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Artist’s impression of high redshift galaxy

Studying star formation in the early universe can give us clues about what the universe was like when the earliest massive galaxies were forming. How efficiently were these first galaxies making stars only a billion years after the Big Bang?

Lighting Up the Universe

The universe wasn’t always a treasure trove of galaxies. Not long after the Big Bang, it consisted largely of opaque neutral hydrogen, and the only photons present were either from the cosmic microwave background or emitted during electron transitions in hydrogen atoms. Between redshifts of z = 20 and z = 6 (i.e., between 150 million and 1 billion years after the Big Bang), the first galaxies formed and their stars ionized the hydrogen, allowing light to travel freely through the universe. 

This sounds very neat and straightforward, but the specifics of this process are hazy (note the very large time window!). What sort of stars did the ionizing? Did it happen all at once or more slowly? How long did it take? Pinning down the star formation rates of galaxies at that transition redshift of z ~ 6 can help us answer some of these questions.

Spectra and line maps of HZ10 and LBG-1

Left: spectra of galaxies HZ10 and LBG-1 highlighting the CO and carbon lines studied. The red line in the HZ10 spectrum is the Gaussian fit to the detected CO. The blue histogram is the CO emission, scaled down by a factor of 40. Right: integrated line maps of HZ10 and LBG-1 with contours showing CO (white lines) and carbon (black lines). The maps are created by integrating the individual spectra that compose the map, collapsing the data from three dimensions to two. The maps are scaled based on the carbon detections. [Pavesi et al. 2019]

The star formation rate of a galaxy is governed by many factors, not least of which are inflows and outflows of gas and the makeup of the gas contained within the galaxy. Measuring flows in early galaxies is beyond the capability of current instruments, but a lot can be learned by studying the cold, star-formation-fueling molecular gas that lies within galaxies.

And that’s precisely what a group of scientists led by Riccardo Pavesi (Cornell University) did. Using observations taken by the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), Pavesi and collaborators studied molecular features in a sample of galaxies at z = 5–6.

Reading the Lines

For this study, Pavesi and collaborators leverage particular emission lines associated with carbon monoxide (CO), carbon, and nitrogen. This emission can appear in long-wavelength observations when the galaxies in question are very distant. The authors also study the overall radiation emanating from dust in their target galaxies.

A notable finding from the analysis is the highest redshift detection of CO emission, which comes from a galaxy at z ~ 5.7. However, this galaxy, HZ10, also stands out for another reason: it appears to contain a large reservoir of gas, perfect for making stars.

Ratio of molecular gas mass to stellar mass versus redshift for galaxies between z = 0 and z = 6 based on CO measurements

The ratio of molecular gas mass to stellar mass versus redshift for galaxies located at 0 < z < 6, with the calculated masses based on CO measurements. Two galaxies from this study, HZ10 and LBG-1 are highlighted at the high redshift end of the plot, between z = 5 and z = 6. The gray line is a quadratic relation between mass ratio and redshift. The gray points are also HZ10 and LBG-1 with their mass ratios based on a different interpretation of the CO measurements. [Pavesi et al. 2019]

Star-ting and Stopping

The presence of unused star-making material so early in the universe is significant. Previous measurements of star formation at z > 5 found that early galaxies formed stars very efficiently — but because these studies probed only the most luminous of galaxies, it was unclear whether this result would hold for more typical galaxies at this redshift.

HZ10’s large gas reservoir indicates that this “normal” galaxy has a much lower star-formation efficiency than the brighter galaxies we’ve previously studied at this redshift; it actually shares more characteristics with galaxies at z ~ 3. HZ10 offers some of the first evidence that galaxies with lower star formation efficiencies exist at z > 3.

As with most new discoveries, we need larger samples and high quality observations to better understand this — stay tuned!

Citation

“Low Star Formation Efficiency in Typical Galaxies at z = 5–6,” Riccardo Pavesi et al 2019 ApJ 882 168. https://doi.org/10.3847/1538-4357/ab3a46

Omega Centauri

The globular cluster Omega Centauri makes for an impressive sight — millions of stars gravitationally bound into a beautiful sphere, its core alight from the glow of densely packed bodies. A recent study has unveiled a new discovery at the heart of this cluster: five long-anticipated pulsars.

What Lies At the Core

globular clusters

Image of two globular clusters (can you spot them? Look carefully!) in the Milky Way. Omega Centauri is brighter and more massive than either of these — or any other Milky Way globular cluster. [ESO/D. Minniti/VVV Team]

Located just 17,000 light-years away, Omega Centauri is an intriguing object of study. Though we know of more than 200 globular clusters — compact spheres of old stars — that lie in the outer regions of the Milky Way, Omega Centauri is the most massive and the most luminous. Its properties have led scientists to speculate that this cluster was once a dwarf galaxy that was captured by the Milky Way and had its outer stars stripped away.

Omega Centauri’s large mass and unusual formation history open two interesting possibilities:

  1. The cluster might contain massive black holes.
    Theory predicts that the conditions at the center of massive stellar clusters are ripe for collisions that drive the growth of intermediate-mass black holes with perhaps hundreds to tens of thousands of solar masses.
  2. The cluster core might show evidence of dark matter annihilation.
    If Omega Centauri was once a dark-matter-dominated dwarf galaxy, then its relatively close distance makes it an excellent target to search for dark matter annihilating at its center.
pulsar

Artist’s illustration of a pulsar, a fast-spinning, magnetized neutron star. [NASA]

Pulsars as Probes

How can we explore these possibilities? Our best bet would be to study the motions and signatures of radio pulsars — rapidly rotating, magnetized neutron stars — in the cluster’s core. The motions of these dense objects would provide information about the core’s dynamics, potentially revealing the gravitational influence of lurking massive black holes. The signatures of the pulsars’ emission could also tell us about the interstellar medium of the cluster, constraining particle dark matter annihilation models.

But though radio pulsars are common in the cores of other globular clusters, they’ve remained elusive in Omega Centauri. A tantalizing hint came in 2010 with the discovery of a gamma-ray source in the cluster’s core — but years of searching for pulsed radio emission from this location turned up nothing.

A Population Found

pulse profiles

The pulse profiles of the five newly discovered millisecond radio pulsars in Omega Centauri’s core. Click to enlarge. [Dai et al. 2020]

A new study led by Shi Dai (CSIRO Australia Telescope National Facility) has now used the high sensitivity of the Ultra-Wideband Low receiver on the Parkes radio telescope in Australia to up the search intensity. The result? The team found five faint millisecond pulsars hiding in Omega Centauri’s core.

These newly discovered pulsars have spin periods that range from 4.1 to 6.8 milliseconds. While four of them are isolated objects, the fifth lies in an eclipsing binary system with a very low-mass star, orbiting once every 2.1 hours.

These pulsars may just be the tip of the iceberg: Dai and collaborators found additional compact sources in deep radio continuum images of Omega Centauri’s center, suggesting there may be more pulsars awaiting discovery. In the meantime, tracking the five pulsars now known will give us an excellent opportunity to probe the properties of this massive, bright cluster and learn more about its secrets.

Citation

“Discovery of Millisecond Pulsars in the Globular Cluster Omega Centauri,” Shi Dai et al 2020 ApJL 888 L18. doi:10.3847/2041-8213/ab621a

Parker Solar Probe

What might we learn about the Sun if we could fly a spacecraft close enough to dip down and skim through its atmosphere? Thanks to the Parker Solar Probe, we don’t have to speculate!

The Parker Solar Probe (PSP) is a telescope designed to orbit the Sun at least 24 times, dipping closer and closer to our star’s surface over its mission lifetime. Its first few orbits have already been completed at a distance of about 35.7 solar radii from the Sun’s center. Just this past month, PSP used the gravitational pull of Venus to drop its orbit to 27.8 solar radii — and by 2024, after several such maneuvers, PSP will be flying just 8.86 solar radii (that’s less than 4 million miles) from the Sun’s surface, soaring through the Sun’s tenuous outer atmosphere.

Solar corona

A view of the solar corona during the 2015 total solar eclipse in Svalbard, Norway. [S. Habbal, M. Druckmüller and P. Aniol]

This innovative spacecraft will bring us our closest look yet at the magnetic structure and heating of this outer atmosphere — the Sun’s corona — and give us the chance to better explore the solar wind, the stream of energetic particles that flows off of the Sun and pervades our solar system.

Though PSP’s orbit still has a lot further to drop, it’s already flying closer to the Sun than any other spacecraft ever has! This means we’ve already been able to do some remarkable science in the year and a half since its mission began. A new special issue of the Astrophysical Journal Supplement Series now presents roughly 50 studies detailing the findings from PSP’s first two orbits around the Sun.

A few broad categories of topics explored among these articles are:

  • switchbacks

    Illustration of magnetic switchbacks in the solar wind, first discovered by Parker Solar Probe. Click to view an animation. [NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez]

    Switchbacks

    On large scales, the solar wind looks like a smooth flow of particles streaming radially outward from the Sun. But on scales close to the Sun’s surface, this flow is much more complex. PSP has measured a phenomenon termed “switchbacks” — rapid reversals in the direction of the magnetic field that governs the solar wind flow. Several articles detail what PSP has revealed about this phenomenon.

  • Plasma physics

    The high time and frequency resolutions of PSP’s instruments allow the probe to capture unprecedented observations of different plasma phenomena in the ionized gas close in around the Sun. PSP’s first two orbits have produced data on various wave modes, electron holes, magnetic reconnection, radio bursts, microinstabilities, plasma turbulence, and more. Several articles in this issue are devoted to analysis of these detections.

  • Small energetic-particle events

    Some solar activity can rapidly accelerate particles to enormous speeds. Since such energetic-particle events can pose a serious hazard to spacecraft and astronauts, we want to better understand what triggers them and how the particles are accelerated. PSP detected a large number of small energetic-particle events associated with various phenomena — and several articles in this issue detail what we’ve learned from these observations.

  • Plasma structures

    filament eruption

    A huge variety of plasma structures — like this erupting filament — can be witnessed on the Sun. [NASA’s Goddard SFC]

    When plasma is ejected from the Sun, it erupts into space in a variety of structures. PSP carries a camera system that has imaged the complex features of smaller plasma structures; in this special issue, these observations are analyzed and even combined with data from other Sun-watching spacecraft to build three-dimensional views of the structures. From this, we can better understand how magnetic fields govern the geometry and motions of the ionized gas emitted from the Sun.

  • Dust-free zone

    Though dust pervades our solar system, theory predicts that close to the Sun, the high temperatures should prevent dust from existing. Several articles describe PSP’s observations that suggest thinning dust levels; data from PSP’s future travels even closer to the Sun will hopefully confirm the presence of a dust-free zone and determine where, exactly, it lies.

solar cycle

We’re likely at a solar minimum right now in between two activity cycles, as shown here in the predictions made from sunspot observations over the last several cycles. The Sun should become progressively more active over the course of PSP’s mission lifetime. Click to enlarge. [David Hathaway, NASA, Marshall Space Flight Center]

These observations are just the start of what we can hope to learn from the Parker Solar Probe. We should expect to see many updates to our understanding of the corona and the solar wind as PSP explores regions closer to the Sun, as solar activity increases (we’re currently at a solar cycle minimum), and as in-flight calibrations of the PSP instruments continue. Stay tuned!

Citation

Special ApJS Issue on Parker Solar Probe

“Introduction,” Marcia Neugebauer 2020 ApJS 246 19. doi:10.3847/1538-4365/ab67cf

Comet 2I/Borisov

What do we know about the second object to visit us from another stellar system? Detailed Hubble images have given us plenty to consider!

Extrasolar Guests Among Us

'Oumuamua

Artist’s interpretation of interstellar asteroid 1I/’Oumuamua. [M. Kornmesser/ESO]

When asteroid 1I/‘Oumuamua tore through our solar system last year, it was a one-of-a-kind event: the first time we had observed an object from another stellar system pass through our own. But nature likes to keep us on our toes — and it wasn’t long before the next interstellar object paid us a visit.

In October of 2019, comet 2I/Borisov was first spotted. The body swung through perihelion — the point closest to the Sun — in December and then sped along on its way back out through our solar system. It’s expected to reach the distance of Jupiter in July of 2020, and the distance of Saturn by March of 2021.

Apples and Oranges, Asteroids and Comets

Our two interstellar visitors thus far, however, are surprisingly dissimilar. Unlike ‘Oumuamua, Borisov doesn’t have an obviously elongated, tumbling shape. And though observations of ‘Oumuamua showed it to be entirely inactive, Borisov has the appearance of a typical solar system comet: it has a prominent coma — a cloud of gas and dust — around it, and its spectrum contains weak lines that demonstrate ongoing outgassing.

As comet Borisov approached perihelion, a team of scientists led by David Jewitt (UC Los Angeles) imaged it with the Hubble Space Telescope, capturing this body at a distance of just 2.4 AU from the Sun. These observations provide us with a stunningly detailed look at an interstellar object.

A Comet Under Scrutiny

What have we learned from these new data?

  • comet 2I/Borisov

    Composite Hubble image of comet Borisov taken on 12 October 2019 shows the coma and extensive tail of dust trailing from the comet. The 5” scale bar corresponds to a distance of 10,000 km. [Jewitt et al. 2020]

    Size
    Jewitt and collaborators use three independent constraints — the comet’s surface brightness, the rate of its acceleration from forces other than gravity, and its gas production — to measure the size of the comet’s nucleus. They show that this body is likely much smaller than originally thought: just 200–500 meters in radius, as opposed to the 2–16 km estimated from initial observations.
  • Density
    The authors show that the above constraints also dictate a minimum density for the comet nucleus of at least 25 kg/m3 (for comparison, Earth’s density is 5,500 kg/m3). This rules out extreme low-density models for this body like some of those proposed to explain ‘Oumuamua’s unexpected acceleration (which have proposed densities of as little as 0.01 kg/m3).
  • Spin-up
    The small radius for Borisov renders this body susceptible to being spun up by torques from asymmetric outgassing. Jewitt and collaborators show that in the ~0.6 years Borisov will spend heating up within 3 AU of the Sun — and, consequently, outgassing as its water sublimates — the spin of the comet could change significantly, perhaps even causing the body to break up entirely! We can keep an eye out for spin changes as we observe it in the future.

There’s still plenty to learn as we continue to track this newest interstellar object — and we can surely expect more visitors to drop in for future observations!

Citation

“The Nucleus of Interstellar Comet 2I/Borisov,” David Jewitt et al 2020 ApJL 888 L23. doi:10.3847/2041-8213/ab621b

quasar

Black holes come in a variety of sizes — from a mass of a few Suns, to millions or even billions of solar masses. As these vastly different black holes feast on accreting matter, do they behave in the same way?

Shining Structures

state transition model

An example model in cross-section of the possible geometry of an accreting black hole before and after a state transition. The two emitting components are the hot coronal gas enveloping the black hole and the thin accretion disk that lies at its midplane. [Adapted from Ruan et al. 2019]

When a black hole feeds, it emits a lot of light: from the accreting disk of material that swirls around its midplane, from the hot corona of gas surrounding the black hole, and sometimes from fast-moving jets emitted from its poles. These different components all have different temperatures, which means their light peaks at different wavelengths. This means that we can use the shape of a black hole’s spectrum to learn about the physical geometry of an accreting black hole system.

Feasting black holes don’t always maintain the same geometry, though! Black holes transition between different states over time — each with its own physical geometry and spectrum shape. We think that these accretion state transitions are related to changes in the amount of gas a black hole consumes, but there’s still a lot left to learn about this process.

two types of accreting black holes

Illustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxy’s center (bottom). [Top: ESA/NASA/Felix Mirabel; Bottom: ESO/M. Kornmesser]

It’s a Matter of Scale

One major question is whether the geometry of an accreting black hole behaves the same way through state transitions regardless of the black hole’s size. We observe accreting black holes on two dramatically different scales: black holes of 5–15 solar masses that consume material from a companion star, and supermassive black holes feeding on material from the gas-rich environment at the centers of galaxies.

Do the geometries of stellar-mass black holes directly scale up to supermassive ones?

An Alternative to Waiting

The simplest way to answer this question would be to compare the spectra of stellar-mass and supermassive black holes before and after a state transition, to see if the spectra evolve in the same way. But though state transitions take place on timescales of days for stellar-mass black holes, they take 10,000–100,000 years for supermassive black holes — far too long to observe.

spectral behavior

This plot shows spectral behavior of accreting black holes over a range of relative brightnesses. The predicted values for supermassive black holes (faint orange and blue data points), which are based on observations of state transitions in stellar-mass black holes, are well matched to the observed behavior of the sample of accreting supermassive black holes (solid red line and blue data points). Click to enlarge. [Ruan et al. 2019]

A team of scientists led by John Ruan (McGill University, Canada) has taken the next best approach: the team collected a large sample of supermassive black holes in different accretion states. From this, they built a statistical picture of how the spectra of these systems evolve through a state transition, and they compared this to the evolution predicted if accreting supermassive black holes really are just scaled-up versions of stellar-mass black holes.

Same Structures, Different Sizes

From their sample — which, critically, includes a set of very faint accreting supermassive black holes called changing-look quasars to represent the low end of the luminosity scale — Ruan and collaborators find that supermassive black holes do exhibit the expected spectral features based on predictions from stellar-mass black holes.

What does this mean? The structures of accreting black holes seem to directly scale across eight orders of magnitude in mass and across different accretion states. If true, this has the pretty awesome implication that we can use the observable, short-timescale transitions in nearby, stellar-mass black holes to model the long-timescale behavior of feeding supermassive black holes in the distant universe.

Citation

“The Analogous Structure of Accretion Flows in Supermassive and Stellar Mass Black Holes: New Insights from Faded Changing-look Quasars,” John J. Ruan et al 2019 ApJ 883 76. doi:10.3847/1538-4357/ab3c1a

Exoplanet K2-18b

Water is critical to life as we know it on Earth. So naturally, finding evidence of liquid water on a planet in its star’s habitable zone is extremely relevant to searches for extraterrestrial life. Thus far, we’ve only discovered water vapor in the atmospheres of massive, short-period gas giants — but new observations of sub-Neptune K2-18b have now changed that.

Hubble Has Its Eye on You

The Kepler spacecraft was a planet-finding expert, using the transit method to identify thousands of exoplanets and even more exoplanet candidates. K2-18b is one of Kepler’s more notable finds, a sub-Neptune orbiting within the habitable zone of its star.

K2-18b’s white-light curve

Fits to K2-18b’s white-light curve (top) and an example spectral light curve (bottom) with data points from the Hubble observations overplotted. The white-light curve considers points across all available wavelengths while the spectral light curve considers points across a specific wavelength range. The wavelength range for each plot can be seen in the lower right. [Adapted from Benneke et al. 2019]

With an orbital period of 33 days and an M dwarf (K2-18) as its host, K2-18b receives about as much radiation as the Earth does. It occupies an odd space in exoplanet demographics, being fairly large and massive but still close to its star. This makes K2-18b a good target for atmospheric studies.

For exactly that purpose, a team of scientists led by Björn Benneke (Université de Montréal) used the Hubble Space Telescope to observe nine transits of K2-18b over three years — and they found something interesting!

Characterize, Characterize, Characterize

To begin, Benneke and collaborators re-determined the properties of K2-18b and its host. Aside from Hubble data, the team also used new and archived observations of K2-18b taken by Kepler and the Spitzer Space Telescope (rest well, good instrument) along with Gaia data of K2-18.

One result of this analysis was that the measured radius of K2-18b was bumped up from 2.29 Earth-radii to 2.61 Earth-radii. A previous study gives the planet’s mass as nearly nine Earth masses — which means that K2-18b is very much a sub-Neptune and not a super-Earth as had been thought earlier.

A High Chance of Clouds

Benneke and collaborators then analyzed the features of the observed transits in K2-18’s spectra. The spectra in this study are known as transmission spectra, since they are spectra of light that is emitted by the host star and passes through the planet’s atmosphere. Benneke and collaborators produced a single spectrum for K2-18b based on eight observed transits.

K2-18b transmission spectrum

The transmission spectrum of K2-18b based on observations (the data points) and modeling. The red line indicates the best fit to the model, the blue line indicates the median fit, and the shaded blue areas indicate confidence intervals. The water feature is located at 1.4 μm. [Benneke et al. 2019]

The most exciting thing to come out of K2-18b’s spectrum is… statistically significant evidence of water! Models suggest that this water exists in a hydrogen-rich atmosphere and that its presence in the spectrum is caused by clouds. Other gases detected in the spectrum include nitrogen, carbon dioxide, and methane.

So if there are clouds, could there be rain? Possibly! Due to K2-18b’s close orbit around a cool star, the incoming sunlight and atmospheric temperature for this planet are similar to Earth’s. Benneke and collaborators demonstrate that it’s likely that water vapor could therefore condense into liquid droplets in K2-18b’s atmosphere.

The James Webb Space Telescope will help us probe K2-18b’s atmosphere further, answering some of the questions this study has raised.

Citation

“Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b,” Björn Benneke et al 2019 ApJL 887 L14. https://doi.org/10.3847/2041-8213/ab59dc

ASKAP

New evidence deepens the mystery of fast radio bursts (FRBs), the brief flashes of radio emission stemming from unknown sources beyond our galaxy. Scientists have now discovered faint repeat bursts from one of the brightest FRBs, previously thought to have been a one-off event.

To Repeat or Not to Repeat

It was over a decade ago that scientists noticed the first enigmatic, millisecond-duration burst of radio waves from outside of the Milky Way. Since then, we’ve discovered about 100 FRB sources and even identified the host galaxies for several of them. Nonetheless, we still don’t know what causes FRBs, or even whether they’re all the same type of phenomenon.

FRB 121102

FRB 121102, the first fast radio burst found to repeat, was also the first to be localized in the sky. [Gemini Observatory/AURA/NSF/NRC]

FRB properties span a wide range, but one of the biggest distinguishing features has been repetition. While most discovered FRBs have been one-off events — a single bright flash and no evidence of any additional emission from the same region either before or after — around ten FRBs have been found to repeat.

We successfully localized one repeating FRB to a distant low-mass, low-metallicity dwarf galaxy. The two non-repeating bursts that we’ve localized, on the other hand, are associated with very massive host galaxies. Does this distinction mean that repeating and non-repeating bursts make up two different classes of FRBs? Or are FRBs all the same type of source, and the difference in host galaxies is just random variation?

Recently, a team of scientists led by Pravir Kumar (Swinburne University of Technology, Australia) has added one more clue to the puzzle: observations of weak repeat bursts from an FRB thought to be non-repeating.

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]

What Are We Missing?

Kumar and collaborators were testing a simple theory: What if FRBs all repeat, but we don’t have the sensitivity to detect the fainter bursts?

In this scenario, supposed one-off FRBs are actually just the most energetic bursts from repeating sources. If we carefully study very sensitive observations of the region around a non-repeating burst, the team reasoned, we might find evidence of other bursts from the same source.

The authors chose FRB 171019 as their target — one of the brightest bursts found in a recent survey conducted with the Australian Square Kilometre Array Pathfinder (ASKAP). Kumar and collaborators used ASKAP itself, as well as the 64-meter Parkes radio telescope and the 110-meter Green Bank Telescope, to conduct follow-up observations of the 10’ x 10’ region FRB 171019 was determined to have originated from.

Faint Flashes Found

timeline of FRB 171019 observations

Timeline of the ASKAP, Parkes, and Green Bank Telescope observations in the direction of FRB 171019. Red circles mark observed bursts. Click to enlarge. [Kumar et al. 2019]

Though no additional bursts were found in the follow-up ASKAP or Parkes data, two faint bursts were visible in the 820 MHz Green Bank Telescope data, occurring 9 and 20 months after the initial ASKAP burst detection. The inferred distances are consistent with that of FRB 171019, but they are a whopping factor of ~590 fainter than the original burst!

This discovery lends credence to the idea that more seemingly one-off bright FRBs may actually have faint repetitions that we’ve simply missed — and these sources may be found to repeat if we conduct follow-up with more sensitive telescopes. Understanding this brings us one step closer to discovering the nature of these mysterious sources.

Citation

“Faint Repetitions from a Bright Fast Radio Burst Source,” Pravir Kumar et al 2019 ApJL 887 L30. doi:10.3847/2041-8213/ab5b08

Hubble NGC 5189

Not all laboratory astrophysics occurs in labs down here on Earth; sometimes, the lab is in space! A new study has used a space laboratory to confirm a new atomic process — with far-reaching implications.

Cat's Eye nebula

The Cat’s Eye planetary nebula, as imaged in X-rays and optical light. [X-ray: NASA/CXC/SAO; Optical: NASA/STScI]

Balancing a Plasma

Throughout our universe, cosmic soups of electrons and ions — astrophysical nebulae — fill the spaces surrounding dying stars, hot and compact binaries, and even supermassive black holes. The atoms in these nebulae cycle within a delicate balance: they are ionized (electrons are torn off) by the high-energy photons emitted from the hot nearby sources, and then they recombine (electrons are recaptured), emitting glowing radiation in the process.

After many years of research into atomic processes, we thought that we’d pretty well pinned down the ways in which this photoionization and recombination takes place. This is crucial, since these rates go into models that we use to determine abundances — which, in turn, informs our understanding of stellar evolution, nucleosynthesis, galactic composition and kinematics, and cosmology.

But what if we’re missing something?

A New Process

RER diagram

A diagram of how Rydberg Enhanced Recombination works. Click to enlarge. [Nemer et al. 2019]

In 2010, a team of scientists proposed exactly this: that we’re missing an additional type of recombination process that occurs frequently in astrophysical plasmas throughout the universe.

The catch? This type of recombination — which they termed Rydberg Enhanced Recombination, or RER — had never before been detected, and it’s effectively impossible to study in Earth-based laboratories. Only in cold, low-density cosmic environments like astrophysical nebulae do the conditions necessary for RER exist.

Laboratories in Space

When Earth-based labs fail, it’s time to look to space! A team of scientists led by Ahmad Nemer (Auburn University; Princeton University) recently went on the hunt for astrophysical laboratories showing evidence of RER.

First, Nemer and collaborators developed detailed models of how RER would work, under what conditions it would be effective, and what observable spectral lines this process would produce.

symbiotic binary

Illustration of a symbiotic binary system, consisting of a white dwarf and a red giant. [NASA, ESA, and D. Berry (STScI)]

With sample spectra in hand, they then explored the high-resolution optical spectra of several planetary nebulae (the clouds of ionized plasma that surround dying, low-mass stars) and ultraviolet spectra of symbiotic binaries (systems where ionized plasma surrounds a white dwarf accreting mass from a red giant).

Time for an Update

Space lab success! In eight of the planetary nebulae and one of the symbiotic binaries, the authors found spectral lines that provide evidence of the RER process at work, with relative strengths that agree nicely with predictions.

This confirmation of a predicted new atomic process represents a remarkable discovery with far-reaching implications. Nemer and collaborators show that the addition of RER contributions into our current models of ionization balance makes a significant difference in estimated elemental abundances of astrophysical nebulae — which means we may have a lot of work ahead of us to update our past research!

Thanks to the power of laboratories in space, however, we now have a clearer idea of what we’ve been missing.

Citation

“First Evidence of Enhanced Recombination in Astrophysical Environments and the Implications for Plasma Diagnostics,” A. Nemer et al 2019 ApJL 887 L9. doi:10.3847/2041-8213/ab5954

white dwarf giant planet

It’s nearly eight billion years in the future.

The Sun, having exhausted its source of fuel, has dramatically expanded into a red giant and then puffed off its outer layers, leaving its dense, scalding hot core exposed. This core — a white dwarf — initially clocks in at nearly 100,000 K (180,000 °F), bathing its surroundings in harsh extreme-ultraviolet (EUV) radiation at levels that are up to a million times brighter than the present-day Sun.

Earth and the other inner, rocky planets were swallowed up by the ballooning Sun long ago. But how have the giant planets of our solar system — Jupiter, Saturn, Uranus, and Neptune — fared since this unavoidable apocalypse?

A Hostile Environment

atmospheric erosion

Artist’s impression of a close-in hot Jupiter being evaporated by its nearby host. Giant planets on much longer orbits can be evaporated at similar rates by the extreme ultraviolet radiation of a white dwarf. [NASA’s Goddard SFC]

According to Matthias Schreiber (University of Valparaíso, Chile) and collaborators, if our giant planets survive the Sun’s evolution into a white dwarf, they still have another challenge ahead: their atmospheres will be dramatically eroded by the bright EUV radiation emitted by the new white dwarf.

Through a series of calculations, Schreiber and collaborators show that giant planets in a solar system like ours — even with their large orbital distances — will lose mass while orbiting a solar white dwarf at a rate that’s comparable to that measured for short-period hot Jupiters and warm Neptunes orbiting ordinary stars.

Signatures of Pollution

How will future aliens be able to spot this evaporation of our solar system’s giant planets?

When left alone, white dwarfs usually have pristine outer layers of hydrogen and helium, because heavier elements rapidly sink to the stars’ centers under the strong gravity. If we observe signatures of metals in a white dwarf’s spectrum, therefore, this is likely a sign that the star recently accreted debris from its environment — perhaps from the evaporated atmosphere of a distant gas giant.

Schreiber and collaborators demonstrate that a fraction of the evaporated material from giant planets in our solar system will accrete onto the Sun-turned-white-dwarf, polluting its outer layers at a level that can be detected with optical and ultraviolet spectroscopy for millions of years. Alien scientists could therefore observe these signatures and determine the composition of the outer atmospheres of our giant planets.

Glimpsing Our Future

pollution abundances

There is good agreement between prediction (orange) and observation (blue) for pollution abundances in hot white dwarf atmospheres, for white dwarfs of different temperatures. The predictions assume a giant planet occurrence rate of 65%. [Schreiber et al. 2019]

So what does this prediction mean for us now? We can detect systems currently undergoing our own solar system’s future!

About a third of the hot white dwarfs we’ve observed with adequate ultraviolet spectroscopy show signs of polluting volatiles — and the amount of polluting material is hotter for hotter white dwarfs. Schreiber and collaborators use a population model to show that the observed numbers and trends are beautifully consistent with the pollution being caused by evaporated material from giant planets, assuming that at least 50% of hot white dwarfs host at least one giant planet.

Soon, we’ll be able to use more detailed models and observations of these systems to better understand the composition of the evaporated planets — but in the meantime, we can enjoy this insightful glimpse into our solar system’s future.

Citation

“Cold Giant Planets Evaporated by Hot White Dwarfs,” Matthias R. Schreiber et al 2019 ApJL 887 L4. doi:10.3847/2041-8213/ab42e2

Habitable zone

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

A Limited Habitable Zone for Complex Life

Published June 2019

Main takeaway:

A team of scientists led by Edward Schwieterman (UC Riverside) has demonstrated that the habitable zone — the range of orbital distances around its host star at which a planet can support life — becomes significantly narrower when exploring where complex life can exist, as opposed to just microbial life.

habitable zone

Some constraints on the habitable zone, with a few sample exoplanets plotted for reference. The darkest blue region shows where risk of CO2 and CO toxicities are lowest and a planet would be most likely to be able to support complex life. [Schwieterman et al. 2019]

Why it’s interesting:

By identifying planets that lie in their host stars’ habitable zones, we can narrow down our list of targets in our search for life beyond Earth. But traditionally, “habitable zone” merely refers to the range of distances at which a planet can support liquid water — and this definition ignores the many additional constraints that could limit the ability of different life forms to exist on a planet. Schwieterman and collaborators’ work explores one of those constraints by considering what orbital distances would result in a buildup of toxic gases — carbon monoxide and carbon dioxide — in the atmospheres of planets that would make them unfit for complex life as we know it.

Why this significantly limits our options:

Schwieterman and collaborators’ work indicates that carbon dioxide toxicity alone limits the habitable zone for simple animal life to half of the traditional habitable zone around a Sun-like star, and that region shrinks to less than a third of the traditional habitable zone for more complex life like humans. In addition, carbon monoxide toxicity is a significant problem for planets that receive a large amount of ultraviolet radiation — like those orbiting M dwarfs. This suggests that the entire habitable zones of M-dwarf stars may be ruled out in the search for some complex life.

Citation

Edward W. Schwieterman et al 2019 ApJ 878 19. doi:10.3847/1538-4357/ab1d52

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