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Pulsar

In 2006, pulsar PSR 1846–0258 unexpectedly launched into a series of energetic X-ray outbursts. Now a study has determined that this event may have permanently changed the behavior of this pulsar, raising questions about our understanding of how pulsars evolve.

Between Categories

A pulsar — a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation — can be powered by one of three mechanisms:

  1. Rotation-powered pulsars transform rotational energy into radiation, gradually slowing down in a predictable way.
  2. Accretion-powered pulsars convert the gravitational energy of accreting matter into radiation.
  3. Magnetars are powered by the decay of their extremely strong magnetic fields.

Astronomical classification often results in one pesky object that doesn’t follow the rules. In this case, that object is PSR 1846–0258, a young pulsar categorized as rotation-powered. But in 2006, PSR 1846–0258 suddenly emitted a series of short, hard X-ray bursts and underwent a flux increase — behavior that is usually only exhibited by magnetars. After this outburst, it returned to normal, rotation-powered-pulsar behavior.

Since the discovery of this event, scientists have been attempting to learn more about this strange pulsar that seems to straddle the line between rotation-powered pulsars and magnetars.

Unprecedented Drop

One way to examine what’s going on with PSR 1846–0258 is to evaluate what’s known as its “braking index,” a measure of how quickly the pulsar’s rotation slows down. For a rotation-powered pulsar, the braking index should be roughly constant. The pulsar then slows down according to a fixed power law, where the slower it rotates, the slower it slows down.

In a recent study, Robert Archibald (McGill University) and collaborators report on 7 years’ worth of timing observations of PSR 1846–0258 after its odd magnetar-like outburst. They then compare these observations to 6.5 years of data from before the outburst. The team finds that the braking index for this bizarre pulsar dropped suddenly by 14.5σ after the outburst — a change that’s unprecedented both in how large and how long-lived it’s been.

Why is this a problem? Many of the quoted properties of pulsars (like ages, magnetic fields, and luminosities) are determined based on models that envision pulsars as magnetic dipoles in a vacuum. But if this is the case, a pulsar’s braking index should be constant — or, in more realistic scenarios, we might expect it to change slightly over the span of thousands of years. The fact that PSR 1846–0258 underwent such a drastic change during its outburst poses a significant challenge to these models of pulsar behavior and evolution.

 

Citation

R. F. Archibald et al 2015 ApJ 810 67. doi:10.1088/0004-637X/810/1/67

Kepler Orrery

To date, we’ve discovered nearly 2000 confirmed exoplanets, as well as thousands of additional candidates. Amidst this vast sea of solar systems, how special is our own? A new study explores the answer to this question.

Analyzing Distributions

Knowing whether our solar system is unique among exoplanetary systems can help us to better understand future observations of exoplanets. Furthermore, if our solar system is typical, this allows us to be optimistic about the possibility of life existing elsewhere in the universe.

In a recent study, Rebecca Martin (University of Nevada, Las Vegas) and Mario Livio (Space Telescope Science Institute) examine how normal our solar system is, by comparing the properties of our planets to the averages obtained from known exoplanets.

Comparing Properties

So how do we measure up?

  1. Planet densities

    Densities of planets as a function of their mass. Exoplanets (N=287) are shown in blue, planets in our solar system are shown in red. [Martin&Livio 2015]

    Planet masses and densities
    Those of the gas giants in our solar system are pretty typical. The terrestrial planets are on the low side for mass, but that’s probably a selection effect: it’s very difficult to detect low-mass planets.
  2. Age of the solar system
    Roughly half the stars in the disk of our galaxy are younger than the Sun, and half are older. We’re definitely not special in age.
  3. Orbital locations of the planets
    This is actually a little strange: our solar system is lacking close-in planets. All of our planets, in fact, orbit at a distance that is larger than the mean distance observed in exoplanetary systems. Again, however, this might be a selection effect at work: it’s easier to detect large planets orbiting very close to their stars.
  4. Eccentricities of the planets’ orbits
    Our planets are on very circular orbits — and that actually makes us somewhat special too, compared to typical exoplanet systems. There is a possible explanation though: eccentricity of orbits tends to decrease with more planets in the system. Because we’ve got eight, it might be unsurprising that their eccentricities are so low.
  5. Super-Earths
    We don’t have any planets in the range of 1-10 times the mass of Earth, which is pretty unusual — super-Earths have a high occurrence rate among exoplanets.

In summary, the authors find that for the most part, we’re a pretty typical solar system. Our most unusual features are the lack of a super-Earth, the lack of any close-in planets, and the low eccentricities of our planets. The fact that we’re fairly average means that, from a habitability standpoint, there’s probably nothing special about our little corner of the galaxy. So perhaps life elsewhere is a possibility!

Citation

Rebecca G. Martin and Mario Livio 2015 ApJ 810 105. doi:10.1088/0004-637X/810/2/105

Collapsar

One of the big puzzles in astrophysics is how supermassive black holes (SMBHs) managed to grow to the large sizes we’ve observed in the very early universe. In a recent study, a team of researchers examines the possibility that they were formed by the direct collapse of supermassive stars.

Formation Mystery

SMBHs billions of times as massive as the Sun have been observed at a time when the universe was less than a billion years old. But that’s not enough time for a stellar-mass black hole to grow to SMBH-size by accreting material — so another theory is needed to explain the presence of these monsters so early in the universe’s history. A new study, led by Tatsuya Matsumoto (Kyoto University, Japan), poses the following question: what if supermassive stars in the early universe collapsed directly into black holes?

Previous studies of star formation in the early universe have suggested that, in the hot environment of these primordial times, stars might have been able to build up mass much faster than they can today. This could result in early supermassive stars roughly 100,000 times more massive than the Sun. But if these early stars end their lives by collapsing to become massive black holes — in the same way that we believe massive stars can collapse to form stellar-mass black holes today — this should result in enormously violent explosions. Matusmoto and collaborators set out to model this process, to determine what we would expect to see when it happens!

Energetic Bursts

The authors modeled the supermassive stars prior to collapse and then calculated whether a jet, created as the black hole grows at the center of the collapsing star, would be able to punch out of the stellar envelope. They demonstrated that the process would work much like the widely-accepted collapsar model of massive-star death, in which a jet successfully punches out of a collapsing star, violently releasing energy in the form of a long gamma-ray burst (GRB).

Because the length of a long GRB is thought to be proportional to the free-fall timescale of the collapsing star, the collapse of these supermassive stars would create much longer GRBs than are typical of massive stars today. Instead of the typical long-GRB length of ~30 seconds, these ultra-long GRBs would be 104–106 seconds.

Interestingly, we have already detected a small number of ultralong GRBs; they make up the tail end of the long GRB duration distribution. Could these detections be signals of collapsing supermassive stars in the early universe? According to the authors’ estimates, we could optimistically expect to detect roughly one of these events per year — so it’s entirely possible!

Citation

Tatsuya Matsumoto et al 2015 ApJ 810 64. doi:10.1088/0004-637X/810/1/64

Hot Jupiter with bow shock

As hot Jupiters whip around their host stars, their speeds can exceed the speed of sound in the surrounding material, theoretically causing a shock to form ahead of them. Now, a study has reported the detection of such a shock ahead of transiting exoplanet HD 189733b, providing a potential indicator of the remarkably strong magnetic field of the planet.

Rushing Planets

Due to their proximity to their hosts, hot Jupiters move very quickly through the stellar wind and corona surrounding the star. When this motion is supersonic, the material ahead of the planet can be compressed by a bow shock — and for a transiting hot Jupiter, this shock will cross the face of the host star in advance of the planet’s transit.

In a recent study, a team of researchers by Wilson Cauley of Wesleyan University report evidence of just such a pre-transit. The team’s target is exoplanet HD 189733b, one of the closest hot Jupiters to our solar system. When the authors examined high-resolution transmission spectra of this system, they found that prior to the optical transit of the planet, there was a large dip in the transmission of the first three hydrogen Balmer lines. This could well be the absorption of an optically-thick bow shock as it moves past the face of the star.

Tremendous Magnetism

Operating under this assumption, the authors create a model of the absorption expected from a hot Jupiter transiting with a bow shock ahead of it. Using this model, they show that a shock leading the planet at a distance of 12.75 times the planet’s radius reproduces the key features of the transmission spectrum.

This stand-off distance is surprisingly large. Assuming that the location of the bow shock is set by the point where the planet’s magnetospheric pressure balances the pressure of the stellar wind or corona that it passes through, the planetary magnetic field would have to be at least 28 Gauss. This is seven times the strength of Jupiter’s magnetic field!

Understanding the magnetic fields of exoplanets is important for modeling their interiors, their mass loss rates, and their interactions with their host stars. Current models of exoplanets often assume low-value fields similar to those of planets within our solar system. But if the field strength estimated for HD 189733b’s field is common for hot Jupiters, it may be time to update our models!

Bonus

Check out this video from Cauley’s website, which provides an action view of the transit data for HD 189733b and the possible bow shock leading it. The upper panel shows the transit as viewed from the side, the right panel shows a top-down view of the orbit, and the plot shows the transmission data (points) and model (solid lines) for the three hydrogen lines monitored. All sizes and distances are to scale.

Citation

P. Wilson Cauley et al 2015 ApJ 810 13. doi:10.1088/0004-637X/810/1/13

X-shaped radio source candidates

X-shaped radio galaxies (XRGs) have been used as a measure of the frequency with which supermassive black holes (SMBHs) in the centers of galaxies coalesce after galaxy mergers. But a new study suggests that this approach may be over-predicting the merger rate — as well as the gravitational wave background resulting from it.

Flipping Axes

The dominant contributor to the gravitational wave background in low frequencies is thought to be the coalescence of two SMBHs during galaxy mergers. Predictions of the strength of the gravitational wave background — and, accordingly, our ability to detect it — require us to know how often such mergers occur.

X examples

A closer examination of three candidate XRGs. Top panel: J0702+5002, a possible case of backflow deflection. Middle panel: J0845+4031, a possible case of jet-axis precession. Bottom panel: J1043+3131, a possible case of a genuine jet-axis reorientation due to SMBHs coalescing. [Roberts et al. 2015]

One approach to estimating merger frequency is to determine the abundance of extended radio sources with an X-shaped appearance. XRGs are predicted to contain SMBHs whose axes have been reoriented as a result of absorbing a second SMBH during a merger. Since these sources emit jets, we see both the old and the new axes of the emission, which form an X.

But there’s a problem with using XRGs to estimate the merger frequencies, according to a new study led by David Roberts of Brandeis University: many of the sources identified might not be caused by mergers. Roberts and his coauthors set out to examine and characterize the extended structure of known XRGs to determine how many are actually signposts of SMBH coalescence.

Alternate Causes

The authors used archival data from the Very Large Array, which was available for 52 sources identified as candidate XRGs in the NRAO FIRST survey. By examining the high-resolution images of the archival data, the authors determined that in most of the sources, the X shape was probably not caused by the sudden flip of jet axes during SMBH coalescence. Instead, it was caused either by slow precession or drift of the axes, or by deflection of backflows from the jets as they impact the thermal halo of the host galaxy.

In the end, only 11 of the 52 sources examined were determined to be genuine cases of sudden axis reorientations. This means only 1.3% of extended radio sources appear to be indicators of a recent galaxy merger — significantly lower than the 7% abundance predicted by previous studies. The merger rate, rather than being the ~1 Gyr-1 galaxy-1 previously predicted, is therefore closer to 0.13 Gyr-1 galaxy-1.

What can we learn from this about gravitational waves? If it’s indeed the case that the number of galaxy mergers has been overestimated by studies using XRGs as signposts, then the associated gravitational wave background may be substantially smaller than these studies have predicted.

Citation

David H. Roberts et al 2015 ApJ 810 L6. doi:10.1088/2041-8205/810/1/L6

Comet 67P

Comet 67P/Churyumov–Gerasimenko — made famous by the explorations of the Rosetta mission — has been displaying puzzling activity as it hurtles toward the Sun. However, recent modeling of the comet by a group of scientists from the Côte d’Azur University may now explain what’s causing 67P’s activity.

Shadowed Activity

67P temperature model

A model of comet 67P, with the colors indicating the rate of change of the temperature on the comet’s surface. The most rapid temperature changes are seen at the comet’s neck, in the same locations as the early activity seen in the Rosetta images. [Alí-Lagoa et al. 2015]

Between June and September of 2014, Rosetta observed comet 67P displaying “early activity” in the form of jets of dust emitted from near the “neck” of the comet (its narrowest point). Such activity is usually driven by the sublimation of volatiles from the comet’s surface as a result of sun exposure. But the neck of the comet is frequently shadowed as the comet rotates, and it receives significantly less sunlight than the rest of the comet. So why would the early activity originate from the comet’s neck?

The authors of a recent study, led by Victor Alí-Lagoa, hypothesize that it’s precisely because the neck is receiving alternating sunlight/shadows that it’s displaying activity. They suggest that thermal cracking of the surface of the comet is happening faster in this region, due to the rapid changes in temperature that result from the shadows cast by the surrounding terrain. The cracking exposes subsurface ices in the neck faster than in other regions, and the ensuing sublimation of that ice is what creates the activity we’re seeing.

Temperature Models

To test their hypothesis, the authors study the surface temperatures on comet 67P by means of a thermophysical model — a model used to calculate the temperatures on an airless body, both on and below the surface. The model takes into account factors like thermal inertia (how quickly the body’s temperature responds to changes in the incident energy), shadowing, and self-heating between parts of the surface in contact.

67P surface temperatures

Plot of the modeled temperature of two typical surfaces on the comet: one from the neck region (solid line) and one from the head region (dashed line). Unlike the head, the neck displays drastic drops in temperature as a result of shadowing. [Alí-Lagoa et al. 2015]

Using this model, the authors find that the temperatures behaved as they predicted: the shadows falling on the comet’s neck causes this region to experience very rapid temperature changes relative to the rest of the body. The authors also found a definite correlation between the regions of most rapid temperature variations and the regions of the comet that show signs of activity in Rosetta images. This provides strong evidence that thermal cracking is indeed taking place in the shadowed regions of the neck, gradually eroding away the surface.

Should this model prove correct, it’s a step toward understanding the evolution of comets like 67P. In addition, the results from this study imply that thermal cracking might happen faster than previously estimated in shadowed regions of other atmosphereless bodies, both near Earth and in the asteroid belt.

Citation

V. Alí-Lagoa et al 2015 ApJ 810 L22. doi:10.1088/2041-8205/810/2/L22

Cosmic web

Scientists have recently identified a connection between metal-poor regions in a set of dwarf galaxies and bursts of star-formation activity within them. These observations provide long-awaited evidence supporting predictions of how stars formed in the early universe and in dwarf galaxies today.

Metal-Poor Clues

The primary driver of star formation over cosmic history is thought to be the accretion onto galaxies of cold gas streaming from the cosmic web. The best way to confirm this model would be to observe a cloud of cosmic gas flowing into an otherwise-quiescent galaxy and launching a wave of star formation. But because cold gas doesn’t emit much radiation, it’s difficult to detect directly.

Now, a team of scientists have found a clever way around this problem: they searched galaxies for a correlation between areas of active star formation and metal-poor regions. Why? Because metal-poor regions could be a smoking gun indicating a recently accreted cloud of cold gas from the cosmic web.

Impacting Clouds

Metallicity and Star Formation Rates

Distribution of metallicity along the major axis of one of the target galaxies. The red bar in the top image shows the position of the spectrograph slit along the galaxy, with the arrow showing the direction of growing distance in the plot below. The plot shows the metallicity variation (red symbols) and star-formation rate (blue line) along the galaxy’s major axis. The metallicity drop coincides with the brightest knot of the galaxy. [Sánchez Almeida et al. 2015]

The authors of this study, led by Jorge Sánchez Almeida (Instituto de Astrofísica de Canarias and University of La Laguna, Spain), used the Great Canary Telescope to obtain high-quality spectra of ten dwarf galaxies with especially low average metallicities. They aligned the spectrograph slit along the major axes of the galaxies in order to measure abundances as a function of position within each galaxy.

The team found that, in nine out of the ten cases, the galaxies displayed sharp drops (by factors of 3–10) in metallicity along a portion of their lengths. The metallicity drops corresponded to bright knots representing starburst regions, in which surface star formation rates are larger than that of the rest of the galaxy by factors of 10–100.

The authors conclude that in these galaxies, a cold cosmic gas cloud with low metallicity impacted the galaxy’s outer region. This impact caused the cloud to compress, triggering the star formation we now observe. At the same time, the gas from the cloud diluted the region’s metallicity, resulting in the low abundances now measured. The authors determine that the cloud impacted within the last 100 Myr — otherwise enough time would have passed for the gas to mix azimuthally as it rotated around the galaxy.

 

Citation

J. Sánchez Almeida et al 2015 ApJ 810 L15. doi:10.1088/2041-8205/810/2/L15

Pre-transitional disk

The protoplanetary disk around DoAr 44 is fairly ordinary in most ways. But a recent study has found that this disk contains water vapor in its inner regions — the first such discovery for a disk of its type.

Drying Out Disks

DoAr 44 is a “transitional disk”: a type of protoplanetary disk that has been at least partially cleared of small dust grains in the inner regions of the disk. This process is thought to happen as a result of dynamical interactions with a protoplanet embedded in the disk; the planet clears out a gap as it orbits.

protoplanetary disks

A schematic of the differences between a full protoplanetary disk, a pre-transitional disk, and a transitional disk. [Catherine Espaillat]

Classical protoplanetary disks surrounding young, low-mass stars often contain water vapor, but transitional disks are typically “dry” — no water vapor is detected from the disk inner regions. This is probably because water vapor is easily dissociated by far-UV radiation from the young, hot star. Once the dust is cleared out from the inner regions of the disk, the water vapor is no longer shielded from the UV radiation, so the disk dries out.

Enter the exception: DoAr 44. The disk in this system doesn’t have a fully cleared inner region, which labels it “pre-transitional”. It’s composed of an inner ring out to 2 AU, a cleared gap between 2 and 36 AU, and then the outer disk. What makes DoAr 44 unusual, however, is that it’s the only disk with a large inner gap known to harbor detectable quantities of water vapor. The authors of this study ask a key question: where is this water vapor located?

Unusual System

Led by Colette Salyk (NOAO and Vassar College), the authors examined the system using the Texas Echelon Cross Echelle Spectrograph, a visiting instrument on the Gemini North telescope. They discovered that the water vapor emission originates from about 0.3 AU — the inner disk region, where terrestrial-type planets may well be forming.

Both dust-shielding and water self-shielding seem to have protected this water vapor from the harsh radiation of the central star, and the authors model this shielding to place constraints on the composition of the disk’s inner regions. They conclude that DoAr 44 has maintained similar physical and chemical conditions to classical protoplanetary disks in its terrestrial-planet forming regions, in spite of having formed a large gap.

Why has DoAr 44 succeeded at maintaining its water vapor, unlike other transition disks? The authors propose that gas might be migrating across the gap in the disk, replenishing the inner disk from the outer. Future observations are planned to help better understand the overall architecture of the gap, as well as the implications of these detections for any possible planets embedded in the disk.

Citation

Colette Salyk et al 2015 ApJ 810 L24. doi:10.1088/2041-8205/810/2/L24

Kuiper Belt

A feature of the Kuiper Belt known as the “kernel” has yet to be adequately explained by solar system formation models. In a recent study, a theorist at the Southwest Research Institute proposes a new explanation for how Neptune arrived at its current orbit — and how this planet’s migration in the early years of the solar system might have created the kernel.

Orbital Jump

The kernel is a concentration of orbits within the Kuiper Belt that all have semimajor axes of roughly a ≈ 44 AU, low eccentricities, and low inclinations. How this collection of objects formed — and why they exist where they do — is difficult to explain with current models, however. Kernel objects aren’t in resonance with any of the larger bodies, so why are they concentrated at that specific distance? In this study, David Nesvorný proposes that the kernel resulted from Neptune’s outward migration through the solar system.

In the currently favored model of our solar system’s formation, the outermost gas giant planets formed closer to the Sun and then migrated out to their current locations. Nesvorný ran a series of simulations of this migration to test the theory that a discontinuity in Neptune’s movement outward — i.e., a sudden jump in the planet’s orbital distance — could explain the presence of the Kuiper Belt’s kernel.

Gas Giant Orbits

Results from a previous study, in which the authors evolved the four gas giant planets plus a fifth giant planet (blue) initially on an orbit between Saturn and Uranus. At 18.3 Myr, a close encounter with the fifth planet causes Neptune’s orbit (pink) to jump outwards by ~0.4 AU, and the fifth planet is then ejected from the solar system by Jupiter. [Nesvorný 2015]

Resonant Population

Nesvorný was successful in finding a model that reproduced the kernel, as well as other observed features of the solar system today. In his model, Neptune began its journey closer to the Sun, at a distance of roughly 24 AU, and it migrated fairly rapidly outward to about 28 AU. As it traveled, it swept up bodies in the outer disk in a 2:1 resonance. These bodies migrated along with it, from an original location of around 40 AU out to 44 AU.

At this point, something occurred to cause Neptune’s orbit to suddenly jump outward by roughly half an AU. The 2:1 population, unable to keep up, were released from the resonance with Neptune here, and they remained orbiting at 44 AU to this day. These bodies are what we now observe as the Kuiper Belt kernel. Neptune, meanwhile, continued to migrate slowly, eventually reaching its current orbit of roughly 30 AU.

Fifth Planet

But what happened to make Neptune’s distance suddenly jump? Nesvorný speculates that Neptune may have had a close encounter with another giant planet — one not currently in our solar system. In a previous study, he inserted a fifth gas giant planet into his simulations with an initial orbit between Saturn and Uranus. Neptune’s orbit jumped by about half an AU when it interacted with the planet at 28 AU, and the fifth planet was subsequently ejected from the solar system by Jupiter.

Citation

David Nesvorný 2015 AJ 150 68. doi:10.1088/0004-6256/150/3/68

Tidal disruption event

In a tidal disruption event (TDE), an unfortunate star passes too close to a dormant supermassive black hole (BH) and gets torn apart by tidal forces, feeding the BH for a short time. Oddly, we’re not finding nearly as many TDEs — typically detected due to their distinctive observational signatures — as theory says we should. A recent study suggests that we might be missing many of these events, due to the way the streams of shredded stars fall onto the BHs.

Signatures of Shredding

When a BH tears a star apart, the star’s material is stretched out into what’s known as a tidal stream. That stream continues on a trajectory around the BH, with roughly half the material eventually falling back on the BH, whipping around it in a series of orbits. Where those orbits intersect each other, the material smashes together and circularizes, forming a disk that then accretes onto the BH.

What does a TDE look like? We don’t observe anything until after the tidal streams collide and the material begins to accrete onto the BH. At that point we observe a sudden peak in luminosity, which then gradually decreases (scaling roughly as time-5/3) as the tail end of what’s left of the star accretes and the BH’s food source eventually runs out.

So why have we only been observing about a tenth as many TDEs as theory predicts we should see? By studying the structure of tidal streams in TDEs, James Guillochon (Harvard-Smithsonian Center for Astrophysics) and Enrico Ramirez-Ruiz (UC Santa Cruz) have found a potential reason — and the culprit is general relativity.

Dark Years

The authors run a series of simulations of TDEs around black holes of varying masses and spins to see what form the resulting tidal streams take over time. They find that precession of the tidal stream due to the BH’s gravitational effects changes how the stream interacts with itself, and therefore what we observe. Some cases behave like what we expect for what’s currently considered a “typical” TDE — but some don’t.

tidal streams

Example from simulations of a tidal stream’s path around the black hole. Relativistic precession causes this tidal stream to wind around the black hole 13 times before it finally collides with itself. [Guillochon&Ramirez-Ruiz 2015]

For cases where the relativistic effects are small (such as BHs with masses less than a few 106 solar masses), the tidal stream collides with itself after only a few windings around the BH, quickly forming a disk. The disk forms far from the BH, however, so it takes a long time to accrete. As a result, the observed flare can take 100 times longer to peak than what’s typically expected for a TDE, so we might be failing to identify these sources as TDEs.

Furthermore, for cases where the BH is both massive and has a spin of a ≳ 0.2, the tidal stream doesn’t collide with itself right away. Instead, it can take many windings around the BH before the first intersection. In these cases, it may potentially be years after a star gets ripped apart before the material accretes and we’re able to observe the event!

 

Citation

James Guillochon and Enrico Ramirez-Ruiz 2015 ApJ 809 166. doi:10.1088/0004-637X/809/2/166

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