Features RSS

TDE

What happens when a magnetized star is torn apart by the tidal forces of a supermassive black hole, in a violent process known as a tidal disruption event? Two scientists have broken new ground by simulating the disruption of stars with magnetic fields for the first time.

partial disruption simulation

The magnetic field configuration during a simulation of the partial disruption of a star. Top left: pre-disruption star. Bottom left: matter begins to re-accrete onto the surviving core after the partial disruption. Right: vortices form in the core as high-angular-momentum debris continues to accrete, winding up and amplifying the field. [Adapted from Guillochon & McCourt 2017]

What About Magnetic Fields?

Magnetic fields are expected to exist in the majority of stars. Though these fields don’t dominate the energy budget of a star — the magnetic pressure is a million times weaker than the gas pressure in the Sun’s interior, for example — they are the drivers of interesting activity, like the prominences and flares of our Sun.

Given this, we can wonder what role stars’ magnetic fields might play when the stars are torn apart in tidal disruption events. Do the fields change what we observe? Are they dispersed during the disruption, or can they be amplified? Might they even be responsible for launching jets of matter from the black hole after the disruption?

Star vs. Black Hole

In a recent study, James Guillochon (Harvard-Smithsonian Center for Astrophysics) and Michael McCourt (Hubble Fellow at UC Santa Barbara) have tackled these questions by performing the first simulations of tidal disruptions of stars that include magnetic fields.

In their simulations, Guillochon and McCourt evolve a solar-mass star that passes close to a million-solar-mass black hole. Their simulations explore different magnetic field configurations for the star, and they consider both what happens when the star barely grazes the black hole and is only partially disrupted, as well as what happens when the black hole tears the star apart completely.

Amplifying Encounters

For stars that survive their encounter with the black hole, Guillochon and McCourt find that the process of partial disruption and re-accretion can amplify the magnetic field of the star by up to a factor of 20. Repeated encounters of the star with the black hole could amplify the field even more.

The authors suggest an interesting implication of this idea: a population of highly magnetized stars may have formed in our own galactic center, resulting from their encounters with the supermassive black hole Sgr A*.

turbulent magnetic field

A turbulent magnetic field forms after a partial stellar disruption and re-accretion of the tidal tails. [Adapted from Guillochon & McCourt 2017]

Effects in Destruction

For stars that are completely shredded and form a tidal stream after their encounter with the black hole, the authors find that the magnetic field geometry straightens within the stream of debris. There, the pressure of the magnetic field eventually dominates over the gas pressure and self-gravity.

Guillochon and McCourt find that the field’s new configuration isn’t ideal for powering jets from the black hole — but it is strong enough to influence how the stream interacts with itself and its surrounding environment, likely affecting what we can expect to see from these short-lived events.

These simulations have clearly demonstrated the need to further explore the role of magnetic fields in the disruptions of stars by black holes.

Bonus

Check out the full (brief) video from one of the simulations by Guillochon and McCourt (be sure to watch it in high-res!). It reveals the evolution of a star’s magnetic field configuration as the star is partially disrupted by the forces of a supermassive black hole and then re-accretes.

Citation

James Guillochon and Michael McCourt 2017 ApJL 834 L19. doi:10.3847/2041-8213/834/2/L19

SN Refsdal

Type Ia supernovae that have multiple images due to gravitational lensing can provide us with a wealth of information — both about the supernovae themselves and about our surrounding universe. But how can we find these rare explosions?

Clues from Multiple Images

When light from a distant object passes by a massive foreground galaxy, the galaxy’s strong gravitational pull can bend the light, distorting our view of the background object. In severe cases, this process can cause multiple images of the distant object to appear in the foreground lensing galaxy.

gravitational lensing

An illustration of gravitational lensing. Light from the distant supernova is bent as it passes through a giant elliptical galaxy in the foreground, causing multiple images of the supernova to appear to be hosted by the elliptical galaxy. [Adapted from image by NASA/ESA/A. Feild (STScI)]

Observations of multiply-imaged Type Ia supernovae (explosions that occur when white dwarfs in binary systems exceed their maximum allowed mass) could answer a number of astronomical questions. Because Type Ia supernovae are standard candles, distant, lensed Type Ia supernovae can be used to extend the Hubble diagram to high redshifts. Furthermore, the lensing time delays from the multiply-imaged explosion can provide high-precision constraints on cosmological parameters.

The catch? So far, we’ve only found one multiply-imaged Type Ia supernova: iPTF16geu, discovered late last year. We’re going to need a lot more of them to develop a useful sample! So how do we identify the mutiply-imaged Type Ias among the many billions of fleeting events discovered in current and future surveys of transients?

Searching for Anomalies

SNe Ia absolute magnitudes

Absolute magnitudes for Type Ia supernovae in elliptical galaxies. None are expected to be above -20 in the B band, so if we calculate a magnitude for a Type Ia supernova that’s larger than this, it’s probably not hosted by the galaxy we think it is! [Goldstein & Nugent 2017]

Two scientists from University of California, Berkeley and Lawrence Berkeley National Laboratory have a plan. In a recent publication, Daniel Goldstein and Peter Nugent propose the following clever procedure to apply to data from transient surveys:

  1. From the data, select only the supernova candidates that appear to be hosted by quiescent elliptical galaxies.
  2. Use the host galaxies’ photometric redshifts to calculate absolute magnitudes for the supernovae in this sample.
  3. Select from this only the supernovae above the maximum absolute magnitude expected for Type Ia supernovae.

Supernovae selected in this way are likely tricking us: their apparent hosts are probably not their hosts at all! Instead, the supernova is likely behind the galaxy, and the galaxy is just lensing its light. Using this strategy therefore allows us to select supernova candidates that are most likely to be distant, gravitationally lensed Type Ia supernovae.

ZTF and LSST finds

Redshift distribution of the multiply-imaged Type Ia supernovae the authors estimate will be detectable by ZTF and LSST in their respective 3- and 10-year survey durations. [Goldstein & Nugent 2017]

A convenient aspect of Goldstein and Nugent’s technique is that we don’t need to be able to resolve the lensed multiple images for discovery. This is useful, because ground-based optical surveys don’t have the resolution to see the separate images — yet they’ll still be useful for discovering multiply-imaged supernovae.

Future Prospects

How useful? Goldstein and Nugent use Monte Carlo simulations to estimate how many multiply-imaged Type Ia supernovae will be discoverable with future survey projects. They find that the Zwicky Transient Facility (ZTF), which will begin operating this year, should be able to find up to 10 using this technique in a 3-year search. The Large Synoptic Survey Telescope (LSST), which should start operating in 2022, will be able to find around 500 multiply-imaged Type Ia supernovae in a 10-year survey.

Citation

Daniel A. Goldstein and Peter E. Nugent 2017 ApJL 834 L5. doi:10.3847/2041-8213/834/1/L5

hot Jupiter

Two new, large gas-giant exoplanets have been discovered orbiting close to their host stars. A recent study examining these planets — and others like them — may help us to better understand what happens to close-in hot Jupiters as their host stars reach the end of their main-sequence lives.

Oversized Giants

HAT-P-65b

Unbinned transit light curves for HAT-P-65b. [Adapted from Hartman et al. 2016]

The discovery of HAT-P-65b and HAT-P-66b, two new transiting hot Jupiters, is intriguing. These planets have periods of just under 3 days and masses of roughly 0.5 and 0.8 times that of Jupiter, but their sizes are what’s really interesting: they have inflated radii of 1.89 and 1.59 times that of Jupiter.

These two planets, discovered using the Hungarian-made Automated Telescope Network (HATNet) in Arizona and Hawaii, mark the latest in an ever-growing sample of gas-giant exoplanets with radii larger than expected based on theoretical planetary structure models.

What causes this discrepancy? Did the planets just fail to contract to the expected size when they were initially formed, or were they reinflated later in their lifetimes? If the latter, how? These are questions that scientists are only now starting to be able to address using statistics of the sample of close-in, transiting planets.

HAT-P-66b

Unbinned transit light curves for HAT-P-66b. [Hartman et al. 2016]

Exploring Other Planets

Led by Joel Hartman (Princeton University), the team that discovered HAT-P-65b and HAT-P-66b has examined these planets’ observed parameters and those of dozens of other known close-in, transiting exoplanets discovered with a variety of transiting exoplanet missions: HAT, WASP, Kepler, TrES, and KELT. Hartman and collaborators used this sample to draw conclusions about what causes some of these planets to have such large radii.

The team found that there is a statistically significant correlation between the radii of close-in giant planets and the fractional ages of their host stars (i.e., the star’s age divided by its full expected lifetime). The two newly discovered hot Jupiters with inflated radii, for instance, are orbiting stars that are roughly 84% and 83% through their life spans and are approaching the main-sequence turnoff point.

Late-Life Reinflation

host star age vs. planet radius

Fractional age of the host stars of close-in transiting exoplanets vs. the planet’s radius. There is a statistically significant correlation between age and planet radius. [Adapted from Hartman et al. 2016]

Hartman and collaborators propose that the data support the following scenario: as host stars evolve and become more luminous toward the ends of their main-sequence lifetimes, they deposit more energy deep into the interiors of the planets closely orbiting them. These close-in planets then increase their equilibrium temperatures — and their radii reinflate as a result.

Based on these results, we would expect to continue to find hot Jupiters with inflated radii primarily orbiting closely around older stars. Conversely, close-in giant planets around younger stars should primarily have non-inflated radii. As we continue to build our observational sample of transiting hot Jupiters in the future, we will be able to see how this model holds up.

Citation

J. D. Hartman et al 2016 AJ 152 182. doi:10.3847/0004-6256/152/6/182

Exomoon

In other solar systems, the radiation streaming from the central star can have a destructive impact on the atmospheres of the star’s close-in planets. A new study suggests that these exoplanets may also have a much harder time keeping their moons.

Where Are the Exomoons?

Moons are more common in our solar system than planets by far (just look at Jupiter’s enormous collection of satellites!) — and yet we haven’t made a single confirmed discovery of a moon around an planet outside of our solar system. Is this just because moons have smaller signals and are more difficult to detect? Or might there also be a physical reason for there to be fewer moons around the planets we’re observing?

Led by Ming Yang, a team of scientists from Nanjing University in China have explored one mechanism that could limit the number of moons we might find around exoplanets: photoevaporation.

photoevaporation

Artist’s illustration of the process of photoevaporation, in which the atmosphere of a planet is stripped by radiation from its star. [NASA Goddard SFC]

Effects of Radiation

Photoevaporation is a process by which the harsh high-energy radiation from a star blasts a close-in planet, imparting enough energy to the atoms of the planet’s atmosphere for those atoms to escape. As the planet’s atmosphere gradually erodes, significant mass loss occurs on timescales of tens or hundreds of millions of years.

How might this process affect such a planet’s moons? To answer this question, Yang and collaborators used an N-body code called MERCURY to model solar systems in which a Neptune-like planet at 0.1 AU gradually loses mass. The planet starts out with a large system of moons, and the team tracks the moons’ motions to determine their ultimate fates.

Escaping Bodies

moon orbit evolution

Evolution of the planet mass (top) in a simulation containing 500 small moons. The evolution of the semimajor axes of the moons (middle) and their eccentricities (bottom) are shown, with three example moons, starting at different radii, highlighted in blue, red and green. The black dotted line shows how the critical semimajor axis for stability evolves with time as the planet loses mass. [Yang et al. 2016]

Yang and collaborators find that the photoevaporation process has a critical impact on whether or not the moons remain in stable orbits. As the photoevaporation drives mass loss of the planet, the planet’s gravitational influence shrinks and the orbits of its exomoons expand and become more eccentric. Eventually these orbits can reach critical values where they’re no longer stable, often resulting in systems with only one or no surviving moons.

The team finds that even in the best-case scenario of only small moons, no more than roughly a quarter of them survive the simulation still in orbit around their planet. In simulations that include larger moons further out, the system is even more likely to become unstable as the planet loses mass, with more moons ultimately escaping.

What happens to the moons that escape? Some leave the planet–moon system to become planet-like objects that remain in orbit around the host star. Others are smashed to bits when they collide with other moons or with the planet. And some can even escape their entire solar system to become a free-floating object in the galaxy!

Based on their simulations, the authors speculate that exomoons are less common around planets that are close to their host stars (<0.1 AU). Furthermore, exomoons are likely less common in solar systems around especially X-ray-luminous stars (e.g., M dwarfs) that can more easily drive photoevaporation. For these reasons, our best chances for finding exomoons in future missions will be around stars that are more Sun-like, orbiting planets that aren’t too close to their hosts.

Citation

Ming Yang et al 2016 ApJ 833 7. doi:10.3847/0004-637X/833/1/7

Photograph of a blue planet.

How would the Kepler mission see a star like the Sun? We now know the answer to this question due to a creative approach: a new study has used the Kepler K2 mission to detect signals from the Sun reflected off of the surface of Neptune.

stellar oscillations

Asteroseismology uses different oscillation modes of a star to probe its internal structure and properties. [Tosaka]

Information in Oscillations

Kepler’s most glamorous work is in discovering new planets around other stars. To successfully do this, however, the spacecraft is also quietly doing a lot of very useful work in the background, characterizing the many stars in our vicinity that planets might be found around.

One of the ways Kepler gets information about these stars is from oscillations of the stars’ intensities. In asteroseismology, we look at oscillatory modes that are caused by convection-driven pressure changes on the inside of the star. All stars with near-surface convection oscillate like this — including the Sun — and by measuring the oscillations in intensity of these stars, we can make inferences about the stars’ properties.

A Planetary Mirror

We do this by first understanding our Sun’s oscillations especially well (made easier by the fact that it’s nearby!). Then we use asteroseimic scaling relations — determined empirically — that relate characteristics like mass and radius of other stars to those of the Sun, based on the relation between the stars’ oscillation properties to the Sun’s.

The trouble is, those oscillation properties are difficult to measure, and different instruments often measure different values. For this reason, we’d like to measure the Sun’s oscillations with the same instrument we use to measure other stars’ oscillations: Kepler.

light curve

Top panel: Kepler K2 49-day light curve of Neptune. Bottom panel: power density spectrum as a function of frequency (grey). Neptune’s rotation frequencies and harmonics appear toward the left side (blue); the excess power due to the solar modes is visible toward the bottom right. The green curve shows the direct observations of solar oscillations simultaneously made by VIRGO/SPM. [Gaulme et al. 2016]

A team led by Patrick Gaulme (New Mexico State University, New Mexico Institute of Mining and Technology, and Apache Point Observatory) have now done this — but not with direct Kepler observations of the Sun. Instead, Kepler was pointed at Neptune for a total of 49 days, during which time it measured the reflection of the Sun’s oscillations off of the planet’s surface. These observations mark the first indirect detection of solar oscillations in intensity.

Measuring Solar Properties

The success of this technique for observing solar oscillations represents a remarkable technical performance. The oscillations the team observed by Kepler in the reflection from Neptune are consistent with the solar oscillations that were measured directly with programs like the Birmingham Solar Oscillations Network (BiSON) and SOHO/VIRGO/SPM.

What can we learn from the oscillations? The authors treated the detection of the Sun as though it were any other star being observed: they used the asteroseismic scaling relations to estimate the star’s mass and radius. Based on the oscillations they measured, they found a mass for the Sun between 1.11 ± 0.05 and 1.16 ± 0.09 solar masses, and a radius between 1.04 ± 0.02 and 1.05 ± 0.03 solar radii.

The fact that these values are a little high (roughly 13.8% too high for mass and 4.3% for the radius) illustrates the highly stochastic nature of stellar oscillations. Still, it provides a useful reference point, and it also gives us a valuable look at how Kepler would see a star like the Sun.

Citation

P. Gaulme et al 2016 ApJL 833 L13. doi:10.3847/2041-8213/833/1/L13

galaxies around the Milky Way

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Parkes H I Zone of Avoidance Survey

Published February 2016

 

Main takeaway:

883 galaxies have been discovered within a few hundred million light-years of us, hiding behind the Milky Way. The galaxies were found by a team led by Lister Staveley-Smith (International Center for Radio Astronomy Research, University of Western Australia) using the 64-m Parkes radio telescope in Australia.

Zone of Avoidance galaxies

Distribution of the galaxies discovered in the Zone of Avoidance. Radial distance is measured by the recessional velocities of the galaxies. [Staveley-Smith et al. 2016]

Why it’s interesting:

These new galaxies were discovered in what’s known as the “Zone of Avoidance”, a gap that extends roughly 5° above and 5° below the galactic plane. The Zone of Avoidance has been excluded from many past surveys because the stars and dust of the Milky Way prevent us from being able to identify background galaxies in this region.  But the Parkes radio telescope — equipped with an innovative new receiver — was able to peer through the foreground of the Milky Way to detect the hidden galaxies behind it.

What this could teach us:

The discovery of hundreds of new galaxies may help explain the gravitational anomaly known as the Great Attractor region, a diffuse concentration of mass roughly 250 million light-years away that is pulling the Milky Way and hundreds of thousands of other galaxies toward it.

Citation

L. Staveley-Smith et al 2016 AJ 151 52. doi:10.3847/0004-6256/151/3/52

flare ribbons

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Nature of CME-Flare-Associated Coronal Dimming

Published June 2016

 

Main takeaway:

The Solar Dynamics Observatory (SDO) observed a large solar eruption at the end of December 2011. Scientists Jianxia Cheng (Shanghai Astronomical Observatory and the Chinese Academy of Sciences) and Jiong Qiu (Montana State University) studied this coronal mass ejection and the associated flaring on the Sun’s surface. They found that this activity was accompanied by dimming in the Sun’s corona near the ends of the flare ribbons.

Why it’s interesting:

The process of coronal dimming isn’t fully understood, but Cheng and Qiu’s observations provide a clear link between coronal dimming and eruptions of plasma and energy from the Sun. The locations of the dimming — the footpoints of the two flare ribbons — and the timing relative to the eruption suggests that coronal dimming is caused by the ejection of hot plasma from the Sun’s surface.

How this process was studied:

There are a number of satellites dedicated to observing the Sun, and several of them were used to study this explosion. Data from SDO’s Atmospheric Imaging Assembly (which images in extreme ultraviolet) and its Helioseismic and Magnetic Imager (which measures magnetic fields) were used — as well as observations from STEREO, the pair of satellites orbiting the Sun at ±90° from SDO.

Citation

J. X. Cheng and J. Qiu 2016 ApJ 825 37. doi:10.3847/0004-637X/825/1/37

Dragonfly 44

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra-Diffuse Galaxy Dragonfly 44

Published August 2016

 

Main takeaway:

Using the Keck Observatory and the Gemini North telescope in Hawaii, a team led by Pieter van Dokkum (Yale University) discovered the very dim galaxy Dragonfly 44, located in the Coma cluster. The team estimated the center of this galaxy’s disk to be a whopping 98% dark matter.

Why it’s interesting:

Dragonfly 44, though dim, was discovered to host around 100 globular clusters. Measuring the dynamics of these clusters allowed van Dokkum and collaborators to estimate the mass of Dragonfly 44: roughly a trillion times the mass of the Sun. This is similar to the mass of the Milky Way, and yet the Milky Way has over a hundred times more stars than this intriguing galaxy. It’s very unexpected to find a galaxy this massive that has a dark-matter fraction this high.

What we can learn from this:

How do ultra-faint galaxies like these form? One theory is that they’re “failed” normal galaxies: they have the sizes, dark-matter content, and globular cluster systems of much more luminous galaxies, but they were prevented from building up a normal stellar population. So far, Dragonfly 44’s properties seem consistent with this picture.

Citation

Pieter van Dokkum et al 2016 ApJL 828 L6. doi:10.3847/2041-8205/828/1/L6

IR background

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

LIGO Gravitational Wave Detection, Primordial Black Holes, and the Near-IR Cosmic Infrared Background Anisotropies

Published May 2016

 

Main takeaway:

A study by Alexander Kashlinsky (NASA Goddard SFC) proposes that the cold dark matter that makes up the majority of the universe’s matter may be made of black holes. These black holes, Kashlinsky suggests, are primordial: they collapsed directly from dense regions of the universe soon after the Big Bang.

Why it’s interesting:

This model would simultaneously explain several observations. In particular, we see similarities in patterns between the cosmic infrared and X-ray backgrounds. This would make sense if accretion onto primordial black holes in halos produced the X-ray background in the same regions where the first stars also formed, producing the infrared background.

What this means for current events:

In Kashlinsky’s model, primordial black holes would occasionally form binary pairs and eventually spiral in and merge. The release of energy from such an event would then be observable by gravitational-wave detectors. Could the gravitational-wave signal that LIGO detected last year have been two primordial black holes merging? More observations will be needed to find out.

Citation

A. Kashlinsky 2016 ApJL 823 L25. doi:10.3847/2041-8205/823/2/L25

triple-imaged galaxy

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Detection of Lyman-Alpha Emission from a Triply Imaged z = 6.85 Galaxy Behind MACS J2129.4−0741

Published May 2016

 

Main takeaway:

A team led by Kuang-Han Huang (University of Caliornia, Davis) discovered a faint galaxy at z = 6.846 located behind the galaxy cluster MACS J2129.4–0741. This galaxy contains only one ten-thousandth the stellar mass of the Milky Way, and it’s the faintest galaxy we’ve found at this great distance.

Why it’s interesting:

This galaxy is roughly 13 billion years old, placing it near the end of the reionization epoch (in which the first stars formed and caused our universe to transition from neutral gas to ionized gas). Examining such a small galaxy at this distance provides valuable information about how the process of reionization may have occurred.

About the discovery:

The newly discovered galaxy was found due to a fortunate alignment with a foreground galaxy cluster. Gravitational lensing by the foreground cluster produced three images of the distant galaxy, which were identified as being the same galaxy due to their similar spectra.

Citation

Kuang-Han Huang et al 2016 ApJL 823 L14. doi:10.3847/2041-8205/823/1/L14

1 … 76 77 78 79 80 … 96