Features RSS

solar corona

The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

The Problem with the Corona

Solar temperatures

The temperatures of different layers of the Sun. Click for a closer look. [ISAS/JAXA]

The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alfvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

DKIST

The Daniel K. Inouye Solar Telescope, located on the summit of Haleakala in Hawaii, is scheduled to be completed in 2018. [Ekrem Canli]

Predicting Observations

Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

A Promising Future

power distribution

The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.

Citation

Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

Neptune’s moon system is not what we would expect for a gas giant in our solar system. Scientists have now explored the possibility that Neptune started its life with an ordinary system of moons that was later destroyed by the capture of its current giant moon, Triton.

An Odd System

Our current understanding of giant-planet formation predicts a period of gas accretion to build up the large size of these planets. According to models, the circumplanetary gas disks that surround the planets during this time then become the birthplaces of the giant planets’ satellite systems, producing systems of co-planar and prograde (i.e., orbiting in the same direction as the planet’s rotation) satellites similar to the many-moon systems of Jupiter or Saturn.

Neptune satellite orbits

Triton’s orbit is tilted relative to the inner Neptunian satellite orbits. [NASA, ESA, and A. Feild (STScI)]

Neptune, however, is quirky. This gas giant has surprisingly few satellites — only 14 compared to, say, the nearly 70 moons of Jupiter — and most of them are extremely small. One of Neptune’s moons is an exception to this, however: Triton, which contains 99.7% of the mass of Neptune’s entire satellite system!

Triton’s orbit has a number of unusual properties. The orbit is retrograde — Triton orbits in the opposite direction as Neptune’s rotation — which is unique behavior among large moons in our solar system. Triton’s orbit is also highly inclined, and yet the moon’s path is nearly circular and lies very close to Neptune.

Triton impacts

The distribution of impact velocities in the authors’ simulations for primordial satellite interactions with Triton, in three cases of different satellite mass ratios. In the low-mass case — a third of the mass ratio of the Uranian satellite system — 88% of simulations ended with Triton surviving on its high-inclination orbit. The survival rate was only 12% in the high-mass case. [Adapted from Rufu et al. 2017]

How did this monster of a satellite get its strange properties, and why is Neptune’s system so odd compared to what we would expect for a gas giant’s satellites? Two scientists, Raluca Rufu (Weizmann Institute of Science, Israel) and Robin Canup (Southwest Research Institute), propose an explanation in which Triton long ago wreaked havoc on a former system of satellites around Neptune.

Destruction After Capture

Rufu and Canup explore the scenario in which Neptune once had an ordinary, prograde system of moons around it that resembled those of the other gas giants. Triton, the authors suggest, may have been a former Kuiper belt object that was then captured by Neptune. The ensuing interactions between retrograde Triton and Neptune’s original, prograde satellite system may have then resulted in the destruction of this original system, leaving behind only Triton and Neptune’s other current satellites.

Nereid, a small irregular moon of Neptune, orbits at an average distance of more than 15 times that of Triton. Models of Triton’s orbital evolution must also account for the preservation of satellites like this one. [NASA]

Using N-body simulations that model a newly captured Triton and a likely primordial prograde system of moons, Rufu and Canup show that if the moons have a mass ratio similar to that of Uranus’s system or smaller, Triton’s interactions with it have a substantial likelihood of reproducing the current Neptunian satellite system. They even demonstrate that the interactions decrease Triton’s initial semimajor axis quickly enough to prevent smaller, outer satellites like Nereid from being kicked out of the system.

If the authors’ picture is correct, then it neatly explains why Neptune’s satellite system looks so unusual compared to Jupiter’s or Saturn’s — which means that our models of how primordial systems of moons form around gas giants still hold strong.

Citation

Raluca Rufu and Robin M. Canup 2017 AJ 154 208. doi:10.3847/1538-3881/aa9184

star formation

Huge reservoirs of cold hydrogen gas — the raw fuel for star formation — lurk in galaxies throughout the universe. A new study examines whether these reservoirs have always been similar, or whether those in distant galaxies are very different from those in local galaxies today.

Left: Optical SLOAN images of the five HIGHz galaxies in this study. Right: ALMA images of the molecular gas in these galaxies. Both images are 30” wide. [Adapted from Cortese et al. 2017]

Molecular or Atomic?

The formation of stars is a crucial process that determines how galaxies are built and evolve over time. We’ve observed that star formation takes place in cold clouds of molecular gas, and that star-formation rates increase in galaxies with a larger surface density of molecular hydrogen — so we know that molecular hydrogen feeds the star-forming process.

But not all cold gas in the interstellar medium of galaxies exists in molecular form. In the local universe, only around 30% of cold gas is found in molecular form (H2) and able to directly feed star formation; the rest is atomic hydrogen (H I). But is this true of galaxies earlier in the universe as well?

Studying Distant Galaxies

Cosmological simulations have predicted that earlier in our universe’s history, the ratio of molecular to atomic hydrogen could be larger — i.e., more cold hydrogen may be in a form ready to fuel star formation — but this prediction is difficult to test observationally. Currently, radio telescopes are not able to measure the atomic hydrogen in very distant galaxies, such as those at the peak of star formation in the universe, 10 billion years ago.

Recently, however, we have measured atomic hydrogen in closer galaxies: those at a redshift of about z ~ 0.2–0.4, a few billion years ago. One recent study of seven galaxies at this distance, using a sample from a survey known as COOL BUDHIES, showed that the hydrogen reservoirs of these galaxies are dominated by molecular hydrogen, unlike in the local universe. If this is true of most galaxies at this distance, it would suggest that gas reservoirs have drastically changed in the short time between then and now.

But a team of scientists from the International Centre for Radio Astronomy Research in Australia, led by Luca Cortese, has now challenged this conclusion.

molecular to atomic gas ratio

Top: molecular vs. atomic hydrogen gas in galaxies between z = 0 and z = 1.5. Bottom: the evolution of the molecular-to-atomic mass ratio with redshift. [Adapted from Cortese et al. 2017]

Adding to the Sample

Cortese and collaborators combined observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and Arecibo to estimate the ratio of molecular to atomic hydrogen in five HIGHz-survey massive star-forming galaxies at a redshift of z ~ 0.2. They then combine these results with those of the COOL BUDHIES survey; they argue that, since the two surveys use different selection criteria, the combination of the two samples provides a fairer view of the overall population of star-forming galaxies at z ~ 0.2.

Intriguingly, the HIGHz galaxies do not show the molecular-gas dominance that the COOL BUDHIES galaxies do. Cortese and collaborators demonstrate that the addition of the HIGHz galaxies to the sample reveals that the gas reservoirs of star-forming disks 3 billion years ago are, in fact, still the same as what we see today, suggesting that star formation in galaxies at z ~ 0.2 is likely fueled in much the same way as it is today.

As telescope capabilities increase, we may be able to explore whether this continues to hold true for more distant galaxies. In the meantime, increasing our sample size within the range that we can observe will help us to further explore how galaxies have formed stars over time.

Citation

Luca Cortese et al 2017 ApJL 848 L7. doi:10.3847/2041-8213/aa8cc3

solar corona

solar corona 2012

Images taken during the solar eclipse in 2012. The central color composite of the eclipsed solar surface was captured by SDO, the white-light view of the solar corona around it was taken by the authors, and the background, wide-field black-and-white view is from LASCO. The white arrows mark the “atypical” structure. [Alzate et al. 2017]

It seems like science is increasingly being done with advanced detectors on enormous ground- and space-based telescopes. One might wonder: is there anything left to learn from observations made with digital cameras mounted on ~10-cm telescopes?

The answer is yes — plenty! Illustrating this point, a new study using such equipment recently reports on the structure and dynamics of the Sun’s corona during two solar eclipses.

A Full View of the Corona

The solar corona is the upper part of the Sun’s atmosphere, extending millions of kilometers into space. This plasma is dynamic, with changing structures that arise in response to activity on the Sun’s surface — such as enormous ejections of energy known as coronal mass ejections (CMEs). Studying the corona is therefore important for understanding what drives its structure and how energy is released from the Sun.

Though there exist a number of space-based telescopes that observe the Sun’s corona, they often have limited fields of view. The Solar Dynamics Observatory AIA, for instance, has spectacular resolution but only images out to 1/3 of a solar radius above the Sun’s limb. The space-based coronagraph LASCO C2, on the other hand, provides a broad view of the outer regions of the corona, but it only images down to 2.2 solar radii above the Sun’s limb. Piecing together observations from these telescopes therefore leaves a gap that prevents a full picture of the large-scale corona and how it connects to activity at the solar surface.

solar corona 2013

Same as the previous figure, but for the eclipse in 2013. [Alzate et al. 2017]

To provide this broad, continuous picture, a team of scientists used digital cameras mounted on ~10-cm telescopes to capture white-light images from the solar surface out to several solar radii using a natural coronagraph: a solar eclipse. The team made two sets of observations: one during an eclipse in 2012 in Australia, and one during an eclipse in 2013 in Gabon, Africa. In a recent publication led by Nathalia Alzate (Honolulu Community College), the team now reports what they learned from these observations.

Building Atypical Structures

The authors’ image processing revealed two “atypical” large-scale structures with sharp edges, somewhat similar in appearance to what is seen near the boundaries of rapidly expanding polar coronal holes. But these structures, visible in the southeast quadrant of the images taken during both eclipses, were not located near the poles.

By analyzing their images along with space-based images taken at the same time, Alzate and collaborators were able to determine that the shape the structures took was instead a direct consequence of a series of sudden brightenings due to low-level flaring events on the solar surface. These events were followed by small jets, and then very faint, puff-like CMEs that might otherwise have gone unnoticed.

impact of puff-like CMEs

Impact of the passage of a series of puff-like CMEs (shown in the LASCO time sequence in the bottom panels) on coronal structures. [Alzate et al. 2017]

The fact that such innocuous transient events in the Sun’s lower atmosphere can be enough to influence the corona’s large-scale structure for timescales of 12–48 hours is a significant discovery. There are roughly 3 CMEs per day during solar maximum, suggesting that atypical structures like the ones discovered in these images are likely very common. These results therefore have a significant impact on our understanding of the solar corona — which goes to show that there’s still a lot we can learn with small telescopes!

Citation

Nathalia Alzate et al 2017 ApJ 848 84. doi:10.3847/1538-4357/aa8cd2

Centaurus A

How do the supermassive black holes that live at the centers of galaxies influence their environments? New observations of a distant active galaxy offer clues about this interaction.

Signs of Coevolution

M-sigma relation

Plot demonstrating the m-sigma relation, the empirical correlation between the stellar velocity dispersion of a galactic bulge and the mass of the supermassive black hole at its center. [Msigma]

We know that the centers of active galaxies host supermassive black holes with masses of millions to billions of suns. One mystery surrounding these beasts is that they are observed to evolve simultaneously with their host galaxies — for instance, an empirical relationship is seen between the growth of a black hole and the growth of its host galaxy’s bulge. This suggests that there must be a feedback mechanism through which the evolution of a black hole is linked to that of its host galaxy.

One proposed source of this coupling is the powerful jets emitted from the poles of these supermassive black holes. These jets are thought to be produced as some of the material accreting onto the black hole is flung out, confined by surrounding gas and magnetic fields. Because the jets of hot gas and radiation extend outward through the host galaxy, they provide a means for the black hole to influence the gas and dust of its surroundings.

Radio-loud AGN model

In our current model of a radio-loud active galactic nuclei, a region of hot, ionized gas — the narrow-line region — lies beyond the sphere of influence of the supermassive black hole. [C.M. Urry and P. Padovani]

Clues in the Narrow-Line Region

The region of gas thought to sit just outside of the black hole’s sphere of influence (at a distance of perhaps a thousand to a few thousand light-years) is known as the narrow line region — so named because we observe narrow emission lines from this gas. Given its hot, ionized state, this gas must somehow be being pummeled with energy. In the canonical picture, radiation from the black hole heats the gas directly in a process called photoionization. But could jets also be involved?

In a recent study led by Ákos Bogdán, a team of scientists at the Harvard-Smithsonian Center for Astrophysics used X-ray observations of a galaxy’s nucleus to explore the possibility that its narrow-line region is heated and ionized not only by radiation, but also by the shocks produced as radio jets collide with their surrounding environment.

Heating from Jets

Chandra Mrk 3

Chandra X-ray data for Mrk 3, with radio contours overplotted. Both wavelengths show S-shaped morphology of the jets, with the X-ray emission enveloping the radio emission. A strong shock is present in the west and a weaker shock toward the east. [Bogdán et al. 2017]

Bogdán and collaborators analyzed deep Chandra X-ray observations of the center of Mrk 3, an early-type galaxy located roughly 200 million light-years away. Chandra’s imaging and high-resolution spectroscopy of the galaxy’s narrow-line region allowed the team to build a detailed picture of the hot gas, demonstrating that it shows similar S-shaped morphology to the gas emitting at radio wavelengths, but it’s more broadly distributed.

The authors demonstrate the presence of shocks in the X-ray gas both toward the west and toward the east of the nucleus. These shocks, combined with the broadening of the X-ray emission and other signs, strongly support the idea that collisions of the jets with the surrounding environment heat the narrow-line-region gas, contributing to its ionization. The authors argue that, given how common small-scale radio jets are in galaxies such as Mrk 3, it’s likely that collisional ionization plays an important role in how the black holes in these galaxies impart energy to their surrounding environments.

Citation

Ákos Bogdán et al 2017 ApJ 848 61. doi:10.3847/1538-4357/aa8c76

protoplanetary disk

Growing a planet from a dust grain is hard work! A new study explores how vortices in protoplanetary disks can assist this process.

When Dust Growth Fails

V1247 Orionis

Top: ALMA image of the protoplanetary disk of V1247 Orionis, with different emission components labeled. Bottom: Synthetic image constructed from the best-fit model. [Kraus et al. 2017]

Gradual accretion onto a seed particle seems like a reasonable way to grow a planet from a grain of dust; after all, planetary embryos orbit within dusty protoplanetary disks, which provides them with plenty of fuel to accrete so they can grow. There’s a challenge to this picture, though: the radial drift problem.

The radial drift problem acknowledges that, as growing dust grains orbit within the disk, the drag force on them continues to grow as well. For large enough dust grains — perhaps around 1 millimeter — the drag force will cause the grains’ orbits to decay, and the particles drift into the star before they are able to grow into planetesimals and planets.

A Close-Up Look with ALMA

So how do we overcome the radial drift problem in order to form planets? A commonly proposed mechanism is dust trapping, in which long-lived vortices in the disk trap the dust particles, preventing them from falling inwards. This allows the particles to persist for millions of years — long enough to grow beyond the radial drift barrier.

Observationally, these dust-trapping vortices should have signatures: we would expect to see, at millimeter wavelengths, specific bright, asymmetric structures where the trapping occurs in protoplanetary disks. Such disk structures have been difficult to spot with past instrumentation, but the Atacama Large Millimeter/submillimeter Array (ALMA) has made some new observations of the disk V1247 Orionis that might be just what we’re looking for.

V1247 Ori model

Schematic of the authors’ model for the disk of V1247 Orionis. [Kraus et al. 2017]

Trapped in a Vortex?

ALMA’s observations of V1247 Orionis are reported by a team of scientists led by Stefan Kraus (University of Exeter) in a recent publication. Kraus and collaborators show that the protoplanetary disk of V1247 Orionis contains a ring-shaped, asymmetric inner disk component, as well as a sharply confined crescent structure. These structures are consistent with the morphologies expected from theoretical models of vortex formation in disks.

Kraus and collaborators propose the following picture: an early planet is orbiting at 100 AU within the disk, generating a one-armed spiral arm as material feeds the protoplanet. As the protoplanet orbits, it clears a gap between the ring and the crescent, and it simultaneously triggers two vortices, visible as the crescent and the bright asymmetry in the ring. These vortices are then able to trap millimeter-sized particles.

V1247 Orionis simulation

Gas column density of the authors’ radiation-hydrodynamic simulation of V1247 Orionis’s disk. [Kraus et al. 2017]

The authors run detailed hydrodynamics simulations of this scenario and compare them (as well as alternative theories) to the ALMA observations of V1247 Orionis. The simulations support their model, producing sample scattered-light images that match well the one-armed spiral observed in previous scattered-light images of the disk.

How can we confirm V1247 Orionis provides an example of dust-trapping vortices? One piece of supporting evidence would be the discovery of the protoplanet that Kraus and collaborators theorize triggered the potential vortices in this disk. Future deeper ALMA imaging may make this possible, helping to confirm our picture of how dust builds into planets.

Citation

Stefan Kraus et al 2017 ApJL 848 L11. doi:10.3847/2041-8213/aa8edc

neutron-star merger

Where were you on Thursday, 17 August 2017? I was in Idaho, getting ready for Monday morning’s solar eclipse. What I didn’t know was that, at the time, around 70 teams around the world were mobilizing to point their ground- and space-based telescopes at a single patch of sky suspected to host the first gravitational-wave-detected merger of two neutron stars.

Sudden Leaps for Science

BH/NS masses

The masses for black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and the neutron stars that merged in GW170817 (orange). [LIGO-Virgo/Frank Elavsky/NorthwesternUniversity]

The process of science is long and arduous, generally occurring at a slow plod as theorists make predictions, and observations are then used to chip away at these theories, gradually confirming or disproving them. It is rare that science progresses forward in a giant leap, with years upon years of theories confirmed in one fell swoop.

14 September 2015 marked the day of one such leap, as the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves for the first time — simultaneously verifying that black holes exist, that black-hole binaries exist, and that they can merge on observable timescales, emitting signals that directly confirm the predictions of general relativity.

As it turns out, 17 August 2017 was another such day. On this day, LIGO observed a gravitational-wave signal unlike its previous black-hole detections. Instead, this was a signal consistent with the merger of two neutron stars.

nuetron-star merger model

Artist’s illustrations of the stellar-merger model for short gamma-ray bursts. In the model, 1) two neutron stars inspiral, 2) they merge and produce a gamma-ray burst, 3) a small fraction of their mass is flung out and radiates as a kilonova, 4) a massive neutron star or black hole with a disk remains after the event. [NASA, ESA, and A. Feild (STScI)]

What We Predicted

Theoretical models describing the merger of two compact objects predict a chirping gravitational-wave signal as the objects spiral closer and closer. Unlike in a black-hole merger, however, the end of the chirp from merging neutron stars should coincide with a phenomenon known as a short gamma-ray burst: a powerful storm of energetic gamma rays produced as the objects finally collide.

According to the models, these gravitational waves and gamma rays will be followed by a kilonova — a transient source visible in infrared, optical, and ultraviolet — which arises from radioactive decay of heavy elements formed in the collision. This source should gradually decay over a timescale of weeks.

Lastly, the merger could create a powerful jet of high-energy particles, which could be visible to us in X-ray and radio wavelengths as it is emitted and interacts with its surrounding environment. We could also detect neutrinos from this outflow.

What We Saw (and Didn’t See)

The localization of the gravitational-wave, gamma-ray, and optical signals of the neutron-star merger detected on 17 August, 2017. [Abbott et al. 2017]

So what did we see on 17 August, 2017 and thereafter? Here’s what was found by the army of collaborations searching in gravitational waves, electromagnetic signals across the spectrum, and neutrinos:

  1. Gravitational Waves
    The gravitational-wave signature of a binary neutron-star merger was observed with all three gravitational-wave detectors currently operating as a part of the LIGO-Virgo collaboration. GW170817’s signal was in the sensitivity band of these detectors for ~100 seconds, arriving first at the Virgo detector in Italy, next at LIGO-Livingston in Louisiana 22 milliseconds later, and finally at LIGO-Hanford in Washington 3 milliseconds after that. These detections localized the source to a region of 31 square degrees at a relatively nearby distance of ~130 million light-years, and they identified the binary components to be neutron stars.
  2. Gamma-Ray Burst
    The Fermi Gamma-Ray Burst Monitor detected a short (~2-second) gamma-ray burst, GRB170817A, which appears to have occurred 1.7 seconds after the merger indicated by the gravitational-wave signal. This source was later identified by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft as well.
  3. observatory map

    Locations of the many observatories that observed the neutron-star merger first detected on 17 August, 2017. [Abbott et al. 2017]

    Electromagnetic Counterpart and Host Galaxy
    Though they were initially foiled by the signal’s location (the localized region of GW170817 only became visible in Chile 10 hours after its detection), the One-Meter, Two-Hemisphere team used the Swope telescope at Las Campanas Observatory in Chile to discover an optical counterpart to the LIGO and Fermi detection, located in the early-type galaxy NGC 4993. Within an hour, five other teams had independently detected the optical source in NGC 4993, with more following after.
    In the subsequent hours, days, and weeks, observatories across the electromagnetic spectrum monitored the transient. The source soon faded from view in the ultraviolet and gradually reddened in the optical and infrared bands. Delayed X-ray emission was discovered ~9 days after the LIGO signal, and a radio counterpart was discovered a week after that.
  4. No Neutrinos
    Though several neutrino observatories searched for high-energy neutrinos in the direction of NGC 4993 in the two-week period following the merger, none were detected.
timeline of GW170817 observations

Summary and timeline of the observations of the neutron-star merger detected on 17 August, 2017 relative to the time tc of the gravitational-wave event. Click for a closer look. [Abbott et al. 2017]

A Spectacular Confirmation

So what do these observations tell us? Our model for neutron-star mergers appears to be remarkably successful! The associated detections of gravitational waves and electromagnetic counterparts have confirmed that merging neutron stars produce the expected gravitational-wave signal, that they are the source of gamma-ray bursts, that some of the heaviest elements in the universe are produced during the collision of these stars, and that jets of high-energy particles are created that subsequently interact with their environment.

As with any interesting scientific discovery, new points of exploration have arisen — we can now wonder why the gamma-ray burst was unusually weak given its close distance, for instance, or why we didn’t detect any neutrinos from the outflow.

In spite of our new questions, the combination of these recent discoveries provide a resounding verification of our understanding of how compact objects merge. The various signals that began on 17 August, 2017 have simultaneously confirmed a stack of carefully constructed theories that were crafted over decades to explain how seemingly unrelated electromagnetic signals might all tie together. It’s a beautiful thing when science works out this well!

For more information, check out the ApJL Focus Issue on this result here:
Focus on The Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817  

Citation

Abbott, B.P. et al 2017 ApJL 848 L12. doi:10.3847/2041-8213/aa91c9

UGC 11411 Hubble

VLA

The Karl G. Jansky Very Large Array, located in Socorro, NM. [John Fowler]

Taking advantage of a program offered by the National Radio Astronomy Observatory (NRAO), an undergraduate class has observed local dwarf galaxies to learn about their properties.

The Benefits of Nearby Dwarfs

If you want to learn about the physical properties of low-mass galactic halos, the best place to look is nearby dwarf galaxies. These objects have the benefit of being close enough that we can resolve individual stars, allowing us to explore the relationship between star formation and the surrounding interstellar medium. They also allow us to directly measure bulk velocities, so we can interpret the distributions of both dark and baryonic matt5ter in these galaxies.

HI images of UGC 11411

HI images of UGC 11411. Left: HI mass surface density. Right: the intensity-weighted velocity field of the HI gas, which reveals the bulk kinematics of the galaxy. [Bralts-Kelly et al. 2017]

Though thousands of local-volume, gas-rich objects have been explored by gas surveys in the past, many have slipped through the cracks due to the varied selection criteria of these different surveys. In a new study, neutral atomic hydrogen observations are presented for the first time for two of these star-forming, gas-rich dwarf galaxies.

A Class in Action

Guided by Professor John Cannon and collaborators at other universities, a class of undergraduates at Macalester College in St. Paul, Minnesota, has coauthored a study of the neutral interstellar medium of these two local dwarf galaxies. The project was made possible by the “Observing for University Classes” program offered by NRAO’s Karl G. Jansky Very Large Array (VLA), in which university classes in observational astronomy can apply for observing time with the VLA.

UGC 11411

Top: a view of UGC 11411’s stars from Hubble. Middle: the locations of the galaxy’s star formation, as traced by SAO’s telescope’s observations of Hα. Bottom: UGC 11411’s neutral interstellar medium distribution (red contour), overlaid on the other two data sets. [Bralts-Kelly et al. 2017]

The students used the VLA to obtain neutral hydrogen spectral-line observations of UGC 11411 and UGC 8245 in February and March of this year. They then processed and analyzed the data, exploring the stellar population and star formation in each galaxy, and using the galaxies’ bulk kinematics to calculate their total dynamical masses.

Dominated by Dark Matter

The authors found that in both galaxies, the greatest bulk of the neutral interstellar medium can be found in the same location as the ongoing star formation. The two galaxies are different in several ways, however: UGC 8245 has a much lower star formation rate than UGC 11411 currently, and though the neutral hydrogen gas and stellar masses are similar for both galaxies, UGC 11411 has a halo that is more than an order of magnitude more massive.

They conclude that UGC 8245 — which has a total mass that is only ~2 times larger than its baryonic mass — is very similar to other low-mass galaxies that have been studied in the past. On the other hand, UGC 11411 — which has a total mass that is at least a factor of 10 larger than its baryonic mass — is significantly more massive than other known local low-mass galaxies, and it is unusually highly dark-matter dominated.

Further explorations of these dwarfs in contrast to one another will continue to reveal information about the low-mass galaxies of the universe.

Citation

Lilly Bralts-Kelly et al 2017 ApJL 848 L10. doi:10.3847/2041-8213/aa8ea0

gamma-ray burst

With the first detection of gravitational waves in 2015, scientists celebrated the opening of a new window to the universe. But multi-messenger astronomy — astronomy based on detections of not just photons, but other signals as well — was not a new idea at the time: we had already detected tiny, lightweight neutrinos emitted from astrophysical sources. Will we be able to combine observations of neutrinos and gravitational waves in the future to provide a deeper picture of astrophysical events?

Signs of a Merger

binary neutron star merger

Artist’s impression of the first stage of a binary neutron star merger. [NASA, ESA, and A. Feild (STScI)]

If the answer is yes, the key will probably be short gamma-ray bursts (SGRBs). Theory predicts that when a neutron star merges with another compact object (either another neutron star or a black hole), a number of signals may be observable. These include:

  • gravitational waves as the binary spirals inward,
  • a brief burst of gamma rays at merger (this is the SGRB),
  • high-energy neutrino emission during the SGRB,
  • optical and infrared emission after the merger in the form of a kilonova, and
  • radio afterglows of the merger remnants.

While we’ve observed the various electromagnetic components of this picture, the multi-messenger part is lacking: gravitational-wave detections haven’t been made in conjunction with electromagnetic counterparts thus far, and the only confirmed astrophysical sources of neutrinos are the Sun and Supernova 1987A.

neutrino fluxes

Pedicted neutrino fluxes during different stages of emission in an SGRB. [Kimura et al. 2017]

Can we expect this to change in the future? A team of authors led by Shigeo Kimura (Pennsylvania State University) has now explored the likelihood that we’ll be able to detect high-energy neutrinos in association with future gravitational-wave events.

Detecting the SGRB Neutrinos

Kimura and collaborators first estimate the flux of high-energy neutrinos expected during various emission phases of an SGRB. They show that a period of late-time emission, known as the “extended emission” phase, may produce high-energy neutrinos more efficiently than the other phases. But would we be able to see these neutrinos?

IceCube vs IceCube-Gen2

A comparison of IceCube’s detection capabilities (top) to those of the planned IceCube-Gen2 (bottom), for different models of neutrino emission during an SGRB. [Kimura et al. 2017]

To answer this, the authors calculate the probability of detection for neutrinos coming from a distance of ~300 Mpc — the predicted sensitivity range of advanced LIGO for gravitational-wave detection from a face-on neutron-star binary. They find that the IceCube Neutrino Observatory could detect neutrinos from around 10% of average extended-emission events — or perhaps up to half in the most optimistic scenario. The planned next iteration of the detector, IceCube-Gen2, should do better, however: Kimura and collaborators estimate that a quarter of the extended emission events will be detectable in the general case, and up to three quarters of them may be seen in the optimistic case.

The authors’ calculations suggest that within several years of operation of IceCube-Gen2, there is a good chance that we’ll be able to simultaneously detect gamma rays, neutrinos, and gravitational waves from bright SGRBs. This will provide us with powerful tools for learning about the physics of these energetic events.

Citation

Shigeo S. Kimura et al 2017 ApJL 848 L4. doi:10.3847/2041-8213/aa8d14

hot Jupiter

Jupiter-like planets with blisteringly close-in orbits are generally friendless, with no nearby planets transiting along with them. Giant planets with orbits a little further out, on the other hand, often have at least one companion. A new study examines the cause of hot Jupiters’ loneliness.

Forming Close-In Giants

planet forming in disk

Artist’s impression of a planet forming within a protoplanetary disk. [NAOJ]

Though we’ve studied close-in giant planets for decades now, we still don’t fully understand how these objects form and evolve. Jupiter-like giant planets could form in situ next to their host stars, or they could form further out in the system — beyond the ice line — and then migrate inwards. And if they do migrate, this migration could occur early, while the protoplanetary disk still exists, or long after, via excitation of large eccentricities.

We can try to resolve this mystery by examining the statistics of the close-in giant planets we’ve observed, but this often raises more questions than it answers. A prime example: the properties of close-in giants that have close-in companion planets orbiting in the same plane (i.e., co-transiting).

About half of warm Jupiters — Jupiter-like planets with periods of 10–30 days — appear to have close-in, co-transiting companions. In contrast, almost no hot Jupiters — Jupiter-like planets with periods of less than 10 days — have such companions. What causes this dichotomy?

schematic of orbit tilt

Schematic of the authors’ model, in which the close-in giant (m1) encounters a resonance with its host star, causing the orbit of the exterior companion (m2) to become tilted. [Spalding & Batygin 2017]

Friendless Hot Jupiters

While traditional models have argued that the two types of planets form via different pathways — warm Jupiters form in situ, or else migrate inward early and smoothly, whereas hot Jupiters migrate inward late and violently, losing their companions in the process — a new study casts doubt on this picture.

Two scientists from the California Institute of Technology, Christopher Spalding and Konstantin Batygin, propose an alternative picture in which both types of planets form through identical pathways. Instead, they argue, a hot Jupiter’s apparent loneliness arises over time through interactions with its host star.

Stellar Interactions Impact Companions

keeping the companion

Semimajor axis for the outer companion (a2) vs that of the close-in giant planet (a1) at three different system ages. Outer companions within the shaded region will not encounter the resonance investigated by the authors, instead remaining coplanar with the inner giant. For this reason, warm Jupiters will have evident companions whereas hot Jupiters will not. [Spalding & Batygin 2017]

Whether giant planets form in situ near their hosts or migrate inward, they can still have close-in, co-transiting companions outside of their orbit shortly after their birth, Spalding and Batygin argue. But after the disk in which they were born dissipates, the orbits of their companions may be altered.

The authors demonstrate that because hot Jupiters are so close to their hosts, these giants eventually encounter a resonance with their stellar hosts’ quadrupole moment, which arises because rotating stars aren’t perfectly spherical. This resonance tilts the orbits of the hot Jupiters’ outer, lower-mass companions, rendering the companions undetectable in transit surveys.

Warm Jupiters, on the other hand, are located just far enough away from their hosts to avoid feeling the effects of this resonance — which allows them to keep their outer companions in the same plane.

Based on their model, Spalding and Batygin make direct predictions for the systems they expect to be observed in large upcoming surveys like the Transiting Exoplanet Survey Satellite (TESS) — which means we should soon have a sense of whether their picture is correct. If it is, it will confirm that the non-sphericity of stars can have significant impact on the dynamics and architecture of exoplanetary systems.

Citation

Christopher Spalding and Konstantin Batygin 2017 AJ 154 93. doi:10.3847/1538-3881/aa8174

1 … 90 91 92 93 94 … 119