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ESO VLTI Image of Antares

As stars evolve from the red giant branch to the red clump, they accumulate lithium on their surfaces. How does this accumulation happen?

The Red Giant Branch… 

Stars with less than eight solar masses, or low mass stars, live fairly placid lives. A low mass star would start off burning hydrogen in its core like all main sequence stars. Once the core hydrogen has been exhausted, the star resorts to burning hydrogen in a shell surrounding its passive core, which is now mostly helium. This stage of life is called the red giant branch (RGB) stage.

While the helium core may not be burning any material, that doesn’t mean it’s not doing anything! The sheer mass of the core means that it collapses in on itself to the point that the only thing holding it up against gravity is something called electron degeneracy — you can’t fit more than one electron in a space meant for only one electron.

Qualitiative HR Diagram Showing the Horizontal Branch

A qualitative stellar evolution track going all the way to the horizontal branch. A low-mass star would begin its life somewhere on the main sequence line before moving up the red giant branch, undergoing helium flashes, and moving on to the horizontal branch. Click to enlarge. [Richard Pogge]

…and the Red Clump

This standoff doesn’t last forever though, as conditions become ideal for helium to ignite and start core burning again. This helium ignition is called a helium flash. The star is now at the horizontal branch stage, where it continues to burn hydrogen in a shell around the helium-burning core.

Multiple helium flashes can occur as the star transitions to the horizontal branch, but the first is the strongest. Cooler horizontal branch stars appear red and tend to cluster in a particular region in brightness–temperature space, aptly called the red clump (RC).

Observations of RGB and RC stars have found that RC stars have more lithium on their surface than RGB stars do. This suggests some enriching process — a process that results in more heavy elements being present in a region — occurs between the RGB and the RC stages. To investigate what could be behind this enriching, Josiah Schwab (University of California, Santa Cruz) used stellar evolution models combined with our knowledge of how material moves in stars.

Mixing Things Up

Evolving lithium abundance in different stellar models

Luminosity versus lithium abundance for evolving stellar models with two different starting masses: 0.9 solar mass (blue) and 1.2 solar masses (orange). The plot shows both standard models with no mixing (solid lines) and models that assume mixing from the helium flash (dashed lines). The stars indicate where the red clump stars occur in the mixing model. The gray ellipse surrounding the stars shows the expected location of red clump stars based on observations. [Schwab 2020]

One way to create lithium in a star is to start with helium. Deep within the star, helium can fuse into beryllium. When the beryllium is transported to cooler regions closer to the star’s surface, it can experience electron capture — when the nucleus of an atom absorbs one of the electrons orbiting it — and become lithium.

Schwab suggested the first helium flash that happens between the RGB and RC stages can trigger internal waves that mix material in the star. In some stars, this mixing would deplete lithium, but with simulations Schwab showed that the opposite happens as stars transition from the RGB to the RC, enhancing the amount of lithium present at the star’s surface.

An observational check for this flash-induced mixing would be to determine lithium abundances for stars that are just beginning to evolve from the RGB to the RC, since the first helium flash occurs right at the start of this transition. More detailed stellar models will also be useful, but for now it seems the core of this mystery is solved!

Citation:

“A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars,” Josiah Schwab 2020 ApJL 901 L18. doi:10.3847/2041-8213/abb45f

stellar binary mass transfer

The nearby star Regulus — the heart of the constellation Leo — has long been known to be in a binary. But though the bright, main-sequence star is easy to spot, we’ve yet to detect Regulus’s companion! A recent study now presents what may be a first look at this mysterious object.

A Future Entwined

If you’re a star in a close binary system, your fate is not your own. Instead, your future is heavily dependent on how you interact with your companion — especially as you both age.

Leo

Regulus is the brightest star in the constellation Leo, visible as the star in the lower right corner of the constellation. The bright object below Leo in this photo is Jupiter. [Till Credner; CC BY 3.0]

Such is likely the case for Regulus, a star just ~80 light-years away. Once thought to be a single star, this bright, blue-white, rapidly rotating main-sequence star has since been shown to wobble — it dances in a 40-day orbit with a too-dim-to-spot partner.

May I Have This Dance?

The dance of an interacting, evolving binary is complex. As the more massive star in the pair ages into the red giant stage, it grows larger in size. Eventually, its radius becomes comparable to the separation between it and its less massive, main-sequence partner. At this point, mass and angular momentum begins siphoning off of the massive star onto its main-sequence partner.

As mass and angular momentum continue to transfer, the orbit of the binary first shrinks, and then expands again. At the end of the transfer, the main-sequence star is now bright and rapidly rotating, and the originally massive star has become nothing more than a faint, stripped-down stellar core.

Evidence suggests that this stellar dance is exactly what has happened with Regulus and its undetected companion— but an observational detection of its dim partner would cement this picture. Now, in a study led by Douglas Gies (Georgia State University), a team of scientists may finally have come up with this proof.

Missing Partner Found

Regulus spectrum

Mean, normalized spectrum of Regulus (generated from 786 CFHT/EsPaDOnS and TBL/NARVAL spectra). Below it are three model spectra used for cross-correlation analysis. [Gies et al. 2020]

Gies and collaborators used the high resolution and high signal-to-noise ratio of the CFHT/EsPaDOnS and TBL/NARVAL spectrographs in Hawaii and France to search for the spectral features of Regulus’s dim companion. Though this object is too faint to detect in individual spectra, the authors combine the information from a large set of spectra, use clever cross-correlation analysis to tease out the signature of the weak signal.

Their analysis paid off: Gies and collaborators’ work revealed weak spectral features that have all the properties expected for the spectral signature of the stripped-down companion of Regulus.

The authors find that this surviving core is tiny — just 0.31 solar mass, compared to the Regulus’s 3.7 solar masses — and scalding hot, at perhaps 20,000 K (35,500 °F)! This stripped core will eventually cool and become a white dwarf star.

Regulus and its faint partner confirm our understanding of how stars in a close binary influence each others’ fate. What’s more, the properties of this pair suggest that there may be many, many cases of faint white dwarfs and their progenitors orbiting around bright, rapidly rotating stars.

Citation

“Spectroscopic Detection of the Pre-White Dwarf Companion of Regulus,” Douglas R. Gies et al 2020 ApJ 902 25. doi:10.3847/1538-4357/abb372

solar system

What thoughts keep you awake at night? If it’s questions about how our solar system is going to end … wow, you really focus on the big picture! But some scientists have wondered the same thing, and they’ve got an answer for you: part of it will be swallowed, and the rest is probably going to disintegrate.

After the Sun Grows Old

Studying the likely fate of our solar system is “one of the oldest pursuits of astrophysics, tracing back to Newton himself,” according to the opening of a recent publication led by Jon Zink (UC Los Angeles). Though the tradition is long, this field is complicated: solving for the dynamical interactions between many bodies is a notoriously difficult problem.

red giant Sun

As the Sun evolves, it will become a red giant star, growing in size until it has engulfed the inner planets. [Roen Kelly]

What’s more, it’s not just the dynamics of unchanging objects that need to be taken into account. The Sun will evolve dramatically as it ages off the main sequence, ballooning up to a size that engulfs the orbits of Mercury, Venus, and Earth and losing nearly half of its mass over the next 7 billion years.

The outer planets will survive this evolution, but they won’t escape unscathed: since the gravitational pull of the Sun’s mass is what governs the planets’ orbits, our Sun’s weight loss will cause the outer planets to drift even farther out, weakening their tether to our solar system.

What happens next? Zink and collaborators play out the scenario using a series of N-body numerical simulations.

A Solar System No More

The authors’ simulations explore what happens to our outer planets after the Sun consumes the inner planets, loses half its mass, and begins its new life as a white dwarf. Zink and collaborators show how the giant planets will migrate outward in response to the Sun’s mass loss, forming a stable configuration in which Jupiter and Saturn settle into a 5:2 mean motion resonance — Jupiter will orbit five times for every two orbits of Saturn.

solar system ejection

This plot shows when each outer planet is ejected from the solar system in the authors’ 10 simulations (represented by different colors). Click to enlarge. [Zink et al. 2020]

But our solar system doesn’t exist in isolation; there are other stars in the galaxy, and one passes near to us roughly every 20 million years. Zink and collaborators include the effects of these other stars in their simulations. They demonstrate that within about 30 billion years, stellar flybys will have perturbed our outer planets enough that the stable configuration will turn chaotic, rapidly launching the majority of the giant planets out of the solar system.

The last planet standing will stick around for a while longer. But within 100 billion years, even this final remaining planet will also be destabilized by stellar flybys and kicked out of the solar system. After their eviction, the giant planets will independently roam the galaxy, joining the population of free-floating planets without hosts.

Our fate, then, is bleak: the combination of solar mass loss and stellar flybys will lead to the complete dissolution of the solar system, according to these simulations. The good news? This fate is many billions of years in the future — so you needn’t lose sleep over it.

Citation

“The Great Inequality and the Dynamical Disintegration of the Outer Solar System,” Jon K. Zink et al 2020 AJ 160 232. doi:10.3847/1538-3881/abb8de

rogue planet

Scientists have long believed that there may be billions to trillions of rogue planets drifting through our galaxy, unattached to any host star. A recent study has now identified one such candidate — potentially the first terrestrial-mass world we’ve spotted on the run.

Severing Attachments

rogue planet

Artist’s impression of a free-floating, Earth-like planet. [Christine Pulliam (CfA)]

We’ve discovered more than 4,000 exoplanets in the last three decades, spanning a dramatic range of masses, sizes, temperatures, compositions, orbital properties, and more. The vast majority of them, however, share one feature: they all orbit a star.

While this may seem like normal behavior — after all, we’re rather attached to our own star, here on Earth — planetary formation models predict that there should be a large population of free-floating planets in our galaxy. According to the models, these typically sub-Earth-mass planets get kicked out from their parent systems through interactions with other bodies (usually bullying gas giants).

How can we observationally confirm this picture? Without the beacon of a host star’s light, free-floating planets are challenging to detect — but they’re discoverable via a method called gravitational microlensing.

Gravitational microlensing illustration

Gravitational microlensing is a powerful tool for detecting exoplanets. This illustration shows the bending of light from a background source by a planetary system in the foreground. [NASA Exoplanet Exploration]

The Lens Is the Thing

When light from a background source passes by a massive body on its way to us, the intervening object acts as a gravitational lens, bending the light.

In the case of microlensing, the intervening lens object is small — a stellar- or planetary-mass object — so the lensing doesn’t produce a resolvable ring of light like in strong lensing. Instead, we see a brief brightening of the background source as the lens passes in front of it. From the shape of the light curve, we can then infer lens and source properties.

Roughly 100 planets have been discovered in microlensing events so far — but in most of these cases, the lensing mass is actually a combination of a planet and its host star. Only a handful of objects have been found so far that might be free-floating planets, and they’ve all been of relatively large mass.

That is, until now.

Short Blip, Small Planet

OGLE-2016-BLG-1928

The 23-yr long OGLE light curve of the microlensing event OGLE-2016-BLG-1928 (top) reveals a single brightening. A closeup of the magnified part of the light curve (bottom) shows the structure of the event and its best-fit model. [Mróz et al. 2020]

A recent study led by Przemek Mróz (California Institute of Technology) presents a new discovery gleaned from data from two gravitational lensing telescopes: the shortest-timescale microlensing event seen yet, OGLE-2016-BLG-1928.

The event was located in high-cadence survey fields, so though the brightening timescale was just 41.5 minutes, the Optical Gravitational Lensing Experiment (OGLE) and the Korea Microlensing Telescope Network (KMTN) managed to capture a joint total of 15 magnified data points. By modeling the light curve, the authors establish that OGLE-2016-BLG-1928 is either a free-floating planet, or its host is located at least 8 au away from it. 

Assuming that the planet is located in the galactic disk (which the authors deem likely based on their data), it’s estimated to weigh ~0.3 Earth mass, or roughly 3 times the mass of Mars.

So how do our prospects look for finding more of these free-floating low-mass planets and verifying the expectation that they’re plentiful? Certainly, this OGLE detection proves it’s possible — and with the power of upcoming observatories like the Nancy Grace Roman Space Telescope, odds are good that we’ll be able to spot more of these drifting terrestrial worlds.

Citation

“A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event,” Przemek Mróz et al 2020 ApJL 903 L11. doi:10.3847/2041-8213/abbfad

solar corona

Sinuous, undulating waves in the Earth’s atmosphere play a large role in driving the weather patterns on our planet. A new study now describes how similar motion can govern the behavior of the Sun — and what we stand to learn from it.

Seeing the Future

Rossby waves jet stream

Visualization of Rossby waves in the Earth’s northern hemisphere jet stream. See the end of the article for a video. [NASA/Goddard Space Flight Center Scientific Visualization Studio]

When you plan a sunny picnic outing for the weekend, you can thank Carl-Gustav Rossby for his role in enabling the weather forecasts you’re now able to check.

In 1939, Rossby first identified large-scale waves in the Earth’s atmosphere. These slow meanders of high-altitude winds are visible as long, persistent undulations in the jet stream that carry cells of warmer or cooler air to different regions of the planet.

Through this transport, Rossby waves are critical in driving the day-to-day weather patterns that we experience at middle and higher latitudes on our planet’s surface. Our understanding of the hydrodynamics of Rossby waves is, consequently, one of the things that enables us to make (approximate) weather predictions on timescales of roughly 14 days.

Waves Far and Wide

But Rossby waves aren’t specific to Earth’s atmosphere; they can arise naturally within any fluids that exhibit differential rotation. Scientists have studied some cases of Rossby waves in detail — like those in the Earth’s oceans, or in Jupiter’s atmosphere. But less is known about the role of Rossby waves on an even larger rotating body: the Sun.

Are the motions of the Sun’s atmosphere governed by these same waves? And if so, can we figure out how to model them similarly to how we model Rossby waves on Earth, thereby unlocking a key to making solar weather predictions on 14-rotation (that’s around a year, given the Sun’s rotation period!) timescales?

solar rossby waves

Observations of Rossby waves on the Sun. [Scott W. McIntosh, NCAR/HAO]

But What About Magnetic Fields?

The answer to the first question is yes: signs of Rossby waves have already been observed on the Sun, in the form of persistent, global velocity patterns that evolve on timescales longer than a solar rotation period, but shorter than a solar cycle.

The answer to the second question, however, is less clear. Why? Because there’s a complicating factor: unlike Earth’s lower atmosphere, the Sun is strongly magnetized. A new study led by Mausumi Dikpati (National Center for Atmospheric Research) now walks us through the basic physics involved in Rossby wave development in the Sun, and discusses how the Sun’s magnetic fields influence those waves.

Sketching a Wavy Picture

MHD Rossby Waves

The propagation of the two classes of magnetic Rossby waves: retrograde waves (left) and prograde waves (right). [Dikpati et al. 2020]

Using a simple model, Dikpati and collaborators show that different waves form in the hydrodrynamic and magnetohydrodynamic cases. When magnetic fields are present, two different classes of waves develop that propagate in opposite directions relative to the mean atmospheric flow. The authors also demonstrate what we should expect for fluid particle trajectories within these waves — which is important for understanding observations.

The basic physics described here is a first step that now needs to be expanded to include more complex interactions. But this starting point demonstrates that Rossby waves likely play an important role in organizing the motions of the Sun’s atmosphere. And once we’ve developed more detailed models of this process, perhaps we’ll be able to check our phones for the solar weather forecast for the year!

Bonus

Check out this wildly awesome NASA-produced simulation showing the development of Rossby waves in the Earth’s northern jet stream.

Citation

“Physics of Magnetohydrodynamic Rossby Waves in the Sun,” Mausumi Dikpati et al 2020 ApJ 896 141. doi:10.3847/1538-4357/ab8b63

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

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