From AAS NOVA: ” When a Star and a Binary Meet”

American Astronomical Society

22 September 2017
Susanna Kohler

What happens when stars interact in dense environments, such as globular clusters like the one pictured here? [HST/NASA/ESA]

What happens in the extreme environments of globular clusters when a star and a binary system meet? A team of scientists has new ideas about how these objects can deform, change their paths, spiral around each other, and merge.

Getting to Know Your Neighbors

Two simulations of the interaction of a white-dwarf–compact-object binary with a single incoming compact object (progressing from left to right). When tides are not included (bottom panel), the system interacts chaotically for a while before the single compact object is ejected and the binary system leaves on slightly modified orbit. When tides are included (top panel), the chaotic interactions eventually result in the tidal inspiral and merger of the binary (labeled in the top diagram and shown in detail in the inset). [Samsing et al. 2017]

Stars living in dense environments, like globular clusters, experience very different lives than those in the solar neighborhood. In these extreme environments, close encounters are the norm — and this can lead to a variety of interesting interactions between the stars and systems of stars that encounter each other.

One common type of meeting is that of a single star with a binary star system. Studies of such interactions often treat all three bodies like point sources, examining outcomes like:

1. All three objects are mutually unbound by the interaction, resulting in three single objects.
2. A flyby encounter occurs, in which the binary survives the encounter but its orbit becomes modified by the third star.
3. An exchange occurs, in which the single star swaps spots with one of the binary stars and ejects it from the system.

Complexities of Extended Objects

But what if you treat the bodies not like point sources, but like extended objects with actual radii (as is true in real life)? Then there are additional complexities, such as collisions when the stars’ radii overlap, general relativistic effects when the stars pass very near one another, and tidal oscillations as gravitational forces stretch the stars out during a close passage and then release afterward.

In a recently published study led by Johan Samsing (an Einstein Fellow at Princeton University), the authors explore how these complexities change the behavior of binary-single interactions in the centers of dense star clusters.

One example — again in the case of a white-dwarf–compact-object binary interacting with a single compact object — of the cross sections for different types of interactions. Exchanges (triangles) are generally most common, and direct collisions (circles) occur frequently, but tidal inspirals (pluses) can occur with similar frequency in such systems. Inspirals due to energy loss to gravitational waves (crosses) can occur as well. [Samsing et al. 2017]

How Tides Change Things

Using numerical simulations with an N-body code, and following up with analytic arguments, Samsing and collaborators show that the biggest change when they include effects such as tides is a new outcome that sometimes results from the chaotic evolution of the triple interaction: tidal inspirals.

Tidal inspirals occur when a close passage creates tidal oscillations in a star, draining energy from the binary orbit. Under the right conditions, the loss of energy will lead to the stars’ inspiral, eventually resulting in a merger. This new channel for mergers — similar to mergers due to energy lost to gravitational waves — can occur even more frequently than collisions in some systems.

Samsing and collaborators demonstrate that tidal inspirals occur more commonly for widely separated binaries and small-radius objects. Highly eccentric white-dwarf–neutron-star mergers, for example, can be dominated by tidal inspirals.

The authors point out that this interesting population of eccentric compact binaries likely results in unique electromagnetic and gravitational-wave signatures — which suggests that further studies of these systems are important for better understanding what we can expect to observe when stars encounter each other in dense stellar systems.
Citation

Johan Samsing et al 2017 ApJ 846 36. doi:10.3847/1538-4357/aa7e32

Related Journal Articles
Further references at the full article 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

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From AAS NOVA: “Pulsar Jackpot in a Star Cluster”

American Astronomical Society

15 September 2017
Susanna Kohler

Markers showing the locations and directions of motion (with blue indicating acceleration toward us and red indicating acceleration away from us) of millisecond pulsars residing in the globular cluster Terzan 5. [B. Saxton (NRAO/AUI/NSF); GBO/AUI/NSF; NASA/ESA Hubble, F. Ferraro]

NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

GBO radio telescope, Green Bank, West Virginia, USA

NASA/ESA Hubble Telescope

Is Terzan 5, a star cluster that lies ~19,000 light-years away, a true globular cluster born in the Milky Way? Or are we seeing the remains of a dwarf galaxy that was captured by our galaxy? New observations by the Green Bank Telescope in West Virginia have tracked the radio signals of a treasure trove of millisecond pulsars — 36 of them — in the heart of Terzan 5. These signals can be used to trace the density distribution of the cluster, revealing where the matter resides. The observations, detailed in a recent article led by Brian Prager (University of Virginia, Charlottesville) and illustrated in the video below (credited to B. Saxton (NRAO/AUI/NSF); GBO/AUI/NSF; NASA/ESA Hubble), suggest that there is no supermassive black hole in the cluster center. This supports the idea that Terzan 5 is a true globular cluster.

Original article: Brian J. Prager et al 2017 ApJ 845 148. doi:10.3847/1538-4357/aa7ed7
Green Bank Observatory release: Pulsar Jackpot Reveals Globular Cluster’s Inner Structure

GBO radio telescope, Green Bank, West Virginia, USA

The Milky Way is chock-full of star clusters. Some contain just a few tens-to-hundreds of young stars. Others, known as globular clusters, are among the oldest objects in the Universe and contain up to a million ancient stars.

Some globular clusters are thought to be fragments of our galaxy, chiseled off when the Milky Way was in its infancy. Others may have started life as standalone dwarf galaxies before being captured by the Milky Way during its formative years.

Regardless of their origins, many globular clusters reside either in or behind the dusty regions of our galaxy. For ground- and space-based optical telescopes, however, this poses a challenge. Though it is possible to observe the cluster as a whole, the dust hinders astronomers’ efforts to study the motions of individual stars. If astronomers could track the motions of individual stars, they could see how “lumpy” the globular cluster is or if it contains something really dense, like a giant black hole at its center.

Fortunately, radio waves — like those emitted by pulsars — are unhindered by galactic dust. So rather than tracing the motions of the stars, astronomers should be able to map the motions of pulsars instead. But, of course, things are never that simple. Though globular clusters are brimming with stars, they contain far fewer pulsars.

“That’s what makes Terzan 5 such an important target of study; it has an unprecedented abundance of pulsars – a total of 37 detected so far, though only 36 were used in our study,” said Brian Prager, a Ph.D. candidate at the University of Virginia in Charlottesville and lead author on a paper appearing in the Astrophysical Journal. “The more pulsars you can observe, the more complete your dataset and the more details you can discern about the interior of the cluster.”

The Terzan 5 cluster is about 19,000 light-years from Earth, just outside the central bulge of our galaxy.

For their research, the astronomers used the National Science Foundation’s (NSF) Green Bank Telescope (GBT) in West Virginia. The GBT is an amazingly efficient instrument for pulsar detection and observation. It has exquisitely sensitive electronics, some specifically optimized for this task, and a 100-meter dish, the largest of any fully steerable radio telescope.

Pulsars are neutron stars – the fantastically dense remains of supernovas — that emit beams of radio waves from their magnetic poles. As a pulsar rotates, its beams of radio light sweep across space in a cosmic version of a lighthouse. If the beams shine in the direction of Earth, astronomers can detect the exquisitely steady pulses from the star.

As the pulsars in Terzan 5 move in relation to Earth – drawn in different directions by the varying density of the cluster — the Doppler effect comes into play. This effect adds a tiny delay to the timing if the pulsar is moving away from Earth. It also shaves off the tiniest fraction of a millisecond if the pulsar is moving toward us.

In the case of Terzan 5, astronomers are particularly interested in a class of pulsars known as millisecond pulsars. These pulsars rotate hundreds of times each second with a regularity that rivals the precision of atomic clocks on Earth.

Pulsars achieve these remarkable speeds by siphoning off matter from a nearby companion star. The infalling matter hits the edge of the neutron star at an angle, increasing the pulsar’s rate of spin in much the same way that a basketball balanced on the tip of a finger can be spun up by striking its side.

Millisecond pulsars are a particular boon to astronomers because they make it possible to detect almost infinitesimally small changes in the timing of the radio pulses.

“Pulsars are amazingly precise cosmic clocks,” said Scott Ransom, an astronomer with the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, and coauthor on the paper. “With the GBT, our team was able to essentially measure how each of these clocks is falling through space toward regions of higher mass. Once we have that information, we can translate it into a very precise map of the density of the cluster, showing us where the bulk of the ‘stuff’ in the cluster resides.”

Previously, astronomers thought that Terzan 5 might be either a warped dwarf galaxy gobbled up by the Milky Way or a fragment of the galactic bulge. If the cluster were a captured dwarf galaxy, it might also harbor a central supermassive black hole, which is one of the hallmarks of all large galaxies and can be found in many dwarf galaxies as well.

The new GBT data, however, show no obvious signs that a single, central black hole is lurking in Terzan 5. “However, we can’t yet say for sure if a smaller, intermediate mass black hole resides there. The new observations also provide better evidence that Terzan 5 is a true globular cluster born in the Milky Way rather than the remains of a dwarf galaxy,” said Ransom.

Future observations using more sophisticated acceleration models may better constrain the origin of Terzan 5.

Contact: Mike Holstine
mholstine@gbobservatory.org

Related Journal Articles, with links

Eight New Millisecond Pulsars in NGC 6440 and NGC 6441 doi: 10.1086/526338
GEMINI/GeMS Observations Unveil the Structure of the Heavily Obscured Globular Cluster Liller 1. doi: 10.1088/0004-637X/806/2/152
The Green Bank Northern Celestial Cap Pulsar Survey. I. Survey Description, Data Analysis, and Initial Results doi: 10.1088/0004-637X/791/1/67
The NANOGrav Nine-year Data Set: Mass and Geometric Measurements of Binary Millisecond Pulsars doi: 10.3847/0004-637X/832/2/167
The Second Fermi Large Area Telescope Catalog of Gamma-Ray Pulsars doi: 10.1088/0067-0049/208/2/17
PSR J1024–0719: A Millisecond Pulsar in an Unusual Long-period Orbit doi: 10.3847/0004-637X/826/1/86

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: “How a Black Widow Consumes Its Companion”

American Astronomical Society

1 September 2017
Susanna Kohler

Artist’s impression of the optical and X-ray emission surrounding the original “Black Widow” pulsar B1957+20. [NASA/CXC/M.Weiss]

Hanging out in a binary system with a hot millisecond pulsar can be hazardous to your health! A new study has examined how these perilous objects can heat and evaporate away their companions.

Panel (a) shows the intrabinary shock and the companion star (the pulsar would lie to the right). Panel (b) shows the companion star and the magnetic field lines funneling into its front pole. [Adapted from Sanchez & Romani 2017]

Predatory Stars

Millisecond pulsars — highly magnetized neutron stars that we detect through their beamed pulses of radiation — lose energy rapidly as they spin slower and slower. When such an object is locked in a binary system with a star or a planetary-mass object, the energy lost by the pulsar can blast its companion, causing it to evaporate.

Such systems, termed “black widows” in an acknowledgement of how the pulsar effectively consumes its partner, show optical emission revealing their strong heating. We hope that by studying these systems, we can learn more about the properties of the energetic winds emitted by pulsars, and by measuring the companion dynamics we can determine the masses of the pulsars and companions in these systems.

Different geometries for the companion’s magnetic field lines can alter the resulting light curve for the system. [Sanchez & Romani 2017]

How Heating Happens

Past models of black widows — necessary to interpret the observations — have generally assumed that the companion’s evaporation was due only to direct heating by the energetic gamma-ray photons emitted by the pulsar. This scenario, however, doesn’t successfully reproduce some of the quirks we’ve observed for these systems, such as very large temperatures and asymmetric light curves.

This picture also ignores the fact that much of the pulsar’s spin-down energy — the energy lost as it gradually spins slower and slower — is carried away by not just the gamma-ray photons, but also a magnetized wind of electrons and positrons. Two scientists at Stanford University, Nicolas Sanchez and Roger Romani, asked the following: how could the particles in the pulsar wind contribute to the heating of a black widow’s companion?

The authors’ heating model, as fit to PSR J1301+0833. [Sanchez & Romani 2017]

A Shock Assists

Sanchez and Romani’s alternative model relies on the fact that somewhere between the pulsar and its companion lies an intrabinary shock — the collision point between the pulsar’s relativistic wind and the companion’s ordinary, baryonic wind. The shock is anchored to the companion via magnetic fields, which provides an entry point for shock particles to be funneled along the magnetic field lines onto the companion’s surface. These energetic particles, in addition to the direct irradiation by the pulsar’s photons, cause the heating of the companion that results in its evaporation.

Sanchez and Romani show via simulations that this model can reproduce the observed light curves of several known black widow systems — including the strange features that the direct-heating model didn’t account for. They then use their model to make estimates for the masses of the pulsars and companions in these systems.

The authors caution that this model is still incomplete, but it illustrates that other sources of heating are important to consider in addition to heating by photons. Applying this and similar models to more black-widow systems will surely help us to better understand how these predatory compact stars cause their companions’ ultimate demise.

Citation

Nicolas Sanchez and Roger W. Romani 2017 ApJ 845 42. doi:10.3847/1538-4357/aa7a02

See the full article here .

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

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: “Collisions Around a Black Hole Mean Mealtime”

American Astronomical Society

4 August 2017 [I do not know how I missed this one.]
Susanna Kohler

Still from a simulation of stars orbiting the supermassive black hole at the center of a galaxy. Stars like these can sometimes be perturbed onto close circular orbits where they very slowly lose mass to the black hole as they spiral inward. [ESO/ S. Gillessen, R. Genzel]

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

Not a Disruption?

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

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

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

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

Inspirals and Outspirals

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

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

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

Results of a Collision

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

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

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

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

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

Citation

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

Related Journal Articles
See the full article for further references complete 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

From AAS NOVA: ” 2017 HEAD: Day 3″

American Astronomical Society

24 August 2017
Susanna Kohler

An artist’s impression of a known ultracompact binary consisting of a white dwarf and a pulsar. Systems like this were among the many discussed in Wednesday’s sessions at the HEAD meeting. [ESO/L. Calçada]

A Chandra X-ray view of our galactic center. [NASA/CXC/MIT/F.K. Baganoff et al.]

SGR A* NASA’s Chandra X-Ray Observatory

NASA/Chandra Telescope

Session: Black Holes Across the Mass Spectrum

Lia Corrales (University of Wisconsin-Madison) opened the first session on Wednesday by discussing Sgr A*, the supermassive black hole at the center of our own galaxy. Compared to the supermassive black holes we discussed on Tuesday, Sgr A* is extremely dim — but it does actively accrete matter, and its flux is therefore variable, exhibiting occasional flares. The problem? Studying this variability is tricky, because our sightline to the galactic center is subject to dust scattering, which can create apparent variability that’s not due Sgr A*. Corrales tackled this problem by making measurements of X-ray transients in the galactic center to map the dust that lies along the sightline to the region. She and her collaborators found that dust scattering within 15 arcseconds of the galactic center accounts for a variation of 6–12% the flux of the source, with variability on timescales of hours — which means this is something we definitely have to account for when studying Sgr A*.

Sjoert Van Velzen (Johns Hopkins University) and Stephen Cenko (NASA GSFC) both discussed aspects of tidal disruption events (TDEs). Van Velzen addressed the question of how to tell whether the events we’re detecting are really TDEs, or if they’re imposters — accretion disk instabilities, a new kind of supernova, or collisions of stars on bound orbits around a black hole. Van Velzen pointed out that at high black-hole mass (>108 solar masses), stars will be swallowed whole instead of torn apart (remember this if you plan to fall into a black hole: it’s better to choose a high-mass one!), so we expect to see a turnover in the distribution of TDEs at high black-hole masses. Though we have only 17 observations of optical/UV TDEs, Van Velzen showed that we do, indeed, see signs of this turnover — indicating that these events are TDEs rather than impostors.

Cenko explained the sleuthing we can do with the ultraviolet spectrum from TDEs: we can infer from the emission lines that TDE disks are much more radially compact and dense than typically seen in quasars. Consistent with what we learned in Tuesday’s talks, there’s evidence in the ultraviolet spectra for low-velocity, outflowing material. And we may even be able to use abundances measured in the UV spectra to learn about the type of star that was disrupted in the event.

Giorgio Matt (University Roma Tre) moved the discussion to smaller black holes, providing an overview of what we hope to learn about microquasars with the Imaging X-ray Polarimetry Explorer (IXPE), a new mission that will launch in early 2021.

NASA/IXPE spacecraft

IXPE will simultaneously provide imaging, spectroscopy, timing, and polarimetry measurements of sources — and the polarimetry is what will make this spacecraft unique. Matt walked us through three things that X-ray polarimetry may help us to learn about microquasars, stellar-mass black holes that are actively accreting mass from a companion star, exhibiting accretion disks and jets:

1.The geometry of the corona
Microquasars are expected to have coronae just like AGN, and we may be able to constrain their geometry based on the amount of polarization we detect from them.
2.The role of the jet
If jets are present and play an important role, then we expect to see much higher polarization levels.
3.The spin of the black hole
Measuring the polarization angle will provide a new way of identifying the black hole’s spin.

Session: Synergies with the Millihertz Gravitational Wave Universe

In this session, Jillian Bellovary (CUNY – Queensborough Community College) began by expanding on a topic first introduced on Monday: growing supermassive black holes in the early universe. Bellovary models this process using cosmological simulations, and she focuses on the direct-collapse method: low-metallicity, low-angular-momentum, massive clouds of gas collapse to form black holes, which then merge to grow. Bellovary and collaborators find that the massive black holes in low-mass, dwarf galaxies are often not directly in the center of the galaxy — they wander around, and ~50% end up off-nuclear. These have very low accretion rates (since they aren’t where the gas is); this will make them difficult to find, but observing them provides us with information about the original seed mass, since less than 10% of their mass is accreted gas. The mergers between massive black holes in low-mass galaxy environments will rarely be 1:1 mass ratios; it is much more common that the mass ratios are 1:10 or lower — which will influence the gravitational-wave signature we can expect to see from these. The upcoming LISA gravitational-wave mission will be critical for detecting these mergers from our early universe.

ESA/eLISA the future of gravitational wave research

The next talk was given by Thomas Maccarone (Texas Tech University) on the topic of ultracompact binaries: binaries consisting of two compact objects, including white dwarfs, neutron stars, or black holes. Our current knowledge of double compact objects is very limited, as we have very few detections of these systems thus far. LISA will provide a look at some of these ultracompact binaries — and LISA’s frequency band is lower than Advanced LIGO’s, meaning that the binaries can be detected at earlier evolutionary stages, when they are evolving slowly enough for electromagnetic follow-up. LISA’s observations of these systems will help us to do astronomy with gravitational waves, including exploring binary evolution, common envelopes, kicks, etc., and probing mass distributions in globular clusters and galaxy clusters.

Last up, Tamara Bogdanovic (Georgia Institute of Technology) walked us through the merger of two galaxies and the supermassive black holes at their centers — mergers that LISA will be able to detect when it launches. Galaxy mergers consist of four stages:

Stage 1: Galactic merger (separation: 100,000–1,000 pc, timescale: Gyrs)
Stage 2: Interactions with gas and stars (separation: 1,000–10 pc, timescale: Myr–Gyr)
Stage 3: Gravitationally bound binary (separation: 10–0.01 pc, timescale: Myrs–Gyrs)
Stage 4: Gravitational-wave phase and coalescence (separation: <0.01 pc, timescale: short!)

How do we explore the later-stage mergers? Bogdanovic reviewed the ways that we can observe sub-parsec supermassive black-hole binaries. Direct imaging is possible, though difficult; we’ve detected one candidate using this method. Searching for quasiperiodic variability in photometry is another option, and ~150 candidates have been found in this way. These systems also have a spectroscopic signature, and another ~dozen candidates have been found by searching for this. Lastly, future detections and non-detections of gravitational-wave emission from final merger stages may result in the discovery of additional systems and provide constraints on those detected by electromagnetic means.

Dissertation Prize Talk: Stellar Death by Black Hole: How Tidal Disruption Events Unveil the High Energy Universe

This year’s HEAD Dissertation Prize winner is Eric Coughlin, who did his PhD at University of Colorado Boulder and is now an Einstein Fellow at UC Berkeley. Coughlin spoke on his theoretical work studying tidal disruption events (TDEs). He noted that in TDEs, immediately after a star is torn apart and begins to accrete onto the supermassive black hole, an initial intense phase of hyperaccretion occurs. Can accretion disks even hold themselves together under this extreme release of energy?

Coughlin and his PhD advisor, Mitch Begelman, came up with a model for how these disks survive: the disks puff up into giant spherical shapes, and then any excess accretion energy is funneled from the disk into bipolar jets. They termed the puffed-up disks “zero-Bernoulli accretion flows” — ZEBRAs for short. Coughlin showed how various simulations have backed up this model, demonstrating the formation of these puffed up, spherical disks as material falls back on a supermassive black hole after the tidal disruption of a star.

Coughlin concluded his talk by presenting simulations from his more recent work, in which he explores what TDEs look like when the star is disrupted not by an isolated black hole, but by a supermassive black hole binary. Initially, the star only experiences the effects of the black hole that is disrupting it, but within short order the tidal stream of material encounters the second black hole and forms a spectacular mess of debris in beautiful patterns. This can lead to re-brightenings in the observed light curve, creating a distinctive signature that should allow us to differentiate these events from disruption events from isolated black holes. You can check out his stunning simulations yourself in the video below (you may need to give it a minute to load, but it’s worth it to watch through the end), and you can visit his website for more movies of his work.

Session: Missions & Instruments

Wednesday afternoon’s first session provided useful introductions to a number of missions, instruments, and data analysis tools for high-energy astrophysics. These included:

Chandra Interactive Analysis of Observations (CIAO)
Antonella Fruscione (Smithsonian Astrophysical Observatory) discussed this modern data-analysis system for examining images produced by the Chandra X-ray Observatory.

360° movies of X-ray data in the galactic center
Christopher Russell (NASA GSFC) shared his unusual 360° movies of our galactic center’s inner parsec, created from hydrodynamic simulations that model X-ray emission from hot stars in the center of the galaxy. You can check it out yourself by visiting this link on your computer, or by searching for “Christopher Russell astronomy” in the youtube app on your phone (recommended for the full 360° experience!).

Compton Spectrometer and Imager (COSI)

The Compton Spectrometer and Imager (COSI) payload just prior to launch from Wanaka
Clio Sleator (SSL, UC Berkeley) introduced this balloon-borne soft gamma-ray detector, which floated for 46 days in 2016. Data from this run included observations of the Crab pulsar and the gamma-ray burst GRB 160530A.

LISA Pathfinder

ESA/LISA Pathfinder

James Thorpe (NASA GSFC) provided us with an overview of the pathfinding mission for the Laser Interferometer Space Antenna (LISA). LISA Pathfinder was intended to test some of the technologies required for LISA, which will rely on incredibly high-precision engineering. Initial data from this pathfinder mission have shown that it’s exceeded the requirements for LISA, which is extremely promising for the future mission!

Arcus

NASA/ARCUS satellite

Randall Smith (Smithsonian Astrophysical Observatory) gave us a cheerful overview of Arcus, a free-flying X-ray satellite that was recently selected by NASA for a concept study as a Medium-Class Explorer mission. If ARCUS makes it to the final phase of the proposal process and launches (as early as 2022), it will significantly improve on views from current missions like Chandra, providing us with new information on the formation and evolution of clusters of galaxies, black holes, and stars.

All-sky Medium Energy Gamma-ray Observatory (AMEGO)

Sara Buson (NASA GSFC) presented on the mission AMEGO, soon to be proposed to the 2020 Decadal Review, which would provide an all-sky survey of emission in the MeV energy band. AMEGO will provide at least a 20x improvement on sensitivity relative to its predecessor, COMPTEL, allowing us to better explore sources like MeV blazars. We hope to use AMEGO to shed light on supermassive black-hole growth, the accretion–jet connection, the MeV background, and much more.

Session: SNR/GRB/Gravitational Waves

This session was termed by the first speaker, Jeremy Schnittman (NASA GSFC), as “the miscellaneous session.” The first two talks focused on intriguing aspects of gravitational-wave astronomy. Schnittman presented his work modeling the complex radiation physics in the time-dependent spacetimes of a binary compact-object system. What would we expect to see when two black holes accreting gas are locked in a close binary? Schnittman modeled this with a radiation transport calculation of the gas accretion onto the merging binary black holes, and then used ray-tracing of photons to determine what a distant observer would see.

Cecilia Chirenti (UFABC) discussed the gravitational waves that are emitted from highly eccentric neutron-star binaries — not from the binary as a whole, but from the oscillation modes that are induced in the individual neutron stars as a result of their close passages. She demonstrated that the proposed Einstein Telescope should be able to detect up to tens of these events per year, and we may be able to use these detections to help constrain the neutron-star equation of state.

ASPERA Einstein Telescope

The next two speakers discussed various aspects of modeling the aftermath of supernovae. Tea Temim (Space Telescope Science Institute) presented her work simulating the interaction of a pulsar wind nebulae (which is generated by the pulsar embedded deep within a supernova remnant) with the supernova reverse shock. By matching her simulations with observations, she hopes to obtain information about the ambient medium, the supernova ejecta, the pulsar properties, and the energetic particle population injected into the interstellar medium.

Brian Williams (Space Telescope Science Institute) then discussed his three-dimensional modeling of the ejecta from Tycho’s supernova remnant, which I’ve written about in a previous AAS Nova post.

Closing out the session, Colleen Wilson-Hodge (NASA MSFC) gave an overview of the time-domain astronomy done with the Fermi Gamma-ray Burst Monitor (GBM). GBM’s large field of view — and the fact that it scans the sky once every ~90 minutes — has allowed it to detect a number of gamma-ray bursts, as well as to regularly monitor pulsars and galactic transients. In the current era of multi-messenger astronomy, GBM has also been used to follow up on gravitational-wave triggers from LIGO and neutrino detections from IceCube. We can only hope that it will prove successful in similar follow-up campaigns in the future!

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: ” 2017 HEAD: Day 2″

American Astronomical Society

23 August 2017
Susanna Kohler

The gamma-ray excess at the heart of M31. [NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF]

Session: AGN 1

Ehud Bahar (Technion) opened the meeting’s first session on active galactic nuclei (AGN) by discussing eclipses of a different kind than the one we observed on Monday. Light from AGN is often obstructed on its path to us by warm, outflowing, intervening material that absorbs some of the AGN’s light. Bahar explained the difference between what he termed “absorbers” and “obscurers”: absorbers are slow and steady outflows from the AGN that change very little over long timescales. These provide us with the opportunity to probe their detailed physics. Obscurers, on the other hand, are fast-moving and transient outflows, briefly causing dramatic drops in the X-ray flux of the AGN.

Artist’s impression of the tidal disruption event ASASSN-14li, in which a supermassive black hole destroyed a star, launching outflows. [NASA GSFC]

Two speakers in the session discussed the idea of particularly fast outflows from AGN: Michael Nowak (MIT Kavli Institute) presented data on ultrafast outflows moving at 5–20% of the speed of light from the AGN PG 1211+143 (that’s 15,000–60,000 km/s, as compared to more typical outflow speeds of 100–1,000 km/s), and Erin Kara (University of Maryland) discussed what we can learn from ultrafast outflows from tidal disruption events. Kara’s talk demonstrated how we can use our observations of a well-studied tidal disruption event, ASASSN-14li, to learn about how an accretion disk around a black hole can transition from a super-Eddington (especially high) accretion phase that launches winds to a sub-Eddington (lower) accretion phase in which the wind is shut off.

Andrew Fabian (University of Cambridge) wrapped up the session by providing an overview of what we know about AGN coronae — the incredibly luminous, compact regions that lie directly above the accretion disks of supermassive black holes. Coronae are the source of the majority of the hard X-ray emission from AGN, and we have used observations of this emission to constrain the size of AGN coronae to a mere 10 gravitational radii. We’ve learned that coronae are extremely hot, at 30–300 keV, and are highly magnetized and dynamic, likely containing outflowing plasma.

Session: The Very High Energy Universe as Viewed with VERITAS and HAWC

HAWC High Altitude Cherenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters, at WikiMiniAtlas 18°59′41″N 97°18′30.6″W.

CfA/VERITAS, a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV – TeV energy range. Located at FLWO in AZ, USA

The session on very high energy observations opened with a talk by Brenda Dingus (LANL). Dingus introduced us to the High Altitude Water Cherenkov (HAWC) gamma-ray observatory, a new observatory located in Mexico that maps the northern sky in high-energy gamma rays. HAWC has a wide field of view, observing roughly 2/3 of the sky each day with long integration times. This means that the observatory is sensitive to the highest energy gamma rays. HAWC has recently released its very first catalog, 2HWC, and this is only the beginning — there is much more science expected from this observatory in the future!

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is another high-energy observatory, located in southern Arizona; Philip Kaaret (University of Iowa) provided us with an overview of this set of telescopes. VERITAS has a narrower field of view than HAWC, but its sensitivity and angular resolution are higher, allowing it to probe sources at a deeper level. It’s therefore often used for follow-up observations of known targets.

So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

NASA/Fermi Telescope

MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

Mid-Career Prize Talk: X-ray Winds from Black Holes

Tuesday afternoon kicked off with the HEAD Mid-Career Prize Talk, given this year by Jon Miller (University of Michigan). Miller spoke in further depth about a topic introduced earlier in the day: winds emitted from black hole disks. He argued that these winds are worth studying because they provide information about how mass is accreted onto black holes, and therefore how the black holes grow and their spins evolve.

The dense and ionized winds from black-hole disks can potentially carry away more mass than is accreted — and this appears to hold true across the mass scale, from X-ray binaries containing stellar-mass black holes to Seyfert galaxies containing supermassive black holes. Miller discussed the different mechanisms that may launch these winds, and how observations indicate that magnetic driving is important, although other forces may also be at work.

ESA/Athena spacecraft

Miller argued that many tests of disk physics are now within reach of data and simulations, such as measurements of disk magnetic fields. He also showed how extreme settings such as tidal disruption events can provide a unique and interesting regime in which to explore disks and winds, as the mass accretion rate in these events changes drastically on observable timescales.

As a final point, Miller discussed how our understanding of black hole disk winds will change with upcoming observatories. Missions like Xarm, ARCUS, ATHENA, Lynx [no image available], etc. will be transformative; ATHENA, for instance, will be able to produce observations outstripping the sensitivity and resolution of any observations obtained so far with current instrumentation, in “less than the time it took you to have lunch today,” Miller explained.

Xarm satelite

NASA/ARCUS

Session: ISM & Galaxies

Xian Hou (Yunnan Observatories) opened the session on the interstellar medium (ISM) and galaxies by discussing our view of M31 (the Andromeda galaxy) with the Fermi Large Area Telescope.

NASA/Fermi LAT

Andromeda Galaxy Adam Evans

M31 is the only other large spiral local galaxy — and it’s nearby, providing an excellent opportunity for resolved analysis of high-energy emission from a large, star-forming, spiral galaxy similar to the Milky Way. The >1 GeV emission tracked by Fermi LAT was found to be concentrated only in the inner region of the galaxy; it is not correlated with interstellar gas or star-formation sites. What could be this emission’s source, then? Hou suggests that possibilities include a population of millisecond pulsars in the galactic center, or annihilation/decay of dark matter.

NuSTAR observations of M31. The bright blue point in the inset is the intermediate-mass pulsar candidate. [NASA/JPL-Caltech]

NASA NuSTAR X-ray telescope

Later in the session, Ann Hornschemeier (NASA GSFC) provided a complementary discussion of observations of M31 — this time in the form of NuSTAR’s deep survey of of our nearest galactic neighbor. Hornschemeier reminded us that before NuSTAR, we were unable to spatially resolve hard X-ray sources (energies over 10 keV) in other galaxies. Now, with NuSTAR, we can resolve point sources — and their hard X-ray color can help us to identify whether they are black hole X-ray binaries, neutron-star X-ray binaries, pulsars, etc. A number of neutron stars were identified in globular clusters in M31, as well as a particularly high energy source that is likely an intermediate-mass X-ray pulsar.

The work done by Francesca Fornasini (Harvard-Smithsonian CfA) and collaborators explores how low-luminosity AGN activity and star formation in its host galaxy are connected. Is there a correlation between these two types of activity? If there’s a positive correlation, we can infer that AGN feedback suppresses star formation; if there is a negative correlation, both types of activity may be fueled by a common mechanism. On the other hand, there may be no correlation at all! Because AGN are variable, and because the relation between AGN activity and star formation rate can vary with other host galaxy properties like stellar mass and redshift, we need a very large sample that covers the whole phase space to test for correlation. Fornasini and collaborators achieve this by building X-ray stacks from data from 123,000 galaxies in the Chandra COSMOS Legacy Survey. Their work is still underway, but thus far it has revealed no correlation between the black-hole accretion rate and the star formation rate of the host galaxies.

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: “Can Radio Telescopes Find Axions?”

American Astronomical Society

16 August 2017
Susanna Kohler

A simulation showing the distribution of dark matter in the universe. [AMNH]

Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

In the search for dark matter, the most commonly accepted candidates are invisible, massive particles commonly referred to as WIMPs. But as time passes and we still haven’t detected WIMPs, alternative scenarios are becoming more and more appealing. Prime among these is the idea of axions.

The Italian PVLAS is an example of a laboratory experiment that attempted to confirm the existence of axions. [PVLAS]

A Bizarre Particle

Axions are a type of particle first proposed in the late 1970s. These theorized particles arose from a new symmetry introduced to solve ongoing problems with the standard model for particle physics, and they were initially predicted to have more than a keV in mass. For this reason, their existence was expected to be quickly confirmed by particle-detector experiments — yet no detections were made.

Today, after many unsuccessful searches, experiments and theory tell us that if axions exist, their masses must lie between 10-6–10-3 eV. This is minuscule — an electron’s mass is around 500,000 eV, and even neutrinos are on the scale of a tenth of an eV!

But enough of anything, even something very low-mass, can weigh a lot. If they are real, then axions were likely created in abundance during the Big Bang — and unlike heavier particles, they can’t decay into anything lighter, so we would expect them all to still be around today. Our universe could therefore be filled with invisible axions, potentially providing an explanation for dark matter in the form of many, many tiny particles.

Artist’s impression of the central core of proposed Square Kilometer Array antennas. [SKA/Swinburne Astronomy Productions]

How Do We Find Them?

Axions barely interact with ordinary matter and they have no electric charge. One of the few ways we can detect them is with magnetic fields: magnetic fields can change axions to and from photons.

While many studies have focused on attempting to detect axions in laboratory experiments, astronomy provides an alternative: we can search for cosmological axions. Now scientists Katharine Kelley and Peter Quinn at ICRAR, University of Western Australia, have explored how we might use next-generation radio telescopes to search for photons that were created by axions interacting with the magnetic fields of our galaxy.

Potential axion coupling strengths vs. mass (click for a closer look). The axion mass is thought to lie between a µeV and a meV; two theoretical models are shown with dashed lines. The plot shows the sensitivity of the upcoming SKA and its precursors, ASKAP and MEERKAT. [Kelley&Quinn 2017]

SKA/ASKAP radio telescope at the Murchison Radio-astronomy Observatory (MRO) in Mid West region of Western Australia

SKA Meerkat telescope, 90 km outside the small Northern Cape town of Carnarvon, SA

Hope for Next-Gen Telescopes

By using a simple galactic halo model and reasonable assumptions for the central galactic magnetic field — even taking into account the time dependence of the field — Kelley and Quinn estimate the radio-frequency power density that we would observe at Earth from axions being converted to photons within the Milky Way’s magnetic field.

The authors then compare this signature to the detection capabilities of upcoming radio telescope arrays. They show that the upcoming Square Kilometer Array and its precursors should have the capability to detect signs of axions across large parts of parameter space.

Kelley and Quinn conclude that there’s good cause for optimism about future radio telescopes’ ability to detect axions. And if we did succeed in making a detection, it would be a triumph for both particle physics and astrophysics, finally providing an explanation for the universe’s dark matter.

Citation

Katharine Kelley and P. J. Quinn 2017 ApJL 845 L4. doi:10.3847/2041-8213/aa808d

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

From AAS NOVA: “How Do Earth-Sized, Short-Period Planets Form?”

American Astronomical Society

7 August 2017
Susanna Kohler

Artist’s impression of COROT-7b, an ultra-short-period planet. [ESO/L. Calçada].

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

Former Hot Jupiters?

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

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

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

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

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

Testing Metallicities

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

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

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

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

They then compare the metallicity distributions of these three groups.

Back to the Drawing Board

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

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

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

NASA/TESS

Citation

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

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

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

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

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

Related Journal Articles
Further references complete with links on 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