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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

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

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