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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

Circumplanetary disk

Giant planets are thought to form in circumstellar disks surrounding young stars, but material may also accrete into a smaller disk around the planet. We’ve never detected one of these circumplanetary disks before — but thanks to new simulations, we now have a better idea of what to look for.

giant planet formation

Image from previous work simulating a Jupiter-mass planet forming inside a circumstellar disk. The planet has its own circumplanetary disk of accreted material. [Frédéric Masset]

Elusive Disks

In the formation of giant planets, we think the final phase consists of accretion onto the planet from a disk that surrounds it. This circumplanetary disk is important to understand, since it both regulates the late gas accretion and forms the birthplace of future satellites of the planet.

We’ve yet to detect a circumplanetary disk thus far, because the resolution needed to spot one has been out of reach. Now, however, we’re entering an era where the disk and its kinematics may be observable with high-powered telescopes (like the Atacama Large Millimeter Array).

To prepare for such observations, we need models that predict the basic characteristics of these disks — like the mass, temperature, and kinematic properties. Now a researcher at the ETH Zürich Institute for Astronomy in Switzerland, Judit Szulágyi, has worked toward this goal.

Simulating Cooling

Szulágyi performs a series of 3D global radiative hydrodynamic simulations of 1, 3, 5, and 10 Jupiter-mass (MJ) giant planets and their surrounding circumplanetary disks, embedded within the larger circumstellar disk around the central star.

disk density, temp, ang mom

Density (left column), temperature (center), and normalized angular momentum (right) for a 1 MJ planet over temperatures cooling from 10,000 K (top) to 1,000 K (bottom). At high temperatures, a spherical circumplanetary envelope surrounds the planet, but as the planet cools, the envelope transitions around 6–4,000 K to a flattened disk. [Szulágyi 2017]

This work explores the effects of different planet temperatures and masses on the properties of the disks. Szulágyi specifically examines a range of planetary temperatures between 10,000 K and 1,000 K for the 1 MJ planet. Since the planet cools as it radiates away its formation heat, the different temperatures represent an evolutionary sequence over time.

Predicted Characteristics

Szulágyi’s work produced a number of intriguing observations, including the following:

  1. For the 1 MJ planet, a spherical circumplanetary envelope forms at high temperatures, flattening into a disk as the planet cools. Higher-mass planets form disks even at high temperatures.
  2. The disk has a steep temperature profile from inside to outside, and the whole disk is too hot for water to remain frozen. This suggests that satellites couldn’t form in the disk earlier than 1 Myr after the planet birth. The outskirts of the disk cool first as the planet cools, indicating that satellites may eventually form in these outer parts and then migrate inward.
  3. The planets open gaps in the circumstellar disk as they orbit. As a planet radiates away its formation heat, the gap it opens becomes deeper and wider (though this is a small effect). For high-mass planets (>5 MJ), the gap eccentricity increases, which creates a hostile environment for satellite formation.

Szulágyi discusses a number of features of these disks that we can plan to search for in the future with our increasing telescope power — including signatures in direct imaging and observations of their kinematics. The results from these simulations will help us both to detect these circumplanetary disks and to understand our observations when we do. These future observations will then allow us to learn about late-stage giant-planet formation as well as the formation of their satellites.

Citation

J. Szulágyi 2017 ApJ 842 103. doi:10.3847/1538-4357/aa7515

starspots

The unusual light curve of the star KIC 8462852, also known as “Tabby’s star” or “Boyajian’s star”, has puzzled us since its discovery last year. A new study now explores whether the star’s missing flux is due to internal blockage rather than something outside of the star.

Mysterious Dips

comets around a star

Most explanations for the flux dips of Boyajian’s star rely on external factors, like this illustrated swarm of comets. [NASA/JPL-Caltech]

Boyajian’s star shows unusual episodes of dimming in its light curve by as much as 20%, each lasting a few to tens of days and separated by periods of typically hundreds of days. In addition, archival observations show that it has gradually faded by roughly 15% over the span of the last hundred years. What could be causing both the sporadic flux dips and the long-term fading of this odd star?

Explanations thus far have varied from mundane to extreme. Alien megastructures, pieces of smashed planets or comets orbiting the star, and intervening interstellar medium have all been proposed as possible explanations — but these require some object external to the star. A new study by researcher Peter Foukal proposes an alternative: what if the source of the flux obstruction is the star itself?

Analogy to the Sun

Decades ago, researchers discovered that our own star’s total flux isn’t as constant as we thought. When magnetic dark spots on the Sun’s surface block the heat transport, the Sun’s luminosity dips slightly. The diverted heat is redistributed in the Sun’s interior, becoming stored as a very small global heating and expansion of the convective envelope. When the blocking starspot is removed, the Sun appears slightly brighter than it did originally. Its luminosity then gradually relaxes, decaying back to its original value.

starspot model

Model of a star’s flux after a 1,000-km starspot is inserted at time t = 0 and removed at time t = ts at a depth of 10,000 km in the convective zone. The star’s luminosity dips, then becomes brighter than originally, and then gradually decays. [Foukal 2017]

Foukal recognized that this phenomenon may also provide an explanation for Boyajian’s star. He modeled how this might occur for Boyajian’s star, demonstrating that if its flux is somehow blocked from reaching the surface and stored in a shallow convective zone, this can account for the 20% dips seen in the star’s light curve.

In addition, these sporadic flux-blocking events would cause Boyajian’s star to constantly be relaxing from the post-blockage enhanced luminosity. This decay — which occurs at rates of 0.1–1% brightness per year for convective-zone depths of tens of thousands of kilometers — would nicely account for the long-term, gradual dimming observed.

What’s blocking the flux? Foukal postulates a few options, including magnetic activity (as with the Sun), differential rotation, sporadic changes in photospheric abundances, and simply random variation in convective efficiency.

Strangely Unique

latest dips

Boyajian’s star’s flux in May and June shows some brand new dips. Note that the team now names them! [Tabetha Boyajian and team]

So why have we only found one star with light curves like Boyajian’s? If these are inherently natural processes in the star, we would expect to have seen more than one such object. This may be selection effect — Boyajian’s star lies at the hot end of the range of stars that Kepler observes — or it may be that the star is reaching the end of its convective lifetime.

Until we discover more cases, the best we can hope for is more data from Boyajian’s star itself. Conveniently, it has continued to keep us on our toes, with new dips in May and June. Perhaps our continued observations will finally reveal the answer to this mystery.

Citation

Peter Foukal 2017 ApJL 842 L3. doi:10.3847/2041-8213/aa740f

Planetary-mass binary

An object previously identified as a free-floating, large Jupiter analog turns out to be two objects — each with the mass of a few Jupiters. This system is the lowest-mass binary we’ve ever discovered.

Tracking Down Ages

TW Hydrae Association

2MASS J11193254–1137466 is thought to be a member of the TW Hydrae Association, a group of roughly two dozen young stars moving together in the solar neighborhood. [University of Western Ontario/Carnegie Institution of Washington DTM/David Rodriguez]

Brown dwarfs represent the bottom end of the stellar mass spectrum, with masses too low to fuse hydrogen (typically below ~75-80 Jupiter masses). Observing these objects provides us a unique opportunity to learn about stellar evolution and atmospheric models — but to properly understand these observations, we need to determine the dwarfs’ masses and ages.

This is surprisingly difficult, however. Brown dwarfs cool continuously as they age, which creates an observational degeneracy: dwarfs of different masses and ages can have the same luminosity, making it difficult to infer their physical properties from observations.

We can solve this problem with an independent measurement of the dwarfs’ masses. One approach is to find brown dwarfs that are members of nearby stellar associations called “moving groups”. The stars within the association share the same approximate age, so a brown dwarf’s age can be estimated based on the easier-to-identify ages of other stars in the group.

An Unusual Binary

Recently, a team of scientists led by William Best (Institute for Astronomy, University of Hawaii) were following up on such an object: the extremely red, low-gravity L7 dwarf 2MASS J11193254–1137466, possibly a member of the TW Hydrae Association. With the help of the powerful adaptive optics on the Keck II telescope in Hawaii, however, the team discovered that this Jupiter-like object was hiding something: it’s actually two objects of equal flux orbiting each other.

Keck binary

Keck images of 2MASS J11193254–1137466 reveal that this object is actually a binary system. A similar image of another dwarf, WISEA J1147-2040, is shown at bottom left for contrast: this one does not show signs of being a binary at this resolution. [Best et al. 2017]

To learn more about this unusual binary, Best and collaborators began by using observed properties like sky position, proper motion, and radial velocity to estimate the likelihood that 2MASS J11193254–1137466AB is, indeed, a member of the TW Hydrae Association of stars. They found roughly an 80% chance that it belongs to this group.

Under this assumption, the authors then used the distance to the group — around 160 light-years — to estimate that the binary’s separation is ~3.9 AU. The assumed membership in the TW Hydrae Association also provides binary’s age: roughly 10 million years. This allowed Best and collaborators to estimate the masses and effective temperatures of the components from luminosities and evolutionary models.

Planetary-Mass Objects

2MASS J1119–1137 among ultracool dwarfs

The positions of 2MASS J11193254–1137466A and B on a color-magnitude diagram for ultracool dwarfs. The binary components lie among the faintest and reddest planetary-mass L dwarfs. [Best et al. 2017]

The team found that each component is a mere ~3.7 Jupiter masses, placing them in the fuzzy region between planets and stars. While the International Astronomical Union considers objects below the minimum mass to fuse deuterium (around 13 Jupiter masses) to be planets, other definitions vary, depending on factors such as composition, temperature, and formation. The authors describe the binary as consisting of two planetary-mass objects.

Regardless of its definition, 2MASS J11193254–1137466AB qualifies as the lowest-mass binary discovered to date. The individual masses of the components also place them among the lowest-mass free-floating brown dwarfs known. This system will therefore be a crucial benchmark for tests of evolutionary and atmospheric models for low-mass stars in the future.

Citation

William M. J. Best et al 2017 ApJL 843 L4. doi:10.3847/2041-8213/aa76df

Mercury

If space rocks are unpleasant to encounter, space dust isn’t much better. Mercury’s cratered surface tells of billions of years of meteoroid impacts — but its thin atmosphere is what reveals its collisional history with smaller impactors. Now new research is providing a better understanding of what we’re seeing.

Micrometeoroids Ho!

The inner solar system is bombarded by micrometeoroids, tiny particles of dust (on the scale of a tenth of a millimeter) emitted by asteroids and comets as they make their closest approach to the Sun. This dust doesn’t penetrate Earth’s layers of atmosphere, but the innermost planet of our solar system, Mercury, doesn’t have this convenient cushioning.

Just as Mercury is affected by the impacts of large meteoroids, it’s also shaped by the many smaller-scale impacts it experiences. These tiny collisions are thought to vaporize atoms and molecules from the planet’s surface, which quickly dissociate. This process adds metals to Mercury’s exosphere, the planet’s extremely tenuous atmosphere.

Modeling Populations

Impactor source distributions

Distribution of the directions from which meteoroids originate before impacting Mercury’s surface, as averaged over its entire orbit. Local time of 12 hr corresponds to the Sun-facing side. A significant asymmetry is seen between the dawn (6 hrs) and dusk (18 hrs) rates. [Pokorný et al. 2017]

The metal distribution in the exosphere provides a way for us to measure the effect of micrometeoroid impacts on Mercury — but this only works if we have accurate models of the process. A team of scientists led by Petr Pokorný (The Catholic University of America and NASA Goddard SFC) has now worked to improve our picture of micrometeoroid impact vaporization on Mercury.

Pokorný and collaborators argue that two meteoroid populations — Jupiter-family comets (short-period) and Halley-type comets (long-period) — contribute the dust for the majority of micrometeoroid impacts on Mercury. The authors model the dynamics and evolution of these two populations, reproducing the distribution of directions from which micrometeoroids strike Mercury during its yearly orbit.

impact geometry

Schematic of Mercury in its orbit around the Sun. The dawn side leads the orbital motion, while the dusk side trails it.

Geometry of an Orbit

Mercury’s orbit is unique in our solar system: it circles the Sun twice for every three rotations on its own axis — so if you were on Mercury, you’d see a single day pass over the span of two years. As with all prograde planets, the edge leading the Mercury’s orbit marks the dawn terminator, while the edge trailing the planet’s orbital motion marks the dusk terminator.

Pokorný and collaborators find a significant asymmetry in the impact vaporization that occurs on Mercury’s dawn side versus its dusk side. This is due to impact geometry (since the dusk side is shielded from impacts in the direction of motion) and seasonal variation of the dust/meteoroid environment around the planet. The authors show that the source of impact vaporization shifts toward the nightside as Mercury approaches aphelion, and toward the dayside when the planet approaches the Sun.

Importance of Long-Period Comets

seasonal variations

Seasonal variations of the relative vaporization rate from the authors’ model (black line) compared to measurements of Mercury’s exospheric abundance of Ca. The contribution of long-period comets is shown by the blue line. [Pokorný et al. 2017]

The dawn/dusk asymmetry and the seasonal variations predicted by the model are all nicely consistent NASA’s MESSENGER spacecraft observations of the metal distribution in Mercury’s exosphere.

What makes Pokorný and collaborators’ model work so well? Their inclusion of the long-period, Halley-type comets is key: the high impact velocity of the micrometeoroids produced by this family play a significant role in shaping the impact vaporization rate of Mercury’s surface.

This work successfully demonstrates that we can use measurements of Mercury’s exosphere as a unique tool to constrain the dust population in the inner solar system.

Citation

Petr Pokorný et al 2017 ApJL 842 L17. doi:10.3847/2041-8213/aa775d

magnetar

The origin of the mysterious fast radio bursts has eluded us for more than a decade. With the help of a particularly cooperative burst, however, scientists may finally be homing in on the answer to this puzzle.

A Burst Repeats

FRB 121102 host

The host of FRB 121102 is placed in context in this Gemini image. [Gemini Observatory/AURA/NSF/NRC]

More than 20 fast radio bursts — rare and highly energetic millisecond-duration radio pulses — have been observed since the first was discovered in 2007. FRB 121102, however, is unique in its behavior: it’s the only one of these bursts to repeat. The many flashes observed from FRB 121102 allowed us for the first time to follow up on the burst and hunt for its location.

Earlier this year, this work led to the announcement that FRB 121102’s host galaxy has been identified: a dwarf galaxy located at a redshift of z = 0.193 (roughly 3 billion light-years away). Now a team of scientists led by Cees Bassa (ASTRON, the Netherlands Institute for Radio Astronomy) has performed additional follow-up to learn more about this host and what might be causing the mysterious flashes.

HST view of FRB 121102 host

Hubble observation of the host galaxy. The object at the bottom right is a reference star. The blue ellipse marks the extended diffuse emission of the galaxy, the red circle marks the centroid of the star-forming knot, and the white cross denotes the location of FRB 121102 ad the associated persistent radio source. [Adapted from Bassa et al. 2017]

Host Observations

Bassa and collaborators used the Hubble Space Telescope, the Spitzer Space Telecsope, and the Gemini North telecsope in Hawaii to obtain optical, near-infrared, and mid-infrared observations of FRB 121102’s host galaxy.

The authors determined that the galaxy is a dim, irregular, low-metallicity dwarf galaxy. It’s resolved, revealing a bright star-forming region roughly 4,000 light-years across in the galaxy’s outskirts. Intriguingly, the persistent radio source associated with FRB 121102 falls directly within that star-forming knot.

Bassa and collaborators also found that the properties of the host galaxy are consistent with those of a type of galaxy known as extreme emission line galaxies. This provides a tantalizing clue, as these galaxies are known to host both hydrogen-poor superluminous supernovae and long-duration gamma-ray bursts.

Linking to the Cause

What can this tell us about the cause of FRB 121102? The fact that this burst repeats already eliminates cataclysmic events as the origin. But the projected location of FRB 121102 within a star-forming region — especially in a host galaxy that’s similar to those typically hosting superluminous supernovae and long gamma-ray bursts — strongly suggests there’s a relation between these events.

gamma-ray burst

Artist’s impression of a gamma-ray burst in a star-forming region. [NASA/Swift/Mary Pat Hrybyk-Keith and John Jones]

The authors propose that this observed coincidence, supported by models of magnetized neutron star birth, indicate an evolutionary link between fast radio bursts and neutron stars. In this picture, neutron stars or magnetars are born as long gamma-ray bursts or hydrogen-poor supernovae, and then evolve into fast-radio-burst-emitting sources.

This picture may finally explain the cause of fast radio bursts — but Bassa and collaborators caution that it’s also possible that this model applies only to FRB 121102. Since FRB 121102 is unique in being the only burst discovered to repeat, its cause may also be unique. The authors suggest that targeted searches of star-forming regions in galaxies similar to FRB 121102’s host may reveal other repeating burst candidates, helping us to unravel the ongoing mystery of fast radio bursts.

Citation

C. G. Bassa et al 2017 ApJL 843 L8. doi:10.3847/2041-8213/aa7a0c

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