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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Tuesday 15 September

Solar and Stellar Astrophysics

A self-consistent orbital architecture for GG Tau A. I. Simultaneous orbital fitting of the hierarchical triple

A. Lacquement, H. Beust, G. Duchêne et al. (+4 more)

GG Tau $A$ is a young triple system with the close pair, $Ab_1$–$Ab_2$, and $Aa$, surrounded by a massive circumtriple disk with a large inner cavity difficult to explain in a binary framework. We aim to determine the orbital architecture and individual stellar masses of GG Tau $A$ and assess how the available astrometric and disk-based constraints restrict the range of admissible solutions. We performed a joint fit using Oracle, developed specifically for hierarchical stellar systems. All astrometric measurements were placed in a common reference frame, since the historical wide-orbit astrometry is given relative to the unresolved photocenter of the $Ab$ subsystem. The fit included one new wide-orbit astrometric epoch and a prior on the total stellar mass derived from disk kinematics. We then applied, in post-processing, an additional geometrical constraint based on the observed center of the circumtriple disk. The fit yields orbital solutions compatible with the available astrometric and disk-based constraints and provides estimates of the individual stellar masses. The additional wide-orbit epoch only marginally reduces the range of admissible solutions. By contrast, the disk-center constraint leaves the favored orbital architectures largely unchanged but significantly tightens the stellar-mass partition. This yields posterior masses of $0.521^{+0.069}_{-0.051}$, $0.106^{+0.017}_{-0.013}$, and $0.79^{+0.10}_{-0.10}$ $M_\odot$ for $Ab_1$, $Ab_2$, and $Aa$, respectively. The reported values are posterior medians with 16th–84th percentile intervals. A joint treatment of the two orbital levels is required to recover a physically meaningful architecture and constrain the individual stellar masses. The resulting solutions provide a basis for future dynamical modeling of the circumtriple disk and for testing whether the observed cavity can further constrain the system architecture.
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All authors A. Lacquement, H. Beust, G. Duchêne, C. Lawlor, C. Ginski, E. Di Folco, A. Dutrey

Solar and Stellar Astrophysics

Count-based spectral component imaging (CCI) of solar flares in X-rays

Alessia Guidetti, Muriel Zoë Stiefel, Paolo Massa et al. (+3 more)

X-ray emission in solar flares is produced by both multi-thermal plasma and accelerated electrons. Classical imaging approaches reconstruct X-ray intensity maps which contain contributions from multiple spectral components (e.g., hot, superhot, and non-thermal), and do not allow retrieving the morphology of the different components separately. We introduce a novel imaging technique, called "Count-based spectral Component Imaging (CCI)", to jointly reconstruct spatially resolved emission measure maps of the thermal components, and the electron flux distribution of the non-thermal component from data provided by the Spectrometer/Telescope for Imaging X-rays (STIX) aboard Solar Orbiter. We formulate a linear model linking the Differential Emission Measure (DEM), approximated by two thermal components, and the non-thermal electron flux to the observed counts. The resulting inverse problem is solved with the Richardson-Lucy algorithm. We apply CCI to STIX observations of SOL2024-10-01T22 and compare it with the previously developed Spectral Component Imaging (SCI) method, as well as classical imaging approaches. The reconstructed thermal and non-thermal components show good agreement with those obtained using SCI. This proof-of-concept study shows that CCI obtains results consistent with SCI but with fewer inputs. In contrast with SCI, CCI can also be applied to hard X-ray focusing optics imaging.
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All authors Alessia Guidetti, Muriel Zoë Stiefel, Paolo Massa, Anna Maria Massone, Michele Piana, Säm Krucker

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

No statistically significant evidence for a correlation between stellar and planetary composition

Daisy A. Turner, Annelies Mortier, James G. Rogers et al. (+4 more)

The chemical compositions of planets and their hosts stars are intrinsically linked, having formed from the same protostellar material. Characterising this relationship provides key constraints on the processes governing planetary formation and evolution. Focusing on host stars with near-solar chemical abundances limits our understanding of how planetary composition varies across a broader chemical parameter space. Expanding the sample to include stars with compositions markedly different from the Sun is therefore essential for building a complete picture of star–planet compositional connections. Here, we focus specifically on iron-poor hosts since they are more likely to be alpha-enhanced and represent some of the most chemically distinct stars relative to the Sun (i.e., thick disc stars). We present a sample of 45 stars hosting 64 planets across the super-Earth and sub-Neptune regimes, for which we homogeneously obtain new stellar parameters including abundances, re-derive planetary mass and radius, and model the bulk interior compositions of their planetary companions. No statistically significant evidence for a correlation between stellar and planetary composition was found in our sample. We suggest that this null result is primarily driven by the large uncertainties inherent to both compositional proxies, which may obscure an underlying relationship. Quantifying these uncertainties is therefore a critical step that previous studies have not fully addressed. Furthermore, even with improved precision, uncovering such a relation may require a higher-dimensional treatment that accounts for additional parameters such as planetary equilibrium temperature.
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All authors Daisy A. Turner, Annelies Mortier, James G. Rogers, Jo Ann Egger, Alix V. Freckelton, Tim Lichtenberg, Anjali A. A. Piette

Solar and Stellar Astrophysics

Stellar age is not disk age

M. Kuffmeier, A. Winter, A. Kuznetsova et al. (+2 more)

Disk fractions in star-forming clusters decline with stellar age, and are routinely read as the lifetime distribution of primordial protoplanetary disks. We argue that this reading is not warranted, because late infall can replenish or reform disks after collapse. To formalize this, we model the disk fraction as a superposition of primordial and environmentally replenished components, with region-to-region dispersion in the supply rate. Disk fractions cannot distinguish a long-lived disk from a short-lived one that is repeatedly replenished. In Sco–Cen both reproduce the data, the replenished model marginally better. Stellar age therefore need not equal disk age, and stars can host successive disk generations with distinct mass, orientation, and chemistry. A key test is whether regions of the same stellar age show a genuine spread in disk fractions, as environmental variation in the replenishment supply predicts.
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All authors M. Kuffmeier, A. Winter, A. Kuznetsova, M. Vioque, A. Gupta

Solar and Stellar Astrophysics

The evolution of discs and the epoch of formation of giant planets around solar type stars

Raffaele Gratton and Elisabetta Rigliaco

Disc evolution dictates the time for planet formation providing the raw material to build the planets' cores. Dynamical stellar masses derived from the kinematics of the gas in discs provide constraints on the ages of very young stars, offering a powerful approach to studying disc evolution and the epoch of giant-planet formation. We investigated a sample of 33 solar-type stars with bright protoplanetary discs and ages between 1 and 9\,Myr. We modeled the spectral energy distributions and ALMA dust continuum images simultaneously to constrain the stellar radius and effective temperature, accretion rate, disc dust mass, the radii of the inner and outer disc regions (roughly separated at the ice line), and the maximum grain size in the inner disc. Combining the stellar masses and radii with stellar evolutionary models, we derived individual stellar ages. We found that the accretion rate and the dust mass of the inner disc evolve with time. The dust mass of the inner disc correlates with the observed accretion rates for stars younger than 2\, Myr. A depletion of centimetre-sized grains in the inner disc is observed in more than half of the analysed stars with ages between 2 and 5\, Myr indicating the action of a mechanism that inhibits the inward drift of large grains, potentially associated with the progressive growth of giant-planet cores more massive than 30 \MEarth, although the exact mass threshold remains uncertain. In addition, three discs exhibiting strong asymmetries in their ALMA continuum emission are associated with stars with ages of $\sim$2\, Myr. The results provide observational indications of significant changes in the dust distribution during the epoch when giant planets may be forming.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Around the water snowline: I. Effects of dynamical and chemical interplay on mid-infrared water spectra of protoplanetary discs

Till Kaeufer, Sebastiaan Krijt, Edwin A. Bergin et al. (+2 more)

Protoplanetary discs' infrared water spectra (e.g. as observed by JWST) encode information about the underlying water distribution and physical conditions. In this work, we examine and quantify the imprint of various dynamical processes (operating alone or in unison) on water abundances and emerging spectra using a new 2D model called Molecular emission Affected by Gas, Pebble, and Ice Evolution (MAGPIE). This model combines diffusion and advection for water (and associated species) and dust grains with aerodynamically decoupled pebbles, and photodissociation and a gas-phase water formation prescription. In general, we find that transport processes have a major impact on the inner disc's water content and can (re)plenish the layer above the ice reservoir which in static models is water-poor due to photodissociation. In particular, adding diffusion to a chemical model can result in strong changes in the spectrum (e.g. a flux increases of colder compared to warmer water lines) that recent work has associated with pebble drift. Adding pebble drift dramatically increases the inner disc water abundance but, surprisingly, does not readily lead to a further increase of cold water tracers as (1) efficient outward transport of water vapour is inhibited by a conveyor belt effect of incoming pebbles, and (2) the increase in hot water line fluxes overshadows smaller increases of colder lines. We conclude that dynamical processes have a significant but complex impact on water spectra. Future work should explore a range in stellar (e.g. UV irradiation) and disc properties to comprehensively understand the observed diversity of water emission.
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All authors Till Kaeufer, Sebastiaan Krijt, Edwin A. Bergin, Andrea Banzatti, Joe Williams

Solar and Stellar Astrophysics

Promise and pitfalls of plasma emission as an exo-space-weather tool

Ivey Davis

Significant observational effort has been spent on identifying stellar analogues to solar type II and III bursts, which are plasma emission that is produced when coronal mass ejections and fast electron beams, respectively, propagate through the corona and wind. The sensitivity of these bursts to ambient plasma and their association with energetic transient particle flux makes them powerful diagnostics of exo-solar space weather. However, analyses of stellar bursts often rely on the assumption that the coronae of the stars that produce them are similar to the solar corona or that the instability requirements are the same as for the Sun. By extension, the assumption is that the forms of stellar bursts are very similar to that of solar bursts. I introduce a simple framework and the python package, swabs, to predict the shapes of stellar type II and III bursts to understand how they may deviate from the shapes of solar bursts. I present the burst expectations in the range of 10–170 MHz for two well-studied active stars: the M dwarf AD Leonis, and the G dwarf EK Draconis. The results for these stars suggest that the coronae of active stars may lead to bursts that have substantially (1) delayed arrival times to the observing band, (2) shallower drift rates, and (3) longer durations than solar bursts. The hot and fast winds may also inhibit the development of type~II and III bursts, regardless of the Alfvén speed. Non-isothermal winds reduce this inhibition, but may increase the delays and durations of the bursts. This work demonstrates that stellar bursts may deviate radically from their solar analogues. These deviations need to be considered in how we conduct searches for stellar bursts and how we evaluate the coronal conditions that cause the burst structures.
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Solar and Stellar Astrophysics

Triple coronal hard X-ray source observed by STIX during a failed filament eruption

Tomasz Mrozek, Marek Stęślicki, Sylwester Kołomański et al. (+1 more)

{Observations reveal hard X-ray sources in the solar corona with morphologies and locations distinct from typical solar flares. One expected mechanisms for hard X-ray (HXR) production in the solar corona is eruptions of various types. Failed eruptions are promising targets for detecting non-flare-related HXR sources because most of their energy is expected to be dissipated in the corona. } {We investigated the SOL2022-0 to determine the nature of coronal HXR sources associated with the event.} {We used Solar Orbiter's STIX, EUI, and SDO AIA observations. We reconstructed STIX images using the MARLIN algorithm. We used EUI data to cross-check the view of the event from two vantage points. The AIA images enabled kinematic analysis and reconstruction of differential emission measure (DEM) maps. We then used the AIA DEM to predict X-ray emission maps and compared them with STIX images. } {We find temporal and spatial correlations between the failed eruption and HXR sources. During the braking of the filament eruption, we observed non-thermal HXR emission from coronal sources along the path of the eruption. Thermal HXR emission was concentrated in three coronal sources formed with two separate mechanisms: chromospheric evaporation and direct heating. Directly heated sources are the result of the interaction of the eruption with the overlying magnetic field, and were observed to cool for an extremely long time.} {The HXR emission sources related to the interaction between the eruption and the overlying magnetic field might be typical structures in all flares. However, they appear to be weak, since their emission is only 5-20~% of that from the flare coronal source. If footpoint sources were not occulted, then these weak coronal emission sources would not be visible due to the low dynamic range of HXR telescopes.}
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All authors Tomasz Mrozek, Marek Stęślicki, Sylwester Kołomański, Krzysztof Barczynski

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Testing a Pre-Supernova Contribution to $^{44}$Ti in Cassiopeia A

Toshiki Sato, Joshua Issa, Falk Herwig et al. (+4 more)

The radioactive isotope $^{44}$Ti in Cassiopeia A has been used as a tracer of the innermost ejecta and explosion asymmetries in core-collapse supernova explosions. However, recent stellar-evolution models suggest substantial $^{44}$Ti may also be synthesized prior to explosion during oxygen-carbon (O-C) shell mergers, a process proposed for the Cas A progenitor. We investigate this pre-supernova contribution by combining NuSTAR measurements of the spatial and kinematic distribution of $^{44}$Ti with X-ray and infrared ejecta diagnostics. The low- and intermediate-velocity $^{44}$Ti components are more closely associated with O-layer tracers than with shocked Fe-rich material. If the $^{44}$Ti associated with the O-layer tracers was synthesized prior to core collapse, it could contribute up to $\sim7\times10^{-5}~M_\odot$, about half of the total. We find that this amount of $^{44}$Ti can be produced in O-C shell-merger models with ingestion rates and convective velocities higher than those in standard one-dimensional stellar-evolution calculations. Stable Fe-group abundance ratios measured with XRISM provide an additional constraint on the explosive contribution, as three-dimensional models with larger $^{44}$Ti yields tend to predict Ti/Fe and Mn/Cr ratios above the observed values. These results suggest that both pre-supernova shell-merger nucleosynthesis and explosive burning contribute to the observed $^{44}$Ti.
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All authors Toshiki Sato, Joshua Issa, Falk Herwig, Yui Kuboike, J. Martin Laming, Kai Matsunaga, Ryo Sawada

Solar and Stellar Astrophysics

The study of AF And. I. Hydrogen abundance constraints and dynamically-consistent wind model of the hot state

E. O. Dedov, A. E. Kostenkov, A. S. Vinokurov et al. (+1 more)

This paper starts a series of works devoted to an in-depth study of luminous blue variable AF And in the M31 galaxy. Here we present the results of non-LTE modelling of the star in a hot state with CMFGEN code. In addition to the $\beta$-velocity law, widely used in the analysis of stellar atmospheres, we used a velocity distribution obtained from the balance of radial forces in the wind, including centrifugal forces for several models. Such a dynamically consistent approach reduces the parameter space during modelling and, in addition, allows us to estimate the current stellar mass whose inner hydrostatic layers are obscured by a dense wind that contaminates the absorption profiles. Based on our modelling we estimate the parameters of AF And as follows: luminosity $L \approx 6.5\times10^5~\text{L}_\odot$, mass loss rate $\dot{M}$ $\approx 3\times10^{-5}~\text{M}_\odot~\text{yr}^{-1}$, terminal velocity $v_\infty \approx 300~\text{km s}^{-1}$ and effective temperature $T_{\rm eff} \approx 17~\text{kK}$. modelling of the hot state of AF And is of particular interest due to the possibility to place constraints on the ratio of hydrogen and helium abundances for further use in constructing models of the cold state of this star. We conclude that the current hydrogen mass fraction in the wind lies in the range from $20$ to $30$ per cent. Self-consistent modelling gives the current mass $M_*$ estimation in the range $15.6 - 17.7~\text{M}_\odot$. By employing evolutionary tracks, we were able to estimate the initial stellar mass as $M_{*,~init} \approx 50~\text{M}_\odot$.
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All authors E. O. Dedov, A. E. Kostenkov, A. S. Vinokurov, A. N. Sarkisyan

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Population synthesis of low-mass binaries with wind-accreting neutron stars

Marina Afonina and Sergei Popov

We perform population synthesis modeling of neutron star binary systems containing a low-mass main-sequence companion underfilling its Roche lobe. About $10^5$ such systems are expected to exist in the Milky Way. In the majority of such systems, NSs are expected to start accreting the stellar wind matter within a few Gyr, resulting in the appearance of a dim X-ray source. The exact value of this fraction mainly depends on the efficiency of the propeller spin-down, properties of the stellar wind, and common envelope efficiency. Mostly, NSs in such systems are formed in electron capture supernovae. Thus, they obtain a low natal kick. Due to this, the center-of-mass velocities of such binaries are low, too. Only in the case of high common envelope efficiency ($\alpha_\mathrm{CE}\gtrsim 3$), a few tens of long-lived high-velocity binaries can be produced in core-collapse supernovae.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Supernova nucleosynthesis: a review

Shuai Zha, Yudong Luo and Zhanwen Han

Supernovae are major drivers of cosmic chemical evolution. They synthesize heavy elements and disperse them into the interstellar medium via their explosion shocks. Light elements are converted into heavier ones during both presupernova evolution and the explosive event itself. Supernova explosions generate nucleosynthesis environments rich in neutrons, protons, and neutrinos under unique thermodynamic conditions, which can enable the production of heavy elements beyond iron. Modern numerical simulations are constructing increasingly realistic explosion models of various supernova channels, providing more accurate nucleosynthesis conditions and chemical yields. In tandem with advances in large-scale spectroscopic surveys delivering precise, high-resolution stellar abundance data, as well as isotopic ratios from presolar grains and meteorites, our understanding of the supernova role in cosmic nucleosynthesis is poised to advance significantly. We review recent progress in modeling various supernova channels, with particular emphasis on nucleosynthesis yields derived from state-of-the-art simulations. We examine the roles of Type Ia, core-collapse, electron-capture, and pair-instability supernovae in producing intermediate-mass, iron-peak, trans-iron, and very heavy elements, as well as their characteristic chemical imprints. We also outline major theoretical uncertainties that affect yield predictions. We intend this review to serve as a timely reference for theoretical model development and a practical guide for interpreting observational abundance data.
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Solar and Stellar Astrophysics

The Variation of Circumstellar Parameters through Early Stellar Evolution

Michael Connelley, Christian Flores and Bo Reipurth

We report on the evolution of extinction, mass accretion rate, and spectral index from analysis of SpeX observations and AllWISE spectral energy distributions, compared to stellar parameters (mass, age, and veiling) derived from analysis of previously reported and newly analyzed iSHELL observations of the same young stellar objects (YSOs). By using stellar gravity as an age indicator, we are able to infer age without any assumptions about the circumstellar environment. Our sample consists of 109 Class I, II, and III YSOs. The spectral index broadly decreases with time but with sufficient scatter that Class I-III YSOs can all be found at the same age. There is a wide range of extinction (A_V) from nearly 0 to greater than 30 mag for ages up to 5 Myr, showing no clear trend with age. The mass accretion rate declines from a median of 10^-8 solar masses per year at ~1 Myr to less than 10^-10 solar masses per year at ages greater than 10 Myr, with a scatter of the mass accretion rate of +/- 1 dex. Accretion seems to effectively end by ~ 5 Myr, by which time the median mass gained is 0.05 solar masses. We find a wide range of veiling (rK) from zero to three (limited by our need to detect photospheric lines) for Class I and II YSOs, with the median veiling declining after ~2 Myr. Class I YSOs appear to be physically very similar to Class II YSOs, with similar accretion rates and overlapping age and mass distributions. There are no objects with a high spectral index and low extinction, suggesting that once starved of circumstellar material, the spectral index rapidly declines.
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Monday 14 September

Solar and Stellar Astrophysics

Characterizing bright $\delta$ Scuti pulsators using TESS light curves:II. Pulsation amplitude and energy distributions

Prasad Mani, Timothy Bedding, Simon Murphy et al. (+2 more)

Pulsations in $\delta$ Scuti stars exhibit a wide range of excited frequencies and amplitudes, but the underlying principles governing their amplitude distribution remain unclear. We revisit this problem using a global, energy-based perspective. For a complete sample of 3200 bright $\delta$ Scuti stars observed by TESS, integrated power spectral density (PSD) of the Fourier spectrum allows us to measure the overall pulsation signal. From this, we derive a proxy for the total pulsation energy. We uncover a bimodality among higher-frequency delta Scuti stars in their integrated PSD, possibly suggesting the presence of two distinct pulsation regimes. In addition, we identify an apparent upper limit to the total pulsation energy, indicating a constrained energy budget for these pulsations. This feature is not evident when stars are characterized solely by their dominant mode amplitude.
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All authors Prasad Mani, Timothy Bedding, Simon Murphy, Daniel Hey, Tom Love

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve

Takashi J. Moriya, Steve Schulze, Alexander Heger et al. (+2 more)

SN 2018ibb is one of the best pair-instability supernova (PISN) candidates identified to date. It, however, showed unexpected blue flux excess in late-phase spectra, likely originating from the interaction between supernova (SN) ejecta and dense circumstellar matter (CSM). We develop synthetic light-curve models of PISNe interacting with dense CSM and estimate the CSM properties of SN 2018ibb by comparing the synthetic and observed light curves. We found that the bolometric luminosity evolution of SN 2018ibb from 150 d after the peak can be well reproduced by the interaction with the CSM formed by a mass-loss rate of 0.01(v_CSM/1,000 km/s) Msun/yr, where v_CSM is the CSM velocity. The observed luminosity break at around 300 d from the peak indicates that the CSM interaction ended at this time and the dense CSM radius was 8.5e16 cm. The CSM radius then implies that the mass loss of the progenitor was enhanced for 28/(v_CSM/1,000 km/s) yr before explosion. We conclude that PISN progenitors may experience short-term mass-loss enhancement within decades before explosion, similar to what is often observed in progenitors of core-collapse SNe.
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All authors Takashi J. Moriya, Steve Schulze, Alexander Heger, Sergei I. Blinnikov, Marat Sh. Potashov

Solar and Stellar Astrophysics

Using Gravity as an Age Indicator for Young Stars

Michael Connelley, Christian Flores and Bo Reipurth

The age of a young star is difficult to measure and yet is a fundamental property that is essential for understanding stellar evolution. Accurate age indicators across the early phases of low-mass stars are scarce and often rely on properties of the circumstellar environment. We investigate stellar surface gravity as a practical age estimator from the class I to the class III phases of low-mass stars. Gravity increases monotonically as young stars descend their Hayashi tracks. Gravity is a physical property of the central star and is independent of the circumstellar environment or evolutionary models. Also, gravity can be measured for individual objects. We performed two tests for the reliability of gravity as an age indicator: (1) the relative ages of the components of young binary stars and (2) the relative ages of a young moving group. We show that young binary stars have consistent gravities, as expected, if they are coeval. Similarly, we find similar gravities among the TW Hya group and derive a median age of 12.8 +/- 1.2 Myr. In both cases, differences in gravities may reflect real differences in the ages. We show that there is no correlation between veiling emission and gravity, showing that circumstellar disk emission does not affect our ability to infer ages from measurements of photospheric stellar gravity.
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Solar and Stellar Astrophysics

The Post-Outburst Spectrum of L1415-IRS

Lynne A. Hillenbrand, Thomas P. Greene, Evan R. Portnoi et al. (+1 more)

We present near-infrared spectroscopic observations of L1415-IRS, a Class I type young stellar object that underwent a factor of $>$30 photometric outburst in the early 2000s. Ours is the first reported spectroscopy of this deeply embedded source. Both components of L1415-IRS present continuum plus $H_2$ emission. The southern component has additional weak HI Br$\gamma$ and $CO$ emission. Based upon our spectrum and evidence of a declining lightcurve, we hypothesize that either: the outburst was not large enough and therefore hot enough to produce the FU Ori type absorption spectrum commonly interpreted as an accretion disk atmosphere, or the source has already recovered from FU Ori-type equatorial accretion to re-establish T Tauri-type magnetospheric accretion.
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All authors Lynne A. Hillenbrand, Thomas P. Greene, Evan R. Portnoi, Bringfried Stecklum

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Formation of Merging Black Hole Binaries Inside Massive AGN Stars

Sizheng Ma, Douglas N. C. Lin and David A. Velasco-Romero

Stars embedded in the disks of active galactic nuclei (AGN) can grow to hundreds of solar masses, and the same disks are expected to host a population of stellar-mass black holes. A star may therefore capture passing black holes, turning its interior into a potential factory for forming compact binary black hole systems and, ultimately, for the gravitational-wave events seen by our detectors. We test how readily this channel operates using three-dimensional hydrodynamic simulations of black hole–star encounters. As a proof of principle, we adopt a star-to-black-hole mass ratio of $34\!:\!1$, and find that the capture does not disrupt the structure of the star: it heats the star by $10$–$30\%$, and the star loses only $\lesssim1\%$ of its mass throughout. The captured black hole loses its orbital angular momentum rapidly to dynamical friction, sinking to the stellar center within a stellar dynamical time $\lesssim10^{4}\,$s. If the star already harbors a black hole at its center, the two form a bound binary whose gravitational-wave coalescence time falls below $10^{4}\,$yr. Depending on the geometry and speed of the encounter, the resulting orbit may be nearly circular or retain substantial eccentricity. Our calculations confirm that black hole–star encounters in AGN disks are indeed a viable channel for assembling stellar-mass black hole binaries and supplying sources for gravitational-wave detectors.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Rescuing Kepler's False Positives: A Possible Habitable Zone Exoplanet in the Triple Star System KOI-1623

Rita Lanxuan Wang, Daniel Huber, Daniel Hey et al. (+6 more)

The Kepler mission has significantly advanced our understanding of exoplanet demographics. However, Kepler planet candidates showing centroid shifts are often classified as false-positives and thus excluded from demographic analyses. Some of these systems may be erroneous false positives, resulting from unresolved binary stars with planets orbiting one of the stars, thereby introducing biases into past exoplanet demographic studies. Here, we present updated Kepler centroid offset values derived from Gaia Data Release 3 (DR3) and analyze KOI-1623.01, a Kepler centroid offset false positive (COFP) with a clear transit signal and an orbital period of 111 days. High-contrast imaging with Keck/NIRC2 reveals that KOI-1623A has a close companion with a separation of $0.704 \pm 0.002$ arcsec, consisting of a slightly evolved G-type primary and a K-dwarf secondary, while Gaia detects a tertiary companion with common proper motion at a separation of $\sim 8.4$ arcsec. By combining transit photometry, Kepler pixel level data, direct imaging, and Gaia DR3, we find that the transit signal is consistent with a $10.02 ^{+0.44}_{-0.31} R_{\oplus}$ giant planet located in the habitable zone of the secondary. We furthermore use resolved companions from Gaia to estimate that another $\approx 70$ out of $515$ of Kepler's centroid false positives were misclassified and instead are real planets that belong to binary systems.
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All authors Rita Lanxuan Wang, Daniel Huber, Daniel Hey, Nathanael Burns-Watson, Alexander Larsen, Adam Kraus, Trent J. Dupuy, Steven Giacalone, Justin R. Crepp

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Optical Spectroscopy of a Candidate O-Star X-ray Binary in M33

Margaret Lazzarini, Lara Cullinane, Karoline Gilbert et al. (+8 more)

We present new observations of a candidate O-type donor star in a high mass X-ray binary (HMXB) in M33. The candidate donor star is spatially coincident with a hard X-ray point source. The star's optical magnitude and colors are consistent with what is expected for a massive star. The four band optical/UV spectral energy distribution (SED) of the donor star is well fit by an O-giant model with an effective temperature of $\sim$43 kK, mass of 75$_{+9}^{-0.3}$ $M_{\odot}$, and surface gravity of log(g) of $\sim$3.8. We present four epochs of optical spectroscopy of the donor star taken with the Keck/DEIMOS spectrograph over the course of approximately one year. Features in the star's spectrum are consistent with an O-type star. The star's spectrum exhibits a strong H$\alpha$ emission line during all epochs of observation with a variable line profile. We measure the star's radial velocity for each epoch, which we use to fit for a systemic velocity. We find that the systemic velocity is offset from the local gas velocity from HI measurements, suggesting that the system has a peculiar velocity relative to the local gas. We observe a radial velocity shift in the He I absorption lines that is consistent with motion within a binary system.
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All authors Margaret Lazzarini, Lara Cullinane, Karoline Gilbert, Puragra GuhaThakurta, Kyros Hinton, Daniel Stern, O. Grace Telford, Tobin M. Wainer, Benjamin F. Williams, Robert Alexander, Ernesto Ramirez Jr

Solar and Stellar Astrophysics

The JWST Proto-PAH Project: Detection of the methyl radical CH$_{3}$ in the highly evolved C-rich object SMP LMC 011

Jialu Li, D. A. García-Hernández, A. Manchado et al. (+17 more)

We report the first detection of the neutral methyl radical CH$_3$ in a highly evolved C-rich object, SMP LMC 011, using JWST MIRI/MRS. CH$_3$ is well fitted by an excitation temperature of $T_{\rm ex} \simeq$~190\,K and a column density of $N_{\rm tot}(\mathrm{CH_3}) \simeq 5.6\times10^{17}$ cm$^{-2}$. We also report the non-detection of ethane (C$_2$H$_6$), consistent with CH$_3$ reacting preferentially with unsaturated radicals rather than self-recombination, which may be inefficient in SMP LMC 011. Combined with the exceptionally large benzene column density of SMP LMC 011, this supports a methyl-addition route from benzene towards alkyl-substituted aromatics. We propose that the high CH$_3$ abundance is driven by the erosion of hydrogenated amorphous carbon (HAC) dust grains in the dense warm torus by UV photons from the central star and/or by shocks, which release CH$_3$ directly into the gas phase. These results establish CH$_3$ as a key reactive intermediate in the formation of complex hydrocarbons in C-rich circumstellar environments. Chemical models that incorporate methyl-addition reactions are needed for a better understanding of PAH formation pathways in evolved stars. The potential detection of alkyl-substituted aromatics such as toluene (C$_7$H$_8$) and ethylbenzene (C$_8$H$_{10}$) in C-rich evolved stars would provide direct confirmation of CH$_3$-driven aromatic growth in circumstellar environments.
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All authors Jialu Li, D. A. García-Hernández, A. Manchado, Debayan Das, J. Cami, Els Peeters, G. C. Sloan, B. Aringer, J. Bernard-Salas, C. Bhatt, Nicholas Clark, Harriet L. Dinerstein, M. A. Gómez-Muñoz, Kathleen E. Kraemer, M. Matsuura, R. Sahai, N. C. Sterling, Kevin Volk, G. M. Wahlgren, A. A. Zijlstra

Solar and Stellar Astrophysics

Multi-chromatic observations of classical Cepheids using the CHARA Array interferometer: Surface brightness-colour relation, projection factor, and limb-darkening

M. C. Bailleul, N. Nardetto, D. Mourard et al. (+28 more)

The Baade-Wesselink (BW) method compares the linear and angular variations of Cepheids to derive their distance. This method is limited, however, by the projection factor, which relates the observed radial velocity to the true pulsation velocity of the star. Using simultaneous observations from the CHARA Array interferometer in the K, H, and R bands with the MYSTIC, MIRC-X, and SPICA combiners, respectively, we aim to understand the physics of Cepheid atmospheres better. Applying a specific method to multi-chromatic simultaneous interferometric observations of Cepheids, we derived robust limb-darkened angular diameters that were then used to calibrate the SBCR and study the projection factor. We also developed a strategy to measure the limb-darkening of Cepheids in R, H, and K bands. These measurements were then used to constrain the geometrical component of the projection factor. From the limb-darkened angular diameter curves of three Cepheids, we decreased the scatter of the SBCR in $V-K$ colour to 0.0011 magnitude and to 0.0040 for the SBCR in $G_{BP}-G_{RP}$. These SBCRs are particularly robust because they are based on multi-chromatic diameters and are homogeneous, which previous calibrations for Cepheids were not. For the very first time, we derived the limb-darkening of Cepheids in R, H, and K bands. For $\delta$ Cep, we derived an R-based projection factor of $1.275 \pm 0.051$, and the geometrical part obtained from the measured limb-darkening coefficient in R band was estimated to $1.420 \pm 0.016$ (on average), as expected from stellar static and hydrodynamical atmosphere models. The limb-darkening coefficients obtained in H and K band are consistent with models. These results demonstrate that multi-chromatic interferometry can improve the accuracy of the BW method. It is therefore essential to continue the CHARA survey of Cepheids in the coming years.
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All authors M. C. Bailleul, N. Nardetto, D. Mourard, A. Mérand, P. Kervella, A. Gallenne, G. Bras, J. Monnier, K. Sivkova, B. Apostolova, L. Breuval, W. Kiviaho, W. Gieren, V. Hocdé, R. S. Rathour, G. Pietrzyński, P. Bério, F. Morand, D. Salabert, J. Jones, C. Lanthermann, N. Ebrahimkutty, J. Jonàk, H. Nowacki, R. V. Ibañez Bustos, M. Vrard, S. Kraus, N. Anugu, M. Gutierrez, N. Ibrahim, T. Gardner

Solar and Stellar Astrophysics

Stellar activity in a post-merger Giant star

Hans Moritz Günther, C. Melis, K. J. Shen et al. (+3 more)

Binary stars are the progenitors of exotic objects, such as supernovae and gravitational wave sources. However, some systems merge earlier when the primary expands into a giant star at the end of its lifetime on the main sequence and causes its companion to spiral in. TYC 4144-329-2 is a candidate post-merger first ascent giant star with a circumstellar disk seen in the infrared (IR). We detect weak X-ray and far ultraviolet (FUV) emission from TYC 4144-329-2, which we interpret as a signature of weak coronal activity. We also find variability in Ha line profiles and in the optical light curves and we suggest an intermediate geometry where the line-of-sight passes through the upper layers of a flared disk with a time variable column density. While the X-rays must be coronal, the Ha line profiles point to ongoing and variable accretion. We suggest that TYC 4144-329-2's merger was more recent than other stars in this class and that it did not yet have time to develop a deep convection zone. If this scenario holds, TYC 4144-329-2 would be a unique probe of the earliest stages in the evolution of post-merger systems.
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All authors Hans Moritz Günther, C. Melis, K. J. Shen, Eric G. Blackman, K. Hoadley, P. C. Schneider

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Constraining the Delay Time Distribution of the r- and s-Process from Stellar Ages at Solar Metallicity

Meng Zhang and Hans-Walter Rix

We present a new approach to constraining the delay time distribution (DTD) governing r- and s-process element production. Rather than using metallicity as a proxy for time, we work directly with stellar ages at solar metallicity, where chemical equilibrium models are most applicable. We analyze two complementary samples: more than $10,000$ red clump (RC) thin disk stars at solar metallicity from LAMOST, and solar twins with high-precision abundances; both have well-determined stellar ages $\tau$. We fit chemical-equilibrium models with power-law DTDs to the evolution of [Eu/Mg]$(\tau)$ and [Ba/Mg]$(\tau)$ in each sample, taking europium as a canonical r-process element and barium as an s-process element, each referenced to magnesium as a near-instantaneous, CCSNe-dominated element. To validate the method, we first apply it to [Fe/Mg]$(\tau)$ and recover that $\approx\!70\%$ of Fe comes from a delayed channel with a declining DTD $\propto t^{-1.2}$, in agreement with well-established results for iron production in thermonuclear supernovae. It also shows that about $60\%$ of Eu and Ba forming today originate from strongly-delayed processes, and the inferred delayed-production rates of both Eu and Ba rise with delay time, opposite to that of thermonuclear supernovae. These results provide direct temporal constraints on r- and s-process enrichment, complementary to metallicity-based chemical-evolution studies. Because we model abundance referenced to Mg, and the same trend is recovered across stars of different guiding-center radii, this appears unaffected by radial migration and chemical evolution varying with Galactocentric radius. If confirmed, it points to delayed neutron-capture production beyond NS–NS mergers.
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Solar and Stellar Astrophysics

MINCE IV. A detailed analysis of 37 metal-poor and subsolar metallicity stars

L. Sgatti, G. Cescutti, L. Lombardo et al. (+13 more)

The project called Measuring at Intermediate metallicity Neutron-Capture Elements (MINCE) primarily focuses on the photospheric abundances of several neutron-capture (NC) elements from high-quality spectra of metal-poor stars within the metallicity range of ${\rm -2.5 \leq\![Fe/H]\!\leq -1.5}$, aiming to provide homogeneous information on their chemical compositions. Furthermore, based on the Gaia mission, chemical abundances can now be coupled with kinematic properties, enabling investigations of the constituent substructures of the Milky Way. We analyse the fourth sample of MINCE stars in terms of atmospheric parameters and chemical composition. The results are then compared with the previously analysed MINCE samples. The observations were conducted with HARPS-N (${\rm R \approx 120\,000}$) at the Telescopio Nazionale Galileo (TNG) and FIES (${\rm R \approx 67\,000}$ ) at the Nordic Optical Telescope (NOT), providing optical spectra with a high signal-to-noise ratio (S/N) and high resolution. We then used the methods developed in previous MINCE works and Gaia photometric data to obtain the stellar atmospheric parameters. The chemical abundances were analysed using 1D-LTE model atmospheres. We present the photospheric abundances in 37 stars of up to 26 elements, including $\alpha$, iron-peak, light-odd, and 10 NC$-$elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu). Kinematic properties and membership assessments are provided for all stars. The MINCE catalogue is thus expanded by 13 thin-disc stars, 9 thick-disc stars, 1 star in a transition zone between the thin and thick disc, 9 stars of the halo, and 5 Gaia-Sausage-Enceladus stars. The results agree excellently with previous MINCE works. The comparison with a Gaia Sausage Enceladus GCE model is consistent with the scenario of a major accretion event on the Milky Way from a dwarf galaxy.
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All authors L. Sgatti, G. Cescutti, L. Lombardo, C. T. Nguyen, P. Bonifacio, E. Caffau, L. Monaco, R. Lallement, L. Magrini, M. Franchini, C. J. Hansen, A. Mucciarelli, A. Korn, A. Kucinskas, E. Spitoni, S. Lucatello

Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

PYRA and MYTHRA: new statistical analysis for image reconstruction

Julien Drevon, Margaux Abello, Florentin Millour et al. (+2 more)

Image reconstruction in optical interferometry remains a challenging inverse problem due to sparse Fourier sampling and the strong dependence of the reconstructed images on user-defined parameters. We present PYRA (Python for MiRA) and MYTHRA (Mean Astrophysical Images with PYRA), two tools designed to improve the robustness and reproducibility of interferometric imaging. PYRA automatically generates large ensembles of MiRA reconstructions by exploring a wide range of reconstruction parameters, while MYTHRA statistically analyzes the resulting images to identify the most consistent solutions and derive a final averaged image with associated uncertainty estimates. The methodology is validated using the blind datasets of the 2024 Interferometric Imaging Contest, where our reconstruction strategy achieved the best overall score. We further apply the method to VLTI/MATISSE observations of the triple AGB system {\pi}1 Gru and the A[e] supergiant 3 Pup. These results demonstrate that statistical analysis of large reconstruction ensembles provides a powerful framework for assessing image reliability and reducing reconstruction artifacts in optical interferometry.
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All authors Julien Drevon, Margaux Abello, Florentin Millour, Anthony Meilland, Armando Domiciano de Souza

Solar and Stellar Astrophysics

SPAMMS: 3D spectroscopic modeling of stellar surfaces. II. Implementation of Kurucz and TLUSTY model atmospheres

D. Galán-Diéguez, M. Abdul-Masih, C. Allende Prieto et al. (+3 more)

Context. Accurate stellar spectra are essential to derive stellar properties. Traditional model atmospheres often oversimplify phenomena that break spherical symmetry, such as rotational deformation or multiplicity. The Spectroscopic PAtch Model for Massive Stars (SPAMMS) accounts for these effects, but its applicability has been limited by the spectral types covered by its model atmosphere grids. Aims. We aim to extend the parameter space of the model atmosphere grids available to SPAMMS, enabling spectral synthesis across a broader range of stellar types. Methods. We computed specific intensities, $I\left(\lambda,\mu\right)$, for $101$ emergent angles using PRISMAS (Pipeline of Radiative Intensity Synthesis for Meshed Atmospheric Surfaces), and pre-computed LTE and non-LTE atmospheres from two ATLAS9-Kurucz grids and the TLUSTY-based OSTAR2002 and BSTAR2006 models. Results. The intensity grids cover effective temperatures from $3500$ to $55000\,\mathrm{K}$ and surface gravities from $0.0$ to $5.0\,\mathrm{dex}$, spanning O- to K-type stars. They include metallicities from $0$ to $30\,\mathrm{Z_\odot}$ and microturbulent velocities of $1$, $3$, $5$, and $10\,\mathrm{km\,s^{-1}}$. The spectral range extends from $3000$ to $9000\,\mathring{\mathrm{A}}$ with $\Delta\lambda=0.01\,\mathring{\mathrm{A}}$. As a proof of concept, we modelled with SPAMMS a rapidly rotating B-type star and an eclipsing Algol-type binary. Conclusions. The new LTE-Kurucz and NLTE-TLUSTY grids substantially expand the parameter space accessible to SPAMMS. The code can now generate synthetic spectra for a broader range of stellar types and geometries, including rapidly rotating and multiple systems – providing a more comprehensive framework for modelling non-spherical stellar surfaces.
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All authors D. Galán-Diéguez, M. Abdul-Masih, C. Allende Prieto, S.R. Berlanas, A. Herrero, H. Sana

Solar and Stellar Astrophysics · Cross-listed from gr-qc

Deterministic denoising of long-duration gravitational-wave signal candidates with $\alpha$-(de)blending

Przemysław Figura and Michał Bejger

We present a deterministic machine-learning method for denoising candidate signals from all-sky searches for continuous gravitational waves from rotating neutron stars. Using the time-domain $F$-statistic search pipeline, we post-process the resulting $F(f,\dot{f})$ candidate patterns with iterative $\alpha$-(de)blending, a deterministic diffusion-type generative model implemented as a U-Net, well suited to the non-Gaussian, correlated noise of the $F$-statistic output. Two models, trained on data based on simulated 6- and 12-day time-domain segments with software-injected signals similar to the LVK hardware injections, are tested by comparing deblended images to the library of expected signal patterns via the Structural Similarity Index Measure. In general the method recovers the correct sky-position-dependent pattern for signal-to-noise ratios $\rho \gtrsim 4$, moderately below typical all-sky detection thresholds, with recovery depending strongly on pattern morphology, demonstrating that deterministic diffusion-based denoising may serve as a consistency/veto tool ahead of the semi-coherent coincidence stage. We also discuss limitations and possible improvements of the method.
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Solar and Stellar Astrophysics

A Self-consistent Model for the Generation of Coronal Condensations: The Effects of Turbulent Wave Dissipation

M. McMurdo, V. Jercic, T. Van Doorsselaere et al. (+1 more)

Prominences and coronal rain are two manifestations of coronal condensations whose formation mechanisms remain only partially understood. While previous studies have explored how localised steady or stochastic heating can trigger thermal instability and condensations, the role of wave driven energy transport and dissipation has not been extensively investigated. In the solar atmosphere, Alfven and kink waves are an abundant source of energy capable of contributing both to coronal heating and plasma structuring, making them compelling candidates in the formation processes of the structures we observe in the corona. We aim to explore how Alfven and kink wave energy and their dissipation influence the formation and evolution of condensations in coronal magnetic structures. In particular, we investigate each waves role independently of parameterised external heating mechanisms to isolate whether either wave can trigger condensations. We perform 2.5D simulations using the open source MPI AMRVAC code extended with the newly developed UAWSoM module. This module self consistently evolves Alfven and kink wave energy and their feedback on the plasma through wave pressure and heating terms. We examine the conditions under which condensations form, their morphology, and their dynamical evolution. Our results show that for our range of parameters, in 2.5D MHD, Alfven waves did not trigger the formation of prominences or condensations. We find that if the radius of the fine scale structures supporting the kink waves is of the order of 100 km, then kink waves can trigger coronal rain that differs considerably in its morphology from coronal rain triggered by parameterised heating functions. Through the use of UAWSoM, we have established that Alfven and kink waves leave distinct thermodynamic signatures on coronal loops, given the same wave energy injection.
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All authors M. McMurdo, V. Jercic, T. Van Doorsselaere, C. Froment

Solar and Stellar Astrophysics

A new method to cleanly separate hot and cool populations using just Gaia and 2MASS photometry

J. Maíz Apellániz and M. Pantaleoni González

We have developed a photometric method that produces clean hot (intrinsically blue) populations using Gaia and 2MASS photometry alone with [a] a very small (false positives < 1%) contamination from cool (intrinsically red) luminous sources, [b] independent of extinction (limited only by S/N), and [c] with only small losses (false negatives < 10%) of real OB stars. Applying the method to Gaia DR3 data we are able to recover extinguished OB stars down to E(G_BP-G_RP) = 5, producing a catalog of ~2x10^5 candidate OB stars that reaches beyond the Scutum-Centaurus arm. This method can potentially extend the sample of known OB stars by an order of magnitude, which would yield a large leap in our knowledge of the populations of compact-object progenitors, including gravitational-wave sources.
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2609.15264 · 14 Sep · To appear in Highlights of Spanish Astrophysics XIII, Proceedings of the XVII Scientific Meeting of the Spanish Astronomical Society held on 13-17 July 2026, in Tarragona, Spain. The generated file includes two animated pdfs, which require an Adobe-compatible viewer to activate the animation

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