Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
Effects of Physical Cooling on the Structure of Circumbinary Disks
Minjae V. Kim, Julian H. Krolik and Scott C. Noble
Radiation pressure and radiative cooling often play an important role in deciding the structure and dynamics of astrophysical objects. In this paper, we present a radiative MHD simulation of a circumbinary disk (CBD) around an equal-mass SMBBH. It employs an approximate thermodynamic model based on local thermodynamic
equilibrium and
diffusion cooling which captures how the internal energy evolves in an optically thick region. The disk reaches a quasi-steady state within $r=5a$ whose mass distribution differs substantially from the one obtained from simulations without physical thermodynamics: the local maximum in the azimuthally-averaged surface density and the lump are completely erased; the time-averaged inner edge becomes more eccentric; and the vertical density distribution depends strongly on the radius. These changes are attributable to the enhancement of the
magnetic fields caused by the cooling-driven vertical compression of the disk. Our result emphasizes that the basic properties of the inner CBD are sensitive to both radiation and
magnetic physics.
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Solar and Stellar Astrophysics
Tidal Dissipation from Circularization in Kepler Binaries
Joshua Schussler, Torsha Majumder and Kaloyan Penev
Understanding tidal dissipation is a requisite step for explaining the
evolution of systems such as short-period binaries. This understanding has not yet been achieved, and in fact there are many different approaches to modelling tides. By using
Bayesian analysis on a system-by-system basis, we provide additional constraints on dissipation for 105 short-period Sun-like Kepler binaries. We account for period-dependent $Q$ and stellar
evolution, and propagate observational uncertainties to uncertainties in our constraints. We find a group constraint of $\log Q_*' \approx 6.75$, with no apparent dependence on tidal period. We also do not detect mass dependence for $Q$. Our inferred prescription for the tidal dissipation successfully reproduces the unexpected overlap between almost perfectly circular and significantly non-circular orbits in binaries of Sun-like
stars reported by arXiv:2112.05868.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
The Galactic Dynamics of Free-Floating Planets: From Ejection Kicks to Microlensing
Zara Sayed, Stefano Profumo and Nolan Smyth
Free-floating
planets (FFPs) may retain dynamical signatures of the mechanisms that eject them from their host systems. We integrate $10^6$ collisionless FFP test particles for $10^8$~yr in a static, phenomenological Galactic potential, comparing a mass-independent kick model with a mass-coupled prescription ($\sigma_{\rm CB}\propto M^{-0.15}$, $v_{\rm PL}\propto M^{-0.5}$) against a matched no-kick control. The imposed mass dependence is recovered at injection, with the combined-channel median coupled kick decreasing from $10.716$ to $0.708~{\rm km\,s^{-1}}$ across five mass bins—a factor of $\sim15.1$. A mass-dependent trend remains visible after $100$~Myr of Galactic propagation in the matched kicked-versus-control displacement, whose median decreases from $0.7001$ to $0.0539$~kpc across the same bins. The remaining numerical distinction is concentrated in this mass-resolved differential displacement rather than in the bulk phase-space moments: the final velocity dispersions of the null and coupled kicked populations are nearly identical, only a few percent of particles leave the adopted disk region, and the fraction of particles satisfying the formal Galactic-unbound criterion ($E_i>0$) remains negligible in every diagnostic we report. Comparing each kicked population against its matched no-kick control shows that much of the overall kinematic heating arises from the
evolution of the initially warm, nonequilibrium disk rather than from the ejection kick itself, isolating the kick's smaller contribution.
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Solar and Stellar Astrophysics
White Dwarf Natal Kicks as Velocity-Space Random Walks. II. Binary Stellar Evolution and Observational Kick Constraints
Dang Pham, Stephen Majeski, Taeho Ryu et al. (+2 more)
Wide binaries containing white dwarfs (WDs) provide a pathway to exploring post-main-sequence (MS) natal kicks associated with asymmetric mass loss. In this work, we develop a binary population synthesis model that involves episodic mass loss and velocity kicks during the thermally-pulsing asymptotic-giant-branch (TP-AGB) phase of stellar
evolution, when mass loss is expected to be most significant. We compare our synthesized populations against the Gaia DR2 and DR3 data for wide WD-MS and WD-WD binaries, varying the number, strength, and directionality of TP-AGB kicks. We find that the observed separation distribution can constrain the kick strength, but it alone is insufficient to also constrain the kick number and dimensionality. We therefore additionally constrain our models using the observed eccentricity distributions. Models where mass loss induces a random walk in the
star's velocity are most consistent with observed eccentricity constraints for WD-MS binaries. No models are found to agree with the WD-WD eccentricity constraint. Our results suggest that at least 10 randomly directed kicks take place during the TP-AGB phase, leading to a cumulative root-mean-square kick velocity of approximately 0.7 km/s. These findings constrain the nature of WD natal kicks, connecting the orbital distributions of evolved wide binaries to asymmetric mass loss during the final stages of stellar
evolution.
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All authors Dang Pham, Stephen Majeski, Taeho Ryu, Gabriel Tomassini, Andrea Chiavassa
Solar and Stellar Astrophysics
White Dwarf Natal Kicks as Velocity-Space Random Walks. I. Fokker-Planck Theory
Stephen Majeski, Dang Pham, Taeho Ryu et al. (+2 more)
White dwarfs are the most common end state of stellar
evolution, including that of our own Sun, making their formation critical to completing a more comprehensive understanding of stellar
evolution and even determining the eventual fate of the solar system. Wide binaries containing white dwarfs offer a powerful probe: their components evolve largely independently, yet stellar mass loss and velocity impulses (natal kicks) on the white dwarf progenitor leave measurable imprints on orbital properties. Previous population studies typically assume that each white dwarf receives only a single natal kick as it forms. While convenient, this simplification conflicts with simulations finding the formation process to be episodic. Therefore, a more realistic picture is that white-dwarf progenitors experience multiple
stochastic kicks over their
evolution. Here, we present a Fokker-Planck model for the
evolution of binary orbital parameter distributions subject to many, randomly-oriented mass-loss-driven kicks. This model recovers key features of white-dwarf binary observations, including a -2 power law tail in the probability distribution of separations, and the thermalization of binary eccentricities beyond separations of 1000 au. We additionally investigate how white-dwarf kicks impact a myriad of other processes, such as binary dissolution, planetary system
evolution, and even the production of interstellar objects.
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All authors Stephen Majeski, Dang Pham, Taeho Ryu, Gabriel Tomassini, Andrea Chiavassa
Solar and Stellar Astrophysics
Cause of chromospheric opposite polarity intrusions discovered in Sunrise III/SCIP data: MURaM-ChE simulations point to twisted flux ropes
Patrick A. Ondratschek, Damien F. Przybylski, Robert H. Cameron et al. (+33 more)
The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) instrument onboard the balloon-borne Sunrise III observatory provided new high-resolution observations of the solar chromosphere in the Ca II 854.2 nm line. The Stokes-V signal in
magnetic network regions was found to show fine-structured details, which suggests the
magnetic field above the network elements does not simply expand as a unipolar feature but displays opposite-polarity-intrusions (OPIs). These features appear as elongated structures in Stokes-V observations. In this work, we demonstrate that such features appear ubiquitously in a numerical simulation of the solar chromosphere. We use a simulation that is computed with the recently developed chromospheric extension of MURaM (MURaM-ChE) and resembles an enhanced network region. We find that OPIs appear ubiquitously in the vertical component of the
magnetic field at around 1 Mm above the surface and are visible in the synthetic Stokes-V signal of the Ca II 854.2 nm line. The structures have lengths of 2 Mm to 7 Mm and widths of approximately 1 Mm. The
magnetic field configurations associated with the OPI features appear to belong to twisted flux ropes (TFRs) and are visible for most of the time in the presented 21 min time series. Our results show that the
magnetic structure of the chromosphere is more complex than previously thought, with even seemingly simple flux tubes showing embedded twisted fields pointing in the opposite direction. This may help in explaining new high-resolution observations from the Sunrise III mission.
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All authors Patrick A. Ondratschek, Damien F. Przybylski, Robert H. Cameron, H. N. Smitha, Sami K. Solanki, Masahito Kubo, Andreas Lagg, Achim Gandorfer, Jose Carlos del Toro Iniesta, Yukio Katsukawa, Pietro Bernasconi, Thomas Berkefeld, Alex Feller, Tino L. Riethmueller, Alberto Alvarez-Herrero, David Orozco Suarez, Bianca Grauf, Michael Carpenter, Alexander Bell, Valentin Martinez Pillet, Laurent Gizon, Francisco Javier Bailen, Julian Blanco Rodriguez, Juan Sebastian Castellanos Duran, Edvarda Harnes, Johannes Hoelken, Francisco A. Iglesias, Ryohtaroh T. Ishikawa, Yusuke Kawabata, Takuma Matsumoto, Takayoshi Oba, Azaymi L. Siu-Tapia, Hanna Strecker, Dusan Vukadinovic, Hirohisa Hara, Toshifumi Shimizu
Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
The Galactic Phosphorus Survey. I. Chemical Evolution across the Galactic Disk from $\sim$750 FGK Stars
Dionysios Gakis, Keith Hawkins, Zachary G. Maas et al. (+5 more)
Phosphorus (P) is an odd-$Z$ element produced primarily in massive
stars and is also important for planetary chemistry as an element essential to life. However, its
evolution over
cosmic time remains poorly constrained because precise and homogeneous stellar P measurements are scarce. We determine the relationship between [P/Fe] and [Fe/H] over a range of
metallicities across the Galactic disk and assess its variation between different stellar populations using the largest uniformly analyzed sample to date, comprising $\sim$750 FGK
stars with high-resolution near-infrared spectra. Stellar parameters are determined through a spectro-photometric approach and P abundances are derived by fitting synthetic spectra to the P~I 10529.52~Å\ absorption feature. We confirm that [P/Fe] increases toward lower
metallicities down to [Fe/H] $\sim$ -1.0 dex, where it appears to peak, broadly resembling the rise seen in $\alpha$-element abundance ratios over the same
metallicity range and supporting a dominant contribution from massive
stars on short core-collapse
supernova timescales. When separating the sample into Galactic disk populations, thick-disk
stars are systematically enhanced in [P/Fe] relative to the thin disk by $\sim$0.08 dex at fixed
metallicity. We also measure a radial [P/Fe] gradient of $-0.124 \pm 0.006$ dex kpc$^{-1}$ and a vertical gradient of $+0.052 \pm 0.007$ dex kpc$^{-1}$. Although current chemical
evolution models struggle to reproduce the observed trends, this survey increases the number of homogeneously measured stellar P abundances by approximately a factor of three, providing a new
benchmark for Galactic chemical
evolution models and the phosphorus inventory available to planetary systems.
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All authors Dionysios Gakis, Keith Hawkins, Zachary G. Maas, Christopher Sneden, Melike Afşar, Natalie R. Hinkel, Chenguang Sun, Wenwei Liang
Solar and Stellar Astrophysics
The Treasury of Extremely Metal-Poor O Stars
O. Grace Telford, Christiana Erba, Kristen B. W. McQuinn et al. (+10 more)
The Treasury of Extremely Metal-Poor O
Stars (TEMPOS) is a Hubble Space
Telescope survey of hot and massive O-type
stars in nearby, low-
metallicity galaxies ($\lesssim$20% of the solar
metallicity, $Z_\odot$). Understanding massive-
star physics in this regime is essential to interpret observations of metal-poor
galaxies, including both low-mass dwarf
galaxies and chemically unevolved
galaxies in the early Universe. Yet, few far-ultraviolet (FUV) spectra of O
stars of sufficient quality to characterize their fundamental properties and stellar winds exist below 20% $Z_\odot$, and heterogeneous observation design and incomplete coverage of parameter space pose significant barriers to progress. To remedy this, TEMPOS obtained new
Cosmic Origins Spectrograph (COS) FUV spectra of 12 very metal-poor O
stars, building upon archival data to assemble a spectroscopic atlas of 29 homogeneously observed
stars that efficiently samples a wide range of spectral types and luminosity classes. Here, we describe the motivation, sample selection, and observation design for TEMPOS and present the first data release of reduced and coadded COS spectra. We then present initial results on the empirical properties of FUV O-
star spectra below 20% $Z_\odot$, including radial velocities, equivalent widths of photospheric lines, and terminal wind velocities ($v_\infty$). We show that $v_\infty$ correlates with host
galaxy metallicity across $\sim$5-50% $Z_\odot$ and find tentative evidence of a steeper decline in wind strength below $\sim$10% $Z_\odot$. The combined
dataset of FUV spectra and planned releases of photometry and optical spectra from the TEMPOS Treasury program will advance our understanding of both stellar astrophysics and the interstellar medium in the extremely metal-poor regime.
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All authors O. Grace Telford, Christiana Erba, Kristen B. W. McQuinn, Calum Hawcroft, Andreas A. C. Sander, Julia Roman-Duval, John Chisholm, Danielle A. Berg, Varsha Ramachandran, Yong Zheng, Claus Leitherer, Abby Mintz, Evan N. Kirby
Solar and Stellar Astrophysics · Cross-listed from physics.space-ph
Microphysical Diversity in Two Very Closely Spaced Magnetic Switchbacks Observed by Parker Solar Probe
Dipali Vadher, Ankush Bhaskar, Smitha Thampi et al. (+1 more)
Parker Solar Probe observations near the Sun reveal frequent, sudden reversals of the
magnetic field known as switchbacks (SBs). Despite their ubiquity, the internal
plasma structure and associated heating within SBs remain poorly understood. We present a case study of two closely spaced SBs (referred in text as SB_1 and SB_2) observed on 24 January 2020 using high-cadence
magnetic and
plasma measurements.
Magnetic fluctuations are decomposed into components parallel and perpendicular to the mean field, and their power spectra are analyzed to characterize the turbulent cascade. The Partial Variance of Increments (PVI) method is applied to identify intermittent current-sheet-like features. Both SB intervals exhibit clear Alfvenic behavior and enhanced radial flow; however, their microphysics differ: SB_1 shows a higher
proton temperature, larger fluctuation amplitudes, and a denser population of current sheets compared to SB_2. The two events also differ in spectral index, with SB_1 exhibiting a steeper perpendicular slope than SB_2. The elevated intermittency,
proton temperature, and transient $\beta > 1$ excursion in SB_1 suggest that localized dissipation at small-scale structures is a plausible driver of the observed heating. These findings demonstrate that SBs are not uniform kinematic deflections but dynamically evolving
plasma structures whose internal
turbulence may regulate local energy conversion and contribute to the spatially intermittent heating of the near-Sun solar wind.
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All authors Dipali Vadher, Ankush Bhaskar, Smitha Thampi, Kamlesh Pathak
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
Water Cloud and Chemical Modulations in the Coldest Brown Dwarf
Brittany E. Miles, James Mang, Caroline V. Morley et al. (+24 more)
We present high signal-to-noise, medium resolution (R ~ 1000), time-series JWST/NIRSpec spectra of WISE 0855 (265K), the coldest known brown dwarf. Medium resolution, time-series spectroscopy gives us the power to disentangle the effects of chemistry, temperature, and condensates on this cool world. Our observations span 11 hours with a 15 minute cadence covering 2.87 - 5.27 $\mu$m. The strongest time variable spectroscopic feature is carbon monoxide gas absorption producing modulations with a peak-to-peak amplitude up to 10% at some wavelengths. Using principal component analysis, we show that the variations in carbon monoxide and phosphine correlate with one another. By comparing our data to atmospheric and structure models we present evidence of patchy water clouds within the atmosphere of WISE 0855. We find that variations in CO and PH$_{3}$ abundances must originate from quenched atmospheric pressures while variations in water cloud thickness occurs at lower pressures.
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All authors Brittany E. Miles, James Mang, Caroline V. Morley, Andrew J. I. Skemer, Melanie Rowland, Brianna Lacy, Genaro Suarez, Channon Visscher, Dániel Apai, Gordon Bjoraker, Aarynn L. Carter, Jacqueline K. Faherty, Jonathan J. Fortney, Nguyen Fuda, Thomas Geballe, Harshil Kothari, Mary Anne Limbach, Mark Marley, Allison M. McCarthy, Adam Schneider, Johanna Vos, Michael Cushing, Richard Freedman, Michael Line, Roxana Lupu, Emily Martin, Mikayla J. Wilson
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
Daytime seeing variations in Paranal through a SHABAR system. Description of the method and first measurements from the PoET solar telescope
A. M. Silva, B. Wehbe, A. Oliveira et al. (+8 more)
Aims. In this paper we present the software and first seeing measurements from a SHAdow Band Ranger (SHABAR) system that is coupled with the recently comissioned PoET solar
telescope, installed at ESO's VLT platform in the Paranal Observatory. The seeing measurements that are collected by this system play a pivotal role in the observational strategy of PoET, as it employs apertures ranging between 1 and 55 arcseconds to observe resolved solar regions. As such, the usage of the smaller apertures hinges upon good observational conditions, which are assessed through real-time measurements from the instrument. Methods. We employ a SHABAR system, consisting of a series of scintillometers that are non-uniformly spaced in a bar, measuring the scintillation of the incoming light. The scintillation is measured in individual detectors, which are then used to characterize the atmospheric
turbulence at a given range of heights. We use previously derived formulations, following Kolmogorov
turbulence models, to model the collected data and transform it into a seeing measurement. Results. We find that the Paranal daytime seeing can reach values close to 1 arcsecond in the early morning, before the ground temperature increases and creates
turbulence in the lower layers of the atmosphere. Over the day, the seeing gradually worsens, with the median seeing reaching values larger than 4 arcseconds in the afternoon. We also find that these variations appears to be stable over the first weeks of observations, yielding compatible results when comparing 37 independent days of operations, with a median seeing of 2.37 arcseconds.
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All authors A. M. Silva, B. Wehbe, A. Oliveira, M. Abreu, C. Cardenas-Conde, A. Cabral, P. Moreno, J. H. C. Martins, M. A. Monteiro, N. C. Santos, R. Gafeira
Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
The Galactic centre G+0.633-0.0604 molecular cloud: A new astrochemical gold mine: II. Chemical richness
D. San Andrés, V. M. Rivilla, L. Colzi et al. (+4 more)
In recent years, the inventory of interstellar molecules has rapidly grown, yet detections of complex species are confined to a few chemically rich sources, raising the question of whether such complexity is genuinely rare or shaped by observational biases. We present the Galactic Centre G+0.633-0.0604 molecular cloud, located in the southern part of the Sgr B2 complex, as a new and very promising laboratory for the discovery of new interstellar molecules. We analysed data from a high-sensitivity molecular line survey of G+0.633, covering nearly 100 GHz of aggregated bandwidth over the 31$-$275 GHz frequency range with the Yebes 40m, IRAM 30m and APEX
radio telescopes. G+0.633 exhibits an exceptionally rich molecular feedstock comparable to that of the chemically rich G+0.693 cloud in the northern part of Sgr B2, establishing it as its 'astrochemical twin'. We report new robust interstellar detections of 15 astrochemically relevant species, most of prebiotic interest, previously identified only in G+0.693 or a very limited number of sources. These species include urea, $C$- and $N$-cyanomethanimine, ethanolamine, glycolamide, carbonic acid, and dimethyl sulphide. Our results reveal that the extraordinary molecular complexity observed in G+0.693 is not unique, but may instead reflect a natural outcome of shock-dominated chemistry in the Galactic Centre. The G+0.633 and G+0.693 clouds exhibit remarkably similar excitation conditions and comparable molecular abundances, establishing them as an ideal benchmarking pair for astrochemical studies. Moreover, G+0.633 offers a clear advantage for molecular observations over G+0.693 owing to the significantly narrower linewidths of its molecular line emission, nearly half as broad as those characteristic of G+0.693, making it an exceptionally powerful target for guiding the search for increasingly complex molecules in the ISM.
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All authors D. San Andrés, V. M. Rivilla, L. Colzi, M. Sanz-Novo, S. Martín, I. Jiménez-Serra, S. Zeng
Solar and Stellar Astrophysics
Gaia method paper: an algorithm to detect flare events in Gaia time-series
E. Distefano, A. C. Lanzafame, A. F. Lanza et al. (+14 more)
The third Gaia data release (GDR3) includes the gdr3_rotmod catalogue of 474026
stars with variability that is attributed to
magnetic activity analogous to that observed in the Sun, arising from rotational modulation induced by spots and faculae. These
stars are therefore expected to exhibit flaring activity driven by
magnetic reconnection processes. We aim to demonstrate that stellar
flares can be reliably identified and characterised in the sparsely sampled \gaia\ photometric time series. We developed and validated a dedicated
flare-detection pipeline that exploits the simultaneous multi-band photometry in the $G$, $G_{\rm BP}$, and $G_{\rm RP}$ bands. The
algorithm identifies outliers associated with a concurrent increase in brightness and blueing of the stellar colour, and applies consistency criteria to distinguish genuine
flare events from instrumental or calibration artefacts. Applying this method to the gdr3_rotmod catalogue, we detect 3217
flares occurring on 2818
stars out of 474026 analysed sources. All events are provided in a dedicated catalogue together with their photometric amplitudes. For a subset of 348
flares, we estimate an effective temperature using a black-body approximation, and we identify 29 hyper-
flares with amplitudes $A(G)\ge0.75$ mag, which occur preferentially in M dwarfs. Despite the sparse temporal sampling of Gaia, our results demonstrate that its multi-band photometry and spectrophotometric information enable the robust detection of
flares and the basic characterisation of their properties across an all-sky stellar sample. This work establishes the methodological foundation for future Gaia
flare catalogues and provides a homogeneous, complementary view of stellar flaring activity alongside high-cadence missions such as Kepler and TESS.
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All authors E. Distefano, A. C. Lanzafame, A. F. Lanza, S. Messina, I. Pagano, M. Audard, G. Jevardat de Fombelle, B. Holl, I. Lecoeur-Taibi, N. Mowlavi, K. Nienartowicz, L. Rimoldini, D. W. Evans, M. Riello, P. García-Lario, P. Gavra, L. Eyer
Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
Evolution and mergers of eccentric white dwarf binaries encountering intermediate-mass black holes
Adarsh Pandey, Aryabrat Mahapatra and Tapobrata Sarkar
We study how a close passage past an intermediate-mass black hole (IMBH) changes the
evolution of an eccentric white dwarf (WD) binary that has nominal Roche lobe overflow at internal pericenter. Using three-dimensional smoothed particle hydrodynamics, we follow binaries with eccentricity $e_{\rm in}=0.6$ and mass ratios $q=0.2$ – $0.7$ on parabolic, retrograde orbits around a Schwarzschild IMBH at two encounter strengths, $\beta^b=0.5$ and $1.5$. We compare them with the same binaries evolved in isolation. The IMBH greatly speeds up the merger. Every IMBH-perturbed binary that merges does so during or shortly after its second internal pericenter passage, which is always less than the time taken to merge in isolation. The merger outcome is not monotonic with encounter strength. The $q=0.2$ binary retains two bound cores for $\beta^b=1.5$ but merges at $\beta^b=0.5$. In the deeper encounter it settles onto a less eccentric post-encounter orbit, showing that the timing of the encounter relative to the inner orbit plays an important role. Deep encounters also produce transient reversals of the internal orbital angular momentum for $q=0.2$ – $0.4$, while the material bound to the accretor gains more
spin angular momentum than that bound to the donor. The specific orbital angular momentum of the binary CM around the IMBH changes by only a few $\times10^{-4}$ of its initial value. We also identify He-origin material that can burn helium on a dynamical timescale, amounting to $\sim15$ – $17\%$ of the He-origin mass for $q=0.7$. IMBH encounters can therefore quantitatively change the
evolution of eccentric WD binaries.
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Solar and Stellar Astrophysics
Investigating the inferred spatiotemporal evolution of magnetic fields during an X1-class flare
Dimitri Arramy, Jaime de la Cruz Rodríguez, Adur Pastor Yabar et al. (+1 more)
Several observational studies have reported sudden temporal changes in chromospheric
magnetic fields inferred during solar
flares using the weak-field approximation (WFA) and nonlocal thermodynamic
equilibrium (NLTE) inversions. We investigate whether these variations reflect the rapid
evolution of the solar atmosphere or can be influenced by opacity changes caused by
flare heating. We perform spatially coupled and regularized NLTE inversions of high-resolution observations and use the inferred atmospheric models to study the formation of the Ca II 854.2 nm line. We also analyze snapshots from a 3D radiative magnetohydrodynamic
flare simulation to investigate the
evolution of the mapping between column mass and geometrical height. We find that opacity effects alone cannot, in many cases, explain the temporal variations in the reconstructed
magnetic fields, suggesting that they reflect real solar
evolution. However, in approximately 36% of the regions exhibiting rapid
magnetic-field changes, opacity effects are sufficiently strong to bias their detection. Ejected or condensed cold material can also shift the line formation to a different depth, producing an apparent sudden change in the inferred
magnetic field. The simulation further shows that, once the
flare ribbon forms, a given column mass maps to deeper geometrical layers. These results highlight the need for caution when interpreting the temporal
evolution of chromospheric
magnetic fields during
flares, as neither the WFA nor NLTE inversions based on 1D hydrostatic
equilibrium can fully account for these effects.
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All authors Dimitri Arramy, Jaime de la Cruz Rodríguez, Adur Pastor Yabar, Jorrit Leenaarts
Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
The 2025.6 X-ray Minimum of Eta Carinae
Amit Kashi and Noam Soker
We present observations of the recent 2025.6 X-ray minimum and recovery of $\eta$~Carinae, obtained with NICER/XTI, Swift/XRT, XRISM/Xtend, and Chandra/HRC-I. We construct light curves in the soft (0.5–2 keV) and hard (2–10 keV) bands and derive hardness ratios. The X-ray emission reached its minimum about 10 days earlier than in the previous five cycles. As in previous events, the spectrum softened during the decline, while the X-ray luminosity varied on short timescales. These findings strengthen the conclusion that enhanced line-of-sight absorption, such as that expected during an eclipse, does not have a significant role in producing the spectroscopic event. Instead, they indicate that the X-ray decline is caused by
stochastic variations and instabilities in the two stellar winds and their interaction, destruction of the colliding winds region, and
accretion of primary wind onto the secondary. The main process behind the deep X-ray minimum near periastron is the
accretion of mass lost by the primary
star onto the secondary
star.
Accretion at periastron passages in $\eta$ Carinae implies that
accretion occurred at much higher rates during the nineteenth-century Great and Lesser Eruptions, when the primary
star lost mass at much higher rates. Such
accretion likely launches jets. The broader implication is that
accretion events in binary systems, possibly with jet launching, power other luminous blue variable giant eruptions and intermediate-luminosity optical transients.
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Solar and Stellar Astrophysics
Meerkat measurement of the radio emission from massive stars in the Galactic plane I. Wolf-Rayet stars
Mathilde Tasseroul, Michaël De Becker, Paula Benaglia et al. (+1 more)
Massive
stars, including Wolf-Rayet
stars, are predominantly found in binary systems. These systems are known to emit both thermal emission from stellar winds and, occasionally, non-thermal emission produced by relativistic electrons accelerated in the wind-wind interaction region. We intend to provide the most complete census of
radio emission from WR
stars in the Galactic plane, using the SARAO MeerKAT Galactic Plane Survey (SMGPS) complemented by the MeerKAT Galactic Center Survey. Our main motivation is to identify hints of synchrotron
radio emission indicative of particle-accelerating colliding-wind binaries (PACWBs). We compiled an input catalogue of 428 WR
stars positionally covered by the SMGPS and the MGCS. Using the survey data, we measured the
radio emission at 1.3 GHz for detected WR
stars. We also measured the upper limits for objects located in sufficiently low
radio background regions. For detected objects, we searched for a
radio excess by comparing the measured fluxes to two different evaluators of the thermal emission from massive
star winds. We detected 23 targets and determined the upper limits for 279 WR
stars. Among the detected objects, 15 display a (significant or potential)
radio excess that cannot be explained by unresolved circumstellar emission of any kind. After removing already known PACWBs, we report the identification of 12 potential new PACWB candidates. Our study has led to the compilation of the most extensive catalogue of Galactic WR
radio emission to date, based on a homogeneous
dataset covering the Galactic plane. The low detection rate indicates either a low occurrence rate of synchrotron emission or substantial attenuation by turnover processes that are clearly dominated by free-free absorption from the WR wind material. Our results open the door to dedicated follow-up observations aimed at ascertaining the nature of the identified
radio excesses.
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All authors Mathilde Tasseroul, Michaël De Becker, Paula Benaglia, Anandmayee Tej
Solar and Stellar Astrophysics
Highly bipolar planetary nebula Hen 2-57 and its possible symbiotic core
J. Merc, J. Mikołajewska, K. Iłkiewicz et al. (+1 more)
We present a multi-wavelength analysis of the highly bipolar object Hen 2-57 based on new optical spectroscopy from the Southern African Large
Telescope (SALT), complemented by archival imaging, and long-term time-series photometry from OGLE and NEOWISE. The nebula exhibits a pronounced hourglass morphology with a
symmetry axis at a position angle of $110\pm5^{\circ}$ and an average lobe opening angle of $\sim$$53^{\circ}$. We also identify pronounced north–south brightness asymmetry, with the southern parts of lobes consistently brighter. The spectral energy distribution shows a strong mid-infrared excess that cannot be reproduced by a single-temperature blackbody, indicating the presence of a dense circumstellar dust structure, likely in the form of a disk or
torus, analogous to that observed in M 2-9. Optical spectroscopy reveals a complex kinematic structure in which the central core is redshifted relative to both the eastern ($55\pm4\ \mathrm{km\,s^{-1}}$) and western ($46\pm3\ \mathrm{km\,s^{-1}}$) lobes. While standard diagnostic diagrams place the extended lobes in the planetary nebula regime, emission-line diagnostics of the central core are consistent with those of symbiotic
stars. The same region also exhibits a steep Balmer decrement, indicative of significant internal dust extinction. Long-term OGLE photometry shows no evidence for coherent stellar pulsations, whereas NEOWISE photometry reveals a gradual mid-infrared fading of $\sim$0.25 mag between 2010 and 2024. Together, these properties support a scenario in which Hen 2-57 hosts a dust-enshrouded D-type symbiotic central engine actively shaping the surrounding bipolar nebula, making it a close analogue of M 2-9.
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All authors J. Merc, J. Mikołajewska, K. Iłkiewicz, A. Udalski
Solar and Stellar Astrophysics
On the Constancy of Binary Star Fractions in Local Star Clusters
Priya Hasan and Mohammed Saifuddin
We investigate the unresolved binary-
star fraction in a complete sample of 376 open clusters located within 1 kpc of the Sun using Gaia DR3 and ASteCA based synthetic colour magnitude diagram modelling. The binary fraction is a global photometric quantity, sensitive primarily to unresolved binaries that produce measurable broadening above the single-
star main sequence. We find that the median binary fraction of the sample is nearly constant, $0.29\pm 0.03$, with no clear dependence on cluster age, core radius, limiting radius or median mass. This apparent constancy suggests that the global binary content of nearby open clusters is regulated by competing dynamical processes, including disruption of soft binaries, survival and hardening of close binaries, mass segregation, evaporation of single
stars, and long-term cluster dissolution. The result is consistent with theoretical expectations and previous observational studies indicating that cluster binary populations can evolve internally while maintaining a relatively stable global binary fraction. Our findings provide an observational constraint for models of open-cluster formation and dynamical
evolution and highlight the need for future work combining photometric, astrometric, spectroscopic, and N-body approaches to distinguish between primordial and dynamically formed binary populations.
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Solar and Stellar Astrophysics
The Contribution of Starspots to the Lithium Spread in NGC 2264
Yan Luo, Gregory J. Herczeg, Min Fang et al. (+2 more)
Star clusters with ages of a few Myr have a previously unexplained dispersion in the lithium equivalent widths. In this work, we show that much of the lithium spread of the young cluster NGC 2264 can be attributed to the observational presence of starspots. We demonstrate that the equivalent width of Li I 6708 Angstrom changes with observed spot coverage, using a sample of 15 Weak-Lined T Tauri
Stars (WTTSs) with multi-epoch high-resolution ESPaDOnS spectra and photometry. To quantify the contribution of starspots, we construct an empirical two-component starspot model. When differences in visible spot coverage are taken into account, 68% of the WTTSs fall within the model-predicted EW(Li) dispersion for a range from 0-90% visible spot coverage. We find no correlation between the age spread or
gravity and Li I equivalent width in NGC 2264. Our results suggest that invoking starspots can explain much of the apparent lithium spread in NGC 2264 and reduces but does not fully eliminate the need to invoke rotation-driven lithium depletion or other non-standard explanations at such young ages. Future simultaneous measurements of lithium and beryllium in young
stars may provide an independent test of this scenario.
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All authors Yan Luo, Gregory J. Herczeg, Min Fang, Qinghui Sun, Xiaoting Fu
Solar and Stellar Astrophysics
Multi-thermal Structure of Solar Jets based on Combined EUV and Ha Observations
Lei Huang, Yang Guo, Zining Ren et al. (+1 more)
This study investigates the multi-thermal structure of solar jets by combining extreme ultra-violet (EUV) and Ha observations, addressing whether cool and hot components originate from the same jet, their morphological similarities and differences, and the role of mini-filaments. We analyze 70 jet events simultaneously observed by Atmospheric Imaging Assembly (AIA) and Chinese Ha Solar Explorer (CHASE) from 2023 May 25 to 2026 January 15. The events are classified into four types based on whether or not there are significant differences between the two wavebands in terms of length and width; nonlinear force-free field extrapolations are applied to examine
magnetic fields. We find that the cool and hot components correspond to different temperature parts of a unified jet, showing spatiotemporal coincidence and kinematic synchronization. Morphologically, 30% show no significant difference in length/width, 37% differ in one dimension, and 33% differ in both. About 80% involve mini-filament eruptions; jets with filaments more often exhibit morphological differences (filament non-expansion, hot shell thickness, insufficient kinetic energy, or material rarefaction). In contrast, jets without filaments tend to show consistent morphologies; when discrepancies occur, they are mainly linked to small
magnetic field
curvature, which reduces
magnetic tension and leads to less acceleration of cool material. These results highlight the complex interaction between thermal, dynamic, and
magnetic factors in shaping jet morphology.
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All authors Lei Huang, Yang Guo, Zining Ren, Mingde Ding