On the Identification of N-rich Metal-poor Field Stars with Future Chinese Space Station Telescope

During the long term evolution of globular clusters(GCs), some member stars are lost to the field. The recently found nitrogen-rich(N-rich) metal-poor field stars are promising candidates of these GC escapees, since N enhancement is the fingerprint of chemically enhanced populations in GCs. In this work, we discuss the possibility of identifying N-rich metal-poor field stars with the upcoming Chinese Space Station Telescope(CSST). We focus on the main survey camera with NUV, u, g, r, i, z, y filters and slitless spectrograph with a resolution about 200.The combination of UV sensitive equipment and prominent N-related molecular lines in the UV band bodes well for the identification: the color–color diagram of(u-g) versus(g-r) is capable of separating N-rich field stars from normal halo stars, if metallicity can be estimated without using the information on u-band photometry.Besides, the synthetic spectra show that a signal-to-noise ratio of 10 is sufficient to identify N-rich field stars. In the near future, a large sample of N-rich field stars found by CSST, combined with state-of-the-art N-body simulations will be crucial to deciphering GC-Galaxy co-evolution.

Rotating Massive Strangeon Stars and X-Ray Plateau of Short GRBs

Strangeon stars,which are proposed to describe the nature of pulsar-like compact stars,have passed various observational tests.The maximum mass of a non-rotating strangeon star could be high,which implies that the remnants of binary strangeon star mergers could even be long-lived massive strangeon stars.We study rigidly rotating strangeon stars in the slowly rotating approximation,using the Lennard-Jones model for the equation of state.Rotation can significantly increase the maximum mass of strangeon stars with unchanged baryon numbers,enlarging the mass-range of long-lived strangeon stars.During spin-down after merger,the decrease of radius of the remnant will lead to the release of gravitational energy.Taking into account the efficiency of converting the gravitational energy luminosity to the observed X-ray luminosity,we find that the gravitational energy could provide an alternative energy source for the plateau emission of X-ray afterglow.The fitting results of X-ray plateau emission of some short gamma-ray bursts suggest that the magnetic dipole field strength of the remnants can be much smaller than that of expected when the plateau emission is powered only by spin-down luminosity of magnetars.

Free Energy of Anisotropic Strangeon Stars

Can pulsar-like compact objects release further huge free energy besides the kinematic energy of rotation?This is actually relevant to the equation of state of cold supra-nuclear matter,which is still under hot debate.Enormous energy is surely needed to understand various observations,such asγ-ray bursts,fast radio bursts and softγ-ray repeaters.In this paper,the elastic/gravitational free energy of solid strangeon stars is revisited for strangeon stars,with two anisotropic models to calculate in general relativity.It is found that huge free energy(>10^(46)erg)could be released via starquakes,given an extremely small anisotropy((p_(t)-p_(r))/p_(r)~10^(-4),with pt/pr the tangential/radial pressure),implying that pulsar-like stars could have great potential of free energy release without extremely strong magnetic fields in the solid strangeon star model.

Period–Luminosity Relationship forδScuti Stars Revisited

The Gaia DR3 parallax approach was used to estimate the absolute parameters of 2375δScuti stars from the ASAS catalog.The selected stars have a variety of observational characteristics,with a higher than 80%probability of beingδScuti stars.We have displayed all the stars in the Hertzsprung-Russell diagram along with theδScuti instability strip,the Zero Age Main Sequence and the Terminal Age Main Sequence.Then,we determined which fundamental and overtone modes each star belongs to using pulsation constant(Q)calculations.In addition,we evaluated the parameters in the Q calculation equation using three machine learning methods,which showed that surface gravity and temperature have the greatest effect on its calculation.The Period-Luminosity(P-L)relationship of theδScuti stars was also revisited.Eventually,using least squares linear regression,we made four linear fits for fundamental and overtone modes and updated their relationships.

Variable Stars in the 50BiN Open Cluster Survey.Ⅲ.NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way.Variable stars in open clusters provide a unique way to accurately probe the internal structure,temporal and dynamical evolutionary stages of individual stars and the host cluster.The most powerful tool for such studies is time-domain photometric observations.This paper follows the route of our previous work,concentrating on a photometric search for variable stars in NGC 884.The target cluster is the companion of NGC869,forming the well-known double cluster system that is gravitationally bound.From the observation run in 2016 November,a total of 9247 B-band CCD images and 8218Ⅴ-band CCD images were obtained.We detected a total of 15 stars with variability in visual brightness,including five Be stars,three eclipsing binaries,and seven of unknown types.Two new variable stars were discovered in this work.We also compared the variable star content of NGC 884 with its companion NGC 869.

Life Origin in the Milky Way Galaxy: III. Spatial Distribution of Overheated Stars in the Solar Neighborhood

A concept of ensemble averaged stellar reactors is developed to study the dynamics of processes occurring in stars, allocated in the ~200 pc solar neighborhood. According to the effective temperature value, four stellar classes are identified, for which the correlation coefficients and standard deviation are counted. The theory of the buoyancy terrestial elements is generalized to stellar systems. It was suggested that stars are over-heated due to the shift parameters of the nuclear processes occurring inside the stars, which leads to the synthesis of transuranium elements until the achievement of a critical nuclear mass and star explosion. The heavy transuranium elements sink downward and are concentrated in the stellar depth layers. The physical explanation of the existence of the critical Chandrasekhar star limit has been offered. Based on the spatial analysis of overheated stars, it was suggested that the withdrawal of the stellar reactor from the equilibrium state is a consequence of extragalactic compression inside the galaxy arm due to the arm spirality (not to be confused with the spirality of the galaxy itself).

Velocity Curve Analysis of Spectroscopic Binary Stars AI Phe,GM Dra,HD 93917 and V502 Oph by Nonlinear Regression

We introduce a new method to derive the orbital parameters of spectroscopic binary stars by nonlinear least squares of （o - c）. Using the measured radial velocity data of the four double lined spectroscopic binary systems, AI Phe, GM Dra, HD 93917 and V502 Oph, we derived both the orbital and combined spectroscopic elements of these systems. Our numerical results are in good agreement with the those obtained using the method of Lehmann-Filhés.

The Binarity of Early-type Stars from LAMOST medium-resolution Spectroscopic Survey

Massive binaries play significant roles in many fields.Identifying massive stars,particularly massive binaries,is of great importance.In this paper,by adopting the technique of measuring the equivalent widths of several spectral lines,we identified 9382 early-type stars from the LAMOST medium-resolution survey and divided the sample into four groups,T1(~O-B4),T2(~B5),T3(~B7)and T4(~B8-A).The relative radial velocities RVrelwere calculated using Maximum Likelihood Estimation.The stars with significant changes of RVreland at least larger than 15.57 km s-1 were identified as spectroscopic binaries.We found that the observed spectroscopic binary fractions for the four groups are 24.6%±0.5%,20.8%±0.6%,13.7%±0.3%and 7.4%±0.3%,respectively.Assuming that orbital period(P)and mass ratio(q)have intrinsic distributions as f(P)∝Pπ(1<P<1000 days)and f(q)∝qκ(0.1<q<1),respectively,we conducted a series of Monte-Carlo simulations to correct observational biases for estimating the intrinsic multiplicity properties.The results show that the intrinsic binary fractions for the four groups are 68%±8%,52%±3%,44%±6%and 44%±6%,respectively.The best estimated values forπare-1±0.1,-1.1±0.05,-1.1±0.1 and-0.6±0.05,respectively.Theκcannot be constrained for groups T1 and T2 and is-2.4±0.3 for group T3 and-1.6±0.3 for group T4.We confirmed the relationship of a decreasing trend in binary fractions toward late-type stars.No correlation between the spectral type and orbital period distribution has been found yet,possibly due to the limitation of observational cadence.

Spectroscopic study of Be-shell stars:4 Her and 88 Her

We present an optical spectroscopic study based on 41 spectra of 4 Her and 32 spectra of 88 Her, obtained over a period of 6 months. We estimate the rotational velocity of these stars from HeI lines in the blue spectral region （4000-4500 A）. We find that these stars are likely to be rotating at a fractional critical rotation of -0.80. We measure the average Ip/lc ratio to quantify the strength of the Ha line and obtain 1.63 for 4 Her and 2.06 for 88 Her. The radius of the Ha emission region is estimated to be Ra/R. -5.0, assuming a Keplerian disk. These stars are thus found to be fast rotators with a relatively small Hoe emission region. We detect V/R variation of the Ha spectral line during the observed period. We re-estimate the periods for both stars and obtain a period of ,-46 d and its harmonic of 23.095 d for 4 Her, and a period of -86 d for 88 Her. As these two cases are shell stars with binaries and have low Ha EW with the emission region closer to the central star, the V/R variation and a change in period may be an effect of the binary on the circumstellar disk.

Application of a Probabilistic Neural Network in Radial Velocity Curve Analysis of the Spectroscopic BinaryStars ROXR1 14, RX J1622.7-2325Nw, RR Lyn, 12 Boo and HR 6169

Using measured radial velocity data of five double-lined spectroscopic binary systems ROXR1 14, RX J1622.7-2325Nw, RR Lyn, 12 Boo and HR 6169, we find corresponding orbital and spectroscopic elements via a Probabilistic Neural Network (PNN). Our numerical results are in good agreement with those obtained by others using more traditional methods.

The First Photometric and Spectroscopic Study of Contact Binary V2840 Cygni

The first photometric,spectroscopic and period variation studies of neglected short-period eclipsing binary V2840Cygni are presented.High mass ratio contact binaries(HMRCBs),especially those in the weak-contact configuration,are vital when probing the evolutionary models of contact binaries(CBs) using stellar parameters.The photometric solutions reveal the weak-contact nature of V2840 Cygni with a high mass ratio(~1.36),motivating us to investigate the nature of such binaries.The period variation study of V2840 Cygni spanning 15 yr shows a secular period decrease at a rate of ~5.5 × 10^(-7) day yr^(-1),indicating mass transfer between the components.The superimposed cyclic variation provides a basic understanding of the possible third body(P_(3)- 8 yr,m_(3)- 0.51 M_(⊙)).Following the derived parameters,the evolution of the system is discussed based on the thermal relaxation oscillation(TRO) model.It is found that V2840 Cygni falls in a special category of HMRCBs,which validates TRO.To characterize the nature of HMRCBs,a catalog of 59 CBs with high mass ratios has been compiled along with their derived parameters from the literature.For all the HMRCBs in the study,a possible correlation between their contact configuration and observed period variations for relative log J_(rel) is discussed.The spectroscopic study of V2840 Cygni provides evidence of the presence of magnetic activity in the system and the existence of ongoing mass transfer which is additionally deduced from the period variation study.The LAMOST spectra of 17 HMRCBs are collected to interpret the stellar magnetic activity in such systems.

Identifying Symbiotic Stars with Machine Learning

Symbiotic stars are interacting binary systems, making them valuable for studying various astronomical phenomena, such as stellar evolution, mass transfer, and accretion processes. Despite recent progress in the discovery of symbiotic stars, a significant discrepancy between the observed population of symbiotic stars and the number predicted by theoretical models. To bridge this gap, this study utilized machine learning techniques to efficiently identify new symbiotic star candidates. Three algorithms(XGBoost, LightGBM, and Decision Tree)were applied to a data set of 198 confirmed symbiotic stars and the resulting model was then used to analyze data from the LAMOST survey, leading to the identification of 11,709 potential symbiotic star candidates. Out of these potential symbiotic star candidates listed in the catalog, 15 have spectra available in the Sloan Digital Sky Survey(SDSS) survey. Among these 15 candidates, two candidates, namely V^(*)V603 Ori and V^(*)GN Tau, have been confirmed as symbiotic stars. The remaining 11 candidates have been classified as accreting-only symbiotic star candidates. The other two candidates, one of which has been identified as a galaxy by both SDSS and LAMOST surveys, and the other identified as a quasar by SDSS survey and as a galaxy by LAMOST survey.

Application of a probabilistic neural network in analysis of the radial velocity curve of spectroscopic binary stars

Using measured radial velocity data of five double-lined spectroscopic binary systems,HD 89959,HD 143705,HD 146361,HD 165052 and HD 152248, we find corresponding orbital and spectroscopic elements via a Probabilistic Neural Network.Our numerical results are in good agreement with those obtained by others using more traditional methods.

Sodium Abundances in Very Metal-poor Stars

Chemical composition of very metal-poor(VMP)stars can provide observational constraints on current models of nucleosynthesis and the chemical evolution of the Galaxy.It has been found that the scatter of[Na/Fe]versus[Fe/H]in VMP stars is very large in contrast with most other elements.Moreover,a negative slope in[Na/Fe]versus[Fe/H]was found for giants,which is very unlikely according to the theory of nucleosynthesis.For the sample of93 VMP stars in the metallicity range-4.25<[Fe/H]<-1.64 we obtained NLTE sodium abundances using the line profile fitting method by employing accurate atmospheric parameters determined when taking into account NLTE line formation for both FeⅠand FeⅡ.Originally selected from the LAMOST low-resolution spectral database,the spectra of stars were obtained with the High Dispersion Spectrograph of the Subaru Telescope.For 57turn-off stars in metallicity domain-3.04<[Fe/H]<-1.64,we obtained mean[Na/Fe]=-0.29±0.14 and positive slope 0.09±0.06.For 21 giants distributed over metallicity-3.59<[Fe/H]<-2.19,we found mean[Na/Fe]=-0.35±0.1 and positive slope 0.07±0.07.Our[Na/Fe]trend is lower by~0.2 dex,compared to the modern GCE model.We believe the GCE model should be adjusted,by considering the associated scatter.Twelve stars in our sample are found to be outliers,with too low or too high Na abundances.

New Cases of Superflares on Slowly Rotating Solar-type Stars and Large Amplitude Superflares in G-and M-type Main Sequence Stars

In our previous work,we searched for superflares on different types of stars while focusing on G-type dwarfs using entire Kepler data to study statistical properties of the occurrence rate of superflares.Using these new data,as a byproduct,we found 14 cases of superflare detection on 13 slowly rotating Sun-like stars with rotation periods of24.5–44 days.This result supports the earlier conclusion by others that the Sun may possibly undergo a surprise superflare.Moreover,we found 12 and seven new cases of detection of exceptionally large amplitude superflares on six and four main sequence stars of G-and M-type,respectively.No large-amplitude flares were detected in A,F or K main sequence stars.Here we present preliminary analysis of these cases.The superflare detection,i.e.,an estimation of flare energy,is based on a more accurate method compared to previous studies.We fit an exponential decay function to flare light curves and study the relation between e-folding decay time,τ,versus flare amplitude and flare energy.We find that for slowly rotating Sun-like stars,large values ofτcorrespond to small flare energies and small values ofτcorrespond to high flare energies considered.Similarly,τis large for small flare amplitudes andτis small for large amplitudes considered.However,there is no clear relation between these parameters for large amplitude superflares in the main sequence G-and M-type stars,as we could not establish clear functional dependence between the parameters via standard fitting algorithms.

Optical Spectroscopy of Classical Be Stars in Old Open Clusters

We performed the optical spectroscopy of 16 classical Be stars in 11 open clusters older than 100 Myr.Ours is the first spectroscopic study of classical Be stars in open clusters older than 100 Myr.We found that the Hαemission strength of most of the stars is less than 40?,in agreement with previous studies.Our analysis further suggests that one of the stars,[KW97]35–12,might be a weak Hαemitter in nature,showing Hαequivalent width of-0.5?.Interestingly,we also found that the newly detected classical Be star LSⅢ+4737b might be a component of the possible visual binary system LSⅢ+4737,where the other companion is also a classical Be star.Hence,the present study indicates the possible detection of a binary Be system.Moreover,it is observed that all 16 stars exhibit a lesser number of emission lines compared to classical Be stars younger than 100 Myr.Furthermore,the spectral type distribution analysis of B-type and classical Be stars for the selected clusters points out that the existence of CBe stars can depend on the spectral type distribution of B-type stars present in these clusters.

Analysis of X-Ray Emission from OB Stars. IV. About X-Ray Emission from Inhomogeneous Winds of OB Stars

We study the origin of X-ray emission from OB stars due to collisions of stellar winds and/or inhomogeneities in the winds. The low-resolution X-ray spectra of a big sample of OB stars were fitted by both the stationary APEC/MEKAL models and by this model with an additional PSHOCK component describing the nonstationary X-ray emission. These spectra were also described by two-temperature PSHOCK models. More than ~50% of considered spectra can be described by the above-mentioned model combinations including the PSHOCK model and the quality of the fits appears to be better for O stars. The plasma temperature of the PSHOCK component is about 1–5 keV with the ionization timescale τ_(u)~ 10^(8)–10^(13)s cm^(-3). The temperature of the PSHOCK component increases with the momentum and kinetic energy of the stellar wind by a power law with an index ~0.12–0.14.Such dependencies were not revealed through modeling by the stationary APEC/MEKAL models only. At the same time the X-ray luminosity of OB stars depends on momentum and kinetic energy of their winds similarly either for stationary or for nonstationary models. We conclude that many O stars and some B stars can be sources of the nonstationary X-rays formed in their inhomogeneous stellar wind.

An Update of the Catalog of Radial Velocity Standard Stars from the APOGEE DR17

We present an updated catalog of 46,753 radial velocity(RV)standard stars selected from the APOGEE DR17.These stars cover the Northern and Southern Hemispheres almost evenly,with 62%being red giants and 38%being main sequence stars.These RV standard stars are stable on a baseline longer than 200 days(with 54%longer than one year and 10%longer than five years)with a median stability better than 215 m s~(-1).The average number of observations of those stars is 5 and each observation is required to have signal-to-noise ratio greater than50 and RV measurement error smaller than 500 m s~(-1).Based on the new APOGEE RV standard star catalog,we have checked the RV zero-points(RVZPs)for current large-scale stellar spectroscopic surveys including RAVE,LAMOST,GALAH and Gaia.By careful analysis,we estimate their mean RVZP to be+0.149 kms~(-1),+4.574 km s~(-1)(for LRS),-0.031 km s~(-1)and+0.014 kms~(-1),respectively,for the four surveys.In the RAVE,LAMOST(for MRS),GALAH and Gaia surveys,RVZP exhibits a systematic trend with stellar parameters(mainly[Fe/H],T_(eff),log g,G_(BP)-G_(RP)and G_(RVS)).The corrections to those small but clear RVZPs are of vital importance for these massive spectroscopic surveys in various studies that require extremely high RV accuracies.

Asteroseismology of 16 Kepler Solar-like Stars:Stellar Parameters and the Effects of Element Diffusion

Element diffusion has small but significant effects on the structure of the stellar interior.It is interesting to investigate the effects of element diffusion using asteroseismology.We have constructed two grids of stellar models,one with diffusion and one without,for solar-like stars with masses between 0.9 and 1.4 solar masses,and varied initial helium abundance and metallicity.The oscillation frequencies of all stellar models have also been calculated.Piecewise Hermite cubic polynomials are adopted to interpolate stellar p-mode frequencies at an arbitrary age on a stellar evolutionary track.We have investigated 16 Kepler solar-like stars by comparing the model frequencies with observations.The suggested ranges of stellar parameters and some global variables are obtained.For all stars,the best model reproduces the observational frequencies with aχ^(2)of the order of unity.It has been found that element diffusion is important in modeling solar-like stars.Without diffusion,the best value of the initial helium abundance is below the primordial helium abundance from Big Bang nucleosynthesis.When diffusion is taken into account,the required initial helium abundance increases to be higher than the primordial abundance.Diffusion also generally improves the frequency fitting results by reducing the minimum ofχ^(2).Investigation of the second difference of the oscillation frequencies on KIC 8694723 and KIC 10454113 indicates that the current model of element diffusion may underestimate the strength of settling.

Spectroscopic Survey of Hα Emission Line Stars Associated with Bright Rimmed Clouds

The results of a spectroscopic survey of Hα emission line stars associated with fourteen bright rimmed clouds are presented. Slit-less optical spectroscopy was carried out with the Inter University Centre for Astronomy and Astrophysics (IUCAA) 2 m telescope and IUCAA Faint Object Spectrograph and Camera (IFOSC). Hα emission line was detected from 173 objects. Among them 85 objects have a strong Hα emission line with its equivalent width larger than 10 ?. Those are classical T Tauri stars. 52 objects have a weak Hα emission line with its equivalent width less than 10 ? and do not show intrinsic near-infrared excess. Those are weak-line T Tauri stars. On the other hand, 36 objects have a weak Hα emission line (?), although they show intrinsic near-infrared excess. Such objects are not common in low-mass star forming regions. Those are misfits of the general concept on formation process of a low-mass star, in which it evolves from a classical T Tauri star to a weak-line T Tauri star. Those might be weak-line T Tauri stars with a flared disk in which gas is heated by ultraviolet radiation from a nearby early-type star. Alternatively, we propose pre-transitional disk objects as their evolutional stage.