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active galaxy Centaurus A

composite infrared and X-ray image of a molecular cloud with young stars

Clouds of cold gas, like Cepheus B shown here at infrared (red, blue, and green) and X-ray (violet) wavelengths, provide the fuel for star formation. [X-ray: NASA/CXC/PSU/K. Getman et al.; IR: NASA/JPL-Caltech/CfA/J. Wang et al.]

How does the presence of an accreting supermassive black hole affect its host galaxy’s ability to form stars? A new study examines the supply of star-forming gas in more than 10,000 galaxies to find out.

Stymied Star Formation?

Stars form from cold, dense hydrogen gas. Any process that heats this gas, disperses it, or makes it turbulent has the potential to disrupt the star-formation process. For this reason, astronomers have long suspected that the radiation and particle jets from active galactic nuclei — extremely luminous galactic centers powered by a supermassive black hole accreting matter — can inhibit star formation in the galaxies they inhabit.

If this is the case, galaxies with active galactic nuclei should have smaller reservoirs of cold, star-forming hydrogen gas than those without, either because the gas has been heated up or blown away. However, searches for a decline in cold gas have so far come up short. Can a new analysis find trends that have gone unnoticed in previous studies?

Grouping Galaxies

A team led by Hong Guo (Shanghai Astronomical Observatory, China) analyzed a massive sample of galaxies observed by the Sloan Digital Sky Survey and Arecibo Observatory to determine what effect — if any — the presence of an active galactic nucleus has on the host galaxy’s supply of cold, star-forming gas. Guo and collaborators grouped the >10,000 galaxies in their sample according to their masses, star-formation rates, and luminosity of their active galactic nuclei.

plots showing how galaxies in the sample were categorized

Separation of the sample into galaxies with and without active galactic nuclei. The classification is shown according to the Eddington parameter (ratio of luminosity and black hole mass) on the left and according to the luminosity on the right. Click to enlarge. [Guo et al. 2022]

Using a stacking technique to determine the average mass of cold hydrogen gas contained in the galaxies in each bin, the authors compared the supply of star-forming gas between galaxies with active galactic nuclei and those without. The authors find that cold gas is depleted in active galactic nuclei-hosting galaxies with masses up to 10 billion solar masses. For galaxies in this mass range, the depletion of cold gas is greatest in galaxies with high star-formation rates and highly luminous active galactic nuclei. However, this trend is weak or absent in higher-mass galaxies — those from 10 to 100 billion solar masses.

A High-Mass Hypothesis

plots of neutral hydrogen mass as a function of star-formation rate, galaxy mass, and whether or not the galaxy has an active galactic nucleus

Active galactic nucleus (AGN) hosting galaxies with masses up to 10 billion solar masses have less star-forming gas available than galaxies without AGN (left column). This trend is not robust among more massive galaxies (center and right columns). Click to enlarge. [Guo et al. 2022]

Why did Guo and collaborators find that the presence of an active galactic nucleus correlates with a lower supply of star-forming gas in certain galaxies, while previous works found no such trend in any galaxies? The authors speculate that binning their galaxy sample by mass and star-formation rate allowed them to uncover the trend; since the mass of star-forming gas also depends on a galaxy’s star-formation rate, lumping galaxies of all star-formation rates together might mask any correlation.

As to why only the galaxies in the lowest mass bin showed this behavior, Guo and coauthors suggest that high-mass galaxies may simply be too large for their entire reservoir of cold gas to be heated or disrupted while the active galactic nucleus is “on.” In this case, an active galactic nucleus might have a large impact on the inner regions of the galaxy, but that impact may not extend far enough out to be observed. Future high-resolution surveys of cold gas should illuminate this issue further, especially in low-mass galaxies where active galactic nuclei have an outsize impact.

Citation

“Cold Gas Reservoirs of Low- and High-mass Central Galaxies Differ in Response to Active Galactic Nucleus Feedback,” Hong Guo et al 2022 ApJL 933 L12. doi:10.3847/2041-8213/ac794f

Artist's depiction of two black holes nearing a merger.

The detection of gravitational waves paved a new avenue for the study of binary black holes across cosmic time. What can we learn about the evolution of black hole binaries from the systems we’ve detected with gravitational waves?

Examining Gravitational-Wave Events

plot of the masses of the compact objects discovered with gravitational waves

The rapidly expanding “stellar graveyard,” a plot that shows the masses of compact objects observed via gravitational waves and other means. Click to enlarge. [Visualization: LIGO-Virgo-KAGRA / Aaron Geller / Northwestern]

With our detections of merging black hole binaries ever increasing, we can start to answer fundamental questions about how these fascinating systems form and evolve. And since black holes are an endpoint of stellar evolution, studying how the population of black holes may have changed over time can also provide insights into stellar evolution.

In a new publication, a team led by Sylvia Biscoveanu (Massachusetts Institute of Technology) took advantage of the wealth of black hole data to search for trends in the spins of black holes in binary systems. Using a collection of 69 binary black hole events in the third Gravitational-Wave Transient Catalog, the team aimed to determine if the spins of the black holes are correlated with their masses or with the redshift at which the binary lies.

plot of black hole spin distribution for several redshift values

Each shaded region indicates the 90% credible region for the black hole effective spin distribution at a different redshift (z). Click to enlarge. [Biscoveanu et al. 2022]

Calculations from a Curated Catalog

Biscoveanu and collaborators found that the average spin of binary black holes varied little with redshift (a proxy for cosmic time) or mass, but the distribution of spins broadened at higher redshifts. In other words, black holes in binary pairs had the same average spin 10 billion years ago as they do today, but there were more black holes spinning more rapidly  — both with higher positive and higher negative spin — in the past than there are now.

The team analyzed synthetic black hole data and applied new models to existing data to rule out the possibility that the broadening is 1) caused by the increase in uncertainty in the spin measurements of high-redshift binary systems, 2) a reflection of an underlying correlation between other factors, such as black hole mass and redshift, or 3) a consequence of applying the wrong model to the data. These analyses suggested that the trend is real. In fact, a distribution that widens as redshift increases should be easier to rule out than other trends, since rapidly spinning black holes are overall easier to detect than those that spin slowly, so failing to detect them at high redshift would rule out a broadening distribution.

three plots of black hole spin distributions for different redshift values

Black hole spin distributions recovered using three different models. These plots demonstrate that the observed trend is not the result of the authors’ initial model choice. [Biscoveanu et al. 2022]

Taking a Broad View of Black Hole Spin

A broadening of the spin distribution with redshift could have many physical causes. It could indicate that there are several formation pathways for black hole binaries, and each pathway has a different redshift dependence and different spin distribution. Another possibility is that black hole binaries might form via only one pathway but then evolve in such a way that the spins change over time.

However, while both hypotheses can lead to black holes with high positive spin at high redshift, they can’t yet explain the increase in black holes with high negative spin, which are necessary to create the broad spin distribution we observe. One possibility is that these systems with negative spin — in which the spin of an individual black hole is off kilter with respect to the angular momentum of the binary system — could form if black holes in the early universe got a larger gravitational “kick” when they’re born than they do today. There’s still much to investigate, and as our catalog of binary black hole systems grows, our answers are likely to evolve further!

Citation

“The Binary Black Hole Spin Distribution Likely Broadens with Redshift,” Sylvia Biscoveanu et al 2022 ApJL 932 L19. doi:10.3847/2041-8213/ac71a8

Star surrounded by a planet-forming disk

In mystery thriller books, the authors always lead you to suspect that the culprit is someone outside the group: the gardener or the locksmith, perhaps. Sometimes, however, the answer is right in front of you, and the perpetrator is in the inner circle. A group of astronomers has recently reached the same conclusion: that an ongoing orbital alignment mystery seen in some stellar systems isn’t caused by disruptions from the outside, but rather comes from within the stellar systems themselves. 

A cartoon of a protoplanetary disk: the star is at the middle surrounded a small gap and then by the inner disk, there's another gap, and then the outer disk is present. The angular momentum vectors all point up but at slightly different angles.

A schematic of the geometry of a misaligned system, showing the angular momentum direction of the star, the inner disk, and the outer disk. [Epstein-Martin et al. 2022]

A Mystery Arises 

When a stellar system forms, everything is thought to be aligned: the star forms, it ignites nuclear fusion, and all of the leftover gas and dust orbit in a single plane and in the same direction that the star spins. This theoretical picture initially seemed to fit the planetary systems we had found…. that is, until recent space missions started uncovering thousands of new planets and began to tilt this theory on its head. All of a sudden, astronomers were discovering stars whose spin axes were misaligned with the orbits of their planets. But how does this situation arise? Shouldn’t the angular momentum of the system extend to the stellar spin axes, and everything should be aligned? According to recent exoplanet discoveries, apparently not!  

These questions remain a hot topic in the field of planet formation. One proposed explanation is that an external companion star could exert a torque on a star-forming region and misalign everything. In counterpoint, a team of astronomers led by Marguerite Epstein-Martin (California Institute of Technology and Columbia University) posit that the troublemaker was instead internal: forces within the disk itself.

A plot of stellar age [years]on the x-axis (going from ~10^5 to 10^7) against angular momentum (in AU^2 solar masses/yr) on the y-axis (going from ~10^-3 to ~10^1). The star is the bottom curve then the inner disk (A_in = 5 - 10 AU, a shaded region), and the outermost disk (A_out = 40 - 90 AU, a shaded region). All slowly slopes downward to the right.

The angular momentum ranges for the three components in the authors’ modeled system of a star and its surrounding disk. The yellow line shows a solar-mass star, the red region shows the inner disk, and the purple region shows the range of values for an outer disk. This figure clearly shows the hierarchy of angular momentum in the system. [Epstein-Martin et al. 2022]

An Unexpected Suspect 

Protoplanetary disks are typically modeled as rigid objects. However, recent observations show ~85% of the disks that can be resolved contain gaps with misalignments between the inner and outer disks, which result from the formation of massive planets or the presence of a stellar binary companion. The team theorized that in these cases of an inner and outer disk misalignment, the outer disk can play the role of a stellar companion and influence the dynamics of the inner disk–star orientation. From there, the team used equations to model these systems and found that there’s a hierarchy to the angular momentum within the system: the outer disk has the largest angular momentum and will apply a torque on the inner disk, which will itself exert a torque on the star. This is analogous to the dynamics between a star, an unbroken disk, and an outside companion that caused the misalignment in previous theories.  

Identification of the Culprit 

By using a series of complex equations that represent the dynamics in these misaligned systems, the team determined that, given the timescale of the contraction of the star and the lifetime of the disk, there would be just enough time for the disruptions to take place that cause the star’s misalignment with its eventual planets’ orbits. Though this does fit observations, the team notes that they employed several simplifications and assumptions. Overall, this study provides a new avenue for the formation of disks misaligned from the spins of their host star. So next time, don’t get distracted by the outside characters, because the culprit could come from within! 

Citation 

“Generating Stellar Obliquity in Systems with Broken Protoplanetary Disks,” Marguerite Epstein-Martin et al 2022 ApJ 931 42. doi: 10.3847/1538-4357/ac5b79 

Collage of binary star systems

Many stars travel through space with a binary companion, and large-scale surveys allow us to study enormous numbers of these stellar pairs. What do these surveys tell us about the characteristics of binary stars in the Milky Way?

hubble image of interacting galaxies with tidal tails

Tidal forces are perhaps best known for generating tidal tails and streams in interacting galaxies, but a galaxy’s tidal pull can have subtle effects on binary star systems as well. [NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA]

Stars Awash in the Galactic Tide

The orbital parameters of a binary star system — namely, the distance between the stars and how eccentric (non-circular) their orbits are — can encode information about the formation and evolution of the binary system as well as the evolution of the stars themselves. The orbits of binary stars are susceptible to outside influence, too; gravitational nudges from passing stars, nearby gas clouds, and the overall tidal pull of the galaxy can change a binary system’s orbits over time.

When we examine the orbits of binary systems in the Milky Way with observations from the sky-mapping Gaia spacecraft, we find unexpected trends in the orbital parameters of binary systems near the Sun. Namely, among binary systems separated by large distances (>1,000 au), there are more systems with highly eccentric orbits than expected. What’s the origin of this trend?

Nature vs. Nurture

plot of initial and final eccentricity distribution functions from the model

The final distribution of eccentricities (black lines) and best-fitting power laws (green lines) acquired from various initial distributions (red lines). These results show that a superthermal eccentricity distribution, as is seen in binary systems near the Sun, can only arise from a distribution that is initially superthermal. [Adapted from Hamilton 2022]

As Chris Hamilton (Institute for Advanced Study) explains in a recent research article, understanding the current orbits of binary stars in the Milky Way requires separating the effects of nature (the eccentricities that the binary systems are born with) and nurture (the outside gravitational effects of passing stars and the background galactic pull).

Hamilton approached this problem by modeling the effects of the Milky Way’s overall gravitational pull on populations of synthetic binary stars in the outer regions of the galaxy. In order to test the effects of nature as well as nurture, Hamilton modeled populations with different initial eccentricity distributions: uniform (all eccentricities are equally common), thermal (the binaries have reached statistical equilibrium through gravitational interactions; represented by the gray lines in the figure to the right), subthermal (fewer eccentric binaries than the thermal case), and superthermal (more eccentric binaries than the thermal case, as we see near the Sun).

Maybe They’re Born with It, Maybe It’s the Tidal Influence of the Milky Way

The model results show that the tidal pull of the Milky Way tends not to change the eccentricity distribution of a population of binary stars. Put another way, this means that the high number of wide, eccentric binary systems in the solar neighborhood can’t have been caused by the Milky Way’s gravitational influence — another factor, such as the combined effects of individual gravitational nudges from passing stars and gas clouds, must have caused this trend, or binary systems in the solar neighborhood must be born with a similar distribution of eccentricities.

As is so often the case, there’s plenty more work to be done to understand this issue fully. In particular, modeling the effects of gravitational tugs from passing stars and applying new techniques to study the time evolution of binary systems will be critical to reaching a conclusion.

Citation

“On the Phase-mixed Eccentricity and Inclination Distributions of Wide Binaries in the Galaxy,” Chris Hamilton 2022 ApJL 929 L29. doi:10.3847/2041-8213/ac6600

photograph of earth's aurora taken from the international space station

Researchers have applied a powerful simulation tool to a fundamental question about the solar wind: how do sparse plasmas exchange energy?

A Tenuous Topic

Earth and the solar wind

This artist’s impression of the solar wind shows a constant torrent of particles filling the heliosphere and streaming past Earth. [NASA Goddard’s Conceptual Image Lab/Greg Shirah]

In a dense plasma, particles exchange energy through collisions and eventually reach thermal equilibrium. Space plasmas, however, aren’t dense enough to trade much energy in this way — at Earth’s location, the solar wind is less dense than the best vacuum we can create on Earth.

Particles bounce around equally in all directions in a substance in thermal equilibrium, but in a tenuous, magnetized plasma like the solar wind, the particles can move at very different speeds parallel and perpendicular to the magnetic field. Since temperature is a measure of the average kinetic energy of a collection of particles, this means the temperature of a plasma can be different in the direction parallel to the magnetic field lines than it is perpendicular to them!

Early plasma theory predicted that the solar wind plasma would be tens of times hotter parallel to the magnetic field than perpendicular to it. In reality, at Earth’s location, the temperature parallel to the magnetic field is, on average, just 20% hotter than the temperature perpendicular to the field lines. With few to no collisions to help the plasma come to thermal equilibrium, how is this possible?

An Unstable Solution

photograph of clouds exhibiting the kelvin-helmholtz instability

The Kelvin–Helmholtz instability, which plays an important role in the atmospheres of stars and planets alike, is revealed by rare, wave-like patterns in clouds. [UCAR]

In a new publication, a team led by Rodrigo López (University of Santiago, Chile) used simulations to explore the impact of a plasma instability on the temperature of solar wind plasma. Plasma instabilities kick in when certain physical conditions are present, and they can have a big impact on a plasma’s large-scale characteristics, like temperature and density. While instabilities might seem abstract, they actually play a role in many common situations; weather systems, volcanic clouds, and lava lamps are all examples of instabilities at work. In each of these cases, what we observe is the result of the system spiraling away from equilibrium when it is disturbed.

Similar to these examples, instabilities can arise in a plasma in which the temperature parallel to the magnetic field is much higher than the temperature perpendicular to the magnetic field. In today’s article, López and collaborators investigated what happens to this setup when the fire hose instability comes into play. Previous work has explored the impact of the fire hose instability in plasmas where the electrons or the protons have a much higher temperature parallel than perpendicular, but relatively little work has explored what happens when both types of particles have this feature.

Approaching Equilibrium

plot of simulation results

The change in the perpendicular to parallel temperature ratio over time for the simulated electrons (top) and protons (bottom). In Case 1, the electrons start out with a temperature ratio of 1. In Cases 2 and 3, the electron temperature ratios are 0.4 and 0.3, respectively. [Adapted from López et al. 2022]

Using plasma physics equations and two-dimensional particle-in-cell simulations, López and coauthors found that when solar wind protons and electrons have much higher temperatures in the parallel direction, the fire hose instability takes hold of the protons much faster than it would if only the protons had this feature, while the electrons behave similarly regardless of what the protons are up to. In addition, the protons’ parallel and perpendicular temperatures draw closer when the electrons undergo the fire hose instability as well, suggesting that the electrons’ behavior is an important factor in explaining the observed temperatures in the solar wind.

Overall, the authors’ findings confirm that the fire hose instability plays an important role in moderating the temperature of the solar wind plasma, and future work should consider the influence that electrons have on the behavior of protons in the solar wind and other sparse plasmas.

Citation

“Mixing the Solar Wind Proton and Electron Scales. Theory and 2D-PIC Simulations of Firehose Instability,” R. A. López et al 2022 ApJ 930 158. doi:10.3847/1538-4357/ac66e4

Globular star cluster featuring multiple bright stars

Crab Nebula; an extended structure containing filaments of different colors (representing different elements)

Hubble Space Telescope image of the Crab Nebula, the remnant of a supernova that took place in the year 1054 AD. [NASA, ESA, J. Hester and A. Loll]

Just by knowing the mass of a star, can we predict if it will end its life in fire (a supernova) or ice (a white dwarf that eventually fades into a cool black dwarf)? A team led by astronomers at the University of British Columbia tries to answer that question by observing white dwarfs in order to find exactly where that dividing line is between a death of fire and ice. 

…But Which Road Leads to a White Dwarf?

When a star runs out of fuel, it can either eject its outer layers in an explosion so violent that it outputs more energy than the Sun will in its 10 billion years of life, or the star may simply expand and settle down into a stable star called a white dwarf about the size of our moon. What determines which route the star takes is its mass: lower masses die a death of ice, higher masses of fire. Though we believe the dividing line is somewhere around 8 solar masses, this number doesn’t always agree with what we observe.  

Color-magnitude diagram with F225W - F336W plotted on the x-axis with values ranging from -2 to 5, and F225W plotted on the y-axis with values ranging from ~26 to ~15. A roughly vertical line is shown toward the middle left and a cluster of points forming a trend that points up toward the upper left is also shown.

The color–magnitude diagram of the Milky Way globular cluster 47 Tucanae. The x-axis shows the color, the left y-axis shows the apparent magnitude at 47 Tucanae’s distance, and the right y-axis shifts the cluster to the distance of the Large Magellanic Cloud. This diagram shows that even at the distance of the Large Magellanic Cloud, these massive WDs are detectable. [Richer et al. 2022]

In Two Words I Can Sum Up Everything I’ve Learned About Stars: They Evolve 

If all stars greater than 8 solar masses end their lives in the fire of a supernova, we would see a lot more supernova explosions (specifically, Type II supernovae) than we actually do. This dearth of Type II supernovae could indicate that the maximum mass of a star that can end its life as a white dwarf is actually closer to 12 solar masses rather than 8. Constraining this mass limit of stars that can become white dwarfs could inform the formation rate of compact objects as well as the metal content of galaxies. The more massive a star is, the more massive its white dwarf remnant is. Therefore, by hunting for massive white dwarfs, we can effectively hunt for massive progenitor stars that weren’t heavy enough to end in a supernova. A team led by Harvey Richer at the University of British Columbia has looked deep into young open star clusters outside our own galaxy to try to identify massive white dwarfs. 

Previous searches for massive white dwarfs in young Milky Way open clusters only found white dwarfs up to 1.1 solar masses, which come from stars no larger than 6.2 solar masses. To probe whether even more massive stars can become white dwarfs, Richer and coauthors searched young clusters in the Large Magellanic Clouds. The team looked at four Magellanic Cloud clusters in which stars of 5.7 to 10.2 solar masses were just about to enter the asymptotic giant phase (a late evolutionary stage in an intermediate–mass star’s life at which point the star has exhausted its main fuel source), which would mean the white dwarfs in these clusters must have come from stars more massive than that. They also chose these specific clusters because of their distance; the Magellanic Clouds are far enough away that there would be new clusters to search, but not so distant that Gaia parallaxes are unreliable and there is confusion with field white dwarfs.  

The Universe Is Lovely, Dark, and Deep, But We Need More Data To Put This Mystery To Sleep

Two plots both showing distance from the center cluster in pixels vs. cumulative fraction. The left graph shows background objects, all stars, white dwarfs, and bright stars in NGC 2164, and the right image shows them for NGC 330. The right image shows the majority increasingly diagonally up to the right, whereas the left plot shows more spread out lines (all pointing up toward the right but getting there more slowly/gradually)

Distributions of the various populations of stars in two of the clusters. The white dwarfs in the leftmost panel are the five potential white dwarf candidates. [Richer et al. 2022]

The team found five potential candidates in the oldest of the four clusters they studied by looking at the ages and populations of the clusters. These stars represent the first extragalactic single white dwarfs ever discovered. This study demonstrated that it is possible to detect white dwarfs in nearby galaxies with only moderate exposure times with Hubble. However, to study them spectroscopically and determine their masses and ages, the team needs more resolution, which will come with future 30+ meter telescopes. Confirmation of these heavy white dwarfs may finally lead us to the point where the roads of stellar evolution diverged.

Citation 

“When Do Stars Go Boom?” Harvey B. Richer et al 2022 ApJL 931 L20. doi:10.3847/2041-8213/ac6585 

image of coronal loops on the Sun

Though solar flares are common — the Sun releases as many as 20 per day — these explosive events remain mysterious. What can models tell us about the uncertain origins of the X-rays we see at the onset of a solar flare?

time evolution of high-energy flux and observations of coronal loops

Time evolution of flux in several energy bands (left) for the coronal loops shown on the right. Contour lines indicate the 25–35 keV flux. Click to enlarge. [Adapted from Shabalin et al. 2022]

A High-Energy Inquiry

In the early stages of a flare, solar plasma begins to glow in high-energy X-rays. These X-rays are associated with coronal loops — plasma suspended on arching magnetic field lines above an active region, where electrons in the hot, dense plasma travel at breakneck speed. Most of the X-rays arise from the base of these loops, where the density of the plasma is highest. However, observations show that X-rays can also arise from the top of the loop, high in the Sun’s tenuous upper atmosphere, or corona, where the density is far lower. What causes this emission?

diagram of the collapsing trap model and plot of the time evolution of the magnetic field

Diagram of how magnetic field lines move in the collapsing trap model (left) and an example of how the magnetic field strength at the top of the loop varies with time as the field lines collapse (right). Click to enlarge. [Adapted from Shabalin et al. 2022]

It’s a (Collapsing) Trap!

Alexander Shabalin (Ioffe Institute) and collaborators approached this question by testing ways to generate X-ray emission in the rarefied environment of the solar corona, focusing on the collapsing trap model.

The “trap” in this model is a magnetic one: when magnetic field lines near the top of a coronal loop rearrange into a new configuration, the field lines lower down retract, snapping the “trap” shut and ensnaring any electrons that are present. The electrons bounce back and forth between the boundaries of this trap, and as the boundaries draw closer, the electrons’ velocities skyrocket. In theory, this process could concentrate and accelerate electrons enough for them to generate the X-ray emission we observe.

A Numerical Solution

Shabalin and coauthors numerically solved a system of equations that describe how the trapped electrons behave as the magnetic field in the coronal loop evolves. Their investigations focused on the effect of electrons traveling with different orientations relative to the field lines, and they explored how much these electrons were accelerated as individual strands of the coronal loop collapsed in sequence.

plot of flux percentage as a function of time

Percentage of the total 29–58 keV flux of the coronal loop that arises from the top of the loop as a function of time. The red dotted line shows the collapsing trap model results and the black solid lines shows a model in which the plasma density and magnetic field along the loop are constant with time. [Adapted from Shabalin et al. 2022]

For a magnetic trap that takes 8 seconds to collapse, the team found that electron energies could increase by 20–200%, depending on model inputs. These higher-energy electrons increased X-ray emission in the 29–58 keV range, boosting the loop top’s contribution to the overall X-ray flux by 20–70%. The authors also found that their results depended greatly on the energies and orientations of the electrons before they became ensnared in the trap; when the captured electrons had lower energies, the collapsing trap increased their energies by a larger factor, and boosting the energies of electrons traveling perpendicular to the field lines was especially important.

The results show that the collapsing trap model is a viable explanation for the bright X-ray sources seen in the solar corona at the onset of a solar flare — and as the twenty-fifth solar cycle ramps up toward solar maximum in 2025, there should be plenty of solar flares to test these predictions!

Citation

“Early-stage Coronal Hard X-Ray Source in Solar Flares in the Collapsing Trap Model,” Alexander N. Shabalin et al 2022 ApJ 931 27. doi:10.3847/1538-4357/ac65fe

composite image of the active galaxy Centaurus A

Stars have been singing the same song since the beginning of the universe: you’re born, you fuse hydrogen into helium, you drift off the main sequence, and finally, you’re recycled into the cosmos. Under the right conditions, though, stars could become immortal. How is this possible, and what does it mean for these stars’ surroundings?

Live Fast, Die Never

illustration of an active galactic nucleus with dusty disk and polar winds.

Artist’s illustration of the surroundings of a supermassive black hole at the heart of an active galaxy. [ESO/M. Kornmesser]

Many galaxies host an active galactic nucleus — a luminous disk of gas and dust circling a central supermassive black hole. Extreme though this environment may be, stars can live within these disks, and astronomers suggest that some of these stars might be immortal.

As these stars churn hydrogen into helium in their cores, they constantly replenish their hydrogen stores from the surrounding disk. As a result, they never run out of fuel, never leave the main sequence, and never die. Now, a team led by Adam Jermyn (Flatiron Institute) has explored how these extreme stars might affect the evolution of the disk that surrounds them.

Schematic of stars being captured in an active galactic nucleus disk

The disk around an active galactic nucleus (AGN) captures most stars within rcap. The stars within rmax grow and become immortal, while those outside the disk or beyond rmax accrete little gas. Click to enlarge. [Jermyn et al. 2022]

Drinking from the Stellar Fountain of Youth

Jermyn and collaborators considered disks with short (0.1 million years) and long (10 million year) lifetimes, estimating that these disks would capture 1,000 and 20,000 stars, respectively, from the inner regions of their galaxies. The short-lived disk only contains enough mass to raise 300 of these stars to immortality, while the long-lived disk can support all 20,000 stars.

In both disks, the immortal stars grow to 300 solar masses and have massive convective cores. The constant churning brings fresh hydrogen to their cores and transports helium outward to their surfaces. From there, fierce stellar winds carry the helium-rich gas out into the disk, boosting the abundance of helium near the black hole. The consequences of this chemical enhancement aren’t yet clear — it might rob immortal stars of their superpowers, since sucking up helium-rich material will make them burn through their hydrogen faster than it can be replenished — but measuring helium abundances in active galactic nuclei may provide a way to test the degree of chemical enrichment.

Immortal No More

diagram of the wind-fed disk model

This illustration shows that the gas in stellar winds feeds the disk in the innermost regions where the escape velocity is high and escapes from the disk in the outer regions, where the escape velocity is low. Click to enlarge. [Jermyn et al. 2022]

Do immortal stars help or hinder the disk’s survival? Both outcomes are possible. These stars’ winds likely replenish the inner regions of the disk, but they may also drive material to escape the outer regions of the disk. In addition, active galactic nuclei don’t remain active forever — as the disk begins to dissipate, the stars shed much of their mass, giving the disk a last boost before the stars cross back to the mortal realm and evolve into black holes.

The authors note that their estimates are still uncertain, but it’s clear that immortal stars can play an important role in the evolution of the innermost regions of a galaxy. Future work might explore the consequences of helium-enriched gas spiraling around a supermassive black hole or assess the impacts of stars that form within the disk itself. Clearly, immortal stars provide plenty of work for modelers and observers alike — immortality might be beyond our reach, but at least we can live vicariously through these stars!

Citation

“Effects of an Immortal Stellar Population in AGN Disks,” Adam S. Jermyn et al 2022 ApJ 929 133. doi:10.3847/1538-4357/ac5d40

astrophotograph of the pelican nebula

Since 2018, the Zwicky Transient Facility has kept tabs on fleeting events and variable objects in the universe, scanning the entire northern sky every two days. What can this expansive survey tell us about the variability of young stellar objects?

Stellar Surveys

map of the nebular complex with locations of the sample stars overlaid

Full sample of young stellar objects overlaid on an image of the North America Nebula (light-colored area on the left) and the Pelican Nebula (light-colored area on the right). The final sample of variable stars is indicated by the darker blue circles. [Hillenbrand et al. 2022]

Stars of all ages show a broad variety of variability, from slow pulsations to quick bursts of accretion. Young stellar objects seem especially prone to interesting variations, perhaps thanks to the influence of their natal nebulae. In a new publication, Lynne Hillenbrand (California Institute of Technology) led a team of astronomers, which included four high school students, on a hunt for variability in young stellar objects.

The team surveyed stars in the North America and Pelican Nebulae, which lie 2,590 light-years away in the direction of the brightest star in the constellation Cygnus. Though they bear different names, the nebulae are actually connected, separated visually by a dark band of dust. The North America and Pelican Nebulae complex contains thousands of candidate young stellar objects, dozens of which have been monitored for signs of variability. Astronomers have previously used Zwicky Transient Facility data to study individual young stellar objects, but never a large sample — until now.

example plots showing different values of the flux asymmetry and quasiperiodicity parameters

Examples of light curves with low, medium, and high (from left to right) values of the flux asymmetry parameter (M, top row) and quasiperiodicity parameter (Q, bottom row). Click to enlarge. [Hillenbrand et al. 2022]

Digging In to the Data

Hillenbrand and collaborators analyzed more than two years of Zwicky Transient Facility observations of 392 young stars, searching for signs of variability and classifying each star’s quasiperiodicity and flux asymmetry. As the names suggest, quasiperiodicity refers to how periodic or random an object’s variation is, while flux asymmetry quantifies how symmetrical an object’s light curve is. Both metrics are valuable ways to describe a variable star’s behavior, distinguishing those that vary smoothly and reliably from those that suddenly and haphazardly burst or flare.

The team found that 323 of the stars in their sample vary in brightness, and 15% do so with a regular period and symmetric light curves. Another 39% of the stars have symmetric light curves but vary either quasiperiodically or randomly. Roughly 14% of the stars are “bursters” and 29% are “dippers,” with abrupt increases and decreases in brightness, respectively.

The team also examined the colors of the objects, since color changes can give us clues as to why an object’s brightness is varying. The color analysis suggested that dippers might be due to changes in the dusty, light-absorbing material surrounding the young stars, while bursters may signal accretion episodes.

Broad Applicability

plot of flyx asymmetry versus quasiperiodicity

Categorizations of all stars in the sample according to their flux asymmetry and quasiperiodicity. Click to enlarge. [Hillenbrand et al. 2022]

This work not only provides information about hundreds of young stars, but also reflects on the metrics we use to study them. The quasiperiodicity index was designed for data from space telescopes, and some studies suggest that it can’t be used with less precise and lower cadence ground-based data. However, Hillenbrand and collaborators make a compelling case that applying thoughtful boundary conditions makes the quasiperiodicity index viable for ground-based applications as well.

This study marks the first time that Zwicky Transient Facility data have been used to investigate a large sample of young stellar objects, but surely not the last — be on the lookout for more exciting results from this survey!

Citation

“A Zwicky Transient Facility Look at Optical Variability of Young Stellar Objects in the North America and Pelican Nebulae Complex,” Lynne A. Hillenbrand et al 2022 AJ 163 263. doi:10.3847/1538-3881/ac62d8

Map of the observing area for the fourth phase of the Optical Gravitational Lensing Experiment overlaid on a picture of the milky way

Researchers have reanalyzed nearly 10,000 light curves from the Optical Gravitational Lensing Experiment. The resulting catalog, which is publicly available, provides new opportunities to study black holes, exoplanets, and much more.

Making Sense of Microlensing

illustration of a gravitational microlensing event

An illustration of a gravitational microlensing event. In this case, the lensing object is a star with an exoplanet in orbit around it. Click here to see an animation of this event. [NASA]

When one astronomical object passes in front of another, the background object’s light is lensed, or focused, by the foreground object’s gravity, and we see a temporary jump in the background object’s brightness. This process of gravitational microlensing can clue us in to the presence of objects that emit little or no light, like black holes, exoplanets, and dark matter candidate objects, as they pass in front of stars or other luminous sources. Gravitational microlensing events are one of the most promising ways to find isolated stellar-mass black holes, which have long been difficult to track down.

The Optical Gravitational Lensing Experiment (OGLE) has observed more than 10,000 microlensing events since the survey began in 1992. But recording the event is just the first step to understanding what caused it. Researchers model microlensing light curves to estimate the properties of the objects involved, but many factors can complicate these calculations; our vantage point — Earth — is in constant motion, stars vary in brightness for a multitude of reasons, and instruments aren’t perfect. How can we account for all those factors and extract useful information from a microlensing light curve?

plot of the magnitude of a target of the OGLE survey as a function of time

An example of a gravitational microlensing event drawn from the sample analyzed in this work. In this event, OGLE BLG 156.7.141434, the brightness of the background source is variable. Click to enlarge. [Golovich et al. 2022]

New and Improved

That’s where today’s article comes in. In a new publication, a team led by Nathan Golovich (Lawrence Livermore National Laboratory) reanalyzed nearly 10,000 microlensing events in the third and fourth OGLE catalogs. The team’s new model accounts for Earth’s motion — which affects our perception of how quickly the background and foreground objects move relative to one another — as well as variability in the brightness of the background object and systematic instrumental effects.

This type of model has been applied to a single microlensing event, but it has never been used on a full survey because of the immense computing power it requires — Golovich and coathors used about a million hours of computer processing time to analyze their sample! The team showed that their model was able to separate the desired signal from competing factors like Earth’s motion and the variability of the object being lensed, greatly reducing sources of bias.

A Curated Catalog

plot of a microlensing event and models fit to that event's light curve

An example light curve and model fit for OGLE BLG 156.7.141434, the same event shown in the previous figure. Click to enlarge. [Golovich et al. 2022]

What does this updated catalog mean for the search for isolated black holes? Golovich and collaborators used the open-source Population Synthesis for Compact object Lensing Events tool (PopSyCLE) to simulate microlensing events and identify locations in parameter space that isolated black holes are likely to inhabit. Based on the results of these simulations, the authors estimate that 50% or more of the 390 OGLE events in that region of parameter space are most likely due to foreground black holes.

The catalog — the largest of its kind, to date — is free to anyone who wishes to use it; if you’re interested in black holes, exoplanets, or any other kind of dark object, there’s no better time to be on the hunt!

Citation

“A Reanalysis of Public Galactic Bulge Gravitational Microlensing Events from OGLE-III and -IV,” Nathan Golovich et al 2022 ApJS 260 2. doi:10.3847/1538-4365/ac5969

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