From AAS NOVA: ” Globular Clusters for Faint Galaxies”

American Astronomical Society

21 July 2017
Susanna Kohler

This Hubble image of Dragonfly 44, an extremely faint galaxy, reveals that it is surrounded by dozens of compact objects that are likely globular clusters. [van Dokkum et al. 2017]

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.

NASA/ESA Hubble Telescope

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]

Faint-Galaxy Mystery

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.

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.

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

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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Nearby Hot Stars May Change Our View of Distant Sources”

American Astronomical Society

19 July 2017
Susanna Kohler

Clumps of hydrogen gas in the Helix Nebula have been drawn out into long, ionized streamers, as visible in this Hubble image. Could gas like this be responsible for the twinkling of distant quasars? [C. R. O’Dell/K. Handron/NASA/Manly Astrophysics]

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.

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.]

NASA/Spitzer Telescope

Unexplained Variability

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 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.

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

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Further references complete with links on the full article.

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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

AAS NOVA: ” Featured Image: Central Black Holes in Late-Type Galaxies”

American Astronomical Society

The images above show just 8 of 51 different nearby, late-type galaxies found to host X-ray cores near their centers. The main images are optical views and the insets show Chandra X-ray images of the same galaxies.

NASA/Chandra Telescope

The cross marks identify the near-infrared/optical nucleus of each galaxy, and the green ellipses show the source regions for the X-rays. A recent publication led by Rui She (Tsinghua University, China) presents a search for low-mass (<106 solar masses) black holes lurking in the centers of nearby late-type, low-mass galaxies. Many of the 51 X-ray cores discovered represent such hidden black holes. The authors use the statistics of this sample to estimate that at least 21% of late-type galaxies like those studied here host low-mass black holes at their centers. You can view the full set of X-ray core hosts below; for more information, check out the paper linked at the bottom of the page.

All 51 X-ray cores (displayed in 3 sets); see the article below for the originals.

Citation

Rui She et al 2017 ApJ 842 131. doi:10.3847/1538-4357/aa7634

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Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Speeding Clouds May Reveal Invisible Black Holes”

American Astronomical Society

14 July 2017
Susanna Kohler

Artist’s illustration of a speeding black hole plunging through a dense molecular cloud. This process could form a high-velocity compact cloud of gas. [Keio University].

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.

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.]

Peculiar Clouds

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.

Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

NASA/Chandra Telescope

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.

East Asia Observatory James Clerk Maxwell telescope, Maunakea, Hawaii, USA

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).

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

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Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “WASP-12b and Its Possible Fiery Demise”

American Astronomical Society

12 July 2017
Susanna Kohler

Artist’s illustration of WASP-12b, demonstrating the planet’s possible extended and escaping outer atmosphere that results from its close orbit. [NASA/ESA/G. Bacon]

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
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.

An illustration of apsidal precession. [Mpfiz]

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.

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.

CfA 1.2 meter Whipple telescope located in Amado, Arizona on Mount Hopkins

CfA Whipple 1.2 meter telescope interior, located in Amado, Arizona on Mount Hopkins

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.

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]

Future Prospects
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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Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Exploring Disks Around Planets”

American Astronomical Society

10 July 2017
Susanna Kohler

A 3-Jupiter-mass giant planet and its circumplanetary disk carve a gap in the circumstellar disk in this radius v. azimuth plot from a computer simulation. [Adapted from Szulágyi 2017]

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.

Elusive Disks

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]

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 [ALMA]).

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

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.

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:

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.
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.
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

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Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Discovery of a Free-Floating Double Planet?”

American Astronomical Society

5 July 2017
Susanna Kohler

Artist’s impression of a low-mass binary system. A newly discovered system may be so low-mass as to contain planets rather than stars! [Gemini Observatory/Jon Lomberg Illustration]

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

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 [Science] 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 Observatory, Mauna Kea, Hawaii, USA

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

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]

he 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

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See further references complete with links at the full article.

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “A New Clue in the Mystery of Fast Radio Bursts”

American Astronomical Society

28 June 2017
Susanna Kohler

Artist’s impression of a magnetized neutron star. Could these objects be responsible for fast radio bursts? [ESO/L. Calçada].

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

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

GEMINI/North GMOS

Gemini/North telescope at Mauna Kea, Hawaii, USA

NOAO Gemini Planet Imager on Gemini South

Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile

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.

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]

NASA/ESA Hubble Telescope

NASA/Spitzer Telescope

Host Observations

Bassa and collaborators used the Hubble Space Telescope, the Spitzer Space Telecsope, and the Gemini North telescope [above] 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.

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

NASA/SWIFT Telescope

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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Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: ” Hidden Black Holes Revealed?”

American Astronomical Society

19 June 2017
Susanna Kohler

Artist’s illustration of the thick dust torus thought to surround supermassive black holes and their accretion disks. [ESA / V. Beckmann (NASA-GSFC)]

Supermassive black holes are thought to grow in heavily obscured environments. A new study now suggests that many of the brightest supermassive black holes around us may be escaping our detection as they hide in these environments.

The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, the dusty torus obscures the central engine from view. [Urry & Padovani, 1995]

A Torus Puzzle

The centers of galaxies with bright, actively accreting supermassive black holes are known as active galactic nuclei, or AGN. According to a commonly accepted model for AGN, these rapidly growing black holes and their accretion disks are surrounded by a thick torus of dust. From certain angles, the torus can block our direct view of the central engines, changing how the AGN appears to us. AGN for which we can see the central engine are known as Type 1 AGN, whereas those with an obscured central region are classified as Type 2.

Oddly, the fraction of AGN classified as Type 2 decreases substantially with increasing luminosity; brighter AGN seem to be more likely to be unobscured. Why? One hypothesis is that the torus structure itself changes with changing AGN luminosity. In this model, the torus recedes as AGN become brighter, causing fewer of these AGN to be obscured from our view.

But a team of scientists led by Silvia Mateos (Institute of Physics of Cantabria, Spain) suggests that we may instead be missing the bigger picture. What if the problem is just that many of the brightest obscured AGN are too well hidden?

Geometry Matters

Type 2 AGN fraction vs. torus covering factor for AGN in the authors’ three luminosity bins. The black line shows the 1-to-1 relation describing the expected Type 2 AGN fraction; the black data points show the observed fraction. The red points show the best-fit model including the “missing” AGN, and the inset shows the covering-factor distribution for the missing sources. [Mateos et al. 2017]

Mateos and collaborators built a sample of nearly 200 X-ray-observed AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). They then determined the intrinsic fraction of these AGN that were obscured (i.e., classified as Type 2) at a given luminosity, for redshifts between 0.05 ≤ z ≤ 1.

ESA/XMM Newton

The team next used clumpy torus models to estimate the distributions of AGN covering factors, the geometric factor that describes the fraction of the sky around the AGN central engine that’s obscured.

The pointing directions for AGN should be randomly distributed, and geometry then dictates that the covering factor distributions combined over the total AGN population should match the intrinsic fraction of AGN classified as Type 2 AGN. Instead, the sample from BUXS reveals a “missing” population of high-covering-factor tori that we have yet to detect in X-rays.

Missing Sources

When they include the missing AGN, Mateos and collaborators find that the total fraction of Type 2 AGN is around 58%. They also show that more of these AGN are missing at higher luminosities. By including the missing ones, the total fraction of obscured AGN therefore has a much weaker dependence on luminosity than we thought — which suggests that the receding torus model isn’t necessary to explain observations.

Mateos and collaborators’ results support the idea that the majority of very bright, rapidly accreting supermassive black holes at redshifts of z ≤ 1 live in nuclear environments that are extremely obscured. These black holes are so well embedded in their environments that they’ve escaped detection in X-ray surveys thus far.

Citation

S. Mateos et al 2017 ApJL 841 L18. doi:10.3847/2041-8213/aa7268

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See the full article for a list of further references with links.

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Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: ” Maxing Out the Mass of Early Stars”

American Astronomical Society

16 June 2017
Susanna Kohler

An artist’s impression of the first generation of stars forming in our universe. A new study examines just how large some of these early stars could have grown. [NASA/JPL-Caltech/R. Hurt (SSC)]

Primordial supermassive stars might be responsible for the earliest supermassive black holes in our universe. But just how big can a star grow before it inevitably collapses into a black hole?

Artist’s illustration of a high-redshift quasar, a black hole feeding on the material around it. [ESO/M. Kornmesser]

The Puzzle of Distant Quasars

Quasars — supermassive black holes that are actively feeding — have been observed with enormous sizes (billions of solar masses) at very large distances (redshifts of z > 6). These monsters pose a problem: how could they have accreted so much mass in so little time since the beginning of the universe?

One theory is that these black holes formed from the direct collapse of stars. The larger the original star before collapse, the better the chances that the resulting black hole will be able to grow quickly. But even theorized Pop III stars (which have hundreds of solar masses) would have to accrete at rates higher than believed possible to achieve the black-hole masses we observe so quickly. For this reason, the commonly invoked explanation now is supermassive stars.

Early Giants

Supermassive stars are theoretical stars that formed in the very different environment of the early universe.

A composite infrared and X-ray image showing a molecular cloud and newly formed stars around Cepheus B. [X-ray: NASA/CXC/PSU/K. Getman et al.; IR: NASA/JPL-Caltech/CfA/J. Wang et al.]

In ordinary star formation, halos of gas cool primarily due to emission by molecules. When these clouds cool, they fragment and then collapse into normal-sized stars.

In the supermassive star-formation scenario, hydrogen molecules in primordial halos are broken down — possibly by ultraviolet radiation from nearby star formation. This prevents the halos from cooling by molecular emission, instead allowing them to grow to an enormous 107–108 solar masses before they start cooling due to atomic emission. At this point they finally collapse to form a star.

Stars forming via this scenario quickly grow to be very massive, as the halo material falls onto the core at catastrophic rates of 0.01–10 solar masses per year. After a short period of this rapid accretion, the supermassive star then collapses into a black hole due to instability. But how massive could such a star grow before its collapse?

Simulating Growth

To answer this question, a team of scientists led by Tyrone Woods (Monash University, Australia) ran stellar-evolution simulations of the birth, growth, and eventual collapse of accreting, non-rotating supermassive stars. Their simulations included the effects of nuclear burning and captured the hydrodynamics of the instability that causes the stars to collapse into black holes, allowing the authors to follow the whole evolution.

The final mass at collapse of a star as a function of its accretion rate. Most stars collapse due to instability during hydrogen burning. [Woods et al. 2017]

Woods and collaborators found that for accretion rates above 0.1 solar masses per year, the supermassive stars generally collapsed into black holes at masses of 150,000–330,000 solar masses. Since the final mass at collapse grows only logarithmically with accretion rate, the upper end of this range represents an approximate upper limit on the mass of supermassive stars.

This also sets the maximum mass of the supermassive black holes formed by direct collapse of stars in the early universe. At hundreds of thousands of solar masses, these first quasars provide much more plausible seeds than Pop III stars for growing the billion-solar-mass monsters we observe at high redshifts. Supermassive stars may indeed be the key to the formation of the first and most luminous quasars in our universe.

Citation

T. E. Woods et al 2017 ApJL 842 L6. doi:10.3847/2041-8213/aa7412

Related Journal Articles
See the full article for further references with links.

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009