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globular cluster

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

tidal inspiral

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.

interaction cross sections

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

fast radio burst

How frequently do fast radio busts occur in the observable universe? Two researchers have now developed a new estimate.

Extragalactic Signals

In 2007, scientists looking through archival pulsar data discovered a transient radio pulse — a flash that lasted only a few milliseconds. Since then, we’ve found another 22 such “fast radio bursts” (FRBs), yet we still don’t know what causes these energetic signals.

FRB 121102

Artist’s illustration of the Very Large Array pinpointing the location of FRB 121102. [Bill Saxton/NRAO/AUI/NSF/Hubble Legacy Archive/ESA/NASA]

Recently, some clues have finally come from FRB 121102, the only FRB ever observed to repeat. The multiple pulses detected from this source over the last five years have allowed us to confirm its extragalactic origin and pinpoint an origin for this FRB: a small, low-mass, metal-poor dwarf galaxy located about three billion light-years away.

Is FRB 121102 typical? How frequently do such bursts occur, and how frequently can we hope to be able to detect them in the future? And what might these rates tell us about their origins? Two scientists from the Harvard-Smithsonian Center for Astrophysics, Anastasia Fialkov and Abraham Loeb, have now taken a phenomenological approach to answering these questions.

Influencing Factors

Fialkov and Loeb argue that there are three main factors that influence the rate of observable FRBs in the universe:

  1. The spectral shape of the individual FRBs
    FRB 121102 had a Gaussian-like spectral profile, which means it peaks in a narrow range of frequencies and may not be detectable outside of that band. If this is typical for FRBs, then signals of distant FRBs may become redshifted to outside of the frequency band that we observe, making them undetectable.
  2. FRB detection rates

    FRB detection rates in the 1.25–3.5GHz band predicted by the authors’ models (red and blue solid and dashed lines), as a function of the flux limit for detection (top) and as a function of the FRB host’s redshift (bottom). Grey circles mark our detections of FRBs thus far. [Fialkov & Loeb 2017]

    The FRB luminosity function
    FRBs may all have the same intrinsic brightness (like Type Ia supernovae, for instance). Alternatively, there may be many more faint and dim FRBs than bright ones (like the distribution of galaxy luminosities). This difference affects the number of FRBs we could detect.
  3. The host galaxy population
    Are FRBs most commonly hosted by low-mass galaxies like FRB 121102? Or do they occur in high-mass galaxies as well? This affects the number of FRBs we would expect to observe at different redshifts.

Future Hope

By exploring a range of models that vary these three factors, Fialkov and Loeb find estimates for the rate of FRBs that would appear in the 500 MHz–3.5 GHz frequency band probed by observatories like Parkes, Arecibo, and the Australian Square Kilometre Array Pathfinder (ASKAP).

Fialkov and Loeb find that, when we account for faint sources, one FRB may occur per second across the sky in this band. The authors show that future low-frequency radio telescopes with higher sensitivity, such as the Square Kilometre Array, should be able to detect many more of these sources, helping us to differentiate between the models and narrow down the properties of the bursts and their hosts. This, in turn, may finally reveal what causes these mysterious signals.

Citation

Anastasia Fialkov and Abraham Loeb 2017 ApJL 846 L27. doi:10.3847/2041-8213/aa8905

quasar

How did the first supermassive black holes grow alongside their host galaxies in the early universe? New observations from the Atacama Millimeter/Submillimeter Array (ALMA) have provided us with a detailed look at one quasar, which may help us to answer this question.

Quasar-Starburst Systems

ALMA

The Atacama Large Millimeter/Submillimeter Array. [ESO/C. Malin]

The hunt for quasars — active and bright galactic nuclei — at high redshifts is on, with roughly 200 discovered thus far at redshifts of 5.1 < z < 7. These galaxies, whose light was emitted at a time when the universe was roughly less than a billion years in age, can tell us about conditions in this early time.

In particular, maybe 30% of these known quasars demonstrate active star formation in addition to significant black-hole activity. These quasar-starburst systems are unique laboratories that we can use to explore how the first supermassive black holes formed and grew along with their host galaxies in the period of time close to the end of cosmic reionization.

New observations from ALMA have now revealed a detailed look at one such quasar, ULAS J1319+0950, located at a redshift of z = 6.13. The observations, presented in a paper led by Yali Shao (Peking University and the National Radio Astronomy Observatory), have provided intriguing insight about early supermassive black hole growth.

ALMA maps

ALMA maps of the [C II] gas line emission (top) and the dust continuum (bottom). [Shao et al. 2017]

Measuring Gas and Dust

Previous observations of the galaxy ULAS J1319+0950 gave us some tantalizing hints as to its structure and dynamics, but the limited spatial resolution of these observations prevented us from drawing firm conclusions. Shao and collaborators now combine the previous Cycle 0 ALMA observations of ULAS J1319+0950 with new, high-resolution observations from Cycle 1 to draw a detailed picture of where the dust and the atomic and ionized gas (which traces the star formation) reside in this galaxy’s core.

The authors find that the [C II] line emission is more irregular than that of the dust continuum, suggesting that the dust and the gas have different distributions in the central region of this quasar. Shao and collaborators also detect signatures that indicate that the [C II] emission line originates from a rotating gas disk. By modeling the emission, they estimate the speed of the gas’s rotation, allowing them to obtain a dynamical mass for the galaxy.

A Speedily-Grown Black Hole

velocity map

Mean gas velocity map, revealing the net rotation of the gas. The authors used this rotation to measure the dynamical mass of the galaxy. [Shao et al. 2017]

Shao and collaborators estimate the galaxy’s dynamical mass to be ~130 billion solar masses. This yields a black-hole-to-host-galaxy mass ratio of ~2% — which is roughly 4 times higher than is typical for present-day galaxies. We can therefore infer that in the early evolution phase of this galaxy, the black hole grows the bulk of its mass before the formation of most of the stellar mass.

Are these findings true only for ULAS J1319+0950? Or could this provide us with insight into how all supermassive black holes grew in the early universe? With the significant power of observatories like ALMA on our side, future detections should soon reveal the answer.

Citation

Yali Shao et al 2017 ApJ 845 138. doi:10.3847/1538-4357/aa826c

Black Widow pulsar

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.

Predatory Stars

Intrabinary shock

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]

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.

magnetic field geometries

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?

A Shock Assists

PSR J1301+0833

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

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

Little Cub and NGC 3359

The discovery of an extremely metal-poor star-forming galaxy in our local universe, dubbed Little Cub, is providing astronomers with front-row seats to the quenching of a near-pristine galaxy.

NGC 3359 and Little Cub hydrogen

SDSS image of NGC 3359 (left) and Little Cub (right), with overlying contours displaying the location of hydrogen gas. Little Cub’s (also shown in the inset) stellar mass lies in the blue contour of the right-hand side. The outer white contours show the extended gas of the galaxy, likely dragged out as a tidal tail by Little Cub’s interaction with NGC 3359. [Hsyu et al. 2017]

The Hunt for Metal-Poor Galaxies

Low-metallicity, star-forming galaxies can show us the conditions under which the first stars formed. The galaxies with the lowest metallicities, however, also tend to be those with the lowest luminosities — making them difficult to detect. Though we know that there should be many low-mass, low-luminosity, low-metallicity galaxies in the universe, we’ve detected very few of them nearby.

In an effort to track down more of these metal-poor galaxies, a team of scientists led by Tiffany Hsyu (University of California Santa Cruz) searched through Sloan Digital Sky Survey data, looking for small galaxies with the correct photometric color to qualify a candidate blue compact dwarfs, a type of small, low-luminosity, star-forming galaxy that is often low-metallicity.

Hsyu and collaborators identified more than 2,500 candidate blue compact dwarfs, and next set about obtaining follow-up spectroscopy for many of the candidates from the Keck and Lick Observatories. Though this project is still underway, around 100 new blue compact dwarfs have already been identified via the spectroscopy, including one of particular interest: the Little Cub.

Little Cub

This tiny star-forming galaxy gained its nickname from its location in the constellation Ursa Major. Little Cub is perhaps 50 or 60 million light-years away, and Hsyu and collaborators find it to be one of the lowest-metallicity star-forming galaxies in our local universe. The galaxy contains ~100,000 solar masses of stars and it is notably gas-rich — with nearly 100 times the stellar mass in neutral gas.

The environment of Little Cub is also interesting: it appears to be just a couple hundred thousand light-years away from the grand design spiral galaxy NGC 3359. The galaxies’ proximity and kinematics suggest that Little Cub may be a companion of NGC 3359, and Little Cub’s morphology indicates that the larger galaxy may be tidally stripping gas from it.

Little Cub spectra

Emission-line spectra of Little Cub from Keck Observatory. [Hsyu et al. 2017]

A First Passage?

If Little Cub is indeed being tidally stripped by NGC 3359, then it’s surprising that the small galaxy still contains so much hot, star-forming gas; timescales for tidal stripping of this sort are thought to be very short. Hsyu and collaborators therefore speculate that we may have caught Little Cub in the early stages of its first passage around NGC 3359, allowing us to witness the quenching of a near-pristine satellite by a Milky-Way-like galaxy.

This quenching process is thought to commonly happen around other massive host galaxies in the universe — including around our own Milky Way, where nearly all satellite galaxies within roughly a million light-years are already quiescent and contain little neutral gas. Little Cub provides us with a rare opportunity to watch this process in action in our nearby universe, and it will be an intriguing laboratory for testing our understanding of dwarf satellite galaxy evolution.

Citation

Tiffany Hsyu et al 2017 ApJL 845 L22. doi:10.3847/2041-8213/aa821f

dark matter

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.

A Bizarre Particle

PVLAS

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

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.

SKA

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.

Hope for Next-Gen Telescopes

axion parameter space

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]

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

DCT

Who says there’s no romance in the outer solar system? A new study has identified 2004 TT357 as a body that may be made up of two separate objects in contact with each other.

Identification Challenges

A large fraction of both the near and distant small bodies in our solar system are predicted to be in binary systems. When these systems are nearby and have large separations, we can use telescopes like Hubble to observe them and identify the two separate components. But when binaries are far away and have small separations, Hubble doesn’t have the resolution to tell that they’re not a single object.

67P

Comet 67P/Churyumov–Gerasimenko is an example of a suspected contact binary in the inner solar system. [ESA]

Contact binaries are objects consisting of two lobes in contact with each other, like the bi-lobed, peanut-shaped comet 67P/Churyumov–Gerasimenko. These systems occur when two objects gravitate toward each other until the point where they touch.

Though Hubble can’t recognize distant contact binaries because the components are too close together, we can potentially identify them from their characteristic light curves. But this is a challenging process, and so far we’ve only found one confirmed trans-Neptunian-object (TNO) contact binary and one candidate — despite predictions that 10–30% of TNOs could be contact binaries.

Now, new observations from the 4.3-m Lowell Observatory Discovery Channel Telescope, presented in a study led by Audrey Thirouin (Lowell Observatory), have resulted in the identification of a potential new TNO contact binary.

light curve

Light curve for 2004 TT357. The double-peaked light curve (with a rotational period of 7.79 hr) is plotted over two cycles. [Adapted from Thirouin et al. 2017]

Changing Brightness

2004 TT357 is a TNO in our distant solar system orbiting with a semimajor axis of ~55 AU. Thirouin and collaborators’ observations of this object allowed them to construct a light curve for 2004 TT357 that revealed a ~7.8-hour period and a significant amplitude variation. The authors explore three potential causes for this variation: the object has a varying albedo; it has an elongated, ellipsoidal shape; or it is a contact binary.

Asteroids and TNOs tend to have albedo variations of no more than 10–20%. Since 2004 TT357’s light curve varies by ~80%, it seems unlikely that the albedo is the cause of the variability. And while the authors show that we can’t rule out 2004 TT357 having an elongated ellipsoid body, the morphology of the light curve favors the explanation that this object is a contact binary.

A Dense Binary

Hubble observations

Hubble observations of 2004 TT357, which (unsurprisingly, given the distance) reveal no evidence of the object being a binary. [Thirouin et al. 2017]

If 2004 TT357 is a contact binary, what can we learn about it? Its components most likely have a very unequal mass ratio in which one lobe is only 45% the mass of the other, according to the authors’ estimates. Its density could be quite high — 2 g/cm3 — which means it likely has a rocky composition.

Future observations of this system at different observing angles may allow us to confirm whether 2004 TT357 truly is a contact binary. For now, we’ll just have to hold out hope that two lonely TNOs have found each other in the vastness of space.

Citation

Audrey Thirouin et al 2017 ApJ 844 135. doi:10.3847/1538-4357/aa7ed3

Binary neutron star merger

neutron star

A rapidly spinning, highly magnetized neutron star is one possible outcome when two smaller neutron stars merge. [Casey Reed/Penn State University]

When two neutron stars collide, the new object that they make can reveal information about the interior physics of neutron stars. New theoretical work explores what we should be seeing, and what it can teach us.

Neutron Star or Black Hole?

So far, the only systems from which we’ve detected gravitational waves are merging black holes. But other compact-object binaries exist and are expected to merge on observable timescales — in particular, binary neutron stars. When two neutron stars merge, the resulting object falls into one of three categories:

  1. a stable neutron star,
  2. a black hole, or
  3. a supramassive neutron star, a large neutron star that’s supported by its rotation but will eventually collapse to a black hole after it loses angular momentum.
distributions

Histograms of the initial (left) and final (right) distributions of objects in the authors’ simulations, for five different equations of state. Most cases resulted primarily in the formation of neutron stars (NSs) or supramassive neutron stars (sNSs), not black holes (BHs). [Piro et al. 2017]

Whether a binary-neutron-star merger results in another neutron star, a black hole, or a supramassive neutron star depends on the final mass of the remnant and what the correct equation of state is that describes the interiors of neutron stars — a longstanding astrophysical puzzle.

In a recent study, a team of scientists led by Anthony Piro (Carnegie Observatories) estimated which of these outcomes we should expect for mergers of binary neutron stars. The team’s results — along with future observations of binary neutron stars — may help us to eventually pin down the equation of state for neutron stars.

Merger Outcomes

Piro and collaborators used relativistic calculations of spinning and non-spinning neutron stars to estimate the mass range that neutron stars would have for several different realistic equations of state. They then combined this information with Monte Carlo simulations based on the mass distribution of neutron-star binaries in our galaxy. From these simulations, Piro and collaborators could predict the distribution of fates expected for merging neutron-star binaries, given different equations of state.

The authors found that the fate of the merger could vary greatly depending on the equation of state you assume. Intriguingly, all equations of state resulted in a surprisingly high fraction of systems that merged to form a neutron star or a supramassive neutron star — in fact, four out of the five equations of state predicted that 80–100% of systems would result in a neutron star or a supermassive neutron star.

Lessons from Observations

frequency bands

The frequency bands covered by various current and planned gravitational wave observatories. Advanced LIGO has the right frequency coverage to be able to explore a neutron-star remnant — if the signal is loud enough. [Christopher Moore, Robert Cole and Christopher Berry]

These results have important implications for our future observations. The high predicted fraction of neutron stars resulting from these mergers tells us that it’s especially important for gravitational-wave observatories to probe 1–4 kHz emission. This frequency range will enable us to study the post-merger neutron-star or supramassive-neutron-star remnants.

Even if we can’t observe the remnant’s behavior after it forms, we can still compare the distribution of remnants that we observe in the future to the predictions made by Piro and collaborators. This will potentially allow us to constrain the neutron-star equation of state, revealing the physics of neutron-star interiors even without direct observations.

Citation

Anthony L. Piro et al 2017 ApJL 844 L19. doi:10.3847/2041-8213/aa7f2f

COROT-7b

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?

hot Jupiter

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.

star sample

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

metallicity distributions

Metallicity distributions of the three statistical samples. The hot-Jupiter hosts (orange) have different distribution than the others; it is weighted more toward higher metallicities. [Winn et al. 2017]

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.

Citation

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

stellar orbits

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?

tidal disruption event

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?

orbit and radius evolution

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

EMRI orbits

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

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