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ASKAP

What causes fast radio bursts? Scientists are still hunting for the answer to this question — but the discovery of one burst’s nearby home may bring us a little closer to a solution.

Mysterious Pulses

More than a decade after their first discovery, fast radio bursts continue to pose a tantalizing mystery. These bizarre millisecond-duration pulses have been determined to be extragalactic in origin, yet we still don’t know what causes them. Could they be from merging black holes or neutron stars? Especially energetic supernovae? Collapsing pulsars? Or even something exotic, like cosmic string interactions?

FRB 121102

Visible-light image of the host galaxy of FRB 121102, previously the only fast radio burst that has been localized. [NRAO/ Gemini Observatory/AURA/NSF/NRC]

To narrow down the options, we must first understand the environments producing these extragalactic bursts. That’s more easily said than done, however — until now, though we’ve discovered dozens of fast radio bursts, we’ve only managed to localize one to its host galaxy.

Typical or Unusual?

The only localized burst, FRB 121102, resides in a bright, star-forming region in the outskirts of a low-metallicity dwarf galaxy. But FRB 121102 is unusual among fast radio bursts: it’s the only burst observed to repeat, and its environment appears to be more highly magnetized than those of other bursts.

Is FRB 121102 is representative of the general population of fast radio bursts, or do its unique characteristics mean that it’s caused by something completely different than the rest of the population? Our best bet for answering this question is to find the homes of more bursts — and now we may be in luck.

A Plausible Home Found

Led by Elizabeth Mahony (CSIRO Astronomy and Space Science, Australia), a team of scientists has discovered the likely home of a recent fast radio burst, FRB 171020. This burst’s dispersion measure — a measure of the amount of matter the signal traveled through to get to us — is the lowest of all known fast radio bursts, indicating that this burst originated relatively close to us.

localization of FRB 171020

Australia Telescope Compact Array (ATCA) radio continuum image indicating the localization region of FRB 171020. Small circles mark candidate host galaxies for the burst. The blue circle marks the most likely candidate, ESO 601–G036. The inset shows a zoomed-in view of this galaxy. [Mahony et al. 2018]

By searching the plausible volume of space from which the signal could have come, Mahony and collaborators found that its most likely host is the galaxy ESO 601–G036, located just 120 million light-years away.

Missing Persistence

Intriguingly, ESO 601–G036 is both similar to and different from the host galaxy of FRB 121102. The two galaxies are alike in size, they have similarly low metallicities, and they have similar star formation rates.

But FRB 121102’s host galaxy harbors a persistent compact radio source — a source that continuously emits bright radio emission. Some astronomers have even proposed that detecting such persistent radio emission may be a way to identify fast-radio-burst hosts in the future. In contrast, ESO 601–G036 shows no sign of a persistent radio source — nor does any other galaxy in the volume of space from which FRB 171020 could have originated.

The nearness of the potential home found for FRB 171020 will provide us with a convenient opportunity to learn about this host environment in more detail in the future. Meanwhile, the contrast of this galaxy to the host galaxy of FRB 121102 seems to support the idea that there may be different types of fast radio bursts with different origins.

Citation

“A Search for the Host Galaxy of FRB 171020,” Elizabeth K. Mahony et al 2018 ApJL 867 L10. doi:10.3847/2041-8213/aae7cb

HL Tauri

Bright rings and dark gaps are common features in images of protoplanetary disks. How we interpret these features is key to our understanding of how planetary systems form and evolve — so what do these rings and gaps really mean?

HD 163296

ALMA image of continuum (dust) emission from HD 163296, the subject of today’s paper. [ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)]

Mind the Gap

The dark lanes that punctuate the bright millimeter emission of protoplanetary disks are often thought to signal the presence of baby planets, which sweep up gas and dust as they orbit their parent star. As exciting as this scenario is, other possibilities exist; the gaps in emission could arise due to gravitational instabilities, the growth of dust grains, or the trapping of dust in high-pressure regions.

How can we distinguish between gaps opened by planets and those generated through other methods? And if the gaps are associated with newly formed planets, how can we reliably extract the properties of the planets — their orbital distances and masses — from the observed gaps in dust emission?

To explore these issues, a team of astronomers led by Nienke van der Marel (National Research Council Herzberg Institute for Astrophysics, Canada) considered the case of the disk surrounding the young star HD 163296.

Interpreting ALMA

HD 163296

Comparison of ALMA observation of HD 163296 to the same image, enhanced, and the two models. Click to enlarge. [Adapted from van der Marel, Williams & Bruderer 2018]

HD 163296 was the subject of much attention earlier in 2018 when two teams independently found evidence for planets embedded in its disk. Van der Marel and coauthors explored the origin of the observed gaps in the disk by modeling the expected emission for two scenarios:

  1. The observed gaps in emission are due to a deficit of dust particles inside the gap caused by dust accretion onto a young planet.
  2. The observed gaps in emission are caused by an increase in the size of the dust grains near snowlines — where it’s cold enough for water, carbon monoxide, and other volatiles to freeze into solids.

Their models show that either planets or snowlines can cause the gaps and rings in the disk around HD 163296. Luckily, we can distinguish between the two scenarios by considering the gas distribution; although the emission is similar in the two models, the presence of planets would decrease both the gas and dust density, whereas the presence of a snowline would not affect the gas density.

van der Marel, Williams & Bruderer 2018 Fig. 4

Azimuthally averaged carbon monoxide emission for four modeled gap depths. Shallower gaps correspond to Saturn-mass planets, while deeper gaps correspond to Jupiter-mass planets. Click to enlarge. [van der Marel, Williams & Bruderer 2018]

Taking Cues from Carbon Monoxide

A common tracer of gas in protoplanetary disks is carbon monoxide. When the authors modeled the emission from the carbon monoxide gas, they found something unexpected: as the carbon monoxide density in the gaps decreased, the emission increased.

A closer look at the chemistry reveals why: when gaps are introduced, more of the star’s ultraviolet radiation can reach the gas, which increases its temperature. The increased temperature returns frozen carbon monoxide molecules to the gas phase, which results in increased emission. However, if the gap is sufficiently deep, the warming of the gas isn’t enough to compensate for the decreased density, and the emission decreases.

Because the gap depth is linked to the planet mass, this result underscores the importance of caution when interpreting disk observations. Hopefully, future observations with ALMA can disentangle the roles of planets and snowlines in generating gaps and rings!

Citation

“Rings and Gaps in Protoplanetary Disks: Planets or Snowlines?” Nienke van der Marel, Jonathan P. Williams, and Simon Bruderer 2018 ApJL 867 L14. doi:10.3847/2041-8213/aae88e

X-ray binary

New observations have captured a feeding black hole in our galaxy as it bursts onto the scene.

Bursting Binaries

X-ray binary

This representation of an X-ray binary shows the accretion disk that surrounds the black hole. According to models, instabilities in this accretion disk can lead to the binary going into outburst. [NASA/R. Hynes]

Some of the most easily discoverable stellar-mass black holes in our own galaxy are those in X-ray binaries: binary star systems that consist of a star in orbit with a compact object like a black hole or a neutron star. In such a system, mass is siphoned off the donor star, forming an accretion disk around the feeding compact object. The accreting material emits at X-ray wavelengths, providing these systems with their signature emission.

X-ray binaries come in two main types, depending on the size of the donor star: low-mass and high-mass. In a low-mass X-ray binary (LMXB), the donor star typically weighs less than one solar mass. One type of LMXB, known as a transient/outbursting LMXB, has a peculiar quirk: though they often go undetected in their dim, quiescent accretion state, these sources exhibit sudden outbursts in which the brightness of the system increases by several orders of magnitude in less than a month.

Where do theses outbursts begin? What causes the sudden eruption? What else can we learn about these odd sources? Though theorists have built detailed models of transient LMXBs, we need observations that can confirm our understanding. In particular, most observations only capture transient LMXBs after they’ve transitioned into an outbursting state. But a sneaky telescope has now caught one source in the process of waking up.

Black hole LMXB

ASASSN-18ey’s position on an X-ray vs. optical luminosity diagram, shown with orange and yellow markers as it evolves through its outburst, place it within the region dominated by black hole LMXBs (blue markers) throughout the outburst. [Tucker et al. 2018]

Sudden Discovery

The All-Sky Automated Survey for SuperNovae (ASAS-SN, pronounced “assassin”) regularly scans the sky hunting for transient sources. In March of this year, it spotted a new object: ASASSN-18ey, a system roughly 10,000 light-years away that shows all the signs of being a new black-hole LMXB.

ASASSN-18ey’s initial discovery prompted a flurry of follow-up observations by astronomers around the world. As of 1 October 2018, the tally had hit more than 360,000 observations — giving ASASSN-18ey the potential to be the best-studied black-hole LMXB outburst to date.

In a recent publication led by Michael Tucker (Institute for Astronomy, University of Hawai’i), a team of scientists details what we know about ASASSN-18ey so far, and what it can tell us about how LMXBs behave.

Confirming Models

What makes ASASSN-18ey unique is its discovery in optical wavelengths before X-ray. Tucker and collaborators use the various observations of this source to determine that there was a ~7.2-day lag between the flux rises in the optical and the X-ray light curves.

ASASSN-18ey light curves

The full light curves of ASASSN-18ey from ASAS-SN, ATLAS, and Swift show the source’s rapid rise to outburst state. Click to enlarge. [Adapted from Tucker et al. 2018]

This week-long delay in the two flux increases is predicted by theoretical models in which the LMXB outbursts arise from an instability in the accretion disk surrounding the compact object. Being able to measure this lag for ASASSN-18ey even allowed Tucker and collaborators to determine precisely where in the disk the instability first arose: at a radius of maybe 10,000 km from the black hole — also consistent with models.

Further observations of ASASSN-18ey as it continues to evolve will undoubtedly shed more light on the state transitions and behavior of LMXBs. In the meantime, we can enjoy this sneaky look at a new black hole LMXB system on the rise.

Citation

“ASASSN-18ey: The Rise of a New Black Hole X-Ray Binary,” M. A. Tucker et al 2018 ApJL 867 L9. doi:10.3847/2041-8213/aae88a

Puppis A

Compact objects — the extremely dense remnants left behind after the death of massive stars — continually surprise us with their wide variety of properties and behaviors. Now one compact object known for its stability and predictability has thrown a further hitch into our understanding.

What’s Left Behind

pulsar

Artist’s impression of the strong magnetic fields and beams of radiation emitted by a typical pulsar. [NASA]

When massive stars explode in spectacular supernovae, material collapses onto their cores to leave behind a dense neutron star or black hole. These objects can take a variety of forms, however — from quiet, unexciting bodies emitting little radiation, to pulsars: strongly magnetized neutron stars that bright shine pulses of radiation across us as they rotate.

One particularly puzzling type of body is something we — perhaps unoriginally — termed central compact objects, or CCOs. CCOs lie at the heart of supernova remnants, and they’re detected by their surface X-ray emission. Unlike typical pulsars, however, CCOs are not detectable in other wavelengths, and they do not have strong surface magnetic fields.

1E 1207.4–5209 glitch

Pulse-phase residuals for pulsar 1E 1207.4–5209 reveal a glitch in its rotation at the end of September 2015. [Gotthelf & Halpern 2018]

Pulsar Hiccups

An additional feature distinguishing CCOs from pulsars is their stable and slow spin-down rate. As spinning neutron stars age, they lose energy, spinning slower and slower. In typical pulsars, this steady spin-down can be interrupted by hiccups known as glitches, during which the spin frequency of the pulsar suddenly jumps back up — perhaps due to surface starquakes, or motions of the neutron-star interior.

CCOs, however, are very stable rotators that lose their energy more gradually than typical pulsars. Until now, no one has observed glitches in neutron stars that have spin-down rates as small as those measured in CCOs. But new observations of the CCO 1E 1207.4–5209, presented in a recent publication by Columbia Astrophysics Laboratory researchers Eric Gotthelf and Jules Halpern, have now changed this.

Buried Fields?

Using XMM-Newton and Chandra X-ray data, Gotthelf and Halpern identify a major glitch in 1E 1207.4–5209’s rotation that occurred at the end of September 2015. This unexpected hiccup provides further evidence of the relation between CCOs and more typical pulsars, and it lets us examine the mechanisms that may be at work in the evolution of spinning neutron stars.

pre- and post-glitch spectra

Pre- and post-glitch spectra of pulsar 1E 1207.4–5209 from XMM-Newton show no significant change in line centroids — and therefore magnetic field strength — following the glitch. [Gotthelf & Halpern 2018]

Gotthelf and Halpern compare the spectrum of 1E 1207.4–5209 before and after the glitch and show that there was no change in this object’s weak surface magnetic field. A magnetic-field cause for the glitch isn’t off the table yet, though: one model for CCO formation proposes that CCOs are born like normal pulsars with strong magnetic fields, but these fields are buried and hidden when material from the supernova falls back onto them. This resulting strong internal field could gradually diffuse to the surface, eventually causing a glitch like the one we observed.

Gotthelf and Halpern stress that we should continue to watch 1E 1207.4–5209 in the future, particularly to see if the glitch might have caused it the CCO to transition into a more typical radio-bright pulsar. In the meantime, we have a few more mysteries to puzzle over as we ponder these odd stellar remnants.

Citation

“The First Glitch in a Central Compact Object Pulsar: 1E 1207.4–5209,” E. V. Gotthelf and J. P. Halpern 2018 ApJ 866 154. doi:10.3847/1538-4357/aae152

'Oumuamua

’Oumuamua is back in the headlines again.

When the minor object 1I/2017 ‘Oumuamua was discovered in October of 2017, it was already speeding away from the Sun. ‘Oumuamua’s path, however, quickly identified it not as a solar-system body, but as an asteroid that originated from somewhere beyond our tiny corner of the universe.

Since ‘Oumuamua’s discovery, scientists have scrambled to interpret the bizarre observations of this body. For starters, ‘Oumuamua has an unusual shape: though we have no high-resolution images, light curves suggest that this tumbling asteroid is at least five times as long as it is wide. More recently, however, it’s ‘Oumuamua’s acceleration that has captured the attention of scientists.

Though the asteroid’s trajectory through our solar system was originally assumed to be solely governed by ordinary gravitational forces, recent research suggests that there’s something else at work. ‘Oumuamua’s motion can’t be explained by gravity alone; instead, the asteroid seems to be experiencing an additional acceleration away from the Sun that’s dependent on the Sun–asteroid distance.

What does some of the latest research say about ‘Oumuamua in light of this new development?

‘Oumuamua’s Origin

Encounter distance vs. velocity

Encounter distance vs. velocity between ‘Oumuamua and the four main home candidate stars (plotted in blue, green, red, and yellow), for different models of ‘Oumuamua’s trajectory. [Bailer-Jones et al. 2018]

Previous studies have extrapolated ‘Oumuamua’s path backwards, searching for a nearby star system from which this asteroid may have originated, with no luck: ‘Oumuamua’s trajectory did not seem to line up with any nearby stars.

But the extra acceleration measured for ‘Oumuamua changes this picture. Past studies assumed that ‘Oumuamua was only influenced by gravity as it traveled, but a new study led by Coryn Bailer-Jones (Max Planck Institute for Astronomy, Germany) has traced its path backwards while also taking into account the asteroid’s observed extra boost.

The result? Using Gaia DR2 data, Bailer-Jones and collaborators found at least four plausible host stars for ‘Oumuamua within a few light-years, and future observations are likely to reveal additional candidates.

comet 67P/Churyumov-Gerasimenko

Image of comet 67P/Churyumov-Gerasimenko outgassing as it is heated by the Sun. Could similar processes be occurring on ‘Oumuamua? [ESA/Rosetta/MPS for OSIRIS Team]

Could ‘Oumuamua Be a Comet?

Though puzzling for an asteroid, the additional acceleration measured for ‘Oumuamua is naturally expected for icy comets. Comets passing near the Sun heat up, causing outgassing; this evaporating material then adds a boost to the body’s motion away from the Sun. 

Observations of ‘Oumuamua contradicted the comet theory early on: there’s no observational evidence of outgassing, no coma or dust tails, and no cometary emission lines. But perhaps we just missed the signs, and ‘Oumuamua is somehow a comet in disguise?

That’s unlikely, according to author Roman Rafikov (University of Cambridge, UK and Princeton’s Institute for Advanced Study). In a recent study, Rafikov examined what would happen if ‘Oumuamua were outgassing like a comet. He found that, for reasonable assumptions about ‘Oumuamua’s properties, torques caused by outgassing would cause the body’s spin to evolve rapidly. This process would have spun ‘Oumuamua up to the point where it couldn’t hold itself together anymore, causing it to fly apart before it passed through our solar system.

How About a Giant Sail?

So if ‘Oumuamua’s acceleration can’t be explained by gravity, and it doesn’t seem likely that it’s outgassing, then what is causing the extra boost? Authors Shmuel Bialy and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics) propose an alternative: solar radiation pressure. In a new study, Bialy and Loeb suggest that ‘Oumuamua could have an even more unusual shape than we initially thought: it may be tens of meters in surface area, but less than a millimeter thick.

Bialy and Loeb demonstrate that if this is true, the pressure of photons from the Sun pushing on this large surface could be enough to explain ‘Oumuamua’s extra acceleration, and they demonstrate that such a body could have survived interstellar transit without being eroded or broken apart.

light sail

Artist’s impression of an artificial light sail, a thin spacecraft that can be propelled by radiation pressure. [Josh Spradling / The Planetary Society]

If Bialy and Loeb’s model is correct, then what is ‘Oumuamua? Such a large, thin object would represent a new class of interstellar material that we haven’t previously observed in our solar system, but perhaps something of this shape could arise naturally in the interstellar medium or in protoplanetary disks. Bialy and Loeb also consider the possibility of an artificial origin, proposing that ‘Oumuamua could be a piece of tech debris — a solar sail discarded by an alien civilization and now tumbling through interstellar space. 

The Puzzle Continues

So: is it aliens? Probably not, folks. But does ‘Oumuamua present a large number of puzzles that force us to continue to carefully employ the scientific method, evaluate possible uncertainties, and search for new explanations? Definitely.

We still have so much to learn about this bizarre asteroid. But in many ways, the puzzle of ’Oumuamua beautifully encapsulates the process of science — both in the excitement of an unsolved mystery and in the importance of critical thinking and methodical evaluation of possibilities.

Keep following along as our understanding of ‘Oumuamua continues to evolve!

Citation

“Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 ‘Oumuamua,” Roman R. Rafikov 2018 ApJL 867 L17. doi:10.3847/2041-8213/aae977

“Plausible Home Stars of the Interstellar Object ‘Oumuamua Found in Gaia DR2,” Coryn A. L. Bailer-Jones et al 2018 AJ 156 205. doi:10.3847/1538-3881/aae3eb

“Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?,” Bialy and Loeb 2018 ApJL, in press. https://arxiv.org/abs/1810.11490

Helix Nebula (NGC 7293)

The behavior of electrons in tenuous interstellar nebulae is up for discussion. What is the best way to describe the energies of electrons in these environments?

Orion Nebula (M42)

The gas of the Orion Nebula (M42) is ionized by the young high-mass stars at its center. H II regions like the Orion Nebula may host non-Maxwellian electron energy distributions. [NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team]

Energetic Electrons

H II regions and planetary nebulae are bubbles of ionized gas surrounding young high-mass stars and dying low- to intermediate-mass stars, respectively. We can calculate the density, temperature, and composition of these nebulae by measuring the strengths of their emission lines, but we rely on assumptions about the plasma to interpret the observed line strengths.

Typically, we assume that the electrons in the diffuse, highly irradiated environments of H II regions and planetary nebulae adhere to a Maxwell-Boltzmann distribution, which describes the velocities of a system of particles in thermodynamic equilibrium. However, the observed line strengths don’t always match their theoretically predicted values, causing some astronomers to wonder if this assumption is correct.

A proposed alternative to the Maxwell-Boltzmann distribution is the κ-distribution, which has more particles with high velocities and has been used to describe electron populations in the hot, tenuous solar wind. Which distribution is a better fit for H II regions and planetary nebulae?

Draine & Kreisch 2018 Fig. 5

The calculated steady-state solution for an H II region. The steady-state solution only deviates significantly from a Maxwellian distribution above ~13 eV. [Draine & Kreisch 2018]

May the Best Distribution Win

Bruce Draine and Christina Kreisch of Princeton University approached this problem by deriving the steady-state electron energy distribution in H II regions and planetary nebulae from first principles.

The authors show that the steady-state electron energy distribution is very nearly Maxwellian. While there is a lingering high-energy tail, it contains only ~0.000005% of the electrons in the planetary nebula case and even fewer in the H II region case — not enough to cause the observed departure from theoretical line ratios.

However, it’s not enough to show that the steady-state solution is consistent with the expected Maxwellian distribution. The conditions in the plasma must allow the system to reach the steady-state solution within a reasonable amount of time. To explore this, the authors modeled the time evolution of a population of electrons with a highly nonthermal distribution.

Draine & Kreisch 2018 Fig. 7

Time evolution of the electron energy distribution (left) and the evolution of the distribution relative to a Maxwellian (right). Click to enlarge. [Draine & Kreisch 2018]

Going Steady

Assuming typical values for H II regions and planetary nebulae, Draine and Kreisch find that the distribution quickly relaxes to the steady-state solution. For Orion-Nebula-like conditions — ~3,000 electrons per cubic centimeter — the relaxation time is only 30 seconds. For the more highly irradiated environs of planetary nebula NGC 7293 (the Helix Nebula), the relaxation time is longer, but still short enough to reasonably assume that the steady-state solution will be achieved.

These results show that the Maxwellian distribution is still the best way to describe electrons in H II regions and planetary nebulae. What is causing the unexpected emission line ratios, then? The authors point out that our models assume that the emission arises from plasma with only one temperature — but in reality, the electron temperature likely varies spatially over the region from which we observe the emission.

Citation

“Electron Energy Distributions in H II Regions and Planetary Nebulae: κ-distributions Do Not Apply,” B. T. Draine and C. D. Kreisch 2018 ApJ 862 30. doi:10.3847/1538-4357/aac891

star field

Let’s be honest: nature is messy. Natural forms are complex, and simple models are just approximations — there are no truly spherical cows. And yet … it seems there might actually be some true blackbody stars.

Messy Spectra

Just like you can approximate a complex 3D shape — like a cow — by modeling it as a sphere, stars and planets can be simply approximated by modeling them as perfect blackbodies. Blackbodies are objects that absorb all radiation that shines on them, and they emit their own radiation with a characteristic spectrum that depends only on temperature and spans all electromagnetic wavelengths.

typical stellar spectrum

This spectrum of a solar-like star shows just how far a typical star’s spectrum (red) deviates from the ideal blackbody (blue). [Michael Richmond]

But just as nature doesn’t make true spherical cows, real stars are far from perfect blackbodies. Though blackbody-like radiation might leave the surface of a star, the gas of the star’s atmosphere absorbs and emits light, creating deep absorption and emission lines in the spectrum we observe. These lines, in fact, are how we classify stars — the O, B, A, F, G, K, and M spectral types for stars are determined based on what the lines muddying a star’s spectra tell us about its properties.

Sometimes, however, nature apparently is simple. Two scientists, Nao Suzuki and Masataka Fukugita of the University of Tokyo, have now discovered 17 stars that are ideal blackbodies: the stars have no distinct spectral features from infrared to ultraviolet wavelengths.

The Hunt for No Features

The first of these bizarre stars was found by accident — it was stumbled upon in a Sloan Digital Sky Survey (SDSS) catalog of quasars. Following up on this unexpected discovery, Suzuki and Fukugita hunted through nearly 800,000 star-like objects in SDSS archives, looking for other blackbody spectra that showed large proper motions (implying the objects are probably nearby stars) and no spectral features.

blackbody star

Example of data and residuals for one of the authors’ blackbody stars. The top panel shows the observed spectrum (grey), the spectrum with noise smoothed (blue), and the blackbody fit (red). The bottom panel shows the photometric data from which the fit parameters are derived. [Suzuki & Fukugita 2018]

The authors then explored Galaxy Evolution Explorer (GALEX) ultraviolet spectra and Wide-field Infrared Survey Explorer (WISE) infrared spectra for their candidates, to ensure that the candidates’ blackbody behavior extends beyond just the visible-light spectrum. This selective approach produced 17 objects that met all criteria.

The 17 blackbody stars pose an intriguing puzzle: what are these oddly ideal bodies? Suzuki and Fukugita argue that the stars’ properties are consistent with those of a special type of compact object — a DB white dwarf — that has a temperature too low (a cool ~10,000 K) to develop helium absorption features.

Time to Calibrate

What can we do with these objects? When nature hands you a beautifully perfect blackbody spectrum, there’s clearly only one course of action: use it to calibrate all your instruments!

Suzuki and Fukugita used their blackbody stars to carefully examine the zero point that was set for each of SDSS’s five photometry passbands, as well as the consistency of these zero points with those of the ultraviolet photometry for GALEX and the infrared photometry for WISE.

Just like that, a simple model proves unexpectedly relevant — and spherical-cow stars provide a useful calibration measure for current and future instruments. If only all of nature were so kind!

Citation

“Blackbody Stars,” Nao Suzuki and Masataka Fukugita 2018 AJ 156 219. doi:10.3847/1538-3881/aac88b

multi-planet system

Our search for worlds beyond our own solar system has revealed thousands of exoplanets in an incredible variety of sizes and configurations. But a new study has revealed that there may be a treasure trove of additional planets hiding where we can’t look as easily: close in around low-metallicity stars.

Exploring Architecture

In recent years, we’ve determined that the vast majority of stars in our galaxy host at least one planet. Generally, we observe two main types of exoplanetary system architectures close in around stars:

  1. hot jupiters, massive planets with very short orbital periods, and
  2. compact multi-planet systems, systems containing multiple small planets on tight orbits.
hot Jupiter

Artist’s impression of a hot-Jupiter exoplanet. [NASA]

Intriguingly, these two types of architectures seem to be largely mutually exclusive: where we see hot Jupiters, we’re unlikely to see any close companions, and where there are compact multi-planet systems, there are rarely nearby massive planets.

Led by John Brewer (Yale University and Columbia University), a team of scientists has now explored this odd trend more carefully by investigating how system architecture trends with the metallicities of host stars. Can we draw conclusions about what types of planets we expect to find in different systems?

planet system frequencies

Frequency of compact multi-planet systems (blue) increases with decreasing metallicity as a fraction of known planet hosts. Hot-Jupiter (orange) and cool-Jupiter (green) systems, on the other hand, become more frequent as metallicity increases. [Brewer et al. 2018]

An Unexpected Trend

Brewer and collaborators constructed a catalog of 716 stars known to host 1,148 planets. The team next obtained uniform high-resolution optical spectra for each of these stars with the Keck HIRES spectrograph, which they used to determine the abundances of heavy metals in the stars. They then compared the abundances for hosts of different system architectures.

Previous studies had already showed that hot Jupiters are preferentially found around higher metallicity stars, and the results from Brewer and collaborators’ sample confirmed this, showing a distinctive rise in the fraction of hosts that have both hot and cold Jupiters at higher metallicities.

More surprising, however, were the team’s results for compact multi-planet systems. While the frequency of these systems appears to remain roughly constant for stars around and above solar metallicities, the authors’ data show a large spike in frequency for compact multi-planet systems around stars of very low metallicities.

Surveys Past and Future

planet host metallicities

In the authors’ sample, stars with low metallicity or a high ratio of Si/Fe do not seem to form hot Jupiters, and they are increasingly likely to host compact multi-planet systems. [Brewer et al. 2018]

These results have a number of interesting implications for planet-formation models. In addition, they suggest that we’ve underestimated how many compact multi-planet systems are out there.

How have we missed this? To optimize for finding easy-to-detect hot Jupiters, past radial-velocity exoplanet surveys have primarily targeted high-metallicity hosts. But while current surveys lack the precision to detect the small planets of compact multi-planet systems, that will soon change with the introduction of new, extreme-recision radial-velocity instruments.

Brewer and collaborators’ results suggest that targeting low-metallicity stars with these upcoming surveys is the way to go — and there may be many more compact systems for us to find than we’d ever realized!

Another tantalizing detail is that these low-metallicity hosts tend to be older stars in our galaxy, suggesting that their planetary systems have had a long time to develop. This is good news for astrobiology enthusiasts: we may soon have a new reservoir of small planets to explore for life.

Citation

“Compact Multi-Planet Systems are more Common around Metal-Poor Hosts,” John M. Brewer et al 2018 ApJL 867 L3. doi:10.3847/2041-8213/aae710

supernova

A recent study has discovered three of the fastest stars known in the Milky Way. But these stars may be more than just speeders — they might also be evidence of how Type Ia supernovae occur.

Seeking a Source

Type Ia progenitor scenarios

Two competing theoretical models for the progenitors of Type Ia supernova explosions: the single-degenerate model (top) and the double-degenerate model (bottom). Today’s study focuses on a double-degenerate model in which a one white dwarf explodes in a binary pair, flinging the other one out into space. [NASA/CXC/SAO and GSFC/D. Berry]

Given the extent to which we rely on Type Ia supernovae as standard candles used to measure vast distances, you might think that we’ve got them fairly well figured out. But these stellar explosions are complicated, and it turns out that we don’t know some of the most fundamental things about them! Scientists are still working hard to find answers about what systems Type Ia supernovae originate from, and how the explosions are caused.

Led by astronomer Ken Shen (University of California, Berkeley), a team of astronomers has explored one particular model for Type Ia supernovae further: the “dynamically driven double-degenerate double-detonation” model — or D6, for short. In this scenario, a pair of white dwarfs orbit each other in a binary system. Two back-to-back detonations then cause one of the white dwarfs to explode as a supernova while the other white dwarf survives and is flung free of the explosion site.

Shen and collaborators note that if the D6 model proves to be the primary means of producing Type Ia supernovae, then there’s an observable outcome: there should be white dwarfs speeding throughout our galaxy that were suddenly liberated by the supernova explosions of their companions.

hypervelocity white dwarfs

Posterior probability distributions for the total galactocentric velocities for estimated for the three hypervelocity white dwarf candidates: D6-1, D6-2, and D6-3. [Shen et al. 2018]

Hunt for Speeders

Based on the estimated supernova rate in our galaxy and the properties of binary white dwarfs, Shen and collaborators predict that there should be ~30 hypervelocity white dwarfs within ~3,000 light-years of us. But how to spot these compact stars speeding across the sky? With one of the best tools in the business: Gaia.

Shen and collaborators combed through the numbers from the Gaia mission’s second data release, which presents the astrometric parameters of more than a billion stars across the sky. In this treasure trove of information, they discovered seven candidates that they then followed up with ground-based observations. After ruling out four as ordinary stars, the authors were left with three candidate hypervelocity white dwarfs.

Associated Remnant?

D6-2 orbital solution

The past (blue) and future (red) trajectories calculated for the hypervelocity white dwarf candidate D6-2 suggest it was previously located coincident with the G70.0–21.5 supernova remnant (green dashed circle). [Shen et al. 2018]

The three candidates have total galactocentric velocities between 1,000 and 3,000 km/s (that’s 2.2 to 6.7 million miles per hour!), making them some of the fastest known stars in the Milky Way. That alone is enough to qualify them as potential progenitors of Type Ia supernovae via the D6 model — but Shen and collaborators look for one more clue: whether they can be tracked back to a supernova remnant.

Two of the candidates show no sign of having traveled from a nearby remnant — not necessarily surprising, as the remnants could be very faint, or even have already dissipated completely. But the third candidate can be tracked back to a location within the faint, old supernova remnant G70.0–21.5.

While not yet a smoking gun, these hypervelocity white dwarfs represent important support for the D6 model. And continued follow-up of additional candidates — as well as new candidates discovered in future Gaia releases — may further confirm this model for how Type Ia supernovae occur.

Citation

“Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-Degenerate Double-Detonation Type Ia Supernovae,” Ken J. Shen et al 2018 ApJ 865 15. doi:10.3847/1538-4357/aad55b

Earth and the solar wind

The solar wind extends outward from the solar corona, suffusing interplanetary space with plasma and magnetic fields. While the solar wind has traditionally been designated as either “fast” or “slow” based on its velocity, a new study suggests that there may be a better way to characterize this highly variable plasma flow.

Coronal hole

Coronal holes, like the one clearly visible as a dark region in this X-ray image of the Sun from Solar Dynamics Observatory, are thought to be the source of the fast solar wind. [NASA/AIA]

Slow vs. Fast

The fast solar wind is thought to originate from coronal holes — regions of open solar magnetic field lines. The slow solar wind has been associated with streams of coronal plasma emitted from near the Sun’s equator, but this source location for the slow solar wind is still up for debate.

The formation mechanism for the slow solar wind is also uncertain; one of the persistent questions of solar physics is whether the slow and fast solar wind form in fundamentally different ways.

Solving the mysteries of where and how the slow solar wind forms may rely on first finding a better definition of what constitutes the slow and fast solar wind. While regions of slow and fast solar wind have traditionally been separated based only on velocity, the parameters of the solar wind — such as the density, temperature, and ionization state — vary broadly for a given solar wind speed.

Ko, Roberts & Lepri 2018 Fig. 1

Comparison of solar wind proton speed, components of the proton velocity, and standard deviation in the components of the proton velocity. HCS and PS mark the times of heliospheric current sheet and pseudostreamer crossings, respectively. Low proton speeds are associated with low fluctuations in the proton velocity, while high speeds are associated with high fluctuations in the proton velocity. Click to enlarge. [Ko, Roberts & Lepri 2018]

An ACE up Their Sleeve

Yuan-Kuen Ko of the Naval Research Laboratory and collaborators argue that there is a better way to distinguish between the different states of the solar wind.

By analyzing data from NASA’s Advanced Composition Explorer (ACE), a solar and space exploration mission launched more than two decades ago, Ko and collaborators found that the slow and fast solar wind may be better distinguished by the magnitude of their velocity fluctuations rather than their absolute velocities. To demonstrate this, the authors compared the velocity fluctuation, δvT, to other observed solar wind properties. With the exception of the plasma beta — the ratio of the thermal pressure to the magnetic pressure — δvT correlates well with all observed solar wind properties.

Ko and collaborators also explored the effect the phase of the solar cycle has on solar wind parameters by comparing data from two time intervals: one from the period during which solar activity is declining, and one near solar minimum. The authors found that while the absolute values of the solar wind parameters during epochs of low δvT varied between the two phases, their overall behavior did not; parameters that increased with increasing δvT did so during both the declining phase of the solar cycle and solar minimum.

Ko, Roberts & Lepri 2018 Fig. 10

The three slow-solar-wind formation scenarios implied by the results. Click to enlarge. [Ko, Roberts & Lepri 2018]

More Solar Data Headed Our Way

What does this mean for the formation of the slow solar wind? Ko and collaborators derive three potential slow-solar-wind formation scenarios from their findings, none of which are mutually exclusive.

Distinguishing between these scenarios will have to wait — but not for long. Luckily, the next decade brings two highly anticipated spacecraft that will increase our understanding of the solar corona and solar wind, including the formation of the slow solar wind: NASA’s Parker Solar Probe, which started its journey to the Sun in August 2018, and ESA’s Solar Orbiter, which is scheduled to launch in February 2020.

Citation

“Boundary of the Slow Solar Wind,” Yuan-Kuen Ko, D. Aaron Roberts, and Susan T. Lepri 2018 ApJ 864 139. doi:10.3847/1538-4357/aad69e

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