Solar and Stellar Astrophysics
Visual Orbits of Spectroscopic Binaries with the CHARA Array. V. HD 210763 and HD 221950
Kathryn V. Lester, Josephine Singleton, Alexandra M. Twomey et al. (+16 more)
We present the visual orbits and dynamical masses of two longer period spectroscopic binary
stars, HD 210763 and HD 221950, using long baseline interferometry with the CHARA Array and high resolution spectroscopy with the APO 3.5 m and CTIO 1.5 m
telescopes. By combining the astrometric and radial velocity observations, we solve for the full, three-dimensional orbits and determine the stellar masses to within 0.5% uncertainty and the distance to within 0.2% uncertainty. For HD 210763, we found component masses of M1 = 1.748 Msun and M2 = 1.492 Msun. For HD 221950, we found masses of M1 = 1.098 Msun and M2 = 1.031 Msun. We then estimate the effective temperature and radius of each component
star through disentangling and spectral energy distribution analyses. We compare the observed stellar parameters to the predictions of the stellar
evolution models and estimate the system ages. The primary component of HD 210763 is at the end of the main sequence while the secondary component is on the main sequence, providing a tight age constraint for this system at 1.6 Gyr. Both components of HD 221950 are still on the main sequence with an age of 3.76 Gyr. These systems have longer orbital periods, beyond the tidal circularization limit, and therefore are better proxies for single
stars and tests of stellar
evolution models than short period, eclipsing systems.
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All authors Kathryn V. Lester, Josephine Singleton, Alexandra M. Twomey, Viktoria Sargent, Sally V. Shepherd, Douglas R. Gies, Gail H. Schaefer, Cyprien Lanthermann, Jeremy Jones, Katherine Shepard, Narsireddy Anugu, Claire L. Davies, Todd J. Henry, Hodari-Sadiki Hubbard-James, Wei-Chun Jao, Stefan Kraus, John D. Monnier, Leonardo A. Paredes, Benjamin R. Setterholm
Solar and Stellar Astrophysics
New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion
L. D. Matthews, A. K. Dupree, A. M. S. Richards et al. (+1 more)
We present new spatially resolved observations of the red supergiant Betelgeuse ($\alpha$ Orionis) at seven frequencies: 22 GHz ($\lambda$1.36cm); 44 GHz ($\lambda$6.8mm); 107 GHz ($\lambda$2.8mm); 136 GHz ($\lambda$2.2mm); 224 GHz ($\lambda$1.3mm); 338 GHz ($\lambda$0.89mm); and 485 GHz ($\lambda$0.62mm) using the Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array. These observations (spanning 2021-2026) enable a characterization of the mean brightness temperature ($T_B$), shape, and
symmetry of the
star's extended atmosphere. At 22 GHz and 44 GHz $T_B$ has increased following the historic lows observed in 2019, just prior to the Great Dimming of late 2019/early 2020. We find that $T_B$ peaks at a larger projected radius ($r\sim2.7R_{\star}$) compared with measurements prior to 2016, consistent with a possible
evolution in the atmospheric density profile. In addition we find a systematic decrease in $T_B$ from $\approx3500$K at $\sim2.7R_{\star}$ to $T_{B}\approx$2220K at $\sim1.2R_{\star}$, confirming a previously reported temperature minimum between the photosphere and chromosphere. At all observed frequencies $T_B$ is lower than temperatures inferred from ultraviolet measurements at comparable radii, indicating the continued presence of cooler
plasma ($T\le T_{\rm eff}$) mixed with the warmer chromospheric material. The new measurements yield a global spectral index $\alpha=1.35\pm$0.03, with no evidence for a change in $\alpha$ over more than two decades in frequency. Recently, evidence of a close-in companion to Betelgeuse has been reported, with an orbital separation $r\sim2.3R_{\star}$. We have identified a correlation between the ellipticity of Betelgeuse, as obtained from
radio measurements, and the companion's orbital phase. The results are consistent with periodic perturbations of the atmosphere by the companion.
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All authors L. D. Matthews, A. K. Dupree, A. M. S. Richards, W. R. F. Dent
Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
Structural quasi-universality in highly magnetized differentially rotating neutron stars
Pratul Manna and Surajit Kalita
Universal relations among macroscopic properties of
neutron stars provide a powerful framework to probe their internal structures while minimizing uncertainties associated with the equation of state (EoS). Although such relations have been extensively studied for uniformly rotating
stars, their extension to differentially rotating and strongly magnetized configurations is relatively less explored. We systematically investigate
equilibrium configurations for a wide range of EoSs, rotation profiles, and
magnetic field strengths with XNS numerical simulation code. For sequences with fixed angular momentum, we analyze the dependence of normalized moment of inertia on compactness. We establish a generalized quasi-universal relation between the moment of inertia and compactness that remains remarkably insensitive to the underlying EoS across uniformly rotating, differentially rotating, and strongly magnetized toroidal configurations. For sequences with fixed angular momentum, the normalized moment of inertia exhibits a quasi-universal dependence on compactness, with deviations primarily due to
magnetic field strength and degree of differential rotation. We derive analytic expressions for these dependencies, enabling a unified phenomenological model applicable over a wide range of stellar configurations. As astrophysical applications, we quantify the systematic bias in magnetar luminosity estimates and the rotational kinetic energy of post-merger remnant of GW170817. These results extend quasi-universal relations beyond the standard assumptions of uniform rotation and weak magnetization, providing a robust framework for interpreting observations of highly magnetized and differentially rotating
neutron stars, and enabling more reliable constraints on their astrophysical properties.
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Solar and Stellar Astrophysics
The Bimodal Mass Ratio Distribution of the Hyades
Henri M.J. Boffin
The distribution of stellar mass ratios, q, in binary systems provides critical insights into the dynamical history and
star-formation processes of open clusters. Here, I re-evaluate the recently published mass ratio distribution (MRD) of a sample of spectroscopic binaries in the Hyades cluster, which was based on stellar positions in colour-magnitude diagrams. I demonstrate that the mass ratios derived in that work are statistically inconsistent with a random distribution of orbital inclinations. Furthermore, several systems yielded non-physical results. By applying a Richardson-Lucy deconvolution to the spectroscopic mass functions and assuming a random distribution of inclinations, I re-derive the MRD for this sample, showing that it is statistically different from a uniform distribution. I further find a significant dependence on the primary mass: systems with lower-mass primaries exhibit a peak near q~1, whereas more massive primaries show a distribution heavily skewed toward low mass ratios (q~0.15). These findings highlight the potential pitfalls of photometric mass ratio derivations and underscore the need for further verification with future data releases such as Gaia DR4.
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Solar and Stellar Astrophysics · Cross-listed from gr-qc
On Modeling Anisotropic Quark Stars: The Role of Anisotropy in Radial Oscillation Spectra
G. Panotopoulos
We model the compact object Cen X-3, which is considered to be a good strange
quark star candidate of known mass and radius, incorporating a negative anisotropic factor, and we compute the frequencies of the ten lowest radial oscillation modes. We introduce the anisotropy in three different manners and investigate its impact on the spectra.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
Cloud Scale Star Formation and Gas Scaling Relations in the Milky Way
Alphesunny Sarkar, Tapas Baug, Ariful Hoque et al. (+2 more)
We investigate cloud-scale
star formation in the Milky Way using a sample of 45 molecular clouds (sizes of $5-240$ pc) in the inner Galactic plane, spanning heliocentric distances of $1.1-14.4$ kpc. Masses of these clouds are derived from $^{12}$CO and $^{13}$CO emission, while stellar masses are estimated using the young stellar object (YSO) population. The studied molecular clouds have masses ranging from $\sim10^{3}$ to $2.3\times10^{6}$ $\rm M_\odot$, with
star formation efficiencies (SFE) up to 0.33. We find a tight, nearly linear scaling of the
star formation rate (SFR) with the cloud mass, indicating that more massive clouds form proportionally more
stars. The SFE, however, shows a declining trend with cloud mass. The relation between the
star formation rate surface density ($\Sigma_{\rm SFR}$) and gas surface density ($\Sigma_{\rm gas}$) exhibits substantial cloud-to-cloud scatter, indicating that the canonical Kennicutt–Schmidt law is not strongly recovered at the scale of individual molecular clouds. Incorporating the cloud free-fall time ($\rm t_{ff}$) into the
star formation scaling relation highlights its important role in regulating
star formation, although the observed relations suggest that the
star formation efficiency per free-fall time is not universal. In particular, the SFE decreases with increasing gas mass available per free-fall time. We discuss the implications of our results in the context of recent theoretical models of molecular cloud
evolution and
star formation.
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All authors Alphesunny Sarkar, Tapas Baug, Ariful Hoque, Suchetana Chatterjee, Chayan Mondal
Solar and Stellar Astrophysics
Something new under the Sun: A magnetically driven CH/CN anti-correlation
Y. Momany, L. Monaco, S. Villanova et al. (+11 more)
Spectroscopic monitoring surveys have shown that the
magnetic cycle alters the photospheric structure of the Sun, perturbing the formation of molecular lines. In parallel, the study of Globular Clusters has remained centred on the multiple stellar populations conundrum, a phenomenon defined-most notably-by the anti-correlation between CH and CN molecular bands, serving to distinguish between first-population and second-population
stars. We systematically monitored the behaviour of the CH and CN molecular bands as a function of the solar
magnetic cycle. We demonstrate that the disc-integrated solar spectrum exhibits a distinct CH/CN anti-correlation driven by the
magnetic cycle. Our results suggest that the spectroscopic and photometric anomalies defining the multiple populations conundrum may be physically linked to a differential spectral response to varying degrees of surface
magnetic coverage.
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All authors Y. Momany, L. Monaco, S. Villanova, S. Zaggia, I. Saviane, L. Girardi, G. Volpato, G. Costa, N.R. Landin, I. Yegorova, M. Montalto, M. Dima, F. R. Herpich, F. Almeida-Fernandes
Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
Radial abundance gradients of 18 elements in Galactic open clusters from infrared MWM spectra A detailed analysis of 655 giants in 133 clusters
S. Bijavara Seshashayana, H. Jönsson, V. D'Orazi et al. (+3 more)
Open clusters are powerful tools for studying the Milky Way. While large spectroscopic surveys now provide spectra for many cluster members, automated pipelines and heterogeneous membership selections can introduce systematics and inflate apparent cluster scatter. Therefore, a homogeneous re-analysis with careful membership control and an explicit treatment of departures from Local Thermodynamic
Equilibrium is valuable for establishing robust abundance gradients. The aim is to derive precise Galactic radial abundance gradients for multiple elements using open cluster giants, and to investigate how these gradients depend on cluster age. We re-analysed high-resolution infrared APOGEE Milky Way Mapper spectra from DR19 of the Sloan Digital Sky Survey for 655 open cluster members selected from Gaia data that satisfied strict quality cuts on signal-to-noise ratio. Stellar parameters and 18 elemental abundances were obtained using spectrum fitting with the Python version of Spectroscopy Made Easy, applying Non-Local Thermodynamic
Equilibrium corrections for several key atomic species. Further quality control of the results was made by visual inspection of all fitted synthetic spectra. The
metallicity of the clusters decreases with Galactocentric radius, following a global slope close to -0.06 dex/kpc. Beyond 10-11 kpc, there is modest flattening. In addition to the elements analysed in the \cite{otto2026} study, we derive open-cluster gradients for V, Cu, Zn and Yb using APOGEE spectra.
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All authors S. Bijavara Seshashayana, H. Jönsson, V. D'Orazi, K. Cunha, P. Frinchaboy, J. M. Otto
Solar and Stellar Astrophysics
You're Gonna Need a Bigger Core: Calibrating Massive Star Models against Galactic OB-type Stars
Thibault Lechien, Selma E. de Mink, Stephen Justham et al. (+3 more)
The
evolution of massive
stars above 8 M$_\odot$ depends critically on the amount of mixing above the convective core during the main sequence. However, current models typically extrapolate results from lower-mass
stars, where constraints from asteroseismology and eclipsing binary systems are more readily available. A new opportunity to study the
evolution of massive
stars and their distribution in the Hertzsprung–Russell diagram arises by combining the IACOB spectroscopic sample of over 900 Galactic OB-type
stars with Gaia distances. We use this homogeneously analyzed sample to place population-level constraints on main-sequence
evolution. We analyze the data by forward modeling stellar
evolution tracks with MESA and applying
Bayesian inference. This enables us to 1) determine a physically-motivated, data-driven location of the terminal-age main sequence, 2) constrain convective boundary mixing and resulting core masses, and 3) provide a set of massive
star models calibrated against modern data. We explore how boundary mixing depends on mass and find that it is well described by a constant overshooting parameter in the mass range of 12 to 40 M$_\odot$, with $\alpha_{\mathrm{ov}} = 0.33 \pm 0.02$, or $f_{\mathrm{ov}} = 0.028 \pm 0.003$ in the step and exponential overshooting schemes respectively. We find evidence against a continuation of the trend to increase with mass that is found at lower masses. Instead, the data does not exclude a decreasing trend at the high mass end. We find that the resulting helium core masses are 10 to 40% larger than other commonly used overshooting prescriptions. Combining our findings with existing observational constraints for low- and intermediate-mass
stars, we propose a new mass-dependent overshooting prescription for a wide range of masses. Our calibration and model set are particularly useful for population and spectral synthesis applications.
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All authors Thibault Lechien, Selma E. de Mink, Stephen Justham, Abel de Burgos, Ruggero Valli, Sergio Simón-Díaz
Solar and Stellar Astrophysics
A catalogue of high angular resolution and contrast polarimetric maps of 45 nearby AGB stars with SPHERE/ZIMPOL
Nekolgne Aymard Badolo, Eric Lagadec, Mamadou N'Diaye et al. (+7 more)
We present the largest catalogue of asymptotic giant branch (AGB)
stars, 45 targets in total, observed in polarized light at high angular resolution (~ 20 milliarcsec). The main goal of the study is to detect and characterize dust shells in the close environment of nearby AGB
stars. This work also aims to systematically classify the AGB
star circumstellar morphologies obtained with the SPHERE instrument installed at the Very Large
Telescope (VLT), thanks to its Zurich Imaging Polarimeter (ZIMPOL). We extracted and analyzed polarized intensity maps for 45 AGB
stars, constructed from polarimetric observation data obtained with the SPHERE/ZIMPOL instrument. An ellipse fitting method was applied to characterize the circumstellar envelopes. Stellar parameters (luminosity, effective temperature, surface
gravity, extinction,
metallicity) were compiled and recalculated when necessary from spectral energy distribution (SED) fitting using the Python SED fitting tool (PySSED) software. These data were then used to train a random forest machine learning model to determine the most discriminating variables for a resolved envelope around a given
star. We constructed polarization maps for all
stars in the sample, revealing a wide diversity of circumstellar morphologies. We detected 16 dusty circumstellar envelopes, including three never observed before. They display a wide range of morphologies, all of them showing a clear departure from spherical
symmetry, indicating interaction with a companion or asymmetric mass ejections. The random forest model identified optimal thresholds for several physical parameters, thus providing robust criteria to anticipate SPHERE's ability to resolve dust envelopes around AGB
stars. These results facilitate the selection of targets for future observations and contribute to a better understanding of the
evolution mechanisms of circumstellar envelopes.
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All authors Nekolgne Aymard Badolo, Eric Lagadec, Mamadou N'Diaye, Sie Zacharie Kam, Iain McDonald, Alexis Matter, Jean Koulidiati, Lyu Abe, Marcel Carbillet, Thierry Fusco
Solar and Stellar Astrophysics
Statistical insights on the decorrelation lengths of solar wind parameters at L1 point under varying conditions
Yogesh, Aparupa Apsara Baruah, Dibyendu Chakrabarty et al. (+3 more)
Understanding the spatial coherence of solar wind
plasma and
magnetic field properties is essential for interpreting multi-spacecraft observations and for characterizing the large-scale structure of heliospheric transients. In this study, we quantify the spatial correlation of six key solar wind parameters - interplanetary
magnetic field components, bulk flow speed,
proton number density, and the alpha-to-
proton abundance ratio - using simultaneous measurements from the ACE and Wind spacecraft as a function of their instantaneous separation distance. The analysis is performed separately for intervals of background solar wind, Interplanetary Coronal Mass Ejections (ICMEs), and Stream Interaction Regions (SIRs). The decay of the Pearson correlation coefficient with distance is modeled using an exponential function to infer characteristic de-correlation length scales. We find that the bulk solar wind speed is the most spatially coherent parameter in all regimes, while
plasma composition exhibits the weakest coherence.
Magnetic field coherence shows strong dependence on solar wind structure: ICMEs display near-unity correlations and the largest
magnetic coherence scales, consistent with organized, flux-rope-like configurations, whereas SIRs exhibit reduced coherence - particularly in the north - south
magnetic field component - reflecting compressed and turbulent
plasma. The background solar wind exhibits intermediate behavior, with large-scale coherence in bulk
plasma properties but shorter coherence lengths in
magnetic fluctuations. These results provide a quantitative framework for distinguishing solar wind structures based on their spatial coherence properties and have important implications for multi-point solar wind studies and space weather applications.
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All authors Yogesh, Aparupa Apsara Baruah, Dibyendu Chakrabarty, Kalyan Bhuyan, Aakash Gupta, Gregory G. Howes
Solar and Stellar Astrophysics
Towards understanding stellar variability at the sub m/s level: granulation-induced variability across the optical spectrum
Cis Lagae, Ginger Frame, Heather M. Cegla et al. (+5 more)
Detecting the radial velocity signal of Earth-mass
exoplanets requires the characterisation and removal of granulation-induced radial velocity variability from spectroscopic observations. By coupling three-dimensional (3D) hydrodynamic (HD) simulations to a radiative transfer code, we can isolate and study the effect of granulation on stellar lines. In this study we isolated the impact of granulation on spectral line shapes and shifts for the largest and most diverse synthetic spectral line sample to date. Our aims were twofold. First, we quantified how granulation affects the temporal
evolution of shapes and shifts of 72 unblended spectral lines in two wavelength regions: 5500-5600 Å and 6100-6200 Å, from disc centre to the stellar limb. Second, we investigated if spectral lines behave coherently in their line shape variability. We find that weak lines show the largest radial-velocity variability due to granulation, up to 40 m/s at disc centre and 50 m/s at the stellar limb. On the other hand, strong lines exhibit larger variability in equivalent width than weak lines across the stellar disc. In addition, the equivalent width and line depth of a spectral line are strongly linearly correlated with its radial velocity. While granulation affects all three of these quantities, planetary Doppler shifts only affect radial velocities, opening the door to new granulation-mitigation methods. Lastly, we find that the radial velocity and equivalent width of most spectral lines in our sample evolve coherently in time, with the Fe I and Ca I lines behaving the most similarly. Due to the coherency, line blends will not significantly affect the temporal behaviour of RV and line shape, as induced by granulation. Besides, the coherency between spectral lines offer the opportunity to create disc-integrated spectra of many lines simultaneously, which will be explored in future work.
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All authors Cis Lagae, Ginger Frame, Heather M. Cegla, Veronika Witzke, René Holzreuter, Sergiy Shelyag, Christopher Watson, Alexander Shapiro
Solar and Stellar Astrophysics
Performance of radio-based detection to operational monitoring M5+ class solar flares
Julia van Ravenswaaij, Ines Wilms, Ivan Ricardo et al. (+1 more)
Early detection of major solar
flares is critical for defense operations due to their potential to disturb radar and
radio systems. Typically, soft X-ray flux is used to monitor and classify solar
flares, but since this flux has to be measured in space, it means that its availability itself is dependent on space weather conditions. For this reason, in this paper, we investigated the feasibility of using ground
radio observations to monitor major (M5+ class) solar
flares. We made use of
datasets from the GOES-16 satellite and the
Radio Solar
Telescope Network in the time range between March 2023 and March 2025. An elastic net regularized logistic regression model was trained on this data, optimized through a grid search and with incorporated class weighting for class imbalance. It was found that especially higher frequencies (8800 MHz) had a reasonable ability in monitoring and predicting major
flares (precision and recall for
flare events are 53% and 65%, respectively - implying that roughly one third of
flares were not detected - with signals appearing, on average, 3 to 4 minutes before the M5 threshold is exceeded).
Radio measurements at super high frequencies can thus serve as an alternative method to monitor major solar flaring activity.
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All authors Julia van Ravenswaaij, Ines Wilms, Ivan Ricardo, Michaela Brchnelova
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
Ammonium salt formation and abundance in protoplanetary disks
Maxime Ruaud, Jean-Christophe Loison and Uma Gorti
Ammonium salts may represent an important reservoir of volatile species in Solar system primitive bodies, but the question of how and when these salts can form during the
star formation process remains unknown. In this paper, we use thermo-chemical models to study the formation of ammonium salts during the protoplanetary disk stage. We show that ammonium salts form efficiently in the inner disk midplane (i.e. $r \lesssim 50 $ au), inside the comet forming region. In this region, our model predicts that almost all the available nitrogen is in the form of salts (i.e. mainly in ammonium cyanate) at the surface of grains after evolving for 10 Myrs. For sulfur, we show that almost all the available S is in the form of ammonium hydrosulfide in the inner disk midplane. We show that inside $r\sim 30$ au, ammonium salt formation is enhanced by a
cosmic-ray-driven sink effect that progressively converts gas-phase CO and N$_2$ into carbon dioxide and salts, respectively, at the surface of grains on a timescale $\gtrsim 1$ Myr. This impacts the location of the CO and N$_2$ radial snowlines which both shift closer to the
star as a function of time.
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Solar and Stellar Astrophysics
A new methodology for inferring the plasma conditions in solar flare energetic electron source regions from in situ electron energy spectra
Samuel Carter, Natasha L. S. Jeffrey, Robert T. Wicks et al. (+1 more)
The conditions within solar
flares that lead to efficient electron acceleration are not well constrained. It is not clear whether the populations accelerated out into the heliosphere and inward into the chromosphere originate in the same regions. By analysing the energy distributions of heliospheric populations, modelling suggests that it should be possible to see evidence of their originating region(s), including the presence of hot, dense
flare plasma. By creating and utilising a novel in situ spectral analysis package called INSPEX we have performed this analysis for
flare electrons observed in situ on 09/10/2021, constructing both peak flux and fluence spectra from combined Solar Orbiter in situ electron measurements. We compare how differing methodologies for combining the
datasets influence the spectral shapes and the retrieved parameters over an energy range of 0.5-80 keV. We fit different functions to the multi-component form of the energy spectra, testing combinations of thermal and/or power law components, comparing the fit statistics. We find that the spectra can be fitted with two distinct thermal curves at energies below 20 keV, corresponding to typical corona/active region and flaring material temperatures, varying between 1.4 - 4.1 MK and 12.5 - 23.1 MK depending on the rebinning window and peak flux extraction method. This study showcases how INSPEX can provide a novel and user-friendly methodology for studying electron spectra with different instrumentation, allowing investigation of multiple spectral types and signatures of acceleration and transport. This first application provides a
benchmark case for the analysis of similar
flares.
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All authors Samuel Carter, Natasha L. S. Jeffrey, Robert T. Wicks, Georgios Nicolaou
Solar and Stellar Astrophysics
A multi-epoch spectral and photometric study to understand the nature of the interesting Be star candidate HD 249179
Suman Bhattacharyya, Blesson Mathew, S Muneer et al. (+2 more)
HD 249179, a B5
star with unclear classification as either a classical Be
star or a high-mass X-ray binary (HMXB) system, is investigated using the first multi-epoch spectral and photometric analysis over 5.5 years (2017-2023). Optical spectroscopy from LAMOST, BeSS, and the Himalayan Chandra
Telescope show large H-alpha variability (-17.6 to -3.2 angstrom, in six days, returning to -31.3 angstrom, in 2022) and the presence of Paschen and O I lines, confirming active circumstellar disc dynamics. H-alpha double peak profiles show weak V/R ratio variations 0.92–1.28 indicating one-armed density waves. TESS observations presents hybrid beta Cephei/slowly pulsating B-type variability with primary frequencies of 3.5 c/d and 1.5-1.6 c/d, and an outburst-like brightening with phase-dependent frequency
evolution. Simultaneous anti-correlation between optical brightness and H$\alpha$ equivalent width suggest moderate disc inclination. Gaia DR3 CMD analysis rules out HD 249179 as a BeXRB but instead places it in the classical Be-
star region. The BeppoSAX survey, despite previous X-ray cataloging, found no emission above the sensitivity limits. Our analysis shows that HD 249179 is most likely a classical Be
star with variable circumstellar disc activity and hybrid pulsations, and that there is no strong evidence for an HMXB classification.
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All authors Suman Bhattacharyya, Blesson Mathew, S Muneer, Sreeja S Kartha, Prakruthi Prasad
Solar and Stellar Astrophysics
The location and propagation of fine structures in type II solar radio bursts
S. Normo, K. Bhandari, A. Nadiger et al. (+1 more)
Solar eruptions such as coronal mass ejections can drive collisionless shocks that are good particle accelerators. Electrons accelerated by these shocks can be observed remotely via the electromagnetic emission they generate at low
radio frequencies. The
radio signatures of shock-accelerated electrons at the Sun are type II
radio bursts that can be used to track the propagation of the shock wave in the solar corona and beyond. However, type II
radio bursts can have complex morphologies in dynamic spectra, being composed of numerous fine time and frequency structures. Here, we aim to determine the location and propagation of the fine structures composing type II bursts using
radio imaging from the Nançay Radioheliograph. We investigate the origin of a type II
radio burst that was only co-temporal with a
flare and a coronal wave, and it was not associated with a CME eruption. The type II burst still showed complex morphology. We find that emission lanes and fine structures composing the type II burst originate from multiple locations around the
flare site. The source regions also move in peculiar non-uniform propagation directions following the shock expansion. Our findings are consistent with the idea that multiple
radio emission source regions form as a shock propagates through the solar corona.
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All authors S. Normo, K. Bhandari, A. Nadiger, D. E. Morosan
Solar and Stellar Astrophysics
Alfvénic Motions in a Stratified Open Flux Tube: Transition from Propagating to Locally Standing Motions and Implications for the Kelvin-Helmholtz Instability
Mingzhe Guo, Bo Li, Yuhang Gao et al. (+1 more)
Standing transverse waves in closed coronal structures have been widely studied as a possible route to energy dissipation, with resonant absorption transferring kink wave energy to localized Alfvénic motions and the Kelvin-Helmholtz instability (KHI) accelerating the formation of small dissipative scales. However, it remains unclear whether the same mechanism applies to the open corona, given the long-standing consensus that the KHI tends to be prohibited for propagating Alfvénic waves. Within the framework of magnetohydrodynamics (MHD), we perform three-dimensional MHD simulations of boundary-driven kink waves in a gravitationally stratified open flux tube extending from the chromosphere into the corona. We find that propagating waves in open
magnetic structures can also drive the system toward a turbulent state, with KH
vortices clearly identifiable across the flux tube. This occurs because resonant absorption transfers energy from the propagating kink waves to azimuthal Alfvénic motions near the tube boundary, and wave reflection off the gradient of the Alfvén speed subsequently enables these boundary motions to acquire a locally standing character. Our results provide a possible answer to the long-standing question of whether and how propagating waves in open
magnetic structures can generate nonlinear turbulent fine structures despite their globally propagating nature.
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All authors Mingzhe Guo, Bo Li, Yuhang Gao, Yao Chen
Solar and Stellar Astrophysics
Deformation of the CME and CME-driven shock due to interaction with the ambient solar wind: I. Modelling with Cone Model
A. Niemela, A. Valentino and J. Magdalenic
Context. The near-Earth environment is continuously impacted by the solar wind and the transients embedded in it. The largest eruptions of
plasma and
magnetic field at the Sun are Coronal Mass Ejections (CMEs), which, upon reaching Earth, can induce geomagnetic storms and disrupt our technologically advanced societies. Aims. We aim to improve our understanding of how CMEs and CME-driven shock waves interact with the ambient solar wind, and how the resulting deformations in time and space affect the accuracy of space weather forecasting. Methods. We studied two Earth-directed CMEs, observed on December 7, 2020 and October 28, 2021. To model the solar wind and CME propagation in the inner heliosphere, we used the state-of-the-art 3D MHD model EUHFORIA with the cone CME model, focusing on the regions along the main direction of propagation (MDP) and along the Sun-Earth line. Results. The deformations of the CME and the CME-driven shock can be significant. Both CMEs propagate in a variable background wind and are structured differently even at very close angular distances. The first, in a mildly structured wind, shows its stronger deformation predominantly away from the MDP; the second, in a more complex environment, develops a strongly structured shock at very different locations, including regions close to the MDP. Conclusions. These interactions also affect the characteristics of the CME-driven shock, such as the gas compression ratio across it. CMEs observed as flank encounters at Earth can be strongly affected by the ambient solar wind, with the expected differences in the arrival time at Earth reaching up to 16 hours.
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