Low-mass Stars Research Articles

Molecular inventory of the environment of a young eruptive star. Case study of the classical FU Orionis star V1057 Cyg

II ) sources, in contrast to wide-band coverages at optical and near-infrared wavelengths We carried out the first dedicated wide-band millimeter line survey toward the low-mass young eruptive star and classical FUor V1057 Cyg, which has the highest observed peak accretion rate among FUors. This source is known to have a molecular outflow, and it is associated with dense material. This makes it a good candidate for a search for molecular species. We performed a wide-band spectral line survey of V1057 Cyg with the IRAM 30 m telescope from ∼72 to ∼263,GHz (with a spatial resolution between ∼36arcsec and ∼10arcsec), complemented by on-the-fly maps of selected molecules. We also recorded additional spectra around 219, 227, 291, and 344,GHz (with a spatial resolution between ∼30arcsec and ∼19arcsec) with the APEX 12 m telescope. We conducted simple radiative transfer and population diagram analyses to derive the column densities and excitation temperatures. We constructed integrated-intensity maps of the emission from several molecular species, including those that reveal outflows. These maps and a ^12CO (3--2) position-velocity diagram provide insight into the past outburst activity of the source. We identified mainly simple C-, N-, O-, and S-bearing molecules, deuterated species, molecular ions, and complex organic molecules. Several molecular species (HCN, HC_3N, and HNC) trace large-scale (∼2arcmin) structures in the environment of V1057 Cyg with indications of small-scale fragmentation that remains unresolved by the single-dish data. The position-velocity diagram of ^12CO shows concentrated knots, which may indicate past episodic outburst activity. We calculated the dynamical timescale of the outflow and found it to be on the order of a few ten thousand years (between 15,000 and 22,000,years), similar to other eruptive stars. This suggests that the outflow cannot result from the ongoing outburst alone, since the source has been in the current outburst for less than a century. The population diagrams for species such as CH_3OH, H_2CO, and HC_3N indicate rotational temperatures that range from 8,K to 15,K and column densities that range from 1.4times10^12,cm^-2 to 2.8times10^13,cm^-2. 0/I ) FUors, e.g., V883 Ori. The results of our line survey show that V1057 Cyg is a good candidate for future interferometric observations aimed at resolving emission extents to constrain molecular freeze-out and to search for emission lines of water and additional complex organic molecules. Our observations highlight the potential of millimeter line surveys to characterize the chemistry of eruptive stars and their environments, including more evolved sources, and to complement optical and near-infrared studies in this way to improve current statistics of the molecular inventories of these objects.

Exoplanet Occurrence Rate with Age for FGK Stars in Kepler

Abstract We measure the exoplanet occurrence rate as a function of isochrone and gyrochronology ages using confirmed and candidate planets identified in Q1–17 DR25 Kepler data. We employ Kepler's pipeline detection efficiency to correct for the expected number of planets in each age bin. We examine the occurrence rates for planets with radii 0.2 ≤ Rp ≤ 20 R ⊕ and orbital periods 0.2 ≤ P ≤ 100 days for FGK stars with ages between 1.5 and 8 Gyr using the inverse detection-efficiency method. We find no significant trend between the occurrence rate and stellar age; a slight decreasing trend (within 1.5σ–2.5σ) only emerges for the low-mass and metal-rich stars that dominate our sample. We isolate the effects of mass and metallicity on the occurrence rate trend with age, but find the results to be inconclusive, due to weak trends and the small sample size. Our results hint that the exoplanet occurrence rate may decrease over time due to dynamical instability from planet–planet scattering or planet ejection, but accurate ages and larger sample sizes are needed to resolve a clear relation between the occurrence rate and age.

Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars

Abstract Gaia astrometry of nearby stars is precise enough to detect the tiny displacements induced by substellar companions, but radial velocity (RV) data are needed for definitive confirmation. Here we present RV follow-up observations of 28 M and K stars with candidate astrometric substellar companions, which led to the confirmation of two systems, Gaia-4b and Gaia-5b, identification of five systems that are single lined but require additional data to confirm as substellar companions, and the refutation of 21 systems as stellar binaries. Gaia-4b is a massive planet (M = 11.8 ± 0.7 M J) in a P = 571.3 ± 1.4 day orbit with a projected semimajor axis a 0 = 0.312 ± 0.040 mas orbiting a 0.644 ± 0.02M ⊙ star. Gaia-5b is a brown dwarf (M = 20.9 ± 0.5M J) in a P = 358.62 ± 0.20 days eccentric e = 0.6423 ± 0.0026 orbit with a projected angular semimajor axis of a 0 = 0.947 ± 0.038 mas around a 0.34 ± 0.03M ⊙ star. Gaia-4b is one of the first exoplanets discovered via the astrometric technique, and is one of the most massive planets known to orbit a low-mass star.

GJ 2126 b: A highly eccentric Jovian exoplanet

We report the discovery of GJ,2126,b a highly eccentric (e = 0.85) Jupiter-like planet orbiting its host star every $272.7$ days. The planet was detected and characterized using $112$ radial velocity (RV) measurements from HARPS (High Accuracy Radial velocity Planet Searcher), provided by HARPS-RVBank. This planet orbits a low-mass star and ranks among the most eccentric exoplanets discovered, placing it in a unique region of the parameter space of the known exoplanet population. This makes it a valuable addition to the exoplanet demographics, helping to refine our understanding of planetary formation and evolution theories.

BiSpeD: Binary Spectral Disentangling applied to binary low-mass star searches

Context. In a companion paper, we present a novel method for the spectroscopic detection of low-luminosity secondary companions of binary stars. An interesting application field is the identification of very low-mass stars as companions of late-type main-sequence stars. Aims. To provide a simple tool based on our method, we developed Binary Spectral Disentangling (BiSpeD), a PYTHON repository that provides a toolkit for processing and analyzing spectroscopic observations. The main task of BiSpeD is find2c, which aims to calculate the mass ratio (q) of a single-lined spectroscopic binary from the analysis of a sample of observed spectra. In the same process, an estimate of the secondary effective temperature is obtained. In addition, the toolkit includes other spectral measurement tasks, including spectral disentangling and radial velocity (RV) measurement via cross-correlation. Methods. The BiSpeD package was tested on a sample of 41 late-type stars observed by the High-Accuracy Radial Velocity Planetary Searcher (HARPS). For each star in the sample, we measured the RV for the primary companion and determined the orbital parameters. We then applied the find2c task, which defines a grid of mass ratios and generates the corresponding grid of solutions that reconstruct the secondary spectrum using iterative spectral disentangling. Finally, a modified cross-correlation algorithm compares these secondary spectra with synthetic templates to find the best solution. Results. Our analysis led to the detection of 12 faint companions from the 41 candidates, including five stars below 0.3 M⊙. Among our positive detections are two cases of binaries formed by a red giant primary and a main-sequence secondary. We show that reliable mass ratio values can be obtained even in cases where the orbital period is unknown and the RV amplitude is only a few km s−1.

BPASS stellar evolution models incorporating α-enhanced composition – I. Single star models from 0.1 to 316 M⊙

Abstract Stellar evolution modelling is fundamental to many areas of astrophysics including stellar populations in both nearby and distant galaxies. It is heavily influenced by chemical composition. Observations of distant galaxies and nucleosynthesis calculations show that α-process elements are enriched faster than iron group elements. We present a dense grid of single-star models calculated using the bpass stellar evolution code and covering masses (0.1 ≤ M/M⊙≤316), metallicity mass fractions (10−5 ≤ Z ≤ 0.04) and α-to-iron abundance ratios (−0.2 ≤$[\alpha /\rm {Fe}]$≤+0.6). By comparing Solar-scaled models to ones enriched in α-process elements, we find that stellar radii, surface temperatures, Main Sequence lifetimes, supernova progenitor properties and supernova rates are all sensitive to changes in $[\alpha /\rm {Fe}]$. Lifetimes of low-mass stars differ by up to 0.4 dex, while surface temperatures of massive stars at the end of the Main Sequence also differ by around 0.4 dex. Inferred supernova rates when $[\rm {Fe}/\rm {H}]$ is unknown can be highly uncertain. Models with different $[\alpha /\rm {Fe}]$ but comparable iron abundances show smaller variations, indicating that while iron primarily defines the course of evolution; α-enhancement nonetheless has an impact of up to 0.1 dex on stellar properties. Such changes are small for individual stars, but have a large cumulative effect when considering an entire stellar population as demonstrated by isochrone fitting to nearby clusters. Changes in radii and lifetimes have further consequences for a stellar population including binary stars, as they influence the timing, nature and occurrence rate of mass transfer events.

Modelling chemical clocks. Theoretical evidences of the space and time evolution of [s/α] in the Galactic disc with Gaia-ESO survey

Chemical clocks based on s-process element/α element ratios are widely used to estimate the ages of Galactic stellar populations. However, the s/α versus age relations are not universal, varying with metallicity, location in the Galactic disc, and specific s-process elements. Moreover, current Galactic chemical evolution models struggle to reproduce the observed s/α increase at young ages, particularly for Ba. Our aim is to provide chemical evolution models for different regions of the Milky Way (MW) disc in order to identify the conditions required to reproduce the observed s/H s/Fe and s/α versus age relations. We adopted a detailed multi-zone chemical evolution model for the MW including state-of-the-art nucleosynthesis prescriptions for neutron-capture elements. The s-process elements were synthesised in asymptotic giant branch (AGB) stars and rotating massive stars, while r-process elements originate from neutron star mergers and magneto-rotational supernovae. Starting from a baseline model that successfully reproduces a wide range of neutron-capture element abundance patterns, we explored variations in gas infall/star formation history scenarios, AGB yield dependencies on progenitor stars, and rotational velocity distributions for massive stars. We compared the results of our model with the open clusters dataset from the sixth data release of the Gaia -ESO survey. A three-infall scenario for disc formation aligns better with the observed trends. The models capture the rise of s/α with age in the outer regions but fail towards the inner regions, with larger discrepancies for second s-process peak elements. Specifically, Ba production in the last 3 Gyr of chemical evolution would need to increase by slightly more than half to match the observations. The s-process contribution from low-mass ( 1.1 M_⊙) AGB stars helps reconcile predictions with data but it requires a too-strong increase that is not predicted by current nucleosynthesis calculations, even with a potential i-process contribution. Variations in the metallicity dependence of AGB yields either worsen the agreement or show inconsistent effects across elements, while distributions of massive star rotational velocities with lower velocity at high metallicities fail to improve results due to balanced effects on different elements. The predictions of our model confirm, as expected, that there is no single relationship s/α versus age and that it varies along the MW disc. However, the current prescriptions for neutron-capture element yields are not able to fully capture the complexity of evolution, particularly in the inner disc.

Development of Mass and Energy Rate (Density) of Dissipative Systems Over Their Lifetimes: A Comparison of a Low-Mass Star, Like Our Sun, a Human and the Roman Empire

Energy rate density (ERD) corresponds to the energy flow through a dissipative system to maintain its energy gradient and is defined as the energy rate (ER), normalised to mass. ERD has been proposed by Chaisson [2002] as a single, simple metric for complexity of a wide variety of systems over big time. The main objective of this study is to assess the usefulness of this metric over the lifetimes of dissipative systems. For this purpose, mass and ER(D) data have been collected over the lifetimes of a low-mass star, like our Sun, a human, and the Roman Empire, as representative examples of dissipative systems from the cosmological, biological, and cultural realms. Proxies are very useful for further interpretation of the ER(D) time profiles. ER(D) of a low-mass star as a contracting proto-star (< 0.05 Gyr) as well as during its red giant stage and helium flashes (11 to 12.5 Gyr) is much larger than during its stable appearance as a yellow dwarf star in its main sequence (0.05 to 11 Gyr). ERD of a human peaks at 1.5 yr, reflecting the large relative growth of a baby. ER of a human represents the biochemical activities of the human body, especially of physical activities, and reaches a maximum when a human is in the prime of its life between 20 and 30 yr. Cognitive and emotional performance do not impose any additional energy requirements. For the Roman Empire ERD decreased over time, because the growth rate of the total mass including that of man-made constructs (note that Chaisson only uses mass of humans) was larger than that of ER. ER per capita hardly varied, because human (slave) labour was the main energy source powering the empire. The Roman Empire was at the height of its power between 100 and 200 CE, which is best reflected by ER or simply population. Thus, it is concluded that ERD runs in to several issues, when it is applied as a metric of complexity of systems over their lifetimes. ERD does not capture information processing and, as a result, underestimates the complexity of biological and cultural systems. This hampers its application in a BH context, including the issues of the Big History periodization. Alternative complexity metrics related to information processing need to be developed.

Analyses of Multiple Balmer Emission Lines from Accreting Brown Dwarfs and Very Low Mass Stars

Abstract A planetary growth rate, i.e., the mass accretion rate, is a fundamental parameter in planet formation, as it determines a planet's final mass. Planetary mass accretion rates have been estimated using hydrogen lines, based on the models originally developed for accreting stars, known as the accretion flow model. Recently, Aoyama et al. introduced the accretion shock model as an alternative mechanism for hydrogen line emission. However, it remains unclear which model is more appropriate for accreting planets and substellar objects. To address this, we applied both models to archival data consisting of 96 data points from 76 accreting brown dwarfs and very-low-mass stars, with masses ranging from approximately 0.02 to 0.1 M ⊙, to test which model best explains their accreting properties. The results showed that the emission mechanisms of 15 data points are best explained by the shock model, while 55 data points are best explained by the flow model. For the 15 data points explained by the planetary shock model, the shock model estimates up to several times higher mass accretion rates than the flow model. As this trend is more pronounced for planetary-mass objects, it is crucial to determine which emission mechanism is dominant in individual planets. We also discuss the physical parameters that determine the emission mechanisms and the variability of line ratios.

ALMA detections of circumstellar disks in the giant H ii region M17. Probing the intermediate- to high-mass pre-main-sequence population

Our current understanding is that intermediate- to high-mass stars form in a way similar to low-mass stars, through disk accretion. The expected shorter formation timescales, higher accretion rates, and increasingly strong radiation fields compared to their lower-mass counterparts may lead to significantly different physical conditions that play a role in disk formation, evolution, and the possibility of (sub)stellar companion formation therein. We searched for the mm counterparts of four intermediate- to high-mass ($4-10$ young stellar objects (YSOs) in the giant H ii region M17 at a distance of 1.7 kpc. These objects expose their photospheric spectrum such that their location on the pre-main-sequence (PMS) is well established. They have a circumstellar disk that is likely remnant of the formation process. With ALMA we detected, for the first time, these four YSOs in M17, in Band 6 and 7, as well as four other serendipitous objects. In addition to the flux measurements, the source size and spectral index provide important constraints on the physical mechanism(s) producing the observed emission. We applied different models to estimate the dust and gas mass contained in the disks. All our detections are spatially unresolved, constraining the source size to $<120$,au, and have a spectral index in the range $0.5-2.7$. The derived (upper limits on) the disk dust masses are on the order of a few M_⊕, and estimations of the upper limits on the gas mass vary between 10^-5 and 10^-3 Our modeling suggests that the inner disks of the target YSOs are dust depleted. In two objects (B331 and B268) free-free emission indicates the presence of ionized material around the star. The four serendipitous detections are likely low-mass YSOs. We compared the derived disk masses of our M17 targets to those obtained for YSOs in low-mass star-forming regions (SFRs) and Herbig stars, as a function of stellar mass, age, luminosity, and outer disk radius. The M17 sample, though small, is both the most massive and the youngest sample, yet has the lowest mean disk mass. The studied intermediate- to high-mass PMS stars are surrounded by low-mass compact disks that likely no longer offer a significant contribution to either the final stellar mass or the formation of a planetary system. Along with the four serendipitous discoveries, our findings show the capability of ALMA to probe disks in relatively distant high-mass SFRs, and offer tentative evidence of the influence of the massive star formation environment on disk formation, lifetime, and evolution.

Variation of the low-mass end of the stellar initial mass function with redshift and metallicity

Abstract We report the stellar mass functions obtained from 20 radiation hydrodynamical simulations of star cluster formation in 500 M⊙ molecular clouds with metallicities of 3, 1, 1/10 and 1/100 of the solar value, with the clouds subjected to levels of the cosmic microwave background radiation that are appropriate for star formation at redshifts z = 0, 3, 5, 7, and 10. The calculations include a thermochemical model of the diffuse interstellar medium and treat dust and gas temperatures separately. We find that the stellar mass distributions obtained become increasingly bottom light as the redshift and/or metallicity are increased. Mass functions that are similar to a typical Galactic initial mass function are obtained for present-day star formation (z = 0) independent of metallicity, and also for the lowest-metallicity (1/100 solar) at all redshifts up to z = 10, but for higher metallicities there is a larger deficit of brown dwarfs and low-mass stars as the metallicity and redshift are increased. These effects are a result of metal-rich gas being unable to cool to as lower temperatures at higher redshift due to the warmer cosmic microwave background radiation. Based on the numerical results we provide a parameterisation that may be used to vary the stellar initial mass function with redshift and metallicity; this could be used in simulations of galaxy formation. For example, a bottom-light mass function reduces the mass-to-light ratio compared to a typical Galactic stellar initial mass function, which may reduce the estimated masses of high-redshift galaxies.

Machine learning-based identification of Gaia astrometric exoplanet orbits

ABSTRACT The third Gaia data release (DR3) contains ∼170 000 astrometric orbit solutions of two-body systems located within ∼500 pc of the Sun. Determining component masses in these systems, in particular of stars hosting exoplanets, usually hinges on incorporating complementary observations in addition to the astrometry, e.g. spectroscopy and radial velocities. Several Gaia DR3 two-body systems with exoplanet, brown-dwarf, stellar, and black hole components have been confirmed in this way. We developed an alternative machine learning approach that uses only the Gaia DR3 orbital solutions with the aim of identifying the best candidates for exoplanets and brown-dwarf companions. Based on confirmed substellar companions in the literature, we use semisupervised anomaly detection methods in combination with extreme gradient boosting and random forest classifiers to determine likely low-mass outliers in the population of non-single sources. We employ and study feature importance to investigate the method’s plausibility and produced a list of 20 best candidates, of which two are exoplanet candidates and another five are either very massive brown dwarfs or very low mass stars. Three candidates, including one initial exoplanet candidate, correspond to false-positive solutions where longer-period binary star motion was fitted with a biased shorter-period orbit. We highlight nine candidates with brown-dwarf companions for preferential follow-up. The companion around the Sun-like star G 15-6 could be confirmed as a genuine brown dwarf using external radial-velocity data. This new approach is a powerful complement to the traditional identification methods for substellar companions among Gaia astrometric orbits. It is particularly relevant in the context of Gaia DR4 and its expected exoplanet discovery yield.

4FGL J1544.2−2554: A new spider pulsar candidate

Context. Spider pulsars are millisecond pulsars in tight binary systems in which a low-mass companion star is heated and ablated by the pulsar wind. Observations of these objects allow one to study stellar evolution with the formation of millisecond pulsars and the physics of superdense matter in neutron stars. However, spiders are rare due to difficulties related to their discovery when using typical radio search techniques. The Fermiγ-ray source 4FGL J1544.2−2554 was recently proposed as a pulsar candidate, and its likely X-ray and optical counterparts, with the galactic coordinates l ≈ 344.​​°76, b ≈ 22.​​°59 and the magnitude G ≈ 20.6, were found using the eROSITA and Gaia surveys. Aims. Our goals are to study whether the source is a new spider pulsar and to estimate its fundamental parameters. Methods. We performed the first optical time series multi-band photometry of the object. We used the Lomb-Scargle periodogram to search for its brightness periodicity and fitted its light curves with a model of direct heating of the binary companion by the pulsar wind. Results. The source shows a strong brightness variability with a period of ≈2.724 h and an amplitude of ≳2.5 mag, and its light curves have a single broad peak per period. These features are typical for spider pulsars. The curves are well fitted by the direct heating model, resulting in an orbit inclination of the presumed spider system of ≈83°, a companion mass of ≈0.1 M⊙, ‘day-side’ and ‘night-side’ temperatures of ≈7200 K and ≈3000 K, a Roche lobe filling factor of ≈0.65, and a distance of ≈2.1 kpc. Conclusions. Our findings suggest that 4FGL J1544.2−2554 is a spider pulsar. This result encourages the search for pulsar millisecond pulsations in radio and γ-rays to confirm its nature.

Population synthesis of hot-subdwarf B stars with COMPAS: parameter variations and a prescription for hydrogen-rich shells

Abstract Subdwarf B stars are a well-known class of hot, low-mass stars thought to be formed through interactions in stellar binary systems. While different formation channels for subdwarf B stars have been studied through a binary population synthesis approach, it has also become evident that the characteristics of the found populations depend on the initial set of assumptions that describe the sometimes poorly constrained physical processes, such as common envelope episodes or angular momentum loss during mass transfer events. In this work we present a parameter study of subdwarf B populations, including a novel analytic prescription that approximates the evolution of subdwarf B stars with hydrogen-rich outer shells, an element previously overlooked in rapid binary population synthesis. We find that all studied parameters strongly impact the properties of the population, with the possibility of igniting helium below the expected core-mass value near the tip of the red giant branch strongly affecting the total number of subdwarf B candidates. Critically, our newly proposed prescription for the evolution of subdwarf B stars with hydrogen-shells helps to reconcile theoretical predictions of surface gravity and effective temperature with observational results. Our prescription is useful in the context of rapid binary population synthesis studies and can be applied to other rapid binary population synthesis codes’ output.

Starspots as an Explanation for the Mysterious IYJ Continuum Excess Emission in Classical T Tauri Stars

Abstract An accurate estimation of the continuum excess emission from accretion spots and inner circumstellar disk regions is crucial for a proper derivation of fundamental stellar parameters in accreting systems. However, the presence of starspots can make disentangling the complicated multicomponent emission in these systems challenging. Subtraction of a single-temperature spectral template is insufficient to account for the composite stellar emission, as we demonstrated in a recent campaign involving weak-lined T Tauri stars. Here, we model the moderate-resolution near-infrared spectra of classical T Tauri stars, presenting new spectral models that incorporate spotted stars plus emission from accretion hot spots and a warm inner disk, allowing us to simultaneously reconstruct the entire 0.8–2.4 μm spectrum of our 16 targets. Using these models, we re-derive the continuum excess emission. Our results indicate that accounting for starspots resolves the need to include a previously proposed intermediate-temperature component in the IYJ excess and highlights the importance of a proper treatment of starspots in studies of accreting low-mass stars.

PDS 70b Shows Stellar-like Carbon-to-oxygen Ratio

Abstract The ~5 Myr PDS 70 is the only known system with protoplanets residing in the cavity of the circumstellar disk from which they formed, ideal for studying exoplanet formation and evolution within its natal environment. Here, we report the first spin constraint and C/O measurement of PDS 70b from Keck/KPIC high-resolution spectroscopy. We detected CO (3.8σ) and H2O (3.5σ) molecules in the PDS 70b atmosphere via cross correlation, with a combined CO and H2O template detection significance of 4.2σ. Our forward-model fits, using BT-Settl model grids, provide an upper limit for the spin rate of PDS 70b (<29 km s−1). The atmospheric retrievals constrain the PDS 70b C/O ratio to 0.28 − 0.12 + 0.20 (<0.63 under 95% confidence level) and a metallicity [C/H] of − 0.2 − 0.5 + 0.8 dex, consistent with that of its host star. The following scenarios can explain our measured C/O of PDS 70b in contrast with that of the gas-rich outer disk (for which C/O ≳ 1). First, the bulk composition of PDS 70b might be dominated by dust+ice aggregates rather than disk gas. Another possible explanation is that the disk became carbon enriched after PDS 70b was formed, as predicted in models of disk chemical evolution and as observed in both very low-mass stars and older disk systems with JWST/MIRI. Because PDS 70b continues to accrete and its chemical evolution is not yet complete, more sophisticated modeling of the planet and the disk, and higher-quality observations of PDS 70b (and possibly PDS 70c), are necessary to validate these scenarios.

Search for hot subdwarf stars from SDSS images using a deep learning method: SwinBayesNet

Hot subdwarfs are essential for understanding the structure and evolution of low-mass stars, binary systems, astroseismology, and atmospheric diffusion processes. In recent years, deep learning has driven significant progress in hot subdwarf searches. However, most approaches tend to focus on modelling with spectral data, which are inherently more costly and scarce compared to photometric data. To maximise the reliable candidates, we used Sloan Digital Sky Survey (SDSS) photometric images to construct a two-stage hot subdwarf search model called SwinBayesNet, which combines the Swin Transformer and Bayesian neural networks. This model not only provides classification results but also estimates uncertainty. As negative examples for the model, we selected five classes of stars prone to confusion with hot subdwarfs, including O-type stars, B-type stars, A-type stars, white dwarfs (WDs), and blue horizontal branch stars (BHBs). On the test set, the two-stage model achieved F1 scores of 0.90 and 0.89 in the two-class and three-class classification stages, respectively. Subsequently, with the help of $Gaia$ DR3, a large-scale candidate search was conducted in SDSS DR17. We found 6804 hot-subdwarf candidates, including 601 new discoveries. Based on this, we applied a model threshold of 0.95 and Bayesian uncertainty estimation for further screening, refining the candidates to 3413 high-confidence objects, which include 331 new discoveries.

The physical mechanism for the formation of lithium-rich red clump stars: Rotation, thermohaline mixing, and internal gravity waves

About 0.2-2 percent of red clump stars are revealed as Lithium-rich stars and thus the surface abundance of lithium clearly increases in some red clump stars. The physical mechanism of the enrichment of lithium on the surface of these stars has not yet been explained satisfactorily by the evolutionary models of single stars. Our aim is to investigate how rotation, thermohaline mixing, and internal gravity waves have an important impact on the surface chemical abundance of lithium-rich red giants. The equations for angular momentum transport and the chemical element diffusion for rotating stars have been implemented in this paper. The diffusion coefficients of rotationally induced instabilities, thermohaline mixing, and internal gravitational waves have been included in the diffusion equation of chemical elements. Rotational mixing, thermohaline mixing, and internal gravity waves have been invoked to explain this feature. Rotation impacts the evolution of the surface abundance of Lithium, but it seems an unlikely explanation for a ubiquitous mixing event occurring between the tip of the red giant star and the red clump star. Thermohaline mixing can explain the observed behaviour of $ C/^ C$ and $ N/^ C$ and lithium in low-mass stars that are more luminous than the red-giant branch bump, and its efficiency is decreasing with the increasing initial stellar mass. The internal gravity wave- (IGW-) induced mixing is located between the hydrogen-burning shell, and the outer convective envelope, and it is mainly triggered by turbulent convective motion. This physical process is beneficial to transfer the large amount of $ Be$ to the cool envelope where it is converted to $ Li$. Therefore, IGW-induced mixing could play a main role in explaining the red clump star with lithium enrichment. Rotation can indirectly increase the above effect by making the core-helium-burning lifetime longer. Thermohaline mixing is much smaller than the one of IGWs during the evolution of red clump stars.

Neutron star g modes in the relativistic Cowling approximation

ABSTRACT Mature neutron stars are expected to exhibit gravity g modes due to stratification caused by a varying matter composition in the high-density core. By employing the BSk equation-of-state family, and working within the relativistic Cowling approximation, we examine how subtle differences in the nuclear matter assumptions impact on the g-mode spectrum. We investigate the possibility of detecting individual g-mode resonances during a binary inspiral with current and next-generation ground-based detectors, like Cosmic Explorer and the Einstein Telescope. Our results suggest that these resonances may be within the reach of future detectors, especially for low-mass stars with $M\lesssim 1.4\,\mathrm{ M}_\odot$.

Asymmetry in the tidal tails of open star clusters from direct N-body integrations in Milgrom-law dynamics

Numerical simulations of star clusters in quasilinear modified Newtonian dynamics (QUMOND) orbiting in a Galactic disk potential show that the leading tidal arm of open star clusters typically contain more members than the trailing arm. However, these type of simulations are performed by solving the field equations of QUMOND and have already proven impractical for star cluster masses of around 5000\,$M_ Nearby star clusters exhibit (maximum) masses of 1000\,$M_ (or approx 1000 particles) and cannot be simulated reliably in field-theoretical formulations of modified Newtonian dynamics (MOND) at present. Differences in the formation and evolution of tidal tails of open star clusters in the Newtonian and in the MONDian context are explored in the case of an equal-mass $n=400$ particle cluster ($M_ tot =200\,M_ sun$). To handle particle numbers below the QUMOND-limit, we simulated the star cluster in Milgrom-law dynamics (MLD). Milgrom's law $g_ N M |/a_0)a_ M $ has been postulated to be valid for discrete systems in vectorial form, while MLD resembles QUMOND in that the acceleration of a particle outside any isolated mass concentration scales inversely with the distance. However, in MLD, an internally Newtonian binary will follow a Newtonian rather than a MONDian path around the Galactic centre. To suppress the Newtonisation of compact subsystems in the star cluster, the gravitational force is softened below particle distances of 0.001\,pc approx 206\,AU. Thus, MLD can only be considered as an approximation of a full MOND-theoretical description of discrete systems that are internally in the MOND regime. The MLD equations of motion are integrated by the standard Hermite scheme that is generally applied to Newtonian $N$-body systems. In this work, we have extended this scheme to solve for the accelerations and jerks associated with Milgrom's law. We found that the tidal tails of a low-mass star cluster are populated asymmetrically in the MLD treatment, similarly to what is seen in QUMOND simulations of higher mass star clusters. In the MLD simulations, the leading tail hosts up to twice as many members than the trailing arm, while the low-mass open star cluster dissolves approximately 25<!PCT!> faster than in the respective Newtonian case. Furthermore, the numerical simulations show that the Newtonian integrals of motion are not conserved in MLD. However, the case of an isolated binary in the deep MOND limit can be handled analytically. The velocity of the Newtonian centre of mass does not increase continuously, but instead it wobbles around the constantly moving MLD centre of mass.