsilon Andromedae b

What can we learn about exoplanets from high-resolution, ground-based observations? A new view of the nearby upsilon Andromedae system has revealed a great deal about the system’s closest-in exoplanet — including the presence of water vapor in its atmosphere.

Search for Wobbles

spectroscopic binary

Illustration of how spectral lines shift when observing two objects that orbit each other. Click here to see a simulation of this process. [R. Pogge, OSU]

The upsilon Andromedae system is located roughly 44 light-years from Earth. In 1997, a hot Jupiter exoplanet was discovered orbiting the primary star, and more planets were found not long after — making this the first multiple-planet system discovered around a main-sequence star.

These planets, however, were not discovered due to transits; their orbital planes are not aligned with our line of sight to the star. Instead, the hidden planets were first detected via the star’s spectrum. The radial velocity method of detecting exoplanets searches for telltale periodic shifts of a star’s spectral lines, which are induced by the orbiting planets’ gravitational tugs.

In recent years, ground-based spectroscopy has become ever more powerful; thus revisiting old systems with higher resolution instruments can often open a whole new world of data to us. In the case of a recent study, a team of astronomers led by Danielle Piskorz (California Institute of Technology) revisited upsilon Andromedae with the high-resolution Near Infrared Spectrometer (NIRSPEC) at the Keck telescope in Hawaii. Their goal: to gather data about upsilon Andromedae b, the closest-in planet in the system.

upsilon Andromedae b orbit

Top-down schematic of the orbit of upsilon Andromedae b around its star and the location in the orbit of the authors’ observations. [Piskorz et al. 2017]

An Unusual Architecture

Piskorz and collaborators obtained 13 different sets of observations of upsilon Andromedae with NIRSPEC across three different wavelength bands. By treating the star–planet system as though it were a spectroscopic binary, the authors’ high-resolution observations allowed them to resolve not only the stellar spectrum, but also the spectral lines from the hot Jupiter exoplanet itself.

Obtaining a thermal spectrum of the planet permitted the team to break the usual observational degeneracy that occurs with exoplanet observations: they were able to disentangle the planet mass and its orbital inclination angle. Piskorz and collaborators found that the planet is roughly 1.7 Jupiter masses and its orbit is inclined ~24° relative to our line of sight.

upsilon Andromedae system

Artist’s illustration of the closest three planets in the upsilon Andromedae system. The system also has a distant red-dwarf binary companion, as well as a possible fourth planet. [NASA/ESA/A. Feild (STScI)]

These measurements of the orbital structure of upsilon Andromedae are critical for understanding this unusual system. With non-coplanar planets and a distant red-dwarf companion, the upsilon Andromedae system has long been suspected to lie on the precipice of instability. The new measurements of upsilon Andromeda b’s orbital properties will help us to better understand how the system may have formed, evolved, and survived to today.

Water Found

One of the biggest benefits of spectroscopy of an exoplanet is the potential to learn about its atmospheric composition. Using their NIRSPEC observations of upsilon Andromedae b and detailed atmospheric modeling, Piskorz and collaborators found that the planet’s opacity structure is dominated by water vapor at the wavelengths they probed.

This detection of water vapor in upsilon Andromedae b’s atmosphere and the constraints on the planet’s orbital properties demonstrate the power and potential of ground-based, high-resolution spectroscopy for characterizing exoplanets and constraining the architecture of distant solar systems.

Citation

Danielle Piskorz et al 2017 AJ 154 78. doi:10.3847/1538-3881/aa7dd8

white dwarf pollution

Where do the metals come from that pollute the atmospheres of many white dwarfs? Close-in asteroids may not be the only culprits! A new study shows that distant planet-size and icy objects could share some of the blame.

Pollution Problems

white dwarf debris

Artist’s impression of rocky debris lying close around a white dwarf star. [NASA/ESA/STScI/G. Bacon]

When a low- to intermediate-mass star reaches the end of its life, its outer layers are blown off, leaving behind its compact core. The strong gravity of this white dwarf causes elements heavier than hydrogen and helium to rapidly sink to its center in a process known as sedimentation, leaving an atmosphere that should be free of metallic elements.

Therefore it’s perhaps surprising that roughly 25–50% of all white dwarfs are observed to have atmospheric pollution by heavy elements. The short timescales for sedimentation suggest that these elements were added to the white dwarf recently — but how did they get there?

Bringing Ice Inward

In the generally accepted theory, pre-existing rocky bodies or an orbiting asteroid belt survive the star’s evolution, later accreting onto the final white dwarf. But this scenario doesn’t explain a few observations that suggest white dwarfs might be accreting larger planetary-size bodies and bodies with ices and volatile materials.

exciting eccentricity

Dynamical evolution of a Neptune-like planet (a) and a Kuiper belt analog object (b) in wide binary star systems. Both have large eccentricity excitations during the white dwarf phase. [Stephan et al. 2017]

How might you get large or icy objects — which would begin on very wide orbits — close enough to a white dwarf to become disrupted and accrete? Led by Alexander Stephan, a team of scientists at UCLA now suggest that the key is for the white dwarf to be in a binary system.

Influence of a Companion

In the authors’ model, the white-dwarf progenitor is orbited by both a distant stellar companion (a common occurrence) and a number of large potential polluters, which could have masses between that of a large asteroid up to several times the mass of Jupiter. These potential polluters have very wide orbits that allow them to maintain ice and volatile materials.

At the end of the progenitor’s lifetime it loses a significant amount of mass, causing the orbits of the surviving objects in the system to expand. After this stage, the stellar companion gravitationally perturbs the potential polluters onto extremely eccentric orbits, bringing these massive and long-period objects close enough accrete onto what is now the white dwarf.

The Need for Observations

orbital parameters

The likelihood distributions for orbital parameters of the systems that result in white dwarfs polluted by Neptune-like planets and Kuiper-belt-analog objects. The black arrows mark the parameters for one of the few observed systems, WD 1425+540, for comparison. [Stephan et al. 2017]

By running large Monte Carlo simulations, Stephan and collaborators demonstrate that this scenario can successfully produce accretion of both Neptune-like planets and Kuiper-belt-analog objects. Their simulation results indicate that ~1% of all white dwarfs should accrete Neptune-like planets, and ~7.5% of all white dwarfs should accrete Kuiper-belt-analog objects.

While these fractions are broadly consistent with observations, it’s hard to say with certainty whether this model is correct, as observations are scant. Only ~200 polluted white dwarfs have been observed, and of these, only ~15 have had detailed abundance measurements made. Next steps for understanding white-dwarf pollution certainly must include gathering more observations of polluted white dwarfs and establishing the statistics of what is polluting them.

Citation

Alexander P. Stephan et al 2017 ApJL 844 L16. doi:10.3847/2041-8213/aa7cf3

OH231

The gas expelled by dying stars gets twisted into intricate shapes and patterns as nebulae form. Now a team of researchers might have some answers about how this happens.

What’s a Pre-Planetary Nebula?

AGB on HR diagram

This H-R diagram for the globular cluster M5 shows where AGB stars lie: they are represented by blue markers here. The AGB is one of the final stages in a low- to intermediate-mass star’s lifetime. [Lithopsian]

When a low- to intermediate-mass star approaches the end of its lifetime, it moves onto the Asymptotic Giant Branch (AGB) in the Herzsprung-Russell diagram. As the star exhausts its fuel here, it shrugs off its outer layers. These layers of gas then encase the star’s core, which is not yet hot enough to ionize the gas and cause it to glow.

Instead, during this time the gas is relatively cool and dark, faintly reflecting light from the star and emitting only very dim infrared emission of its own. At this stage, the gas represents a pre-planetary nebula. Only later when the stellar core contracts enough to heat up and emit ionizing radiation does the nebula begin to properly glow, at which point it qualifies as a full planetary nebula.

Images of OH231 in optical light (top) and 12CO (bottom) taken from the literature. [See Balick et al. 2017 for full credit]

Unexpected Shapes

Pre-planetary nebulae are a very short-lived evolutionary stage, so we’ve observed only a few hundred of them — which has left many unanswered questions about these objects.

One particular mystery is that of their shapes: if these nebulae are formed by stars expelling their outer layers, we would naively expect them to be simple spherical shells — and yet we observe pre-planetary nebulae to have intricate shapes and patterns. How does the star create these asymmetric shapes? A team of scientists led by Bruce Balick (University of Washington, Seattle) has now used simulations to address this question.

Injecting Mass

Balick and collaborators use 3D hydrodynamic simulations to model one particular pre-planetary nebula, OH231, which lies ~4,200 light-years away and is about 1.4 light-years long. This is a well studied nebula, so the team had many observations that their model needed to successfully replicate: the nebula’s shapes, dimensions, overall geometry, locations of shocks, timescales, and even velocity gradients are known.

The authors’ model included mass injection from the central source into the ambient gas in three different ways:

  1. clumps: spherical knots injected all at once,
  2. cylindrical jets: thin outflows with parallel streamlines, and
  3. sprays: conical outflows with diverging streamlines.

Explanation from a Champagne Bottle

simulations of OH231

Panel A: best-fitting simulations of OH231 200, 400, and 800 yr after the clump and spray are launched. Panel B: example from the same family of solutions, in which the mass is reduced by a factor of 10. Click for a closer look. [Balick et al. 2017]

Balick and collaborators found that by injecting the mass in these three ways with a specific order and spacing, they were able to find a family of solutions that very well replicated observations of OH231. In the best-fitting model, combinations of pairs of clumps are embedded within sprays of brief duration and launched into static ancient AGB winds. The authors compare the setup to the ejection of the cork and the spray of high-pressure fluid when a bottle of champagne is opened.

These simulations successfully map out all but perhaps the first century of the nebula’s evolution and give us some of the best insight yet into how these short-lived objects are formed. The authors are now working to reproduce these simulations for other pre-planetary nebulae, with the goal of piecing together common attributes of their ejection histories.

Citation

Bruce Balick et al 2017 ApJ 843 108. doi:10.3847/1538-4357/aa77f0

G2 vs Sgr A*

Remember the excitement three years ago before the gas cloud G2’s encounter with the supermassive black hole at the center of our galaxy, Sgr A*? Did you notice that not much was said about it after the fact? That’s because not much happened — and a new study suggests that this isn’t surprising.

An Anticipated Approach

G2, an object initially thought to be a gas cloud, was expected to make its closest approach to the 4.6-million-solar-mass Sgr A* in 2014. At the pericenter of its orbit, G2 was predicted to pass as close as 36 light-hours from the black hole.

G2 simulations

Log-scale column density plots from one of the authors’ simulations, showing the cloud at a time relative to periapsis (t=0) of −5, −1, 0, 1, 5, and 10 yr (left to right, top to bottom). [Morsony et al. 2017]

This close brush with such a massive black hole was predicted to tear G2 apart, causing much of its material to accrete onto Sgr A*. It was thought that this process would temporarily increase the accretion rate onto the black hole relative to its normal background accretion rate, causing Sgr A*’s luminosity to increase for a time.

Instead, Sgr A* showed a distinct lack of fireworks, with very minimal change to its brightness after G2’s closest approach. This “cosmic fizzle” has raised questions about the nature of G2: was it really a gas cloud? What else might it have been instead? Now, a team of scientists led by Brian Morsony (University of Maryland and University of Wisconsin-Madison) have run a series of simulations of the encounter to try to address these questions.

No Fireworks

Morsony and collaborators ran three-dimensional hydrodynamics simulations using the FLASH code. They used a range of different simulation parameters, like cloud structure, background structure, background density, grid resolution, and accretion radius, in order to better understand how these factors might have affected the accretion rate and corresponding luminosity of Sgr A*.

Sgr A* accretion rate

Accretion rate vs. time for two of the simulations, one with a wind background and one with no wind background. The accretion rate in both cases displays no significant increase when G2 reaches periapsis. [Morsony et al. 2017]

Based on their simulations, the authors showed that we actually shouldn’t expect G2’s encounter to have caused a significant change in Sgr A*’s accretion rate relative to its normal rate from background accretion: with the majority of the simulation parameters used, only 3–21% of the material Sgr A* accreted from 0–5 years after periapsis is from the cloud, and only 0.03–10% of the total cloud mass is accreted.

Not Just a Cloud?

By comparing their simulations to observations of G2 after its closest approach, Morsony and collaborators find that to fit the observations, G2 cannot be solely a gas cloud. Instead, two components are likely needed: an extended, cold, low-mass gas cloud responsible for most of the emission before G2 approached pericenter, and a very compact component such as a dusty stellar object that dominates the emission observed since pericenter.

The authors argue that any future emission detected should no longer be from the cloud, but only from the compact core or dusty stellar object. Future observations should help us to confirm this model — but in the meantime these simulations give us a better sense of why G2’s encounter with Sgr A* was such a fizzle.

Citation

Brian J. Morsony et al 2017 ApJ 843 29. doi:10.3847/1538-4357/aa773d

Dragonfly 44

The origin of ultra-diffuse galaxies (UDGs) has posed a long-standing mystery for astronomers. New observations of several of these faint giants with the Hubble Space Telescope are now lending support to one theory.

Faint-Galaxy Mystery

HST UDGs

Hubble images of Dragonfly 44 (top) and DFX1 (bottom). The right panels show the data with greater contrast and extended objects masked. [van Dokkum et al. 2017]

UDGs — large, extremely faint spheroidal objects — were first discovered in the Virgo galaxy cluster roughly three decades ago. Modern telescope capabilities have resulted in many more discoveries of similar faint galaxies in recent years, suggesting that they are a much more common phenomenon than we originally thought.

Despite the many observations, UDGs still pose a number of unanswered questions. Chief among them: what are UDGs? Why are these objects the size of normal galaxies, yet so dim? There are two primary models that explain UDGs:

  1. UDGs were originally small galaxies, hence their low luminosity. Tidal interactions then puffed them up to the large size we observe today.
  2. UDGs are effectively “failed” galaxies. They formed the same way as normal galaxies of their large size, but something truncated their star formation early, preventing them from gaining the brightness that we would expect for galaxies of their size.

Now a team of scientists led by Pieter van Dokkum (Yale University) has made some intriguing observations with Hubble that lend weight to one of these models.

Coma UDGs

Globulars observed in 16 Coma-cluster UDGs by Hubble. The top right panel shows the galaxy identifications. The top left panel shows the derived number of globular clusters in each galaxy. [van Dokkum et al. 2017]

Globulars Galore

Van Dokkum and collaborators imaged two UDGs with Hubble: Dragonfly 44 and DFX1, both located in the Coma galaxy cluster. These faint galaxies are both smooth and elongated, with no obvious irregular features, spiral arms, star-forming regions, or other indications of tidal interactions.

The most striking feature of these galaxies, however, is that they are surrounded by a large number of compact objects that appear to be globular clusters. From the observations, Van Dokkum and collaborators estimate that Dragonfly 44 and DFX1 have approximately 74 and 62 globulars, respectively — significantly more than the low numbers expected for galaxies of this luminosity.

Armed with this knowledge, the authors went back and looked at archival observations of 14 other UDGs also located in the Coma cluster. They found that these smaller and fainter galaxies don’t host quite as many globular clusters as Dragonfly 44 and DFX1, but more than half also show significant overdensities of globulars.

UDG relations

Main panel: relation between the number of globular clusters and total absolute magnitude for Coma UDGs (solid symbols) compared to normal galaxies (open symbols). Top panel: relation between effective radius and absolute magnitude. The UDGs are significantly larger and have more globular clusters than normal galaxies of the same luminosity. [van Dokkum et al. 2017]

Evidence of Failure

In general, UDGs appear to have more globular clusters than other galaxies of the same total luminosity, by a factor of nearly 7. These results are consistent with the scenario in which UDGs are failed galaxies: they likely have the halo mass to have formed a large number of globular clusters, but they were quenched before they formed a disk and bulge. Because star formation never got going in UDGs, they are now much dimmer than other galaxies of the same size.

The authors suggest that the next step is to obtain dynamical measurements of the UDGs to determine whether these faint galaxies really do have the halo mass suggested by their large numbers of globulars. Future observations will continue to help us pin down the origin of these dim giants.

Citation

Pieter van Dokkum et al 2017 ApJL 844 L11. doi:10.3847/2041-8213/aa7ca2

Helix Nebula gas

As if it weren’t enough that quasars — distant and bright nuclei of galaxies — twinkle of their own accord due to internal processes, nature also provides another complication: these distant radio sources can also appear to twinkle because of intervening material between them and us. A new study has identified a possible source for the material getting in the way.

Unexplained Variability

Helix Nebula

A Spitzer infrared view of the Helix nebula, which contains ionized streamers of gas extending radially outward from the central star. [NASA/JPL-Caltech/Univ. of Ariz.]

Distant quasars occasionally display extreme scintillation, twinkling with variability timescales shorter than a day. This intra-day variability is much greater than we can account for with standard models of the interstellar medium lying between the quasar and us. So what could cause this extreme scattering instead?

The first clue to this mystery came from the discovery of strong variability in the radio source PKS 1322–110. In setting up follow-up observations of this object, Mark Walker (Manly Astrophysics, Australia) and collaborators noticed that, in the plane of the sky, PKS 1322–110 lies very near the bright star Spica. Could this be coincidence, or might this bright foreground star have something to do with the extreme scattering observed?

Diagram of scintillation

Diagram explaining the source of the intra-day radio source variability as intervening filaments surrounding a hot star. [M. Walker/CSIRO/Manly Astrophysics]

Swarms of Clumps

Walker and collaborators put forward a hypothesis: perhaps the ultraviolet photons of nearby hot stars ionize plasma around them, which in turn causes the extreme scattering of the distant background sources.

As a model, the authors consider the Helix Nebula, in which a hot, evolved star is surrounded by cool globules of molecular hydrogen gas. The radiation from the star hits these molecular clumps, dragging them into long radial streamers and ionizing their outer skins.

Though the molecular clumps in the Helix Nebula were thought to have formed only as the star evolved late into its lifetime, Walker and collaborators are now suggesting that all stars — regardless of spectral type or evolutionary stage — may be surrounded by swarms of tiny molecular clumps. Around stars that are hot enough, these clumps become the ionized plasma streamers that can cause interference with the light traveling to us from distant sources.

Significant Mass

To test this theory, Walker and collaborators explore observations of two distant radio quasars that have both exhibited intra-day variability over many years of observations. The team identified a hot A-type star near each of these two sources: J1819+3845 has Vega nearby, and PKS 1257–326 has Alhakim.

line-of-sight stars

Locations of stars along the line of site to two distant quasars, J1819+3845 (top panel) and PKS 1257–326 (bottom panel). Both have a nearby, hot star (blue markers) radially within 2 pc: Vega (z = 7.7 pc) and Alhakim (z = 18 pc), respectively. [Walker et al. 2017]

By modeling the systems of the sources and stars, the authors show that the size, location, orientation, and numbers of plasma concentrations necessary to explain observations are all consistent with an environment similar to that of the Helix Nebula. Walker and collaborators find that the total mass in the molecular clumps surrounding the two stars would need to be comparable to the mass of the stars themselves.

If this picture is correct, and if all stars are indeed surrounded by molecular clumps like these, then a substantial fraction of the mass of our galaxy could be contained in these clumps. Besides explaining distant quasar scintillation, this idea would therefore have a significant impact on our overall understanding of how mass in galaxies is distributed. More observations of twinkling quasars are the next step toward confirming this picture.

Citation

Mark A. Walker et al 2017 ApJ 843 15. doi:10.3847/1538-4357/aa705c

plunging black hole

Several small, speeding clouds have been discovered at the center of our galaxy. A new study suggests that these unusual objects may reveal the lurking presence of inactive black holes.

Peculiar Clouds

high-velocity compact clouds

a) Velocity-integrated intensity map showing the location of the two high-velocity compact clouds, HCN–0.009–0.044 and HCN–0.085–0.094, in the context of larger molecular clouds. b) and c) Latitude-velocity and longitude-velocity maps for HCN–0.009–0.044 and HCN–0.085–0.094, respectively. d) and e) spectra for the two compacts clouds, respectively. Click for a closer look. [Takekawa et al. 2017]

Sgr A*, the supermassive black hole marking the center of our galaxy, is surrounded by a region roughly 650 light-years across known as the Central Molecular Zone. This area at the heart of our galaxy is filled with large amounts of warm, dense molecular gas that has a complex distribution and turbulent kinematics.

Several peculiar gas clouds have been discovered within the Central Molecular Zone within the past two decades. These clouds, dubbed high-velocity compact clouds, are characterized by their compact sizes and extremely broad velocity widths.

What created this mysterious population of energetic clouds? The recent discovery of two new high-velocity compact clouds, reported on in a paper led by Shunya Takekawa (Keio University, Japan), may help us to answer this question.

Two More to the Count

Using the James Clerk Maxwell Telescope in Hawaii, Takekawa and collaborators detected the small clouds near the circumnuclear disk at the centermost part of our galaxy. These two clouds have velocity spreads of -80 to -20 km/s and -80 to 0 km/s and compact sizes of just over 1 light-year. The clouds’ similar appearances and physical properties suggest that they may both have been formed by the same process.

Takekawa and collaborators explore and discard several possible origins for these clouds, such as outflows from massive protostars (no massive, luminous stars have been detected affiliated with these clouds), interaction with supernova remnants (no supernova remnants have been detected toward the clouds), and cloud–cloud collisions (such collisions leave other signs, like cavities in the parent cloud, which are not detected here).

black hole masses

Masses and velocities of black holes that could create the two high-velocity compact clouds fall above the red and blue lines here. [Takekawa et al. 2017]

Revealed on the Plunge

As an alternative explanation, Takekawa and collaborators propose that these two small, speeding clouds were each created when a massive compact object plunged into a nearby molecular cloud. Since we don’t see any luminous stellar counterparts to the high-velocity compact clouds, this suggests that the responsible objects were invisible black holes. As each black hole tore through a molecular cloud, it dragged some of the cloud’s gas along behind it to form the high-velocity compact cloud.

Does this explanation make sense statistically? The authors point out that the number of black holes predicted to silently lurk in the central ~30 light-years of the Milky Way is around 10,000. This makes it entirely plausible that we could have caught sight of two of them as they revealed their presence while plunging through molecular clouds.

If the authors’ interpretation is correct, then high-velocity compact clouds provide an excellent opportunity: we can search for these speeding bodies to potentially discover inactive black holes that would otherwise go undetected.

Citation

Shunya Takekawa et al 2017 ApJL 843 L11. doi:10.3847/2041-8213/aa79ee

WASP-12b

Jupiter-like planets on orbits close to their hosts are predicted to spiral ever closer to their hosts until they meet their eventual demise — and yet we’ve never observed orbital decay. Could WASP-12b provide the first evidence?

Undetected Predictions

Since the discovery of the first hot Jupiter more than 20 years ago, we’ve studied a number of these peculiar exoplanets. Despite our many observations, two phenomena predicted of hot Jupiters have not yet been detected, due to the long timescales needed to identify them:

  1. Tidal orbital decay
    Tidal forces should cause a hot Jupiter’s orbit to shrink over time, causing the planet to eventually spiral into its host star. This phenomenon would explain a number of statistical properties of observed star-planet systems (for instance, the scarcity of gas giants with periods less than a day).
  2. apsidal precession

    An illustration of apsidal precession. [Mpfiz]

    Apsidal precession
    The orbits of hot Jupiters should be apsidally precessing on timescales of decades, as long as they are at least slightly eccentric. Since the precession rate depends on the planet’s tidally deformed mass distribution, measuring this would allow us to probe the interior of the planet.

A team of scientists led by Kishore Patra (Massachusetts Institute of Technology) think that the hot Jupiter WASP-12b may be our first chance to study one of these two phenomena. The question is, which one?

WASP-12b

WASP-12b has orbital period of 1.09 days — one of the shortest periods observed for a giant planet — and we’ve monitored it for a decade, making it a great target to test for both of these long-term effects.

WASP-12b residuals

Timing residuals for WASP-12b. Squares show the new data points, circles show previous data from the past decade. The data are better fit by the decay model than the precession model, but both are still consistent. [Patra et al. 2017]

Patra and collaborators made transit observations with the 1.2-m telescope at the Fred Lawrence Whipple Observatory in Arizona and occultation observations with the Spitzer Space Telescope. These two new sets of observations, combined with the decade of previous observations, allowed the authors to fit models to WASP-12b’s orbit over time.

The results show that a constant period for WASP-12b is firmly ruled out — this planet’s orbit is definitely changing over time. The observations are best fit by a model in which the planet’s orbit is tidally decaying, but a 14-year apsidal precession cycle can’t be definitively ruled out.

Future Prospects

WASP-12b futures

Possible futures for WASP-12b’s orbit, based on the decay model (red) and the precession model (blue). We should be able to differentiate between these models with a few more years of observations. [Patra et al. 2017]

If the planet’s orbit is decaying, then the authors show that its period will shrink to zero within 3.2 million years, suggesting that we’re currently witnessing the last ~0.2% of the planet’s lifetime. Supporting the orbital-decay hypothesis are independent observations that suggest WASP-12b is approaching a point of tidal disruption — it appears to have an extended and escaping exosphere, for instance.

While we can’t yet state for certain that WASP-12b’s orbit is decaying, the authors argue that we should be able to tell conclusively with a few more years of observations. Either of the two outcomes above — orbital decay or apsidal precession — would have exciting scientific implications, however: if WASP-12b’s orbit is decaying, we can measure the tidal dissipation rate of the star. If its orbit is apsidally precessing, we may be able to measure the tidal deformability of an exoplanet. Future observations of this hot Jupiter should prove interesting!

Citation

Kishore C. Patra et al 2017 AJ 154 4. doi:10.3847/1538-3881/aa6d75

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