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artist's impression of a gas giant exoplanet

Astronomers have discovered a curious exoplanet with an extremely low bulk density — nearly 15 times less dense than Jupiter and 60 times less dense than Earth. The first spectrum of this planet’s atmosphere gives clues to the cause of this unusual quality.

An Exceptional Exoplanet

black and white photograph of neptune's rings

Voyager 2 captured this shot of Neptune’s rings in 1989. Given its distance from its host star, HIP 41378 would likely have rings that are rocky like Neptune’s rather than icy like Saturn’s. [NASA/JPL]

HIP 41378 f is one of the least dense exoplanets known, floating in at just 0.09 grams per cubic centimeter, but it’s not yet clear why this planet is so loosely packed. HIP 41378 f might be an example of a rare class of exoplanets called super-puffs, which contain far more gas than expected given their masses. On the other hand, the planet’s low bulk density could be just a trick of the light curve — if the planet has rings, its radius might appear artificially large, deflating its calculated bulk density.

Both possibilities are exciting, since we’ve only discovered a handful of super-puff planets, and we’ve yet to definitively detect rings around an exoplanet. Now, a team led by Munazza Alam (Carnegie Earth & Planets Laboratory and Center for Astrophysics | Harvard & Smithsonian) has collected the first near-infrared transmission spectrum of HIP 41378 f’s atmosphere to gain a better understanding of this unusual planet.

transmission spectra and modeled spectra

Transmission spectrum for HIP 41378 f (black circles) compared to model results (colored lines). The three panels show the same data and models over three different wavelength ranges. Click to enlarge. [Alam et al. 2022]

Puffy Planet, Rocky Rings, or Something Else?

Alam and collaborators used the Hubble Space Telescope to measure the light that filters through HIP 41378 f’s atmosphere as the planet makes its 19-hour transit across the face of its parent star. The team found that HIP 41378 f’s spectrum is nearly featureless, lacking the characteristic dips that signal absorption of light by molecules in the atmosphere.

Using one-dimensional atmospheric models, the authors were able to rule out a clear atmosphere rich in hydrogen and helium. However, they found that the planet’s nearly flat spectrum is consistent with multiple scenarios: HIP 41378 f might have an atmosphere exceptionally rich in elements heavier than helium, a layer of haze, or rings. In the ringed planet case, the authors calculated that HIP 41378 f’s true radius would be about 60% smaller than the current estimate, leading to a bulk density of 1.2 grams per cubic centimeter, which is roughly the density of Jupiter and Uranus.

Transit Opportunities

plot of transit times and transit timing variations

Previous and predicted transit times for HIP 41378 f. [Alam et al. 2022]

HIP 41378 f’s lack of spectral features doesn’t mean we can’t learn more about the planet’s atmosphere. Measuring the planet’s transmission spectrum at longer wavelengths might help distinguish between the possibilities, since a hazy atmosphere would induce transits of different depths at different wavelengths, while the presence of rings would produce less variation.

The authors explored the possibility of observing HIP 41378 f with JWST and found that the telescope’s infrared instruments are sensitive enough to distinguish between the competing scenarios. With an orbital period of roughly 1.5 Earth years, there will only be a few opportunities to catch HIP 41378 f transiting in front of its parent star during JWST’s planned five-year mission. Alam and collaborators used the timing of the transit they observed to update the prediction for HIP 41378 f’s future transits; late 2022 and mid 2024 will bring new opportunities to study this enticing faraway world!

Citation

“The First Near-infrared Transmission Spectrum of HIP 41378 f, A Low-mass Temperate Jovian World in a Multiplanet System,” Munazza K. Alam et al 2022 ApJL 927 L5. doi:10.3847/2041-8213/ac559d

Neutron Star Merger

The merger of two neutron stars releases an enormous amount of energy and reconfigures the magnetic field of the whole binary system. How well do we need to know the initial conditions of the system to predict the outcome of the merger? 

When Worlds Collide 

In 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first observation of a neutron star–neutron star merger by detecting the ripples in spacetime released as the two massive objects collided and sent the signal ringing through the cosmos. This detection showed that neutron star–neutron star mergers are powerful enough to emit gravitational waves and allowed us to observe properties of these neutron stars such as their mass and radius. 

Simulations of the magnetic field of the merger (shown as spirals getting tighter over time)

Magnetic field evolution over the course of the merger. Time increases from left to right, showing snapshots at 2, 5, 10, and 20 milliseconds after the merger. Each row is a different simulation, and the darker the area in the image, the more intense the magnetic field. [Aguilera-Miret et al. 2022]

Does the End Point to the Means? 

Though the dynamics of colliding neutron stars are fairly well understood, a few open questions remain, such as how the magnetic field amplifies and reorganizes during the merger. This is important because the amplification and reorganization is necessary for the production of the jet associated with short gamma-ray bursts.

Studying this critical process is difficult, as it involves capturing fluctuations and instabilities on a very small scale and requires precise knowledge of the initial parameters of the system. A team led by Ricard Aguilera-Miret (University of the Balearic Islands / Institute of Space Studies of Catalonia) performed complex simulations to tackle the question of how much the initial magnetic field configuration of the system affects the end product of the merger. 

A plot showing time vs. magnetic energy. The four models stars at different places on the y-axis and converge as time goes on.

The magnetic field energy for various simulations. This shows that no matter what energy the system begins with, the final energy will be roughly the same. [Aguilera-Miret et al. 2022]

Magnetic Merging 

Using supercomputers, the team explored the effect of different initial magnetic configurations on the final magnetic field strengths of the simulated binary neutron star mergers. As they moved time forward, exploring up to 30 milliseconds after the merger, they found that the initial topology of the system does not affect the end product because small-scale turbulence erases any memory of magnetic fields greater than 1012 G within a few milliseconds of the merger. This creates a new conundrum, as it shows that we can’t infer the initial magnetic field of a system by observing it post-merger. 

These simulations show that using a simplified magnetic field model is acceptable in binary neutron star mergers, as long as the magnetic field isn’t too large, because it doesn’t make a difference in the final configuration. Further observations of neutron star–neutron star mergers will provide a test of this theory. 

Citation 

“Universality of the Turbulent Magnetic Field in Hypermassive Neutron Stars Produced by Binary Mergers,” Ricard Aguilera-Miret et al 2022 ApJL 926 L31. doi:10.3847/2041-8213/ac50a7

artist's impression of a star being ripped apart by a black hole

Astronomers may have discovered the fifth instance of a new kind of high-energy outburst seen in distant galaxies. How did they track down the source, and what does this discovery mean for our understanding of these rare eruptions?

Rare Events

Only four galaxies in the universe are currently thought to produce quasi-periodic eruptions: enormous outbursts of X-ray photons that occur roughly every few hours. Two of these sources are associated with galaxies in which the central supermassive black hole is actively consuming material from its surroundings, while the other two sources are quiescent galaxies. Despite their differences, all four sources have eruptions of similar duration, frequency, and energy dependence, which suggests that they share a common origin.

However, the source of these events remains uncertain, and further detections are needed to narrow in on a cause. In today’s article, Joheen Chakraborty (Columbia University and MIT Kavli Institute for Astrophysics and Space Research) and coauthors describe their hunt, discovery, and analysis of a potential fifth quasi-periodic eruptor.

plot of X-ray count rate and amplitude

Count rate and amplitude measured by three detectors on XMM-Newton during the potential quasi-periodic eruptions (left) and during a quiescent phase (right). Note that the y-axis range is different between the two columns. Click to enlarge. [Adapted from Chakraborty et al. 2021]

Slewing Past a Source

Chakraborty and collaborators searched for quasi-periodic eruptions in archival observations from the X-ray Multi-Mirror Mission (XMM-Newton), a space telescope that has been observing the X-ray sky since 2000. Using an algorithm that was originally developed to detect tiny variations in the timing of exoplanet transits, the team found a single convincing source out of nearly 12,000 observations: the galaxy XMMSL1 J0249-041244, referred to as J0249. XMM-Newton first detected this source in 2004 as part of its slew survey, which observes objects that happen to fall en route from one target to the next.

The team’s algorithm identified one and a half X-ray flares from 2006 that resembled the quasi-periodic eruptions seen in other galaxies. That’s not the only interesting thing about this source: it has also been tagged as a potential site of a tidal disruption event, in which a black hole rips apart and consumes a star. Is it possible that tidal disruption events are the underlying cause of quasi-periodic eruptions? The authors dig into the observations of this doubly intriguing galaxy to find out more.

And Then There Were Five

three plots of eruption properties

Comparison of the properties of J0249’s eruptions (purple symbols) to those of the previously detected sources. Click to enlarge. [Adapted from Chakraborty et al. 2021]

In many ways, J0249 behaved similarly to the four previously discovered quasi-periodic eruptors. Its flares were largest in low-energy X-rays and less prominent at higher energies, and its flux was fairly constant outside of the flaring episodes. However, the authors also detected potential dips in the ultraviolet emission during the X-ray flares, which marks the first time a quasi-periodic eruptor candidate has shown hints of variability at longer wavelengths. This might suggest that J0249’s emitting region is larger than that of the previously detected sources, but it may also indicate the presence of an accretion disk that is unrelated to the eruptions.

What does the future hold for J0249? In 2021, the team requested a final XMM-Newton observation long enough to capture 2-3 eruptions, but there was no sign of the flares seen in 2006. This may mean that the eruptions were linked to a tidal disruption event, and they ceased when the star was finally engulfed. With only five sources found so far, there’s plenty we don’t yet know about about quasi-periodic eruptions. Hopefully, further detections will clarify the role that tidal disruption events play in these rare outbursts!

Citation

“Possible X-Ray Quasi-periodic Eruptions in a Tidal Disruption Event Candidate,” Joheen Chakraborty et al 2021 ApJL 921 L40. doi:10.3847/2041-8213/ac313b

illustration of RS Ophiuchi

A new star appears in the constellation Ophiuchus roughly every 15 years when the surface of a white dwarf ignites in a burst of nuclear fusion. Can X-ray observations tell us what happens in the aftermath of these explosions?

Hello Again, RS Oph

six-panel plot showing count rate as a function of location on the sky

Images of RS Oph obtained after processing. The expanding lobes that extend in the east–west direction are clearly visible in the 2009 and the two 2011 observations, which are combined into one for the analysis. The black outlines indicate the location of a known artifact in the imaging system. [Adapted from Montez et al. 2022]

The RS Ophiuchi (RS Oph) binary system, composed of a red giant and a white dwarf, is usually hundreds of times too faint to see with the unaided eye. Occasionally, though, it briefly winks into view. RS Oph is a rare example of a recurrent nova, only a handful of which are known in the Milky Way. Outbursts occur when a white dwarf in a binary system steals enough material from the atmosphere of its puffed-up companion to ignite a brief flash of nuclear fusion on its surface, releasing a thousand times the Sun’s yearly energy output in just a few days.

RS Oph has flared up every 9–21 years since its first recorded explosion in 1898, with the two most recent outbursts occurring in 2006 and 2021. The aftermath of RS Oph’s 2006 eruption has been observed all across the electromagnetic spectrum, revealing an expanding ring of circumstellar material and a massive bipolar outflow. A new article led by Rodolfo Montez Jr. (Center for Astrophysics ∣ Harvard & Smithsonian) introduces X-ray observations from just a few years after the 2006 outburst, giving us an unprecedented view of the system’s expanding outflows.

three panel plot of X-ray count rate

Profiles derived by integrating X-ray images along the declination axis. [Montez et al. 2022]

Expanding Our Knowledge

Montez and collaborators investigated images and spectra taken by the Chandra X-ray Observatory in 2007, 2009, and 2011 to study the X-ray-emitting plasma flowing outward from the white dwarf. They detected jet-like structures spreading out from the system to the east and west, mirroring the structures seen at other wavelengths.

The jets were too close to the RS Oph system to be discernible in 2007, but they grew visibly between 2009 and 2011, traveling across the sky at a rate of 1.1 milliarcseconds per day. The distance to RS Oph is still uncertain, making it difficult to measure the precise velocity of the outflow, but the authors estimate that this apparent movement corresponds to 6,000 kilometers per second — meaning that the material was launched at roughly 2% the speed of light!

No Slowing Down

How did the bipolar jets evolve as they expanded? Based on archival multiwavelength images, the jets appear to have expanded linearly over time, without slowing down. They also appear not to have cooled down; modeling of the X-ray spectra suggests that the outflow maintained a constant temperature of 2 million kelvin.

modeled and observed X-ray spectra

Model output (orange lines) and data (blue symbols) for the 2009 observations (top panel) and the two 2011 observations (bottom panel). [Montez et al. 2022]

The constant velocity and temperature imply that the flow expanded freely without collecting enough gas and dust to slow it down. This suggests that the 2006 outburst expanded into a cavity left by the previous eruption, which occurred in 1985, and that repeated eruptions might generate a series of shells and cavities surrounding the binary.

As data trickle in after RS Oph’s most recent flare-up in August 2021, astronomers are already beginning to discern the presence of new outflows. With luck, the next few years will bring many observations of this intriguing system, allowing astronomers to search for new structures that might arise from the latest outburst.

Citation

“Expanding Bipolar X-Ray Structure After the 2006 Eruption of RS Oph,” R. Montez Jr. et al 2022 ApJ 926 100. doi:10.3847/1538-4357/ac4583

ultraviolet image of a solar filament on the sun

On May 30, 2017, a tendril of twisted plasma fought the Sun’s gravitational pull and escaped into the solar system. What can space- and ground-based observations tell us about this event?

A Filament by Any Other Name

extreme-ultraviolet image of the Sun

A solar prominence extends from the Sun’s surface in the lower left quadrant of this image. If this prominence were viewed against the Sun’s disk, the relatively cool plasma would appear dark against the solar surface. [NASA/STEREO]

Solar physics is ideal for people who love terminology. In what other field can you find sunquakes, switchbacks, and supergranules? Even more intriguing are terms that change based on your perspective: filament and prominence refer to the same structure, just seen from different angles. When viewed against the disk of the Sun, a strand of solar plasma suspended by magnetic field lines is called a filament. When that same structure is silhouetted against the blackness of space, it’s called a prominence.

Sometimes, filaments explode into space just hours after they form, often accompanied by other types of solar outbursts like flares or coronal mass ejections. On other occasions, they can linger quietly for days or months before erupting or slowly sinking back toward the Sun’s surface. Today’s article analyzes an extremely long solar filament that loitered for a week before erupting.

three panel plot showing the velocity of the filament compared to background regions

Evolution of the filament’s line-of-sight velocity over time. Several hours before the eruption (left panel), the filament’s velocity was close to zero. As the eruption progressed (center and right panels), the filament’s outward velocity increased. Click to enlarge. [Wang et al. 2022]

From Quiescent to Eruptive

A team led by Shuo Wang (New Mexico State University) studied the evolution of a 500,000-kilometer-long solar filament that erupted from the Sun in May 2017. Wang and collaborators analyzed spectra and images from telescopes on Earth and in space to understand how the velocity of the filament changed in the hours leading up to its eruption. Understanding how the velocity of an erupting filament evolves over time is important for discerning the mechanism that caused it to erupt as well as predicting when an Earth-directed eruption might hit us.

The team found that the filament didn’t launch into space from a complete standstill, but rather had a (relatively) low upward velocity of 6.3 kilometers per second in the hours before the eruption. Rather than being a coherent structure, the filament contained multiple discrete threads of plasma, which showed their own velocity evolution. As the eruption drew nearer, the velocity distribution of these threads became heavily skewed toward higher velocities. Finally, the filament broke free of the Sun with a velocity of 430 km/s — fast enough to reach Earth’s orbit in just four days.

Launching an Investigation

plot of velocity of the filament as a function of time

Velocity of the filament over time, as derived from the multiple data sets analyzed in this work. The solid and dashed green lines show the exponential and linear fits to the data, respectively. Click to enlarge. [Adapted from Wang et al. 2022]

What caused the eruption of this solar filament? The answer may be hinted at by the filament’s velocity, which Wang and collaborators found to increase exponentially over time. Previous numerical simulations suggest that an exponential increase in velocity could be due to an instability such as the kink instability, which kicks in when a curled rope of plasma becomes twisted too tightly.

The question of what caused the filament’s initial, low-velocity motion away from the Sun may be harder to answer since there is a gap in the observations between the filament’s quiescent phase, when its velocity was zero, and when it began to move upward at 6.3 km/s. Other events have also shown slow upward motion hours before the eruption, so further analysis of other eruptive solar filaments may help us understand why some filaments erupt while others sink back to the surface unperturbed.

Bonus

Check out this video from the authors’ article, which shows the evolution of the filament over the course of 19 hours from two different angles.

Citation

“Velocities of an Erupting Filament,” Shuo Wang et al 2022 ApJ 926 18. doi:10.3847/1538-4357/ac3a04

photograph of an asteroid meteorite sitting on ice

Astronomers wait years for robotic explorers to bring back samples of asteroids — but sometimes, asteroids come to us. What can the meteorites that rain down on Earth tell us about their parent asteroids?

Bringing Outer Space Down to Earth

photograph of a person in the desert with a meteorite

NASA scientist Peter Jenniskens spies an asteroid meteorite in the Nubian Desert in Sudan. The vast majority of meteorites are found in deserts, including Antarctica. [NASA/SETI/P. Jenniskens]

When chunks of asteroids fall to Earth’s surface as meteorites, astronomers seize the opportunity to investigate these remnants of the early solar system. Especially interesting are meteorites called carbonaceous chondrites, which make up less than 5% of all meteorites collected on Earth. Abundance studies suggest that Earth likely accreted many carbonaceous chondrites during its formation, meaning that these meteorites may have been an important source of volatile materials like hydrogen and nitrogen.

In a new article, a team led by Wataru Fujiya (Ibaraki University, Japan) analyzed a slice of the Jbilet Winselwan meteorite, a 6-kilogram carbonaceous chondrite found in Western Sahara in 2013. The team performed laboratory tests to determine the age and composition of the meteorite, as well as what minerals are present, providing clues as to the history of its parent asteroid.

photograph of a sliced meteorite

A sliced fragment of the Jbilet Winselwan meteorite. [MJCato; CC BY-SA 4.0]

A Slice of Early Solar System Life

Fujiya and collaborators found that the meteorite contains calcite — a crystalline form of the main component of eggshells, pearls, and chalk — which forms only in the presence of liquid water. Using radioactive dating, the team determined that the calcite in their sample likely formed just 2.6 million years after the solar system formed, meaning that at that point in time, the parent asteroid was warm enough for ice to melt into a liquid.

In fact, the team found signs that the asteroid warmed up far beyond 0°C; some of the minerals had decomposed, which laboratory tests suggest happens at temperatures above 300°C. Space is much chillier than that — what heated this asteroid so quickly after the solar system’s formation?

Sun-Warmed Asteroids, or Something Else?

Modeled thermal history of the meteorite’s parent asteroid. The black and white lines mark temperatures of 0°C and 70°C, respectively. Note that the color scale in the figure gives the temperature in kelvin. [Fujiya et al. 2022]

The authors identified three possible heat sources: solar radiation, impacts by other asteroids, and the decay of radioactive materials. Given the rapid formation of the calcite crystals, the authors concluded that radioactive decay is the most likely heat source for this asteroid; impact heating and solar heating occur intermittently or slowly over billions of years, while radioactive materials churn out heat for just a few million years.

As a test, Fujiya and collaborators modeled the thermal evolution of an asteroid heated by the decay of a radioactive form of aluminum. They found that 20% of the asteroid reached 300°C — hot enough to cause the mineral decomposition seen in the Jbilet Winselwan meteorite.

Additionally, their model showed that while the inner regions of the asteroid were warm enough to have liquid water 0.3 million years after formation, the exterior regions remained cool for a further 0.3–1.0 million years. This explains how dissimilar meteorites could arise from the same asteroid; the Jbilet Winselwan meteorite likely arose from the interior of its parent asteroid, while meteorites that are chemically similar but lack signs of heating might come from the exteriors of asteroids.

Citation

“Hydrothermal Activities on C-Complex Asteroids Induced by Radioactivity,” Wataru Fujiya et al 2022 ApJL 924 L16. doi:10.3847/2041-8213/ac448f

Solid neutron star emitting a burst of radiation

We’re used to watching magnetars throw temper tantrums which involve large outbursts of energy in the form of X-rays. One magnetar, however, recently exhibited some unusual behavior during such an outburst. What’s causing this strange demeanor? 

Whoa, Magnets. 

When it comes to extreme objects in the universe, it doesn’t get more extreme than a magnetar. Take a massive star’s core, crush it into the size of a small city, spin it as fast as a blender, and give it a magnetic field a trillion times Earth’s, and you have yourself a magnetar. The magnetic fields of magnetars are highly complex, and disturbances in these magnetic fields can output vast amounts of energy in the form of X-rays over the span of months or years. A team led by George Younes from the Goddard Space Flight Center / Universities Space Research Association has now monitored a particularly misbehaving magnetar: one that, during such an outburst, changed its behavior in a way that no other magnetar has before. 

Temporal evolution of the pulse profile, showing 3 components on top that morph into one at the bottom

The profile evolution of the magnetar with time with the earliest profile is shown on the top. The vertical lines show the original center of each of the components. [Younes et al. 2022]

A Swift Discovery  

Magnetar SGR 1830-0645 was discovered in late 2020 by the Swift/Burst Array Telescope, a highly sensitive instrument that can pinpoint a burst within seconds of its discovery, after it released a short X-ray burst. It seemed like a normal magnetar with a rotation period of ~10 seconds and a magnetic field strength of ~1014 G (for reference, the magnetic field of Earth is ~0.5 G and the magnetic field of our Sun is ~1 G). While monitoring this energetic event, astronomers noted something odd: this magnetar’s thermal pulse profile, which shows the thermal energy emitted throughout each rotation of the star, gradually changed from having three peaks to having only one. Changes in profile structure have been seen before in magnetars, but the profiles usually get more complex instead of simplifying…so what’s going on?

Crustal Cracking or Magnetospheric Meandering? 

The team observed the source during the first 37 days of the outburst using the Neutron star Interior Composition Explorer (NICER) instrument. They found that the temperature of the star didn’t change, yet the hotspots on the surface (where the emission is thought to come from) get smaller during the outburst. This points to one of two things: either crustal motions or a twisting of the magnetosphere.

Small array of detectors on a rectangular panel mounted on the International Space Station.

The NICER instrument aboard the International Space Station. [NASA]

In the case of crustal motions, magnetic stresses build up under the magnetar’s crust, shifting it much like how tectonic plate motions cause earthquakes. This could cause changes in the active regions where the emission is generated (and, as a bonus, observations of this effect could also tell us about the density of the magnetar’s interior, which is still a bit of a mystery). In the other case, the magnetic field lines in the magnetosphere get twisted and when they untwist, a burst of energy is released (think of a rubber band: when it gets tightly twisted and then let go of, it releases a bunch of energy). It’s also likely that these two mechanisms could both be correct and both be happening at the same time.

Was SGR 1830-0645’s odd behavior during its outburst a one-off event where two mechanisms both happened to come together, or is this common behavior that’s only now detectable due to the NICER instrument’s high-cadence observations? More magnetar observations will tell! 

Citation 

“Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting,” George Younes et al 2022 ApJL 924 L27. doi:10.3847/2041-8213/ac4700 

infrared image of the stars and protostars in the orion nebula

photograph of the constellation orion

Visible-light image of the constellation Orion and the Orion nebula — located below the three stars of the “belt” — which is part of the Orion molecular cloud complex. [Wikipedia]

Many mature stars have stellar companions, living out their lives in close-knit clusters or swinging through space in binary pairs. This suggests that most stars form in small groups, but the onset of star formation has long been difficult to study. Now, astronomers have turned arrays of sensitive radio telescopes toward one of the most active star-forming regions in the Milky Way to investigate star formation at its earliest stages.

Star Formation in Our Backyard

When it comes to studying young stars, there’s no better place than the Orion molecular cloud complex: a network of active star-forming regions located just over a thousand light-years away. The Orion molecular clouds contain hundreds of protostars still siphoning gas from their nascent nebulae, making it a perfect arena for studying star formation.

The two leading theories for how stars form are disk fragmentation and turbulent fragmentation. The disk fragmentation theory suggests that a rotating disk of star-forming material can splinter into multiple stars. The turbulent fragmentation theory posits that small fluctuations within a dense clump of gas can ripple outward and induce a gas cloud to collapse. Key to distinguishing between these hypotheses are their length scales; disk fragmentation is thought to produce stars separated by roughly 100 au, while turbulent fragmentation likely generates more widely separated companions.

A First Look at New Stars

Studying star formation at small spatial scales is challenging, because short-wavelength light emitted by young stars is heavily obscured by gas and dust, and long-wavelength observations are inherently lower in resolution. Luckily, the advent of radio telescope arrays has increased the achievable resolution of radio images and allowed astronomers to probe smaller scales than ever before. A team led by John Tobin (National Radio Astronomy Observatory) has used the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array (VLA) to investigate stellar companionship in the Orion molecular clouds at the earliest stages of star formation — and to explore what these findings mean for how stars form.

plots of radio flux from protostars

Example of observations from ALMA (left) and the VLA (right) of a young protostellar system. [Adapted from Tobin et al. 2022]

Tobin and collaborators surveyed 328 protostars in the Orion molecular clouds, including 94 in the earliest phase of protostar evolution. The team used an iterative algorithm to search for protostars with companions within 20–10,000 au. (For context, if the Sun had a companion at 10,000 au, it would be located in our solar system’s Oort cloud.) The authors found that roughly 30% of all systems surveyed contained multiple stars, with binary systems being more common than triple- or quadruple-star systems.

Competing Creation Scenarios

plot of fraction of companion protostars observed in orion as a function of separation in astronomical units

Separation distribution for multiple-star systems containing the youngest (Class 0) protostars. The dotted curve approximates the distribution for Sun-like stars in the field. [Adapted from Tobin et al. 2022]

The authors noted that the companion separation distribution for the youngest protostars has two peaks — one around 100 au and another around 3,000 au. This suggests that multiple formation mechanisms are at play in these systems; stars that form at large (>500 au) separations via turbulent fragmentation can migrate inward over time, but comparisons with simulations suggest that there are more close-in protostellar companions than can be accounted for by this migration.

The team concluded that more than half of all companions within 500 au likely formed through disk fragmentation, while those at larger separations likely only formed due to turbulent fragmentation. Hopefully, future studies of this rich protostar data set will reveal even more insights about star formation!

Citation

“The VLA/ALMA Nascent Disk And Multiplicity (VANDAM) Survey of Orion Protostars. V. A Characterization of Protostellar Multiplicity,” John J. Tobin et al 2022 ApJ 925 39. doi:10.3847/1538-4357/ac36d2

illustration of a quasar

Supermassive black holes power the bright nuclei of young galaxies in the early universe — quasars. But how do these black holes gain supermassive status in less than a billion years?

Supermassive Centers

simulation of galaxies during the epoch of reionization in the early universe

Simulation of galaxies ionizing hydrogen gas (bright areas) during the epoch of reionization. [M. Alvarez (http://www.cita.utoronto.ca/~malvarez), R. Kaehler, and T. Abel/ESO; CC BY 4.0]

Quasars are truly superlative objects — they are the brightest objects we know of, and some astronomers claim that they’re also the most interesting! Quasars are so luminous that we can observe them at outrageous distances — the most distant quasar discovered shines from roughly 13 billion light-years away. This vast distance means that we see quasars as they were when the universe was less than a billion years old, on the edge of the epoch of reionization, when the first stars and galaxies suffused the universe with photons and put an end to the cosmic dark ages.

These ultra-bright objects are thought to be the nuclei of young galaxies, powered by the accretion of material onto a central supermassive black hole. The presence of supermassive black holes so early in the universe’s history poses a challenge for theorists. How, exactly, does a black hole amass so much material in just a few hundred million years?

plot of absolute magnitude versus redshift

Redshifts and absolute magnitudes of the quasars in this study compared to other studies. Higher redshifts correspond to larger distances and farther back in time. [Yang et al. 2021]

Sensitive Spectroscopy

The size of a supermassive black hole in the early universe is determined by the masses of the smaller black hole “seeds” from which it forms as well as the rate at which it accretes gas from its surroundings. In order to determine the masses and accretion rates of young supermassive black holes, a team led by Jinyi Yang (Steward Observatory, University of Arizona) analyzed infrared spectra of 37 quasars with redshifts between 6.3 and 7.64 — roughly 700 to 900 million years after the Big Bang.

Yang and collaborators calculated the masses of the black holes in their sample to be in the range of 300 million to 3.6 billion solar masses and found that they accrete at 0.26 to 2.3 times the Eddington limit — the theorized point at which the outward push of radiation generated by accretion is so strong that it balances the inward pull of gravity.

Gauging Growth

black hole redshift versus mass

Measured black hole masses (red squares) with black hole growth tracks for different seed masses and accretion rates. [Yang et al. 2021]

The authors estimate that the black holes in their sample must have arisen from black hole seeds no smaller than 1,000 to 10,000 solar masses — but it’s not clear what process could generate these seeds. The collapse of the first generation of stars is one possibility, but these massive stars likely only formed black holes of a few hundred solar masses. Another option is the collapse of gas clouds directly into black holes without first forming stars, but this process is thought to be extremely rare.

Even if the seeds are small, rapid accretion could still bulk these early-universe black holes up to the masses we observe. However, in order to reach billions of solar masses, early-universe black holes would need nearly a billion years of sustained accretion at a rate far exceeding the Eddington limit — anywhere from a few to a few thousand times this limit, depending on the seed mass — and most of the quasars in this study are accreting too slowly. The jury is still out on how supermassive black holes in the early universe gain their impressive size — perhaps in addition to being the most luminous and most interesting objects in the universe, quasars are also the most mysterious!

Citation

“Probing Early Supermassive Black Hole Growth and Quasar Evolution with Near-infrared Spectroscopy of 37 Reionization-era Quasars at 6.3 < z ≤ 7.64,” Jinyi Yang et al 2021 ApJ 923 262. doi:10.3847/1538-4357/ac2b32

photograph of a dusty arm of the milky way galaxy

Did you know that the Sun has a twin? Actually, it has a lot of them — solar twins are born all across the Milky Way. What can we learn about the chemical evolution of our galaxy from these stellar lookalikes?

Familiar Stars

Periodic table of elements showing the likely origin of each element

Periodic table showing the origin of each chemical element. Those produced by the r-process are shaded orange and attributed to supernovae in this image; though supernovae are one proposed source of r-process elements, an alternative source is the merger of two neutron stars. [Cmglee, CC BY-SA 3.0]

The solar neighborhood is populated with stars that have similar temperatures, masses, and radii to the Sun. Though solar twins may seem identical, they vary in a few key ways: they have different ages, they were born at varying distances from the galactic center, and they contain different amounts of heavy elements. Because solar twins span a wide range of ages, astronomers can study them to understand how chemical abundances in the Milky Way have changed over time.

In particular, solar twins provide a way to study elements that form mainly through the r-process: the rapid capture of multiple neutrons in a hot, dense environment. It’s not yet clear where these elements form — core-collapse supernovae were an early contender, but some astronomers now believe that neutron star mergers are the major source of these elements. These two processes differ greatly in their timescales: supernovae could generate r-process elements in just a few million years, while neutron star mergers would likely require billions of years. By tracking the abundances of r-process elements over cosmic time, astronomers can estimate how quickly these elements form — and what produces them.

three plots of elemental abundances of solar twins

Elemental abundances of solar twins in three age groups. Abundances are reported as the log of the ratio with respect to iron (e.g., if X/Fe = 10, [X/Fe] = 1.0). The green bars in the top panel show abundances measured in galactic bulge stars; the correlation suggests that the oldest solar twins originated in the bulge. [Tsujimoto 2021]

Heavy-Metal Patterns Revealed

In order to search for signs of r-process enrichment, Takuji Tsujimoto (National Astronomical Observatory of Japan) analyzed the elemental abundances of 79 solar twins with ages ranging from just 700 million years to 8.7 billion years. Tsujimoto measured the abundances of 28 elements — as light as carbon and as heavy as dysprosium — and found that solar twins of the same age showed the same chemical abundance patterns, but those patterns varied between different age groups.

Both the youngest and oldest solar twins showed signs of enhanced r-process elements compared to iron, while the middle-aged solar twins do not show this intriguing abundance pattern. What does this imply about the sources of r-process elements?

A Resurgent Source

To understand this pattern, Tsujimoto modeled the abundance of europium — an element produced almost exclusively through the r-process — as a function of time. Tsujimoto found that a two-population model was necessary to reproduce the abundance patterns seen in the data, meaning that there are likely two important sources of r-process elements — one that proceeds quickly, leading to enhanced abundances in the oldest twins, and one that proceeds slowly, leading to enhanced abundances in the youngest twins.

plot of europium abundance over time for models and the solar twins studied here

Modeled and observed (gray crosses) europium abundances. [Tsujimoto 2021]

Tsujimoto found that a fast process like supernovae must play an important role in producing r-process elements, though a solely neutron-star origin isn’t out of the question; recently, models have predicted that a subset of neutron star binaries could merge in less than a billion years. For now, the mystery of the origin of r-process elements remains unsolved, though future investigations of distant neutron star mergers may crack the case.

Citation

“Two Sites of r-process Production Assessed on the Basis of the Age-tagged Abundances of Solar Twins,” Takuji Tsujimoto 2021 ApJL 920 L32. doi:10.3847/2041-8213/ac2c75

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