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stellar flare and exoplanet

As the exoplanet count continues to increase, we are making progressively more measurements of exoplanets’ outer atmospheres through spectroscopy. A new study, however, reveals that these measurements may be influenced by the planets’ hosts.

Spectra From Transits

Exoplanet spectra taken as they transit their hosts can tell us about the chemical compositions of their atmospheres. Detailed spectroscopic measurements of planet atmospheres should become even more common with the next generation of missions, such as the James Webb Space Telescope (JWST), or Planetary Transits and Oscillations of Stars (PLATO).

But is the spectrum that we measure in the brief moment of a planet’s transit necessarily representative of its spectrum all of the time? A team of scientists led by Olivia Venot (University of Leuven in Belgium) argue that it might not be, due to the influence of the planet’s stellar host.

atmospheric composition

Atmospheric composition of a planet before flare impacts (dotted lines), during the steady state reached after a flare impact (dashed lines), and during the steady state reached after a second flare impact (solid lines). [Venot et al. 2016]

The team suggests that when a host’s flares impact upon a planet’s atmosphere (especially likely in the case of active M-dwarfs that commonly harbor planetary systems), this activity may modify the chemical composition of the planet’s atmosphere. This would in turn alter the spectrum that we measure from the exoplanet.

Modeling Atmospheres

Venot and collaborators set out to test the effect of stellar flares on exoplanet atmospheres by modeling the atmospheres of two hypothetical planets orbiting the star AD Leo — an active and flaring M dwarf located roughly 16 light-years away — at two different distances. The team then examined what happened to the atmospheres, and to the resulting spectra that we would observe, when they were hit with a stellar flare typical of AD Leo.

absorption

The difference in relative absorption between the initial steady-state and the instantaneous transmission spectra, obtained during the different phases of the flare. The left plot examines the impulsive and gradual phases, when the flare first impacts and then starts to pass. The peak photon flux occurs at 912 seconds. The right plot examines the return to a steady state over 1012 seconds, or roughly ~30,000 years. [Adapted from Venot et al. 2016]

The authors found that the planets’ atmospheric compositions were significantly affected by the incoming stellar flare. The sudden increase in incoming photon flux changed the chemical abundances of several important molecular species, like hydrogen and ammonia — which resulted in changes to the spectrum that would be observed during the planet’s transit.

Permanent Impact

In addition to demonstrating that a planet’s atmospheric composition changes during and immediately after a flare impact, Venot and collaborators show that the chemical alteration isn’t temporary: the planet’s atmosphere doesn’t fully return to its original state after the flare passes. Instead, the authors find that it settles to a new steady-state composition that can be significantly different from the pre-flare composition.

For a planet that is repeatedly hit by stellar flares, therefore, its atmospheric composition never actually settles to a steady state. Instead it is continually and permanently modified by its host’s activity.

Venot and collaborators demonstrate that the variations of planetary spectra due to stellar flares should be easily detectable by future missions like JWST. We must therefore be careful about the conclusions we draw about planetary atmospheres from measurements of their spectra.

Citation

Olivia Venot et al 2016 ApJ 830 77. doi:10.3847/0004-637X/830/2/77

Andromeda

You might think that small satellite galaxies would be distributed evenly around their larger galactic hosts — but local evidence suggests otherwise. Are satellite distributions lopsided throughout the universe?

Satellites in the Local Group

The distribution of the satellite galaxies orbiting Andromeda, our neighboring galaxy, is puzzling: 21 out of 27 (~80%) of its satellites are on the side of Andromeda closest to us. In a similar fashion, 4 of the 11 brightest Milky Way satellites are stacked on the side closest to Andromeda.

It seems to be the case, then, that satellites around our pair of galaxies preferentially occupy the space between the two galaxies. But is this behavior specific to the Local Group? Or is it commonplace throughout the universe? In a recent study, a team of scientists led by Noam Libeskind (Leibniz Institute for Astrophysics Potsdam, Germany) set out to answer this question.

Properties of the galaxies included in the authors’ sample. Left: redshifts for galaxy pairs. Right: Number of satellite galaxies around hosts. [Adapted from Libeskind et al. 2016]

Properties of the galaxies included in the authors’ sample. Left: redshifts for galaxy pairs. Right: Number of satellite galaxies around hosts. [Adapted from Libeskind et al. 2016]

Asymmetry at Large

Libeskind and collaborators tested whether this behavior is common by searching through Sloan Digital Sky Survey observations for galaxy pairs that are similar to the Milky Way/Andromeda pair. The resulting sample consists of 12,210 pairs of galaxies, which have 46,043 potential satellites among them. The team then performed statistical tests on these observations to quantify the anisotropic distribution of the satellites around the host galaxies.

Libeskind and collaborators find that roughly 8% more galaxies are seen within a ~15° angle facing the other galaxy of a pair than would be expected in a uniform distribution. The odds that this asymmetric behavior is randomly produced, they show, are lower than 1 in 10 million — indicating that the lopsidedness of satellites around galaxies in pairs is a real effect and occurs beyond just the Local Group.

Caution for Modeling

Probability that satellites are located at an angle of θ degrees from the direction pointing toward the other galaxy in the pair. There are more satellites found in the space between the pair than predicted by a uniform distribution. [Libeskind et al. 2016]

Probability that satellites are located at an angle of θ degrees from the direction pointing toward the other galaxy in the pair. There are more satellites found in the space between the pair than predicted by a uniform distribution. [Libeskind et al. 2016]

What might cause this asymmetric distribution? The authors suggest the primary cause is that galaxies in pairs are not necessarily relaxed halos in equilibrium — a case in which spherical symmetry would apply. Instead, these are likely merging, dynamically active pairs of galaxies, so we cannot assume that they have axially symmetric halos.

Simulations of Local-Group-like pairs of galaxies will be the next step needed to understand how such asymmetries in the distribution of satellites form and evolve. Meanwhile, the results presented here suggest that the commonly adopted axially symmetric models of the Milky Way (and other galaxies in pairs) should be used with caution, as they may not be capturing the true shape of the halo.

Citation

Noam I. Libeskind et al 2016 ApJ 830 121. doi:10.3847/0004-637X/830/2/121

double pulsar

Have you been contributing your computer idle time to the Einstein@Home project? If so, you’re partly responsible for the program’s recent discovery of a new double-neutron-star system that will be key to learning about general relativity and stellar evolution.

Arecibo

The 305-m Arecibo Radio Telescope, built into the landscape at Arecibo, Puerto Rico. [NOAO/AURA/NSF/H. Schweiker/WIYN]

The Hunt for Pulsars

Observing binary systems containing two neutron stars — and in particular, measuring the timing of the pulses when one or both companions is a pulsar — can provide highly useful tests of general relativity and binary stellar evolution. Unfortunately, these systems are quite rare: of ~2500 known radio pulsars, only 14 of them are in double-neutron-star binaries.

To find more systems like these, we perform large-scale, untargeted radio-pulsar surveys — like the ongoing Pulsar-ALFA survey conducted with the enormous 305-m radio telescope at Arecibo Observatory in Puerto Rico. But combing through these data for the signature of a highly accelerated pulsar (the acceleration is a clue that it’s in a compact binary) is very computationally expensive.

pulse profile

PSR J1913+1102’s L-band pulse profile, created by phase-aligning and summing all observations. [Adapted from Lazarus et al. 2016]

To combat this problem, the Einstein@Home project was developed. Einstein@Home allows anyone to volunteer their personal computer’s idle time to help run the analysis of survey data in the search for pulsars. In a recent publication led by Patrick Lazarus (Max Planck Institute for Radio Astronomy), the Einstein@Home team announced the discovery of the pulsar PSR J1913+1102 — a member of what seems to be a brand new double-neutron-star system.

An Intriguing Discovery

Lazarus and collaborators followed up on the discovery to obtain timing measurements of the pulsar, which they found to have a spin period of 27.3 ms. They measured PSR J1913+1102 to be in a 4.95-hr, nearly circular (e~0.09) binary orbit with a massive companion that, based on its properties, is most likely another neutron star. The team wasn’t able to detect pulsations from the companion, but that could mean that its beam doesn’t cross the Earth, or it’s very faint, or it’s simply no longer active as a pulsar.

DNS orbital evolution

Orbital evolution of the six known double-neutron-star systems that will coalesce within a Hubble time, including J1913+1102 (black solid line). They move toward the origin as they lose energy to gravitational waves and approach merger. Shown are current positions (black dots), estimates of the positions when the compact binaries were formed (grey dots), and future evolution. [Lazarus et al. 2016]

Lazarus and collaborators use their observations of the system to argue that PSR J1913+1102 was likely spun up (“recycled”) by accretion of matter from its companion’s progenitor. The companion then exploded in the second supernova of the system, providing a very small kick — hence the low eccentricity of the system — and resulting in the current double-neutron-star binary we observe.

Lessons from PSR J1913+1102

Observations of compact binaries such as this one can reveal a wealth of information. Besides providing clues about how the binary evolved, precise timing measurements (now being made) will also allow powerful tests of general relativity. One of the measurements that may be possible by the end of this year will provide information about the orbital decay of the binary — expected to continue for ~0.5 Gyr until the system merges — due to the emission of gravitational waves.

In the meantime, you can bet that Einstein@Home will continue hunting for more systems like PSR J1913+1102 and its companion!

Citation

P. Lazarus et al 2016 ApJ 831 150. doi:10.3847/0004-637X/831/2/150

Milky Way

The Milky Way’s dense central bulge is a very different environment than the surrounding galactic disk in which we live. Do the differences affect the ability of planets to form in the bulge?

Exploring Galactic Planets

gravitational microlensing

Schematic illustrating how gravitational microlensing by an extrasolar planet works. [NASA]

Planet formation is a complex process with many aspects that we don’t yet understand. Do environmental properties like host star metallicity, the density of nearby stars, or the intensity of the ambient radiation field affect the ability of planets to form? To answer these questions, we will ultimately need to search for planets around stars in a large variety of different environments in our galaxy.

One way to detect recently formed, distant planets is by gravitational microlensing. In this process, light from a distant source star is bent by a lens star that is briefly located between us and the source. As the Earth moves, this momentary alignment causes a blip in the source’s light curve that we can detect — and planets hosted by the lens star can cause an additional observable bump.

Milky Way bulge

Artist’s impression of the Milky Way galaxy. The central bulge is much denser than the surrounding disk. [ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt]

Relative Abundances

Most source stars reside in the galactic bulge, so microlensing events can probe planetary systems at any distance between the Earth and the galactic bulge. This means that planet detections from microlensing could potentially be used to measure the relative abundances of exoplanets in different parts of our galaxy.

A team of scientists led by Matthew Penny, a Sagan postdoctoral fellow at Ohio State University, set out to do just that. The group considered a sample of 31 exoplanetary systems detected by microlensing and asked the following question: are the planet abundances in the galactic bulge and the galactic disk the same?

A Paucity of Planets

To answer this question, Penny and collaborators derived the expected distribution of host distances from a simulated microlensing survey, correcting for dominant selection effects. They then compared the distribution of distances in this model sample to the distribution of distances measured for the actual, observed systems.

planet hosts

Histogram and cumulative distribution (black lines) of distance estimates for microlensing planet hosts. Red lines show the distributions predicted by the model if the disk and bulge abundances were the same. [Penny et al. 2016]

Intriguingly, the two distributions don’t match when you assume that the planet abundances in the disk and the bulge are the same. The relative abundances appear to be higher in the disk than in the bulge, according to the team’s results: the observations agree with a model in which the bulge/disk abundance ratio is less than 0.54.

What’s to Blame?

There are a few ways to interpret this result: 1) distance measurements for the sample of planets discovered by microlensing have errors, 2) the model is too simplified; it needs to also include dependence of planet abundance and detection sensitivity on properties like host mass and metallicity, or 3) the galactic bulge actually has fewer planets than the disk.

Penny and collaborators suspect some combination of the first two interpretations is most likely, but an actual paucity of planets in the galactic bulge can’t be ruled out. Performing similar analysis on a larger sample of microlensing planets — expected from upcoming, second-generation microlensing searches — and obtaining more accurate distance measurements will help us to address this puzzle more definitively in the future.

Citation

Matthew T. Penny et al 2016 ApJ 830 150. doi:10.3847/0004-637X/830/2/150

SN 1994D

One of the major challenges for modern supernova surveys is identifying the galaxy that hosted each explosion. Is there an accurate and efficient way to do this that avoids investing significant human resources?

Why Identify Hosts?

supernova host

One problem in host galaxy identification. Here, the supernova lies between two galaxies — but though the centroid of the galaxy on the right is closer in angular separation, this may be a distant background galaxy that is not actually near the supernova. [Gupta et al. 2016]

Supernovae are a critical tool for making cosmological predictions that help us to understand our universe. But supernova cosmology relies on accurately identifying the properties of the supernovae — including their redshifts. Since spectroscopic followup of supernova detections often isn’t possible, we rely on observations of the supernova host galaxies to obtain redshifts.

But how do we identify which galaxy hosted a supernova? This seems like a simple problem, but there are many complicating factors — a seemingly nearby galaxy could be a distant background galaxy, for instance, or a supernova’s host could be too faint to spot.

confusion

The authors’ algorithm takes into account “confusion”, a measure of how likely the supernova is to be mismatched. In these illustrations of low (left) and high (right) confusion, the supernova is represented by a blue star, and the green circles represent possible host galaxies. [Gupta et al. 2016]

Turning to Automation

Before the era of large supernovae surveys, searching for host galaxies was done primarily by visual inspection. But current projects like the Dark Energy Survey’s Supernova Program is finding supernovae by the thousands, and the upcoming Large Synoptic Survey Telescope will likely discover hundreds of thousands. Visual inspection will not be possible in the face of this volume of data — so an accurate and efficient automated method is clearly needed!

To this end, a team of scientists led by Ravi Gupta (Argonne National Laboratory) has recently developed a new automated algorithm for matching supernovae to their host galaxies. Their work builds on currently existing algorithms and makes use of information about the nearby galaxies, accounts for the uncertainty of the match, and even includes a machine learning component to improve the matching accuracy.

Gupta and collaborators test their matching algorithm on catalogs of galaxies and simulated supernova events to quantify how well the algorithm is able to accurately recover the true hosts.

Successful Matching

The matching algorithm’s accuracy (“purity”) as a function of the true supernova-host separation, the supernova redshift, the true host’s brightness, and the true host’s size. [Gupta et al. 2016]

The matching algorithm’s accuracy (“purity”) as a function of the true supernova-host separation, the supernova redshift, the true host’s brightness, and the true host’s size. [Gupta et al. 2016]

The authors find that when the basic algorithm is run on catalog data, it matches supernovae to their hosts with 91% accuracy. Including the machine learning component, which is run after the initial matching algorithm, improves the accuracy of the matching to 97%.

The encouraging results of this work — which was intended as a proof of concept — suggest that methods similar to this could prove very practical for tackling future survey data. And the method explored here has use beyond matching just supernovae to their host galaxies: it could also be applied to other extragalactic transients, such as gamma-ray bursts, tidal disruption events, or electromagnetic counterparts to gravitational-wave detections.

Citation

Ravi R. Gupta et al 2016 AJ 152 154. doi:10.3847/0004-6256/152/6/154

Pop III stars

What impact did X-rays from the first binary star systems have on the universe around them? A new study suggests this radiation may have played an important role during the reionization of our universe.

Ionizing the Universe

During the period of reionization, the universe reverted from being neutral (as it was during recombination, the previous period) to once again being ionized plasma — a state it has remained in since then. This transition, which occurred between 150 million and one billion years after the Big Bang (redshift of 6 < z < 20), was caused by the formation of the first objects energetic enough to reionize the universe’s neutral hydrogen.

soft X-ray background

ROSAT image of the soft X-ray background throughout the universe. The different colors represent different energy bands: 0.25 keV (red), 0.75 keV (green), 1.5 keV (blue). [NASA/ROSAT Project]

Understanding this time period — in particular, determining what sources caused the reionization, and what the properties were of the gas strewn throughout the universe during this time — is necessary for us to be able to correctly interpret cosmological observations.

Conveniently, the universe has provided us with an interesting clue: the large-scale, diffuse X-ray background we observe all around us. What produced these X-rays, and what impact did this radiation have on the intergalactic medium long ago?

The First Binaries

A team of scientists led by Hao Xu (UC San Diego) has suggested that the very first generation of stars might be an important contributor to these X-rays.

This hypothetical first generation, Population III stars, are thought to have formed before and during reionization from large clouds of gas containing virtually no metals. Studies suggest that a large fraction of Pop III stars formed in binaries — and when those stars ended their lives as black holes, ensuing accretion from their companions could produce X-ray radiation.

The evolution with redshift the authors find for the mean X-ray background intensities. Each curve represents a different observed X-ray energy (and the total X-ray background is given by the sum of the curves). The two panels show results from two different calculation methods. [Xu et al. 2016]

The evolution with redshift of the mean X-ray background intensities. Each curve represents a different observed X-ray energy (and the total X-ray background is given by the sum of the curves). The two panels show results from two different calculation methods. [Xu et al. 2016]

Xu and collaborators have now attempted to model to the impact of this X-ray production from Pop III binaries on the intergalactic medium and determine how much it could have contributed to reionization and the diffuse X-ray background we observe today.

Generating a Background

The authors estimated the X-ray luminosities from Pop III binaries using the results of a series of galaxy-formation simulations, beginning at a redshift of z ~ 25 and evolving up to z = 7.6. They then used these luminosities to calculate the resulting X-ray background.

Xu and collaborators find that Pop III binaries can produce significant X-ray radiation throughout the period of reionization, and this radiation builds up gradually into an X-ray background. The team shows that X-rays from Pop III binaries might actually dominate more commonly assumed sources of the X-ray background at high redshifts (such as active galactic nuclei), and this radiation is strong enough to heat the intergalactic medium to 1000K and ionize a few percent of the neutral hydrogen.

If Pop III binaries are indeed this large of a contributor to the X-ray background and to the local and global heating of the intergalactic medium, then it’s important that we follow up with more detailed modeling to understand what this means for our interpretation of cosmological observations.

Citation

Hao Xu et al 2016 ApJL 832 L5. doi:10.3847/2041-8205/832/1/L5

SuperTIGER launch

cosmic-ray impact

Illustration of cosmic-ray nuclei impacting Earth’s atmosphere and decaying into lighter particles. [ESA]

The SuperTIGER (Trans-Iron Galactic Element Recorder) experiment flew over Antarctica for 55 days, collecting millions of galactic cosmic rays. What can it tell us about the origins of these high-energy particles?

High-Energy Impacts

Galactic cosmic rays are immensely high-energy protons and atomic nuclei that impact our atmosphere, originating from outside of our solar system. Where do they come from, and how are they accelerated? These are both open topics of research.

One of the leading theories is that cosmic-ray source material is primarily a mixture of material that has been ejected from massive stars — either from supernovae or in stellar wind outflows — and normal interstellar medium (ISM). This material is then accelerated to cosmic-ray energies by supernova shocks.

charge histogram

Number of nuclei of each element detected by SuperTIGER. Note the change of scale between the two plots (click for a closer look)! [Murphy et al. 2016]

How can we test this model? An important step is understanding the composition of galactic cosmic rays: what elemental nuclei are they made up of? If abundances are similar to solar-system abundances, then the material is likely mostly ISM. If the abundances of rarer heavy elements are high, however, then the material is more likely to have come from massive stars in star-forming regions.

Balloon-Borne Detections

Enter SuperTIGER, an experiment designed to collect cosmic rays and measure the abundances of the rare heavy elements — those with atomic number between iron (Z=26) and zirconium (Z=40).

SuperTIGER path

The path that SuperTIGER took over Antarctica during its flight, with a different color denoting each circuit around the pole. Note where it got stuck in an eddy over the Transarctic Mountains at the end of its second circuit! [Columbia Scientific Balloon Facility]

To gather galactic cosmic rays, the detector must be above the Earth’s atmosphere; interactions with the atmosphere cause these particles to decay into lower-energy secondary particles upon impact. In addition, the detector must operate for a long time in order to make meaningful abundance measurements: millions of cosmic-ray detections can result in just a few hundreds of detections of heavy-element nuclei.

The SuperTIGER team solved these problems by flying their instrument on a high-altitude scientific balloon at 127,000 feet. The project launched in Antarctica, taking advantage of the wind patterns circulating around the pole to maximize the flight time. In the end, SuperTIGER flew for 55 days, shattering the record for longest flight of a heavy-lift scientific balloon (which was previously 42 days). During this time, the experiment detected a whopping 50 million cosmic rays.

Pinning Down Abundances

elemental abundances

Measured elemental abundances for galactic cosmic rays, from SuperTIGER (orange) and other experiments. [Murphy et al. 2016]

Now, in a publication led by Ryan Murphy (Washington University in St. Louis), the team has detailed the results of SuperTIGER’s observations. Their measurements represent the tightest constraints on the abundances of galactic cosmic ray nuclei made in this charge and energy range, and the abundances are consistent with a model in which source material consists of a mixture of 19% material from massive stars and 81% normal ISM.

These results support the idea that a significant fraction of cosmic-ray material originates in associations of young, massive stars, and that the remainder is made up of solar-system-abundance ISM that’s injected and accelerated — possibly by supernova shocks — into cosmic rays.

Citation

R. P. Murphy et al 2016 ApJ 831 148. doi:10.3847/0004-637X/831/2/148

hot Jupiter

We know that the large masses of stars govern the orbits of the planets that circle them — but a large, close-in planet can also influence the rotation of its host star. A recently discovered, unusual hot Jupiter may be causing its star to spin faster than it should.

Exotic Planets

Hot Jupiters are gas giants of roughly Jupiter’s size that orbit close in to their host stars. Though these planets are easy to detect — their large sizes and frequent transits mean surveys have a good chance of catching them — we haven’t found many of them, suggesting that planetary systems containing hot Jupiters are fairly unusual.

HATS-18 light curve

The period-folded light curve of HATS-18, revealing the transit of the hot Jupiter HATS-18b. The period is P=0.8378 days. [Penev et al. 2016]

Studying this exotic population of planets, however, can help us to better understand how gas giants form and evolve in planetary systems. New observations of hot Jupiters may also reveal how stars and close-in planets interact through radiation, gravity, and magnetic fields.

The recent discovery of a transiting hot Jupiter a little over 2000 light-years away therefore presents an exciting opportunity!

A Speeding Giant

The discovery of HATS-18b, a planet of roughly 2 times Jupiter’s mass and 1.3 times its radius, was announced in a study led by Kaloyan Penev (Princeton University). The planet was discovered using the HATSouth transit survey network, which includes instruments in Chile, Namibia, and Australia, and follow-up photometry and spectroscopy was conducted at a variety of ground-based observatories.

HATS-18b’s properties are particularly unusual: this hot Jupiter is zipping around its host star — which is very similar to the Sun — at the incredible pace of one orbit every 0.84 days. HATS-18b’s orbit is more than 20 times closer to its host star than Mercury’s is to the Sun, bringing it so close it nearly grazes the star’s surface!

orbit vs. stellar rotation period

Size of the planetary orbit relative to the stellar radius as a function of the stellar rotation period, for transiting planets with orbital periods shorter than 2 days and masses greater than 0.1 Jupiter masses. HATS-18b is denoted by the red star. [Penev et al. 2016]

Tidal Interactions

What happens when a massive planet orbits this close to its star? Tidal interactions between the star and the planet cause tidal dissipation in the star, resulting in decay of the planet’s orbit. But there may be an additional effect of this interaction in the case of HATS-18b, the authors claim: the planet may be transferring some of its angular momentum to the star.

As stars age, they should gradually spin slower as they lose angular momentum via stellar winds. But Penev and collaborators note that this exoplanet’s host star, HATS-18, spins roughly three times as fast as its inferred age suggests it should. The authors conclude that the angular momentum lost by the planet as its orbit shrinks is deposited in the star, causing the star to spin up.

HATS-18 is an excellent laboratory for studying how very short-period planets interact with their stars — in fact, Penev and collaborators have already used their observations of the system to constrain models of tidal dissipation from Sun-like stars. Additional observations of HATS-18 and other short-period systems should allow us to further test models of how planetary systems form and evolve.

Citation

K. Penev et al 2016 AJ 152 127. doi:10.3847/0004-6256/152/5/127

LISA

Artist’s impression of the European Space Agency’s Laser Interferometer Space Antenna, currently planned for a 2034 launch. [NASA]

How are black-hole binaries built? Observations of gravitational waves from these systems — made using the European Space Agency’s upcoming mission, the Laser Interferometer Space Antenna (LISA) — may be able to reveal their origins.

Formation Channels

There are two primary places where stellar-mass black-hole binaries are thought to form:

  1. In isolation in the galactic field, as the components of a stellar binary independently evolve into black holes but remain bound to each other.
  2. In dense stellar environments like globular clusters, where the high density of already-formed black holes can cause a pair to dynamically interact and form a binary before being ejected from the cluster.

Can we differentiate between these origins based on future detections of gravitational waves from black-hole binaries? A team of scientists led by Katelyn Breivik (CIERA, Northwestern University) thinks that we can!

GW spectrum

The gravitational-wave spectrum and how we detect it (click for a closer look!). While ground-based interferometers like LIGO detect black-hole binaries in the final moments before merger, LISA’s lower frequency band will allow it to detect binaries earlier in their inspiral. [NASA Goddard SFC]

Differentiation by Eccentricity

Breivik and collaborators believe that the key clue is the binary’s eccentricity. Gravitational-wave emission will eventually circularize all black-hole binaries during their inspiral. But in the first formation scenario, binary evolution processes like tidal circularization and mass transfer will reduce the binary’s eccentricity early on — whereas in the second scenario, the binaries that form in globular clusters may retain eccentricity in their orbits long enough that we can detect it.

Ground-based interferometers won’t be up to this task; by the time the binary orbits shrink enough to evolve into the LIGO frequency band, the orbits won’t have measurable eccentricity anymore. But the upcoming space-based LISA mission, which will operate in a lower frequency band, might be able to pick up this signature.

To determine if LISA can pull it off, Breivik and collaborators simulate two populations of binary black holes: one evolved in isolation in galactic fields, and the other formed dynamically in globular clusters and then ejected. The authors then explore the evolution of these populations’ masses and eccentricities as their orbits narrow into the LISA-detectable frequency band.

Eccentricity evolution

Eccentricity evolution tracks as a function of gravitational-wave frequency for black-hole binaries formed in dynamical scenarios (black) and in isolation (blue for those with a common-envelope episode, green for those without). Eccentricities above 10-2 are measurable for all binaries; those above 10-3 are measurable for 90%. LISA’s frequency band is shown in grey. [Breivik et al. 2016]

Separating Populations

Breivik and collaborators find that LISA will be able to make several important distinctions. First, if LISA detects binary black holes with eccentricities of e > 0.01 at frequencies above 10-2 Hz, we can be fairly certain that these originated from dynamical processes in dense stellar environments.

For binary black holes detected with eccentricities of e > 0.01 at lower frequencies, they could either have formed in dense stellar environments or they could have formed in isolation. Based on this study’s results, however, those with measurable eccentricities that formed in isolation most likely originated from a common-envelope formation. Measuring eccentricities of such systems in the future could provide constraints on the physics of how this formation mechanism works.

Though the field of gravitational-wave astronomy is only just beginning, its future is promising! Theoretical studies like this one will help us to extract a greater understanding from the observations we can expect down the road.

Bonus

Check out this beautiful simulation from Northwestern Visualization and Carl Rodriguez (a co-author on the above study) that shows what the formation of a binary black hole in a globular cluster might look like!

Citation

Katelyn Breivik et al 2016 ApJL 830 L18. doi:10.3847/2041-8205/830/1/L18

solar cycle

Solar-like stars exhibit magnetic cycles; our Sun, for instance, displays an 11-year period in its activity, manifesting as cyclic changes in radiation levels, the number of sunspots and flares, and ejection of solar material. Over the span of two activity cycles, the Sun’s magnetic field flips polarity and then returns to its original state.

TRAPPIST-1

An artist’s illustration comparing the Sun to TRAPPIST-1, an ultracool dwarf star known to host several planets. [ESO]

But what about the magnetic behavior of objects near the cooler end of the stellar main sequence — do they exhibit similar activity cycles?

Effects of a Convecting Interior

Dwarf stars have made headlines in recent years due to their potential to harbor exoplanets. Because these cooler stars have lower flux levels compared to the Sun, their habitable zones lie much closer to the stars. The magnetic behavior of these stars is therefore important to understand: could ultracool dwarfs exhibit solar-like activity cycles that would affect planets with close orbits?

stellar interiors

The differences in internal structure between different mass stars. Ultracool dwarfs have fully convective interiors. [www.sun.org]

There’s a major difference between ultracool dwarfs (stars of spectral type higher than M7 and brown dwarfs) and Sun-like stars: their internal structures. Sun-like stars have a convective envelope that surrounds a radiative core. The interiors of cool, low-mass objects, on the other hand, are fully convective.

Based on theoretical studies of how magnetism is generated in stars, it’s thought that the fully convective interiors of ultracool dwarfs can’t support large-scale magnetic field formation. This should prevent these stars from exhibiting activity cycles like the Sun. But recent radio observations of dwarf stars have led scientist Matthew Route (ITaP Research Computing, Purdue University) to question these models.

A Reversing Field?

During observations of the brown dwarf star J1047+21 in 2010–2011, radio flares were detected with emission primarily polarized in a single direction. The dwarf’s flares in late 2013, however, all showed polarization in the opposite direction. Could this be an indication that J1047+21 has a stable, global dipolar field that flipped polarity in between the two sets of observations? If so, this could mean that the star has a magnetic cycle similar to the Sun’s.

stellar types

Artist’s impression showing the relative sizes and colors of the Sun, a red dwarf (M-dwarf), a hotter brown dwarf (L-dwarf), a cool brown dwarf (T-dwarf) similar to J1047+21, and the planet Jupiter [Credit: NASA/IPAC/R. Hurt (SSC)]

Inspired by this possibility, Route conducted an investigation of the long-term magnetic behavior of all known radio-flaring ultracool dwarfs, a list of 14 stars. Using polarized radio emission measurements, he found that many of his targets exhibited similar polarity flips, which he argues is evidence that these dwarfs are undergoing magnetic field reversals on roughly decade-long timescales, analogous to those reversals that occur in the Sun.

If this is indeed true, then we need to examine our models of how magnetic fields are generated in stars: the interface between the radiative and convective layers may not be necessary to produce large-scale magnetic fields. Understanding this process is certainly an important step in interpreting the potential habitability of planets around ultracool dwarfs.

Citation

Matthew Route 2016 ApJL 830 L27. doi:10.3847/2041-8205/830/2/L27

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