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Title: Phase-Dependent Magnetic Coherence in the Turbulent Interstellar Medium
Authors: Iryna S. Butsky et al.
First Author’s Institution: Stanford University
Status: Accepted to ApJ

The space between stars isn’t empty, because it’s threaded with magnetic fields that shape how gas moves and steer cosmic rays as they race across the galaxy. As a result, understanding how the magnetic field is structured within different components of the interstellar medium, specifically clouds of different temperatures, is a key part of understanding how the interstellar medium we see today takes shape.

Because magnetic fields are invisible, astronomers need tracers to reveal their structure. One such tracer is the emission from dust grains. Dust grains are not spherical, and a dust grain’s long axis tends to align at a right angle to the magnetic field, causing dust grains to emit light that vibrates in a particular direction. This is called polarized emission. If the magnetic field in a region is very ordered, the grains all align the same way, and a large fraction of their emitted light vibrates in the same direction, giving a high polarization fraction. This fraction is therefore used as a proxy for whether the field can maintain a consistent direction on large scales (an ordered/regular magnetic field).

Comparing the polarization fraction with other tracers of gas lets astronomers infer the field’s structure in the interstellar medium. Previous observations found that the polarization fraction correlates with the fraction of cold gas, but not with the total amount of gas along the line of sight. This suggests that the field is preferentially ordered within cold clouds rather than warm clouds. Curiously, this seemed to contradict other studies that found the field to be more ordered in the warm clouds. Such disagreements are where simulations become useful: because the true magnetic field is known in a simulation, astronomers can test which interpretation the observations actually support.

In this article, the authors use high-resolution numerical simulations to reproduce the relationship between polarization fraction and cold cloud fraction. They find that as long as the line of sight crosses fewer than about 20 clouds, the polarization fraction correlates with the cold cloud fraction, but not with the total neutral gas (Figure 1). This matches earlier observations, where the correlation shows up at high galactic latitudes, which point out of the galactic plane and therefore cross only a few cold structures. When too many clouds lie along the line of sight, each cloud carries an ordered field pointing in a different direction. Averaged together, these vibration directions cancel out, lowering the polarization fraction and washing the correlation away.

Since the simulation reproduces the observational result, does it therefore agree with the conclusion that magnetic fields are genuinely more ordered in the cold neutral medium? The authors argue that the answer depends on how you measure “order.” Comparing the magnetic field direction in individual dense clouds, they find that cold clouds have more ordered fields when compared against warm clouds of similar mass, but the conclusion flips when comparing clouds of similar length (Figure 2).

This apparent paradox comes from the different nature of cold and warm clouds. Cold clouds are denser and more compact, occupying much smaller physical scales than warm clouds. So to match the length of a warm cloud, you have to string together several cold structures, sampling more of the field’s variation and making the cold gas look disordered. Measured per unit length, then, warm clouds appear more ordered. Measured per unit mass, cold clouds appear more ordered. Crucially, when we observe dust emission, we are naturally weighting by mass, so real observations effectively trace the polarization fraction per unit mass, the regime in which the cold clouds are the more coherent phase.

In the end, this work shows that the ordering of interstellar magnetic fields depends on both the gas phase and how we choose to measure it. Cold clouds carry more coherent fields per unit mass, which is exactly the quantity dust observations trace, resolving an apparent contradiction between earlier studies. It is a reminder that even our best observations come with hidden assumptions, and that simulations, where the true field is known, are essential for interpreting what the sky is really telling us.

Original astrobite edited by Viviana Cáceres.

About the author, Sandy Chiu:

I’m a PhD candidate at the University of Michigan, Ann Arbor. I’m interested in numerical simulations of cosmic rays feedback in galaxies and their comparison with observation.

The post The Invisible Architecture Between the Stars appeared first on AAS Nova.

Light and dark regions speckle the Sun and other stars, complicating the interpretation of exoplanet transmission spectra. New modeling suggests that faculae — bright areas on the visible surface of the Sun — appear dark on cooler stars, adding a new wrinkle to this challenge.

Dark Spot, Bright Spot

It’s an adage as old as exoplanet science itself: know thy star, know thy planet. This recommendation is key when using transmission spectroscopy to tease out the components of an exoplanet’s atmosphere; the backdrop to this process is a rotating star dotted with ever-changing active regions, and the better the star’s behavior is understood, the more reliable the extracted spectrum becomes.

The Sun is freckled with dark spots — sunspots — as well as bright regions called faculae. Sunspots form where the Sun’s magnetic field bubbles up, creating pockets of cooler, less luminous plasma. Solar faculae form in regions of concentrated magnetic field, creating bright spots that line the dark crevasses between convective cells.

Other stars also have starspots and faculae, but thus far, astronomers have no direct observations of faculae on any star except the Sun. Out of necessity, researchers interpreting exoplanet spectra assume that faculae on other stars are similar to those on the Sun — but are they actually?

Would a Facula on Any Other Star Look as Bright?

To explore what faculae might look like on stars other than the Sun, Alexander Shapiro (University of Graz; Max Planck Institute for Solar System Research) and collaborators turned to three-dimensional radiative magnetohydrodynamics simulations and radiative transfer modeling. Their aim was to simulate the appearance and spectra of faculae on stars of spectral type G2, K2, M0, and M4 — from Sun-like to substantially smaller and cooler than the Sun.

The modeled faculae on the Sun-like G2 star were bright, as expected. On the K2 and M0 star, these features were fainter, with the faculae on the M0 star being roughly as bright as the surrounding stellar surface. On the M4 star, the faculae were darker than surrounding areas, upending the assumption that faculae on all stars are brighter than the stellar surface.

Exoplanetary Effects

The transition from bright faculae on Sun-like stars to dark faculae on M4-class stars directly affects interpretations of exoplanet transmission spectra. The existence of dark faculae increases the derived exoplanet radius, whereas bright faculae on Sun-like stars decrease the derived radius. The contamination due to the faculae is wavelength dependent, with the largest contamination occurring at shorter wavelengths. This contamination, if not fully accounted for, can mimic spectral features from an exoplanet’s atmosphere.

What determines whether faculae appear brighter or darker than their surroundings? Using a qualitative framework, Shapiro and coauthors found that the appearance of faculae can be traced to a few competing factors: the strong magnetic fields within faculae prevent the incursion of hotter material (making the spots appear darker), while the magnetic field lowers the opacity of the gas in that region, allowing us to peer deeper and glimpse hotter material (making the spots appear brighter). On cooler stars, effects related to higher surface gas pressure tilt the balance in favor of darker faculae.

Citation

“The Curious Case of Dark Faculae on M Dwarf Stars,” A. I. Shapiro et al 2026 ApJ 1008 24. doi:10.3847/1538-4357/ae7105

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Earth itself may be one of the world’s largest detectors for ultralight dark matter. Earth may already contain one of the largest dark matter experiments ever built. Instead of relying only on specialized equipment inside a laboratory, scientists have found a way to use the planet’s magnetic field, atmosphere, and surface as parts of a [...]

A previously overlooked set of spin symmetries has revealed that magnetism does not fit neatly into just two categories. For more than a century, physicists divided collinear magnets into two basic types. The discovery of altermagnetism has added a third, earning Professor Jairo Sinova of Johannes Gutenberg University Mainz (JGU), Dr. Libor Šmejkal and Professor [...]

Researchers have developed a magnetic method for influencing deep-brain circuits without permanently implanted electrodes. A magnetic field may offer a new way to reach circuits buried deep within the brain without permanently implanting electrodes. An international research team injected magnetic nanoplatelets into a targeted brain region and used them to improve movement problems in mice [...]

The first direct measurements of exoplanet magnetic fields reveal that ultra-hot Jupiters possess strong magnetism that shapes their atmospheres and may provide new insights into planetary habitability. Astronomers have achieved a major advance in the study of worlds beyond our Solar System by estimating the strength of the magnetic fields surrounding seven ultra-hot Jupiters. The [...]

Astronomers studying the atmospheres of several intensely heated exoplanets uncovered an unexpected pattern that may reveal a hidden property of these distant worlds. Astronomers have uncovered the clearest evidence so far that some planets beyond our Solar System possess magnetic fields. By measuring atmospheric winds on seven extremely hot gas giants, researchers found signs that [...]

The Sun’s activity history, the sources of tiny jets, and how magnetic fields might impact space weather forecasts: today’s Monthly Roundup introduces a trio of recent research articles from the Astrophysical Journal that tackle hot topics in solar physics.

From 1755 to 2020: Reconstructing Centuries of Solar Activity

Since the 1970s, regular monitoring of the Sun’s magnetic field has allowed researchers to decipher the Sun’s behavior as it cycles through periods of low and high activity. But what resources exist for studying the Sun’s past behavior? To investigate the long-term behavior of the Sun, researchers reach for historical records of sunspots, which stretch back to the 1700s.

Bibhuti Kumar Jha (Southwest Research Institute) and collaborators explored whether historical records and modern statistics could be used to reconstruct the behavior of the solar magnetic field centuries into the past. Jha’s team used recorded sunspot numbers from 1755 to 2020 as an input for their newly developed Synthetic Active Region Generator, which generates synthetic solar active regions based on known statistics of these regions. This catalog of active regions becomes the input for the Advective Flux Transport model, which simulates the active regions as they emerge and evolve, allowing for an estimation of the magnetic field across the Sun’s disk.

Jha and coauthors found that this method successfully reproduced the expected large-scale behavior of the Sun’s magnetic field across multiple solar cycles, such as the reversal of the polarity of the solar magnetic field near the peak of the activity cycle. While this work highlights the impressive results obtained from combining historical sunspot records with modern sunspot statistics, the team pointed out that this method cannot accurately reproduce all quantities because of the nonlinear nature of the underlying physics. Future work will refine the model further, and in the meantime, the team’s simulated historical magnetic field maps will be made publicly available.

Where Do the Tiniest Coronal Jets Come From?

The Sun’s atmosphere is filled with an ever-shifting maze of magnetic field lines, which reconfigure and release pent-up magnetic energy in a process called magnetic reconnection. Magnetic reconnection can launch particles and heat plasma, and it’s thought to be responsible for driving jets on a variety of scales in the Sun’s tenuous outer atmosphere, or corona. Annu Bura (Indian Institute of Astrophysics; Pondicherry University) and coauthors investigated the physical origins of the smallest of these reconnection driven jets, picoflare jets, using data from the Extreme Ultraviolet Imager on board Solar Orbiter. Solar Orbiter has ventured within 0.3 au of the Sun, and its Extreme Ultraviolet Imager has returned exceptionally precise high-energy images of our home star.

Bura and collaborators identified picoflare jets by the presence of a hot, bright spire coupled with a cool, dark streak, forming a structure shaped like the letter “Y” upside down. They found that the bright and dark features were on average 650 and 490 kilometers wide, respectively, with a typical jet length of 16,900 kilometers. They also found that the bright features moved more quickly than the dark features, and the jets tended to last about 6 minutes. Overall, these measurements place the jets in between the scales expected for picoflare jets and slightly larger features called jetlets, though the energies are consistent with expectations for picoflare jets.

Using radiation magnetohydrodynamics simulations, the team explored the origins of their sample of jets. The simulations suggested that the bright features arose from hot, tenuous coronal plasma, while the accompanying dark features were due to cooler, denser plasma venturing upward from deeper in the Sun’s atmosphere, in the chromosphere. Overall, the simulations and observations support a picture in which picoflare-scale jets are driven by magnetic reconnection after magnetic flux emerges through the Sun’s surface. This echoes the formation route of larger coronal jets, suggesting that this process may apply to a wide range of physical scales.

Investigating the Influence of the Solar Polar Magnetic Field

The vast majority of our observations of the Sun have come from within a few degrees of the ecliptic plane, either from telescopes situated on Earth’s surface or from spacecraft that stuck close to the plane of our solar system. With rare but notable exceptions — the Ulysses spacecraft, which measured the Sun’s magnetic field and plasma environment from a near-polar orbit, and Solar Orbiter, which took the first-ever image of the Sun’s poles in November 2025 — the Sun’s poles have remained hidden from our instruments.

Our limited knowledge of the Sun’s polar magnetic field has consequences for our ability to model space weather events like coronal mass ejections. A team led by Xiao Zhang (Chinese Academy of Sciences; University of Chinese Academy of Sciences) has used numerical simulations to investigate how solar polar magnetic field conditions influence the propagation of a coronal mass ejection (CME) through the solar system.

The team simulated a CME from 4 December 2021, which was observed by the BepiColombo spacecraft at Mercury and Tianwen-1 and MAVEN at Mars. The simulations showed that as the polar magnetic field strength increases, the CME’s movement through the solar system becomes slower. CMEs typically expand as they billow outward from the solar corona and sweep past the planets; increasing the polar magnetic field strength slowed the CME’s expansion as well. A stronger polar magnetic field also alters the background solar wind, making it slower and denser. Overall, these results demonstrate that the strength of the Sun’s poorly constrained polar magnetic field can have a significant impact on the movement and evolution of a CME. In future work, Zhang’s team plans to expand their investigation to a larger sample of CMEs.

Citation

“Historical Reconstruction of Solar Surface Magnetism from Cycles 1–24 Using the Synthetic Active Region Generator and the Advective Flux Transport Model,” Bibhuti Kumar Jha et al 2026 ApJ 997 279. doi:10.3847/1538-4357/ae279a

“On the Origin of Coronal Picoflare Jets,” Annu Bura et al 2026 ApJ 1000 94. doi:10.3847/1538-4357/ae48e6

“Influence of Solar Polar Magnetic Fields on the Propagation of Coronal Mass Ejections,” Xiao Zhang et al 2026 ApJ 1000 200. doi:10.3847/1538-4357/ae4c54

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Astronomers have discovered the most compact cataclysmic variable system containing a strongly magnetized white dwarf. The extreme closeness of the system suggests that the companion may be a metal-poor star — the first time such a star has been paired with a strongly magnetized white dwarf.

Meet the Cataclysmic Variable

Cataclysmic variables are binary star systems that contain a white dwarf and a companion star in uncomfortably close quarters. In these systems, the companion star transfers gas to the white dwarf, resulting in sudden, irregular, and often repeated outbursts as the stolen gas ignites on the scorching surface of the white dwarf.

These bound-together stars typically orbit one another on orbits ranging from about 80 minutes to 10 hours. Though theorists place the minimum orbital period around 76 minutes, a handful of cataclysmic variable systems have cropped up with periods below this limit. The systems that have limboed under the limit are thought to have stellar companions that are more compact than typical main-sequence stars, allowing the white dwarf to nestle in closer.

Exploring Below the Limit

Discovered in 2019 by the Gaia spacecraft, Gaia19bxc is a cataclysmic variable that fluctuates with a period of 64.42 minutes. If this variability is linked to the orbital period of the system, that would place it well below the theoretical minimum period. Adding to the intrigue, early observations also hinted that Gaia19bxc’s white dwarf is strongly magnetic, making the system what’s called a polar. In polar systems, the white dwarf’s magnetic field diverts the accreted matter toward the white dwarf’s poles as it is collected, rather than into a disk around the white dwarf’s equator.

Now, Ilkham Galiullin (Kazan Federal University) and collaborators have analyzed photometry and spectra of Gaia19bxc from the Zwicky Transient Facility, the Hale Telescope, and the Keck I telescope to investigate the nature of this unusual system.

The team’s analysis confirmed that the stars of Gaia19bxc orbit one another every 64.42 minutes, cementing the system’s place below the period minimum for typical cataclysmic variables. The system’s double-peaked light curves and evidence for an accretion stream — rather than an accretion disk — confirm the system’s polar nature, implying a magnetic field strength greater than 10 million Gauss. This makes Gaia19bxc the most closely orbiting system to contain a strongly magnetized white dwarf.

A Sign of Discoveries to Come

These observations illuminate the nature of the white dwarf — but what about the companion star?

Galiullin and coauthors saw no evidence for metal lines in Gaia19bxc’s spectrum, nor did they see spectral features arising from a hot companion star. These findings suggest that the companion is an old, cool, compact, metal-poor star, which would make Gaia19bxc the first known polar to contain a metal-poor star. It’s also one of only a handful of cataclysmic variables to contain a metal-poor star and be below the theoretical period minimum.

Though Gaia19bxc is currently in a class of its own, it may not be for long; with the start of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time rapidly approaching, many more cataclysmic variable systems as faint as or fainter than Gaia19bxc may soon be discovered.

Citation

“Optical Spectroscopy of the Most Compact Accreting Binary Harboring a Magnetic White Dwarf and a Hydrogen-Rich Donor,” Ilkham Galiullin et al 2025 ApJL 990 L57. doi:10.3847/2041-8213/adff82

The post Shortest-Period Polar Cataclysmic Variable Discovered appeared first on AAS Nova.

Researchers say the isolated white dwarfs Gandalf and Moon-Sized define a new class of stellar remnant because they share five traits, including X-ray emission. Across the immense scale of the Universe, a single unusual object can prompt astronomers to look for others like it, sometimes leading to the recognition of an entirely new class of [...]

Scientists have achieved a first in studying lanthanum superhydrides, a class of materials that could help unlock superconductivity at much higher temperatures. The dream of transmitting electricity without energy loss has driven decades of superconductivity research. Some of the most promising candidates yet are superhydrides, hydrogen-rich materials that, under immense pressures, have exhibited superconducting behavior [...]