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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

first stars

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.

Implications of Symmetry and Pressure in Friedmann Cosmology. I. Formalism

Published August 2019

Main takeaway:

Two scientists at University of Hawai’i at Mānoa, Kevin Croker and Joel Weiner, have reexplored Friedmann’s equations — the set of equations that describe the expansion of the universe — under a more precisely specified set of fundamental assumptions. Using their revised formalism, they show that the universe’s growth rate can be influenced by the relatively small pressure contributions of compact objects left behind after a star’s death.

Why it’s interesting:

NS merger

According to the authors’ calculations, contributions from compact, relativistic objects like neutron stars, illustrated above, could affect the growth rate of the universe. [NASA/Goddard Space Flight Center/Dana Berry]

In previous studies, it’s been assumed that the universe’s matter is all alike and evenly distributed — an assumption that allows us to ignore the details of small structures like stars and galaxies when calculating the evolution of the universe as a whole. But Croker and Weiner’s calculations shows that the averaged contributions of massive, compact objects could affect the universe’s expansion rate after all — and in exchange, the universe’s evolution may affect the energy gain or loss of these compact objects over time. This work provides a new link between the small-scale structures and large-scale evolution of the universe.

What this work says about black holes:

Croker and Weiner’s model has an interesting side note: it has revived interest in an alternative picture of how we conceive of black holes. In the 1960s, Russian physicist Erast Gliner proposed that large stars would collapse into GEODEs — Generic Objects of Dark Energy — at the ends of their lifetimes. These objects would look like black holes from the outside, but on the inside, they would contain a bubble of dark energy instead of a singularity. Croker and Weiner have found evidence supporting Gliner’s hypothesis by demonstrating that if just a fraction of the oldest stars in our universe collapsed into GEODEs instead of black holes, the averaged contribution of these objects today would naturally produce the required uniform dark energy to produce the expansion of the universe we observe. In addition, collisions of GEODEs could naturally explain LIGO’s gravitational-wave observations.

Citation

K. S. Croker and J. L. Weiner 2019 ApJ 882 19. doi:10.3847/1538-4357/ab32da

black hole

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.

Indication of Another Intermediate-mass Black Hole in the Galactic Center

Published January 2019

Main takeaway:

By analyzing the internal motions of a cloud of molecular gas using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of scientists led by Shunya Takekawa (Nobeyama Radio Observatory, NAOJ, Japan) has found evidence for a 30,000 solar-mass black hole drifting near our galaxy’s center.

Why it’s interesting:

LIGO black holes

Simulated image of two merging black holes. One theory predicts that supermassive black holes grow from the mergers of IMBH seeds. [SXS Lensing]

A weight of 30,000 times that of the Sun would categorize this object as an intermediate-mass black hole (IMBH) — a black hole of 100 to 100,000 solar masses, a category that we’ve never directly observed. Though we’ve long suspected that supermassive black holes —black holes of millions to billions of solar masses that often lurk at the centers of galaxies, like our own Sgr A* at the center of the Milky Way — may grow via the mergers of IMBH seeds, we’ve lacked evidence for this theory. The possible IMBH discovered by Takekawa and collaborators lies within 20 light-years of Sgr A*, suggesting that such a merger may be in our galaxy’s future.

Why this discovery may lead to others:

The discovery of this black hole is interesting for another reason: it was found despite the fact that it’s extremely faint. Theory predicts that there are perhaps a hundred million black holes in the Milky Way. Since the vast majority of these objects are dark, however, we’ve found very few of them — only a few dozen exceptions that are bright due to active accretion. Takekawa and collaborators found the faint intermediate-mass black hole not by looking for its emission, but by examining the motions of a gas cloud around it: the gas streams of the cloud were being flung in unexpected directions due to the gravitational forces of an unseen source. By using this same technique in the future, we may be able to discover additional otherwise-invisible black holes.

Citation

Shunya Takekawa et al 2019 ApJL 871 L1. doi:10.3847/2041-8213/aafb07

Kuiper Belt

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.

Shepherding in a Self-gravitating Disk of Trans-Neptunian Objects

Published January 2019

Main takeaway:

Scientists Antranik Sefilian (University of Cambridge, UK) and Jihad Touma (American University of Beirut, Lebanon) have demonstrated that the odd clustering of orbits we observe in the outer reaches of our solar system could be explained by the collective gravity of a large disk of trans-Neptunian objects — small, icy bodies beyond the orbit of Neptune.

Why it’s interesting:

eTNO orbit clustering

Schematic showing the alignment of the orbits of detached extreme trans-Neptunian objects and the proposed orbit of a hypothetical unseen planet (in green). A new study proposes an alternative to this “Planet Nine”. [Sheppard et al. 2019]

In the last 15 years or so, we’ve spotted several dozen distant solar system bodies with highly elliptical orbits that, perplexingly, seem to preferentially cluster together rather than being randomly distributed. One hypothesis put forward to explain this clustering is the presence of a massive, undiscovered additional planet in our solar system. We’ve spent years searching for Planet Nine, however, and it still remains elusive. Sefilian and Touma’s theory would provide a neat, natural alternative to Planet Nine that explains many of the observed properties of our solar system.

What this explanation looks like:

According to the authors, a disk of icy Kuiper-belt bodies roughly a few to ten times the mass of the Earth can, in combination with the gravitational tugs of our solar system’s known planets, shepherd the bodies in the outer solar system onto their observed orbits.

Is such a disk realistic? While previous estimates of the total mass of objects in the Kuiper belt weigh in at only around only a tenth of an Earth mass, it’s difficult to obtain an accurate measure due to our location in the plane of the solar system. There’s a chance there’s a lot more mass out there that we’re missing, and those small, icy bodies could shape the outer reaches of our solar system without the need for a ninth planet.

Citation

Antranik A. Sefilian and Jihad R. Touma 2019 AJ 157 59. doi:10.3847/1538-3881/aaf0fc

Venus as viewed by Akatsuki

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.

Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope

Published August 2019

Main takeaway:

A new study led by Yeon Joo Lee (The University of Tokyo, Japan) has used an entire suite of instruments — Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope — to study the long-term reflectivity of Venus’s atmosphere. They find that, over the timescale of roughly a decade, variations in the ultraviolet albedo of the atmosphere cause significant changes in aspects of Venus’s climate, like windspeeds at the planet’s cloud tops.

Why it’s interesting:

Venus vs. Earth

A comparison of Venus and Earth. Though they are nearly the same size and density, the two planets evolved very differently. [NASA]

Often cast as a sibling to Earth, Venus’s similarities to our own planet are striking — yet the two worlds have very different atmospheres. Exploring what drives the weather on Venus helps us to understand what makes these two bodies alike and different.

From Lee and collaborators’ study, we can see that, like Earth, Venus has decade-long climate variations. Unlike on Earth, however, most of the Sun’s energy is absorbed by the planet’s atmosphere, rather than its surface. The decade of observations analyzed in this study provide new insight into Venus’s climate processes, demonstrating that Venusian weather is deeply influenced by ultraviolet absorption in the planet’s atmosphere.

Why these results provide extra intrigue:

Tracking how ultraviolet absorption in Venus’s atmosphere influences the planet’s climate is only part of the puzzle; the other part is understanding what is doing the absorbing. For more than a century, we’ve been aware that Venus has dark patches in its atmosphere that absorb light at ultraviolet to visible wavelengths, peaking at around 360 nm. But what are the patches made of, and why do they absorb ultraviolet light? Lee and collaborators’ analysis may help us to further study these “unknown absorbers” and identify the particles that they’re made up of.

Citation

Yeon Joo Lee et al 2019 AJ 158 126. doi:10.3847/1538-3881/ab3120

quasar

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.

Discovery of the First Low-luminosity Quasar at z > 7

Published February 2019

Main takeaway:

A team of scientists led by Yoshiki Matsuoka (Ehime University, Japan) announced the discovery of the first low-luminosity quasar — a dim supermassive black hole actively feeding at the center of a galaxy — found at a redshift of z > 7. This redshift represents a time when the universe was still in its infancy, at less than a billion years old.

J1243+0100

False color, composite image around the low-luminosity, high-redshift quasar J1243+0100, marked with the cross-hair. [Matsuoka et al. 2019]

Why it’s interesting:

By exploring distant quasars, we can learn about the conditions in the early universe, helping us to address questions like how the earliest black holes were able to grow to their enormous sizes in such a short time, or what sources were responsible for the majority of our universe’s reionization. Matsuoka and collaborators’ discovery is unique because, before now, the few quasars we’ve found at such high redshifts have been extremely luminous. Since we don’t think those sources are representative of the general population of high-redshift quasars, it’s exciting to now have a distant source to study that’s likely to be a more typical example.

How this quasar was spotted:

It’s hard to find high-redshift quasars — especially low-luminosity ones! — because their emission in observed wavelengths of less than 970 nm is almost entirely absorbed by the intergalactic medium. To discover them, you therefore need wide-field, deep imaging at wavelengths longer than this — which is exactly what the Hyper SuprimeCam on the Subaru Telescope in Hawaii provides. Matsuoka and collaborators conducted a broad survey, the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs), which revealed not just the record-setting quasar detailed in this study, but also 82 additional, previously unknown distant quasars.

Citation

Yoshiki Matsuoka et al 2019 ApJL 872 L2. doi:10.3847/2041-8213/ab0216

habitable exoplanet

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.

Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

Published March 2019

Main takeaway:

A team of scientists led by Edward Schwieterman (UC Riverside) used computer simulations of planetary ecospheres/atmospheres to show that a number of planet types of interest in the search for life — like planets similar to early Earth, or planets around M-dwarf stars — can maintain an accumulation of carbon monoxide in their atmospheres.

Why it’s interesting:

It was previously thought that the presence of carbon monoxide in a planet’s atmosphere could be considered an antibiosignature — a signature that indicates that there probably isn’t life present. This is because carbon monoxide represents an unexploited source of free energy; its accumulation was thought to indicate that there’s no life available to take advantage of it. Antibiosignatures are valuable because, in the search for life on planets beyond the solar system, they can quickly tell us where we shouldn’t waste our time looking. But Schwieterman and collaborators’ work now suggests we may need to rethink the assumption that carbon monoxide can be used as such an indicator.

What this means for the search for life:

If carbon monoxide can be present in a planet’s atmosphere even when the world is inhabited, we clearly can’t use the accumulation of this gas to unambiguously rule out targets in the search for extraterrestrial life. Instead, our best bet is to continue to develop a framework that relies on the presence or absence of various combinations of gases. We may also be able to use this information together with novel approaches, like searching for seasonal variation that could be caused by the presence of life, or calculating whether the atmosphere and surface of a planet are out of equilibrium.

Citation

Edward W. Schwieterman et al 2019 ApJ 874 9. doi:10.3847/1538-4357/ab05e1

'Oumuamua

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.

High-drag Interstellar Objects and Galactic Dynamical Streams

Published March 2019

Main takeaway:

Remember the first known interstellar asteroid, 1I/’Oumuamua? A study by scientist Marshall Eubanks (Space Initiatives, Inc.) explores the possibility that the asteroid is a very lightweight, high-drag object that long ago orbited the galaxy, became caught up within dense interstellar gas, and was then released in our direction as part of the Pleiades dynamical stream.

Why it’s interesting:

'Oumuamua velocity

‘Oumuamua’s incoming velocity is consistent with the dynamics of the Pleiades stream. [Eubanks 2019]

This interstellar object visited our solar system for only a brief time, and it displayed a number of perplexing behaviors — like its unexpected acceleration boost as it left the solar system again. Eubanks’ hypothesis provides a plausible natural explanation: if ‘Oumuamua is a body with a large area-to-mass ratio, solar radiation pressure could have provided the acceleration boost (note that this is the same explanation used by Bialy & Loeb to argue that ‘Oumuamua could be a light sail). And this same lightweight, high-drag structure makes it easy for the asteroid to have become entrained in interstellar gas before being sent into our solar system with the Pleiades stream.

Why This Could Provide Cool Future Opportunities:

In Eubanks’ model, ‘Oumuamua may not be unique. Instead, this object could be just one of an entire population of light asteroids with large area-to-mass ratios — all of which are likely to be entrained with dense gas and could be ejected toward us in streams. By searching stellar streams for bodies like this one, we could identify future interstellar visitors like ‘Oumuamua before they arrive, providing us with more time to study them from afar — or even prepare a fly-by mission.

Citation

T. M. Eubanks 2019 ApJL 874 L11. doi:10.3847/2041-8213/ab0f29

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