Astrophysical Research Articles

A theoretical framework for BL Her stars III. A case study: Robust light curve optimization in the Large Magellanic Cloud

In the era of precision stellar astrophysics, classical pulsating stars play a crucial role in determinations of the cosmological distance scale thanks to their period-luminosity (PL) relations. Therefore, it is important to constrain their stellar evolution and pulsation models not only through a comparison of empirical and theoretical PL relations and properties at mean light, but also using their light curve structure over the complete pulsation cycle. We carried out an extensive light curve comparison of BL Her stars using observations from Gaia DR3 and stellar pulsation models computed using mesa-rsp with the goal of obtaining the best-matched observed-model pairs for BL Her stars in the Large Magellanic Cloud (LMC). We used the Fourier decomposition technique to analyze the light curves in the G band obtained from Gaia DR3 and from mesa-rsp and used a robust light-curve-fitting approach to score the observed-model pairs with respect to their pulsation periods and over their Fourier parameter space. We obtain the best-fit models for 48 BL Her stars in the LMC and thereby provide the stellar parameter estimates of these stars, 30 of which we classify as our ``gold sample'' due to their superior light curve fits. We find a relatively flat distribution of stellar masses between 0.5 and 0.65,M_⊙ for the gold sample of observed-model pairs. An interesting result is that the majority of the best-matched models in the gold sample were computed using the convection parameter sets without radiative cooling. The period-Wesenheit (PW) relation for the best-matched gold sample of 30 BL Her models has a slope of -2.805 ± 0.164 and the corresponding period-radius relation a slope of $0.565 ± 0.035$, both in good agreement with the empirical PW and period-radius slopes from BL Her stars in the LMC, respectively. We also used the Wesenheit magnitudes of the 30 best-matched observed-model pairs to estimate a distance modulus of LMC = 18.582 ± 0.067 to the LMC, which lies within the bounds of previous literature values. We also discuss the degeneracy in the stellar parameters of the BL Her models that result in similar pulsation periods and light curve structure, and highlight that caution must be exercised while using the stellar parameter estimates.

SN 2023ixf: An average-energy explosion with circumstellar medium and a precursor

The fortunate proximity of the Type II supernova (SN) 2023ixf has allowed astronomers to follow its evolution from almost the moment of the collapse of the progenitor's core. SN,2023ixf can be explained as an explosion of a massive star with an energy of 0.7 erg but with a greatly reduced envelope mass, probably because of binary interaction. In our radiative-transfer simulations, the SN ejecta of 6 interact with circumstellar matter (CSM) of (0.55--0.83) extending to 10^15 cm, which results in a light curve (LC) peak matching that of SN,2023ixf. The origin of this required CSM might be gravity waves originating from convective shell burning, which could enhance wind-like mass loss during the late stages of stellar evolution. The steeply rising low-luminosity flux during the first hours after observationally confirmed non-detection, however, cannot be explained by the collision of the energetic SN shock with the CSM. Instead, we consider it as a precursor that we can fit by the emission from (0.5--0.9) of matter that was ejected with an energy of ∼10^49 erg a fraction of a day before the main shock of the SN explosion reached the surface of the progenitor. The source of this energy injection into the outermost shell of the stellar envelope could also be dynamical processes related to the convective activity in the progenitor's interior or envelope. Alternatively, the early rise of the LC could point to the initial breakout of a highly non-spherical SN shock or of fast-moving asymmetrically ejected matter that was swept out well ahead of the SN shock, potentially in a low-energy, nearly relativistic jet. We also discuss that pre-SN outbursts and LC precursors can be used to study or to constrain energy deposition in the outermost stellar layers by the decay of exotic particles, such as axions, which could be produced simultaneously with neutrinos in the newly formed hot neutron star. A careful analysis of the earliest few hours of the LCs of SNe can reveal elusive precursors and provide a unique window onto the surface activity of massive stars during their core collapse. This can greatly improve our understanding of stellar physics and consequently also offer new tools for searching for exotic particles.

Classification of Periodic Variable Stars from TESS

Abstract The number of known periodic variable stars has increased rapidly in recent years. As an all-sky transit survey, the Transiting Exoplanet Survey Satellite (TESS) plays an important role in detecting low-amplitude variable stars. Using 2 minute cadence data from the first 67 sectors of TESS, we find 72,505 periodic variable stars. We used 19 parameters including period, physical parameters, and light-curve (LC) parameters to classify periodic variable stars into 12 subtypes using the random forest method. Pulsating variable stars and eclipsing binaries are distinguished mainly by period, LC parameters, and physical parameters. Classical Cepheids, Type-II Cepheids, rotational variable stars, eruptive variable stars of the UV Ceti type, and young stellar objects are distinguished mainly by period and physical parameters. Compared to previously published catalogs, 63,106 periodic variable stars (87.0%) are newly classified, including 13 Cepheids, 27 RR Lyrae stars,​​​​​​ ~4600 δ Scuti variable stars, ~1600 eclipsing binaries, ~34,000 rotational variable stars, and about 23,000 other types of variable star. The purity of eclipsing binaries and pulsation variable stars ranges from 94.2% to 99.4% when compared to the variable star catalogs of Gaia Data Release 3 and Zwicky Transient Facility Data Release 2. The purity of rotational variable stars is relatively low at 83.3%. The increasing number of variables stars is helpful to investigate the structure of the Milky Way, stellar physics, and chromospheric activity.

An ultrafast algorithm for ultrafast time-resolved coherent Raman spectroscopy

Time-resolved coherent Raman spectroscopy (CRS) is a powerful non-linear optical technique for quantitative, in-situ analysis of chemically reacting flows, offering unparalleled accuracy and exceptional spatiotemporal resolution. Its application to large polyatomic molecules, crucial for understanding reaction dynamics, has thus far been limited by the complexity of their rotational-vibrational Raman spectra. Progress in developing comprehensive spectral codes for these molecules, a longstanding goal, has been hindered by prohibitively long computation times required for their spectral synthesis. Here, we present an algorithm that achieves a million-fold improvement in computation time compared to existing methods. The algorithm demonstrates remarkable accuracy, with an approximation error below 0.1% across all tested probe delays, at both room temperature (296 K) and elevated temperatures (1500 K). This result could greatly expand the application of time-resolved CRS, particularly in plasma research, as well as in broader atmospheric and astrophysical sciences.

Diversity in Hydrogen-rich Envelope Mass of Type II Supernovae. I. Plateau Phase Light-curve Modeling

Abstract We present a systematic study of Type II supernovae (SNe II) originating from progenitors with effective temperatures (T eff) and luminosities closely resembling red supergiants (RSGs) observed in pre-supernova (SN) images and in the Galaxy. Using Modules for Experiments in Stellar Astrophysics, we compute a large grid of massive stars with T eff ranging from 3200 to 3800 K at their RSG phases, with hydrogen envelopes artificially stripped to varying extents (3–10 M ⊙). The light curves of SNe IIP resulting from the explosions of these Galactic-RSG–like progenitors are modeled using STELLA. Our survey of the light curves reveals that partial stripping of the hydrogen envelope creates diversity in the magnitude and duration of SNe IIP light curves, without affecting the position of the RSG progenitor on the Hertzsprung–Russell diagram. For these Galactic-RSG-like progenitor models, we establish an indicator based on the light-curve properties to estimate the hydrogen envelope mass. Additionally, we discuss the effects of material mixing and 56Ni heating. Applying our model grid to a large sample of approximately 100 observed SNe IIP reveals a considerably broader range of hydrogen-rich envelope masses than predicted by standard stellar wind models. This finding suggests that if SNe IIP are explosions of Galactic-like RSGs to explain the diversity in the observed light curves, a significant fraction of them must have experienced substantial mass loss beyond the standard mass-loss prescription prior to their explosions. This finding highlights the uncertainties involved in massive star evolution and the pre-SN mass-loss mechanism.

Examining the brightness variability, accretion disk, and evolutionary stage of the binary OGLE-LMC-ECL-14413

Several intermediate-mass close binary systems exhibit photometric cycles longer than their orbital periods, potentially due to changes in their accretion disks. Past studies indicate that analyzing historical light curves can provide valuable insights into disk evolution and track variations in mass transfer rates within these systems. Our study aims to elucidate both short-term and long-term variations in the light curve of the eclipsing system with a particular focus on the unusual reversals in eclipse depth. We aim to clarify the role of the accretion disk in these fluctuations, especially in long-cycle changes spanning hundreds of days. Additionally, we seek to determine the evolutionary stage of the system and gain insights into the internal structure of its stellar components. We analyzed photometric time series from the Optical Gravitational Lensing Experiment (OGLE) project in the $I$ and $V$ bands, and from the MAssive Compact Halo Objects (MACHO) project in the $B_ M $ and $R_ M $ bands, covering a period of 30.85 years. Using light curve data from 27 epochs, we constructed models of the accretion disk. An optimized simplex algorithm was employed to solve the inverse problem, deriving the best-fit parameters for the stars, orbit, and disk. We also utilized the Modules for Experiments in Stellar Astrophysics ( MESA ) software to assess the evolutionary stage of the binary system, investigating the progenitors and potential future developments. We found an orbital period of 38.15917 pm 0.00054 days and a long-term cycle of approximately 780 days. Temperature, mass, radius, and surface gravity values were determined for both stars. The photometric orbital cycle and the long-term cycle are consistent with a disk containing variable physical properties, including two shock regions. The disk encircles the more massive star and the system brightness variations align with the long-term cycle at orbital phase 0.25. Our mass transfer rate calculations correspond to these brightness changes. MESA simulations indicate weak magnetic fields in the donor star's subsurface, which are insufficient to influence mass transfer rates significantly.

AstroMLab 1: Who wins astronomy jeopardy!?

We present a comprehensive evaluation of proprietary and open-weights large language models using the first astronomy-specific benchmarking dataset. This dataset comprises 4,425 multiple-choice questions curated from the Annual Review of Astronomy and Astrophysics, covering a broad range of astrophysical topics.11Jeopardy is a popular American quiz show where contestants are tested on their knowledge across various subjects. Other similar shows include Who’s Still Standing (▪) in China and University Challenge in the UK, among others. Our analysis examines model performance across various astronomical subfields and assesses response calibration, crucial for potential deployment in research environments. Claude-3.5-Sonnet outperforms competitors by up to 4.6 percentage points, achieving 85.0% accuracy. For proprietary models, we observed a universal reduction in cost every 3-to-12 months to achieve similar score in this particular astronomy benchmark. open-weights models have rapidly improved, with LLaMA-3-70b (80.6%) and Qwen-2-72b (77.7%) now competing with some of the best proprietary models. We identify performance variations across topics, with non-English-focused models generally struggling more in exoplanet-related fields, stellar astrophysics, and instrumentation related questions. These challenges likely stem from less abundant training data, limited historical context, and rapid recent developments in these areas. This pattern is observed across both open-weights and proprietary models, with regional dependencies evident, highlighting the impact of training data diversity on model performance in specialized scientific domains. Top-performing models demonstrate well-calibrated confidence, with correlations above 0.9 between confidence and correctness, though they tend to be slightly underconfident. The development for fast, low-cost inference of open-weights models presents new opportunities for affordable deployment in astronomy. The rapid progress observed suggests that LLM-driven research in astronomy may become feasible in the near future.

The ESO UVES/FEROS Large Programs of TESS OB pulsators. II. The physical origin of macroturbulence

Spectral lines of hot massive stars are known to exhibit large excess broadening in addition to rotational broadening. This excess broadening is often attributed to macroturbulence, whose physical origin is a matter of active debate in the stellar astrophysics community. We aim to shed light on the physical origin of macroturbulent line broadening by looking into the statistical properties of a large sample of O- and B-type stars both in the Galaxy and the Large Magellanic Cloud (LMC). We deliver newly measured macroturbulent velocities for 86 stars from the Galaxy in a consistent manner with 126 stars from the LMC. We composed a total sample of 594 stars with measured macroturbulent velocities by complementing our sample with archival data for the Galactic O- and B-type stars in order to gain better coverage of the parameter space. Furthermore, we computed an extensive grid of mesa models to compare, in a statistical manner, the predicted interior properties of stars (such as convection and wave propagation) with the inference of macroturbulent velocities from high-resolution spectroscopic observations. We find evidence for subsurface convective zones that formed in the iron opacity bump (FeCZ) being connected to observed macroturbulent velocities in hot massive stars. Additionally, we find the presence of two principally different regimes where, depending on the initial stellar mass, different mechanisms may be responsible for the observed excess line broadening. Stars with initial masses above 30$M_ odot $ exhibit macroturbulent velocities that are in line with FeCZ properties, as indicated by the trends in both observations and models. For stars below 12$M_ odot $, alternative mechanisms are needed to explain macroturbulent broadening, such as internal gravity waves (IGWs). Finally, in the intermediate range between 12 and 30$M_ odot $, IGWs tunnelling through subsurface convective layers combined with the presence of FeCZ-driven convection suggests that both processes could contribute to the observed macroturbulent velocities. This intermediate regime presents a region where the interplay between these two (or more) mechanisms remains to be fully understood.

Blue large-amplitude pulsators formed from the merger of low-mass white dwarfs

Context. Blue large-amplitude pulsators (BLAPs) are a recently discovered group of hot stars pulsating in radial modes. Their origin needs to be explained, and several scenarios for their formation have already been proposed. Aims. We investigate whether BLAPs can originate as the product of a merger of two low-mass white dwarfs (WDs) and estimate how many BLAPs can be formed in this evolutionary channel. Methods. We used the Modules for Experiments in Stellar Astrophysics (MESA) code to model the merger of three different double extremely low-mass (DELM) WDs and the subsequent evolution of the merger product. We also performed a population synthesis of Galactic DELM WDs using the COSMIC code. Results. We find that BLAPs can be formed from DELM WDs provided that the total mass of the system ranges between 0.32 and 0.7 M⊙. BLAPs born in this scenario either do not have any thermonuclear fusion at all or show off-centre He burning. The final product evolves to hot subdwarfs and eventually finishes its evolution either as a cooling He WD or a hybrid He/CO WD. The merger products become BLAPs only a few thousand years after coalescence, and it takes them 20–70 thousand years to pass the BLAP region. We found the instability of the fundamental radial mode to be in fair agreement with observations, but we also observed instability of the radial first overtone. The calculated evolutionary rates of period change can be both positive and negative. From the population synthesis, we found that up to a few hundred BLAPs born in this scenario can exist at present in the Galaxy. Conclusions. Given the estimated number of BLAPs formed in the studied DELM WD merger scenario, there is a good chance to observe BLAPs that originated through this scenario. Since strong magnetic fields can be generated during mergers, this scenario could lead to the formation of magnetic BLAPs. This fits well with the discovery of two likely magnetic BLAPs whose pulsations can be explained in terms of the oblique rotator model.

Solar convective velocities: Updated helioseismic constraints

Modeling heat transport by convection is one of the most challenging aspects of solar and stellar physics. The literature currently provides apparently inconsistent observational estimates of the strength of large-scale convective flows in the upper layers of the solar convection zone. In addition, the large-scale convective flows predicted from numerical simulations are substantially stronger than some of the observational inferences in the literature. The current work aims to provide a consistent presentation of some of the main results in the literature both from observations and simulations. To achieve this aim, we carry out an analysis of published estimates of the strength of solar convection at different spatial scales. In particular, we employ a consistent set of conventions to compute the kinetic energy density in the east-west flows. This establishes a clear baseline for future work. The main conclusion is that there are inconsistencies between different observational results and also differences between observations and simulations. This conclusion is important as it demonstrates a need to determine the sources of the inconsistencies between different observational inferences and also to determine the missing ingredients in simulations of solar subsurface convection.

Physical Models for the Astrophysical Population of Black Holes: Application to the Bump in the Mass Distribution of Gravitational-wave Sources

Gravitational-wave observations of binary black holes have revealed unexpected structure in the black hole mass distribution. Previous studies employ physically motivated phenomenological models and infer the parameters that control the features of the mass distribution that are allowed in their model, associating the constraints on those parameters with their physical motivations a posteriori. In this work, we take an alternative approach in which we introduce a model parameterizing the underlying stellar and core-collapse physics and obtaining the remnant black hole distribution as a derived by-product. In doing so, we constrain the stellar physics necessary to explain the astrophysical distribution of black hole properties under a given model. We apply this to the mapping between initial mass and remnant black hole mass, accounting for mass-dependent mass loss using a simple parameterized description. Allowing the parameters of the initial mass–remnant mass relationship to evolve with redshift permits correlated and physically reasonable changes to features in the mass function. We find that the current data are consistent with no redshift evolution in the core–remnant mass relationship, but place only weak constraints on the change of these parameters. This procedure can be applied to modeling any physical process underlying the astrophysical distribution. We illustrate this by applying our model to the pulsational pair instability supernova (PPISN) process, previously proposed as an explanation for the observed excess of black holes at ∼35 M ⊙. Placing constraints on the reaction rates necessary to explain the PPISN parameters, we concur with previous results in the literature that the peak observed at ∼35 M ⊙ is unlikely to be a signature from the PPISN process as presently understood.

The red giant branch tip in the SDSS, PS1, JWST, NGRST, and Euclid photometric systems

We used synthetic photometry from Gaia DR3 BP and RP spectra for a large selected sample of stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) to derive the magnitude of the red giant branch (RGB) tip for these two galaxies in several passbands across a range of widely used optical photometric systems, including those of space missions that have not yet started their operations. The RGB tip is estimated by fitting a well motivated model to the RGB luminosity function (LF) within a fully Bayesian framework, allowing for a proper representation of the uncertainties of all the involved parameters and their correlations. By adopting the best available distance and interstellar extinction estimates, we provide a calibration of the RGB tip as a standard candle for the following passbands: Johnson-Kron-Cousins I (mainly used for validation purposes), Hubble Space Telescope F814W, Sloan Digital Sky Survey i and z, PanSTARRS 1 y, James Webb Space Telescope F090W, Nancy Grace Roman Space Telescope Z087, and Euclid IE, with an accuracy within a few per cent, depending on the case. We used theoretical models to explore the trend of the absolute magnitude of the tip as a function of colour in the different passbands (beyond the range spanned by the LMC and SMC), as well as its dependency on age. These calibrations can be very helpful to obtain state-of-the-art RGB tip distance estimates to stellar systems in a very large range of distances directly from data in the natural photometric system of these surveys and/or missions, without recurring to photometric transformations. We have made the photometric catalogues publicly available for calibrations in additional passbands or for different approaches in the estimate of the tip, as well as for stellar populations and stellar astrophysics studies that may take advantage of large and homogeneous datasets of stars with magnitudes in 22 different passbands.

An analysis of spectroscopic, seismological, astrometric, and photometric masses of pulsating white dwarf stars

Context. A central challenge in the field of stellar astrophysics lies in accurately determining the mass of stars, particularly when dealing with isolated ones. However, for pulsating white dwarf stars, the task becomes more tractable due to the availability of multiple approaches such as spectroscopy, asteroseismology, astrometry, and photometry, each providing valuable insights into the mass properties of white dwarf stars. Aims. Numerous asteroseismological studies of white dwarfs have been published, focusing on determining stellar mass using pulsational spectra and comparing it with spectroscopic mass, which uses surface temperature and gravity. The objective of this work is to compare these mass values in detail and, in turn, to compare them with the mass values derived using astrometric parallaxes or distances and photometry data from Gaia, employing astrometric and photometric methods. Methods. Our analysis involves a selection of pulsating white dwarfs with different surface chemical abundances that define the main classes of variable white dwarfs. We calculated their spectroscopic masses, compiled seismological masses, and determined astrometric masses. We also derived photometric masses, when possible. Subsequently, we compared all the sets of stellar masses obtained through these different methods. To ensure consistency and robustness in our comparisons, we used identical white dwarf models and evolutionary tracks across all four methods. Results. The analysis suggests a general consensus among the four methods regarding the masses of pulsating white dwarfs with hydrogen-rich atmospheres, known as DAV or ZZ Ceti stars, especially for objects with masses below approximately 0.75 M⊙, although notable disparities emerge for certain massive stars. For pulsating white dwarf stars with helium-rich atmospheres, called DBV or V777 Her stars, we find that astrometric masses generally exceed seismological, spectroscopic, and photometric masses. Finally, while there is agreement among the sets of stellar masses for pulsating white dwarfs with carbon-, oxygen-, and helium-rich atmospheres (designated as GW Vir stars), outliers exist, where mass determinations by various methods show significant discrepancies. Conclusions. Although a general agreement exists among different methodologies for estimating the mass of pulsating white dwarfs, significant discrepancies are prevalent in many instances. This shows the need to redo the determination of spectroscopic parameters and the parallax and/or improve asteroseismological models for many stars.

Evolution of lithium in the disc of the Galaxy and the role of novae

Context. Lithium plays a crucial role in probing stellar physics, stellar and primordial nucleosynthesis, and the chemical evolution of the Galaxy. Stars are considered to be the main source of Li, yet the identity of its primary stellar producer has long been a matter of debate. Aims. In light of recent theoretical and observational results, we investigate in this study the role of two candidate sources of Li enrichment in the Milky Way, namely asymptotic giant branch (AGB) stars and, in particular, novae. Methods. We utilised a one-zone Galactic chemical evolution (GCE) model to assess the viability of AGB stars and novae as stellar sources of Li. We used recent theoretical Li yields for AGB stars, while for novae we adopted observationally inferred Li yields and recently derived delay time distributions (DTDs). Subsequently, we extended our analysis by using a multi-zone model with radial migration to investigate spatial variations in the evolution of Li across the Milky Way disc and compared the results with observational data for field stars and open clusters. Results. Our analysis shows that AGB stars clearly fail to reproduce the meteoritic Li abundance. In contrast, novae appear as promising candidates within the adopted framework, allowing us to quantify the contribution of each Li source at the Sun’s formation and today. Our multi-zone model reveals the role of the differences in the DTDs of Type Ia supernovae and novae in shaping the evolution of Li in the various galactic zones. Its results are in fair agreement with the observational data for most open clusters, but small discrepancies appear in the outer disc.

Optimised use of interferometry, spectroscopy, and stellar atmosphere models for determining the fundamental parameters of stars

Context. Thanks to recent progress in the field of optical interferometry, instrument sensitivities have now reached the level achieved in the domain of new space missions dedicated to exoplanet and stellar studies. Combining interferometry with other observational approaches enables the determination of stellar parameters and helps improve our understanding of stellar physics. Aims. In this paper, we aim to demonstrate a new way of using stellar atmosphere models for a joint interpretation of spectroscopic and interferometric observations. Methods. Starting from a discrete grid of one-dimensional (1D) stellar atmosphere models, we developed a training algorithm, based on an artificial neural network, capable of estimating the spectrum and intensity profile of a star over a range of wavelengths and viewing angles. A minimisation algorithm based on the trained function allowed for the simultaneous fitting of the observational spectrum and interferometric complex visibilities. As a result, coherent and precise stellar parameters can be extracted. Results. We show the ability of the trained function to match the modelled intensity profiles of stars in the effective temperature range of 4500–7000 K and surface gravity range of 3 to 5 dex, with a relative precision to the model that is better than 0.05%. Using simulated interferometric data and actual spectroscopic measurements, we demonstrated the performance of our algorithm on a sample of five benchmark stars. Using this method, we achieved an accuracy within 0.5% for the angular diameter, radius, and surface gravity, and within 20 K for the effective temperature. Conclusions. This paper demonstrates a new method of using interferometric data combined with spectroscopic observations. This approach offers an improved determination of the radius, effective temperature, and surface gravity of stars.

A “Wonderful” Reference Dataset of Mira Variables

The conditions in Mira variable atmospheres make them wonderful laboratories to study a variety of stellar physics such as molecule–grain formation, dust production, shock chemistry, stellar winds, mass loss, opacity-driven pulsation, and shocks. We were awarded an NSF grant to analyze over a decade of synoptic observations from the Palomar Testbed Interferometer (PTI) of 106 Miras to curate a Mira Reference Dataset. The Miras included in this dataset include M-types, S-types, and C-types, and span a wide range of pulsation periods. PTI measured K-band angular sizes that when combined with a distance allow us to directly determine fundamental stellar parameters such as effective temperature, radial size, and bolometric flux. Supplementing observations with interferometric measurements of the stars opens the Mira laboratory to a wealth of different experiments. We provide two case studies to serve as examples of the power of the Mira Reference Dataset. The first case study describes combining PTI measurements with Spitzer IRS spectra of M-type Miras, which allowed us to fully characterize CO2 gas in their atmospheres. The second case study examines how PTI narrow-band data can be used to study phase-dependent pulsation effects on the stellar atmosphere. We provide a list of all the Miras (with coordinates) included in the set for anyone who would like to add them to their observing programs. All the data we produce and collate for this Mira Reference Dataset will be hosted and curated on a website open to the public so that other researchers and citizen scientists can participate in expanding the utility and body of knowledge on this set of “wonderful” stars.

Asteroseismology of the Nearby K Dwarf σ Draconis Using the Keck Planet Finder and TESS

Asteroseismology of dwarf stars cooler than the Sun is very challenging owing to the low amplitudes and rapid timescales of oscillations. Here we present the asteroseismic detection of solar-like oscillations at 4-minute timescales ( νmax∼4300 μHz) in the nearby K dwarf σ Draconis using extreme-precision Doppler velocity observations from the Keck Planet Finder and 20 s cadence photometry from NASA’s Transiting Exoplanet Survey Satellite. The star is the coolest dwarf star to date with both velocity and luminosity observations of solar-like oscillations, having amplitudes of 5.9 ± 0.8 cm s−1 and 0.8 ± 0.2 ppm, respectively. These measured values are in excellent agreement with established luminosity−velocity amplitude relations for oscillations and provide further evidence that mode amplitudes for stars with T eff < 5500 K diminish in scale following an (L/M)1.5 relation. By modeling the star’s oscillation frequencies from photometric data, we measure an asteroseismic age of 4.5 ± 0.9 (ran) ± 1.2 (sys) Gyr. The observations demonstrate the capability of next-generation spectrographs and precise space-based photometry to extend observational asteroseismology to nearby cool dwarfs, which are benchmarks for stellar astrophysics and prime targets for directly imaging planets using future space-based telescopes.

Equilibria and the protomodel of the Sun’s atmosphere by Karl Schwarzschild in hindsight

The concepts of radiative and adiabatic equilibria, introduced by Karl Schwarzschild in his seminal paper Ueber das Gleichgewicht der Sonnenatmosphäre published in January 1906, are the founding blocks of the theory of radiative transfer, stellar structure, and solar physics. Careful reading of the paper and its later English translation reveals small formal inaccuracies and ambiguities but with no consequences whatsoever for the final outcomes and conclusions. This paper offers their adjustments with respective derivations using contemporary formalism and sets Schwarzschild’s paper in context with a historical and modern perspective. Particular attention is paid to Schwarzschild’s largely forgotten limb-darkening formula for adiabatic equilibrium. The paper also reproduces Schwarzschild’s radiative equilibrium protomodel of the Sun’s atmosphere in graphical form and compares it with modern models presented in some of the most cited papers in stellar and solar physics.

Ultraluminous X-ray sources with He star companions

ABSTRACT Ultraluminous X-ray sources (ULXs) are non-nuclear point-like objects observed with extremely high X-ray luminosity that exceeds the Eddington limit of a $\rm 10\, M_\odot$ black hole. A fraction of ULXs has been confirmed to contain neutron star (NS) accretors due to the discovery of their X-ray pulsations. The donors detected in NS ULXs are usually luminous massive stars because of the observational biases. Recently, the He donor star in NGC 247 ULX-1 has been identified, which is the first evidence of a He donor star in ULXs. In this paper, we employed the stellar evolution code mesa (Modules for Experiments in Stellar Astrophysics) to investigate the formation of ULXs through the NS+He star channel, in which a He star transfers its He-rich material onto the surface of an NS via Roche lobe overflow. We evolved a large number of NS+He star systems and provided the parameter space for the production of ULXs. We found that the initial NS+He star systems should have $\sim 0.7\!-\!2.6 \, \mathrm{M}_\odot$ He star and $\sim 0.1\!-\!2500\, \mathrm{d}$ orbital period for producing ULXs, eventually evolving into intermediate-mass binary pulsars. According to binary population synthesis calculations, we estimated that the Galactic rate of NS ULXs with He donor stars is in the range of $\sim 1.6\!-\!4.0\times 10^{-4}\, {\rm yr}^{-1}$, and that there exist $\sim 7-20$ detectable NS ULXs with He donor stars in the Galaxy.

Unravelling the Enigmatic Nexus: Black Holes, Dark Matter, and the Interplay of Light, Gravity, and Electromagnetic Forces in Astrophysics and Astronomy

This research introduces a mathematical model positioning our physical reality within a ten-dimensional hyperspace, in which time acts as the unifying coordinate linking three-dimensional electric, magnetic, and gravitational spaces. Each domain is characterized by its respective field—electric, magnetic, or gravitational—and governed by intrinsic divergences and rotations, leading to a universal property known as Force Density, expressed in [N/m³]. This Force Density facilitates interactions among the distinct spaces while adhering to the principle of equilibrium, which posits that cumulative force densities from disturbances must consistently sum to zero. Building upon Einstein&amp;apos;s General Relativity—which describes the curvature of spacetime by gravitational fields and assumes a constant light speed—this study proposes a perspective wherein light speed may vary during coherent laser beam interactions, prompting a re-examination of gravitational and luminous interactions across scales. The proposed model integrates the Stress-Energy Tensor and Gravitational Tensor, introducing a new tensor representation for black holes, termed Gravitational Electromagnetic Confinements, incorporating electromagnetic energy gradients and Lorentz transformations. This framework transcends traditional General Relativity, particularly evident in gravitational lensing. By reinterpreting Einstein&amp;apos;s incorporation of the Gravitational Constant within the Energy-Stress Tensor, this work harmonizes gravity and light, offering insights into black hole solutions resonating with John Archibald Wheeler&amp;apos;s 1955 research. Empirical data from Galileo satellites and MASER frequency measurements underscore discrepancies between established theories and this new model, enhancing the precision of gravitational observations. Through the confluence of Quantum Physics and General Relativity, as seen in approaches like String Theory, this interdisciplinary endeavor revisits the gravitational constant &amp;quot;G,&amp;quot; redefining it while bridging theoretical frameworks, thus paving the way for breakthroughs in astronomical and astrophysical sciences.