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Sculptor Dwarf Galaxy

Though the majority of the mass in the universe lies in dark matter, many mysteries remain about its nature. A new study suggests that the smallest and faintest galaxies hold the key to unlocking how dark matter interacts within the universe.

Core-Cusp Conundrum

Surrounding the Milky Way are tiny faint galaxies — known as satellite galaxies — millions of times smaller than the massive galaxy they orbit. Despite their small sizes, these satellites are of huge importance in understanding the behavior of dark matter. In the leading cosmological model, called lambda cold dark matter (ΛCDM), dark matter is cold, collisionless, and interacts with normal matter only by way of gravity.

Core vs cusp diagram

From simulations, the expected shape (cusp) of a galaxy’s dark matter density profile is shown with the red curve, while the observed shape (core) is shown with the blue curve. As we approach the galactic center, a cuspy profile peaks at high density, and the cored profile levels off at a lower density. Click to enlarge. [Lexi Gault]

Galaxy formation and evolution simulations using the ΛCDM model predict galaxies to form within dark matter halos that have density distributions that peak in the center, or reach a cusp. While this seems to be true on large scales like galaxy clusters, individual galaxies often appear to have smoother dark matter distributions. When enough normal matter enters the picture, feedback from newborn stars, tidal interactions, and other processes can flatten out the dark matter distribution, transforming the central cusp into a central core.

However, according to ΛCDM, there should exist very low-mass galaxies — like the faintest satellite galaxies that orbit the Milky Way — that do not have enough mass to change an originally cuspy dark matter distribution into a cored one. If observations reveal that even these small galaxies have cored dark matter distributions, that may mean that dark matter may not behave as expected in the ΛCDM paradigm.

Dark Matter Distributions

In order to investigate dark matter distributions at the smallest scales, Jorge Sánchez Almeida (Institute of Astrophysics of the Canary Islands; University of La Laguna) and collaborators analyze the stellar count distribution of six ultra-faint dwarf satellites of the Milky Way and the Large Magellanic Cloud. These galaxies have cored stellar surface density distributions, suggesting that their underlying dark matter distributions may be cored as well.

Histograms showing slope distributions

Histograms showing how each considered dark matter profile compares to the observations, where NFW corresponds to a cuspy distribution, Plummer corresponds to a cored distribution, and the ρ230 is in the middle of the two. The the histograms plot the distributions of the innermost slopes for the fits where a slope close to 0 means the profile flattens, and a more negative slope means a steeper rise in the center. The cored (Plummer) fit aligns best with the observed galaxies. Click to enlarge. [Modified from Sánchez Almeida et al 2024]

Through estimating the dark matter distribution function — a function that describes how dark matter is spread out in a given galaxy — for the given sample galaxies’ stellar surface densities, the authors find that a cored dark matter distribution agrees with the observations. The expected cusp profile fit is significantly different from the observed properties of these small galaxies. 

Beyond Standard

Given that a cored dark matter distribution seems to fit best for these ultra-faint galaxies, the authors suggest that dark matter may not behave as the standard cold dark matter model anticipates. Instead, dark matter may be warm, self-interacting, or fuzzy. However, could other factors be driving the cored distributions of these galaxies?

The author’s analysis hinges on several assumptions that could blur their results. To address these concerns, they carefully consider properties like stellar feedback contributions, tidal interactions, velocity distributions, and other properties important to their analysis. When changing these assumptions slightly, in ways still physically reasonable for very low-mass galaxies, the ultra-faint dwarfs still appear to reside in cored dark matter distributions.

What does this mean? While the standard ΛCDM model explains large-scale structures of the universe well, it does not adequately characterize what is observed on smaller scales — like the Milky Way’s satellites. Though the exact nature of dark matter is still unknown, cold and collisionless dark matter does not likely produce the ultra-faint galaxies’ cored profiles. Thus, further studying the lowest-mass galaxies in the universe may reveal new characteristics of dark matter beyond the standard model.

Citation

“The Stellar Distribution in Ultrafaint Dwarf Galaxies Suggests Deviations from the Collisionless Cold Dark Matter Paradigm,” Jorge Sánchez Almeida et al 2024 ApJL 973 L15. doi:10.3847/2041-8213/ad66bc

simulation of a tidal disruption event

There are many ways for a star to meet its end: suddenly and spectacularly, as a supernova; gradually and gracefully, as a white dwarf wreathed in a planetary nebula; and gruesomely, torn apart by a black hole. Tidal disruption events, in which a star gets too close to a black hole and is ripped apart by tidal forces, play out over the course of months or years in galaxies across the universe.

First predicted by theorists in the 1970s, the first tidal disruption event candidate was discovered in X-ray data collected in 1990–1991 by the ROSAT All Sky Survey. While early candidates were identified by their bright, transient X-ray emission, the majority of candidates today are found by optical surveys. As the number of known tidal disruption events grows, so does the diversity of these events, creating new sub-classes like partial tidal disruption events, in which just a fraction of a star’s atmosphere is torn away, or repeating tidal disruption events, in which a star is progressively peeled apart each time it draws near a black hole.

Researchers have analyzed observations of dozens of tidal disruption events, and the number of known events is growing rapidly thanks to wide-field searches for astrophysical transients. The upcoming Rubin Observatory’s Legacy Survey of Space and Time alone is predicted to discover 1,000 tidal disruption events per year. Today, we’ll explore three research articles that probe various aspects of tidal disruption events, seeking to understand these untimely ends of unsuspecting stars.

Signature of a Tilted Disk

When a supermassive black hole rips a star apart, some of the star’s gas is cast off into the galaxy, and some of it collects in an accretion disk around the black hole. This searingly hot disk is the source of the ultraviolet and X-ray emission we see from afar, and variations in the disk’s emission can tell us about the particular characteristics of a tidal disruption event.

plot of X-ray counts per second and ultraviolet flux density for AT2020ocn

Demonstration of the event’s different behavior at X-ray (top) and ultraviolet (bottom) wavelengths. Click to enlarge. [Adapted from Cao et al. 2024]

Zheng Cao (SRON Netherlands Institute for Space Research; Radboud University) and collaborators used models of a tilted accretion disk to interpret observations of a highly variable tidal disruption event candidate discovered in 2020. In the first hundred days after its discovery, AT 2020ocn churned out multiple X-ray flares before settling down into calmer behavior. Some observations showed semi-regular X-ray brightness changes with a cadence of about 17 days, which the team suspected were due to precession of the accretion disk. Curiously, AT 2020ocn showed no sign of flares in the ultraviolet, instead fading gradually from its initial brightness at those wavelengths.

To learn more, Cao’s team analyzed and modeled ultraviolet and X-ray observations of AT 2020ocn from Swift Observatory, XMM-Newton, and the Neutron star Interior Composition Explorer (NICER). The team found that AT 2020ocn’s behavior can be explained by a star that was initially traveling at an angle relative to the direction of the black hole’s spin, producing an accretion disk tilted in the same direction as the star’s motion. Over time, the inner part of the disk becomes aligned with the black hole’s spin, making the disk precess over time and changing its orientation from our point of view. The timeframe of these changes match what’s predicted as well: flares lasting 1–10 days, with the flaring behavior persisting for less than 200 days overall.

Cao and coauthors noted that their modeling has limitations. For example, the model handles a change in the disk’s tilt by changing the observer’s position rather than manipulating the actual tilt of the disk, which may not fully capture the dynamic behavior of a tilted, precessing disk. Similarly, it’s possible that the inner disk is also warped, creating a complicated scenario in which the disk periodically partially obscures itself.

In addition to enhancing our understanding of AT 2020ocn’s behavior, the team’s findings may apply to other tidal disruption events with flares that turn off after a few months and ultraviolet emission that doesn’t flare at all.

Modeling a Missing Structure

While the bright ultraviolet and X-ray emission of a tidal disruption event is readily explained by a super-heated accretion disk circling a black hole, the origins of these events’ optical emission is less understood. The optical emission from a tidal disruption event appears to arise from surprisingly cool, slow-moving gas tens to hundreds of astronomical units from the black hole. Where is this emission coming from?

plots showing the evolution of the gas column density during a tidal disruption event

The evolution of the gas column density over the course of a year during a tidal disruption event. You can see an animation of this figure here. [Price et al. 2024]

To answer this question, a team led by Daniel Price (Monash University) modeled the destruction of a solar-mass star by a million-solar-mass black hole, tracing the changes in the debris and its emission properties over a full year of simulation time. Using general relativistic smoothed particle hydrodynamics modeling, the team observed the star being stretched into a stellar spaghetti noodle, half of which collects around the black hole and half of which is cast away. As the narrow stream of gas circles the black hole, it collides with itself, causing some of the material to fall toward the black hole and some of it to form an outflow.

Over time, the outflow takes the shape of a thin expanding bubble. This structure closely resembles a hypothesized featured called an Eddington envelope, which is thought to capture high-energy emission from the disk and accretion stream, reprocessing it to generate the optical emission researchers observe. This is the first time such a structure has formed self-consistently in simulations, representing a breakthrough in the understanding of tidal disruption event emissions.

A Chemical Conundrum

In addition to illuminating the physics of accretion and outflows in general, tidal disruption events can also clue us in to the properties of individual stars. Tidal disruption events often show prominent spectral lines, allowing researchers to study the chemical compositions of individual stars in the cores of distant galaxies — a population that’s otherwise difficult to examine closely.

The chemical analyses of certain tidal disruption events have revealed surprisingly large ratios of nitrogen to carbon (N/C ratio) in the disrupted stars. As Brenna Mockler (The Observatories of the Carnegie Institution for Science) and collaborators explain, a high N/C ratio seems to suggest that the disrupted star was a relatively massive one (>2 solar masses), bulky enough for the CNO cycle to take place in its interior. The CNO cycle produces energy by fusing hydrogen into helium and shifts the proportions of carbon and nitrogen, gradually increasing the amount of nitrogen relative to carbon. However, recent research suggests that the N/C ratios of certain events are even larger than what’s expected for the disruption of a massive star.

calculated N/C ratios for tidally disrupted stars

Comparison of the N/C ratios relative to the solar value for stripped stars (red lines; the different line styles represent stars at different stages of their helium-burning lifetimes) and normal stars (blue lines; the different line styles represent results for different metallicities). Click to enlarge. [Mockler et al. 2024]

To explore possible reasons, Mockler and collaborators simulated the disruption of a star that has lost its outer envelope — a stripped star. These stars are thought to be created in interacting binary systems. Unlike typical main-sequence stars, stripped stars have outer layers that are poor in hydrogen and rich in helium and nitrogen, and thus have higher N/C ratios. Mockler’s team compared the results of the simulated tidal disruptions of two main-sequence stars with masses of 2 solar masses (one with the same abundance of metals as the Sun and one with three times the metal abundance) and a stripped star that was initially 3 solar masses but has dwindled to just 0.58 solar mass.

The team found that the 2-solar-mass main-sequence stars, regardless of metallicity, could not attain the extreme N/C ratios seen in some events. For example, the tidal disruption event ASASSN-14li appears to have an N/C ratio greater than 300 times the Sun’s ratio. This is similar to the predicted N/C ratio for stripped stars but 10 or more times larger than predictions for a 2-solar-mass main-sequence star. The N/C ratios of tidally disrupted stripped stars appear to decrease over time, while the observed N/C ratios of normal main-sequence stars were found to increase over time, providing another avenue to determine the identity of a star after its demise.

Citation

“Tidal Disruption Event AT2020ocn: Early Time X-Ray Flares Caused by a Possible Disk Alignment Process,” Z. Cao et al 2024 ApJ 970 89. doi:10.3847/1538-4357/ad496f

“Eddington Envelopes: The Fate of Stars on Parabolic Orbits Tidally Disrupted by Supermassive Black Holes,” Daniel J. Price et al 2024 ApJL 971 L46. doi:10.3847/2041-8213/ad6862

“Tidal Disruption Events from Stripped Stars,” Brenna Mockler et al 2024 ApJL 973 L9. doi:10.3847/2041-8213/ad6c34

simulation snapshot showing neutron stars about to merge

The collapse of an extremely dense, highly magnetized stellar remnant into a black hole is an extreme event. New simulations explore what happens when a magnetar collapses into a black hole, providing a potential connection between these events and gamma-ray bursts.

Becoming a Black Hole

Crab Nebula

A multi-wavelength view of the Crab Nebula, the remnant of a supernova that birthed a neutron star. The neutron star powers a pulsar wind nebula, shown in blue. [X-Ray: NASA/CXC/J.Hester (ASU); Optical: NASA/ESA/J.Hester & A.Loll (ASU); Infrared: NASA/JPL-Caltech/R.Gehrz (Univ. Minn.)]

When massive stars die, they collapse in upon themselves and can leave behind their condensed cores in the form of neutron stars. For many stars, the journey ends here: over time, neutron stars slowly cool, spin down, and fade. Others have a different final destination: if one neutron star collides with another, the remnant of the merger can collapse further, becoming a black hole.

This process can be nearly instantaneous, the merger remnant shrinking down to a black hole in just milliseconds. Sometimes, though, the merger remnant survives for several hours, coalescing into a rapidly spinning, highly magnetized neutron star called a magnetar. As the magnetar sheds energy through its outflowing magnetized wind, it’s no longer able to support itself, and it collapses into a black hole. What a distant observer might see when this happens is not yet clear.

Waves, Shocks, and Rays

To understand what happens during a magnetar’s collapse, Elias Most (California Institute of Technology) and collaborators turned to complex general relativistic magnetohydrodynamic simulations. Magnetar collapse been modeled before, but this study improves upon previous work by tackling a full magnetohydrodynamic approach to the problem, allowing the team to investigate the role that shocks play during and after the collapse. Because astrophysical shocks provide a way to accelerate particles and generate high-energy radiation, a powerful shock from a collapsing magnetar could be a source of gamma-ray bursts: brief, powerful flashes of gamma rays of unknown origin.

plot of waves around a newborn black hole

Magnetohydrodynamic waves in the magnetosphere after the magnetar collapses. The newborn black hole is at the center, and its spin axis lies along the z-axis. Green lines show the magnetic field. [Most et al. 2024]

The team modeled a rotating magnetar with a surface magnetic field strength of 1016 Gauss, about 15 quadrillion times stronger than Earth’s magnetic field. After introducing a perturbation into the system, the magnetar collapsed, forming a black hole and launching a near-lightspeed wave into the magnetar’s magnetosphere. The wave then accelerated plasma trapped in the magnetosphere, launching a shock. The so-called “monster shock” generated by the collapse of a magnetar may be the strongest shock in the universe.

Bundles of Bursts

The team’s simulations showed that a hot, powerful, magnetized electron–positron plasma outflow explodes outward in the wake of the monster shock. As they hurry outward, the electron–positron pairs collide, annihilating one another and releasing gamma rays. This flash of gamma rays likely lasts a few milliseconds, similar to the duration of many gamma-ray bursts.

To make this possibility even more interesting, Most and collaborators showed that the newborn black hole’s ring-down — a fleeting period in which the event horizon of the black hole vibrates — would imprint slight variability on the gamma-ray signal. This could explain the tiny variations seen in some gamma-ray bursts.

Not only does this scenario predict the formation of a powerful gamma-ray burst, it might also produce another type of mysterious astrophysical signal: a fast radio burst. This connection is only tenuous, as Most’s team notes that plasma surrounding the collapsed magnetar would likely swallow the radio signal before it escaped. Only under certain conditions could a fast radio burst or a gamma-ray burst escape the plasma’s grasp and make its way to our telescopes.

Citation

“Monster Shocks, Gamma-Ray Bursts, and Black Hole Quasi-normal Modes from Neutron-Star Collapse,” Elias R. Most et al 2024 ApJL 974 L12. doi:10.3847/2041-8213/ad7e1f

galaxies of the Hubble Ultra Deep Field

Galaxies are arrayed along strands of dark matter. New research uses cosmological simulations to explore which galaxies are the best tracers of these dark-matter filaments.

Seeking the Invisible Trees

Trying to figure out the large-scale structure of the universe is a bit like trying to map a forest from within it — except most of the trees are invisible. The luminous galaxies that populate our universe tend to be arranged along strands of invisible dark matter: a hypothetical form of matter that appears to interact with normal matter only gravitationally. While we can readily spot the galactic beacons, the underlying structure is more difficult to determine.

Mapping out the large-scale dark-matter structure of the universe is critical for being able to test our theories of cosmology. What’s the best way to map the dark-matter distribution in our universe?

Simulating Structure

Sang Hyeok Im (Seoul National University) and collaborators investigated the dark-matter-tracing abilities of two types of galaxies: Lyman-alpha emitters and Lyman-break galaxies. These are high-redshift galaxies that can be identified using photometry rather than spectroscopy, requiring (relatively) little telescope time to map their positions.

spectral energy distributions of Lyman-break galaxies

Example spectral energy distributions for Lyman-break galaxies at three different redshifts. [Im et al. 2024]

To determine how well the locations of these galaxies match the hidden dark-matter distribution, Im’s team used the results of the Horizon Run 5 cosmological simulation, which gave them access to both the underlying dark-matter structure and the overlying galaxy distribution. They collected samples of Lyman-alpha emitters and Lyman-break galaxies at redshifts of z = 2.4, 3.1, and 4.5, corresponding to roughly 2.7, 2.1, and 1.3 billion years after the Big Bang, respectively. These are also the target redshifts of the One-hundred-deg2 DECam Imaging in Narrow-bands (ODIN) survey, an ongoing effort to map the large-scale structure of the universe using Lyman-alpha emitters.

Promising Probes

Using a structure-finding code, Im’s team first extracted the simulated three-dimensional dark-matter structure — the “invisible trees” that astronomers hope to probe through observations. Then, they determined the distributions of Lyman-alpha emitters and Lyman-break galaxies around those dark-matter filaments. As a comparison, they also calculated how concentrated the dark-matter particles themselves were around the dark-matter filaments.

comparison of filaments traced by dark matter, Lyman-alpha emitters, Lyman-break galaxies, and massive galaxies

Example of the filaments traced by dark matter (orange), Lyman-alpha emitters (red), Lyman-break galaxies (blue), and massive galaxies (green). The underlying dark-matter distribution is shown in the background of each image. Click to enlarge. [Im et al. 2024]

This analysis showed that Lyman-alpha emitters and Lyman-break galaxies both follow the filamentary structure of the dark matter. Remarkably, both types of galaxies are more concentrated around the underlying dark-matter filaments than the dark-matter particles themselves. This may be because galaxies develop only in the most dark-matter-dense regions, leading to a narrow distribution along the spine of the dark-matter filaments, while dark-matter particles have no such restriction.

Critically, Im’s team found that Lyman-alpha-emitting and Lyman-break galaxies mirror the distribution of the general galaxy population, showing that the galaxy types studied here are an appropriate tracer of the underlying dark-matter distribution. Im and collaborators suggest that Lyman-alpha emitters are an especially promising way to trace fine filamentary structures, since the narrow-band observations used to select these galaxies allow for a more refined redshift estimate.

This work has determined that two types of commonly detected high-redshift galaxies trace the three-dimensional structure of the cosmic web. Future work will examine the two-dimensional distributions of the different galaxy types to provide a way to compare the statistical properties of the distributions against observations, such as those from the ODIN survey.

Citation

“Testing Lyα Emitters and Lyman-Break Galaxies as Tracers of Large-Scale Structures at High Redshifts,” Sang Hyeok Im et al 2024 ApJ 972 196. doi:10.3847/1538-4357/ad67d2

Main-sequence stars with brighter than expected mid-infrared emission can signal the presence of a debris disk, rubble from planetary collisions, or even a theorized sign of a technologically advanced civilization. New research demonstrates a data-driven method to identify mid-infrared excesses in main-sequence stars.

An Excess of Emission

Fomalhaut debris disk

This image combines observations from the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array to show the dusty debris disk surrounding the star Fomalhaut. [ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope A. Fujii/Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble); CC BY 4.0]

Young stars swaddled in gas and dust are known to shine extra brightly in the mid-infrared, but as stars age, this mid-infrared exuberance is expected to fade. When it doesn’t, that signals something interesting. Extreme debris disks resulting from collisions between planets or planetesimals provide one explanation for excess infrared light from mature stars; as rubble and dust billow from the collision, the dust captures and reprocesses the star’s light, re-emitting it in the mid-infrared and causing a potentially detectable excess. Only a handful of extreme debris disk candidates have been identified.

Mid-infrared excesses are hypothesized to signal something even wilder: the presence of a Dyson sphere — a hypothetical artificial structure created by an advanced civilization to harness the power of their home star. Similar to dust and rubble, the components of a Dyson sphere would collect starlight and re-emit it at infrared wavelengths, potentially producing a mid-infrared bump.

Regardless of the cause, excess mid-infrared emission from mature Sun-like stars is something to investigate. But how do we find stars with this feature?

Taking Cues from Data

Gabriella Contardo (International School for Advanced Studies, Italy) and David Hogg (New York University; Flatiron Institute; Max Planck Institute for Astronomy) began their search for mid-infrared excesses with an expansive set of observations from the Gaia spacecraft, the Two Micron All Sky Survey, and the Wide-field Infrared Survey Explorer (WISE). After trimming these data sets down to include only main-sequence Sun-like stars, and to exclude objects that might be contaminated by close neighbors or are too dusty, they reduced the number of stars in their sample from 18,751,187 to 4,898,812.

plot showing the difference between observed and predicted magnitudes in two infrared bands

Difference between observed and predicted magnitude in the WISE W1 and W2 bands (3.4 and 4.6 microns, respectively). Click to enlarge. [Adapted from Contardo & Hogg 2024]

To identify mid-infrared excesses in this sample, the team needed an estimate of what the mid-infrared fluxes of these stars should be. Rather than using models, which can be computationally intensive and require making assumptions about the objects, Contardo and Hogg let the data lead the way.

Their data-driven method involves splitting the five million stars into eight sub-samples, each of which is used to train a separate random forest algorithm. Each algorithm “learns” what the mid-infrared emission “should” be from the stars in its sample, then predicts the mid-infrared emission of the stars in the other seven sub-samples. When a star’s actual mid-infrared emission is brighter than predicted, it gets flagged.

To Be Continued

locations of the 53 stars in the final sample

Locations of the 53 stars in the final mid-infrared excess sample. Click to enlarge. [Contardo & Hogg 2024]

This analysis yielded a preliminary sample of 127 objects with mid-infrared excess. Ultimately, after applying additional cuts to remove crowded objects, duplicate sources, and other complications, Contardo and Hogg landed on a sample of 53 objects with interesting infrared behavior. These objects’ mid-infrared emission ranged from 0.5% to 10% higher than expected, spanning the values predicted for extreme debris disks and rubble left over from planetary collisions. In fact, one of the 53 objects has already been highlighted by previous work as an extreme debris disk candidate.

What happens now? To identify the stars that are the most promising hosts of extreme debris disks, Contardo and Hogg listed ways to pin down the ages of the stars in their sample, which may rule out stars whose mid-infrared excess is due to their youth. They also proposed to compare the mid-infrared behavior of their stellar sample to Dyson sphere models, exploring whether the observed stellar behavior matches the predictions for these hypothetical structures.

Citation

“A Data-Driven Search for Mid-infrared Excesses Among Five Million Main-Sequence FGK Stars,” Gabriella Contardo and David W. Hogg 2024 AJ 168 157. doi:10.3847/1538-3881/ad6b90

Artist's rendition of white dwarf merger

As the Gaia spacecraft has mapped more and more of the Milky Way, astronomers have uncovered some of the fastest-moving stars in the galaxy. Can simulations link these stars to the elusive origins of Type Ia supernovae? 

Type Ia Supernova Origins

Occurring in binary star systems with at least one white dwarf, Type Ia supernovae are key cosmological distance indicators and have allowed astronomers to study the expansion of the universe. Despite their importance, the details behind these explosions and the characteristics of their progenitor systems remain unclear. 

One proposed mechanism to launch a Type Ia supernova is the double-detonation scenario, in which the white dwarf accretes helium from a helium-rich donor star. Forming a thin shell around the carbon-oxygen core, the siphoned helium eventually detonates, sending shock waves through the core, causing it to also detonate. In the wake of the powerful explosion, the donor star launches across the Milky Way, forever changed. 

Recent Gaia discoveries of a runaway helium-burning star and hypervelocity stars — stars that zoom through the galaxy much faster than the general stellar population — suggest that the double-detonation scenario may be responsible for a number of Type Ia supernovae. Can double-detonation simulations predict the observed properties of these fast-moving stars, further uncovering Type Ia supernova origins?   

Supernova Ejecta Effects

Simulation snapshots of helium white dwarf donor model.

Simulation snapshots showing fraction of donor material (left) and total density (right) for a helium white dwarf donor model. The bottom panel shows that, though much of the donor’s material has expanded, a large fraction is still bound to the donor star as indicated by the gray lines in the left panel. The impacts of shock waves can be seen as concentric shells in the density distribution on the right. Click to enlarge. [Wong et al. 2024]

As a helium-rich donor star is bombarded with material and energy from its exploding white dwarf companion, interactions with the supernova ejecta can leave lasting impacts on the donor star’s trajectory through the galaxy as well as the star’s properties and evolution. Motivated by this interaction and the Gaia observations of hypervelocity stars, Tin Long Sunny Wong (University of California, Santa Barbara) and collaborators performed hydrodynamical simulations that track, with novel clarity, the lasting imprints supernova ejecta leave on their companions. 

The authors’ analysis shows that as the supernova ejecta crashes into the donor star, some of the donor star’s material is swept up and pulled in the direction of the supernova’s propagation. The supernova shock wave passes through the donor star, both compressing and pushing the star away from the explosion center. As the shock front moves on, the donor star attempts to return to equilibrium, contracting and expanding, sending smaller shock waves into its surroundings. 

For each progenitor stellar type simulated, the authors find that the donor stars become puffed up with lower densities and larger radii. The donors also lose some of their original mass but acquire a small portion of supernova ejecta material — consistent with the observed metal-polluted atmospheres and larger radii of hypervelocity stars. 

HR diagram showing evolutionary tracks of simulated donor stars

Postexplosion evolution for each simulated donor star type (labeled in figure legend) in luminosity-temperature space (Hertzsprung-Russell diagram). Four observed stars of interest are plotted, showing intriguing agreement between the well-studied hypervelocity star D6-2 and the expected evolution for a helium white dwarf donor companion. Click to enlarge. [Wong et al. 2024]

Postexplosion Evolution

Particularly important to the identification of donor stars in the field is how these stars evolve over longer timescales and how we may observe them today. The authors performed further simulations to track the temperature and luminosity changes for each simulated donor star from ~10 years to 100 million years after the supernova event. Intriguingly, some of the observed hypervelocity stars seem to fall near the predicted evolutionary tracks, suggesting that these stars could have been ejected by Type Ia supernovae.

This study provides important evidence for the possible double-detonation scenario of Type Ia supernovae. As simulations continue to improve, the ability to identify the progenitor systems of these energetic events becomes more promising. 

Citation

“Shocking and Mass Loss of Compact Donor Stars in Type Ia Supernovae,” Tin Long Sunny Wong et al 2024 ApJ 973 65. doi:10.3847/1538-4357/ad6a11

brown dwarf Gliese 229 B as seen by the Hubble Space Telescope

Astronomers recently discovered a companion to Gliese 229 B, the first confidently identified brown dwarf. This discovery resolves the conflict between Gliese 229 B’s observed mass and the predictions of evolutionary models, potentially illuminating the nature of other poorly understood brown dwarf systems as well.

First in Its Class

illustration of a brown dwarf

An illustration of a brown dwarf. Brown dwarfs aren’t actually brown, likely spanning a range of colors from reddish-orange to nearly black. [NASA/JPL-Caltech]

In 1995, Gliese 229 B became the first object to be unambiguously classified as a brown dwarf: an object that bridges the gap between planets and stars. At roughly 13–80 times the mass of Jupiter, brown dwarfs aren’t massive enough to sustain fusion of hydrogen in their cores, as stars do, but they are able to burn a heavier form of hydrogen called deuterium, setting them apart from planets. (The most massive brown dwarfs can burn lithium as well.) After exhausting their supply of deuterium, brown dwarfs steadily cool, sliding down the spectral-type ladder. The youngest and most massive brown dwarfs occupy late M spectral types, while older or less massive brown dwarfs are classified as L, T, or Y dwarfs.

While improved telescopes have advanced our understanding of brown dwarfs, there’s still much we don’t know about these objects, and attempts to study and classify brown dwarfs have been confounded by their complex properties. This is the case for the first confirmed T-class brown dwarf, Gliese 229 B, which recently became the subject of an astronomical mystery.

A Mass Mystery

Soon after Gliese 229 B was discovered, researchers used substellar evolution models to interpret the object’s spectrum and luminosity and estimate its mass at 30–50 Jupiter masses. More than two decades later, refined observations of the brown dwarf’s orbit around its red dwarf host star allowed researchers to calculate its mass dynamically. The newly calculated mass — 71 Jupiter masses — was troubling. According to models of how substellar objects cool as they age, it simply wasn’t possible for a 71-Jupiter-mass object of Gliese 229 B’s age to have cooled to its present temperature.

plot of count density versus radial velocity

The large relative radial velocity between Gliese 229 A and 229 B and the large difference in Gliese 229 B’s radial velocity between the two time periods provides firm evidence for the existence of an unseen companion. [Whitebook et al. 2024]

This conflict between dynamical mass measurements and evolutionary model predictions led researchers to suspect that Gliese 229 B is actually a binary system — a brown dwarf harboring an unseen companion. In March and November of 2022, Samuel Whitebook (University of California, Santa Barbara; California Institute of Technology) and coauthors turned one of the giant telescopes of Keck Observatory toward the Gliese 229 system, using the sensitive High Resolution Echelle Spectrometer to search for evidence of a companion tugging on Gliese 229 B. The team found a clear difference in Gliese 229 B’s radial velocity compared to expectations for an orderly orbit around its host star. Its radial velocity changed by 11σ between the observations, completely ruling out the possibility that Gliese 229 B is a single object.

Single No More

likelihood distribution of orbital period and mass of the companion object Gliese 229 Bb

Likelihood distribution of the orbital period and mass for the companion object. [Whitebook et al. 2024]

What do these observations tell us about the newfound companion? While it’s not possible to fully pin down the properties of the companion object from current observations, Whitebook’s team estimated the companion’s mass to be somewhere between 15 and 35 Jupiter masses with an orbital period between a few days and 60 days. Future observations will refine the companion’s orbit and provide an accurate estimate of the masses of the two components.

In addition to solving the mystery of Gliese 229 B, this discovery may help to explain other seemingly over-massive T dwarfs orbiting main-sequence stars, several of which have been discovered in the past decade. If future work reveals that these too-massive T dwarfs are actually pairs of brown dwarfs, that may suggest that T dwarfs orbiting main-sequence stars are more likely to host companions than T dwarfs in the field, which are usually solo.

Citation

“Discovery of the Binarity of Gliese 229B, and Constraints on the System’s Properties,” Samuel Whitebook et al 2024 ApJL 974 L30. doi:10.3847/2041-8213/ad7714

V838 Monocerotis Hubble image

Stars orbiting one another on extremely wide orbits seem destined to remain apart forever — but new research shows that the subtle influence of the Milky Way’s gravitational pull can bring them crashing together.

Loosely Connected Stars

Many stars in our galaxy travel through space with a binary partner, but some binary pairs are closer than others. Surveys suggest that as many as 12% of nearby Sun-like stars are members of wide binary systems with separations of 100 to 100,000 au or more. Unlike close stellar binaries that might influence each other’s evolution through accretion, stellar winds, and radiation, wide binaries are expected to evolve as if their stellar partner didn’t exist.

Though unaffected by their partners, stars in loosely connected binary systems can be swayed by the subtle influence of the Milky Way’s background gravitational pull, or they can be knocked off course by passing stars. How do these factors affect the evolution of wide binary systems?

Unexpectedly Close

A team led by Jakob Stegmann (Max Planck Institute for Astrophysics) used relativistic N-body simulations to study how the background gravitational pull of the Milky Way and nudges from passing stars affect the orbits and evolution of stars in wide binary systems.

Plot showing outcomes for wide binaries with different separations and distances from the galactic center

Typical outcomes for wide binaries as a function of initial binary separation and distance from the galactic center. Click to enlarge. [Stegmann et al. 2024]

The team simulated 100,000 binary systems with varying initial separations and allowed them to evolve for as long as 14 billion years. The outcome depended on the initial separation of the binary. Binaries orbiting within 100 au of one another weren’t much affected by the galactic tide or by stars whizzing by. Wide binaries with separations of 100,000 au (about 1.6 light-years) or greater were swept up in the galactic tide and torn apart. In between these two extremes, though, the binaries had the potential to end up closer than before.

Swept Up in the Galactic Tide

Many of the binaries underwent wild eccentricity oscillations that brought their orbits from nearly circular to narrow and elongated. This means that initially widely separated stars can undergo close encounters, mass-transfer events, and even mergers.

plot showing the evolution of a widely spaced binary system's separation over time

An example of the evolution of a binary’s separation. This particular binary collided after 2.3 billion years, producing gravitational waves. Click to enlarge. [Adapted from Stegmann et al. 2024]

The precise outcome depended on the masses of the stars involved. High-mass stars evolved into neutron stars or black holes long before the slow eccentricity evolution brought them close, leading to compact-object mergers. The authors estimated that 0.4–5% of observed binary black hole gravitational-wave events could be due to mergers brought about this way. Slightly less massive stars evolved into white dwarfs before merging, generating Type Ia supernovae, though this process likely accounts for just 0.01–0.1% of observed Type Ia supernovae. Low-mass stars interacted while they were still on the main sequence, leading to divergent outcomes: they either tidally interacted and were drawn into close, stable orbits, or they collided, producing luminous red novae.

There are more intricacies to the model results than can be covered in this brief highlight, so be sure to check out the full research article for all the details!

Citation

“Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-Wave Sources,” Jakob Stegmann et al 2024 ApJL 972 L19. doi:10.3847/2041-8213/ad70bb

Globular cluster 47 Tucanae

Massive star clusters have moved about the Milky Way for billions of years, and a recent study finds that more complicated dynamics must be considered when using these clusters to investigate the evolutionary history of the galaxy.

Globular Cluster and Galaxy Evolution

Born from giant molecular gas clouds, globular clusters are large, dense collections of tens of thousands to millions of stars. These systems are very stable and long-lived, making them some of the oldest residents of the Milky Way and important relics of our galaxy’s ancient structure that has long since evolved. 

Within the galaxy, globular clusters tend to fall into two groups — those that formed in the Milky Way (in situ) and those that were accreted during mergers. One clue often used to identify a globular cluster’s origin is its current location in the Milky Way: globular clusters formed in situ are typically found near the galactic center and accreted globular clusters tend to lie more so in the galactic halo. But could the Milky Way’s long-term evolution have also played a role in these globular clusters’ present positions?

Trapped in Resonance

Milky Way Artistic Rendition

An artist’s rendition of the Milky Way showing a strong central bar. Click to enlarge. [NASA/JPL-Caltech]

The presence of a central bar in the Milky Way has strong dynamical implications for objects that orbit it. Over time, as friction slows the rotating bar, objects can be pushed and pulled, altering their orbits and even forcing them to migrate to different locations altogether. Globular clusters that happen to be trapped in resonance with the bar — having orbital speeds that match or are multiples of the rotation speed of the bar — may be susceptible to more dramatic changes over time than those that reside outside of resonances. 

Using Gaia Data Release 3 measurements and dynamical simulations, Adam Dillamore and collaborators (University of Cambridge) explore how the Milky Way’s barred center influences its surroundings. The authors find that the globular clusters most significantly impacted are those caught in resonance. As the central bar slows, these clusters are transported farther away from the galactic center, making a cluster’s current location less indicative of its origins.

Understanding Observations

How do the authors’ findings fit in with previous observations? Of note is Messier 22, a globular cluster with a metallicity spread akin to those anticipated in the Milky Way’s first massive star clusters. After becoming entangled in resonance with the central bar, Messier 22 likely migrated from the very center of the Milky Way outward, further indicating an ancient origin.

Simulation snapshots of the globular cluster 47 Tucanae, where the simulated stars are shown in on-sky coordinates. From left to right, the panels show the impacts of no central bar, a steady central bar, and a slowing central bar on the resulting spatial distribution of stars in the cluster. The presence of a central bar creates a diffuse halo, corresponding to the prior observations of the central region of the cluster marked with the black circles. Click to enlarge. [Dillamore et al 2024]

In addition to exploring the impact of a central bar on globular clusters’ orbits, the authors also investigated how a bar affects globular clusters’ shape and density. Their simulations show that being trapped in resonances with the central bar strips stars and spreads them out into a diffuse halo — a picture that’s consistent with observations of clusters like 47 Tucanae.

The Milky Way does not rest in a steady state, but instead changes over time, impacting the dynamics of its constituents. When it comes to using globular clusters to characterize the galaxy’s past, taking resonances and other perturbations into account is imperative to accurately determine the Milky Way’s evolutionary history. 

Citation

“Trojan Globular Clusters: Radial Migration via Trapping in Bar Resonances,” Adam M. Dillamore et al 2024 ApJL 971 L4. doi:10.3847/2041-8213/ad60c8

A rendering of two dark planets in front of a bright yellow star.

Although some planets calmly follow the lead of their host stars and always stay more or less aligned with the star’s equator, others are more unruly and are found circling their parent in any direction they choose. A recent study adds evidence to the claim that “warm Jupiters”, or massive planets that are a little farther from their host star than their hot-Jupiter cousins, are almost always well-behaved no matter what else is going on around them.

Disorderly and Dramatic

Hot Jupiters, or massive planets that orbit incredibly close to their host stars, have pretty tough lives. Thanks to a ceaseless stellar blowtorch constantly scorching one hemisphere, their atmospheres can be hot enough to melt steel. Even among this tortured group, some have it worse than others. While some stars, like our sun, are relatively cool, others are more massive and therefore hotter. Hot Jupiters around these hot stars must tolerate even more extreme conditions than their (relatively) cooler counterparts.

Perhaps, then, we can forgive this charred group for acting out. In recent years, astronomers have noticed that hot Jupiters around hot stars tend to orbit within planes that are severely misaligned to the one set by their stars’ equators. Some of these planets have been found on nearly polar orbits, meaning they move nearly perpendicularly to the direction of the star’s spin. Their counterparts around cooler stars, however, seem to mostly follow the rules and stay well aligned with their star.

This pattern invites some obvious questions: Why the difference? Is there something about a star’s temperature that would determine the geometry of its planets’ orbits?

Cool and Collected

To answer these and others, a team of astronomers have spent the past few years measuring the alignments of “warm Jupiters”, or planets about as massive as hot Jupiters but slightly farther from their host stars. The latest study in this effort, led by Xian-Yu Wang of Indiana University, brings the total sample of measured warm Jupiters up to 23 planets. None of these worlds are misaligned; in other words, warm Jupiters, even when around hot stars, never set off on their own.

A plot showing stellar temperature on the X-axis and angle of misalignment on the Y-axis. Hot Jupiters are shown on the top half, where we see more misaligned systems around hotter stars. Warm Jupiters, which are all aligned, are shown on the bottom. Click to enlarge. [Wang et al. 2024]

This consistency allows the team to put together a self-consistent story that not only explains their data, but also the outstanding mystery of how hot Jupiters ended up on such inhospitable orbits. In their model, most planets form in quiet disks that are aligned with the star’s spin. In some systems, the planets jostle one another around, sending one or more careening toward the star on wild, eccentric orbits. These unlucky worlds will eventually end up on tight, circular, but tipped-over orbits thanks to tidal interactions and gravitational dynamics: in other words, they’ll become misaligned hot Jupiters. As for the hot Jupiters we see that are aligned, that’s where the stellar temperature comes into play. Cool stars will slowly, over time, wrestle any misaligned planets onto lower-inclination orbits, again thanks to tides, while hot stars are powerless to alter the orbits of their planets.

Through this combination of new observations and intensive modeling of planetary dynamics, astronomers continue to build a fuller picture of planetary formation, and ultimately, how any planets got to where they are today.

Citation

“Single-star Warm-Jupiter Systems Tend to Be Aligned, Even around Hot Stellar Hosts: No Teff–λ Dependency,” Xian-Yu Wang et al 2024 ApJL 973 L21. doi:10.3847/2041-8213/ad7469

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