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SMBH

Supermassive black holes (SMBHs) lurk in the centers of galaxies, and we’ve measured their masses to range from hundreds of thousands to ten billion solar masses. But is there a maximum mass that these monsters are limited to?

Observed Maximum

Since the era when the first SMBHs formed, enough time has passed for them to potentially grow to monstrous size, assuming a sufficient supply of fuel.

Instead, however, we observe that SMBHs in the centers of the largest local-universe galaxies max out at a top mass of a few times 1010 solar masses. Even more intriguingly, this limit appears to be redshift-independent: we see the same maximum mass of a few 1010 solar masses for SMBHs fueling the brightest of quasars at redshifts up to z~7.

accretion rates

Accretion rate (solid) and star formation rate (dashed) vs. radius in a star-forming accretion disk, for several different values of black-hole mass. Though accretion rates start out very high at large radius, they drop to just a few solar masses per year at small radii, because much of the gas is lost to star formation in the disk. [Inayoshi & Haiman 2016]

So why don’t we see any giants larger than around 10 billion solar masses, regardless of where we look? Two astronomers from Columbia University, Kohei Inayoshi (Simons Fellow) and Zoltán Haiman, suggest that there is a limiting mass for SMBHs that’s set by small-scale physical processes, rather than large processes like galaxy evolution, star formation history, or background cosmology.

Challenges for Accretion

Growing an SMBH that’s more massive than 1010 solar masses requires gas to be quickly funneled from the outer regions of the galaxy (hundreds of light-years out), through the large accretion disk that surrounds the black hole, and into the nuclear region (light-year scales): the gas must be brought in at rates as high as 1,000 solar masses per year.

Modeling this process, Inayoshi and Haiman demonstrate that at such high rates, the majority of the gas instead gets stuck in the disk, causing star formation at radii of tens to hundreds of light-years and never getting close enough to fuel the SMBH. The remaining trickle of gas that does accrete onto the SMBH is not enough to allow it to grow to more than 1011 solar masses in the age of the universe.

Cygnus A

Cygnus A provides a stunning example of the tremendous jets that can be launched from SMBHs at the center of galaxies. [NRAO]

What’s more, for a large enough SMBH, this trickle of gas can become so small relative to the black hole mass that the physics of the accretion itself changes, causing the inner disk to puff up and launching strong outflows and jets. Once this transition occurs, the black-hole feeding is suppressed, preventing the SMBH from growing any larger.

The authors show that the critical mass for this transition is 1–6 x 1010 solar masses — consistent with the maximum masses that we’ve observed for SMBHs in the wild. This consistency supports the idea that the small-scale physics around the SMBH may be setting its size limit, rather than the large-scale environment around the galaxy.

Citation

Kohei Inayoshi and Zoltán Haiman 2016 ApJ 828 110. doi:10.3847/0004-637X/828/2/110

Phobos and Deimos

A new study examines the possibility that Mars’s two moons formed after a large body slammed into Mars, creating a disk of debris. This scenario might be the key to reconciling the moons’ orbital properties with their compositions.

Conflicting Evidence

Formation scenarios

The different orbital (left) and spectral (right) characteristics of the Martian moons in the three different formation scenarios. Click for a better look! Phobos and Deimos’s orbital characteristics are best matched by formation around Mars (b and c), and their physical characteristics are best matched by formation in the outer region of an impact-generated accretion disk (rightmost panel of c). [Ronnet et al. 2016]

How were Mars’s two moons, Phobos and Deimos, formed? There are three standing theories:

  1. Two already-formed, small bodies from the outer main asteroid belt were captured by Mars, intact.
  2. The bodies formed simultaneously with Mars, by accretion from the same materials.
  3. A large impact on Mars created an accretion disk of material from which the two bodies formed.

Our observations of the Martian moons, unfortunately, provide conflicting evidence about which of these scenarios is correct. The physical properties of the moons — low albedos, low densities — are consistent with those of asteroids in our solar system, and are not consistent with Mars’s properties, suggesting that the co-accretion scenario is unlikely. On the other hand, the moons’ orbital properties — low inclination, low eccentricity, prograde orbits — are consistent with bodies that formed around Mars rather than being captured.

In a recent study, a team of scientists led by Thomas Ronnet and Pierre Vernazza (Aix-Marseille University, Laboratory of Astrophysics of Marseille) has attempted to reconcile these conflicting observations by focusing on the third option.

Moons After a Large Impact

In the third scenario, an impactor of perhaps a few percent of Mars’s mass smashed into Mars, forming a debris disk of hot material that encircled Mars. Perturbations in the disk then led to the formation of large clumps, which eventually agglomerated to form Phobos and Deimos.

Circum-Mars accretion disk

The authors find that Phobos and Deimos most likely formed in the outer regions of the accretion disk that was created by a large impact with Mars. [Adapted from Ronnet et al. 2016]

In the study conducted by Ronnet, Vernazza, and collaborators, the authors investigated the composition and texture of the dust that would have crystallized in an impact-generated accretion disk making up Mars’s moons. They find that Phobos and Deimos could not have formed out of the extremely hot, magma-filled inner regions of such a disk, because this would have resulted in different compositions than we observe.

Phobos and Deimos could have formed, however, in the very outer part of an impact-generated accretion disk, where the hot gas condensed directly into small solid grains instead of passing through the magma phase. Accretion of such tiny grains would naturally explain the similarity in physical properties we observe between Mars’s moons and some main-belt asteroids — and yet this picture is also consistent with the moons’ current orbital parameters.

The authors argue that the formation of the Martian moons from the outer regions of an impact-generated accretion disk is therefore a plausible scenario, neatly reconciling the observed physical properties of Phobos and Diemos with their orbital properties.

Citation

T. Ronnet et al 2016 ApJ 828 109. doi:10.3847/0004-637X/828/2/109

NS merger

Where do the heavy elements — the chemical elements beyond iron — in our universe come from? One of the primary candidate sources is the merger of two neutron stars, but recent observations have cast doubt on this model. Can neutron-star mergers really be responsible?

Elements from Collisions?

element origins

Periodic table showing the origin of each chemical element. Those produced by the r-process are shaded orange and attributed to supernovae in this image; though supernovae are one proposed source of r-process elements, an alternative source is the merger of two neutron stars. [Cmglee]

When a binary-neutron-star system inspirals and the two neutron stars smash into each other, a shower of neutrons are released. These neutrons are thought to bombard the surrounding atoms, rapidly producing heavy elements in what is known as r-process nucleosynthesis.

So could these mergers be responsible for producing the majority of the universe’s heavy r-process elements? Proponents of this model argue that it’s supported by observations. The overall amount of heavy r-process material in the Milky Way, for instance, is consistent with the expected ejection amounts from mergers, based both on predicted merger rates for neutron stars in the galaxy, and on the observed rates of soft gamma-ray bursts (which are thought to accompany double-neutron-star mergers).

Challenges from Ultra-Faint Dwarfs

Recently, however, r-process elements have been observed in ultra-faint dwarf satellite galaxies. This discovery raises two major challenges to the merger model for heavy-element production:

  1. When neutron stars are born during a core-collapse supernova, mass is ejected, providing the stars with asymmetric natal kicks. During the second collapse in a double-neutron-star binary, wouldn’t the kick exceed the low escape velocity of an ultra-faint dwarf, ejecting the binary before it could merge and enrich the galaxy?
  2. Ultra-faint dwarfs have very old stellar populations — and the observation of r-process elements in these stars requires mergers to have occurred very early in the galaxy’s history. Can double-neutron-star systems merge quickly enough to account for the observed chemical enrichment?

Small Kicks and Fast Mergers

kick velocities

Fraction of double-neutron-star systems that remain bound, vs. the magnitude of the kick they receive. A typical escape velocity for an ultra-faint dwarf is ~15 km/s; roughly 55-65% of binaries receive smaller kicks than that and wouldn’t be ejected from an ultra-faint dwarf. [Beniamini et al. 2016]

Led by Paz Beniamini, a team of scientists from the Racah Institute of Physics at the Hebrew University of Jerusalem has set out to answer these questions. Using the statistics of our galaxy’s double-neutron-star population, the team performed Monte Carlo simulations to estimate the distributions of mass ejection and kick velocities for the systems.

Beniamini and collaborators find that, for typical initial separations, more than half of neutron star binaries are born with small enough kicks that they remain bound and aren’t ejected — even from small, ultra-faint dwarf galaxies.

The team also used their statistics to calculate the time until merger for the population of binaries, finding that ~90% of the double-neutron-star systems merge within 300 Myr, and around 15% merge within 100 Myr — quick enough to enrich even the old population of stars.

This population of systems that remain confined to the galaxy and merge rapidly can therefore explain the observations of r-process material in ultra-faint dwarf galaxies. Beniamini and collaborators’ work suggests that the merger of neutron stars is indeed a viable model for the production of heavy elements in our universe.

Citation

Paz Beniamini et al 2016 ApJ 829 L13. doi:10.3847/2041-8205/829/1/L13

blue straggler/white dwarf

A new study has examined how the puzzling wide binary system HS 2220+2146 — which consists of two white dwarfs orbiting each other — might have formed. This system may be an example of a new evolutionary pathway for wide white-dwarf binaries.

Evolution of a Binary

More than 100 stellar systems have been discovered consisting of two white dwarfs in a wide orbit around each other. How do these binaries form? In the traditional picture, the system begins as a binary consisting of two main-sequence stars. Due to the large separation between the stars, the stars evolve independently, each passing through the main-sequence and giant branches and ending their lives as white dwarfs.

hierarchical triple

An illustration of a hierarchical triple star system, in which two stars orbit each other, and a third star orbits the pair. [NASA/JPL-Caltech]

Because more massive stars evolve more quickly, the most massive of the two stars in a binary pair should be the first to evolve into a white dwarf. Consequently, when we observe a double-white-dwarf binary, it’s usually a safe bet that the more massive of the two white dwarfs will also be the older and cooler of the pair, since it should have formed first.

But in the case of the double-white-dwarf binary HS 2220+2146, the opposite is true: the more massive of the two white dwarfs appears to be the younger and hotter of the pair. If it wasn’t created in the traditional way, then how did this system form?

Two From Three?

Led by Jeff Andrews (Foundation for Research and Technology-Hellas, Greece and Columbia University), a team of scientists recently examined this system more carefully, analyzing its spectra to confirm our understanding of the white dwarfs’ temperatures and masses.

Based on their observations, Andrews and collaborators determined that there are no hidden additional companions that could have caused the unusual evolution of this system. Instead, the team proposed that this unusual binary might be an example of an evolutionary channel that involves three stars.

formation model

The authors’ proposed formation scenario for H220+2146. In this picture, the inner binary merges to form a blue straggler. This star and the remaining main-sequence star then evolve independently into white dwarfs, forming the system observed today. [Andrews et al. 2016]

An Early Merger

In the model the authors propose for HS 2220+2146, the binary system began as a hierarchical triple system of main-sequence stars. The innermost binary then merged to form a large star known as a “blue straggler” — a star that, due to the merger, will evolve more slowly than its larger mass implies it should.

The blue straggler and the remaining main-sequence star, still in a wide orbit, then continued to evolve independently of each other. The smaller star ended its main-sequence lifetime and became a white dwarf first, followed by the more massive but slowly evolving blue straggler — thus forming the system we observe today.

If the authors’ model is correct, then HS 2220+2146 would be the first binary double white dwarf known to have formed through this channel. ESA’s Gaia mission, currently underway, is expected to discover up to a million new white dwarfs, many of which will likely be in wide binary systems. Among these, we may well find many other systems like HS 2220+2146 that formed in the same way.

Citation

Jeff J. Andrews et al 2016 ApJ 828 38. doi:10.3847/0004-637X/828/1/38

comet swarm

KIC 8462860

Photometric time series for a neighboring star that’s 25” NNW of Boyajian’s Star. No significant long-term dimming is seen — which constrains the size of potential material obscuring Boyajian’s Star. [Wright et al. 2016/Benjamin Montet]

What’s causing the mysterious light-curve dips of the so-called “alien megastructure” star, Boyajian’s Star? A recent study analyzes a variety of possible explanations to determine which ones are the most plausible.

An Unusual Light Curve

Earlier this year, astronomer Tabetha Boyajian reported on the unusual light curve of the star KIC 8462852. This star, now nicknamed “Tabby’s Star” or “Boyajian’s Star”, shows unusual dips on day-long timescales that are too large to be explained by planet transits or similar phenomena.

In addition to these short dips in luminosity, recent observations have also indicated that the star has faded by roughly 20% over the past hundred years. What could be causing both the short-term dips in the star’s light and the long-term dimming over a century?

Alien megastructures

Could the dimming be caused by an ‘alien megastructure’ built by an extraterrestrial civilization? The authors find that a spherical structure is very unlikely. [Danielle Futselaar/SETI International]

Alien Megastructures? Or Another Explanation?

Boyajian’s Star was vaulted into the media spotlight when astronomer Jason Wright (Pennsylvania State University and University of California, Berkeley) proposed that its unusual light curve could potentially be explained by a surrounding megastructure built by an extraterrestrial civilization.

Now Wright is back with co-author Steinn Sigurd̵sson (Pennsylvania State University). In a new study, Wright and Sigurd̵sson analyze an extensive list of explanations for the puzzling apparent behavior of Boyajian’s Star, based on our latest knowledge about this strange object.

The Realm of Possibilities

Here are just a few possible causes of Boyajian’s Star’s dimming, as well as the authors’ assessment of their plausibility. For the full list, see the authors’ original article, or check out Wright’s own summary of the article here!

  1. Pulsations, polar spots, and other stellar variability: unlikely
    The authors show that the variety of timescales observed for dimming events make scenarios involving stellar variations unlikely.
  2. Circumstellar material: unlikely
    Material orbiting the star (like comets) would explain some of the light-curve dips, but it can’t explain the long-term dimming observed.
  3. Post-merger return to normal: unclear
    Perhaps Boyajian’s Star recently merged with a brown dwarf or other star? Now it could be gradually dimming as it returns to its normal brightness, and restructuring of the star’s material could cause the short-term dips. Though this scenario is possible, the timescales for the brightness changes are shorter than we would expect.
  4. Artificial structures: unclear
    Spherical swarms of structures would intercept the star’s light and re-radiate it in infrared. Since long-wavelength observations have found no evidence of such radiation, the authors declare spherical geometries to be unlikely. Other structure geometries can’t yet be ruled out, though.
  5. Small-scale interstellar medium (ISM) structure: plausible
    Small-scale density variations in the ISM between us and Boyajian’s Star could cause the dimming we observe, but the fact that nearby stars don’t show similar dimming sets tight limits on the size of such ISM clumps.
SED

Spectral energy distribution of Boyajian’s Star. The upper-limit arrows on the right-hand side indicate that big clouds of megastructures are unlikely, because we would detect their heat as they re-radiate the star’s light in infrared. [Wright et al. 2016]

Looking to the Future

Of the possible locations for the source of the dimming, Wright and Sigurd̵sson deem the interstellar space between us and Boyajian’s Star to be the most likely culprit. They identify several future lines of research that could help us further eliminate possibilities, however, including a study of the ISM toward Boyajian’s Star, a hunt for similar variations in stars near in the sky to Boyajian’s Star, and infrared observation of the star with JWST to search for heat signatures.

Citation

Jason T. Wright and Steinn Sigurd̵sson 2016 ApJ 829 L3. doi:10.3847/2041-8205/829/1/L3

CME at Earth

Coronal mass ejections (CMEs), enormous releases of energy from the Sun, can have significant space-weather implications for Earth. Do similar storms from smaller stars — M dwarfs like V374 Peg, or the nearby Proxima Centauri — mean bad news for the planets that these stars host?

Volatile Stars

Habitable zones

Difference in habitable-zone sizes for different stellar types. [NASA]

When plasma is released from the Sun in the form of a CME traveling toward Earth, these storms can be powerful enough to disrupt communications and navigational equipment, damage satellites, and cause blackouts — even with our planetary magnetic field to protect us! How might planets in the habitable zone of M-dwarf stars fare against similar storms?

The first danger for an M dwarf’s planets is that the habitable zone lies much closer to the star: it can range from 0.03 to 0.4 AU (i.e., within Mercury’s orbit). Being so close to the star definitely makes a planet in an M dwarf’s habitable zone vulnerable to storms.

Probability of impact

Colors indicate the probability of CME impact, for different different stellar latitudes where the CME originated vs. orbital inclination of the planet, (a) without any deflection, and (b) taking into account the CME deflection by the star’s magnetic field. Hanging out in an orbit aligned with the current sheet turns out to be a bad idea. [Adapted from Kay et al. 2016]

What about the storms themselves? You might think that because M dwarfs are cooler stars, they would be quieter, releasing fewer CMEs with less energy. Surprisingly, the opposite is true: M dwarfs are significantly more active than solar-type stars, and the CMEs are typically ten times more massive than those released from the Sun. Impacts from these powerful outbursts could easily strip any existing planet atmosphere, making a planet much less likely to be habitable. To make matters worse, M dwarfs can remain magnetically active for billions of years: even a star like Proxima Centauri, which is nearly 5 billion years old, is still relatively active.

Dodging Deflected Storms

Interestingly, an important factor in the survival of an M dwarf’s habitable-zone planet is the plane in which the planet’s orbit lies. A team of scientists led by Christina Kay (NASA Goddard’s Solar Physics Laboratory and Boston University) recently modeled CMEs from V374 Peg, a mid-type M dwarf of roughly a third of the Sun’s mass and radius, to determine how the CMEs propagate and the probability that they’ll impact a hypothetical planet in the star’s habitable zone.

The team shows that traveling CMEs tend to be deflected by the star’s magnetic field. Instead of propagating purely radially outward, the CMEs are pushed toward the astrospheric current sheet — the minimum point of the background magnetic field — which moves around, but is generally located toward the stellar equatorial plane.

Kay and collaborators find that planet orbits roughly aligned with the current sheet therefore have a higher probability of getting hit by a CME: around 10%. In contrast, planets with higher-inclination orbits have CME impact probabilities around 1%. These probabilities translate to an impact rate of about 0.5–5 times per day for a habitable-zone planet around a mid-type M dwarf — which is 2–20 times the average at Earth during solar maximum!

Magnetic field strength needed

Minimum planetary magnetic field strength required to sustain a magnetosphere twice the size of the planetary radius for different CME masses and speeds, for a 1 kG (left) and 20 kG (right) initial CME magnetic field strength. A typical CME requires a field strength of 10–100 G. [Adapted from Kay et al. 2016]

Is There Hope for Planet Habitability?

With this many CME impacts even outside of the current-sheet plane, how can a planet hope to survive? The key lies in having a strong magnetic field to protect the planet. Such a field would deflect the charged particles from the CME, preventing the CME from stripping the planet’s atmosphere.

Kay and collaborators calculate that a habitable-zone mid-type M-dwarf exoplanet would need a planetary magnetic field between tens and hundreds of Gauss — 1 to 2 orders of magnitude more than that of Earth — to protect itself from these CMEs: difficult to muster, but not impossible!

These results provide some interesting food for thought as we continue to discover new exoplanets orbiting M-dwarf stars.

Citation

C. Kay et al 2016 ApJ 826 195. doi:10.3847/0004-637X/826/2/195

filament

A team of scientists has now uncovered half of the entire “skeleton” of the Milky Way, using an automated method to identify large filaments of gas and dust hiding between stars in the galactic plane.

galactic distribution

Galactic distribution of 54 newly discovered filaments, plotted along with colored lines indicating six relevant spiral arms in our galaxy. The upper two plots show the consistency of the filaments’ motion with the spiral arms, while the lower shows their location within the galactic plane. [Wang et al. 2016]

The Search for Nessie and Friends

The Milky Way’s interstellar medium is structured hierarchically into filaments. These structures are difficult to observe since they largely lie in the galactic plane, but if we can discover the distribution and properties of these filaments, we can better understand how our galaxy formed, and how the filaments affect star formation in our galaxy today.

Some of the largest of the Milky Way’s filaments are hundreds of light-years long — like the infrared dark cloud nicknamed “Nessie”, declared in 2013 to be one of the “bones” of the Milky Way because of its position along the center of the Scutum-Centaurus spiral arm.

Follow-up studies since the discovery of Nessie (like this one, or this) have found a number of additional large-scale filaments, but these studies all use different search methods and selection criteria, and the searches all start with visual inspection — by humans — to identify candidates.

What if we could instead automate the detection process and build a homogeneous sample of the large filaments making up the skeleton of the Milky Way?

Automated Detection

This is exactly what a team of astronomers led by Ke Wang (European Southern Observatory) has done. The group used a customization of an algorithm called a “minimum spanning tree” — the technique used to optimize the cost of internet networks, road networks, and electrical grids in our communities — to perform an automated search of data from the Bolocam Galactic Plane Survey. The search was designed to identify long filaments that are coherent both in physical and velocity space.

Using this method, Wang and collaborators found a total of 54 large-scale filaments that met all of their criteria. The survey covered nearly half of the galactic plane, and the team estimates that there may be a total of ~200 large-scale filaments like these in the Milky Way.

masses and lengths of new filaments

Histograms of the mass and length of the newly discovered filaments (N=54). The distributions for the filaments that are bones (N=13) are overplotted in red. [Adapted from Wang et al. 2016]

A Catalog of Bones and More

The authors generated a catalog of the newly discovered filaments, determining properties like their masses (1,000–100,000 solar masses), lengths (30–900 light-years), aspect ratios, temperatures, and more. They then used this catalog to make several statistical observations:

  1. The filaments are widely distributed across the galactic disk, with roughly 50% located within 65 light-years of the galactic plane (for reference, the Sun is 82 light-years “above” the galactic plane).
  2. Roughly a 1/3 of the filaments are part of the Milky Way’s skeleton, lying along the centers of our galaxy’s spiral arms.
  3. Around 1% of the molecular interstellar medium in our galaxy is confined in large filaments like these.
  4. The formation of massive stars occurs more favorably in large filaments, compared to elsewhere in our galaxy.

This catalog is an important building block in our understanding of the structure of the interstellar medium of our galaxy. The authors next plan to extend this census to the rest of our galaxy, providing us with the best picture yet of the skeleton of the Milky Way.

Bonus

Check out all 54 of the filaments discovered by Wang and collaborators in the gif below (or follow the link to the article to view the original images)! Submillimeter dust emission is shown in red, and Spitzer/WISE 24/22 µm emission is shown in cyan. The connected dots show how the filament was identified by the minimum spanning tree algorithm.

gif of all filaments

Citation

Ke Wang (王科) et al 2016 ApJS 226 9. doi:10.3847/0067-0049/226/1/9

Chariklo rings

We’ve recently discovered narrow sets of rings around two minor planets orbiting in our solar system. How did these rings form? A new study shows that they could be a result of close encounters between the minor planets and giants like Jupiter or Neptune.

Unexpected Ring Systems

Positions of the centaurs in our solar system (green). Giant planets (red), Jupiter trojans (grey), scattered disk objects (tan) and Kuiper belt objects (blue) are also shown. [WilyD]

Positions of the centaurs in our solar system (green). Giant planets (red), Jupiter trojans (grey), scattered disk objects (tan) and Kuiper belt objects (blue) are also shown. [WilyD]

Centaurs are minor planets in our solar system that orbit between Jupiter and Neptune. These bodies — of which there are roughly 44,000 with diameters larger than 1 km — have dynamically unstable orbits that cross paths with those of one or more giant planets.

Recent occultation observations of two centaurs, 10199 Chariklo and 2060 Chiron, revealed that these bodies both host narrow ring systems. Besides our four giant planets, Chariklo and Chiron are the only other bodies in the solar system known to have rings. But how did these rings form?

Scientists have proposed several models, implicating collisions, disruption of a primordial satellite, or dusty outgassing. But a team of scientists led by Ryuki Hyodo (Paris Institute of Earth Physics, Kobe University) has recently proposed an alternative scenario: what if the rings were formed from partial disruption of the centaur itself, after it crossed just a little too close to a giant planet?

Tidal Forces from a Giant

Hyodo and collaborators first used past studies of centaur orbits to estimate that roughly 10% of centaurs experience close encounters (passing within a distance of ~2x the planetary radius) with a giant planet during their million-year lifetime. The team then performed a series of simulations of close encounters between a giant planet and a differentiated centaur — a body in which the rocky material has sunk to form a dense silicate core, surrounded by an icy mantle.

simulation outcomes

Some snapshots of simulation outcomes (click for a closer look!) for different initial states of the centaur internal structure, its spin, and the distance of closest approach of the centaur to the giant planet. Blue and red represent icy and silicate material, respectively. [Hyodo et al. 2016]

The outcomes of the close encounters are diverse, depending strongly on the internal structure and spin of the minor planet and the geometry of the encounter. But the team finds that, in many scenarios, the centaur is only partially destroyed by tidal forces from the giant as it passes close by.

In these cases the icy mantle and even some of the centaur’s core can be ripped away and scattered, becoming gravitationally bound to the largest remaining clump of the core. The particles travel in highly eccentric orbits, gradually damping as they collide with each other and forming a disk around the remaining core. Further dynamical evolution of this disk could easily shape the rings that we observe today around Chariklo and Chiron.

If Hyodo and collaborators’ scenario is correct, then Chariklo and Chiron are differentiated bodies with dense silicate cores, and their rings are either pure water ice, or a mixture of water ice and a small amount of silicate. Future observations of these minor planets will help to test this model — and observations of other centaurs may discover yet more ring systems hiding in our solar system!

Bonus

Check out this awesome animation from ESO showing an artist’s impression of the ring system around Chariklo! [ESO/L. Calçada/M. Kornmesser]

Citation

Ryuki Hyodo et al 2016 ApJ 828 L8. doi:10.3847/2041-8205/828/1/L8

coronal hole

Coronal holes are where the fast solar wind streams out of the Sun’s atmosphere, sending charged particles on rapid trajectories out into the solar system. A new study examines how the distribution of coronal holes has changed over the last 40 years.

coronal hole schematic

Coronal holes form where magnetic field lines open into space (B) instead of looping back to the solar surface (A). [Sebman81]

Source of the Fast Solar Wind

As a part of the Sun’s natural activity cycle, extremely low-density regions sometimes form in the solar corona. These “coronal holes” manifest themselves as dark patches in X-ray and extreme ultraviolet imaging, since the corona is much hotter than the solar surface that peeks through from underneath it.

Coronal holes form when magnetic field lines open into space instead of looping back to the solar surface. In these regions, the solar atmosphere escapes via these field lines, rapidly streaming away from the Sun’s surface in what’s known as the “fast solar wind”.

Coronal Holes Over Space and Time

Automated detection of coronal holes from image-based analysis is notoriously difficult. Recently, a team of scientists led by Ken’ichi Fujiki (ISEE, Nagoya University, Japan) has developed an automated prediction technique for coronal holes that relies instead on magnetic-field data for the Sun, obtained at the National Solar Observatory’s Kitt Peak between 1975 and 2014. The team used these data to produce a database of 3335 coronal hole predictions over nearly 40 years.

distribution of coronal holes

Latitude distribution of 2870 coronal holes (each marked by an x; color indicates polarity), overlaid on the magnetic butterfly map of the Sun. The low-latitude coronal holes display a similar butterfly pattern, in which they move closer to the equator over the course of the solar cycle. Polar coronal holes are more frequent during solar minima. [Fujiki et al. 2016]

Examining trends in the coronal holes’ distribution in latitude and time, Fujiki and collaborators find a strong correlation between the total area covered by low-latitude coronal holes (holes closer to the Sun’s equator) and sunspot activity. In contrast, the total area of high-latitude coronal holes (those near the Sun’s poles) peaks around the minimum in each solar cycle and shrinks around each solar maximum.

Predicting the Impact of the Solar Wind

Why do these observations matter? Coronal holes are the source of the fast solar wind, so if we can better predict the frequency and locations of coronal holes in the future, we can make better predictions about how the solar wind might impact us here on Earth.

periodicity

Periodicity of high-latitude (orange) and low-latitude (blue) coronal-hole areas, and periodicity of galactic cosmic rays detected at Earth (black). The cosmic rays track the polar coronal-hole area behavior with a 1-year time lag. [Fujiki et al. 2016]

In one example of this, Fujiki and collaborators show that there’s a distinct correlation between polar coronal-hole area and observed galactic cosmic rays. Cosmic rays from within our galaxy have long been known to exhibit a 22-year periodicity. Fujiki and collaborators show that the periodicity of the galactic cosmic-ray activity tracks that of the polar coronal-hole area, with a ~1-year lag time — which is equivalent to the propagation time of the solar wind to the termination shock.

Polar coronal holes are therefore a useful observable indicator of the dipole component of the solar magnetic field, which modulates the incoming cosmic rays entering our solar system. This coronal hole database will be a useful tool for understanding the source of solar wind and the many ways the wind influences the Earth and our solar system.

Citation

K. Fujiki et al 2016 ApJ 827 L41. doi:10.3847/2041-8205/827/2/L41

dark matter

Could the dark matter in our universe be “warm” instead of “cold”? Recent observations have placed new constraints on the warm dark matter model.

What’s the Deal with Cold/Warm/Hot Dark Matter?

MACHOs

An example of cold dark matter: MACHOs, massive objects like black holes that are hiding in the halo of our galaxy. [Alain r]

Nobody knows what dark matter is made of, but we have a few theories. The objects or particles that could make up dark matter fall into three broad categories — cold, warm, and hot dark matter — based on something called their “free streaming length,” or how far they moved due to random motions in the early universe.

Neutrinos are an example of hot dark matter: very light particles with free streaming lengths much longer than the size of a typical galaxy. Cold dark matter could consist of objects like black holes or brown dwarfs, or particles like WIMPs — all of which are very heavy and therefore have free streaming lengths much shorter than the size of a galaxy.

Warm dark matter is what’s in between: middle-mass particles with free streaming lengths roughly the size of a galaxy. There aren’t any known particles that fit this description, but there are theorized particles such as sterile neutrinos or gravitinos that do.

WDM model vs obs 1

Cumulative mass functions at z = 6 for different values of the warm dark matter particle mass mX. The shaded boxs on the left correspond to the observed number density of faint galaxies within different confidence levels. [Menci et al. 2016]

Smoothing Out the Universe

The widely favored model is lambda-CDM, in which cold dark matter makes up the missing matter in our universe. This model nicely explains much of what we observe, but it still has a few problems. The biggest issue with lambda-CDM is that it predicts that there should be many more small, dwarf galaxies than we observe.

While this could just mean that we haven’t yet managed to see all the existing, faint dwarf galaxies, we should also consider alternative models — the warm dark matter model chief among them.

In the early universe, small density perturbations on sub-galactic scales produce dwarf galaxies in the lambda-CDM model. But in the warm dark matter model, the longer free streaming length of the dark matter particles smooth out some of those small perturbations. This results in the formation of fewer dwarf galaxies — which fits better with our current observations.

Limits on Warm Dark Matter

So how can we test this alternative model? The maximum number density of dark-matter halos predicted by the warm dark matter model at a given redshift depends on the mass of the candidate dark matter particle: a larger particle mass means that more halos form. We therefore can set lower limits on the mass of dark matter particles in a two-step process:

  1. Calculate the maximum number density of dark matter halos predicted by models, and
  2. Compare this to the measured abundance of the faintest galaxies at a given redshift.
WDM model vs obs 2

Another way of looking at it: for different values of the dark matter particle mass mX, this shows the maximum number density of dark matter halos predicted at z = 6. The shaded areas represent the observed number density of faint galaxies at different confidence levels. [Menci et al. 2016]

Recently, unprecedented new Hubble observations of ultra-faint, lensed galaxies in the Hubble Frontier Fields at z~6 have allowed for the discovery of more faint galaxies at this redshift than ever before. Now, a team of scientists led by Nicola Menci (INAF Rome) have used these observations to set a new limit on the lowest mass that candidate dark matter particles can have.

Menci and collaborators find that these new observations constrain the particle masses to be above 2.9 keV at the 1σ confidence level. These constitute the tightest constraints on the mass of candidate warm dark matter particles derived to date, and they even allow us to rule out some production mechanisms for theorized particles.

Extending this analysis to other clusters with deep observations will only improve the constraints, bringing us ever closer to understanding what dark matter is made of.

Citation

N. Menci et al 2016 ApJ 825 L1. doi:10.3847/2041-8205/825/1/L1

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