Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
NGC 6426: Colour-Magnitude diagram and variable star population as indicators of metallicity, distance, and age
A. Arellano Ferro, R. Michel, D. Deras et al. (+3 more)
We present new VI CCD photometry of the globular cluster NGC 6426. The membership status of all point sources and, in particular, of all the variables in the field of view is discussed, including four recently detected variables. The double mode nature of variables V3 and V9 is confirmed and their periods are calculated. The presence of a cluster member anomalous Cepheid (V17) is reported. Employing only likely members we produce a decontaminated colour-magnitude diagram (CMD) differentially dereddened, which enables us to compare it with theoretical predictions and to study the variable
stars distribution. The mean cluster
metallicity and distance were calculated via the RR Lyrae V-band light curve
Fourier decomposition and an I-band calibration of the Period-Magnitude-
Metallicity relation. Further, a comparison using a
Bayesian approach between theoretical isochrones of Dartmouth Stellar
Evolution Database (DSED), with a deep HST CMD, gave us independent estimates of distance modulus,
metallicity, colour excess, and cluster age.
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All authors A. Arellano Ferro, R. Michel, D. Deras, M. Vaca Montejano, S. Muneer, I. Bustos Fierro
Solar and Stellar Astrophysics
Understanding how Coronal Heating and Magnetic Field Scaling Affect Global Properties of the Low and Middle Corona
Caroline L. Evans, Cooper Downs and Donald Schmit
The
magnetic field of the Sun is a key driver of coronal heating and solar wind acceleration. In tandem, these influence the observable morphology of the corona, along with defining the
magnetic connectivity and magnitude of open
magnetic flux. Here,we investigate how coronal heating affects the magnetothermal state of the 3D global corona by analyzing a series of data-constrained, thermodynamic magnetohydrodynamic (MHD) simulations for solar minimum conditions. We vary the overall heat deposited in the corona via modifying the input Poynting flux of a wave-
turbulence-driven (WTD) model for coronal heating and by scaling the inner boundary
magnetic field. We quantify the
magnetic, thermodynamic, and combined magnetothermal effects on our simulations. We find that increasing the overall amount of heat leads to a higher magnitude of open flux and a more visibly open white-light morphology in the K-Corona between 1.5 and 6 Rsun. We quantify how
plasma beta evolves and drives morphological differences between simulations. We also compare these runs to an extremely high-resolution reference solution that includes small-scale
magnetic information in the surface boundary condition. This latter comparison provides insight into the further development of 'subgrid' corrections for low(er) resolution models to best represent the structure and observables of the corona and inner heliosphere.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
Shortening the Baseline: Characterising Type Ia Supernovae Using $\Delta m_{10}(X)$
Joel M. Velasco, Paolo A. Mazzali, Christopher Ashall et al. (+8 more)
An explorative study of Type Ia
Supernovae (SNe Ia) light curves is presented. $\Delta m_{t}(X)$ decline-rate metrics are used to evaluate alternative characterisations of light curve diversity and the Luminosity-Width Relation (LWR). Using a sample of 362 SNe Ia from the Carnegie
Supernova Project (CSP-I and CSP-II), Gaussian Process regression is used to fit $B$- and $g$-band light curves and measure decline-rates across multiple post-maximum timescales. The LWR is calibrated using a clean subsample of SNe Ia with host-independent distance moduli and Fitzpatrick (1999) reddening corrections. Decline-rates $\Delta m_{10}(B)$ and $\Delta m_{10}(g)$ are found to capture the LWR effectively, while mitigating the severe degeneracy that affects longer timescales for fast-declining ($\Delta m_{15}(B) \geq 1.6$ mag) events. Continuous, piecewise linear models are presented that seamlessly anchor the transition between normal and fast-declining regimes. The decline-rate $\Delta m_{10}(X)$ closely approximates the results of the more complex colour-stretch parameters, requiring less post-maximum temporal coverage than $\Delta m_{15}(B)$ or $s_{BV}$, and offers an even greater sensitivity for slow decliners. $\Delta m_{10}(B)$ provides a precise and practical tool for large-scale photometric surveys and real-time observer classification.photometric surveys and real-time observer classification.
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All authors Joel M. Velasco, Paolo A. Mazzali, Christopher Ashall, Christopher R. Burns, Mark M. Phillips, Lluis Galbany, Nidia Morrell, Maximilian D. Stritzinger, Peter Hoeflich, Eric Y. Hsiao, Syed A. Uddin
Solar and Stellar Astrophysics
Population II Post-AGB Stars. I. Methodology, Initial Results, and the First Luminosity Function
S. Jean Feeser, Robin Ciardullo, Michael H. Siegel et al. (+2 more)
Post-asymptotic giant branch (PAGB)
stars share many advantages with the tip of the red giant branch (TRGB) for Population II distance measurements, including minimal internal reddening, easy identification, a solid theoretical foundation, and an expected weak
metallicity dependence. At visual wavelengths PAGB
stars are the brightest members in old populations, appearing up to $\sim2$ mag brighter than the TRGB and $\sim4$ mag brighter than RR Lyrae
stars. Stellar
evolution theory predicts that these objects occupy a narrow luminosity strip across the top of the Hertzsprung-Russell diagram, making them excellent standard candles. Observations in Galactic globular clusters (GCs) support this prediction, but a comprehensive survey is required to define their luminosity function and test for systematic errors. In this work, we construct a multi-wavelength catalog, spanning the ultraviolet (UV) to infrared (IR), for PAGB
stars in 39 Galactic GCs. We validate cluster membership with Gaia astrometry, identify 21 objects across 16 GCs using ground-based uBVI and space-based UV photometry, and perform
Bayesian spectral energy distribution (SED) fitting with stellar atmosphere models to estimate their effective temperatures, luminosities, and radii. We find that PAGB
stars occupy a narrow luminosity range centered at $\log(L_{\mathrm{bol}}/L_{\odot})=3.25\pm0.03$, with dispersion $\sigma=0.12$ dex, supporting their use as reliable Population II distance indicators. Comparisons with theoretical tracks show excellent agreement with evolutionary models. We confirm that PAGB
stars occur preferentially in metal-poor clusters with blue horizontal branches. Our results demonstrate that PAGB
stars provide a powerful probe of late stellar
evolution and a promising Population II standard candle.
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All authors S. Jean Feeser, Robin Ciardullo, Michael H. Siegel, C. J. Hapich, Akshat S. Chaturvedi
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
EK Cha b: multi-tracer detection of a protoplanet candidate in a gas-rich environment
Felipe Alarcón, C. Swastik, Stefano Facchini et al. (+2 more)
Despite sustained community efforts and advances in instrumentation and high-contrast imaging, the search for protoplanets remains difficult, with only two widely accepted systems to date in Class II systems (PDS 70 and WISPIT 2). Here we present the detection of EK Cha b, a new protoplanet candidate, identified through CO emission, millimeter continuum and 0.82 micron broadband photometry, linked to EK Cha, a young late-type M
star. The detection of a new protoplanet candidate offers an additional opportunity to advance our understanding of
planet formation. By exploiting the wealth of archival data available for nearby
star-forming regions, we aim to place EK Cha b in context, characterize its properties, and motivate future campaigns targeting similar systems. We analyzed high-spatial-resolution archival observations of EK Cha from HST, VLT/NACO, VLT/MUSE, and ALMA. We searched for EK Cha b in broadband continuum and gas tracers, reporting the measured flux with uncertainties, and upper limits for non-detections. EK Cha b shows a spectrally-resolved footprint that matches the CO J=2–1 emission, together with 0.82 micron and millimeter continuum emission. EK Cha b is spectrally resolved in CO J=2–1 emission, kinematically consistent with a circumplanetary disk. Best fits from evolutionary models place EK Cha b as a planetary mass companion, although deeper observations are required to fully confirm its nature. Although giant
planets are uncommon around M dwarfs, these
stars host the highest
planet occurrence rate while remaining severely underrepresented in protoplanet searches. EK Cha b demonstrates that such systems are now within reach, and the advent of extremely large
telescopes and improved high-contrast imaging – where the lower host luminosity relaxes the required contras– will open this overlooked regime to systematic exploration.
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All authors Felipe Alarcón, C. Swastik, Stefano Facchini, Edwin A. Bergin, Gabriele Cugno
Solar and Stellar Astrophysics
Atypical White Dwarfs in Open Clusters
David R. Miller, Ilaria Caiazzo, Jeremy Heyl et al. (+1 more)
Open clusters are powerful laboratories for investigating the formation and
evolution of white dwarfs (WDs), as their well-constrained ages link WDs to their progenitor
stars. This is particularly valuable for atypical WDs, including
magnetic WDs and those with non-hydrogen-dominated spectra, whose origins can be difficult to disentangle in the field. We assemble the largest sample to date of atypical WDs in open clusters: eight non-
magnetic WDs with helium-dominated spectra, five with featureless spectra, and five
magnetic WDs showing hydrogen or helium lines. We find no significant difference between the initial–final mass relations of hydrogen- and helium-atmosphere WDs, nor robust spatial or kinematic differences. The strongest difference from the field is a deficit of non-DAs between $0.8$ and $1.0\,\mathrm{M}_\odot$, where the field predicts $6.0^{+0.8}_{-0.7}$ WDs, yet none are observed, with a probability of only $0.37\%$. Massive
magnetic WDs show a similar deficit: only one of 20 cluster WDs with $M_{\mathrm{WD}}\geq1.0\,\mathrm{M}_\odot$ and $T_{\mathrm{eff}}\geq20{,}000\,{\rm K}$ is
magnetic, compared with 10 of 16 in the field. Together, these deficits are consistent with an additional field population of massive merger remnants, particularly high-mass
magnetic WDs and warm/hot DQs. Such objects may be underrepresented in clusters because young systems provide limited time for certain classes of delayed mergers, while older clusters lose members or dissolve; the field accumulates binary
evolution products over Galactic timescales.
Magnetic cluster WDs consistent with single-
star evolution further suggest mass-dependent
magnetic-field emergence, with fields appearing early in some massive WDs while young lower-mass WDs remain consistent with delayed emergence.
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All authors David R. Miller, Ilaria Caiazzo, Jeremy Heyl, Pier-Emmanuel Tremblay
Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
Accreting Neutron Stars in Low-Mass X-ray Binaries
M. C. Baglio, S. E. Motta, A. P. Nitindala et al. (+1 more)
Neutron star low-mass X-ray binaries (NS LMXBs) are systems in which a
neutron star accretes matter from a Roche-lobe-filling companion via an
accretion disc, releasing gravitational energy with an efficiency of order 10-20% of the rest-mass energy. Unlike black holes,
neutron stars possess a solid surface and a
magnetic field, making NS LMXBs uniquely rich laboratories for high-energy astrophysics and dense-matter physics. In this chapter we review the current understanding of
accretion onto
neutron stars in LMXBs, tracing matter from the outer disc radii to the stellar surface across five decades of observational discovery. We survey the principal classes of accreting
neutron star systems - canonical atoll and Z-sources, ultracompact X-ray binaries, accreting millisecond
pulsars, and transitional millisecond
pulsars - and discuss how their spectral states, timing properties, and multiwavelength behaviour encode the physics of the inner
accretion flow. We examine thermonuclear Type-I X-ray bursts as probes of nuclear burning and
neutron star structure, and review the boundary and spreading layers where accreted matter decelerates onto the stellar surface. We discuss relativistic jets and ionised disc winds as regulators of the
accretion-ejection cycle. Finally, we present X-ray polarimetric results from the Imaging X-ray Polarimetry Explorer (IXPE), which for the first time provide new, though model-dependent, constraints on the geometry of the emission regions - disc, corona, spreading layer, and jets - that spectroscopy alone cannot resolve, alongside complementary optical and infrared polarimetric observations.
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All authors M. C. Baglio, S. E. Motta, A. P. Nitindala, J. Poutanen
Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
Global and Local Infall in the ASHES Sample (GLASHES). III. Physical Parameters of Contracting Motion onto Dense Cores in 70 $\mu$m Dark Massive Clumps
Kaho Morii, Patricio Sanhueza, Qizhou Zhang et al. (+3 more)
Core growth is a key process in high-mass
star formation. As part of the GLASHES (Global and Local Infall in the ASHES Sample) survey, we present infall velocities and mass infall rates derived from blue-asymmetric line profiles toward cores embedded in massive 70 \textmu m-dark clumps. Robust Hill5 model fits were obtained for 70 of the 135 cores exhibiting blue-asymmetric profiles, yielding infall velocities of 0.11–1.60 km s$^{-1}$ and mass infall rates ranging from $\sim$10$^{-5}$ to $5\times10^{-3}\,M_\odot\,{\rm yr}^{-1}$. Both quantities increase systematically from prestellar candidates to warm cores and outflow-associated cores. The inferred infall velocities are close to the cores' free-fall velocities (mean $v_{\rm in}/v_{\rm ff}\sim0.75$) and $t_{\rm in}=R/v_{\rm in}$ is comparable to the free-fall time, indicating dynamical fast contraction. Hill5 infall velocity correlates moderately with the non-thermal velocity dispersion and Mach number measured from optically thin tracers, indicating that organized infall contributes to the observed line widths. The mass infall rate follows a power-law relation with core mass, $\dot{M}=1.3\times10^{-4}\,(M/M_\odot)^{0.83\pm 0.06}\,M_\odot\,{\rm yr}^{-1}$, across $0.1\,M_\odot \lesssim M \lesssim 30\,M_\odot$. The power-law index increases from $\sim$0.65 for prestellar cores to $\sim$1.10 for outflow-associated cores, likely tracing an
evolution in the dominant physical process. The Hill5 infall velocities and mass infall rates for the GLASHES cores are larger than those of cores in nearby low-mass
star forming regions but smaller than those of larger scale high-mass clumps. These results provide the first statistical constraints on core-scale mass accumulation during the earliest stages of high-mass
star formation.
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All authors Kaho Morii, Patricio Sanhueza, Qizhou Zhang, Philip C. Myers, James M. Jackson, Fumiaki Yoshida
Solar and Stellar Astrophysics
Kinematic and Dynamical Properties of Solar Neighborhood White Dwarfs from SDSS DR19 and Gaia DR3
O. Caliskan, M. Kilic and S. Bilir
We present a comprehensive kinematic and dynamical analysis of 5,837 DA white dwarfs with SDSS DR19 spectroscopy and Gaia DR3 astrometry. Using complementary spatial distributions, kinematic properties, and orbital parameters, we show that the sample is overwhelmingly dominated by the thin disk, with only minor contributions from the thick disk and the
halo. Velocity dispersions and orbital eccentricities increase systematically from the thin disk to the
halo, reflecting progressively hotter Galactic populations. Total stellar ages available for 4,829 white dwarfs reveal a clear age-velocity dispersion relation with power-law indices of 0.06-0.17, confirming that white dwarfs preserve the secular dynamical heating history of the Galactic disk. We derive a thin-disk scale-height of 123-229 pc and radial scale-length of 3.2-3.5 kpc, while the thick disk has a scale height of 865 pc and contributes approximately 6-7 % of the local stellar density. Reconstructed birth radii indicate that changes in the guiding radii represent an important component of the present-day radial redistribution of nearby white dwarfs, although the relative contributions of radial migration and orbital blurring cannot be uniquely separated from these displacement measures. These results establish white dwarfs as powerful tracers of the kinematic, dynamical, and structural
evolution of the Milky Way.
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Solar and Stellar Astrophysics
Photometric and Spectroscopic Study of Young Stellar Objects in IC 1848E Region
Shubham Yadav, Neelam Panwar, Lalita Sharma et al. (+1 more)
We present a comprehensive analysis combining multi-epoch mid-infrared (MIR) photometric and optical spectroscopic observations of young stellar candidates associated with elongated, elephant trunk-like structures (ETLSs) in the IC 1848E
star-forming region. A sample of 15 candidate YSOs, initially identified by Chauhan et al. (2011), was investigated using multi-epoch NEOWISE $W1$ and $W2$ photometry to characterize their variability properties. After imposing stringent positional and photometric quality constraints, a final set of eight robust YSOs was retained and classified into three variability categories: non-variable, secular, and
stochastic. One source,
Star #9, exhibits a pronounced curved variability in the MIR light-curve and color-magnitude domains, indicative of systematic
evolution of its circumstellar disk and/or
accretion processes. Optical spectroscopic data for this object show strong H$\alpha$ emission and prominent forbidden lines, providing clear evidence of active mass
accretion and outflow phenomena. The combined photometric and spectroscopic diagnostics establish a close link between infrared variability, circumstellar disk
evolution, and
accretion-driven activity in YSOs associated with ETLSs in IC 1848E.
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All authors Shubham Yadav, Neelam Panwar, Lalita Sharma, C. Rishi
Solar and Stellar Astrophysics
ROSE (Red-giant Oscillations Spectra Estimator): A Modular Machine-Learning Framework for Automated Asteroseismic Characterisation. I. $\nu_{\max}$ and $\Delta\nu$ from TESS
Nipun Ghanghas, Dinil B. Palakkatharappil, Rafael A. Garcia et al. (+1 more)
Space-based photometry from Kepler and TESS has delivered oscillation spectra for hundreds of thousands of red giants, and PLATO and the Roman Galactic Bulge Time-Domain Survey will add more. We present ROSE, a modular machine-learning framework for automated asteroseismic characterisation of red giants, and apply two of its modules to the frequency of maximum oscillation power, $\nu_{\rm max}$, and the large frequency separation, $\Delta\nu$. Each module is a neural network bundled with its own preprocessing transform and output binning, so that each parameter is measured in the representation that displays it most effectively: $\nu_{\rm max}$ from the power-density spectrum, and $\Delta\nu$ from the power spectrum of the power spectrum taken over a window centred on $\nu_{\rm max}$. Both are trained only on synthetic spectra generated at a one-year baseline and both return probability distributions rather than point estimates. This enables us to retrieve asymmetric uncertainty and a reliability index. Validating against held-out synthetic spectra, Kepler spectra at the training resolution and TESS continuous-viewing-zone giants, we obtain robust dispersions of 1.4, 3.3 and 4.4% in $\nu_{\rm max}$ and 0.35, 0.61 and 1.3% in $\Delta\nu$, with the $\nu_{\rm max}$ uncertainties close to their nominal coverage. The fully automated pipeline was applied to 274,721 TESS red-giant candidates observed for at least ten sectors, the framework returning reliable $\nu_{\rm max}$ for 56,224
stars and reliable $\Delta\nu$ for 39,939, taking 8 ms per
star. The sample follows $\Delta\nu = 0.278 \nu_{\rm max}^{0.757}$ with a scatter of 6.2%, and the red-clump phase stands out as a distinct overdensity, although neither module is provided an evolutionary label or asked to produce one.
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All authors Nipun Ghanghas, Dinil B. Palakkatharappil, Rafael A. Garcia, Shravan Hanasoge
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
A Recurrent Infrared Modulation in the Warm Post-impact Debris Disk NGC 2547 ID8
Xiaodian Chen, Shu Wang and Zhuochao Huang
Extreme debris disks are signposts of violent rocky collisions, but it remains unclear whether their mid-infrared variability traces transient dust clouds or longer-lived post-impact reservoirs. We analyze a 19 yr mid-infrared record of the young solar analog NGC 2547 ID8, combining four spectra from 2007-2024 with 3-5 micron monitoring through 2026. The optically thin mineral surface remains compositionally stable: broad fits spanning multiple grain sizes give 550-577 K, and a common-basis decomposition with T_surf fixed at 560 K is dominated by 1 micron olivine/Mg-olivine and 0.1 micron forsterite. The W2-equivalent 4.5 micron excess, traced across six recurrent high states, is instead driven mainly by changes in the hot-base continuum component and defines a 956.8 d infrared recurrence timescale with cycle-to-cycle variations in timing and amplitude. This combination supports the interpretation of ID8 as an early post-impact stage in which material on eccentric orbits repeatedly interacts with a warm rocky reservoir, preserving the impact's mineral fingerprint while the interaction geometry evolves. Material on eccentric orbits can persist for decades, and possibly longer, while interacting with debris disks. Longer-baseline, higher-cadence infrared monitoring of ID8 will further constrain the stability of its recurrence timescale and the detailed
evolution of its infrared variability.
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Solar and Stellar Astrophysics
Wave-Growth-Limited Particle Confinement in a Near-Parallel Interplanetary Shock Observed by Parker Solar Probe
A. Kouloumvakos, I. C. Jebaraj, M. A. Malkov et al. (+11 more)
Collisionless shocks confine the particles they accelerate with the upstream wave field they themselves drive. Streaming particles resonantly amplify Alfvén waves, and the waves then confine the same particles by pitch-angle scattering. The rigidity a shock precursor can hold is therefore capped by how fast those waves grow, and that coupling has not previously been constrained in-situ at any interplanetary shock. Here we report Parker Solar Probe measurements of a fast, near-parallel coronal mass ejection-driven shock at 0.24~AU on 2023 March 13 that constrain this wave-particle coupling through the energetic-particle population it accelerates. We measure the spectral folding energy, the boundary between the confined and the upstream-escaping
protons, and find that it falls from $\sim$6 to $\sim$1.5~MeV as $E_\mathrm{fold}\propto\Delta t^{\,\alpha}$ with $\alpha\simeq0.34$, where $\Delta t$ is the time remaining before shock arrival. One precursor convective time before arrival this boundary is at $E_\mathrm{c}\simeq 2.46$~MeV, a characteristic confinement energy of a precursor of scale $L_\mathrm{EP}$. Read through the quasi-linear marginal-stability condition, the same
evolution implies a resonant wave-growth timescale $\tau_\mathrm{growth}(E)\propto E^{1/\alpha}\simeq E^{2.92}$ that exceeds the precursor convective time above $E_\mathrm{c}$. The post-shock pitch-angle anisotropy reverses sign across the same confinement–escape transition at 2–5~MeV. The rigidity to which a shock precursor can confine particles is therefore set, in-situ, by how quickly the streaming population builds its own confining waves.
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All authors A. Kouloumvakos, I. C. Jebaraj, M. A. Malkov, G. D. Berland, M. Gedalin, N. Wijsen, C. M. S. Cohen, O. V. Agapitov, D. J. McComas, D. G. Mitchell, E. Husidic, S. Yadav, M. E. Hill, R. McNutt
Solar and Stellar Astrophysics
On the origin of the short-period Blue Straggler Stars in Collinder 261: a binary evolution approach
M. Echeveste, M. J. Rain, G. Carraro et al. (+3 more)
Building on the recent publication of a fresh catalog of blue straggler
stars in Galactic open clusters using Gaia DR3, we conduct a study focusing on the blue straggler
stars with short-period photometric variability found in the open cluster Collinder 261: nine eclipsing or ellipsoidal binaries and five SX Phe pulsator candidates. Our investigation delves into the potential of mass transfer in binary systems to account for the presence and properties of these
stars. We first measure their photometric periods and classify the variables into eclipsing/ellipsoidal binaries, SX Phe pulsator candidates, and instrumental aliases or blends. Subsequently, we perform a set of simulations based on comprehensive calculations of the structure of both
stars within the binary system, as well as their orbital
evolution and mass transfer rates. To compare with observations, we solved the atmospheric structure of each stellar component and computed filter-weighted integrated fluxes in the Gaia DR3 passbands, obtaining a combined evolutionary sequence that accounts for the unresolved flux of both
stars as seen photometrically. Our investigation includes the study of several parameters, such as the initial mass of both
stars, the initial orbital period, and the efficiency of mass transfer. The aim is to identify potential progenitor candidates matching color-magnitude diagram observations at the same age as that determined for Collinder 261. We find that binary
evolution serves as a viable mechanism for producing these eclipsing blue stragglers and that the efficiency of mass transfer plays a key role in accurately matching the fainter objects.
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All authors M. Echeveste, M. J. Rain, G. Carraro, O. G. Benvenuto, J. A. Panei, M. A. De Vito
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
Astrometric planet search around southern ultracool dwarfs: V. Discovery of the exoplanet candidate PALTA-1 b
J. Sahlmann, P. F. Lazorenko, E. L. Martín et al. (+3 more)
We report the discovery of PALTA-1 b, a sub-Jupiter mass
exoplanet candidate orbiting the nearby early-L dwarf DENIS-P J1253108-570924 (DE1253-57) located at a distance of ~17 pc. The
exoplanet was discovered by monitoring the L dwarf's astrometric motion with FORS2 at the Very Large
Telescope over more than eight years and characterising the deviations from the expected single-source motion. The estimated
planet mass is between 0.54 MJup and 0.77 MJup, depending on the uncertain mass of the ultracool dwarf host. The system's mass ratio is about 0.010, and the relative separation is estimated at 1.6-3.4 au. If confirmed, PALTA-1 b would be one of the very few known examples of a giant
planet orbiting a nearby ultracool dwarf. This discovery finalises a larger astrometric
planet search survey that targeted 20 nearby ultracool dwarfs and previously discovered three new ultracool binary systems.
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All authors J. Sahlmann, P. F. Lazorenko, E. L. Martín, D. Ségransan, M. Mayor, S. Udry
Solar and Stellar Astrophysics
The existence of null points of flow and magnetic fields as prerequisites for the presence of astropauses and their co-location
Dieter H. Nickeler, Kuljeet S. Saddal, Rodrigo Meneses Pacheco et al. (+1 more)
Astropauses can be defined via
magnetic or fluid separatrices. For the boundary layer between the stellar wind and the local interstellar medium, two separatrices can occur, the magnetopause and the hydropause. The question is whether they are identical. To address this, we investigate under which conditions the null points of the
magnetic and velocity field, which define the corresponding separatrices, coincide spatially. We analysed the case of a non-monotonous pressure distribution driving stationary counterstreaming ideal HD flows and demonstrate that the existence of an extremum of the pressure in the 3D case together with a non-degeneracy condition on the velocity field implies that this point is also a stagnation point. Turning to ideal MHD and considering the regularity of all fields, we formulated the momentum equations, revealing their quasi-linearity. This allowed us to prove the occurrence of null points by examining the rank of the Jacobians of the vector fields. We found that for a given null point of either the velocity or the
magnetic field at the pressure extremum, the pendant vector field has a co-located null if the Jacobian of this field is non-singular. We extended the analysis to resistive MHD and further to MHD with general non-ideal terms and proved that if at least one of the first-order momentum equations is ideal, the stagnation point and the
magnetic null point are co-located. Even for general fluid theory, the two null points co-locate as long as the curl of the generalised non-ideal terms is zero at one of them or can be represented by a linear operator in the
magnetic and velocity field. In general, it is not excluded that the velocity and
magnetic nulls are dislocated. However, this would require specific properties of the curl of the non-ideal term, which would reduce the probability of a relaxed quasi-stationary configuration.
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All authors Dieter H. Nickeler, Kuljeet S. Saddal, Rodrigo Meneses Pacheco, Michaela Kraus
Solar and Stellar Astrophysics
A Jupiter-like radio aurora in the L3.5 dwarf LSPM J0036+1821
D. Martín-Carrero, J.B. Climent, P.A. Miles-Páez et al. (+6 more)
Ultracool dwarfs exhibit
radio emission revealing strong
magnetic fields and complex magnetospheric processes. Their auroral emission resembles that observed in magnetized Solar System
planets, providing a framework to investigate their
magnetic topology. We aim to characterize the
magnetic activity, rotation, and atmospheric variability of the L3.5 dwarf LSPM J0036+1821 through a multiwavelength analysis. We conducted a long-term monitoring campaign combining
radio observations from the Very Large Array and very long baseline interferometry arrays with TESS optical photometry, spanning 2019-2023. We analyzed time-series data to derive precise rotation periods and constructed light curves and
radio dynamic spectra. The
radio lightcurves were modeled using an auroral framework analogous to Jupiter's magnetosphere, incorporating main oval and active field line components. We detect persistent, compact
radio emission with significant variability and high circular polarization. Periodogram analysis yields a rotation period of 3.07941 $\pm$ 0.00020h from
radio data, consistent with the TESS optical period (3.07908 $\pm$ 0.00038h). The
radio light curves display stable morphology characterized by asymmetric, rotationally modulated pulses. Our modeling reproduces the emission using a hybrid auroral scenario, with the main oval contributing a stable component and an active field line producing dominant, variable bursts. Short-term variability is explained by small changes in emission cone geometry or longitude of the active field line. The
radio emission of LSPM J0036+1821 is consistent with auroral processes driven by the electron cyclotron maser instability in a structured magnetosphere. The behavior is consistent with a Jupiter-like auroral system, possibly involving companion-driven or internally-driven magnetospheric interactions analogous to those observed in the Solar System.
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All authors D. Martín-Carrero, J.B. Climent, P.A. Miles-Páez, J.Balmón-Antón, J.C. Guirado, M.M. Kao, C. O'Toole, J.S. Pineda, J.M. Vos
Solar and Stellar Astrophysics
A Novel Approach to Identify Unseen Companions: An In-Depth study of GDR3 220_28
T. B. Cysil, Blesson Mathew, B. Shridharan et al. (+6 more)
Emission lines in the spectrum and the excess seen in the spectral energy distribution (SED) at infrared (IR) wavelengths serve as crucial indicators of
accretion activity and the presence of gas and dust in the circumstellar environment of a young
star. As
accretion ceases over several million years, the H$\alpha$ emission diminishes. However, IR excess combined with negligible H$\alpha$ emission could also extend to indicate certain close binary systems, where a circumbinary disk contributes to the observed excess. Additionally, in unresolved binaries, companion
stars can contribute to the photometry, leading to inaccuracies in parameter measurements and skewed positions in the color-magnitude diagram (CMD). We identify a photometrically peculiar object from a survey of the Cepheus
star-forming regions (SFRs), Gaia DR3 2205212960562531328, exhibiting a shift from its expected position in the CMD, as indicated by the Gaia optical photometry. This peculiarity drives our research, prompting an investigation through an approach that combines astrometric accuracy, photometric modeling and spectroscopic studies. The Gaia XP spectrum of the source shows a systematic blue excess, indicating an additional hotter unresolved component. Two-component SED modelling yields a companion with $T_{\rm eff}\sim27000$ K and $\log g\ge4.5$, consistent with sdB
stars. Further CMD and spectroscopic analyses support a binary system consisting of an A0-type PMS
star with an sdB companion. In addition, ZTF photometry shows tentative evidence for a periodicity of 0.498 days. This study paves the way for a systematic search of similar binary systems that would otherwise remain unidentified.
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All authors T. B. Cysil, Blesson Mathew, B. Shridharan, S. Muneer, Pramod Kumar S, S. Nidhi, Sreeja S Kartha, Suman Bhattacharyya, T. Robin
Solar and Stellar Astrophysics
Light curve spectral energy distribution fitting of 13 carbon-rich Mira variables stars in the Small Magellanic Cloud
A. Liberatori, M. Trabucchi, D. Hatzidimitriou et al. (+8 more)
Carbon-rich Mira variables are highly dynamic
stars characterized by large-amplitude radial pulsations and intense mass-loss. Standard spectral energy distribution (SED) fitting often relies on time-averaged observations, which flatten intrinsic variability and yield only approximate stellar and circumstellar parameters. To overcome these limitations, we dynamically trace the physical parameters of 13 carbon-rich Miras in the Small Magellanic Cloud (SMC) throughout their pulsation cycles. Combining time-series photometry from $Gaia$, WISE, VMC, 2MASS, and OGLE, we reconstruct simultaneous multi-band light curves. We extract independent SEDs at 14 distinct phases and model them using the ATHENA-C grid. This phase-resolved approach traces cyclic variations in effective temperature ($T_{\rm eff}$), optical depth ($\tau_{0.55}$), and bolometric luminosity. We find peak-to-peak $T_{\rm eff}$ variations ranging from $\sim 100$ K to $>1000$ K. A strong anti-correlation between $\tau_{0.55}$ and bolometric luminosity indicates the dust condensation zone is pushed outward at maximum brightness, rendering the circumstellar environment optically thinner. We also recover the classical Period-Luminosity relation and derive radial expansion velocities and radius variation amplitudes, which are consistent with independent measurements. Phase-resolved fitting reveals the true dynamic ranges of these parameters, which static fits fail to capture. Although limited by grid sampling and gaps where converged hydrostatic models are unavailable, explicitly accounting for time-dependent behavior provides a much more physically realistic picture of the interplay between stellar pulsation and dust formation. This highlights the critical need for time-resolved methodologies when studying large-amplitude variables.
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All authors A. Liberatori, M. Trabucchi, D. Hatzidimitriou, M.A.T. Groenewegen, A. Bressan, G. Pastorelli, L. Girardi, K. Shepherd, B. Aringer, A. Udalski, A. Nunnari