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black holes in a globular cluster

When the Laser Interferometer Gravitational-Wave Observatory (LIGO) discovered its first merging black holes, astronomers were surprised: these black holes were much larger than we had expected! A new study looks at what these observations might tell us about black holes in star clusters.

X-ray binary

Artist’s impression of an X-ray binary, in which a black hole accretes matter from a stellar companion. [NASA/CXC/M.Weiss]

Unexpected Masses

Before the first gravitational-wave detection in 2015, the theoretical existence of black holes was solidly established within the framework of general relativity. Observational evidence for stellar-mass black holes came from X-ray binaries: binary star systems consisting of a compact object accreting matter from a companion star.

Though we can’t directly observe the black holes in X-ray binaries, we can infer their existence by watching the motions of the binary. By measuring the dynamics of these systems, we’ve obtained mass estimates for the inferred black holes in perhaps two dozen binaries so far; they typically range between about 5 and 20 solar masses.

stellar graveyard

A look at the masses for the black holes and neutron stars we’ve been able to measure. The black holes observed via X-rays (purple) are much less massive than those observed recently via gravitational waves (blue). [LIGO-Virgo/Frank Elavsky/Northwestern U.]

With this precedent, it was quite the surprise when LIGO’s first detection revealed the merger of two black holes of a whopping 31 and 36 solar masses. In the ten mergers LIGO and its European counterpart, Virgo, have detected since then, 16 of the 20 pre-merger black holes have had masses above the range measured for black holes in X-ray binaries.

Two Formation Channels?

What’s creating the dichotomy between the lower black-hole masses measured in X-ray binaries and the higher masses measured from mergers? Some scientists speculate that X-ray-detected and gravitational-wave-detected black holes are dominated by two different formation channels:

  1. A binary star system evolves in isolation, with at least one star eventually becoming a black hole. This channel is proposed for X-ray-detected black holes.
  2. Stars evolve individually within a cluster, and some of the resulting black holes later pair up into binaries via dynamical interactions in the cluster. This channel is proposed for gravitational-wave-detected black holes.

Can LIGO/Virgo observations tell us more about the latter scenario? A team of scientists led by Rosalba Perna (Stony Brook University) have run a series of N-body simulations of initially isolated black holes in a mini-cluster to find out.

Simulating Interactions

model vs LIGO observations

Comparison between the model prediction for the distribution of observed total mass after 10 observations for three models with different initial mass distributions and the LIGO/Virgo observations (black line). The best-fit model (green) is consistent with evolution of a cluster of low-metallicity, massive stars. [Perna et al. 2019]

Perna and collaborators show that dynamical interactions in the cluster preferentially cause the most massive of black holes to come together in more tightly bound binaries. Since tightly bound binaries spiral in more quickly, this cluster preference increases LIGO/Virgo’s chances of preferentially detecting the mergers of heavier black holes.

The team also shows that the particular shape of the distribution of masses measured at merger is dependent upon the distribution of initial black-hole masses in the cluster. By comparing LIGO/Virgo’s observations to their simulations with different initial mass distributions, Perna and collaborators show that the observations are consistent with the distribution expected for a star cluster that initially consists of massive, low-metallicity stars.

While these comparisons are always tricky with only 20 data points, this study can be easily expanded in the future, as LIGO and Virgo continue to amass more observations. For now, however, the dynamical formation channel is looking like a promising explanation for gravitational-wave-detected black holes!

Citation

“Constraining the Black Hole Initial Mass Function with LIGO/Virgo Observations,” Rosalba Perna et al 2019 ApJL 878 L1. doi:10.3847/2041-8213/ab2336

Venus Express

What happens to Venus when an enormous solar eruption slams into the planet? In 2011, the Venus Express spacecraft was on site to find out!

Benefits of a Field

Earth's magnetic field

Schematic illustration of the Earth’s global magnetic field. Venus does not have an intrinsic field. [NASA / Peter Reid / TheUniversity of Edinburgh]

The Earth’s magnetic field does an excellent job of protecting us from the damaging influence of the solar wind. Energetic particles emitted by the Sun are deflected around our planet and channeled to the poles, where they harmlessly light up the sky in haunting aurorae. Even the danger of sporadic solar eruptions — like flares and coronal mass ejections — is largely mitigated by our protective shield.

But our sister planet, Venus, is less fortunate: though similar to Earth in many ways, Venus lacks its own global magnetic field to protect it from the Sun’s onslaught.

What happens to this clouded planet when the Sun sends an enormous interplanetary coronal mass ejection its way?

Venus induced magnetosphere

The interaction of Venus with the magnetized solar wind produces an induced magnetosphere. [Ruslik0]

With a Little Help from the Sun

Venus has a trick up its sleeve: though it doesn’t carry its own magnetic field, it boasts an induced magnetosphere.

As extreme ultraviolet radiation from the Sun lights up Venus’s dayside, it ionizes the planet’s upper atmosphere, forming a plasma known as the ionosphere. When the solar wind — which carries the Sun’s magnetic field with it — encounters Venus, the thermal pressure of the ionosphere pushes back against the magnetic pressure of the solar wind, causing the field lines to drape around Venus and remain supported there.

This induced magnetosphere has a bow shock on the Sun side and a long, trailing magnetotail on the anti-Sun side. The pile-up of magnetic field between the magnetosphere and Venus’s ionosphere — the magnetic barrier — prevents the solar-wind plasma from penetrating deeper down into Venus’s atmosphere.

Front-Row Seats to Action

So Venus isn’t unprotected — but how well does this shield hold up in the face of powerful solar storms? In 2011, we had a orbiter ready to watch the stormy drama up close: the Venus Express spacecraft.

Venus Express, launched in 2005, orbited around Venus’s poles and studied the global space environment around the planet. On 5 November, 2011, an extremely strong interplanetary coronal mass ejection hit Venus while the spacecraft was in orbit — and now, in a publication led by Qi Xu (Macau University of Science and Technology, China), a team of scientists has detailed what the spacecraft learned.

Not Unflappable

Venus's flapping magnetotail

The magnetic field strength (top) and direction (bottom) measured by the Venus Express reveal the rapid flapping motion of the plasma sheet in the magnetotail in response to the interplanetary coronal mass ejection. The red line shows that the Bx component of the magnetic field changed direction 5 times within 1.5 minutes (7:49:30–7:51:00)! [Adapted from Xu et al. 2019]

Venus Express’s data show that the planet’s induced magnetosphere and its ionosphere responded dramatically to the strong solar eruption. Venus’s bow shock was compressed and broadened as the storm hit; the plasma sheet of the magnetotail flapped back and forth rapidly; the magnetic barrier increased in strength; and the ionosphere was excited, jumping to a whopping three times the quiet-Sun plasma density!

Based on their analysis, Xu and collaborators expect that interplanetary coronal mass ejections like this one substantially increase the rate of Venus’s atmospheric loss, violently driving ions from the planet’s gravitational grasp.

We still have a lot to learn about about how our sister planet reacts when solar storms strike, but these observations have shed new light on the dramatic struggle.

Citation

“Observations of the Venus Dramatic Response to an Extremely Strong Interplanetary Coronal Mass Ejection,” Qi Xu et al 2019 ApJ 876 84. doi:10.3847/1538-4357/ab14e1

SN 1987A

When massive stars explode as supernovae, they fling shredded stellar material into the interstellar medium. The explosion should also generate gravitational waves, but astronomers haven’t detected any yet. Is it just a matter of time before we detect gravitational waves from core-collapse supernovae, or are the signals still out of reach?

Cosmic Fireworks

Radice et al. 2019 Fig. 1

Expansion of the simulated supernova shock waves as a function of time since the outer layers of the star bounced off the core. All but the 13-solar-mass simulation exploded successfully. [Radice et al. 2019]

Astronomers have observed supernovae in every part of the electromagnetic spectrum and even captured elusive neutrinos from these events. Despite the wealth of observations, there’s still plenty we don’t know about core-collapse supernovae, including exactly how the explosions happen.

Gravitational-wave observations could be the key to understanding what goes on in the chaotic moments after massive stars collapse. To determine whether we can hope to detect these stellar explosions with our current gravitational-wave observatories — and what we could learn from the observations if we can detect them — a team led by David Radice (Princeton University) conducted three-dimensional simulations of eight supernovae.

Radice et al. 2019 Fig. 3

Computed gravitational-wave spectra. [Adapted from Radice et al. 2019]

When Stars Explode (Digitally)

Radice and coauthors began with models of supernova progenitor stars with masses between 9 and 60 times the mass of the Sun. For each simulated stellar collapse, the authors tracked the shock wave expansion, neutrino luminosity, and gravitational-wave emission as a function of time.

They found that gravitational waves were generated shortly after the collapse by convection in the material just outside the newly formed neutron star — a proto-neutron star — cooling at the supernova’s center. At later times (more than 0.2 seconds after the collapse), the gravitational-wave signal is dominated by oscillations of the proto-neutron star, which are driven by accreted material striking its surface.

The authors also show that the amount of energy radiated away in the form of gravitational waves is correlated with the amount of turbulent energy of the material accreting onto the proto-neutron star. This hints that gravitational-wave observations can provide an estimate of how turbulent the material behind the shock wave is.

These observations can also help us understand the nature of the proto-neutron star itself. The peak frequency of the signal is set by the proto-neutron star’s oscillation frequency, which depends on its interior structure. Clearly, gravitational waves can reveal a lot about what happens in a star’s final moments — but can we even detect them?

Catching a Wave

The authors found that the best possible signal-to-noise ratio for a supernova 33,000 light-years away ranges from 1.5 to 11.5, depending on the mass of the progenitor; for comparison, the signal-to-noise ratio for LIGO’s first detected black-hole merger was 24. While the authors acknowledge that this doesn’t mean that gravitational waves from core-collapse supernovae are undetectable with current observatories, the odds of detecting one are greater with future, more sensitive observatories.

Radice et al. 2019 Fig. 4

Simulated gravitational-wave luminosity as a function of time. [Radice et al. 2019]

In the case of the Einstein Telescope, a proposed gravitational-wave detector with a planned completion date of 2030, the calculated signal-to-noise ratio ranges from 20 to 110. Hopefully, the advent of third-generation detectors in the next couple of decades will bring an explosion of gravitational-wave data, allowing astronomers to peer into the turbulent interiors of collapsing stars.

Citation

“Characterizing the Gravitational Wave Signal from Core-collapse Supernovae,” David Radice et al. 2019 ApJL 876 L9. doi:10.3847/2041-8213/ab191a

Image reveals a tilted oval structure of an orange disk containing a number of concentric gaps and rings.

The stunning substructures of gaps and rings revealed in protoplanetary disks have been attributed to the motions of hidden, newly formed planets. But are we interpreting our observations correctly?

Models of Structure

planet formation

This simulation shows a Jupiter-mass planet forming inside a circumstellar disk. [Frédéric Masset]

When the Atacama Large Millimeter/submillimeter Array (ALMA) came online, one of its first released images was that of HL Tau, a young star surrounded by a protoplanetary disk — a disk that’s structured with a dramatic series of concentric gaps and rings. Since this early image, ALMA has continued to amass observations of disk structure in the inner tens of AU around young stars — and theorists are now left to decide what to make of these.

Multiple explanations for the origin of these structures have been proposed, including snowlines, flows driven by magnetic fields, gravitational instabilities, or dust trapping. But the most popular model suggests that the gaps are driven by the motions of young, invisible planets embedded in the disks.

Challenging Assumptions

Recent studies have suggested that multiple gaps and rings can actually be produced by a single embedded planet. Simulations show that as a planet moves through the disk, it excites multiple spiral density waves. Interactions of these waves with the disk can then carve out several narrow gaps.

But while the basic idea behind these simulations seems sound, two scientists from the Institute for Advanced Study, Ryan Miranda and Roman Rafikov (also of University of Cambridge, U.K.) suggest we need to be a little more careful in how we interpret them.

comparison simulations of planet-disk interactions

Three of the authors’ simulations comparing locally isothermal (left panel of each pair) and non-isothermal disks (right panel of each pair). For low-mass planets (top two pairs), locally isothermal simulations overestimate the contrast of structures. For high-mass planets (bottom pair), locally isothermal simulations also misrepresent the locations of rings and gaps. [Adapted from Miranda & Rafikov 2019]

Because full simulations of disk–planet interactions are computationally inhibitive, numericists make simplifying assumptions in their models. One commonly adopted simplification is to assume that the disk is locally isothermal, i.e., it has a fixed temperature profile. But while this assumption holds in the outer regions of the disk where cooling is efficient, Miranda and Rafikov point out that this isn’t a good model for the poorly cooled inner tens of AU where we observe these ring and gap structures.

Massive Interpretations

What quirks does this assumption introduce? By running a series of comparison simulations of a planet interacting with a locally isothermal and a non-isothermal disk, Miranda and Rafikov show that locally isothermal simulations tend to overestimate the contrast of ring and gap structures produced. This means that using isothermal models to interpret ALMA results would cause us to underestimate the masses of the planets causing the disk structure observed.

What’s more, the authors find that for large planets, the isothermal simulations also misrepresent the locations of the rings. The results in this article suggest a strong need for caution when using locally isothermal simulations to explore the interactions between planets and disks. We’re certainly getting closer to understanding the many complexities of planet formation, but we’ve still got plenty of work to do!

Citation

“On the Planetary Interpretation of Multiple Gaps and Rings in Protoplanetary Disks Seen By ALMA,” Ryan Miranda and Roman R. Rafikov 2019 ApJL 878 L9. doi:10.3847/2041-8213/ab22a7

collapsar

How do we get the heavy elements — elements with atomic mass above iron, like gold, platinum, or uranium — in our universe? A new study suggests that one theorized source, collapsing massive stars, may not be the best option.

Enriching the Universe

The Big Bang produced a universe filled almost exclusively with hydrogen and helium; almost all of the heavier elements in our universe have formed since that time. How and when they formed, however, are still questions we’re working to solve.

element origins

Periodic table showing the origin of each chemical element. Those produced by the r-process are shaded orange and attributed to supernovae in this image; though supernovae are one proposed source of r-process elements, collapsars have been proposed as another. [Cmglee]

We know that the dense, hot cores of stars fuse atoms, producing elements up to iron in mass. But we need more extreme conditions for r-process nucleosynthesis — a set of rapid neutron-capture reactions that we think are responsible for producing about half the atomic nuclei heavier than iron.

Recent research has renewed interest in one potential source of r-process elements: collapsars. Collapsars are massive (>30 solar masses), rapidly rotating stars that suffer catastrophic core-collapses into black holes. In this sudden process, a spinning disk of material accretes onto the core — and conditions in the disk are just right for the r-process. But could collapsars really account for much of the r-process elements in our universe?

Clues from Collapse

abundance ratios

Abundance ratios found by the authors in a sample of low-metallicity stars. The top plot shows good agreement between the collapsar model (red) and observations (black) for the ratio of Mg (not an r-process element) to Fe. But r-process elements Ba, Eu, and Sr show much higher abundances in collapsar models than in the observations. [Macias & Ramirez-Ruiz 2019]

Collapsar r-process nucleosynthesis should leave a visible imprint on the surrounding environment, UC Santa Cruz scientists Phillip Macias and Enrico Ramirez-Ruiz (also University of Copenhagen, Denmark) point out.

In the collapsar model, r-process elements produced in the accreting disk are flung out into the star’s surroundings via disk winds. But collapsars don’t only produce r-process elements — they also create lighter elements like iron, which are spewed from the collapsars via jets. These elements should all then mix, producing a soup of enriched material with a particular ratio of abundances — which will then seed the next generation of stars.

Macias and Ramirez-Ruiz look for signs of this soup imprinted on a sample of 186 very low-metallicity stars that haven’t already been polluted by many additional generations of star formation. If collapsars are the source of most of the r-process material in the universe, then these unpolluted canvases should show the same ratio of r-process elements to iron as the authors calculate from collapsar models.

A Mismatch with the Evidence

Macias and Ramirez-Ruiz find that their stellar sample’s abundance ratios do not match those predicted by the collapsar model — the relative amount of r-process elements would need to be much higher in the observed stars for collapsars to be a good explanation.

Instead, the authors argue that the majority of r-process nucleosynthesis must occur in sources that don’t simultaneously produce iron. One possible source that satisfies this condition is neutron-star mergers, like that observed in the recent gravitational-wave event GW170817. There are challenges to this model as well — but we can hope that future observations will help us to better understand where our universe’s heavy elements come from.

Citation

“Constraining Collapsar r-process Models through Stellar Abundances,” Phillip Macias and Enrico Ramirez-Ruiz 2019 ApJL 877 L24. doi:10.3847/2041-8213/ab2049

gamma-ray bursts

The merger of two compact objects — neutron stars or black holes — could be accompanied by a sudden, immediate flash of radio emission, according to predictions. Can an array of antennas in California help us spot these signals?

neutron-star merger

Artist’s impression of two merging neutron stars producing a gamma-ray burst. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Missing Signal

In recent years, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has opened up a new world with the first detections of gravitational waves from compact-object mergers. In 2017, LIGO announced the detection of a pair of neutron stars colliding — GW170817 — and astronomers around the world rushed to watch the fireworks emitted across the electromagnetic spectrum.

In the time that followed, we saw gamma rays from a short-duration burst emitted during the merger itself; an optical/near-infrared kilonova, powered by the radioactive decay of heavy elements formed in the merger; and a long-lived radio, X-ray, and optical afterglow, caused when ejecta from the merger slammed into the surrounding environment and decelerated. One signal was missing, however: prompt radio emission. 

A Prompt Challenge

OVRO-LWA

A nighttime photo of some of the antennas in the Owens Valley Radio Observatory Long Wavelength Array. [Gregg Hallinan]

According to models, the merger of two neutron stars should produce an immediate flash of radio emission. Unlike belated afterglow radio emission, prompt radio emission should emerge immediately after the merger; models predict that it should arrive within as little as a minute of the gravitational-wave signal.

Catching this signal could provide us with information about the magnetic environment immediately around the binary, the properties of the intergalactic medium, and more. But it’s a tricky prospect — there’s very little lead time between the gravitational-wave detection and the radio signal, and the gravitational-wave source is poorly localized. How can we know where to point, quickly enough to capture the prompt radio emission from a merger?

GW170104 localization

The localization of GW170104 is shown in blue, plotted over the OVRA-LWA’s field of view at the time. The gray area was outside of the array’s field of view. [Callister et al. 2019]

Radio Eyes on the Sky

One approach: all-sky radio monitoring, like that provided by the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). This array of 288 antennas in California continuously scans the sky overhead, taking 13-s integrations that it stores for 24 hours.

This broad coverage and data storage means that even if the alert for a gravitational-wave detection comes hours later, scientists still have access to relevant OVRO-LWA data. We just have to get lucky to have the source in the field of view of the antennas — something that didn’t work out for GW170817, but likely will eventually!

A Successful Nondetection

OVRO-LWA view

The OVRO-LWA view of the radio sky (from 27 to 84 MHz) at the time of GW170104. The contours show the localization of the gravitational-wave source. [Callister et al. 2019]

As a proof of concept, a team of scientists led by Thomas Callister (California Institute of Technology) recently used the OVRO-LWA to perform a search for prompt radio emission from GW170104, a gravitational-wave event for which the majority of the localization region was within the array’s field of view.

Callister and collaborators didn’t spot a signal — but this was expected, since GW170104 was a binary black-hole merger, making it unlikely to produce electromagnetic radiation. The team did find, however, that they were able to place useful constraints on the amount of radio emission the merger could have produced, demonstrating the power of the OVRO-LWA for future observations.

The OVRO-LWA is clearly up to the task of detecting prompt emission from upcoming neutron-star mergers. Now we’ll just have to wait and see if it spots something in the third LIGO observing run, currently underway!

Citation

“A First Search for Prompt Radio Emission from a Gravitational-wave Event,” Thomas A. Callister et al 2019 ApJL 877 L39. doi:10.3847/2041-8213/ab2248

exomoon candidate Kepler-1625b-i

Last October, the first discovery of a potential exomoon was announced. But is Kepler-1625b-i an actual moon in another solar system? Or just an artifact of data reduction?

A Tricky Business

exomoon

Artist’s depiction of an Earth-like exomoon orbiting a gas-giant planet. [NASA/JPL-Caltech]

Moons are a useful diagnostic — they can provide all kinds of information about their host planets, like clues to formation history, evolution, and even whether the planet might be habitable. What’s more, exomoons themselves have been indicated as potential targets in the search for life: while a habitable-zone gas-giant planet might not be an ideal host, for example, such a planet could have moons that are.

Given all we stand to learn from exomoons, it’d be great to find some! But for all that our solar system is chock full of moons (at last count, Jupiter alone hosts 79!), we’ve yet to find any sign of exomoons orbiting planets beyond the solar system. 

Jupiter moons

A montage of Jupiter and its four largest moons. [NASA/JPL]

This may well be because exomoon signals are difficult to spot. Not only would an exomoon’s signal be tiny compared to that of its host planet, but we also would need to separate that signal from the host’s — a tricky business. Throw in some instrument systematics to obscure all the data, and exomoon identification becomes even more of a challenge.

For these reasons, it was a pretty exciting announcement last fall when Columbia University astronomers Alex Teachey and David Kipping presented Kepler-1625b-i, a signal that they argued represented an exomoon around the gas giant Kepler-1625b. But a healthy dose of scientific caution has sent other teams scrambling to explore these data and draw their own conclusions — and one of these groups is calling the exomoon discovery into question.

Waiting for Consensus

Kepler 1625 light curve models

Best-fit models for the Kepler 1625 light curve assuming a planet and no moon (top) or moon (bottom). Data as analyzed by Kreidberg et al. are on the left (blue); data as analyzed by Teachey&Kipping are on the right (red). Kreidberg et al. find that the best fit is given by the no-moon model. Click to enlarge. [Kreidberg et al. 2019]

Led by Laura Kreidberg (Harvard-Smithsonian Center for Astrophysics), a team of astronomers has independently analyzed the same Hubble transit data that Teachey and Kipping used to identify their exomoon candidate. Unlike the other group, however, Kreidberg and collaborators found that the data are best fit by a simple planet transit model — the presence of an exomoon isn’t necessary or indicated.

According to Kreidberg and collaborators, the discrepancy between their results and Teachey and Kipping’s is most likely due to differences in data reduction. Teachey and Kipping have responded to this work with additional analysis in a recent paper submitted to AAS journals, but the debate is far from settled.

So is there an exomoon, or isn’t there? We don’t know yet, but that’s okay!

The case of Kepler 1625 is a beautiful illustration of the messy reality of the scientific process: sometimes the data don’t immediately spell out an answer, and it takes more time, more analysis, and likely more observations before the scientific community reaches a consensus. This isn’t a bad thing, though — this is science being done right! Keep an eye on the story of Kepler 1625b-i going forward; we’re bound to continue to learn about this maybe/maybe-not exomoon.

Citation

“No Evidence for Lunar Transit in New Analysis of Hubble Space Telescope Observations of the Kepler-1625 System,” Laura Kreidberg et al 2019 ApJL 877 L15. doi:10.3847/2041-8213/ab20c8

What happens to binary stars under the influence of a nearby supermassive black hole? A new study shows that things can go one of two ways — being torn apart or becoming closer than ever.

An artist’s impression of an X-ray binary, many of which are seen in the galactic center. [ESA/NASA/Felix Mirabel]

Life in Galactic Downtown

There’s a lot going on in the galactic center. The crowded stellar environs near the Milky Way’s central black hole have long been studied in order to understand how the presence of a nearby black hole can shape stellar populations in our galaxy and others.

A quick peek at the center of the Milky Way reveals that it plays host to an unusually large number of interesting astrophysical phenomena like stellar mergers, hyper-velocity stars, and X-ray sources that could point to cataclysmic variables or binary systems with a neutron-star or stellar-mass-black-hole component.

How do these various unusual objects form in the galactic center? One possibility is that ordinary, garden-variety binary stars do strange things when they evolve in the extreme environment near a supermassive black hole

A histogram of the binary mergers as a function of time in the simulation. [Stephan et al. 2019]

Binary Systems Through the Ages

To explore this scenario, a team of astronomers led by Alexander Stephan (University of California, Los Angeles) simulated the evolution of main-sequence binaries near a supermassive black hole. Specifically, they used Monte Carlo simulations of binary systems in the inner 0.33 light-years of a Milky-Way-like galaxy with a four-million-solar-mass black hole at its center.

Stephan and coauthors considered how the dynamics of binary systems with a range of starting masses would change over time as close encounters with other stars, gentle nudges from more distant objects, and the looming gravitational influence of the black hole warp and disturb their orbits.

While the orbits of the binary systems change over time due to gravitational interactions, the individual stars are busy evolving as well — a process that the authors captured by using a single-star stellar evolution code. Post-main-sequence evolution is important for the dynamics of a binary system since it can lead to mass transfer between the stars or even stellar mergers.

A flowchart of the simulated outcomes of binary evolution. Click to enlarge. [Stephan et al. 2019]

So Long, Partner

If you belong to a binary system and you’re hoping to spend the rest of your days with your companion, the news isn’t good: the authors found that after a few hundred million years, 75% of binary systems have been torn apart by gravitational interactions.

Of the remaining 25%, about half end up merging due to dynamical perturbations or because one star has swelled into a red giant and engulfed the other. The other half have even more exotic outcomes, becoming close binary systems containing white dwarfs, neutron stars, or black holes.

Clearly, the presence of a nearby supermassive black hole shakes up the evolution of otherwise ordinary binary stars: these rare systems are the precursors to Type Ia supernovae, cataclysmic variables, and compact object mergers that can generate gravitational waves.

Citation

“The Fate of Binaries in the Galactic Center: The Mundane and the Exotic,” Alexander Stephan et al 2019 ApJ 878 58. doi:10.3847/1538-4357/ab1e4d

habitable-zone planets

As of last week, the count of confirmed exoplanets officially exceeds 4,000 — and while we’ve learned a lot about planet formation from this wealth of data, it’s also prompted new questions. Could the recent detection of two intriguing new planets shed light on one of these open puzzles?

radius gap

In 2017, a team of scientists led by B.J. Fulton identified a gap in the distribution of radii of small Kepler-discovered planets. [NASA/Ames/Caltech/University of Hawaii (B. J. Fulton]

Mind the Gap

Our growing exoplanet statistics recently revealed a curious trait: there’s a gap in the radius distribution of planets slightly larger than Earth. Rocky super-Earth planets of up to ~1.5 Earth radii are relatively common, as are gaseous mini-Neptunes in the range of ~2–4 Earth radii. But we’ve detected very few planets in between these sizes.

What’s the cause of this odd deficit? One theory is that high-energy radiation emitted by stars early in their lifetimes erodes the atmospheres of planets that are too close in, stripping them of their expansive shells of gas and leaving behind only their dense, rocky cores. Planets that lie further out or start with a thicker shell may be spared this fate, retaining some of their gas for a significantly larger, fluffier construction.

Disentangling Factors

HD 15337 lightcurve

Folded light curves for HD 15337 showing the transits of planets b (top) and c (bottom). [Gandolfi et al. 2019]

This theory can be difficult to test, however, due to the large number of intertwined variables. The super-Earths and mini-Neptunes we’ve observed lie at varying distances from their host stars — but they also orbit around different types of stars with very different radiation histories. It’s hard to tell what role these various factors play in the planets’ evolution.

But a recent discovery from the Transiting Exoplanet Survey Satellite (TESS) may help simplify this picture. With more than 750 planet-candidate detections so far, TESS is rapidly adding to our exoplanet statistics — and two TESS-discovered planets around HD 15337 may be especially useful for better understanding the radius gap.

A Non-Identical Pair

In a publication led by Davide Gandolfi (University of Turin, Italy), a team of scientists carefully analyzes the TESS light curves for HD 15337, as well as archival spectroscopic data from the High Accuracy Radial velocity Planet Searcher. They show that there is evidence for the presence of two planets — HD 15337 b and c — that have similar masses: ~7.5 and ~8.1 Earth masses, respectively.

But while HD 15337 b appears to be a close-in (period of 4.8 days), rocky super-Earth with radius of 1.6 Earth radii and density of 9.3 g/cm3, HD 15337 c lies further out (period of 17.2 days) and is a fluffy mini-Neptune, with a radius of 2.4 Earth radii and a density of 3.2 g/cm3.

atmospheric erosion

Artist’s impression of a star’s high-energy radiation evaporating the atmosphere of its planet. [NASA’s Goddard SFC]

Since these two planets orbit the same star, it seems likely that their different orbital radii are what led to their places on either side of the radius gap. Using a planet atmospheric evolution algorithm, Gandolfi and collaborators show that the properties of the two planets can be produced by high-energy radiation from HD 15337 early in the system’s lifetime.

As our observational statistics for exoplanets continue to grow, it’s exciting to see how these continued discoveries can both raise and address new questions of planet formation and evolution. Who knows what else we’ll learn as detections continue to pile up!

Citation

“The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap,” Davide Gandolfi et al 2019 ApJL 876 L24. doi:10.3847/2041-8213/ab17d9

NGC 6397

Unusually blue and bright stars may not have only themselves to thank for their uniqueness. A new study looks at one way these unconventional objects might form in clusters … with a little help from a friend. 

Cluster Stand-Outs

blue stragglers on an HR diagram

Sketch of a Hertzsprung–Russell diagram for a star cluster. Blue stragglers lie above the main-sequence turnoff point for the rest of the stars in the cluster. [RicHard-59]

A stellar cluster is a group of stars that were all born together and should evolve in a consistent way. According to stellar evolution theory, for a given cluster, the stars of the cluster should fall onto a well-defined track on a Herzsprung–Russell (H–R) diagram — a plot of stellar brightness vs. color — with the stars’ location on the track dependent only on their initial mass.

But a few stars defy this logic. These so-called “blue stragglers” seem to have been left behind as their fellow cluster inhabitants evolved without them; on the H–R diagram, blue stragglers lie alone above the main-sequence turnoff point, shining brighter and bluer than they should be.

Just a Little Boost?

What causes these unorthodox stars? The simplest explanation is that they are main-sequence stars that belatedly received a bump in their mass. Theorists favor two possible formation channels:

  1. Mass transfer from an evolved donor onto a main-sequence star in a binary, which increases the main-sequence star’s mass and consequently causes it to become brighter and hotter.
  2. Collision and merger of two main-sequence stars, which forms a new, more massive main-sequence star that is brighter and hotter than usual.

But these two channels can only explain some observed blue stragglers; other systems — like WOCS ID 7782, a binary consisting of two blue stragglers in a 10-day orbit — are unlikely to have formed in either of these ways.

formation channel for blue stragglers

Schematic detailing the authors’ proposed scenario for the formation of WOCS 7782, in which a binary pair of main-sequence stars have material fed onto them by an evolved outer tertiary companion. [Portegies Zwart & Leigh 2019]

With WOCS ID 7782 in mind, scientists Simon Portegies Zwart (Leiden University) and Nathan Leigh (American Museum of Natural History; Stony Brook University; and University of Concepción, Chile) have now proposed an alternative formation channel.

A Third Star in the Mix

Portegies Zwart and Leigh’s model relies on one important element: a third star. In their proposed scenario, two main-sequence stars in a close binary are orbited by a giant, evolved companion star. As this evolved star ages and overflows its Roche lobe, gas flows from it onto the main-sequence binary, increasing the masses of the two inner stars.

Snapshot from one of the authors’ simulated triple systems. The binary system at left is being fed by gas from the outer tertiary companion on the right. [Portegies Zwart & Leigh 2019]

The authors use simulations to show that the final result of this process can be a close binary with two similar-mass blue stragglers, just as seen in WOCS ID 7782. In this scenario, the outer companion eventually evolves into a hard-to-spot white dwarf on a wide orbit with a period of more than ~5.8 years.

In addition to potentially explaining WOCS ID 7782, Portegies Zwart and Leigh’s model can produce a number of other masses, geometries, and configurations for blue-straggler systems, depending on the initial masses and separations of the binary and the outer companion. This formation scenario — which relies on just a little help from a friend — may therefore go a long way toward explaining the formation of the blue-straggler systems that have stumped us before now.

Citation

“A Triple Origin for Twin Blue Stragglers in Close Binaries,” Simon Portegies Zwart and Nathan W. C. Leigh 2019 ApJL 876 L33. doi:10.3847/2041-8213/ab1b75

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