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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

CI Tau

Blistering hot, giant planets zip around many stars similar to the Sun. New observations of one such planet — likely surrounded by a set of three gas-giant siblings — is now raising questions about the formation of giant planets.

Birthing a Hot Jupiter

Hot Jupiters — gas giants orbiting their hosts at radii of < 0.1 AU — are thought to orbit around 1% of main-sequence solar-type stars. In spite of the many hot Jupiters we’ve discovered, however, we still don’t fully understand how these toasty giants form. Are they born and evolve in situ, alongside their host? Or do they develop further out in the early protoplanetary disk of gas and dust surrounding a young star, and migrate inward later in their lifetimes? What role might outer gas-giant siblings play in this process?

CI Tau continuum observations

Synthesized image of the CI Tau continuum observations, revealing three annular gaps between 10 and 100 AU. The inset shows a 0.35”-wide zoom on the innermost gap, imaged with a finer resolution. [Clarke et al. 2018]

One challenge to answering these questions is we’ve primarily observed older hot Jupiters, rather than those still in the process of forming. But the protoplanetary disk surrounding the young star CI Tau hosts a nascent hot Jupiter — and new observations reveal evidence for additional gas giants in the disk. What can we learn from this system?

A Unique Disk

The Atacama Large Millimeter/submillimeter Array (ALMA) is progressively building its reputation as an imager of the detailed structure within young protoplanetary disks; you might recall one of its first releases, the spectacular image of the concentric gaps and rings of the disk surrounding HL Tau.

In a new study led by Cathie Clarke (University of Cambridge’s Institute of Astronomy, UK), this groundbreaking telescope has been used to image another young star system, CI Tau, which hosts the first hot-Jupiter candidate found still within a protoplanetary disk.

Though CI Tau is roughly the same mass and luminosity as the Sun, it’s only about 2 million years old. The presence of the 11.3-Jupiter-mass giant planet in a close-in orbit in the disk therefore shows that whatever the formation mechanism for hot Jupiters, they must arrive at their close orbits very rapidly!

New Planets, New Challenges

CI Tau model

Synthetic image of the CI Tau continuum emission produced from the authors’ gas and dust hydrodynamical simulation containing three planets. The authors’ models well reproduce the observations of the system. [Clarke et al. 2018]

The hot Jupiter isn’t CI Tau’s only source of intrigue, though: Clarke and collaborators demonstrate that this disk likely hosts another three gas-giant planets of 0.75, 0.15, and 0.4 Jupiter masses, orbiting at 14, 43, and 108 AU. The authors reach this conclusion by fitting detailed dynamical models of the dust and gas to the ALMA data as well as a wealth of supplementary data for the well-studied system.

If confirmed, the presence of these outer planets poses a significant challenge to our understanding of planet formation. Based on current models, the outermost planets shouldn’t have been able to accrete enough material from the outer disk in just 2 million years to reach their current sizes. And even if this mystery could be explained, another exists: the planets’ growth past a certain point should have triggered rapid inward migration.

Though the new observations of CI Tau have raised as many questions as answers, they solidify the association between close-in hot Jupiters and gas giants on wider orbits. As ALMA continues to build a census of young protoplanetary disks, we can hope that future observations will shed further light on the process of building giant planets.

Citation

“High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au,” C. J. Clarke et al 2018 ApJL 866 L6. doi:10.3847/2041-8213/aae36b

neutron-star merger

High-energy radiation released during the merger of two neutron stars last year has left astronomers puzzled. Could a burst of gamma rays from 2015 help us to piece together a coherent picture of both explosions?

A Burst Alone?

When two neutron stars collided last August, forming a distinctive gravitational-wave signal and a burst of radiation detected by telescopes around the world, scientists knew that these observations would change our understanding of short gamma-ray bursts (GRBs). Though we’d previously observed thousands of GRBs, GRB 170817A was the first to have such a broad range of complementary observations — both in gravitational waves and across the electromagnetic spectrum — providing insight into its origin.

GRB energetics

Total isotropic-equivalent energies for Fermi-detected gamma-ray bursts with known redshifts. GRB 170817A (pink star) is a factor of ~1,000 dimmer than typical short GRBs (orange points). GRB 170817A and GRB 150101B (green star) are two of the closest detected short GRBs. [Adapted from Burns et al. 2018]

But it quickly became evident that GRB 170817A was not your typical GRB. For starters, this burst was unusually weak, appearing 1,000 times less luminous than a typical short GRB. Additionally, the behavior of this burst was unusual: instead of having only a single component, the ~2-second explosion exhibited two distinct components — first a short, hard (higher-energy) spike, and then a longer, soft (lower-energy) tail.

The peculiarities of GRB 170817A prompted a slew of models explaining its unusual appearance. Ultimately, the question is: can our interpretations of GRB 170817A safely be applied to the general population of gamma-ray bursts? Or must we assume that GRB 170817A is a unique event, not representative of the general population?

New analysis of a GRB from 2015 — presented in a recent study led by Eric Burns (NASA Goddard SFC) — may help to answer this question.

A Matter of Angles

What does a burst from 2015 have to do with the curious case of GRB 170817A? Burns and collaborators have demonstrated that this 2015 burst, GRB 150101B, exhibited the same strange behavior as GRB 170817A: its emission can be broken down into two components consisting of a short, hard spike, followed by a long, soft tail. Unlike GRB 170817A, however, GRB 150101B is not underluminous — and it lasted less than a tenth of the time.

GRB 150101B

Fermi count rates in different energy ranges showing the short hard spike and the longer soft tail in GRB 150101B. The short hard spike is visible above 50 keV (top and middle panels). The soft tail is visible in the 10–50 keV channel (bottom panel). [Burns et al. 2018]

Intriguingly, these similarities and differences can all be explained by a single model. Burns and collaborators propose that GRB 150101B and GRB 170817A exhibit the exact same two-component behavior, and their differences in luminosity and duration can be explained by quirks of special relativity.

High-speed outflows such as these will have different apparent luminosities and durations depending on whether we view them along their axis or slightly from the side. Burns and collaborators demonstrate that these the two bursts could easily have the same profile — but GRB 150101B was viewed nearly on-axis, whereas GRB 170817A was viewed from an angle.

If this is true, then perhaps more GRBs have hard spikes and soft tails similar to these two; the tails may just be difficult to detect in more distant bursts. While more work remains to be done, the recognition that GRB 170817A may not be unique is an important one for understanding both its behavior and that of other short GRBs.

Citation

“Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail,” E. Burns et al 2018 ApJL 863 L34. doi:10.3847/2041-8213/aad813

galactic outflow

New observations by the Atacama Large Millimeter/submillimeter Array (ALMA) provide a close look at a galaxy that may be in the process of shutting down its star formation.

Transitioning a Galaxy

galaxy types

We think that galaxies transition from blue spirals actively forming stars (left) to red, quiescent ellipticals (right). [Hubble/Galaxy Zoo]

Though we know much more about the processes of galaxy formation and evolution than we did even a decade ago, many key points still elude us. One particular puzzle is that of how star formation ends in a galaxy. We think that galaxies eventually transition from bright, blue, star-forming disks into red and quiescent ellipticals — but what causes star formation in a galaxy to shut down during this transition?

Since galaxies form stars out of cold gas, we could assume that star formation stops only when the cold gas supply is depleted. But observations suggest that star formation can shut down across a galaxy much more quickly than the timescale for using up the gas supply — sometimes turning off within just a few tens of millions of years. Such a rapid shutdown is termed “violent quenching”.

SDSS J1341–0321

Top: Hubble images of SDSS J1341–0321. Bottom: Contours show the location of the galaxy’s molecular gas: all CO J(2 → 1) molecular gas (left), just the gas moving rapidly toward us (middle) and just the gas moving rapidly away from us (right). [Geach et al. 2018]

Options for Violent Quenching

What mechanisms could suddenly prevent cold gas from contracting into stars in the disk of a galaxy? The most efficient approach is the rapid removal or destruction of the molecular gas.

In one common picture of rapid gas removal, powerful jets emitted from the supermassive black hole in a galaxy’s active nucleus (AGN) play a key role. In this model, the jets blow out the galaxy’s molecular gas on short timescales — and in so doing, they both clear out gas available for star formation, and also propel metal-enriched gases into the circumgalactic medium.

But new observations have challenged the picture of AGN-driven outflows as the standard violent-quenching mechanism. In a recent study led by Jim Geach (University of Hertfordshire, UK), a team of scientists presents a new view of a quenching galaxy that doesn’t seem to have an AGN.

Look to the Stars

Using ALMA, Geach and collaborators trace the molecular gas in SDSS J1341–0321, a massive and compact galaxy thought to have recently undergone a major merger and now showing signs of early-stage quenching. Despite this galaxy hosting no evidence of an active nucleus, the ALMA observations reveal an outflow of cool gas moving at a speedy 1,000 km/s relative to the stars.

Antennae Galaxies

The Antennae Galaxies are an example of a starburst galaxy with rapid star-formation activity driven by a recent merger. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration]

Geach and collaborators suggest that this outflow is violent quenching in another form: a powerful stellar outflow currently expelling around 300 solar masses of gas per year. They argue that this outflow was launched within the last 5 million years from a central starburst — a region of compact, vigorous star formation — triggered by SDSS J1341–0321’s recent merger. In this model, the stars themselves blow out all the gas — and once the gas is gone, star formation will turn off and the galaxy will appear red and quiescent.

If this model correctly describes SDSS J1341–0321, the next question is whether similar stellar outflows could account for violent quenching in other compact, massive galaxies across the universe. While we don’t yet know the answer, it seems likely that future high-resolution observations — perhaps also made with ALMA — will help us to find out!

Citation

“Violent Quenching: Molecular Gas Blown to 1000 km s−1 during a Major Merger,” J. E. Geach et al 2018 ApJL 864 L1. doi:10.3847/2041-8213/aad8b6

Acoustic waves in a star with a planet

Aldebaran, the brightest star in the constellation Taurus, was one of the first stars suspected to harbor an exoplanet. The presence of its planetary companion, Aldebaran b, was confirmed in 2015, and the decades of data preceding the discovery might harbor a few more surprises.

p-mode oscillations

A model of acoustic oscillations, also called p-modes, which can be used to infer fundamental properties of stars through asteroseismology. The vertical extent of the oscillation has been exaggerated by a factor of 1,000. [NASA/MSFC]

A Recognizable Target

Like hundreds of other exoplanets, Aldebaran b was discovered via the radial velocity method, in which the tug of a planet causes a detectable shift in the wavelengths of absorption lines in its parent star’s spectrum.

Radial velocity measurements can reveal more than just the presence of planetary companions, however; periodic oscillations of the star itself can be hidden in the radial velocity signal. These oscillations depend on the fundamental parameters of the star: mass, radius, surface gravity, and effective temperature.

To search for stellar oscillations of Aldebaran, a team of astronomers led by Will Farr (University of Birmingham, UK) delved into more than three decades of historical radial velocity measurements.

Farr et al. 2018 Fig. 3

Radial velocity measurements from the Hertzsprung SONG Telescope. The long-period planetary signal is superimposed upon the shorter-period p-mode signal. Click to enlarge. [Farr et al. 2018]

Digging Through the Data

The authors fit to the data a Keplerian model — to confirm the previously discovered planetary signal — and a Continuous Auto-Regressive Moving Average (CARMA) model — to search for stellar oscillations. In addition to verifying the presence of Aldebaran b, Farr and collaborators found evidence for stellar oscillations with maximum power at a frequency of 2.2 microhertz — well within the typical range for a red giant.

The authors followed up on these findings with high-cadence radial velocity observations from the Hertzsprung SONG Telescope and photometry from K2, the revived version of the Kepler Space Telescope. Analysis of both these datasets showed evidence of stellar oscillations consistent with those seen in the more irregularly sampled historical data.

Farr et al. 2018 Fig. 6

A model of the stellar irradiance received at Aldebaran b’s orbital distance over the course of Aldebaran’s main-sequence lifetime. [Farr et al. 2018]

Getting to Know Aldebaran

Farr and collaborators were able to estimate the orbital parameters of Aldebaran b and the mass and age of its parent star. The authors find that Aldebaran has a mass of 1.16 solar masses and an age of 6.4 billion years. This suggests that while Aldebaran is currently more than 500 times more luminous than the Sun, it likely matched the Sun’s output early in its life.

At an orbital distance of 1.5 AU, Aldebaran b probably enjoyed stellar irradiance similar to modern-day Earth in the distant past — about 4.4 billion years ago. However, Aldebaran b is at least 5.8 Jupiter masses (and may be massive enough to be a brown dwarf, depending on the inclination of its orbit), so it’s unlikely to have ever hosted life as we know it.

As Farr and collaborators have shown, it’s possible to extract accurate stellar and orbital parameters from irregularly sampled radial velocity data — which is plentiful thanks to radial-velocity surveys searching for exoplanets. With this technique, we may soon know of thousands of planets around red giant stars!

Citation

“Aldebaran b’s Temperate Past Uncovered in Planet Search Data,” Will M. Farr et al 2018 ApJL 865 L20. doi:10.3847/2041-8213/aadfde

inhomogeneous universe

The universe is expanding — but we’re still not sure how quickly! With past measurements of this expansion rate causing yielding conflict and debate, a new study investigates whether we can resolve the evident tension.

Conflicting Measurements

Hubble constant

Estimated values of the Hubble constant, 2001–2018. Data marked with circles show local, distance-ladder-calibrated measurements; data marked with squares indicate global measurements from the CMB and baryon-acoustic oscillations. Click to enlarge. [Kintpuash]

The first sign that the universe around us is expanding was found in the late 1920s, when astronomers first recorded evidence that distant galaxies appear to be moving away from us at ever faster rates, the more distant they are. This led to the development of the Hubble constant (H0), a value used to quantify this observed rate of expansion — and its value has been debated ever since.

Though we’ve come a long way since our initial, imprecise measurements of H0, today the two primary methods of measuring the Hubble constant remain in tension:

  1. Local measurements can be made by determining the distances and recession speeds of visible objects in the universe; Type Ia supernova surveys provide the standard candles needed for these measurements. Using this approach, scientists obtain an H0 value around us of ~74 (km/s)/Mpc.
  2. Global measurements are made by estimating the Hubble constant from measurements of the cosmic microwave background (CMB), relic radiation from the Big Bang. By fitting a multi-parameter model to Planck-mission observations of the CMB, scientists obtain a slower expansion estimate of ~68 (km/s)/Mpc for H0.

Inhomogeneity to the Rescue?

This discrepancy of nearly 9% between the two measurements — which cannot be brought into agreement by the measurements’ error bars — remains puzzling. Is one or the other group of astronomers making a mistake, or underestimating their errors? Could there be new physics at play in the cosmological model used to interpret the CMB results?

inhomogeneous universe

Expansion rate (left) and density (right) of a simulated inhomogeneous anisotropic universe. [Macpherson et al. 2018]

Some scientists have proposed an alternative explanation: what if the global expansion rate of the universe is not the same as the local rate? One possibility is that we live in a local void, an underdense region of the universe that expands faster than does the universe overall.

To determine whether the tension between the two types of H0 measurements can be explained by such an inhomogeneous universe, a team of scientists led by Hayley Macpherson (Monash University) has explored the behavior of a simulated universe.

A Simulated Universe

Local deviations in H0

The global measurement (Planck measurement; blue solid line) and local measurement (Riess et al. measurement; red solid line) of the Hubble constant can’t be brought into agreement by local deviations in the Hubble constant due to inhomogeneities (blue data showing distribution of various local spheres). [Macpherson et al. 2018]

Macpherson and collaborators simulated the growth of large-scale cosmological structures using numerical relativity. Starting with an inhomogeneous universe, the authors evolved random density fluctuations of the universe from its birth to today, and then investigated what effect these inhomogeneities have on local measurements of the Hubble constant.

The authors find that, in their simulated universe, local measurements of the Hubble constant differ by less than 1% compared to the global value. An inhomogeneous universe therefore cannot explain the nearly 9% difference we measure between the CMB-inferred global and supernova-measured local values of the Hubble constant.

What’s next? It’s back to the drawing board — the mystery of our expanding universe continues to elude us. Here’s hoping that high-precision measurements from future surveys will help us to further refine our understanding!

Citation

“The Trouble with Hubble: Local versus Global Expansion Rates in Inhomogeneous Cosmological Simulations with Numerical Relativity,” Hayley J. Macpherson et al 2018 ApJL 865 L4. doi:10.3847/2041-8213/aadf8c

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