Spectroscopic and photometric studies of a candidate pulsating star in an eclipsing binary:V948 Her

Eclipsing binary stars with a pulsating component are powerful tools that allow us to probe the stellar interior structure and the evolutionary statuses with a good accuracy. Therefore, in this study,spectroscopic and photometric examinations of an eclipsing binary system V948 Her are presented. The primary component of the system is classified to be a candidate δ Scuti variable in the literature. The fundamental stellar, atmospheric and orbital parameters, and the surface abundance of the star were determined and the pulsation behaviour was investigated in this study. The orbital parameters were derived by the analysis of radial velocity and Super WASP light curves. The spectral classification was found to be F2V. The initial atmospheric parameters of the primary component were derived by analysis of the spectral energy distribution and hydrogen lines. The final atmospheric parameters and chemical abundances of the primary component were obtained by using the method of spectrum synthesis. As a result, the final atmospheric parameters were determined as T_(eff)= 7100±200 K, log g= 4.3±0.1 cgs and ξ = 2.2 ± 0.2 km s~(-1). The surface abundance was found to be similar to solar. The fundamental stellar parameters of both components were also obtained to be M = 1.722 ± 0.123, 0.762 ± 0.020 M⊙, R =1.655 ± 0.034, 0.689 ± 0.016 R⊙for primary and secondary components, respectively. The pulsation characteristic of the primary component was examined using Super WASP data and the pulsation period was found to be ~0.038 d. The position of the primary pulsating component was also obtained inside the instability strip of δ Sct stars. The primary component of V948 Her was defined to be a δ Sct variable.     

KIC 5197256: an eclipsing binary containing a δ Scuti variable star

This paper reports analysis of an eclipsing binary system KIC 5197256 with a δ Sct variable component.Utilizing light-curve modeling,several stellar parameters are derived,e.g.,temperature,mass,and mass ratio.The O-C diagram is a straight line with a negative slope which means that its period is almost constant for about 2 yr.Frequency analyses are performed for the residual light curve after subtracting the binary variations.The frequency spectrum reveals that one component star is a δ Scuti variable.Large frequency separation is cross-identified with the histogram graph and the Fourier transform method.Based on the large separation and density relationship,the mean density of the δ Sct component is estimated to be 0.05 g·cm~(-3).Five frequencies with the same frequency spacing in the range of 25 d~(-1)-34 d~(-1) are detected.Statistically,the pulsation amplitudes of δ Sct stars increase with decreasing of rotations,so we propose that KIC 5197256 might have a relatively large rotational velocity,and the frequency f_(10) might be the rotation frequency.     

More than two hundred and fifty thousand spectroscopic binary or variable star candidates discovered by LAMOST

About 786.4 thousand stars were observed by LAMOST twice or more during the first stage of its spectroscopic survey. The radial velocity differences for about 256 thousand targets are larger than10 km s~(-1) and they are possible spectroscopic binary or variable candidates(SBVCs). It is shown that most SBVCs are slightly metal poorer than the Sun. There are two peaks in the temperature distribution of SBVCs around 5760 K and 4870 K, while there are three peaks in the distribution of the gravitational acceleration at 2.461, 4.171 and 4.621 cm s~(-2). The locations of SBVCs on the [Fe/H]-T, [Fe/H]-log g, log g-T and H-R diagrams are investigated. It is found that the detected SBVCs could be classified into four groups. The first group has higher log g～4.621 and lower T ～ 4870 K which are mainly cool red dwarf binaries. The second group of SBVCs has logg around 4.171 cm s~(-2) that includes binaries and pulsating stars such as δSet and γ Dor variables. The gravitational accelerations of the third group of SBVCs are higher and some of them are below the zero-age main sequence. They may be contact binaries in which the primary components are losing energy to the secondaries in the common envelopes and are at a special stellar evolutionary stage.The last group is composed of giants or supergiants with log g around 2.461 cm s~(-2) that may be evolved pulsating stars. One target(C134624.29+333921.2) is confirmed as an eclipsing binary with a period of 0.65 days. A preliminary analysis suggests that it is a detached binary with a mass ratio of 0.46. The primary fills its critical Roche lobe by about 89%, indicating that mass transfer will occur between the two components.     

Time-dependent Behaviour of the Low Amplitude δ Scuti Star HD 52788

Wavelet transform is applied to reanalyze the low amplitude δ Scuti star HD 52788, which exhibits complex light variations with uncertain frequency solutions. We gain an insight into the strong instability of pulsation present in the star. Based on an estimate of the star‘s physical parameters, its evolutionary status is determined. An attempt of asteroseismic modelling failed to predict the observed dense frequencies. Because of its varying pulsation spectrum, HD 52788 is a distinctive and very interesting object among δ Sct stars for testing current models of stellar evolution and pulsation.     

Variable stars in the field of open cluster NGC 2126

We report the results of a time-series CCD photometric survey of variable stars in the field of open cluster NGC 2126. In about a one square degree field covering the cluster, a total of 21 variable candidates are detected during this survey, of which 16 are newly found. The periods, classifications and spectral types of 14 newly discovered variables are discussed, which consist of six eclipsing binary systems, three pulsating variable stars, three long period variables, one RS CVn star, and one W UMa or δ Scuff star. In addition, there are two variable candidates, the properties of which cannot be determined. By a method based on fitting observed spectral energy distributions of stars with theoretical ones, the membership probabilities and the fundamental parameters of this cluster are determined. As a result, five variables are probably members of NGC 2126. The fundamental parameters of this cluster are determined as： metallicity to be 0.008 Z, age log（t） = 8.95, distance modulus （m - M）0 = 10.34 and reddening value E（B - V） = 0.55 mag.     

S1280 and S1284： Two Oscillating Blue Stragglers in the Open Cluster M67

We present results of a time-series CCD photometry of two blue stragglers in the open cluster M67 that are also oscillating variables, S1280 and S1284. The observations obtained on 11 nights confirmed the δ Scuti-like variability of the two stars. Four and five main pulsating frequencies are detected for S1280 and S1284, respectively, through a power spectral analysis. A preliminary mode identification indicates that the two stars are both in radial oscillation. Based on the nature of oscillation, the physical parameters of the two stars are determined, and their evolutionary status discussed.     

A comparison of stellar atmospheric parameters from the LAMOST and APOGEE datasets

We have compared stellar parameters, including temperature, gravity and metallicity, for common stars in the LAMOST DR2 and SDSS DR12/APOGEE datasets. It is found that the LAMOST dataset provides a more well-defined red clump feature than the APOGEE dataset in the Teff versus log g diagram. With this advantage, we have separated red clump stars from red giant stars, and attempt to establish calibrations between the two datasets for the two groups of stars. The results show that there is a good consistency in temperature with a calibration close to the one-to-one line, and we can establish a satisfactory metallicity calibration of[Fe/H]APOGEE= 1.18[Fe/H]LAMOST + 0.11 with a scatter of ～ 0.08 dex for both the red clump and red giant branch samples. For gravity, there is no correlation for red clump stars between the two datasets, and scatters around the calibrations of red giant stars are substantial. We found two main sources of scatter in log g for red giant stars. One is a group of stars with 0.00253 × Teff- 8.67 log g 2.6 located in the forbidden region, and the other is the contaminated red clump stars, which could be picked out from the unmatched region where stellar metallicity is not consistent with position in the Teff versus log g diagram. After excluding stars in these two regions,we have established two calibrations for red giant stars, log g APOGEE = 0.000615 ×Teff,LAMOST+ 0.697 × log g LAMOST- 2.208(σ = 0.150) for [Fe/H] -1 and log gAPOGEE= 0.000874×Teff,LAMOST+0.588×log g LAMOST-3.117(σ = 0.167)for [Fe/H] -1. The calibrations are valid for stars with Teff = 3800- 5400 K and log g = 0- 3.8 dex, and are useful in work aiming to combine the LAMOST and APOGEE datasets in a future study. In addition, we find that an SVM method based on asteroseismic log g is a good way to greatly improve the accuracy of gravity for these two regions, at least in the LAMOST dataset.     

长周期变星的脉冲

Supergiant and AGB LPVs are investigated by calculating both their evolution and pulsations. The fundamental mode is thought to be the most possible pulsation mode for supergiant LPVs, and the first overtone for AGB LPVs. The mixing length and mass loss may result in some effects on the pulsations. A very large mass loss rate is demanded in the late stage of intermediate mass stellar evolution, however, the mass loss rate may not be quite large in the massive stars before their red supergiant phase. There may exist a strong stellar wind which is related with pulsation for long-period variables.     

On the LSP3 estimates of surface gravity for LAMOST-Kepler stars with asteroseismic measurements

Asteroseismology allows for deriving precise values of the surface gravity of stars. The accurate asteroseismic determinations now available for the large number of stars in the Kepler fields can be used to check and calibrate surface gravities that are currently being obtained spectroscopically for a huge number of stars targeted by large-scale spectroscopic surveys, such as the on-going Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) Galactic survey. The LAMOST spectral surveys have obtained a large number of stellar spectra in the Kepler fields. Stellar atmospheric parameters of those stars have been determined with the LAMOST Stellar Parameter Pipeline at Peking University(LSP3), by template matching with the MILES empirical spectral library. In the current work, we compare surface gravities yielded by LSP3 with those of two asteroseismic samples — the largest Kepler asteroseismic sample and the most accurate Kepler asteroseismic sample. We find that LSP3 surface gravities are in good agreement with asteroseismic values of Hekker et al., with a dispersion of ~0.2 dex. Except for a few cases, asteroseismic surface gravities of Huber et al. and LSP3 spectroscopic values agree for a wide range of surface gravities.However, some patterns in the differences can be identified upon close inspection. Potential ways to further improve the LSP3 spectroscopic estimation of stellar atmospheric parameters in the near future are briefly discussed. The effects of effective temperature and metallicity on asteroseismic determinations of surface gravities for giant stars are also discussed.     

Exploring the spectral information content in the LAMOST medium-resolution survey(MRS)

Although high-resolution stellar spectra allow us to derive precise stellar labels(effective temperature, metallicity, surface gravity, elemental abundances, etc.) based on resolved atomic lines and molecular bands, low-resolution spectra have been proved to be competitive in determining many stellar labels at comparable precision. It is useful to consider the spectral information content when assessing the capability of a stellar spectrum in deriving precise stellar labels. In this work, we quantify the information content brought by the LAMOST-II medium-resolution spectroscopic survey(MRS) using the gradient spectra as well as the coefficients-of-dependence(CODs). In general, the wavelength coverage of the MRS well constrains the stellar labels but the sensitivities of different stellar labels vary with spectral types and metallicity of the stars of interest. Consequently, this affects the performance of the stellar label determination from the MRS spectra. By applying the SLAM method to the synthetic spectra which mimic the MRS data, we find that the precision of the fundamental stellar parameters Teff, log g and [M/H] are better when combining both the blue and red bands of the MRS. This is especially important for warm stars because the Hα line located in the red part plays a more important role in determining the effective temperature for warm stars. With blue and red parts together, we are able to reach similar performance to the low-resolution spectra except for warm stars. However, at [M/H] ~-2.0 dex, the uncertainties of fundamental stellar labels estimated from MRS are substantially larger than that from low-resolution spectra. We also tested the uncertainties of T_(eff), log g and [M/H] from MRS data induced from the radial velocity mismatch and find that a mismatch of about 1 km s~(-1), which is typical for LAMOST MRS data, would not significantly affect the stellar label estimates. Finally, reference precision limits are calculated using synthetic gradient spectra, according to which we expect abundances of at least 17 elements to be measured precisely from MRS spectra.     

ASAS J174406+2446.8 is identified as a marginal-contact binary with a possible cool third body

ASAS J174406+2446.8 was originally found as a δ Scuti-type pulsating star with the period P=0.189068 d by ASAS survey.However,the LAMOST stellar parameters reveal that it is far beyond the red edge of pulsational instability strip on the log g-T diagram of δ Scuti pulsating stars.To understand the physical properties of the variable star,we observed it by the 1.0-m Cassegrain reflecting telescope at Yunnan Observatories.Multi-color light curves in B,V,R_c and I_c bands were obtained and are analyzed by using the W-D program.It is found that this variable star is a shallow-contact binary with an EB-type light curve and an orbital period of 0.3781 d rather than a δ Scuti star.It is a W-subtype contact binary with a mass ratio of 1.135(±0.019) and a fill-out factor of 10.4%(±5.6)%.The situation of ASAS J174406+2446.8 resembles those of other EB-type marginal-contact binaries such as UU Lyn,Ⅱ Per and GW Tau.All of them are at a key evolutionary phase from a semi-detached configuration to a contact system predicted by the thermal relaxation oscillation theory.The linear ephemeris was corrected by using 303 new determined times of light minimum.It is detected that the O-C curve shows a sinusoidal variation that could be explained by the light-travel-time effect via the presence of a cool red dwarf.The present investigation reveals that some of the δ Scuti-type stars beyond the red edge of pulsating instability strip on the log g-T diagram are misclassified eclipsing binaries.To understand their structures and evolutionary states,more studies are required in the future.     

盾牌座δ型星的混沌现象的测定与研究

Chaos is universal phenomenon in the nature. Some scientists think that chaos theory is the third revolution in physics of this century, following relativity theory and quantum mechanics. The δSct type stars are pulsating variable stars with light amplitudes in Ⅴ from 0~m.003to 0~m.9 and periods from 0~d.01 to 0~d.2. The spectral types are A0-F5Ⅲ-Ⅴ. Some stars such as ρ Puppis and AD CMi show light curves which repeat beautifully from cycle to cycle.The light curves of many stars, especially the small-amplitude variables, often appear vari-     

Abundances of 23 field RR Lyrae stars

We present stellar parameters and abundances of 15 elements （Na, Mg, A1, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Y and Ba） for 23 field RR Lyrae variables based on high-resolution （R ~ 60 000） and high signal-to-noise （S/N~ 200） spectra obtained using the High Dispersion Spectrograph on the Subaru Telescope. Six stars in the sample have more than one spectrum observed at different pulsation phases. The derived abundance ratios of [X/Fe] for 14 elements （except for Ba） do not vary during the pulsation cycle. An interesting curve of [Ba/Fe] versus phase is detected for the first time and it shows decreasing [Ba/Fe] with increasing temperature at a given metallicity. Combining with data in the literature, abundances of most RR Lyrae stars as a function of [Fe/H] follow the same trends as those of dwarf stars, but [Sc/Fe] and [Y/Fe] ratios of RR Lyrae stars at solar metallicity are lower than those of dwarf stars. The kinematics of RR Lyrae stars indicate that three comparatively metal-rich RR Lyrae stars might originate from the thick disk and they show higher [a/Fe] ratios than RR Lyrae stars with thin disk kinematics. Among 23 RR Lyrae stars, two special objects are found with abnormal abundances; TV Lib has high [a/Fe], [Sc/Fe], [Y/Fe] and [Ba/Fe] ratios while TW Her has solar [a/Fe] but significantly lower [Sc/Fe], [Y/Fe] and [Ba/Fe] ratios as compared with other RR Lyrae stars.     

LASPM:the LAMOST stellar parameter pipeline for M-type stars and application to the sixth and seventh data release(DR6 and DR7)

The molecular-rich atmospheres of M type stars complicate our understanding to their atmospheric properties.Recently,great progress has been made in atmospheric modeling of M-type stars,and we take advantage of the updated BT-Settl model grid to develop a pipeline LAS PM to measure atmospheric parameters(T_(eff),log g,[M/H]) of M-type stars from low-resolution spectra.The pipeline was applied to the sixth and seventh data release(DR6 DR7) of Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST),which released atmospheric parameters for 610419 and 680 185 Mtype spectra,respectively.The key algorithm is to find the best-matching for templates in the synthetic spectral library for an observed spectrum,and then minimizing χ~2 through a linear combination of five best-matching templates.The intrinsic precisions of the parameters were estimated by using the multiple epoch observations for the same stars,which are 118 K,0.20 dex,0.29 dex for T_(eff),log g,and [M/H]respectively.The Teff and log g are consistent with the spectral and luminosity classifications by LAMOST 1D pipeline,and the loci of giants and dwarfs both on spectral index and color-magnitude diagrams show the validity.The metallicities of LASPM are also checked with the selected members of four open clusters(NGC 2632,Melotte 22,ASCC_16,and ASCC_19),which are consistent without any bias.Comparing the results between LASPM and the APOGEE Stellar Parameter and Chemical Abundance Pipeline(ASPCAP),there is a scatter of 73 K,0.22 dex,0.21 dex for T_(eff),log g,and [M/H],respectively.     

An upgraded interpolator of the radial basis function network for spectral calculation based on empirical stellar spectral library

Stellar population synthesis is an important method in galaxy and star-cluster studies. In stellar population synthesis models, a stellar spectral library is necessary for the integrated spectra of the stellar population. Usually, the stellar spectral library is utilized for the transformation between the stellar atmospheric parameters and stellar spectrum. The empirical stellar spectral library has distinct advantages over the theoretical library. However, for the empirical spectral library, the distribution of stars is irregular in the stellar atmospheric parameter space, which makes the traditional interpolator difficult to get accurate results. In this work, we will provide an improved radial basis function interpolator which is implemented to obtain the interpolated stellar spectra based on the empirical stellar spectral library. For this interpolator, we use the relation between the standard variance σ in the Gaussian radial basis function and the density distribution of stars in the stellar atmospheric parameter space to give the prior constraint on this σ. Moreover,we also consider the anisotropic radial basis function by the advantage of the local dispersion of stars in the stellar atmospheric parameter space. Furthermore, we use the empirical stellar spectral library MILES to test this interpolator. On the whole, the interpolator has a good performance except for the edge of the low-temperature region. At last, we compare this interpolator with our work in 2018, and the interpolation result shows an obvious improvement. Users can apply this interpolator to retrieve the interpolated spectra based on the stellar spectral library quickly and easily.     

A triplet of the only pulsation mode detected in the DAV star G132–12

Hydrogen atmosphere pulsating white dwarfs, also known as DAV stars, are the most abundant type of pulsating white dwarfs. High-temperature DAV stars in general exhibit a small number of pulsation modes and stable frequencies. G132–12 is one of the pulsating hydrogen atmosphere white dwarf stars which lies close to the blue edge of the instability strip. Previous researches reported that G132–12 might have only one pulsation mode with the period of 212.69 s. To study the pulsation properties of G132–12 in detail, we carried out a bi-site observation campaign in October 2019. Time series photometric data were collected during around 154 h in total. A Fourier analysis reveals three frequencies which are identified as the triplet of an l = 1 g-mode pulsation with the period of 212.499 s. The rotational period is derived as P_(rot)= 35.0 ± 6.7 h and the inclination of the rotational axis to the line of sight is 70°. G132–12 could be an ideal target for measuring the cooling scale of this white dwarf star with only one excited pulsation mode detected.     

Kinematics of solar neighborhood stars and its dependence on age and metallicity

We have constructed a catalog containing the best available astrometric,photometric, radial velocity and astrophysical data for mainly F-type and G-type stars(called the Astrometric Catalog associated with Astrophysical Data, ACAD). This contains 27 553 records and is used for the purpose of analyzing stellar kinematics in the solar neighborhood. Using the Lindblad-Oort model and compiled ACAD, we calculated the solar motion and Oort constants in different age–metallicity bins. The evolution of kinematical parameters with stellar age and metallicity was investigated directly. The results show that the component of the solar motion in the direction of Galactic rotation(denoted S2) linearly increases with age, which may be a consequence of the scattering processes, and its value for a dynamical cold disk was found to be 8.0 ± 1.2 km s-1. S2 also linearly increases with metallicity, which indicates that radial migration is correlated to the metallicity gradient. On the other hand, the rotational velocity of the Sun around the Galactic center has no clear correlation with ages or metallicities of stars used in the estimation.     

Investigating variable stars in the open cluster NGC 1912 and its surrounding field

In this work,we studied the variable stars in the open cluster NGC 1912 based on the photometric observations and Gaia DR2 data.More than 3600 CCD frames in B,V,R filters were reduced,and we obtained the light curves that span about 63 hours.By analyzing these light curves,we detected 24 variable stars,including 16 periodic variable stars,seven eclipsing binaries and one star whose type is unclear.Among these 24 variable stars,11 are newly discovered,which are classified as sixγDoradus stars,oneδScuti star,three detached binaries and one contact binary.We also confirmed 13 previously known variable stars.Based on cluster members identified by Cantat-Gaudin et al.(2018),we inferred cluster memberships for these detected variable stars.Using Gaia DR2 data,we plotted a new color-magnitude diagram for NGC1912,and showed the nature of variable cluster members in kinematical properties and heliocentric distance.Among the 24 variable stars,seven variables are probable cluster members,which show homogeneity in kinematic characters and space position with the established cluster members.Four of the seven variable cluster members are the previously discovered stars,consisting of twoγDor stars and twoδSct stars.The remaining three variable cluster members,which are allγDor stars,are firstly detected in this work.The main physical parameters of these variable cluster members estimated from the color-magnitude diagram are log(age/yr)=8.75,[Fe/H]=-0.1,m-M=10.03 mag,and E(B-V)=0.307.     

X-ray activity from different types of stars

X-ray emission is an important indicator of stellar activity. In this paper, we study stellar Xray activity using the XMM-Newton and LAMOST data for different types of stars. We provide a sample including 1259 X-ray-emitting stars, of which 1090 have accurate stellar parameter estimations. Our sample size is much larger than those used in previous works. We find a bimodal distribution of the X-ray to optical flux ratio(log(fX/fV)) for G and K stars. We interpret that this bimodality is due to two subpopulations with different coronal heating rates. Furthermore, using the full widths at half maxima calculated from Hα and Hβ lines, we show that these stars in the inactive peaks have smaller rotational velocities. This is consistent with the magnetic dynamo theory that presumes stars with low rotational velocities have low levels of stellar activity. We also examine the correlation between log(fX/fV) and luminosity of the excess emission in the Hα line, and find a tight relation between the coronal and chromospheric activity indicators.     

Radial stellar populations of AGN-host dwarf galaxies in SDSS-Ⅳ MaNGA survey

Based on MaNGA integral field unit(IFU) spectroscopy we search 60 AGN candidates,which have stellar masses M_*≤5 × 10~9 M_⊙ and show AGN ionization signatures in the BPT diagram.For these AGN candidates,we derive the spatially resolved stellar population with the stellar population synthesis code STARLIGHT and measure the gradients of the mean stellar age and metallicity.We find that the gradients of mean stellar age(metallicity) of individual AGN-host dwarfs are diverse in 0-0.5 Re,0.5-1 Re and 0-1 Re.However,the overall behavior of the mean stellar age(metallicity) profiles tend to be flat,as the median values of the gradients are close to zero.We further study the overall behavior of the mean stellar age(metallicity) by plotting the co-added radial profiles for the AGN sample and compare with a control sample with similar stellar mass.We find that the median values of light-weighted mean stellar ages of the AGN sample are as old as 2-3 Gyr within 2 Re,which are about 4-7 times older than those of the control sample.Meanwhile,most of the AGN candidates are low-level AGNs,as only eight sources have L_([O)_(Ⅲ])10~(39.5) erg s~(-1).Hence,the AGNs in dwarf galaxies might accelerate the evolution of galaxies by accelerating the consumption of the gas,resulting in an overall quenching of the dwarf galaxies,and the AGNs also become weak due to the lack of gas.The median values of mass-weighted mean stellar age of both samples within 2 Re are similar and as old as about 10 Gyr,indicating that the stellar mass is mainly contributed by old stellar populations.The gradients of co-added mean stellar metallicity for both samples tend to be negative but close to zero,and the similar mean stellar metallicity profiles for both samples indicate that the chemical evolution of the host galaxy is not strongly influenced by the AGN.