In KIC 9246715, two red-giant stars — twins in nearly every way — circle each other in a 171-day orbit. This binary pair may be a key to learning about masses and radii of stars with asteroseismology, the study of oscillations in the interiors of stars.
Two Ways to Measure
In order to understand a star’s evolution, it is critical that we know its mass and radius. Unfortunately, these quantities are often difficult to pin down!
One of the few cases in which we can directly measure stars’ masses and radii is in eclipsing binaries, wherein two stars eclipse each other as they orbit. If we have a well-sampled light curve for the binary, as well as radial velocities for both stars, then we can determine the stars’ complete orbital information, including their masses and radii.
But there may be another way to obtain stellar mass and radius: asteroseismology. In asteroseismology, oscillations inside stars are used to characterize the stellar interiors. Conveniently, if a star with a convective envelope exhibits solar-like oscillations, these oscillations can be directly compared to those of the Sun. Mass and radius scaling relations — which use the Sun as a benchmark and scale based on the star’s temperature — can then be used to derive the mass and radius of the star.
Test Subjects from Kepler
Solar-like oscillations from KIC 9246715 are shown in red across different resonant frequencies. The oscillations of a single red-giant star with similar properties are shown upside down in grey for reference. [Rawls et al. 2016]
Of course, scaling relations are only useful if we can test them! A team of scientists including Meredith Rawls (New Mexico State University) has identified 18 red-giant eclipsing binaries in the Kepler field of view that also exhibit solar-like oscillations — perfect for testing the scaling relations.
In a recent study led by Rawls, the team analyzed the first of these binaries, KIC 9246715. Using the Kepler light curves in addition to radial velocity measurements from high-resolution ground-based spectroscopy at the Fred Lawrence Whipple Observatory and Apache Point Observatory, Rawls and collaborators established that the two stars have masses of 2.17 and 2.15 solar masses, and radii of 8.4 and 8.3 solar radii.
Not Quite Twins?
Intriguingly, when the authors measured the stellar oscillations from the binary, they were only able to pick out one signal. Using the scaling relations, their measurements reveal that the star producing the oscillations has a mass of 2.17 solar masses and radius of 8.3 radii — consistent with both red giants in the system, within error bars. This provides excellent confirmation of the scaling relations for obtaining mass and radius, but it also raises a new question: why is only one star of this twin system producing oscillations?
Rawls and collaborators have an idea: one star might be more magnetically active than the other, causing the suppression of oscillations in the more active star. The authors’ observations and detailed modeling support this idea, but similar analyses of the rest of the red-giant eclipsing binaries identified in the Kepler field will help to determine if KIC 9246715 is unusual, or if this behavior is common among such systems.
Several months ago, the discovery of WD 1145+017 was announced. This white dwarf appears to be orbited by planetary bodies that are actively disintegrating due to the strong gravitational pull of their host. A follow-up study now reveals that this system has dramatically evolved since its discovery.
Signs of Disruption
Potential planetary bodies orbiting a white dwarf would be exposed to a particular risk: if their orbits were perturbed and they passed inside the white dwarf’s tidal radius, they would be torn apart. Their material could then form a debris disk around the white dwarf and eventually be accreted.
Interestingly, we have two pieces of evidence that this actually happens:
We’ve observed warm, dusty debris disks around ~4% of white dwarfs, and
The atmospheres of ~25-50% of white dwarfs are polluted by heavy elements that have likely accreted recently.
But in spite of this indirect evidence of planet disintegration, we’d never observed planetary bodies actively being disrupted around white dwarfs — until recently.
Unusual Transits
In April 2015, observations by Kepler’s K2 mission revealed a strange transit signal around WD 1145+017, a white dwarf 570 light-years from Earth that has both a dusty debris disk and a polluted atmosphere. This signal was interpreted as the transit of at least one, and possibly several, disintegrating planetesimals.
In a recent follow-up, a team of scientists led by Boris Gänsicke (University of Warwick) obtained high-speed photometry of WD 1145+017 using the ULTRASPEC camera on the 2.4m Thai National Telescope. These observations were taken in November and December of 2015 — roughly seven months after the initial photometric observations of the system. They reveal that dramatic changes have occurred in this short time.
Rapid Evolution
A sample light curve from TNT/ULTRASPEC, obtained in December 2015 over 3.9 hours. Many varied transits are evident (click for a better view!). Transits labeled in color appear across multiple nights. [Gänsicke et al. 2016]
Initial observations of WD 1145+017 showed a significant transit dip (>10%) only every ~3.6 hours, on average. In contrast, in the current observations, every light curve is riddled with numerous transit events that have durations of 3–12 minutes and depths of 10–60%. Many of the transit features overlap, so there are now only short segments of the light curve that don’t appear to be attenuated by debris.
Gänsicke and collaborators use the new data to analyze the transiting bodies. Though some transits are consistent from night to night, most evolve in shape and depth, appearing and disappearing over the course of the observing campaign. This rapid variability, along with the large size of the transiting bodies (several times the size of the white dwarf), support the conclusion that the transiting objects are not solid bodies. Instead, they are likely clouds of gas and dust flowing from smaller bodies that are being disrupted.
Because astronomical timescales are often extremely long, the observations of WD 1145+047 are especially exciting — this is a rare chance to watch a system evolve in real time! Given how rapidly it appears to be changing, continued observations are sure to soon reveal more about the planetary bodies orbiting this white dwarf.
Big news: the Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected its first gravitational-wave signal! Not only is the detection of this signal a major technical accomplishment and an exciting confirmation of general relativity, but it also has huge implications for black-hole astrophysics.
What did LIGO see?
LIGO is designed to detect the ripples in space-time created by two massive objects orbiting each other. These waves can reach observable amplitudes when a binary system consisting of two especially massive objects — i.e., black holes or neutron stars — reach the end of their inspiral and merge.
LIGO has been unsuccessfully searching for gravitational waves since its initial operations in 2002, but a recent upgrade in its design has significantly increased its sensitivity and observational range. The first official observing run of Advanced LIGO began 18 September 2015, but the instruments were up and running in “engineering mode” several weeks before that. And it was in this time frame — before official observing even began! — that LIGO spotted its first gravitational wave signal: GW150914.
One of LIGO’s two detection sites, located near Hanford in eastern Washington. [LIGO]
The signal, detected on 14 September, 2015, provides astronomers with a remarkable amount of information about the merger that caused it. From the detection, the LIGO team has extracted the masses of the two black holes that merged, 36+5-4 and 29+4-4 solar masses, as well as the mass of the final black hole formed by the merger, ~62 solar masses. The team also determined that the merger happened roughly a billion light-years away (at a redshift of z~0.1), and the direction of the signal was localized to an area of ~600 square degrees (roughly 1% of the sky).
Why is this detection a big deal?
This is the first direct detection of gravitational waves, providing spectacular further confirmation of Einstein’s general theory of relativity. But the implications of GW150914 go far beyond this confirmation. This detection is a huge deal for astrophysics because it’s the first direct evidence we’ve had that:
“Heavy” stellar-mass black holes exist. We’ve reliably measured black holes of masses up to 10–20 solar masses in X-ray binaries (binary systems in which a single neutron star or black hole accretes matter from a donor star). But this is the first proof we’ve found that stellar-mass black holes of >25 solar masses can form in nature.
Binaries consisting of two black holes can form in nature. As we’ll discuss shortly, there are two theorized mechanisms for the formation of these black-hole binaries. Until now, however, there was no guarantee that either of those mechanisms worked!
These black-hole binaries can inspiral and merge within the age of the universe. The formation of a black-hole binary is no guarantee that it will merge on a reasonable timescale: if the binary forms with enough separation, it could take longer than the age of the universe to merge. This detection proves that black-hole binaries can form with small enough separation to merge on observable timescales.
What can we learn from GW150914?
Expected increase in sensitivity for LIGO/Virgo detectors is shown as a function of total system mass (x-axis) and surveyed volume (y-axis). The red star indicates the mass of GW150914. [Abbott et al. 2016]
For starters, we can throw out the lower estimates we had on merger rates. This event provides a new inferred binary-black-hole merger rate for the low-redshift universe of 2–400 Gpc-3 yr-1.
Another interesting conclusion about this binary system is that it probably formed in a low-metallicity environment (~ <1/2 solar metallicity). We infer this based on our current understanding of massive-star winds (which drive mass loss) and their dependence on metallicity: had the environment been high-metallicity, it is unlikely that such large black holes would have been able to form.
What can we learn from future gravitational-wave detections?
One of the key questions we’d like to answer is: how do binary black holes form? Two primary mechanisms have been proposed:
A binary star system contains two stars that are each massive enough to individually collapse into a black hole. If the binary isn’t disrupted during the two collapse events, this forms an isolated black-hole binary.
Single black holes form in dense cluster environments and then — because they are the most massive objects — sink to the center of the cluster. There they form pairs through dynamical interactions.
Now that we’re able to observe black-hole binaries through gravitational-wave detections, one way we could distinguish between the two formation mechanisms is from spin measurements. If we discover a clear preference for the misalignment of the two black holes’ spins, this would favor formation in clusters, where there’s no reason for the original spins to be aligned.
The current, single detection is not enough to provide constraints, but if we can compile a large enough sample of events, we can start to present a statistical case favoring one channel over the other.
What does GW150914 mean for the future of gravitational-wave detection?
The fact that Advanced LIGO detected an event even before the start of its first official observing run is certainly promising! The LIGO team estimates that the volume the detectors can probe will still increase by at least a factor of ~10 as the observing runs become more sensitive and of longer duration.
Aerial view of the Virgo interferometer near Pisa, Italy. [Virgo Collaboration]
In addition, LIGO is not alone in the gravitational-wave game. LIGO’s counterpart in Europe, Virgo, is also undergoing design upgrades to increase its sensitivity. Within this year, Virgo should be able to take data simultaneously with LIGO, allowing for better localization of sources. And the launch of (e)LISA, ESA’s planned space-based interferometer, will grant us access to a new frequency range, opening a further window to the gravitational-wave sky.
The detection of GW150914 marks the dawn of a new field: observational gravitational-wave astronomy. This detection alone confirms much that was purely theory before now — and given that instrument upgrades are still underway, the future of gravitational-wave detection looks incredibly promising.
Bonus
This awesome video (produced by SXS lensing) shows an actual simulation of the black-hole merger GW150914. Time is slowed by a factor of 100, compared to the actual merger. The two black holes — of 29 and 36 solar masses — warp the space-time around them, causing the distorted view.
Bitten by the gravitational-wave bug? While we await Thursday’s press conference, here’s some food for thought: if LIGO were able to detect gravitational waves from compact-object mergers, how could we follow up on the detections? A new study investigates whether the upcoming James Webb Space Telescope (JWST) will be able to observe electromagnetic signatures of some compact-object mergers.
Hunting for Mergers
Studying compact-object mergers (mergers of black holes and neutron stars) can help us understand a wealth of subjects, like high-energy physics, how matter behaves at nuclear densities, how stars evolve, and how heavy elements in the universe were created.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is searching for the signature ripples in spacetime identifying these mergers, but gravitational waves are squirrelly: LIGO will only be able to localize wave sources to tens of square degrees. If we want to find out more about any mergers LIGO discovers in gravitational waves, we’ll need a follow-up search for electromagnetic counterparts with other observatories.
The Kilonova Key
One possible electromagnetic counterpart is kilonovae, explosions that can be produced during a merger of a binary neutron star or a neutron star–black hole system. If the neutron star is disrupted during the merger, some of the hot mass is flung outward and shines brightly by radioactive decay.
Kilonovae are especially promising as electromagnetic counterparts to gravitational waves for three reasons:
They emit isotropically, so the number of observable mergers isn’t limited by relativistic beaming.
They shine for a week, giving follow-up observatories time to search for them.
The source location can be easily recovered.
The only problem? We don’t currently have any sensitive survey instruments in the near-infrared band (where kilonova emission peaks) that can provide coverage over tens of square degrees. Luckily, we will soon have just the thing: JWST, launching in 2018!
JWST’s Search
Integration time needed for JWST’s NIRCam to detect a kilonova at 200 Mpc, as a function of time since the merger. Different curves correspond to different NIRCam filters. Note that the total time for follow-up is overwhelmingly dominated by things like telescope slew time, rather than by this exposure time. [Bartos et al. 2016]
In a recent study, a team of authors led by Imre Bartos (Columbia University) evaluate whether JWST will be capable of catching these kilonovae if LIGO finds gravitational wave signals.
Bartos and collaborators calculate that, given the sensitivity of the different filters on JWST’s Near-Infrared Camera, the instrument should easily be able to detect a kilonova 200 Mpc away (a typical distance at which LIGO might be able to find a neutron-star binary). But there’s a catch: 10 deg2 is a really big sky area, and it would take JWST an unfeasible amount of time (days!) to fully cover it.
The authors suggest instead using a targeted search. Since most mergers are expected to be in or near galaxies, JWST could specifically focus the follow-up search on known galaxies within the search area. This approach would bring the total search time down to 12.6 hours, which is within the realm of feasibility. And this time could be reduced even further by concentrating on galaxies most likely to host kilonovae, like those with high star-formation rates.
The conclusion: if LIGO is able to detect gravitational waves, JWST will provide an excellent means to follow up on the detection in the attempt to identify the source.
Small exoplanets tend to fall into two categories: the smallest ones are predominantly rocky, like Earth, and the larger ones have a lower-density, more gaseous composition, similar to Neptune. The planet Kepler-454b was initially estimated to fall between these two groups in radius. So what is its composition?
Small-Planet Dichotomy
Though Kepler has detected thousands of planet candidates with radii between 1 and 2.7 Earth radii, we have only obtained precise mass measurements for 12 of these planets.
Mass-radius diagram (click for a closer look!) for planets with radius <2.7 Earth radii and well-measured masses. The six smallest planets (and Venus and Earth) fall along a single mass-radius curve of Earth-like composition. The six larger planets (including Kepler-454b) have lower-density compositions. [Gettel et al. 2016]
These measurements, however, show an interesting dichotomy: planets with radii less than 1.6 Earth radii have rocky, Earth-like compositions, following a single relation between their mass and radius. Planets between 2 and 2.7 Earth radii, however, have lower densities and don’t follow a single mass-radius relation. Their low densities suggest they contain a significant fraction of volatiles, likely in the form of a thick gas envelope of water, hydrogen, and/or helium.
The planet Kepler-454b, discovered transiting a Sun-like star, was initially estimated to have a radius of 1.86 Earth radii — placing it in between these two categories. A team of astronomers led by Sara Gettel (Harvard-Smithsonian Center for Astrophysics) have since followed up on the initial Kepler detection, hoping to determine the planet’s composition.
Low-Density Outcome
Gettel and collaborators obtained 63 observations of the host star’s radial velocity with the HARPS-N spectrograph on the Telescopio Nazionale Galileo, and another 36 observations with the HIRES spectrograph at Keck Observatory. These observations allowed them to do several things:
Obtain a more accurate radius estimate for Kepler-454b: 2.37 Earth radii.
Measure the planet’s mass: roughly 6.8 Earth masses.
Discover — surprise! — two other, non-transiting companions in the system: Kepler-454c, a planet with a minimum mass of ~4.5 Jupiter masses on a 524-day orbit, and Kepler-454d, a more distant (>10-year orbit) brown dwarf or low-mass star.
Kepler-454b’s newly measured size and mass place it firmly in the category of non-rocky, larger, less dense planets (the authors calculate a density of ~2.76 g/cm3, or roughly half that of Earth). This seems to reinforce the idea that rocky planets don’t grow larger than ~1.6 Earth radii, and planets with mass greater than about 6 Earth masses are typically low-density and/or swathed in an envelope of gas.
The authors point out that future observing missions like NASA TESS (launching in 2017) will provide more targets that can be followed up to obtain mass measurements, allowing us to determine if this trend in mass and radius holds up in a larger sample.
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope has received an upgrade that increased its sensitivity by a whopping 40% — and nobody had to travel to space to make it happen! The difference instead stems from remarkable improvement to the software used to analyze Fermi-LAT’s data, and it has resulted in a new high-energy map of our sky.
Animation (click to watch!) comparing the Pass 7 to the Pass 8 Fermi-LAT analysis, in a region in the constellation Carina. Pass 8 provides more accurate directions for incoming gamma rays, so more of them fall closer to their sources, creating taller spikes and a sharper image. [NASA/DOE/Fermi LAT Collaboration]
Pass 8
Fermi-LAT has been surveying the whole sky since August 2008. It detects gamma-ray photons by converting them into electron-positron pairs and tracking the paths of these charged particles. But differentiating this signal from the charged cosmic rays that also pass through the detector — with a flux that can be 10,000 times larger! — is a challenging process. Making this distinction and rebuilding the path of the original gamma ray relies on complex analysis software.
“Pass 8” is a complete reprocessing of all data collected by Fermi-LAT. The software has gone through many revisions before now, but this is the first revision that has taken into account all of the experience that the Fermi team has gained operating the LAT in its orbital environment.
The improvements made in Pass 8 include better background rejection of misclassified charged particles, improvements to the point spread function and effective area of the detector, and an extension of the effective energy range from below 100 MeV to beyond a few hundred GeV. The changes made in Pass 8 have increased the sensitivity of Fermi-LAT by an astonishing 40%.
Map of the High-Energy Sky
Sky map of the sources in the 2FHL catalog, classified by their most likely association. Click for a better look! [Ackermann et al. 2016]
The first result from the improvements of Pass 8 is 2FHL, the second catalog of high-energy Fermi sources, constructed from 80 months of data from Fermi-LAT. The 2FHL catalog contains 360 sources from across the sky in the 50 GeV–2TeV range. Here are just a few details:
47 of the sources are new — they have not previously been detected by Fermi or ground-based gamma-ray detectors.
86% of the sources can be associated with counterparts at other wavelengths. This includes
75% that are active galactic nuclei, and
11% that originate in our galaxy, the majority of which are associated with objects at the final stage of stellar evolution, such as pulsar wind nebulae and supernova remnants.
Because the quality of Fermi-LAT’s observations is limited by the number of photons collected, longer observing time will only serve to improve the detections in this catalog. And since only 22% of the 2FHL sources have been observed by ground-based gamma-ray detectors (which have much more limited fields of view), this catalog provides an excellent list of candidates that these detectors can now follow up at very high energies.
Bonus
Want to learn more about Pass 8? Check out this video, created by the Fermi team. [NASA’s Goddard Space Flight Center]
Have you ever wondered what springtime is like on Saturn’s largest moon, Titan? A team of researchers has analyzed a decade of data from the Cassini spacecraft to determine how Titan’s gradual progression through seasons has affected its temperatures.
Observing the Saturn System
Though Titan orbits Saturn once every ~16 days, it is Saturn’s ~30-year march around the Sun that sets Titan’s seasons: each traditional season on Titan spans roughly 7.5 years. Thus, when the Cassini spacecraft first arrived at Saturn in 2004 to study the giant planet and its ring system and moons, Titan’s northern hemisphere was in early winter. A decade later, the season in the northern hemisphere had advanced to late spring.
A team scientists led by Donald Jennings (Goddard Space Flight Center) has now used data from the Composite Infrared Spectrometer (CIRS) on board Cassini to analyze the evolution of Titan’s surface temperature between 2004 and 2014.
Changing of Seasons
Surface brightness temperatures (with errors) on Titan are shown in blue for five time periods between 2004 and 2014. The location of maximum temperature migrates from 19°S to 16°N over the decade. Two climate models are also shown in green (high thermal inertia) and red (low thermal inertia). [Jennings et al. 2016]
CIRS uses the decreased opacity of Titan’s atmosphere at 19 µm to detect infrared emission from Titan’s surface at this wavelength. From this data, Jennings and collaborators determine Titan’s surface temperature for five time intervals between 2004 and 2014. They bin the data into 10° latitude bins that span from the south pole (90°S) to the north pole (90°N).
The authors find that the maximum temperature on the moon stays stable over the ten-year period at 94 K, or a chilly -240°F). But as time passes, the latitude with the warmest temperature shifts from 19°S to 16°N, marking the transition from early winter to late spring. Over the decade of monitoring, the surface temperature near the south pole decreased by ~2 K, and that near the north pole increased by ~1 K.
Climate Modeling
Though Titan’s overall temperature trend is expected, the rate of change of its surface temperature doesn’t quite match theoretical climate models: the northern hemisphere lags slightly behind the predicted temperature curve. The authors speculate that this may be due to the effects of seas in Titan’s northern hemisphere. Seas of hydrocarbons (e.g., methane) are thought to account for ~10% of the moon’s surface area at latitudes of 55–90°N. Since the seas have a higher thermal inertia than land, this could explain why temperatures in Titan’s northern hemisphere lag behind the model’s predictions.
The authors hope to gain additional data in the future, as CIRS has another two years of operation planned before the Cassini mission ends. This time span will take us all the way up to Titan’s northern summer solstice; it will be exciting to see what more we can learn from this data!
Accreting, supermassive black holes that reside at galactic centers can power enormous jets, bright enough to be observed from vast distances away. The recent discovery of such a jet in X-ray wavelengths, without an apparent radio counterpart, has interesting implications for our understanding of how these distant behemoths shine.
An Excess of X-Rays
Quasar B3 0727+409 was serendipitously discovered to host an X-ray jet when a group of scientists, led by Aurora Simionescu (Institute of Space and Astronautical Sciences of the Japan Aerospace Exploration Agency), was examining Chandra observations of another object.
The Chandra data reveal bright, compact, extended emission from the core of quasar B3 0727+409, with a projected length of ~100 kpc. There also appears to be further X-ray emission at a distance of ~280 kpc, which Simionescu and collaborators speculate may be the terminal hotspot of the jet.
The quasar is located at a redshift of z=2.5 — which makes this jet one of only a few high-redshift X-ray jets known to date. But what makes it especially intriguing is that, though the authors searched through both recent and archival radio observations of the quasar, the only radio counterpart they could find was a small feature close to the quasar core (which may be a knot in the jet). Unlike what is typical of quasar jets, there was no significant additional radio emission coinciding with the rest of the X-ray jet.
Making Jets Shine
X-ray-to-radio flux ratio vs. redshift, for X-ray quasar jets detected with Chandra. B3 0727+409 is shown in red (with and without the radio knot). The curves represent inverse-Compton scattering models with different magnetic field strengths. [Simionescu et al. 2016]
What does this mean? To answer this, we must consider one of the outstanding questions about quasar jets: what radiation processes dominate their emission? One process possibly contributing to the X-ray emission is inverse-Compton scattering of low-energy cosmic microwave background (CMB) photons off of the electrons in the jet; these photons can scatter up to X-ray energies.
Interestingly, there’s a testable prediction associated with this mechanism. If this process dominates the X-ray emission of quasar jets, then the X-ray-to-radio flux ratio of the jet would increase with redshift as (1+z)4, due to the increased density of CMB photons at higher redshift.
Thus far, our limited detections of high-redshift X-ray quasars have made it difficult to test this prediction, but quasar B3 0727+409 provides an extremely useful data point. When the authors model the radio-to-X-ray flux ratio for the jet, they find that it’s entirely consistent with the inverse-Compton scenario.
This discovery suggests that the inverse-Compton mechanism may indeed be what dominates the X-ray radiation from jets like this one. And since our current observing strategies focus on Chandra follow-up of known bright radio jets, this could mean that there is an entire population of similar systems — with bright X-ray and faint radio emission — that we have missed!
The Friends of Hot Jupiters (FOHJ) project is a systematic search for planetary- and stellar-mass companions in systems that have known hot Jupiters — short-period, gas-giant planets. This survey has discovered that many more hot Jupiters may have companions than originally believed.
Missing Friends
FOHJ was begun with the goal of better understanding the systems that host hot Jupiters, in order to settle several longstanding issues.
The first problem was one of observational statistics. We know that roughly half of the Sun-like stars nearby are in binary systems, yet we’ve only discovered a handful of hot Jupiters around binaries. Are binary systems less likely to host hot Jupiters? Or have we just missed the binary companions in the hot-Jupiter-hosting systems we’ve seen so far?
An additional issue relates to formation mechanisms. Hot Jupiters probably migrated inward from where they formed out beyond the ice lines in protoplanetary disks — but how?
This median-stacked image, obtained with adaptive optics, shows one of the newly-discovered stellar companions to a star hosting a hot Jupiter. The projected separation is ~180 AU. [Ngo et al. 2015]
Observations reveal two populations of hot Jupiters: those with circular orbits aligned with their hosts’ spins, and those with eccentric, misaligned orbits. The former population support a migration model dominated by local planet-disk interactions, whereas the latter population suggest the hot Jupiters migrated through dynamical interactions with distant companions. A careful determination of the companion rate in hot-Jupiter-hosting systems could help establish the ability of these two models to explain the observed populations.
Search for Companions
The FOHJ project began in 2012 and studied 51 systems hosting known, transiting hot Jupiters — with roughly half on circular, aligned orbits and half on eccentric, misaligned orbits. The survey consisted of three different, complementary components:
Study 1
Lead author: Heather Knutson (Caltech)
Technique: Long-term radial velocity monitoring
Searching for: Planetary companions at 1–20 AU from the star
Study 2
Lead author: Henry Ngo (Caltech)
Technique: Adaptive-optics imaging
Searching for: Stellar companions at 50–2000 AU from the star
Study 3
Lead author: Danielle Piskorz (Caltech)
Technique: Spectroscopy
Searching for: Any additional stellar companions at <125 AU from the star
The companion fraction found within Study 2, the adaptive-optics imagine search. The three curves show the total, the systems with hot Jupiters on aligned and circular orbits, and those with hot Jupiters on misaligned and eccentric orbits. [Ngo et al. 2015]
Migration Implications
Using these three different techniques, the team found a significant number of both planetary and stellar companions that had not been previously detected. After correcting their results for completeness, they found a multiple-star rate of ~50% for these systems, resolving the problem of the missing companions. So really, we just weren’t looking hard enough for the companions previously.
Intriguingly, the binary companion rate found for these hot Jupiter systems is higher than the average rate for the field stars (which is below 25% for the semimajor-axis range the FOHJ studies are sensitive to). This suggests that companion stars may indeed play a role in hot Jupiter formation and migration.
That said, none of the three studies found a significant difference in the binary fraction for aligned versus misaligned hot Jupiters — which means that the answer is not as simple as thought, with companion stars causing the misaligned planets. Thus, while hot Jupiters’ “friends” may play a role in their formation and migration, we still have work to do in understanding what that role is.
Dwarf galaxies or globular clusters orbiting the Milky Way can be pulled apart by tidal forces, leaving behind a trail of stars known as a “stellar stream.” One such trail, the Ophiuchus stream, has posed a serious dynamical puzzle since its discovery. But a recent study has identified four stars that might help resolve this stream’s mystery.
Conflicting Timescales
The stellar stream Ophiuchus was discovered around our galaxy in 2014. Based on its length, which appears to be 1.6 kpc, we can calculate the time that has passed since its progenitor was disrupted and the stream was created: ~250 Myr. But the stars within it are ~12 Gyr old, and the stream orbits the galaxy with a period of ~350 Myr.
Given these numbers, we can assume that Ophiuchus’s progenitor completed many orbits of the Milky Way in its lifetime. So why would it only have been disrupted 250 million years ago?
Fanning Stream
Led by Branimir Sesar (Max Planck Institute for Astronomy), a team of scientists has proposed an idea that might help solve this puzzle. If the Ophiuchus stellar stream is on a chaotic orbit — common in triaxial potentials, which the Milky Way’s may be — then the stream ends can fan out, with stars spreading in position and velocity.
The fanned part of the stream, however, would be difficult to detect because of its low surface brightness. As a result, the Ophiuchus stellar stream could actually be longer than originally measured, implying that it was disrupted longer ago than was believed.
Search for Fan Stars
To test this idea, Sesar and collaborators performed a search around the ends of the stream, looking for stars that
are of the right type to match the stream,
are at the predicted distance of the stream,
are located near the stream ends, and
have velocities that match the stream and don’t match the background halo stars.
Histogram of the heliocentric velocities of the 43 target stars. Six stars have velocities matching the stream velocity. Two of these are located in the main stream; the other four may be part of a fan at the end of the stream. [Sesar et al. 2016]
Of the 43 targets for which the authors obtained spectra, four stars met these criteria and are located beyond the main extent of the stream, possibly comprising a fan at the stream’s end. Including these stars as part of the Ophiuchus stream, its length becomes 3 kpc, implying that its time of disruption was closer to 400 million years ago. This relieves the timescale tension but does not resolve it.
That said, the mere evidence of a fan in the Ophiuchus stream suggests that its progenitor may have been on a chaotic orbit. If this is the case, it’s entirely possible that the progenitor could have survived for ~11 Gyr, only to have been disrupted within the last 0.5 Gyr. Detailed modeling and further identification of potential fan stars in the Ophiuchus stream will help to test this idea and resolve the puzzle of this stream.
![Solar-like oscillations from KIC 9246715 are shown in red across different resonant frequencies. The oscillations of a single red-giant star with similar properties are shown upside down in grey for reference. [Rawls et al. 2016]](https://aasnova.org/wp-content/uploads/2016/02/fig25.jpg)
