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ASKAP

As powerful millisecond bursts of radio emission continue to light up our detectors from across the universe, the hunt for the origins of these fast radio bursts continues. It’s the search for light before and after the burst, however, that might prove key to unraveling their mystery.

An Extragalactic Puzzle

fast radio burst

Artist’s impression of the ASKAP radio telescope finding a fast radio burst. Other observatories are shown joining in follow-up observations. [CSIRO/Andrew Howells]

Since the first discovery of fast radio bursts (FRBs) more than a decade ago, we’ve found ~100 of them, including more than 20 that have been observed to repeat. Despite this growing sample — and though we’ve now localized repeating and non-repeating FRBs to distant host galaxies — we still don’t know with certainty what causes them.

Many of the leading origin theories for FRBs come with predictions of other emission that should complement the radio flash. The birth of a magnetized neutron star, for instance, should produce not only an FRB, but also a radio afterglow — steadier radio emission that appears after the burst and then slowly fades over time. 

The fact that we haven’t yet found any radio afterglows definitively associated with FRBs means one of three things: 1) they don’t happen, 2) they’re fainter than we can detect, or 3) they evolve on quicker timescales than we’ve been surveying, so they’ve already faded by the time we look. A new radio array enterprise is focusing on ruling out the third of these possibilities.

ASKAP Milky Way

CRAFT is designed to be able to run simultaneously with other projects on the ASKAP radio array. [CSIRO/Alex Cherney]

Real-Time Eyes on the Skies

To catch a quickly evolving afterglow, we need real-time observations of FRBs — allowing us to monitor the source for radio emission before, during, and immediately after the burst. To this end, a new survey was begun in mid-2019 with the The Australian Square Kilometre Array Pathfinder (ASKAP): the Commensal Real-time ASKAP Fast Transients Survey (CRAFT).

CRAFT runs continuous low-time-resolution observations of the sky while ASKAP is being used for other survey science projects. When an FRB is spotted, the survey automatically saves the data from before and after the FRB itself, so that the survey team can search it for radio precursor and afterglow emission.

In a new study, a team of scientists led by Shivani Bhandari (Australia Telescope National Facility, CSIRO, Australia) reports the first detection of an FRB in this mode: FRB 191001.

The Verdict: No Glow

FRB 191001

ASKAP de-dispersed light curve showing the ms-scale pulse of radiation from FRB 191001. [Adapted from Bhandari et al. 2020]

From the CRAFT data, the authors were able to localize FRB 191001 to the outskirts of a star-forming spiral galaxy at a redshift of z = 0.234 (that’s nearly 3 billion light-years away!). The CRAFT data revealed neither a persistent, compact radio source before the burst, nor a slowly varying radio afterglow after the burst.

What does this mean? The lack of detectable afterglow for FRB 191001 alone doesn’t yet rule out any formation scenarios — but it does demonstrate that the afterglow was either fainter than our detection thresholds, or it didn’t occur at all.

And this CRAFT detection is just the start! With more observations like this one — especially of closer FRBs that would be expected to appear with correspondingly brighter afterglows — we may soon be able to narrow down the options of what causes these mysterious flashes.

Citation

“Limits on Precursor and Afterglow Radio Emission from a Fast Radio Burst in a Star-forming Galaxy,” Shivani Bhandari et al 2020 ApJL 901 L20. doi:10.3847/2041-8213/abb462

Kepler-22b

A critical component of a habitable planet is its ability to stabilize its climate over long timescales. In a new study, scientists explore whether a world covered in water can keep its climate as stable as an Earth-like, continental world.

The Carbon Goes Round and Round

Carbonate-Silicate Cycle

Diagram of the physical and chemical processes (top panel) and feedback loops (bottom panel) associated with the carbonate–silicate cycle. Click to enlarge. [Gretashum]

Over the span of millions of years, a planet’s host star might gradually dim or brighten, or the planet’s volcanic outgassing patterns might slowly shift. If evolution like this also caused dramatic changes in the overall climate of a planet, this would spell bad news for habitability: the planet might not be able to retain liquid water over timescales long enough for life to form and evolve.

So how do you keep a climate stable against these slow shifts? One crucial factor is having a carbonate–silicate cycle. This cycle dictates how carbon is moved around a planet, sometimes burying it deep below the planet’s surface, sometimes releasing it out into the atmosphere.

On Earth, a simplified description of the carbonate–silicate cycle is:

  1. Atmospheric carbon dioxide dissolves in rainwater, forming carbonic acid, which falls to the ground.
  2. Over long timescales, weathering from this weak acid dissolves silicate rocks, and the dissolved products are carried to the oceans, where they accumulate.
  3. Subduction of the seafloor carries the products to great depths, where they reform into silicates and gaseous carbon dioxide.
  4. The carbon dioxide is restored to the atmosphere by volcanism.

When Negative Feedback Is a Good Thing

In an ideal scenario, the carbonate-silicate cycle acts as a planet’s thermostat, with negative feedback loops keeping the temperature of the planet in balance. If oceans freeze over, silicate weathering slows, causing atmospheric carbon dioxide to accumulate and warm the planet via the greenhouse effect. If the planet heats, rainfall increases and silicate weathering speeds up, removing carbon from the atmosphere and cooling the planet.

Exoplanet K2-18b

Illustration of a water world around a cool, dim star. The ability of a planet to stabilize its temperature as its host star evolves is important to habitability. [M. Kornmesser/Hubble/ESA]

This cycle only stabilizes the climate against very slow external changes, like a star’s gradual dimming — so this isn’t the solution to our current global warming crisis caused by fossil fuel emissions. Nonetheless, it’s an important component when considering the general habitability of other worlds.

In a new study, scientists Benjamin Hayworth and Bradford Foley (Pennsylvania State University) consider how this cycle might be affected by the geography of a planet. Will worlds covered in water do a better or worse job of keeping their climates stable?

Stability from the Sea

Climate buffering capacity

Climate buffering capacity of planets with varied ocean coverage, for changes in stellar luminosity. Colored curves correspond to different fractional ocean coverage. Lower values of dT/dL, the change in surface temperature with luminosity, mean the planet is better at stabilizing its climate. [Adapted from Hayworth & Foley 2020]

Hayworth and Foley point out that both continental land and seafloors experience silicate weathering and participate in the carbonate–silicate cycle of a planet. The weathering rates for continental land and the seafloor, however, depend differently on the planet’s surface temperature and the partial pressure of carbon dioxide.

By accounting for these different dependencies in climate and weathering models, the authors show that water worlds — which are dominated by seafloor weathering — are actually better than their continental counterparts at stabilizing planet-wide temperatures against gradual changes in host star luminosity.

This means that temperate climates can exist over a wider range of stellar luminosities for water worlds than for continental planets, and they can stay stable for longer — indicating that these worlds may be worthwhile targets in the search for life.

Citation

“Waterworlds May Have Better Climate Buffering Capacities than Their Continental Counterparts,” Benjamin P. C. Hayworth and Bradford J. Foley 2020 ApJL 902 L10. doi:10.3847/2041-8213/abb882

The Cepheid RS Puppis as Seen by Hubble

Cepheids are pulsating variable stars, meaning that their periodic changes in brightness are associated with changes in physical size. The wavelengths at which Cepheids emit radiation also vary as the stars pulsate. So what’s happening in the X-ray when it comes to Cepheids?

Classical Cepheids and Modern Observations

NASA Cepheids X-Ray Image

X-ray images of the Cepheid δ Cephei and a non-varying nearby star. The variability of δ Cephei is evident from the two images. [NASA]

Cepheid variables — specifically, Classical Cepheid variables (hereafter Cepheids) — hold a special place in astronomy. This is largely thanks to their period–luminosity relations, which were discovered by Henrietta Swan Leavitt over a hundred years ago. 

In short, a Cepheid’s period is related to its brightness in a well-defined way, and we can use that knowledge to determine how far away a given Cepheid is if we measure its period. This property of Cepheids has made them critical to measuring large distances in space, which are extremely valuable in astronomy. So, it’s important that we understand Cepheid behavior very well.

Cepheid emission at shorter wavelengths (e.g., ultraviolet, X-ray) is strong when Cepheids are at their smallest. However, observations taken over the last few years have shown that Cepheids have an unexpected increase in X-ray emission when they’re at their largest.

Shocks in Cepheid atmospheres could be behind this surprise X-ray emission. As Cepheids pulsate, their layers can bump against each other, sending shock waves outward into the stars’ atmospheres. There, the shocks could heat the plasma, causing emission at X-ray wavelengths. To explore the possibility of shocks causing X-ray emission, a group of researchers led by Sofia-Paraskevi Moschou (Center for Astrophysics | Harvard & Smithsonian) simulated Cepheids along with the spectra that would be observed from those simulated stars.

X-ray light curves from three of the Cepheid simulations. The dashed lines indicate one-fourth of the maximum brightness for a given simulation. [Adapted from Moschou et al. 2020]

Shocking Details

Cepheids contain complex environments. Aside from pulsation and shocks, they also experience mass loss driven by winds and magnetic fields. Density and temperature can vary throughout a Cepheid, to say nothing of how radiation permeates through the star.

To account for all these complexities, Moschou and collaborators used modeling software that could simulate Cepheid properties in great detail. They based their simulations on the prototype Cepheid, δ Cephei, which is five times more massive than the Sun. After simulating Cepheids assuming different underlying conditions, Moschou and collaborators then used their results to create corresponding X-ray spectra and light curves.

Shocks for Shorter Periods

It turns out that shocks can produce the observed X-ray emission! However, the varying X-ray emission throughout the Cepheid pulsation cycle suggests that the emission at maximum radius is caused by shocks, while the emission at minimum radius is from another, more consistent process.

δ Cephei is a short-period Cepheid as Cepheids go, so the mechanisms in longer-period Cepheids may present different challenges. As is typical with astronomy, more observations will definitely come in handy!

Citation

“Phase-modulated X-Ray Emission from Cepheids due to Pulsation-driven Shocks,” Sofia-Paraskevi Moschou et al 2020 ApJ 900 157 doi:10.3847/1538-4357/aba8fa

black hole binary

The tally of merging black holes detected by the LIGO-Virgo gravitational-wave detectors continues to grow; the most recent data release brings the total to nearly 50 collisions! But how do these black-hole binaries form in the first place?

Two Formation Channels

black holes in a globular cluster

Still from a simulation showing how black holes might dynamically form as they interact in the chaotic cores of globular clusters. [Carl Rodriguez/Northwestern Visualization]

Before two black holes can collide in a burst of gravitational waves, they must first be bound together in an inspiraling binary pair.

There are two leading theories for how such pairs of black holes might arise in our universe. In isolated binary evolution, two massive stars of a stellar binary independently evolve into black holes. In dynamical encounters, single black holes pair up into binaries through gravitational interactions in the center of a dense, crowded star cluster.

Two Observational Clues

How can we determine which formation channel produced the black-hole binaries we’ve detected so far? Two observational signatures, in particular, could point to a dynamical merger:

  1. Spin misalignment
    Due to conservation of angular momentum, black holes in isolated binaries are expected to have aligned spins. Black holes that pair up via dynamical encounters, on the other hand, are likely to have random, misaligned spins.
  2. Orbital eccentricity
    If a binary evolves in isolation, any initial eccentricity is damped long before the black holes merge. In the dynamical scenario, however, the abruptly formed binaries can merge before their orbits have time to circularize.
Stellar graveyard Nov 2020

The most recent version of the rapidly expanding “stellar graveyard”, a plot that shows the masses of the different components of observed compact binary mergers. GW190521, seen at the top center, is more massive than any other binary merger we’ve observed. Click to enlarge. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

The vast majority of mergers we’ve detected so far have had gravitational-wave signals consistent with low-mass, spin-aligned binaries with circular orbits — preventing us from differentiating between the two formation channels. One recent merger, however, is a promising candidate for further study: GW190521.

One Intriguing Collision

GW190521 has set records as a heavyweight: the merging components were ~85 and ~66 solar masses. These unusually large black holes already hint at a dynamical formation for the binary: it’s easier to explain black holes of this mass if they grew via successive mergers in a dense stellar environment.

Now, a team of scientists led by Isobel Romero-Shaw (Monash University and OzGrav, Australia) has followed up on this clue, modeling the GW190521 signal with a variety of waveforms to explore the binary’s eccentricity and spin alignment.

GW190521

To extract information like black-hole masses, spin alignments, and orbital eccentricities, scientists fit model waveforms to the gravitational-wave signal. Here, the orange, purple, and black lines represent different waveforms that are plotted over the blue LIGO Livingston data for GW190521. [Adapted from R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)]

Romero-Shaw and collaborators show that we can’t currently differentiate between two models: one with non-zero eccentricity and aligned spins, and the other with a circular orbit but misaligned spins. Both models, however, are highly favored over models with circular orbits and aligned spins — which means that a dynamical formation channel is likely for GW190521.

As LIGO-Virgo continues to amass detections, we may soon be able to build a statistical picture of how these black-hole binaries formed. But in the meantime, careful modeling of individual collisions like GW190521 are providing valuable insight.

Citation

“GW190521: Orbital Eccentricity and Signatures of Dynamical Formation in a Binary Black Hole Merger Signal,” Isobel Romero-Shaw et al 2020 ApJL 903 L5. doi:10.3847/2041-8213/abbe26

early quasar

Hungry supermassive black holes in the distant cosmos can help us understand what happened shortly after our universe lit up with its first stars and galaxies. New work now probes the most distant supermassive black hole we’ve seen, searching for more clues.

Our Hazy Past

epoch of reionization

In the schematic timeline of the universe, the epoch of reionization is when the first galaxies and quasars began to form and evolve. Click to enlarge. [NASA]

Our early universe, starting just a few million years after the Big Bang, was a dark place. Space was filled with clouds of neutral hydrogen, but there were no sources of visible light.

At some point a few hundred million years after the birth of the universe, the earliest stars began to form, as well as the first large-scale structures like galaxies. Supermassive black holes grew in the centers of those galaxies, and as the black holes accreted mass, they produced powerful radiation, appearing to us now as distant quasars. Within a billion years of the Big Bang, quasars and stars lit the universe and shaped it into its current form.

The details and precise timeline of these critical evolutionary stages, however, remain uncertain.

Let’s Turn Back Time

One way we can further understand this evolution is by using quasars as cosmic clocks. By peering back in time and exploring the earliest known quasars, we learn about the metallicity of gas at the centers of early galaxies — which reveals when this gas first became enriched with the metals formed by early stars.

In a recent study, a team of scientists led by Masafusa Onoue (Max Planck Institute for Astronomy, Germany) has probed this crucial time using one particularly early clock: quasar ULAS J1342+0928.

AGN model

This schematic of a quasar includes the broad line region, orbiting clouds located very close to the central supermassive black hole. [Urry & Padovani 1995]

Peering into a Galaxy’s Center

ULAS J1342+0928 is the most distant, oldest known quasar; it’s located at a redshift of z = 7.54, which corresponds to a time just 680 million years after the Big Bang. Onoue and collaborators obtained deep near-infrared spectra of this distant source using the Gemini North telescope in Hawaii.

By modeling the spectra, Onoue and collaborators were able to measure the ratios of certain emission lines produced within the broad line region (BLR) of the quasar, a region of clouds that orbit very close to the central black hole.

The ratios of these emission lines can serve as a proxy for the clouds’ metallicity. Since this gas is thought to have originated from the interstellar medium of the host galaxy, the metallicity of BLR gas traces the galaxy’s star formation history, telling us when stars formed and enriched this gas with metals.

Early Metal Pollution

iron enrichment

The Fe II/Mg II line ratio, which traces the iron enrichment of the gas, is very similar for ULAS J1342+0928 (red diamonds, showing two different models) and other, closer quasars. Click to enlarge. [Onoue et al. 2020]

Onoue and collaborators found that ULAS J1342+0928’s BLR gas has similar metallicity to the BLR gas of other quasars located at lower redshifts. This result suggests that the enrichment of gas at the centers of galaxies is already largely completed within just 680 million years of the Big Bang — which constrains our understanding of when and how stars form and evolve in the early universe.

What’s next? We need observations of even more distant quasars to push this limit even farther back in time; while we’ve spotted a handful of galaxies at redshifts above z = 8, we’ll need to keep hunting to find quasars at these larger distances so that we can measure their metallicity.

Citation

“No Redshift Evolution in the Broad-line-region Metallicity up to z = 7.54: Deep Near-infrared Spectroscopy of ULAS J1342+0928,” Masafusa Onoue et al 2020 ApJ 898 105. doi:10.3847/1538-4357/aba193

MWC 758

It’s an old debate: gravity or companions? The spectacular spiral arms we see in some protoplanetary disks are likely caused by one or the other — and a recent study has taken a speedy approach to figuring out which.

What’s Driving Patterns?

Among the many swirling disks of gas and dust we’ve spotted around young, newly formed stars, more than a dozen contain a notable feature: large-scale spiral arms that can span tens to hundreds of astronomical units in size.

spiral disk

Artist’s impression of spiral arms in the disk surrounding a young star. [Institute of Astronomy, University of Cambridge — Amanda Smith & Farzana Meru]

The formation of these arms remains an open question. Are they caused by a gravitational instability that drives a spiral pileup of disk material? Or are they induced by the presence of a hidden planetary companion orbiting within the disk?

Either way, the answer will reveal valuable information about the system. If the spirals are driven by a gravitational instability, then we can constrain the mass of the disk. If they’re caused by the interaction of the disk with a planetary companion, we can infer the mass and location of the planet.

Past explorations of spiral arms have focused on examining static observations of disks. But a team of scientists led by Bin Ren (California Institute of Technology) has now taken a different approach, instead looking at how the arms of one disk move over time.

A Need for Speed … Measurements

Ren and collaborators gathered Very Large Telescope SPHERE observations of MWC 758, a young star and spiral-armed disk located just over 500 light-years away, spanning a baseline of nearly five years. They then fit models to MWC 758’s spiral arms to establish their pattern speeds — the angular speeds at which the arms are rotating.

The authors were looking for one of two potential outcomes linked to the origin of the arms:

  1. If driven by a gravitational instability, the material of the arms will move at the local Keplerian speed, set by the mass of the central star. This means the inner parts of the arms will move quickly and the outer parts more slowly, winding the arms up ever more over time.
  2. If driven by a planetary companion, the arms will move as a solid structure that matches the speed of the companion.

Time for Planet Hunting

MWC 758 driver orbit

The best-fit circular orbit of a planetary companion driving MWC 758’s spiral arms lies at ~170 au for a 1.9-solar-mass central star. [Ren et al. 2020]

Ren and collaborators find that the motions of the arms are not consistent with a gravitational-instability origin — the pattern speed is roughly a factor of 5 too slow for that! Instead, the spiral’s motion is well-fit by a model in which a planetary companion at ~170 au has created and maintains the arms.

Based on imaging of the disk, the authors set upper limits of about 5 Jupiter masses for the hidden planetary companion that’s driving the arms. The MWC 758 system makes an ideal target for future observations with current and upcoming telescopes to try to spot the planet driving its arms — especially now that we know where to look!

Citation

“Dynamical Evidence of a Spiral Arm–driving Planet in the MWC 758 Protoplanetary Disk,” Bin Ren et al 2020 ApJL 898 L38. doi:10.3847/2041-8213/aba43e

ALMA

The incredible power of ALMA, an array of telescopes located in the high deserts of Chile, has revolutionized our understanding of the structures built from gas and dust within our own galaxy. But ALMA can do more than that: it can also catalog the molecular gas and dust of galaxies located in the distant depths of our universe.

HUDF

Hubble Ultra-Deep Field image, a composite of exposures taken 2003-2012 that reveals roughly 10,000 galaxies in just a small section of space. Click to enlarge. [NASA/ESA/H. Teplitz & M. Rafelski (IPAC/Caltech)/A. Koekemoer (STScI)/R. Windhorst (Arizona State University)/Z. Levay (STScI)]

To better understand how galaxies evolve and form stars over time, scientists recently gathered the deepest ALMA observations yet, aiming the array at what is perhaps the most famous field of galaxies in astronomy: the Hubble Ultra-Deep Field (HUDF). This region of sky has been studied via more than a thousand hours of observations using a variety of telescopes, but ALMA provides a new view.

With ALMA, we can gain insight into the cold molecular gas and dust content of galaxies over time — and the remarkable sensitivity of this observatory has been harnessed in a recent, ambitious project called The ALMA Spectroscopic Survey in the Hubble Ultra-Deep Field, or ASPECS.

The newest results from ASPECS are now published in a set of articles in the Astrophysical Journal. Below, we take a brief look at some of the main outcomes of the set, but you can find more details in the original articles and in the press releases linked at the end of the post.

Hidden Star Formation

A first step toward understanding star formation in the distant universe is to figure out which galaxies are forming stars, and at what rates. This is relatively straightforward when the star formation is visible — but often it’s obscured by dust, preventing us from getting clear measurements.

The high sensitivity ALMA is ideal for this situation, however, and ASPECS observations were used to probe the dust-enshrouded star formation in 1,362 galaxies in the HUDF that are located in the redshift range of z = 1.5–10.

Exploring Fuel for Stars

With a clearer picture of star formation over time, it next makes sense to examine the raw material used to make stars.

ALMA HUDF

ASPECS detections within the HUDF. Pink circles denote molecular gas detections, blue squares denote dust, and green diamonds denote faint dust. Click to enlarge. [Aravena et al. 2020]

To do this, ALMA hunted for distant, cold gas and dust, revealing reservoirs in dozens of galaxies — including some unexpected sources with low star formation rates and stellar masses. The unprecedented depth of the ASPECS observations allowed the team to identify nearly all of the cold dust reservoirs from present day out to early cosmic times within the HUDF.

It’s generally understood that once the first stars started forming, star formation activity increased over time until it reached a peak at around z ~ 1–3, or “cosmic noon”. After that point, star formation activity substantially slowed to present day. The ASPECS observations now help us to better understand why.

ASPECS data show that the total amount of molecular gas in the universe increased until cosmic noon, when it began steadily declining. Today, the amount of molecular gas available for star formation is only around a tenth of what was available at cosmic noon! This is neatly consistent with that known cosmic history of star formation: star formation peaked at the same time as the molecular gas density peaked and has declined to present day — and will probably continue to decline until it ceases altogether.

What’s Next?

The ASPECS data is now being used to constrain cosmological models, helping us to better understand how galaxies evolve and stars are born. There’s still a lot of science to be done with the ASPECS observations, and today’s release of publications is just a start! Keep an eye out for more to come.

Bonus

Check out this video that demonstrates the locations of the dusty galaxies examined by ALMA, placed into context within the Hubble Ultra Deep Field.

[STScI, ASPECS Collaboration, Thomas Müller (HdA)]

Read More

ASPECS website
ALMA press release
MPIA press release

Citations

“The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The nature of the faintest dusty star-forming galaxies,” Manuel Aravena et al 2020 ApJ 901 79. doi:10.3847/1538-4357/ab99a2
“The ALMA Spectroscopic Survey in the HUDF: CO Excitation and Atomic Carbon in Star-Forming Galaxies at z=1–3,” Leindert A. Boogaard et al 2020 ApJ 902 109. doi:10.3847/1538-4357/abb82f
“The ALMA Spectroscopic Survey in the HUDF: Multiband constraints on line–luminosity functions and the cosmic density of molecular gas,” Roberto Decarli et al 2020 ApJ 902 110. doi:10.3847/1538-4357/abaa3b
“The Evolution of the Baryons Associated with Galaxies Averaged over Cosmic Time and Space,” Fabian Walter et al 2020 ApJ 902 111. doi:10.3847/1538-4357/abb82e
“The ALMA Spectroscopic Survey Large Program: The Infrared Excess of z=1.5–10 UV-selected Galaxies and the Implied High-Redshift Star Formation History,” Rychard Bouwens et al 2020 ApJ 902 112. doi:10.3847/1538-4357/abb830
“The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Constraining the Molecular Content at log(M*/M⊙) ~ 9.5 with CO Stacking of MUSE-detected z ~ 1.5 Galaxies,” Hanae Inami et al 2020 ApJ 902 113. doi:10.3847/1538-4357/abba2f

HR 6819 wide-field view

A few months ago, scientists announced the indirect detection of the nearest black hole to Earth. But another team is now suggesting a different explanation for this stellar puzzle.

HR 6819: History of a Mystery

From the earliest spectra of HR 6819, scientists identified this source as a bright, early-type Be star — a hot star with emission lines, likely due to the accretion of a circumstellar disk of material. As our ability to resolve detail in stellar spectra has advanced, however, a more complicated picture has emerged.

Studies in the 1980s revealed unexpected narrow absorption lines in HR 6819’s spectra, and a 2003 study showed that these lines were moving over time. This indicated that, though we couldn’t optically resolve them, there were two components of HR 6819: a Be star exhibiting no obvious motion, and a B3 III star on a 40-day orbit.

HR 6819 triple

Artist’s impression of HR 6819 as a triple system. Here, an inner binary consists of a star (blue orbit) and a black hole (red orbit). A third star (blue) circles the inner pair on a wider orbit. [ESO/L. Calçada]

But what was the B3 III star orbiting? In May 2020, scientists announced an answer to the puzzle: HR 6819 must actually be a triple system. The B3 III star, they argued, is orbiting a black hole (which is why we don’t see evidence of it in the spectra), and the Be star is a distant tertiary companion, orbiting too slowly to have detectable motion.

Based on the B3 III star’s orbit, the black hole would need to weigh more than 4 solar masses — and at just 1,120 light-years distance from Earth, this object would be the closest black hole known. But could there be another explanation for HR 6819’s spectra?

Just the Two of Us

In a new study, Georgia State University scientists Douglas Gies and Luqian Wang argue that HR 6819 isn’t a triple system after all. Instead, it’s a simple binary, consisting only of the two known components: the Be star, and the B3 III star.

If HR 6819 is merely a binary, then the Be star should show reflex orbital motion with the same period of 40 days — but this motion could be small, and hard to detect in the complex spectra of the system.

HR 6819 radial velocity

Observed and fitted radial velocity curves for the B3 III star (filled circles) and the disk surrounding the Be star (open circles). The evident orbital motion suggests the two stars comprise a binary system. [Gies & Wang 2020]

To look for it, Gies and Wang analyzed the Hα emission from the accretion disk surrounding the Be star. Using careful spectral modeling, they show that the entire disk can be seen to wiggle back and forth with a period of 40 days, exactly as expected for reflex orbital motion. This motion is roughly an order of magnitude less than that of the B3 III star, which is why it wasn’t previously spotted.

An Unequal Pair

So why is the Be star’s orbital motion so much smaller than that of its B3 III companion? If the Be star has a typical mass of ~6 solar masses, the companion must be only a fraction of a solar mass in size. It may be in a stage of evolution where it’s already donated a significant amount of mass to its companion, and now only the stripped-down remnant remains.

Is the case closed on HR 6819? Far from it! This system continues to challenge our assumptions and gives us the opportunity to practice the process of science as we work to explain our observations.

Citation

“The Hα Emission Line Variations of HR 6819,” Douglas R. Gies and Luqian Wang 2020 ApJL 898 L44. doi:10.3847/2041-8213/aba51c

K2-132b

A star like our Sun is destined to end its main-sequence lifetime by expanding to hundreds of times its current size, evolving into a red giant. But when this apocalypse comes, not all of the star’s planets are doomed! A new study shows how some planets can influence the fates of their siblings.

Surviving the Apocalypse

Six billion years into our future, the Sun will grow in size until its scalding surface reaches roughly the orbit of Earth, engulfing Mercury and Venus in the process. And our own inner planets aren’t the only ones under such a threat — close-in planets around Sun-like stars throughout the universe share this apocalyptic fate.

evolving star

Artist’s illustration of a red-giant star ejecting gas and dust. [JAXA]

But while the innermost planets of such systems will certainly be swallowed or vaporized, and the outermost planets will be largely unfazed by the drama in the distant inner solar system, the planets in between these extremes — at orbital distances of a few au — face a more uncertain future.

Now, new research led by María Ronco (Millennium Nucleus for Planet Formation and Pontifical Catholic University of Chile) suggests that the fate of these middle-ground planets depends strongly on their siblings.

Sibling Rivalry

A number of previous studies have explored the conditions under which planets can survive their hosts’ evolution through the red-giant phase to a white dwarf, but these studies have primarily focused on the outcomes for a single planet trying to ride out the apocalypse.

Ronco and collaborators instead explore how two planets — an inner Neptune-mass planet and an outer Jupiter-mass planet, both orbiting within a system at distances of a few au — might affect each other during their host star’s evolution.

With a Little Help

planet orbit outcomes

Want to test your luck in different scenarios? This grid outlines the fate of the Jupiter-mass planet (large squares) and Neptune-mass planet (small inner squares) as a function of their initial semi-major axes in the authors’ simulations. The dashed lines show mean motion resonances. Two particular cases are circled, wherein the presence of the outer planet changes the fate of the inner Neptune-mass planet. Click to enlarge. [Ronco et al. 2020]

When a star expands to red-giant size, there are two primary effects that can impact planetary orbits: tides induced on the star, which can reduce and circularize the planet orbits, and mass loss from the star, which can expand the planet orbits. But Ronco and collaborators’ models show that when more than one planet is present in the system, the interactions between these planets can have equally important effects on their eventual orbits.

In particular, when both planet orbits are near a mean-motion resonance (i.e., when their orbits have integer ratios, like when one planet orbits twice for every three orbits of its sibling), the Neptune-mass planet is excited to a more eccentric orbit. This has dramatic consequences: for Neptunes that, alone, might have survived the red-giant evolution of their host, the presence of the outer Jupiter can cause them to be engulfed. Similarly, for Neptunes that might have been engulfed alone, the presence of the Jupiter can save them from being swallowed.

Ronco and collaborators’ work demonstrates that when a star evolves through the red-giant phase, the gravitational interactions between its middle-distance planets play an important role in the planets’ survival or destruction. As we continue to explore these influences, we’ll be able to better understand the fates of solar systems like ours and what to expect for planetary architecture around evolved stars.

Citation

“How Jupiters Save or Destroy Inner Neptunes around Evolved Stars,” María Paula Ronco et al 2020 ApJL 898 L23. doi:10.3847/2041-8213/aba35f

turbulent disk

Planets begin their lives shrouded in mystery, embedded in the swirling disks of gas and dust that surround newly born stars. As we try to understand the physical processes at play in these obscured environments, one stands out as a particular unknown: turbulence. New observations have now given us a look at the presence — and absence — of turbulence in planet-forming disks.

A (Theoretical?) Disruption

turbulence

This image captures the transition between laminar and turbulent flow in the convection plume above a candle flame. [Gary Settles]

Turbulence — the same phenomenon that coaxes candle smoke into elaborate swirls, or causes a bumpy airplane ride — can theoretically influence just about every aspect of planet formation and evolution. Models indicate that these unpredictable fluid motions can affect the growth of grains and clumps, the evolution of a protoplanetary disk’s chemistry over time, and even the eventual orbital motion of fully-formed planets.

But are real planet-forming disks actually turbulent? This question is surprisingly difficult to answer, and we’ve managed only a handful of indirect measurements of turbulence in protoplanetary disks so far. A new study led by Kevin Flaherty (Williams College) uses the high resolution of the Atacama Large Millimeter/Submillimeter Array (ALMA) to add a few more data points to the collection, exploring gas motions in the outer regions of three different protoplanetary disks.

DM Tau spectra

ALMA spectrum of the DM Tau CO (2–1) emission (black line). The model that includes turbulence (red) is a significantly better fit to the data than the model with no turbulence (blue). [Adapted from Flaherty et al. 2020]

Smooth Sailing or Rough Seas

By using models to interpret the ALMA observations of carbon monoxide emission from the disks, Flaherty and collaborators were able to place constraints on the amount of turbulence in each of these three planet-forming environments.

The authors show that the disks MWC 480 and V4046 Sgr both have only weak — if any — turbulence. The disk DM Tau, on the other hand, is a different story: it shows gas speeds indicative of significant turbulent motion.

This dichotomy of results is convenient: it gives us an excellent opportunity to explore the similarities and differences between these disks to try to understand what factors lead to a turbulent planet-forming environment instead of a calm one.

Exploring Potential Factors

DM Tau

ALMA image of the dusty disk around the young star DM Tau. [ALMA]

One factor proposed to influence turbulence is the strength of ionizing radiation that reaches the outer disk. DM Tau is the only one of the three systems that doesn’t show evidence of a radiation-blocking inner disk wind, which could mean that more ionizing radiation reaches the disk’s outer edges in DM Tau, driving the turbulence we’ve observed.

Another option is that DM Tau might have stronger magnetic fields than the other systems. It’s also possible that the system’s age — at just a few million years old, DM Tau one of the youngest targets in the sample — could be a factor affecting turbulence strength.

Overall, Flaherty and collaborators suggest that weak turbulence may be a common feature in planet-forming disks — but it’s clear that some outliers like DM Tau exist. More observations like the ones presented here will help us to further understand these mysterious, shrouded planetary nurseries.

Citation

“Measuring Turbulent Motion in Planet-forming Disks with ALMA: A Detection around DM Tau and Nondetections around MWC 480 and V4046 Sgr,” Kevin Flaherty et al 2020 ApJ 895 109. doi:10.3847/1538-4357/ab8cc5

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