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neutron-star merger

Where were you on Thursday, 17 August 2017? I was in Idaho, getting ready for Monday morning’s solar eclipse. What I didn’t know was that, at the time, around 70 teams around the world were mobilizing to point their ground- and space-based telescopes at a single patch of sky suspected to host the first gravitational-wave-detected merger of two neutron stars.

Sudden Leaps for Science

BH/NS masses

The masses for black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and the neutron stars that merged in GW170817 (orange). [LIGO-Virgo/Frank Elavsky/NorthwesternUniversity]

The process of science is long and arduous, generally occurring at a slow plod as theorists make predictions, and observations are then used to chip away at these theories, gradually confirming or disproving them. It is rare that science progresses forward in a giant leap, with years upon years of theories confirmed in one fell swoop.

14 September 2015 marked the day of one such leap, as the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves for the first time — simultaneously verifying that black holes exist, that black-hole binaries exist, and that they can merge on observable timescales, emitting signals that directly confirm the predictions of general relativity.

As it turns out, 17 August 2017 was another such day. On this day, LIGO observed a gravitational-wave signal unlike its previous black-hole detections. Instead, this was a signal consistent with the merger of two neutron stars.

nuetron-star merger model

Artist’s illustrations of the stellar-merger model for short gamma-ray bursts. In the model, 1) two neutron stars inspiral, 2) they merge and produce a gamma-ray burst, 3) a small fraction of their mass is flung out and radiates as a kilonova, 4) a massive neutron star or black hole with a disk remains after the event. [NASA, ESA, and A. Feild (STScI)]

What We Predicted

Theoretical models describing the merger of two compact objects predict a chirping gravitational-wave signal as the objects spiral closer and closer. Unlike in a black-hole merger, however, the end of the chirp from merging neutron stars should coincide with a phenomenon known as a short gamma-ray burst: a powerful storm of energetic gamma rays produced as the objects finally collide.

According to the models, these gravitational waves and gamma rays will be followed by a kilonova — a transient source visible in infrared, optical, and ultraviolet — which arises from radioactive decay of heavy elements formed in the collision. This source should gradually decay over a timescale of weeks.

Lastly, the merger could create a powerful jet of high-energy particles, which could be visible to us in X-ray and radio wavelengths as it is emitted and interacts with its surrounding environment. We could also detect neutrinos from this outflow.

What We Saw (and Didn’t See)

The localization of the gravitational-wave, gamma-ray, and optical signals of the neutron-star merger detected on 17 August, 2017. [Abbott et al. 2017]

So what did we see on 17 August, 2017 and thereafter? Here’s what was found by the army of collaborations searching in gravitational waves, electromagnetic signals across the spectrum, and neutrinos:

  1. Gravitational Waves
    The gravitational-wave signature of a binary neutron-star merger was observed with all three gravitational-wave detectors currently operating as a part of the LIGO-Virgo collaboration. GW170817’s signal was in the sensitivity band of these detectors for ~100 seconds, arriving first at the Virgo detector in Italy, next at LIGO-Livingston in Louisiana 22 milliseconds later, and finally at LIGO-Hanford in Washington 3 milliseconds after that. These detections localized the source to a region of 31 square degrees at a relatively nearby distance of ~130 million light-years, and they identified the binary components to be neutron stars.
  2. Gamma-Ray Burst
    The Fermi Gamma-Ray Burst Monitor detected a short (~2-second) gamma-ray burst, GRB170817A, which appears to have occurred 1.7 seconds after the merger indicated by the gravitational-wave signal. This source was later identified by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft as well.
  3. observatory map

    Locations of the many observatories that observed the neutron-star merger first detected on 17 August, 2017. [Abbott et al. 2017]

    Electromagnetic Counterpart and Host Galaxy
    Though they were initially foiled by the signal’s location (the localized region of GW170817 only became visible in Chile 10 hours after its detection), the One-Meter, Two-Hemisphere team used the Swope telescope at Las Campanas Observatory in Chile to discover an optical counterpart to the LIGO and Fermi detection, located in the early-type galaxy NGC 4993. Within an hour, five other teams had independently detected the optical source in NGC 4993, with more following after.
    In the subsequent hours, days, and weeks, observatories across the electromagnetic spectrum monitored the transient. The source soon faded from view in the ultraviolet and gradually reddened in the optical and infrared bands. Delayed X-ray emission was discovered ~9 days after the LIGO signal, and a radio counterpart was discovered a week after that.
  4. No Neutrinos
    Though several neutrino observatories searched for high-energy neutrinos in the direction of NGC 4993 in the two-week period following the merger, none were detected.
timeline of GW170817 observations

Summary and timeline of the observations of the neutron-star merger detected on 17 August, 2017 relative to the time tc of the gravitational-wave event. Click for a closer look. [Abbott et al. 2017]

A Spectacular Confirmation

So what do these observations tell us? Our model for neutron-star mergers appears to be remarkably successful! The associated detections of gravitational waves and electromagnetic counterparts have confirmed that merging neutron stars produce the expected gravitational-wave signal, that they are the source of gamma-ray bursts, that some of the heaviest elements in the universe are produced during the collision of these stars, and that jets of high-energy particles are created that subsequently interact with their environment.

As with any interesting scientific discovery, new points of exploration have arisen — we can now wonder why the gamma-ray burst was unusually weak given its close distance, for instance, or why we didn’t detect any neutrinos from the outflow.

In spite of our new questions, the combination of these recent discoveries provide a resounding verification of our understanding of how compact objects merge. The various signals that began on 17 August, 2017 have simultaneously confirmed a stack of carefully constructed theories that were crafted over decades to explain how seemingly unrelated electromagnetic signals might all tie together. It’s a beautiful thing when science works out this well!

For more information, check out the ApJL Focus Issue on this result here:
Focus on The Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817  

Citation

Abbott, B.P. et al 2017 ApJL 848 L12. doi:10.3847/2041-8213/aa91c9

UGC 11411 Hubble

VLA

The Karl G. Jansky Very Large Array, located in Socorro, NM. [John Fowler]

Taking advantage of a program offered by the National Radio Astronomy Observatory (NRAO), an undergraduate class has observed local dwarf galaxies to learn about their properties.

The Benefits of Nearby Dwarfs

If you want to learn about the physical properties of low-mass galactic halos, the best place to look is nearby dwarf galaxies. These objects have the benefit of being close enough that we can resolve individual stars, allowing us to explore the relationship between star formation and the surrounding interstellar medium. They also allow us to directly measure bulk velocities, so we can interpret the distributions of both dark and baryonic matt5ter in these galaxies.

HI images of UGC 11411

HI images of UGC 11411. Left: HI mass surface density. Right: the intensity-weighted velocity field of the HI gas, which reveals the bulk kinematics of the galaxy. [Bralts-Kelly et al. 2017]

Though thousands of local-volume, gas-rich objects have been explored by gas surveys in the past, many have slipped through the cracks due to the varied selection criteria of these different surveys. In a new study, neutral atomic hydrogen observations are presented for the first time for two of these star-forming, gas-rich dwarf galaxies.

A Class in Action

Guided by Professor John Cannon and collaborators at other universities, a class of undergraduates at Macalester College in St. Paul, Minnesota, has coauthored a study of the neutral interstellar medium of these two local dwarf galaxies. The project was made possible by the “Observing for University Classes” program offered by NRAO’s Karl G. Jansky Very Large Array (VLA), in which university classes in observational astronomy can apply for observing time with the VLA.

UGC 11411

Top: a view of UGC 11411’s stars from Hubble. Middle: the locations of the galaxy’s star formation, as traced by SAO’s telescope’s observations of Hα. Bottom: UGC 11411’s neutral interstellar medium distribution (red contour), overlaid on the other two data sets. [Bralts-Kelly et al. 2017]

The students used the VLA to obtain neutral hydrogen spectral-line observations of UGC 11411 and UGC 8245 in February and March of this year. They then processed and analyzed the data, exploring the stellar population and star formation in each galaxy, and using the galaxies’ bulk kinematics to calculate their total dynamical masses.

Dominated by Dark Matter

The authors found that in both galaxies, the greatest bulk of the neutral interstellar medium can be found in the same location as the ongoing star formation. The two galaxies are different in several ways, however: UGC 8245 has a much lower star formation rate than UGC 11411 currently, and though the neutral hydrogen gas and stellar masses are similar for both galaxies, UGC 11411 has a halo that is more than an order of magnitude more massive.

They conclude that UGC 8245 — which has a total mass that is only ~2 times larger than its baryonic mass — is very similar to other low-mass galaxies that have been studied in the past. On the other hand, UGC 11411 — which has a total mass that is at least a factor of 10 larger than its baryonic mass — is significantly more massive than other known local low-mass galaxies, and it is unusually highly dark-matter dominated.

Further explorations of these dwarfs in contrast to one another will continue to reveal information about the low-mass galaxies of the universe.

Citation

Lilly Bralts-Kelly et al 2017 ApJL 848 L10. doi:10.3847/2041-8213/aa8ea0

gamma-ray burst

With the first detection of gravitational waves in 2015, scientists celebrated the opening of a new window to the universe. But multi-messenger astronomy — astronomy based on detections of not just photons, but other signals as well — was not a new idea at the time: we had already detected tiny, lightweight neutrinos emitted from astrophysical sources. Will we be able to combine observations of neutrinos and gravitational waves in the future to provide a deeper picture of astrophysical events?

Signs of a Merger

binary neutron star merger

Artist’s impression of the first stage of a binary neutron star merger. [NASA, ESA, and A. Feild (STScI)]

If the answer is yes, the key will probably be short gamma-ray bursts (SGRBs). Theory predicts that when a neutron star merges with another compact object (either another neutron star or a black hole), a number of signals may be observable. These include:

  • gravitational waves as the binary spirals inward,
  • a brief burst of gamma rays at merger (this is the SGRB),
  • high-energy neutrino emission during the SGRB,
  • optical and infrared emission after the merger in the form of a kilonova, and
  • radio afterglows of the merger remnants.

While we’ve observed the various electromagnetic components of this picture, the multi-messenger part is lacking: gravitational-wave detections haven’t been made in conjunction with electromagnetic counterparts thus far, and the only confirmed astrophysical sources of neutrinos are the Sun and Supernova 1987A.

neutrino fluxes

Pedicted neutrino fluxes during different stages of emission in an SGRB. [Kimura et al. 2017]

Can we expect this to change in the future? A team of authors led by Shigeo Kimura (Pennsylvania State University) has now explored the likelihood that we’ll be able to detect high-energy neutrinos in association with future gravitational-wave events.

Detecting the SGRB Neutrinos

Kimura and collaborators first estimate the flux of high-energy neutrinos expected during various emission phases of an SGRB. They show that a period of late-time emission, known as the “extended emission” phase, may produce high-energy neutrinos more efficiently than the other phases. But would we be able to see these neutrinos?

IceCube vs IceCube-Gen2

A comparison of IceCube’s detection capabilities (top) to those of the planned IceCube-Gen2 (bottom), for different models of neutrino emission during an SGRB. [Kimura et al. 2017]

To answer this, the authors calculate the probability of detection for neutrinos coming from a distance of ~300 Mpc — the predicted sensitivity range of advanced LIGO for gravitational-wave detection from a face-on neutron-star binary. They find that the IceCube Neutrino Observatory could detect neutrinos from around 10% of average extended-emission events — or perhaps up to half in the most optimistic scenario. The planned next iteration of the detector, IceCube-Gen2, should do better, however: Kimura and collaborators estimate that a quarter of the extended emission events will be detectable in the general case, and up to three quarters of them may be seen in the optimistic case.

The authors’ calculations suggest that within several years of operation of IceCube-Gen2, there is a good chance that we’ll be able to simultaneously detect gamma rays, neutrinos, and gravitational waves from bright SGRBs. This will provide us with powerful tools for learning about the physics of these energetic events.

Citation

Shigeo S. Kimura et al 2017 ApJL 848 L4. doi:10.3847/2041-8213/aa8d14

hot Jupiter

Jupiter-like planets with blisteringly close-in orbits are generally friendless, with no nearby planets transiting along with them. Giant planets with orbits a little further out, on the other hand, often have at least one companion. A new study examines the cause of hot Jupiters’ loneliness.

Forming Close-In Giants

planet forming in disk

Artist’s impression of a planet forming within a protoplanetary disk. [NAOJ]

Though we’ve studied close-in giant planets for decades now, we still don’t fully understand how these objects form and evolve. Jupiter-like giant planets could form in situ next to their host stars, or they could form further out in the system — beyond the ice line — and then migrate inwards. And if they do migrate, this migration could occur early, while the protoplanetary disk still exists, or long after, via excitation of large eccentricities.

We can try to resolve this mystery by examining the statistics of the close-in giant planets we’ve observed, but this often raises more questions than it answers. A prime example: the properties of close-in giants that have close-in companion planets orbiting in the same plane (i.e., co-transiting).

About half of warm Jupiters — Jupiter-like planets with periods of 10–30 days — appear to have close-in, co-transiting companions. In contrast, almost no hot Jupiters — Jupiter-like planets with periods of less than 10 days — have such companions. What causes this dichotomy?

schematic of orbit tilt

Schematic of the authors’ model, in which the close-in giant (m1) encounters a resonance with its host star, causing the orbit of the exterior companion (m2) to become tilted. [Spalding & Batygin 2017]

Friendless Hot Jupiters

While traditional models have argued that the two types of planets form via different pathways — warm Jupiters form in situ, or else migrate inward early and smoothly, whereas hot Jupiters migrate inward late and violently, losing their companions in the process — a new study casts doubt on this picture.

Two scientists from the California Institute of Technology, Christopher Spalding and Konstantin Batygin, propose an alternative picture in which both types of planets form through identical pathways. Instead, they argue, a hot Jupiter’s apparent loneliness arises over time through interactions with its host star.

Stellar Interactions Impact Companions

keeping the companion

Semimajor axis for the outer companion (a2) vs that of the close-in giant planet (a1) at three different system ages. Outer companions within the shaded region will not encounter the resonance investigated by the authors, instead remaining coplanar with the inner giant. For this reason, warm Jupiters will have evident companions whereas hot Jupiters will not. [Spalding & Batygin 2017]

Whether giant planets form in situ near their hosts or migrate inward, they can still have close-in, co-transiting companions outside of their orbit shortly after their birth, Spalding and Batygin argue. But after the disk in which they were born dissipates, the orbits of their companions may be altered.

The authors demonstrate that because hot Jupiters are so close to their hosts, these giants eventually encounter a resonance with their stellar hosts’ quadrupole moment, which arises because rotating stars aren’t perfectly spherical. This resonance tilts the orbits of the hot Jupiters’ outer, lower-mass companions, rendering the companions undetectable in transit surveys.

Warm Jupiters, on the other hand, are located just far enough away from their hosts to avoid feeling the effects of this resonance — which allows them to keep their outer companions in the same plane.

Based on their model, Spalding and Batygin make direct predictions for the systems they expect to be observed in large upcoming surveys like the Transiting Exoplanet Survey Satellite (TESS) — which means we should soon have a sense of whether their picture is correct. If it is, it will confirm that the non-sphericity of stars can have significant impact on the dynamics and architecture of exoplanetary systems.

Citation

Christopher Spalding and Konstantin Batygin 2017 AJ 154 93. doi:10.3847/1538-3881/aa8174

Eagle Nebula

Molecular clouds — which you’re likely familiar with from stunning popular astronomy imagery — lead complicated, tumultuous lives. A recent study has now found that these features must be rapidly built and destroyed.

Star-Forming Collapse

Carina Nebula

A Hubble view of a molecular cloud, roughly two light-years long, that has broken off of the Carina Nebula. [NASA/ESA, N. Smith (University of California, Berkeley)/The Hubble Heritage Team (STScI/AURA)]

Molecular gas can be found throughout our galaxy in the form of eminently photogenic clouds (as featured throughout this post). Dense, cold molecular gas makes up more than 20% of the Milky Way’s total gas mass, and gravitational instabilities within these clouds lead them to collapse under their own weight, resulting in the formation of our galaxy’s stars.

How does this collapse occur? The simplest explanation is that the clouds simply collapse in free fall, with no source of support to counter their contraction. But if all the molecular gas we observe collapsed on free-fall timescales, star formation in our galaxy would churn a rate that’s at least an order of magnitude higher than the observed 1–2 solar masses per year in the Milky Way.

Destruction by Feedback

Astronomers have theorized that there may be some mechanism that supports these clouds against gravity, slowing their collapse. But both theoretical studies and observations of the clouds have ruled out most of these potential mechanisms, and mounting evidence supports the original interpretation that molecular clouds are simply gravitationally collapsing.

Taurus molecular cloud

A sub-mm image from ESO’s APEX telescope of part of the Taurus molecular cloud, roughly ten light-years long, superimposed on a visible-light image of the region. [ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2. Acknowledgment: Davide De Martin]

If this is indeed the case, then one explanation for our low observed star formation rate could be that molecular clouds are rapidly destroyed by feedback from the very stars they create. But to match with observations, this would suggest that molecular clouds are short-lived objects that are built (and therefore replenished) just as quickly as they are destroyed. Is this possible?

Speedy Building?

In a recent study, a team of scientists led by Mordecai-Mark Mac Low (American Museum of Natural History and Heidelberg University, Germany) explore whether there is a way to create molecular clouds rapidly enough to match the necessary rate of destruction.

Mac Low and collaborators find that some common mechanisms used to explain the formation of molecular clouds — like gas being swept up by supernovae — can’t quite operate quickly enough to combat the rate of cloud destruction. On the other hand, the Toomre gravitational instability, which is a large-scale gravitational instability that occurs in gas disks, can very rapidly assemble gas into clumps dense enough to form molecules.

Barnard 68

A composite of visible and near-infrared images from the VLT ANTU telescope of the Barnard 68 molecular cloud, roughly half a light-year in diameter. [ESO]

A Rapid Cycle

Based on their findings, the authors argue that dense, star-forming molecular clouds persist only for a short time before collapsing into stars and then being blown apart by stellar feedback — but these very clouds are built equally quickly via gravitational instabilities.

Conveniently, this model has a very testable prediction: the Toomre instability is expected to become even stronger at higher redshift, which suggests that the fraction of gas in the form of molecules should increase at high redshifts. This appears to agree with observations, supporting the authors’ picture of a rapid cycle of cloud assembly and destruction.

Citation

Mordecai-Mark Mac Low et al 2017 ApJL 847 L10. doi:10.3847/2041-8213/aa8a61

compact binary

Ultraluminous X-ray sources (ULXs) are astronomical sources of X-rays that, while dimmer than active galactic nuclei, are nonetheless brighter than any known stellar process. What are these beasts and why do they shine so brightly?

Exceeding the Limit

First discovered in the 1980s, ULXs are rare sources that have nonetheless been found in all types of galaxies. Though the bright X-ray radiation seems likely to be coming from compact objects accreting gas, there’s a problem with this theory: ULXs outshine the Eddington luminosity for stellar-mass compact objects. This means that a stellar-mass object couldn’t emit this much radiation isotropically without blowing itself apart.

There are two alternative explanations commonly proposed for ULXs:

  1. Rather than being accreting stellar-mass compact objects, they are accreting intermediate-mass black holes. A hypothetical black hole of 100 solar masses or more would have a much higher Eddington luminosity than a stellar-mass black hole, making the luminosities that we observe from ULXs feasible.
  2. route to ULX

    An example of one of the common routes the authors find for a binary system to become a ULX. In this case, the binary begins as two main sequence stars. As one star evolves off the main sequence, the binary undergoes a common envelope phase and a stage of mass transfer. The star ends its life as a supernova, and the resulting neutron star then accretes matter from the main sequence star as a ULX. [Wiktorowicz et al. 2017]

    They are ordinary X-ray binaries (a stellar-mass compact object accreting matter from a companion star), but they are undergoing a short phase of extreme accretion. During this time, their emission is beamed into jets, making them appear brighter than the Eddington luminosity.

Clues from a New Discovery

A few years ago, a new discovery shed some light on ULXs: M82 X-2, a pulsing ULX. Two more pulsing ULXs have been discovered since then, demonstrating that at least some ULXs contain pulsars — i.e., neutron stars — as the accreting object. This provided strong support for the second model of ULXs as X-ray binaries with super-Eddington luminosity.

But could this model in fact account for all ULXs? A team of authors led by Grzegorz Wiktorowicz (Kavli Institute for Theoretical Physics, UC Santa Barbara and Warsaw University, Poland) says yes.

number of ULXs over time

Time evolution of the number of ULXs since the beginning of star formation, for a star formation burst (left panels) and continuous star formation (right panels), and for solar-metallicity (top panels) and low-metallicity (bottom panels) environments. The heavy solid line shows ULXs with black-hole accretors, the dashed line ULXs with neutron-star accretors, and the solid line the total. [Wiktorowicz et al. 2017]

No Exotic Objects Needed

Wiktorowicz and collaborators performed a massive suite of simulations — made possible by donated computer time from the Universe@Home project — to examine how 20 million binary systems evolve into X-ray binaries. They then determined the number and nature of the ones that could appear as ULXs to us. The authors’ results show that the vast majority of the observed population of ULXs can be accounted for with super-Eddington compact binaries, without needing to invoke intermediate-mass black holes.

Wiktorowicz and collaborators demonstrate that in environments with short star-formation bursts, black-hole accretors are the most common ULX source in the early periods after the burst, but neutron-star accretors dominate the ULX population after a few 100 Myr. In the case of prolonged and continuous star formation, neutron-star accretors dominate ULXs if the environment is solar metallicity, whereas black-hole accretors dominate in low-metallicity environments.

The authors’ results present very clear and testable relations between the companion and donor star evolutionary stage and the age of the system, which we will hopefully be able to use to test this model with future observations of ULXs.

Citation

Grzegorz Wiktorowicz et al 2017 ApJ 846 17. doi:10.3847/1538-4357/aa821d

binary black hole merger

Today promises to be an exciting day in the world of gravitational-wave detections. To keep with the theme, we thought we’d use this opportunity to take a renewed look at an interesting question about the Laser Interferometer Gravitational-Wave Observatory (LIGO) and dark matter: if dark matter is made up of primordial black holes, will LIGO be able to detect them?

LIGO black-hole binary detections

The masses of the black-hole binaries detected by LIGO thus far. [LIGO/Caltech/Sonoma State (Aurore Simonnet)]

Black Holes in the Early Universe

The black holes we generally think about in the context of gravitational-wave detections are black holes formed by the collapse of massive stars. Indeed, LIGO’s detections have thus far been of merging black holes weighing between 7 and 35 solar masses — the perfect sizes to have formed from massive-star collapses.

But another type of black hole has also been proposed: primordial black holes, which theoretically formed in the early universe as a result of the direct collapse of density fluctuations during the Big Bang. These proposed black holes could have initial masses anywhere from an impossibly small 10-8 kg (that’s ~10-38 solar masses) to thousands of solar masses.

Dark Matter as Black Holes

Could dark matter — the missing, unseen matter in our universe — be composed of as-yet-undetected primordial black holes? Over several decades, scientists have narrowed down possible mass ranges for these hypothetical black holes based on models and observations. In a recent publication, Pennsylvania State University scientists Ryan Magee and Chad Hanna now consider these past constraints together with LIGO’s observations thus far.

Magee and Hanna suggest a population of primordial black holes with a mass distribution peaking between 0.06 and 1 solar mass, which they argue could account for all of the missing dark matter. The authors’ model for this population is consistent with LIGO’s observations, as well as the strongest constraints placed by microlensing observations.

LIGO’s Performance

LIGO

The Hanford (top) and Livingston (bottom) LIGO facilities, which work together to detect gravitational-wave signals. [Caltech/MIT/LIGO Lab]

Would LIGO be able to detect the black holes the authors predict? In theory, LIGO is or will be sensitive to signals from the mergers of black holes between ~0.01 and 100 solar masses, even out to extragalactic distances.

Magee and Hanna offer a prediction to test their model: with the mass distribution of black holes predicted by their model, 1% of LIGO’s detections would be of black holes less massive than our Sun. Since such small black holes can’t be formed by stellar collapses, such a detection would be a smoking gun supporting the model of primordial black holes as dark matter.

Based on LIGO’s specs, this prediction suggests that if Magee and Hanna’s model is correct, then within one year of operating advanced LIGO at design sensitivity, we will have found signs of a primordial black hole mass distribution. Now we just have to wait and see how things pan out!

Citation

Ryan Magee and Chad Hanna 2017 ApJL 845 L13. doi:10.3847/2041-8213/aa831c

hypervelocity star

Speeding stars running away from our galaxy pose an intriguing puzzle: where did these stars come from, and how were they accelerated to their great speeds? The recent discovery of two new runaway stars have increased the mystery.

Unexplained Speeders

LAMOST

The LAMOST telescope, located in Xinglong Station, China. [Sheliak]

Hypervelocity stars are rare objects that zip along at unusually high speeds — fast enough to escape the gravitational pull of our galaxy. More than 20 hypervelocity stars have been discovered since the first one was found serendipitously in 2005. But what accelerates these strange stars?

One of the most commonly proposed scenarios is that these objects originated near the center of the Milky Way, and were flung out as a result of dynamical interactions with the central supermassive black hole. Other explanations exist, however — for instance, these stars could be the tidal debris of an accreted and disrupted dwarf galaxy, or they could be the surviving companion stars kicked out in Type Ia supernovae.

Besides wanting to better understand the origin of hypervelocity stars, scientists also care about these speedy objects because of the information they provide about the Milky Way. Measuring the three-dimensional motions of hypervelocity stars can give us a detailed look at the mass distribution of our galaxy — thereby revealing the shape of the Milky Way’s dark matter halo.

Hypervelocity star speeds

The radial velocities and locations of the three LAMOST-detected hypervelocity stars (red), compared to the other 24 known hypervelocity stars (blue). The dashed lines represent two models for the galactic escape velocity curve. [Huang et al. 2017]

Searching for More Hypervelocity Stars

For these reasons, scientists have conducted a number of systematic searches for hypervelocity stars to build up our sample size. Results from the most recent of these searches, conducted by examining the spectroscopic survey data of 6.5 million stars from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), have now been described in a publication led by Yang Huang (Yunnan University and Peking University).

Huang and collaborators narrowed the LAMOST data down to 126 high-mass hypervelocity star candidates. Using distance measurements, they determined the stars’ velocities in the galactic rest frame and eliminated all stars not moving faster than the galactic escape speed. This left three true hypervelocity stars: one that had been previously found in another study, and two that are new discoveries.

Conflicting Results

Hypervelocity star locations

The spatial distribution of the confirmed hypervelocity stars, with the LAMOST detections shown in red and the other 24 known hypervelocity stars shown in blue. The great circles represent planes of young stellar structures near the galactic center. [Huang et al. 2017]

The authors show that the three detected hypervelocity stars are spatially associated with known young stellar structures near the galactic center, supporting a galactic-center origin for hypervelocity stars. But they also find that the time it would have taken two of these stars to travel to their current locations from the galactic center is longer than the stars’ expected lifetimes, posing a new puzzle.

Huang and collaborators suggest that upcoming accurate proper motion measurements of these stars, expected in the next data release from the Gaia mission, will provide direct constraints on their origins. In the meantime, continued systematic searches for hypervelocity stars such as that presented here will ensure that we have a large sample of these speeding objects ready for Gaia’s analysis.

Citation

Y. Huang et al 2017 ApJL 847 L9. doi:10.3847/2041-8213/aa894b

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

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