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An artist’s rendering of a small satellite in orbit above the earth. A telescope barrel extends from an otherwise uninterrupted rectangular prism.

Astronomers have collectively cataloged more than 5,000 planets beyond our Sun, a feat borne from the immense efforts of thousands of scientists. However, not all of these discoveries required the same amount of perseverance: while some planets neatly fell out of abundant high-quality data, other worlds required intensive analysis and years of additional study to earn their places in our archives. A recently discovered planet named TOI-2010 b falls firmly into the latter of these categories.

TESS and the Struggles of Single-Transit Planets

The majority of exoplanets discovered so far were initially flagged by space-based telescopes looking for the repeating dips in starlight caused by circling, intermittently photobombing planets. One of the most productive of these telescopes is NASA’s Transiting Exoplanet Survey Satellite, or TESS, which stares are one chunk of the sky for about a month before moving on to observe a new patch. This is a productive strategy that has netted thousands of exoplanet candidates across the whole sky, but it also lets some planets fall through the cracks. Astronomers need to observe more than one transit of a planet to nail down its period, and many of the most interesting cold planets take longer than a month to complete a lap around their star. Often, TESS catches only one transit of these worlds before it proceeds to the next area of the sky, leaving astronomers in a maddening predicament: they know an interesting planet is there, but they have no idea when it will transit again.

A number of astronomers have taken up the challenge of chasing down these “single-transit” planets, and recently a team led by Christopher Mann (University of Montréal) achieved a significant victory: the discovery and characterization of a Jupiter-like planet with a 141-day period.

Introducing: TOI-2010 b

Two side-by-side time series of a high-quality planetary transit.

Left: The first recorded transit of TOI-2010 b as seen by TESS. Right: Another TESS transit, caught after NEOSat refined the orbital period and late into the team’s analysis. Click to enlarge. [Mann et al. 2023]

Back in 2019, TESS captured a single, beautifully clear transit of a Jupiter-sized planet around a star named TOI-2010. Although only one transit could not uniquely determine the planet’s period, reconnaissance spectra and high-contrast imaging revealed a path forward: TOI-2010 was a good candidate for radial velocity follow-up. So, the team embarked on a three-year campaign to measure the planet’s period. Unfortunately, the resulting constraints were only strong enough to predict that the next transit could take place anytime within a week-long window. No Earth-bound telescope could monitor the sky uninterrupted for that long.

Enter the Near-Earth Object Surveillance Satellite (NEOSat), a suitcase-sized telescope launched and operated by the Canadian Space Agency and the Department of National Defence/Defence Research and Development Canada. In December of 2021, this satellite aimed its 15-cm telescope at TOI-2010 and didn’t look away for six straight days. It caught another transit of TOI-2010 b right near the middle of the predicted window, and in doing so ended the years of initial characterization. Astronomers finally had all of the parameters they needed to follow up the planet sometime in the future.

A two-panel plot of radial velocity amplitudes. The top plot shows a time series, while the lower plot shows the phase-folded measurements. The best-fitting model is plotted alongside the data in both plots.

The radial velocity measurements of TOI-2010, which reveal the tell-tale pattern of a circling planet. Click to enlarge. [Mann et al. 2023]

That information will likely be put to good use soon. As a relatively cold (and therefore rarely found) planet that circles a bright (and therefore easy to follow-up) star, it’s amenable to several different measurements including Doppler spectroscopy and infrared spectroscopy to target the planet’s emission. All of these exciting possibilities are only possible thanks to the determination and patience of this collaboration who made sure that this planet didn’t slip away.

Citation

“Giant Outer Transiting Exoplanet Mass (GOT ‘EM) Survey. III. Recovery and Confirmation of a Temperate, Mildly Eccentric, Single-transit Jupiter Orbiting TOI-2010,” Christopher R. Mann et al 2023 AJ 166 239. doi:10.3847/1538-3881/ad00bc

illustration of a brown dwarf

Somewhere in between stars, which sustain hydrogen fusion in their cores, and giant gaseous planets, which do not, lies an intermediate class of cool, cloudy objects called brown dwarfs. A new look at a nearby brown dwarf shows how challenging it can be to classify these objects.

Neither Star nor Planet

photograph of a brown dwarf among a field of stars

The green object at the center of this image is the first ultra-cool brown dwarf discovered by NASA’s Wide-field Infrared Survey Explorer. [NASA/JPL-Caltech/UCLA]

Brown dwarfs are assigned spectral types M, L, T, and Y, ranging from objects that just barely miss the mass cutoff to be able to fuse hydrogen into helium to those that overlap in mass with the largest planets. The coolest brown dwarfs, those in class Y, might have cloudy atmospheres similar to those of giant planets but easier to study; tiny, faint, and cool, planets are hard to pick out against the bright light of their host stars.

Finding and classifying Y-type brown dwarfs has its own challenges, though. Part of the challenge comes from the fact that even objects assigned the same spectral type show significant differences. To understand whether these variations are due to metallicity, formation mechanism, or other factors, researchers must amass a sample of these hard-to-find objects.

Under Spectral Scrutiny

Enter CWISE J105512.11+544328.3, or W1055+5443 for short. Researchers initially assigned this object a spectral class of T8 — slightly more massive and warmer than a Y-type brown dwarf — but an updated distance measurement placed it closer to Earth, implying a lower luminosity and nudging its spectral type down to Y0. To confirm this spectral-type assignment and learn more about this nearby brown dwarf, Grady Robbins (University of Florida) and collaborators analyzed archival photometry from the Spitzer Space Telescope and collected new spectra using the Keck II telescope.

color and spectral type of brown dwarfs

W1055+5443 (blue star) compared to other brown dwarfs for which Spitzer data exist. [Robbins et al. 2023]

Oddities were immediately apparent, in both its photometry and its spectra: W1055+5443 appears unusually blue in Spitzer’s filters compared to other Y0 brown dwarfs, and its spectral features were difficult to fit with a single template. Previous research has shown that when brown dwarfs are difficult to classify from their spectra, it can mean that what seems to be a single brown dwarf is actually two — but modeling shows that W1055+5443 is most likely a single Y0 brown dwarf, though a peculiar one.

Unusual Properties

As brown dwarfs age, they cool and contract. This means that brown dwarfs that are young tend to have earlier spectral types (M and L) and lower surface gravity, and brown dwarfs that are old tend to have later spectral types (T and Y) and higher surface gravity. Based on its spectral type, W1055+5443 should be an old brown dwarf with high surface gravity, but modeling suggests that its surface gravity is low — is this object unexpectedly young?

near-infrared spectrum

Near-infrared spectrum of W1055+5443 from the Keck II telescope. [Robbins et al. 2023]

Robbins and collaborators found that W1055+5443 is likely a member of the Crius 197 moving group, a collection of ten stars moving together through space. Many of the stars in this group have ages around 180 million years, implying that W1055+5443 is similarly youthful, though some group members are much older. If the 180-million-year estimate proves correct, this suggests that W1055+5443 is a young brown dwarf with a mass just 4–6 times that of Jupiter, placing it well within the realm of planet masses!

The authors note that these estimates are preliminary and await further investigation. Spectroscopy from JWST would allow researchers to pin down whether the object belongs to the Crius 197 moving group, determine its metallicity, and investigate its unusual spectral features, and longer-wavelength photometry would provide an accurate estimate of its temperature.

Citation

“CWISE J105512.11+544328.3: A Nearby Y Dwarf Spectroscopically Confirmed with Keck/NIRES,” Grady Robbins et al 2023 ApJ 958 94. doi:10.3847/1538-4357/ad0043

neutron stars approaching a merger

Gravitational waves from colliding neutron stars have improved our understanding of the interiors of these fantastically compressed objects and helped us measure their radii. How much more precisely will we be able to measure neutron stars with future gravitational wave observatories?

Measuring a Neutron Star

Hubble Space Telescope image of a neutron star

This image from the Hubble Space Telescope shows a neutron star. It was estimated to be no more than 28 kilometers (16.8 miles) across and have a temperature of 1,200,000℉ (670,000℃). [Fred Walter (State University of New York at Stony Brook) and NASA/ESA; CC BY 4.0]

When stars more massive than about eight times the mass of the Sun explode as supernovae, they often leave behind a neutron star: the rapidly spinning, magnetized remnant of the star’s core. Neutron stars are immensely dense and strong, packing more than the mass of the Sun into a sphere the size of a city. Counterintuitively, the more massive the neutron star, the smaller it is. Exactly how a neutron star’s size varies with its mass is described by its equation of state: the relationship between mass, radius, and density.

Already, observations of gravitational waves from colliding neutron stars have helped us hone our estimates of the neutron star equation of state. A 1.4-solar-mass neutron star — around the lower limit of a neutron star’s mass — will have a radius between 10.5 and 13 kilometers. Researchers suspect that future gravitational wave observations will narrow this range further, and new work explores how precisely we’ll be able to measure neutron stars in the future.

Computing Collisions

To probe this question, Daniel Finstad (University of Washington; University of California, Berkeley; Lawrence Berkeley National Laboratory) and collaborators Laurel White and Duncan Brown (both Syracuse University) simulated the gravitational waves produced by many pairs of colliding neutron stars.

plot of neutron star mass versus radius

Soft, medium, and stiff equations of state (blue, orange, and green lines, respectively), as well as the full set of equations of state used in the analysis. [Finstad et al. 2023]

The team modeled three populations of colliding neutron stars with different equations of state, labeled “soft,” “medium,” and “stiff.” These equations of state cover the range of neutron star interiors currently allowed by observations. The stiffness of the equation of state affects both the neutron stars’ sizes and how much they’re deformed by tidal forces as they approach a collision. These changes leave an imprint on the gravitational waves produced in a collision, allowing us to extract the equation of state from gravitational wave observations. Modeling a range of equations of state is important because our ability to measure a neutron star’s equation of state depends on the equation of state itself; “soft” interiors produce signals that are fainter than “stiff” interiors do.

Upcoming Observations

With a suite of simulations in hand, Finstad’s team modeled what future gravitational wave observatories would detect if faced with these synthetic signals. They considered future upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors that would bring them up to their maximum sensitivity as well as the proposed Cosmic Explorer, which would have arms 10 times as long as LIGO’s and therefore be more sensitive.

plot showing how long it would take LIGO–Virgo to measure the neutron star equation of state to within 2%.

Number of years needed for LIGO–Virgo to observe enough mergers to measure the neutron star equation of state to a precision of 2%. Results are shown for stiff, medium, and soft equations of state (green, orange, and blue, respectively), as well as for different values for the neutron star merger rate, shown with the timescales at the top. [Finstad et al. 2023]

Finstad and collaborators found that an upgraded LIGO-Virgo would be able to measure the neutron star equation of state to within a precision of 1.9–0.7%, depending on the stiffness — but it would take 10, 20, or 57 years to observe enough mergers of stiff, medium, or soft neutron stars (respectively) to reach that precision. Cosmic Explorer, on the other hand, would require only a year to amass a similarly large collection of observations, measuring the equation of state to within a precision of 0.56% or better.

Citation

“Prospects for a Precise Equation of State Measurement from Advanced LIGO and Cosmic Explorer,” Daniel Finstad et al 2023 ApJ 955 45. doi:10.3847/1538-4357/acf12f

illustration of the TRAPPIST-1 planets

Editor’s Note: Welcome to the first post of the new Monthly Roundup series! Each month, we’ll be providing a broader view of one astronomical topic by exploring it through the lens of 3-5 recent research articles.

NASA travel poster depicting the TRAPPIST-1 system

The closely spaced rocky planets of the TRAPPIST-1 system inspired scientists and non-scientists alike. [NASA/JPL]

TRAPPIST-1, a cool and unassuming red dwarf star barely larger than Jupiter, became world famous when researchers discovered seven planets orbiting it in 2016–2017. The discovery and characterization of this septuple system was an international effort involving numerous ground- and space-based observatories. Notably, observations from the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) first revealed the innermost two planets and hinted at a third. Then, a three-week observing campaign with the Spitzer Space Telescope clarified that the proposed third planet was actually four or five more planets. At last, 79 days of Kepler Space Telescope data confirmed the presence of the outermost planet and brought the tally to seven planets. All of the planets, cataloged as TRAPPIST-1 b through h, are roughly the size of Earth, and as many as four of them are thought to lie within their star’s habitable zone.

The large number of Earth-like and potentially habitable planets, all orbiting their host star with periods of less than three weeks, and at “only” 41 light-years from Earth, make the TRAPPIST-1 system a prime target for atmospheric characterization. Now, JWST has turned its gigantic golden eye toward this promising system, giving researchers new data to scrutinize and new challenges to overcome.

Through (Atmospheres) Thick and Thin

First up, the innermost of the TRAPPIST-1 planets. Previous observations show that none of the planets have clear (i.e., cloudless) atmospheres made of hydrogen, but we’ve yet to rule out other atmospheres, including those rich in familiar gases like carbon dioxide (CO2), nitrogen, or oxygen.

Jegug Ih (University of Maryland) and collaborators attempted to place constraints on the presence of an atmosphere enveloping TRAPPIST-1 b by analyzing observations taken at 15 microns (1 micron = 10-6 meter) as the planet ducked behind the star. The team modeled these secondary eclipse observations (the primary eclipse happens when the planet passes in front of the star) to extract information about the planet’s atmosphere.

Plot of eclipse depth as a function of wavelength for various atmospheric models

Eclipse spectra of atmospheric models of TRAPPIST-1 b consistent with the JWST observations to within one sigma. Click to enlarge. [Adapted from Ih et al. 2023]

Ih’s team ruled out CO2-containing atmospheres yielding surface pressures higher than about a third of Earth’s, and thin, Mars-like atmospheres that are rich in CO2 also got the boot. If somehow — though the team finds it unlikely — the planet’s atmosphere contains no compounds that strongly absorb mid-infrared light, the planet could even host a thick atmosphere with ten times the surface pressure of Earth’s.

Considering instead the possibility that TRAPPIST-1 b has no atmosphere, Ih and collaborators used the JWST observations to constrain the composition of the planet’s surface. Bare rock is most likely, containing metals and volcanic rocks. Expect to hear more about this planet’s atmosphere and surface soon: additional secondary eclipse observations were performed at a slightly shorter wavelength of 12.8 microns in July 2023.

Stellar Interference

Plots showing transit observations for TRAPPIST-1 b and modeling of its atmosphere.

Two transit observations (panels a and b) with best-fit stellar contamination models (black lines). Atmospheric modeling of contamination-corrected data (panels c and d). Click to enlarge. [Lim et al. 2023]

The holy grail of atmospheric characterization is transmission spectroscopy: assessing the spectral fingerprints left behind as light from the host star passes through a planet’s atmosphere. Olivia Lim (University of Montreal) and collaborators presented the first JWST transmission spectra of TRAPPIST-1 b. The spectra show signs of stellar contamination: features on the star’s surface that complicate the analysis of a planet’s spectrum. In the first of two transits, Lim’s team saw signs of starspots, while the second transit was marred by bright regions called faculae.

To extract information about the planet’s atmosphere from these contaminated spectra, Lim and coauthors used model fitting to account for the starspots and faculae before modeling the planet’s atmospheric spectrum. Using this method, the team could rule out hydrogen-rich, cloudless atmospheres, but they couldn’t draw any conclusions about other atmospheric compositions. The authors noted how important this issue will be going forward, and they called for more observations and better modeling of the surfaces of cool stars, emphasizing one of the tenets of exoplanet science: Know thy star, know thy planet.

Reconsidering TRAPPIST-1 c

Moving farther out from the star, a team led by Andrew Lincowski (University of Washington) analyzed JWST secondary eclipse observations of TRAPPIST-1 c. Previous analysis of these observations suggested that the planet lacks an atmosphere or has a thin atmosphere mainly containing oxygen. Using atmospheric models, Lincowski and collaborators explored a wider range of atmospheric compositions than previous work. Their possible atmospheres included a steam atmosphere, created by water loss from the hot planet — its equilibrium temperature is nearly 400K (260℉ or 127℃) — and a Venus-like CO2 atmosphere that might arise after all the water is lost.

modeled spectra and observed eclipse depth for TRAPPIST-1 c

Modeled spectra (lines) and eclipse depths (circles) at 15 microns compared to the observed eclipse depth (black circle). Click to enlarge. [Lincowski et al. 2023]

Similar to previous work, Lincowski’s team found that TRAPPIST-1 c could host a thin, oxygen-rich atmosphere with a small amount of CO2, but other compositions were possible: thick, oxygen-rich atmospheres with a smidge of CO2; thin, oxygen-rich atmospheres with a hint of CO2 and water; and even steam atmospheres remain in the mix. As always, more data are needed!

As the Inner Planets, So Too the Outer Planets?

Although detailed modeling has found pockets of parameter space in which atmospheres on the two innermost planets of the TRAPPIST-1 system are possible, we still can’t rule out the possibility that these planets have no atmospheres. If TRAPPIST-1 b and c are bare rocks, does that mean the other five planets in the system lack atmospheres, too? Answering this question requires understanding what happened before TRAPPIST-1 settled onto the main sequence, when the star pummeled its planets with atmosphere-eroding high-energy radiation. Joshua Krissansen-Totton (University of Washington) modeled the evolution of the atmospheres and interiors of TRAPPIST-1 b and c to understand which properties, including the amount of high-energy radiation from their host star, result in the two planets being airless. These properties are then applied to other planets in the system to see if they could hang onto their atmospheres until today.

plots showing the probability of TRAPPIST-1 e and f being airless given TRAPPIST-1 b and c being airless

The probability of TRAPPIST-1 e (top) and f (bottom) having atmospheres is largely unaffected by the inner planets being airless. [Krissansen-Totton 2023]

Krissansen-Totton found that because the two innermost planets of the system are so close to the star, they lack atmospheres under a wide range of conditions. These same wide-ranging conditions increase the possibility that TRAPPIST-1 d lacks an atmosphere, but the e and f planets are barely affected. This means that we don’t need to ring the alarm bells if it turns out that the b and c planets don’t have atmospheres: planets farther out don’t necessarily have the same fate. Of course, there is another possibility: that all of the planets in the system started out poor in atmosphere-building atoms and molecules, and they all lack atmospheres today. We’ll have to turn JWST toward the remaining planets to find out.

Life May Find a Way — but Do We Have a Way to Find It?

A pressing, million-dollar question: If one or more planets in the TRAPPIST-1 system were to host life, would JWST even be able to detect it? Victoria Meadows (University of Washington) and collaborators explored that question, using photochemical models to determine whether JWST is sensitive enough to detect the spectral signatures of molecules that indicate the presence of life, called biosignatures.

Meadows’s team modeled spectra of TRAPPIST-1 d and e, both of which may lie within the star’s narrow habitable zone. They investigated two atmospheric compositions: one resembling modern-day Earth and one similar to Earth during the Archean Eon — an ancient period during which Earth’s atmosphere was rich in methane and poor in oxygen.

simulated transmission spectrum of an Archean-like atmosphere

Example of a simulated transmission spectrum and data. Click to enlarge. [Meadows et al. 2023]

By simulating the chemistry of these atmospheres, calculating their spectra, and considering JWST’s observing capabilities, the team found that an atmospheric composition similar to that of modern-day Earth will be challenging to detect, though an Archean-Earth-like atmosphere is more promising. One of the most promising biosignatures for either time period is the presence of both CO2 and methane in amounts that disagree with the predictions of photochemistry. Ultimately, Meadows’s team doesn’t fully rule out the possibility that JWST could detect life on TRAPPIST-1 d or e, though the detection is likely to be challenging: even if we find CO2 and methane in promisingly disequilibrium amounts, we’ll have to be diligent about showing that no other process could be responsible.

Citation

“Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm,” Jegug Ih et al 2023 ApJL 952 L4. doi:10.3847/2041-8213/ace03b

“Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra,” Olivia Lim et al 2023 ApJL 955 L22. doi:10.3847/2041-8213/acf7c4

“Potential Atmospheric Compositions of TRAPPIST-1 c Constrained by JWST/MIRI Observations at 15 μm,” Andrew P. Lincowski et al 2023 ApJL 955 L7. doi:10.3847/2041-8213/acee02

“Implications of Atmospheric Nondetections for Trappist-1 Inner Planets on Atmospheric Retention Prospects for Outer Planets,” Joshua Krissansen-Totton 2023 ApJL 951 L39. doi:10.3847/2041-8213/acdc26

“The Feasibility of Detecting Biosignatures in the TRAPPIST-1 Planetary System with JWST,” Victoria S. Meadows et al 2023 Planet. Sci. J. 4 192. doi:10.3847/PSJ/acf488

A photograph of a closed, spherical telescope dome as seen during the day. Mountains are visible in the background, and several cars are parked in front of a large loading bay.

In astronomy, seeing isn’t always believing, and extra checks are required to make sure we don’t fool ourselves into oversimplifying nature. With the help of citizen scientists, a team of astronomers recently illustrated the value of such thoroughness and demonstrated that three previously observed fuzzy blobs may actually be rare, close-in pairs of brown dwarfs.

Point Sources Confusion

Computer renderings of planets, brown dwarfs, and a small star side by side. All are very similarly sized, though their radii increase in the order given.

A comparison of the size of planets, brown dwarfs, and stars. Click to enlarge. [NASA/JPL-Caltech]

Images of the night sky are undeniably filled with wonders, and features like diffuse nebula and swirling galaxies never fail to impress. However, the most common components of any astronomical image are more mundane: any given image taken with a telescope is filled with small, fuzzy blobs. Astronomers and the public alike most commonly assume these blobs are individual stars, but it’s important to remember that we can’t usually tell by eye alone. Stars are so far away that they are effectively “point sources,” meaning we don’t actually resolve their shapes. They look round and fuzzy not because we actually see the outlines of stars, but instead because our telescope optics and Earth’s atmosphere place caps on the smallest object we can resolve.

This means that what looks like one small, fuzzy blob can actually be two stars packed tightly together, both so far away from Earth that their images are blurred together into one. In rare but exciting cases, these blobs don’t correspond to stars at all, and instead are actually images of one or more brown dwarfs. These objects have masses that fall between stars and planets, and a more complete understanding of their formation would enable better models of both their bigger and smaller cousins. Binaries, or pairs of these objects orbiting each other, are especially useful since long-term radial velocity studies can reveal the masses of each constituent. Astronomers know of only a handful of brown dwarf binaries, so each discovery of a new pair, or even evidence that a given blob might be a pair, is cause for excitement.

WISE Citizens

A photograph of a large silver cylinder sitting vertically within a clean room. It is surrounded by four technicians in protective clothing. They are roughly half the height of the telescope.

The WISE telescope prior to its launch in 2009 from Vandenberg Air Force Base, California. WISE was placed in hibernation in 2011 but reawakened in 2013 to hunt for near-earth objects. [NASA/JPL-Caltech]

For several years now, nearly 80,000 citizen scientists participating in the Backyard Worlds: Planet 9 initiative have been poring over archival images from NASA’s Wide-field Infrared Survey Explorer (WISE) mission. This telescope, which launched in 2009, hibernated in 2011, then reanimated as a near-Earth object hunter in 2013, produced reams of infrared images that could contain brown dwarfs that were missed by the automated processing routines. After volunteers flagged three potential new brown dwarfs, a team of astronomers led by Alexia Bravo (United States Naval Observatory) decided to investigate further.

Bravo and collaborators used the Southern Astrophysical Research (SOAR) Telescope in Chile to collect the spectrum of each target, then compared these to spectra of previously confirmed brown dwarfs. In a happy surprise, none of the “templates” fit their new data very well. These objects were strange: at some wavelengths, they looked like a certain type of large brown dwarf, while at others, they looked like a much smaller and cooler variant.

Two wavelength vs. flux plots, where the data and model agree much better in the bottom panel.

Data and best-fitting models for one of the three candidates. The top panel shows the data (black) and best-fitting model (red) assuming the spectrum is composed of only one brown dwarf. The bottom panel shows how the same data are better matched by a model that allows for a pair of objects. Click to enlarge. [Adapted from Bravo et al. 2023]

The team realized that each of their spectra could be better fit by assuming that they weren’t looking at just one brown dwarf, but a close-together pair. Each of these candidates will require confirmation, and one of them in particular may turn out to be just a highly variable but single brown dwarf. Still, each of these objects could reveal rich information about brown dwarfs with future observations. These detections are also manifestations of the power of questioning the obvious and demonstrate the benefits of enlisting an engaged public to better know our galaxy.

Citation

“An Investigation of New Brown Dwarf Spectral Binary Candidates From the Backyard Worlds: Planet 9 Citizen Science Initiative,” Alexia Bravo et al 2023 AJ 166 226. doi:10.3847/1538-3881/acffc1

spiral galaxy with a tidal tail

Leo I is a distant dwarf galaxy that appears to host a supermassive black hole 100 times more massive than expected for a galaxy of its size. Could tidal stripping explain this galaxy’s black hole–stellar mass mismatch?

An Anomalously Large Black Hole

dwarf galaxy Leo I

The dwarf galaxy Leo I. [Sloan Digital Sky Survey; CC BY 4.0]

The dwarf galaxy Leo I is the most distant satellite of the Milky Way. At 830,000 light-years away, Leo I is so distant that its status as a satellite of our galaxy has been debated, with early studies suggesting it was bound to our neighboring galaxy, Andromeda, instead. Now, Leo I is thought to trace a highly elliptical orbit around the Milky Way that brings it to within about 150,000 light-years of our galaxy’s center.

Leo I is remarkable not just because it’s the Milky Way’s most distant satellite, but also because this 80-million-solar-mass galaxy appears to harbor a 3-million-solar-mass black hole — a black hole nearly as massive as the Milky Way’s central supermassive black hole but in a galaxy more than 10,000 times less massive. To explain this discrepancy, researchers have proposed that Leo I was once a much larger galaxy that has had its stars stolen by passing too close to the Milky Way.

Simulating Stellar Stripping

Fabio Pacucci (Center for Astrophysics | Harvard & Smithsonian; Black Hole Initiative) and collaborators used analytical and dynamical models to explore how Leo I’s past journeys close to the Milky Way might have robbed the smaller galaxy of its stars. Using a simple analytical model, the team estimated that Leo I could have lost 32–57% of its stars after one close passage, though losses up to 78% are possible based on precise observations by the star-mapping Gaia spacecraft.

plot showing the simulated stellar mass as a function of time

Stellar mass (top panel) and distance between Leo I and the Milky Way (bottom panel) over time. [Pacucci et al. 2023]

More detailed N-body simulations backed up this result. The team found that a billion-solar-mass galaxy (an estimate of Leo I’s original mass based on the size of its supermassive black hole) shrinks to roughly Leo I’s current mass over the course of 8 billion years and two close encounters with the Milky Way. Intriguingly, this scenario predicts the formation of two tidal tails — expansive stellar streams created as a galaxy loses its stars — but suggests that these streams would fall along our line of sight and thus be challenging to detect.

Possible, but Probable?

Pacucci’s team notes that this scenario, though possible, may be unlikely for two reasons. First, it requires Leo I to pass very close to the Milky Way on two occasions, at the limit of what is permitted by Gaia observations. Second, Leo I’s metal content — the abundance of elements heavier than helium — matches what we’d expect for a galaxy of its size; a galaxy that used to be more massive should be more metal rich.

plot of model results showing the current location of Leo I's stars

Modeled locations of Leo I’s stars today (green circles) and black hole over the past 8 billion years (purple line). You can see an animation of this figure here. [Adapted from Pacucci et al. 2023]

While these issues may confound the proposed tidal-stripping scenario, they aren’t deal-breakers: though unlikely, multiple close passages aren’t prohibited, and closer analysis of Leo I’s star formation history might explain its meager metal content.

In-depth statistical modeling delving into the likelihood of this scenario awaits future work, and observations may yet dredge up evidence of Leo I’s stellar streams. If future work supports the team’s hypothesis, another possibility waits in the wings: that Leo I will someday relinquish all of its stars to the Milky Way and become a wandering black hole.

Citation

“Extreme Tidal Stripping May Explain the Overmassive Black Hole in Leo I: A Proof of Concept,” Fabio Pacucci et al 2023 ApJL 956 L37. doi:10.3847/2041-8213/acff5e

supernova remnant Puppis A

Across the universe, charged particles are accelerated to nearly the speed of light, attaining energies of quadrillions of electronvolts or more. Within the Milky Way, supernovae have long been considered a likely source of these mind-bogglingly fast particles, called cosmic rays, but new research suggests that supernovae can only accelerate particles up to such high energies for a brief time.

Searching for Galactic PeVatrons

Sources of cosmic rays in the petaelectronvolt (PeV) range — that’s one quadrillion electronvolts, or the energy of a proton traveling at more than 99.9999999999% of the speed of light — are called PeVatrons. The hunt for PeVatrons in the Milky Way has closed in on supernovae, whose expanding shocks provide a way to accelerate particles up to high velocities. But whether supernova shocks can actually accelerate particles up to PeV energies is up for debate. Answering this question is key to understanding where particle acceleration happens in our universe.

Shocks, in expanding supernova remnants or elsewhere, are thought to accelerate particles through a process called diffusive shock acceleration. In this process, charged particles bounce back and forth across a shock as it moves, reflected by fluctuations in the magnetic field. The particles gain energy with each bounce until they escape the shock and zoom away. In the case of supernovae, it’s not yet clear if the magnetic field strength is high enough to hold on to particles until they reach PeV energies.

Setting a Particle Speed Limit

Spectra of accelerated protons under four different conditions

Spectra of protons accelerated by a shock moving into uniform material (top row) and material with a wind-like distribution (bottom row). Results are shown for the cases of diffusing (left column) and escaping (right column) particles driving the increase in the magnetic field. Click to enlarge. [Diesing 2023]

In a recent research article, Rebecca Diesing (University of Chicago) explored exactly how fast a particle accelerated by a supernova shock can travel. Diesing traced the acceleration of a proton by shocks of varying size and velocity interacting with material of varying density. (About 90% of cosmic rays are protons, with electrons and the nuclei of helium atoms making up most of the remainder.)

Diesing’s modeling accounted for the complex interactions between the shock, the particles being accelerated, and the magnetic field. As particles are accelerated, they play a role in shaping the shock itself: the motion of charged particles as they diffuse back and forth across the shock and finally escape it changes the magnetic field, producing fluctuations that boost the magnetic field strength. The increase in the magnetic field strength should allow the shock to hold on to particles longer, accelerating them to higher energies — but is it enough to get particles up to the PeV range?

Predicted maximum energy of accelerated cosmic rays as a function of the supernova remnant's age

Predicted maximum energy of accelerated cosmic rays as a function of the supernova remnant’s age. 1 PeV = 106 GeV. Click to enlarge. [Diesing 2023]

Unlikely Accelerators

Diesing’s calculations show that supernova remnants can only produce particles at PeV energies when the shock velocity is higher than 10,000 km/s or when the material into which the shock expands is dense. These high velocities are attainable by young supernova remnants, but not by the supernova remnants we know of in our galaxy, all of which are centuries old.

This suggests that a small number of supernovae can act as PeVatrons for a short time. To complicate the search for PeVatrons in our galaxy, Diesing found that even supernovae that are no longer producing PeV-energy cosmic rays can still generate high-energy gamma rays, which are often taken as evidence of a PeVatron.

Citation

“The Maximum Energy of Shock-Accelerated Cosmic Rays,” Rebecca Diesing 2023 ApJ 958 3. doi:10.3847/1538-4357/ad00b1

Illustration of a tidal disruption event

When a star wanders too close to a supermassive black hole, the powerful tidal forces can rip the star apart. Tidal disruption events can produce powerful jets, but only sometimes — what determines whether a shredded star produces a jet?

A Star Disrupted

illustration of a tidal disruption event

Illustration of shredded stellar material from a tidal disruption event. [Still image from an animation by NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)]

Tidal disruption events are rare, with only about a hundred events discovered so far, but those that produce powerful jets are rarer still. So far, researchers have only discovered four tidal disruption events accompanied by jets. Jets are identified by their bright X-ray and radio emission, which indicate outflowing material colliding with dense gas surrounding a supermassive black hole. Though jets accompanying tidal disruption events are rare, a growing number of tidal disruption events show signs of jet-like radio emission months or even years after discovery.

Now, researchers have proposed a unified theory that may explain the variety of jet behaviors from tidal disruption events.

Aligning with a New Theory of Jet Production

In a recent research article, Odelia Teboul (The Hebrew University) and Brian Metzger (Columbia University and Flatiron Institute) proposed a new theory for why tidal disruption events are only rarely accompanied by jets. Teboul and Metzger suggested that only stars whose orbits are aligned with the black hole’s spin will produce jets when they are disrupted. Assuming that the stars in the dense neighborhood surrounding the supermassive black hole have orbits randomly oriented relative to the black hole’s spin, this means that only a tiny fraction of tidal disruption events should produce jets at the time of the event — just as we observe.

Diagram illustrating a misaligned tidal disruption event producing a jet

Diagram illustrating a misaligned tidal disruption event producing a jet. [Teboul and Metzger 2023]

What about the tidal disruption events that exhibit jet-like emission months or years later? While only a small fraction of stars start out with their orbits aligned with the black hole’s spin, precession of the accretion disk that forms as the star is ripped apart can bring tilted stellar orbits into alignment with the black hole over time. How long this alignment process takes depends on the mechanism behind it: electromagnetic alignment takes weeks while hydrodynamic alignment takes months or years.

Escape of the Jet Emission

plot of radio flux over time from a tidal disruption event with model fits to the data

Model fit to observations of the tidal disruption event Swift J1644+57. [Teboul and Metzger 2023]

Based on their simulations, Teboul and Metzger find that there are three broad classes of tidal disruption events with different observational signatures. In the first class, the jet escapes before alignment, producing prompt radio and X-ray emission. The X-ray emission varies as the system comes into alignment and dims after alignment. In the second class, the jet escapes as the system achieves alignment, again producing bright emission that grows fainter over time. In the third class, the jet’s escape is delayed, producing faint X-ray and radio emission months later.

Teboul and Metzger found that faint, delayed jets should be much more common than bright, prompt jets, which is exactly what observations show. So perhaps jetted tidal disruption events aren’t as rare as previously thought — the majority of them are just late to the party.

Citation

“A Unified Theory of Jetted Tidal Disruption Events: From Promptly Escaping Relativistic to Delayed Transrelativistic Jets,” Odelia Teboul and Brian D. Metzger 2023 ApJL 957 L9. doi:10.3847/2041-8213/ad0037

photograph of a white dwarf

Studying the highest-mass white dwarfs is key to drawing the line between stars that explode as supernovae and stars that live out their days as white dwarfs. It’s easiest to determine the ages and masses of white dwarfs in star clusters, but many clusters seem to be missing their white dwarfs — can researchers track down a massive white dwarf that once belonged to a nearby cluster?

Kicked Out of a Cluster

Many stars are born in star clusters, but they may not remain there; just as children leave home, so too do stars depart their natal clusters — though children aren’t usually ejected via gravitational interactions. Researchers suspect that gravitational kicks may explain why many star clusters are missing their white dwarfs, the exposed cores of low- to intermediate-mass stars.

stellar evolution schematic

The main stellar evolution pathways. It’s not yet clear exactly which stars explode as supernovae and which become white dwarfs. Click to enlarge. [ESA]

Since it’s far easier to determine the age of a star that belongs to a star cluster than it is for a single star, tracking down white dwarfs that live (or once lived) in well-studied clusters can help us determine their ages and, by extension, the masses of the stars they came from. This helps us understand which stars will shed their atmospheres and spend the rest of their days as white dwarfs and which will end their lives in supernova explosions.

A Search for High-Mass White Dwarfs

To probe the line between stars that become supernovae and stars that become white dwarfs, David Miller (University of British Columbia) and collaborators conducted a search for high-mass white dwarfs associated with the nearby Hyades star cluster. Using precise position and velocity data from the Gaia spacecraft, the team identified stars moving in the same direction as the cluster and traveling on a path that places the star within the cluster during the cluster’s lifetime.

Color–magnitude diagram and white dwarf escapee candidates

Color–magnitude diagram showing the full sample of white-dwarf candidates with the three leading candidates labeled. Click to enlarge. [Miller et al. 2023]

Of these stars, 145 are likely white dwarfs, but not all white dwarfs are of interest in this study — only the most massive white dwarfs provide useful data for defining the line between supernova progenitors and white-dwarf progenitors. The three most massive white dwarfs in the sample weighed in at 1.1, 1.1, and >1.3 solar masses. After consulting a catalog of white dwarfs, Miller’s team ruled that the two lower-mass white dwarfs are likely interlopers, unrelated to the star cluster, while the most massive of the trio was almost certainly a cluster member.

Placing Constraints

plot of observed photometry and synthetic spectrum

Best-fit synthetic spectrum (black line) with synthetic photometry (red circles) and data (blue error bars). Click to enlarge. [Adapted from Miller et al. 2023]

The team took spectra of the single remaining runaway white dwarf candidate and used models to determine its mass, radius, and how long it has been cooling. With a mass of 1.317 solar masses, it’s one of the most massive known white dwarfs, especially among those thought to result from the evolution of a single star. (Other massive white dwarfs may be the product of two stars merging.)

Model fitting indicates that the white dwarf has been cooling for 556 million years. With the age of the Hyades cluster still uncertain, though, Miller’s team could only loosely constrain the progenitor star’s mass to >7.5 solar masses and its lifetime to <40 million years. The existence of this massive white dwarf can instead be used to constrain the cluster’s age, limiting it to <606 million years.

Miller and coauthors note that there’s nothing exceptional about the Hyades cluster except its proximity; this suggests that similarly massive white dwarfs may be more common than expected, waiting to be discovered.

Citation

“An Extremely Massive White Dwarf Escaped from the Hyades Star Cluster,” David R. Miller et al 2023 ApJL 956 L41. doi:10.3847/2041-8213/acffc4

Artist's impression of an active galactic nucleus surrounded by a dusty accretion disk

JWST has uncovered a population of point-like red objects in the early universe that may be active black holes shrouded in dust. Recent follow-up observations of one of these objects suggest that it’s an active supermassive black hole that’s disproportionately large compared to the galaxy it inhabits.

Supermassive Black Holes in the Making

illustration of a gas cloud collapsing to form a black hole

The collapse of a primordial gas cloud into a black hole is a possible way to make supermassive black hole seeds in the 104–105-solar-mass range. [NASA/CXC/M. Weiss; CC BY 4.0]

Most galaxies have a central supermassive black hole, and even galaxies in the first billion years of our universe’s history appear to host black holes millions of times as massive as the Sun. How these black holes get so massive in such a small amount of time is an open question; could the small black holes left behind by dying stars accrete enough gas to go from stellar-mass to supermassive in a few hundred million years, or must the seeds of black holes be far larger?

Both scenarios have challenges: it’s not clear how stellar-mass black holes could sustain the accretion rate necessary to transform into supermassive black holes, and we don’t yet have direct evidence for the formation of massive black hole seeds. To learn more about how supermassive black holes come to be, researchers need to amass observations of these objects in the early universe.

Red Source Revealed

Vasily Kokorev (Kapteyn Astronomical Institute) and collaborators followed up on observations of a reddish point source made through the JWST Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) program. Spectra of the source, named UNCOVER ID 20466, reveal it to be at a redshift of z = 8.5, placing it about 600 million years after the Big Bang.

spectrum of the red point source investigated in this work

Two-dimensional spectrum (top) and one-dimensional extracted spectrum (bottom) of the source investigated in this study. Click to enlarge. [Adapted from Kokorev et al. 2023]

The source’s reddish color and point-like nature suggest that it’s a galaxy hosting an active galactic nucleus — a supermassive black hole accreting gas from its surroundings — that is cocooned in dusty gas. This assessment was backed up by its spectrum, which showed a mixture of broad and narrow emission lines. The broad emission lines come from high-density, high-velocity gas near the central black hole, while the narrow lines come from less dense gas farther from the nucleus. This is the most distant active galactic nucleus for which researchers have clearly detected broad emission lines.

How Large Is the Seed?

Plot of observed black hole masses and modeled black hole masses over cosmic time.

Observed black hole masses and ages (symbols) and the black hole masses achievable as a function of time for accretion at the Eddington rate onto a 100-solar-mass seed (red line) and a 104-solar-mass seed (gray line). Click to enlarge. [Kokorev et al. 2023]

The black hole at the center of this distant galaxy has a mass of more than 100 million solar masses, which amounts to at least 30% of the galaxy’s mass being contained in the black hole. This is a much larger proportion than we see for supermassive black holes in the local universe or for typical black holes in the early universe. How did this black hole get so large so quickly, and why is it so large compared to its home galaxy?

Kokorev’s team favors a scenario in which the black hole was seeded by the collapse of a primordial gas cloud into a 104-solar-mass black hole. In this scenario, the black hole can reach its observed mass in the allotted time without surpassing the Eddington limit, the theoretical maximum accretion rate. This also tends to create black holes that are large compared to their host galaxy.

It’s also possible for the black hole’s seed to be smaller, 100 solar masses or so, if it can accrete at super-Eddington rates for hundreds of millions of years — but exactly how that might happen is unclear, especially without the galaxy’s stellar mass growing in tandem. As JWST reveals more of the high-redshift universe, we’ll gain a better understanding of how black hole seeds are sown and sprouted.

Citation

“UNCOVER: A NIRSpec Identification of a Broad-Line AGN at z = 8.50,” Vasily Kokorev et al 2023 ApJL 957 L7. doi:10.3847/2041-8213/ad037a

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