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27 July 2018

Red Clump Stars and the History of the Galactic Bulge

What’s going on in the galactic bulge? The discovery of the double red clump — two groupings of stars seen in the color-magnitude diagram of the galactic bulge — has raised questions about the structure and formation history of the stars surrounding the center of our galaxy.

A color-magnitude diagram of the galactic bulge from the Two Micron All Sky Survey. In panel b, the red, blue, and black symbols indicate the bright red clump, faint red clump, and background red giant branch stars, respectively. The double red clump is more clearly visible in the luminosity function in panel c. [Lee et al. 2018]

Two Theories for the Double Red Clump

The double red clump, which is separated into bright and faint components, has inspired various theories about its origin:

X-shaped structure scenario: The presence of the double red clump was initially thought to indicate that the Milky Way bulge contains an X-shaped structure similar to that seen in other galaxies. The difference in apparent magnitude between the two components of the double red clump is simply due to the different distances to two of the arms of the structure as seen by an observer on Earth, with the arm reaching toward Earth forming the bright red clump and the arm stretching away from Earth forming the faint red clump.

Multiple-population scenario: Alternatively, the double red clump could be composed of multiple generations of stars that formed in globular clusters, where later generations of stars would be enhanced in helium, nitrogen, and sodium. Stellar evolution models predict that the first-generation stars would be fainter than subsequent generations, so the magnitude difference between the bright and faint components would be due to intrinsic luminosity differences between the two populations.

An illustration of the expected magnitude versus CN strength plots for the competing scenarios. Click to enlarge. [Lee et al. 2018]

A Closer Look at Red Clump Stars

How can we distinguish between these two scenarios? Young-Wook Lee (Yonsei University, South Korea) and collaborators posited that these theories can be separated by taking a closer look at the spectra of the double red clump stars.

Using low-resolution spectroscopy from a 2.5-meter telescope at Las Campanas Observatory, Lee and collaborators set out to measure the CN band strength of nearly 500 stars in the double red clump. Why CN? The CN band strength correlates with the nitrogen abundance, which is expected to be enhanced in later generations of stars.

If the two components of the double red clump represent separate generations of stars, the brighter stars will show the expected nitrogen enhancement. If, on the other hand, the double red clump arises from an X-shaped structure in the galactic bulge, there will be no difference in CN band strength between the bright and faint components.

CN indices for bright red clump (red), faint red clump (blue), and red giant branch (black) stars. Click to enlarge. [Lee et al. 2018]

From Globular Clusters to Galactic Bulges?

By comparing the spectra of the bright and faint red clump stars, Lee and collaborators found that the bright population had stronger CN absorption than the faint population at the 5.3σ level.

While precise parallax distances from Gaia and high-resolution spectroscopy are still needed to completely disentangle the stellar populations near the heart of our galaxy, this result supports the theory that the double red clump contains multiple generations of stars. The abundance patterns inferred from the stars’ spectra imply that the double red clump stars may have formed in young globular clusters, which could have profound implications for how galactic bulges are assembled — in both our home galaxy and afar.

Citation

Young-Wook Lee et al 2018 ApJL 862 L8. doi:10.3847/2041-8213/aad192

25 July 2018

Shocks in the Solar Atmosphere

Despite the Sun’s proximity, there’s still a lot we don’t know about our nearest star! A new study, however, has brought us a step closer to understanding one of its mysteries: how the solar atmosphere is heated.

The temperatures of different layers of the Sun. The Sun’s atmosphere gets progressively warmer with increasing distance from the core, from the photosphere at 6,000 K, to the chromosphere at 10,000 K, to the corona at 1,000,000 K. [ISAS/JAXA]

Blame Shocks for Heating?

The outer layers of the Sun present an interesting conundrum: instead of continuing to drop in temperature with increasing distance from the Sun’s hydrogen-fusing core — the way the Sun’s inner layers do — the atmosphere above the Sun’s surface becomes progressively hotter with distance. How is heat getting delivered from the Sun’s center to these outer layers?

One possible culprit is shocks. Oscillations continuously travel through the Sun’s interior to its surface, launching compressible waves into its atmosphere. These waves can then evolve into shocks as they propagate outward — and the dissipation of these shocks can deposit energy into the Sun’s atmosphere, contributing to its heating.

Besides their potential role in atmospheric heating, these shocks serve a number of other purposes that should encourage us to study them. They are thought to serve as the trigger for observed phenomena, such as the tiny chromospheric jets driven in active regions. Furthermore, they can be used to probe the solar atmosphere — measuring shock properties can tell us more about the structure of the environment through which they propagate.

Theory and Observation Combined

Observational data used by the authors to derive the properties of two successive shock waves observed by IRIS. a) IRIS image, spanning 47” x 47”. The vertical line indicates the location of the spectrograph slit. b) the Doppler velocity of Si IV 1393.76 Å in the region marked by the horizontal white lines in panel (a). c) A wavelength-time plot of Si IV 1393.76 Å. [Adapted from Ruan et al. 2018]

Until now, however, we haven’t observationally measured the key parameters of shocks in the solar atmosphere — like the temperature, density, and speeds of the plasma upstream and downstream of the shock waves.

To address this challenge, a team of scientists at Peking University, the Chinese Academy of Sciences, and KU Leuven in Belgium have now developed a means of quantitatively analyzing the shocks we observe in the Sun’s atmosphere. In a recently published study led by Wenzhi Ruan, the team presents analysis that determine shock properties by using the theoretical physics of shocks in combination with quantities derived from simultaneous imaging and spectroscopic observations.

Testing the Approach

To test their analysis, the team takes two approaches. First, they apply the analysis to real shock observations — imaging and spectroscopy of two successive shock waves captured by the Interface Region Imaging Spectrograph (IRIS) observatory — and demonstrate that the shock properties they find are physically reasonable and consistent with each other.

Next, the authors model shocks using numerical simulations, and they generate synthetic observations of how these shocks would appear to an observatory like IRIS from different viewing angles. They then apply their analysis to the synthetic observations and demonstrate that they can recover the original properties of the shocks in all cases, indicating that their method is reliable and independent of viewing direction.

The authors encourage other researchers to use their analysis code (you can find it here) as a standard approach for quantitatively exploring shocks in the solar atmosphere. This work is a crucial step toward better understanding how these shocks deposit heat, and how they interact with and influence the atmosphere of the Sun.

Citation

Wenzhi Ruan et al 2018 ApJ 860 99. doi:10.3847/1538-4357/aac0f8

20 July 2018

Searching for Exoplanets Around X-Ray Binaries

Finding planets around ordinary stars is great, but what if we could also hunt for planets around the black holes, neutron stars, and white dwarfs that live in binaries with companion stars? A new study shows it’s possible!

A New Place for Planets?

Artist’s impression of an X-ray binary, in which a compact remnant accretes material from a companion star. [NASA]

X-ray binaries are gravitationally interacting binary star systems in which a compact object — a white dwarf, neutron star, or black hole — accretes material from its companion star. In these systems, we can often detect the stars in optical or radio light, but the system additionally shines in X-rays as a result of radiation from the very hot, accreting gas.

Planets orbiting within binaries may be common — we’ve spotted around 70 examples so far of planets orbiting one member of a binary, and another dozen or so in which a planet orbits both members on a circumbinary path. It stands to reason, then, that some planets should survive the evolution of one of the binary stars into a compact remnant, eventually becoming a planet orbiting an X-ray binary.

Several example transit X-ray light curves for circumbinary planets, including an Earth-mass planet (solid lines) and a Jupiter-mass planet (dashed lines), for different values of μ, which relates the mass of the compact remnant to the companion star. Here the companion mass remains fixed and the different panels show different values for the remnant mass. [Imara & Dr Stefano 2018]

Looking at X-Rays

So how would we detect such a planet? Optical and radio searches for planets can be challenging, since a planet transit often means a very small dip in a light curve that might not be detectable. Two scientists from the Harvard-Smithsonian Center for Astrophysics, Nia Imara and Rosanne Di Stefano, propose an alternative: why not specifically look in the X-rays?

Because the area emitting X-rays is very compact — all smaller than the size of a white dwarf’s surface, i.e., the size of the Earth — the expected dip in the X-ray light curve due to a planet transit is quite large. This increases our chances of being able to detect it.

Exploring Challenges

A few challenges exist with this approach. To increase detection odds, the planet would ideally need to be orbiting within a similar plane to that of the binary, and preferably close to the inner cutoff for stable orbits around the binary. This is not an unreasonable assumption, however, given expected orbital dynamics as star systems evolve.

Transit probability versus binary mass ratio, μ, for planet circumbinary orbits around coplanar x-ray binaries with white dwarfs (solid lines), neutron stars (dashed lines), or black holes (dotted lines) as the primaries. The black and magenta lines represent the probability calculations for an Earth-like and Jupiter-like planet, respectively. [Imara & Di Stefano 2018]

Another potential difficulty is that X-ray photons are scarce! We’d have to observe systems for an extended time in order to gather enough light in X-rays to definitively detect light-curve dips. Imara and Di Stefano show, however, that we’ve observed a number of X-ray binaries over several hundred thousand seconds — long enough time periods that the dips would be detectable.

A Positive Outlook

With those challenges in mind, Imara and Di Stefano demonstrate through a series of calculations that circumbinary planets are reasonably likely to transit — transit probabilities range from roughly 0.1%–40%, depending on the mass ratio of the binary and the size of the X-ray-emitting region — and that our detection capabilities are such that we could actually spot these transits with present-day technology.

Future X-ray missions, like the proposed Lynx X-ray space telescope — which may have 50 times the sensitivity of the Chandra X-ray telescope! — will dramatically extend the opportunities for transit detection. Indeed, it seems like the hunt for these exotic exoplanetary systems have very good prospects.

Citation

Nia Imara and Rosanne Di Stefano 2018 ApJ 859 40. doi:10.3847/1538-4357/aab903s

18 July 2018

Shutting Down Star Formation

At some point in a galaxy’s life, it transitions from a star-forming factory into an old, red, inactive relic. Can new observations of a recently transitioned galaxy help us understand what drives that change?

Causes of Quenching

Spiral starburst galaxies (top) may precede red, inactive ellipticals (bottom) evolutionarily. [Adapted from Hubble/Galaxy Zoo]

Across the nearby universe, we see two main types of galaxies: blue, active spirals, and red, quiescent ellipticals. It’s generally believed that these represent two evolutionary states: galaxies first undergo starburst periods in which many young, hot, blue stars are born. Later in their lifetimes, these galaxies then settle into red, inactive states.

But what shuts down the star formation, triggering the transition between these two states? There are a number of possible quenching explanations related to galaxy mergers:

Gas heating
The collision of two galaxies could heat up the gas supply via shocks, preventing it from gravitationally collapsing to form stars.
Compaction
Mergers may lead to compaction, in which the star-forming gas migrates inward and triggers a central starburst. The remainder of the galaxy is depleted of gas and stops forming stars.
Outflows
Mergers can trigger high-velocity outflows driven by radiation from new stars or from a central, feeding black hole. The outflows remove gas from the galaxy, shutting down star formation.
Morphological quenching
A galaxy can stabilize against star formation if the structure of the galaxy changes — say, after a merger — in specific ways, such as if the galactic disk transforms into a spheroid.

A Transitional Galaxy

These different proposed quenching mechanisms should leave distinctive signatures in the stellar populations of recently quenched galaxies. With this in mind, a team of scientists explored SDSS J0912+1523, an intermediate-redshift galaxy that shows signs of having only recently transitioned from a starburst galaxy to a quiescent one.

In a study led by Qiana Hunt, a postbaccalaureate researcher at Princeton University, the scientists combined preexisting ALMA observations of the gas within SDSS J0912+1523 with new Gemini observations of its stellar population. This combination provided a rare opportunity to gain insight into what might shut down a galaxy’s star formation.

Evidence for a Merger

The authors find that SDSS J0912+1523 shows signs of containing two separate cores of stars that now rotate together — which may be evidence of a past merger.

The flux density of SDSS J0912+1523 seems to show two separate cores, which may be the sign of a past merger. [Hunt et al. 2018]

In spite of this indication, none of the quenching scenarios above predict all of the characteristics Hunt and collaborators observed in SDSS J0912+1523. There’s lots of cold molecular gas still present in the galaxy, suggesting that neither depletion nor heating of the gas led to its quenching. Minor, gas-rich mergers or morphological quenching may be candidates still, but the kinematics of the gas and stars suggest that the gas didn’t come from an external origin. And there’s no evidence for strong outflows from SDSS J0912+1523.

For now, it looks like the mechanism that shut down star formation in this galaxy remains a mystery. But Hunt and collaborators are optimistic: a number of follow-up observations could shed more light on the problem — like radio hunts for central black-hole activity or high-resolution Hubble images of the galaxy’s morphology. Once these are conducted, we may soon understand what mechanism turns off star formation in an aging galaxy.

Citation

Qiana Hunt et al 2018 ApJL 860 L18. doi:10.3847/2041-8213/aaca9a

16 July 2018

A Rapidly Spinning Black Hole with a Warped Disk

An X-ray telescope recently installed on the International Space Station has been improving our view of distant high-energy sources, one object at a time. Now, this telescope has provided a detailed look at a black hole feeding off its companion star.

How to Spot Black Holes

Stellar-mass black holes lurk throughout our galaxy — but their darkness makes them understandably difficult to spot. Because these small beasts don’t emit light, we have few observations of the stellar-mass black holes around us, limiting what we can learn about these mysterious objects and their behavior.

An artist’s conception of the NICER telescope installed on the International Space Station. [NASA]

One way that we can observe such black holes is if they exist in an X-ray binary, a binary consisting of a black hole and a donor star. In X-ray binaries, material that is siphoned off the donor star goes to feed the black hole, forming an accretion disk around the black hole as it falls in. As the material in the accretion disk spirals inwards, it radiates strongly in X-rays — resulting in emission that we can observe, even though the black hole itself emits no radiation.

But what is the structure of this accretion disk? How far in toward the black hole does it extend? How fast does the black hole spin at its center? These are among the many questions to which scientists are still seeking answers. In a new study, astronomer Jon Miller (University of Michigan) and collaborators present new views of an X-ray binary that may provide some clues.

Spectrum of MAXI J1535−571, fit with a simple disk blackbody plus power-law model. The additional features visible in the data/model ratio are likely due to reflection from the disk. [Miller et al. 2018]

Eyes on a New X-Ray Source

Miller and collaborators present new observations of the X-ray binary MAXI J1535-571 made with the Neutron Star Interior Composition Explorer (NICER), an X-ray telescope recently installed on board the International Space Station.

NICER’s observations indicate that the black hole in this binary is likely spinning very rapidly — at more than 99% of the maximum possible speed! The light we observe from MAXI J1535-571 appears to include contributions both from the black hole’s accretion disk and from the corona of very hot gas that lies above the disk. Light from the corona reflects off of the accretion disk, providing us with more information about the structure of the disk.

By modeling the observations, Miller and collaborators show that the disk extends nearly all the way inwards to what’s known as the innermost stable circular orbit, the closest stable orbit you can have to a black hole.

Reflections of a Warp

Artist’s impression of a example warped disk of gas and dust surrounding a black hole. [James Gitlin (Space Telescope Science Institute)]

Lastly, the authors point out an additional feature in MAXI J1535-571’s spectrum: a narrow emission line that they suggest might be caused by a warp in the disk, such that the disk no longer lies flat. The warp would locally change the profile of the accretion disk, causing more light from the corona to be reflected to us from that point.

The presence of this potential warp, the extent of the disk, and the spin of the black hole are all pieces of the puzzle that will help us better understand the behavior of stellar-mass black holes feeding off of companions. And we can look forward to the high sensitivity of NICER — which made these observations possible — continuing to produce exciting results in the future!

Citation

J. M. Miller et al 2018 ApJL 860 L28. doi:10.3847/2041-8213/aacc61

13 July 2018

When Stars Run Away

The high-energy catalogs of the Fermi Large Area Telescope contain more than a thousand gamma-ray detections that have never been connected to a source. Some of these gamma rays could stem from very exotic objects: bow shocks of runaway stars.

An artist’s rendering of a blazar. These ultra-luminous objects — associated with the infall of material into a galaxy’s central black hole — are a common source of extragalactic gamma rays. [NASA/Goddard SFC Conceptual Image Lab]

A Shocking Way to Generate Gamma Rays

Runaway stars get their name by careening through interstellar space after being ejected from binary or multiple systems, usually because of gravitational interactions with other stars or a kick from a nearby supernova. As runaway stars plow into the surrounding interstellar medium, a bow shock can form in front of the star. It’s these stellar bow shocks that may be the sources of galactic gamma rays.

Stellar bow shocks generate gamma rays by first accelerating electrons to relativistic speeds. When a relativistic electron collides with a low-energy photon, it transfers energy to the photon, upgrading it to a gamma ray. This process, known as inverse Compton scattering, is the reverse of Compton scattering, through which electrons colliding with high-energy photons are accelerated to relativistic speeds.

Although stellar bow shocks are theoretically capable of producing gamma rays, these ultra-high-energy photons have never been definitively detected. Could the gamma rays from stellar bow shocks be hidden among the unidentified Fermi sources?

Comparison of modeled spectral energy distribution to observations for runaway star Lambda Cephei. Click to enlarge. [Sánchez-Ayaso et al. 2018]

Searching for Missing Gamma-Ray Sources

Estrella Sánchez-Ayaso (Universidad de Jaén, Spain) and collaborators embarked on a search for gamma-ray-emitting stellar bow shocks. They began by comparing the positions of unidentified Fermi sources to those of known bright stars in our galaxy and visually inspecting the matches for signs of a bow shock, which revealed two runaway star candidates.

Sánchez-Ayaso and collaborators then used the known properties of the two stars to determine whether or not it’s feasible for their bow shocks to be the source of the emission seen by Fermi. The authors modeled the gamma-ray emission generated by inverse Compton scattering of infrared photons off of electrons accelerated by the bow shocks.

By matching their model output to the Fermi observations, the authors determined that the physical conditions necessary for the two runaway stars to produce the observed gamma rays were reasonable. This suggests a promising link between the two stellar bow shocks — one of which was discovered as a result of this work — and two of the unidentified Fermi sources.

Fermi error ellipse overlaid on a composite radio, infrared, and optical image of runaway star LS 2355. [Sánchez-Ayaso et al. 2018]

Two Down, One Thousand to Go

With two of the Fermi sources potentially linked to runaway stars, can we expect more matches to be made? While runaway stars are relatively common, the conditions have to be just right for the stars to become gamma-ray sources. They must have very high velocities and be moving through a region of the interstellar medium that is dense and has a weak magnetic field, since electrons spiraling around magnetic field lines lose too much energy through synchrotron radiation to generate gamma rays through inverse Compton scattering.

While more observations are required to confirm that these two stars are the source of the gamma rays seen by Fermi, Sánchez-Ayaso and collaborators have shown that these unique stars are definitely worth exploring!

Citation

E. Sánchez-Ayaso et al 2018 ApJ 861 32. doi:10.3847/1538-4357/aac7c7

11 July 2018

Gaia Identifies a Stellar Gap

Sometimes more-precise measurements are all we need to make new discoveries in old structures! In a new study, data from the Gaia mission has revealed a surprise hidden among main-sequence stars.

Old Diagram with a New Feature

An example HR diagram containing 22,000 stars from the Hipparcos catalog and 1,000 from the Gliese catalog of nearby stars. Main-sequence stars are visible as the diagonal band across the diagram, but data is sparse for M dwarfs in this plot (lower right corner). [Richard Powell]

If you’ve ever taken an introductory astronomy class, you’ve probably encountered the Hertzsprung-Russell (HR) diagram: a diagram on which stellar luminosities are plotted against their colors, which serve as a proxy for their effective temperatures. The resulting positions of stars on the HR diagram reveal distinct stellar evolutionary stages — and perhaps the most striking population is the swath of main-sequence stars that cuts diagonally across the diagram.

Though we’ve constructed HR diagrams for nearby stars for more than a century, they continue to change as our data for these stars improve. In particular, today’s era of precision astrometry has significantly improved the distance measurements for the stars that surround us, allowing them to be placed more accurately on the diagram. The recent second data release from the Gaia mission presented precise astrometry measurements for billions of stars, covering virtually all types on the HR diagram — including M dwarfs, which were sparsely sampled in the past.

A team of scientists led by Wei-Chun Jao (Georgia State University) has now explored this data and discovered a surprise: there’s a gap in the HR diagram at mid-M dwarfs.

Portion of the HR diagram for stars within 100 pc in the Gaia DR2 data set. A narrow low-density gap is visible cutting through the main sequence between the two dashed lines. [Jao et al. 2018]

Mind the Gap

Jao and collaborators plotted a total of nearly 250,000 stars from the Gaia archive on an HR diagram. The new data and improved measurements revealed a previously unseen feature: a narrow, diagonal slice through the main sequence that is underpopulated. The missing stars seem to lie in the middle of the M-dwarf region.

The authors cross-match the stars against the 2MASS catalog, finding that the gap exists in other data and color bands as well — which means it’s not just a weird quirk of Gaia’s photometry. They then check whether the gap exists only in stars at a specific distance. Another no: it’s visible similarly in various populations spanning distances up to 425 light-years.

Transitioning Convection

So what’s causing this unexpected feature? The authors argue that the presence and persistence of the gap suggest that it’s due to some underlying physics that we haven’t yet thought of — which is always an exciting prospect!

M dwarfs transition from being mostly convective to being fully convective, shown here, at masses of around 0.35 solar masses. [NASA/CXC/M.Weiss]

In particular, Jao and collaborators suggest that the gap may be related to a known transition in mid-M dwarfs, from larger stars that are mostly convective with a thin radiative layer, to smaller stars that are fully convective. The authors propose that the missing stars in the gap may be due to subtle changes of structure that occur at this transition between partial and full convection in these M dwarfs.

In the future, the authors propose gathering more data — like dynamical masses, radii, metallicities, rotational periods, and magnetic properties — on stars in and near the gap, to better understand population trends. In the meantime, we can be excited to know that there are still some surprises left for us to discover in old structures, if we just keep improving our data.

Citation

Wei-Chun Jao et al 2018 ApJL 861 L11. doi:10.3847/2041-8213/aacdf6

6 July 2018

When Giant Planets Orbit Evolved Stars

What happens to planets orbiting around stars that age past the main sequence and evolve into red giants? These enormous reddened stars — our Sun’s fate billions of years from now — may have significant impacts on the planet populations around them. A new study explores these impacts by looking at the orbits of gas giants closely circling evolved stars.

An Unexplored Population

The current size of the Sun as a main-sequence star (yellow circle) compared to the size the Sun will be as a red giant (red circle). How do the inflated sizes of evolved stars affect their planets? [Oona Räisänen]

The wealth of data from the Kepler/K2 mission is still coming in — despite the spacecraft being nearly out of fuel! — and we’ve discovered thousands of exoplanets with its help. From these data, we can start to explore different populations of exoplanets: from small rocky worlds orbiting tiny dwarf stars, to gas giants around main-sequence stars like our Sun.

One exoplanet category we’re only just starting to learn about is that of planets orbiting evolved stars — stars that have left the main sequence and grown into enormous red giants. What happens to close-in planets when their star ages and inflates? What fate awaits us when this happens to our own Sun billions of years from now? And how did these planets arrive in their orbits in the first place?

Recent discoveries of gas-giant planets around evolved stars have allowed us to start exploring these planets as a population. In a new study, a team of scientists led by Samuel Grunblatt (Institute for Astronomy, University of Hawaii) explored the properties of two such planets in particular, and then put them into context with other known giant planets around both giant stars and non-evolved stars.

radial velocities of close-in giant planets

Radial velocity observations of Kepler-643 (top), K2-132 (center) and K2-97 (bottom), three systems where close-in giant planets orbit evolved stars. The planets appear to follow a trend, where those on longer orbits are more eccentric than those orbiting their host star more closely. [Grunblatt et al. 2018]

Eccentricities for Two More Giants

Grunblatt and collaborators used radial-velocity measurements of the giant planets K2-97b and K2-132b, made with the HIRES spectrograph on the Keck-I telescope in Hawaii, to explore the orbits of these two planets around their red-giant host star. They then compared these planets’ orbital eccentricities to those of other known giant planets orbiting both giant stars and dwarfs.

The authors find that for close-in planets, those orbiting evolved stars tend to have more eccentric orbits than equivalent planets around main-sequence stars. The difference is slight — a median eccentricity of ~0.152 vs. ~0.056 for close-in giants orbiting giant stars vs. dwarfs — but it’s enough to indicate a statistical difference between the two populations.

Witnessing a Death Spiral

Orbital period vs. eccentricity for giant planets orbiting giant and dwarf stars. Close-in giant planets orbiting giant stars have slightly different eccentricity distributions than those orbiting dwarfs. [Grunblatt et al. 2018]

Why might this be? Grunblatt and collaborators propose that as these planets evolve, their orbits pass into a transient, moderately eccentric phase. As a result of rapid tidal interactions with their evolved hosts — which have much larger stellar radii and convective envelopes than main-sequence stars — the planets’ orbits shrink faster than they circularize. If this is the case, we’re observing these planets as they decay to their eventual fiery death.

How can we confirm this picture in the future? The K2 mission may be nearly over, but the NASA Transiting Exoplanet Survey Satellite (TESS) has only just launched, and it’s expected to observe more than a hundred times as many evolved stars as Kepler/K2. With TESS data on the horizon, we can hope that we’ll soon have the statistics needed to build the picture of planet evolution around giant stars.

Citation

Samuel K. Grunblatt et al 2018 ApJL 861 L5. doi:10.3847/2041-8213/aacc67

2 July 2018

Piecing Together the Light from Colliding Stars

The recent discovery of GW170817 — the first gravitational-wave detection where we also observed electromagnetic signals — has enabled new studies of merging compact objects. What have we since learned about the radiation that emerges from these collisions?

Signs of a Merger

Before GW170817 was observed, our models predicted that when two compact objects — either two neutron stars, or a neutron star and a black hole — merge, a number of observable signals result:

a spike of gravitational waves as the objects conclude their death spiral,
a short gamma-ray burst, an explosion of gamma rays thought to be produced in the relativistic jets launched during the merger,
an afterglow spanning X-rays to radio, caused when the jets slam into the surrounding environment and decelerate, and
an optical/near-infrared transient signal called a kilonova, which occurs when neutron-rich ejected material forms heavy elements that then undergo radioactive decay.

Last August’s binary-neutron-star merger confirmed this picture beautifully: the discovery of gravitational-wave signal GW170817 was followed by detections of short gamma-ray burst SGRB 170817A and later observations of kilonova AT 2017gfo.

But though this was the first time for all of these signals to come together for a source, it wasn’t our first observation of a short gamma-ray burst or a kilonova! How do past detections compare to this new one, and what can we consequently learn about the bursts of radiation from merging compact objects? A team of scientists led by Ben Gompertz (University of Warwick, UK) have now tackled these questions.

Model fit to the kilonova AT 2017gfo. [Gompertz et al. 2018]

Diversity of Kilonovae

Gompertz and collaborators compared the optical and near-infrared light curves of kilonova AT 2017gfo to equivalent light curves for a sample of a dozen nearby short gamma-ray bursts. What they found was a diversity of signals: some with no evidence of a kilonova, despite observations sensitive enough to pick up a signal several magnitudes fainter than AT 2017gfo; some with confirmed or suspected kilonovae brighter than AT 2017gfo; and some with afterglows so bright that they could be masking kilonovae that are brighter still.

How can we explain these vastly different kilonova outcomes from different mergers? The authors demonstrate that neither line-of-sight dust interference nor viewing angles can explain the differences we see in the optical and near-infrared signals.

It’s All in the Starting Players?

Magnitudes of the optical and near-infrared emission for a sample of 12 short gamma-ray bursts, as compared to the kilonova AT 2017gfo. [Gompertz et al. 2018]

Instead, Gompertz and collaborators propose that a dichotomy might exist in the short-gamma-ray-burst population, between those created by the merger of two neutron stars, and those created by the merger of a neutron star with a black hole. A neutron-star–black-hole merger can produce as much as ten times more ejecta than a binary-neutron-star merger — perhaps allowing the former to power observable kilonovae, while the latter produce much fainter signals.

Future observations of these events can confirm or deny this picture, depending on whether the magnitude of kilonova emission continues to display a gap in brightness between two populations, or if it instead forms a continuum. Either way, we can look forward to learning more about these explosive collisions soon!

Citation

B. P. Gompertz et al 2018 ApJ 860 62. doi:10.3847/1538-4357/aac206

29 June 2018

Investigating Confined Solar Flares

Solar flares are often, but not always, associated with coronal mass ejections. Why do coronal mass ejections accompany some solar flares but not others?

Some solar flares are associated with explosive coronal mass ejections, which can disturb Earth’s protective magnetosphere. The resulting geomagnetic storms generate dazzling auroras, but they can also interrupt radio communications and damage power grids. [SOHO (ESA & NASA)]

To Erupt or Not to Erupt

When a solar flare is blissfully unaccompanied by a violent eruption of plasma from the Sun’s surface, we call it a confined flare — the solar atmosphere remains bound to the surface rather than lashing out into interplanetary space. An eruptive flare, on the other hand, occurs when a coronal mass ejection is released along with the solar flare, exploding material out into the Sun’s surroundings.

It’s not yet clear why some solar flares are confined while others are eruptive. One possibility has to do with the arrangement of the Sun’s complex magnetic field. Twisted and tangled field lines can rearrange themselves through a process called magnetic reconnection, which accelerates charged particles and heats the plasma.

If, after reconnection, the magnetic field is either in a low-energy configuration or it’s sufficiently constricted by stronger magnetic fields farther from the Sun’s surface, no eruption will occur and the flare is confined. But how can we figure out what actually happens during confined flares?

Out on a Limb

The July 24, 2016, solar flare as seen in three extreme ultraviolet channels by Solar Dynamics Observatory. Panels a, b, and c show the progression of the flare from the beginning to the peak to the end. Click to enlarge. [Ning et al. 2018]

To better understand what makes solar flares tick, researchers often trace the movement of magnetic field loops before, during, and after the event. However, many flares are observed against the background of the bright solar disk, which can make it difficult to track the behavior of the magnetic loops.

To sidestep this issue, Hao Ning (Shandong University, China) and collaborators investigated the magnetic configuration of a flare extending from the edge of the Sun’s disk — the solar limb — where the bright arcs of plasma are clearly silhouetted against the dark background of the sky.

Ning and collaborators combined observations of a confined flare from the space-based observatories Solar Dynamics Observatory (SDO) and Ramaty High Energy Solar Spectroscopic Imager (RHESSI), as well as the Nobeyama Radioheliograph on the ground.

Time Evolution of a Solar Flare

Light curves extracted from the flaring region showed not one peak but two, which is thought to signal the presence of magnetic reconnection. Why two peaks? Magnetic reconnection in the corona accelerates charged particles, which then interact with the relatively dense chromospheric plasma. This interaction produces high-energy X-rays (the first peak) and heats the plasma, causing it to emit thermally at lower-energy X-ray wavelengths after a short delay (the second peak).

Soft X-ray light curves from RHESSI and hard X-ray light curves from the Geostationary Operational Environmental Satellite (GOES). The two peaks are labeled Stage A and B. The double-peaked behavior is evident at almost all energies. Click to enlarge. [Ning et al. 2018]

What makes this particular confined flare interesting is the range of wavelengths that exhibit the double peak — reaching energies as high as 50 keV (30 million K). This is a departure from previous observations of double-peaked flares, which have a lower-energy secondary peak.

Because of the unusually high energy, Ning and collaborators conclude that the plasma must be heated not only indirectly, by collisions with particles accelerated by reconnection, but also directly, by reconnection itself. This observation of a super-hot plasma component in the second stage of a confined flare is the first of its kind — and surely further observations will reveal new details of how confined flares evolve, continuing to build our picture of what determines confinement vs. eruption!

Citation

Hao Ning et al 2018 ApJ 854 178. doi:10.3847/1538-4357/aaaa69

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