Shallow Decay of X-ray Afterglows in Short GRBs： Energy Injection from a Millisecond Magnetar？

With the successful launch of Swift satellite,more and more data of early X-ray afterglows from short gamma-ray bursts have been collected.Some interesting features such as unusual afterglow light curves and unexpected X-ray flares are revealed.Especially,in some cases,there is a fiat segment in the X-ray afterglow light curve.Here we present a simplified model in which we believe that the flattening part is due to energy injection from the central engine.We assume that this energy injection arises from the magnetic dipole radiation of a millisecond pulsar formed after the merger of two neutron stars.We check this model with the short GRB 060313.Our numerical results suggest that energy injection from a millisecond magnetar could make part of the X-ray afterglow light curve flat.     

Spectroscopic and photometric observations of symbiotic nova PU Vul during 2009–2016

A new set of low-resolution spectral and UBVJHKL-photometric observations of the symbiotic nova PU Vul is presented. The binary has been evolving after its symbiotic nova outburst in 1977 and now it is in the nebular stage. It is found that the third orbital cycle(after 1977) was characterized by great changes in associated light curves. Now, PU Vul exhibits a sine-wave shape in all the light curves(with an amplitude in the U band of about 0.7 mag), which is typical for symbiotic stars in the quiescent state. Brightness variability due to pulsations of the cool component is now clearly visible in the VRI light curves. The amplitude of the pulsations increases from 0.5 mag in the V band to 0.8 mag in the I band. These two types of variability, as well as a very slow change in the physical parameters of the hot component due to evolution after the outburst of 1977, influence the spectral energy distribution(SED)of the system. The variability of emission lines is highly complex. Only hydrogen line fluxes vary with orbital phase. An important feature of the third orbital cycle is the first emergence of the OVI, 6828  Raman scattering line. We determine the temperature of the hot component by means of the Zanstra method applied to the He II, 4686  line. Our estimate is about 150 000 K for the spectrum obtained near orbital maximum in 2014. The VO spectral index derived near pulsation minimum corresponds to M6 spectral class for the cool component of PU Vul.     

A study of variable stars in the open cluster NGC 1582 and its surrounding field

This paper presents Charge-Coupled Device time-series photometric observations of the open cluster NGC 1582 and its surrounding field with Johnson B,V and R filters by using the Nanshan 1 m telescope administered by Xinjiang Astronomical Observatory.19 variable stars and three variable candidates were detected in a 45′×48.75′ field around the cluster.12 of the variable stars are newly-discovered variable objects.The physical properties,classifications and memberships of these 22 objects are studied through their light curves,their positions on the color-magnitude diagram and with archival data from the Naval Observatory Merged Astrometric Dataset.Among these objects,five are eclipsing binary systems,six are pulsating variable stars including one known S Scuti star and one newly-discovered RR Lyrae star.The distance to the RR Lyrae star is estimated to be 7.9 ± 0.3 kpc,indicating that the star is located far behind the cluster.Four variable stars are probable members of the cluster,and 13 of the 22 objects are confirmed to be field stars.     

Discovery of Four Peculiar Variable Stars

Four stars, GSC 1258-0143, GSC 1986-1665, GSC 3045-0892 and GSC 2983-1597 were found to be new variables during a campaign of CCD photometric monitoring of short-period eclipsing binary stars. The variabilities of these new variables are reported. The main characteristics and probable classifications of the variables are discussed through a preliminary analysis on their light curves. Among these stars, GSC 1258-0143 is classified as a RR Lyr star of sub-type a. Its pulsation period is determined to be about 0.5206 days. The other three stars, are definite variables, but their periods and types of light variations remain unknown.     

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.     

Photometric investigation on the W-subtype contact binary V1197 Her

Multi-color light curves of V1197 Her were obtained with the 2.4 meter optical telescope at the Thai National Observatory and the Wilson-Devinney(W-D) program was used to model the observational light curves. The photometric solutions reveal that V1197 Her is a W-subtype shallow contact binary system with a mass ratio of q = 2.61 and a fill-out factor of f = 15.7%. The temperature difference between the primary star and secondary star is only 140 K in spite of the low degree of contact, which means that V1197 Her is not only in geometrical contact configuration but is also already under thermal contact status.The orbital inclination of V1197 Her is as high as i = 82.7?, and the primary star is completely eclipsed at the primary minimum. The totally eclipsing characteristic implies that the determined physical parameters are highly reliable. The masses, radii and luminosities of the primary star(star 1) and secondary star(star2) are estimated to be M1 = 0.30(1) M_⊙, M2 = 0.77(2) M_⊙, R1 = 0.54(1) R_⊙, R2 = 0.83(1) R_⊙,L1 = 0.18(1) L_⊙and L2 = 0.38(1) L_⊙. The evolutionary statuses of the two component stars are drawn in the Hertzsprung-Russell diagram, showing that the less massive but hotter primary star is more evolved than the secondary star. The period of V1197 Her is decreasing continuously at a rate of d P/dt =-2.58 ×10-7 day · year-1, which can be explained by mass transfer from the more massive star to the less massive one at a rate of dM_2/dt=-1.61 × 10~(-7) M_⊙year~(-1). The light curves of V1197 Her are reported to show the O'Connell effect. Thus, a cool spot is added to the more massive star to model the asymmetry in the light curves.     

RT Leonis Minoris： an Unstable W Ursae Majoris System with a Spotted Component

Complete BV light curves of the W UMa type binary RT LMi are presented. From the observations, four times of minimum light were determined. Based on the new times of minimum light and those collected from the literature, changes in the orbital period of the system were found and analyzed with Kalimeris et al.'s method. The result shows that the orbital period possibly oscillates with a cycle of about 64 years and an amplitude of 1.2 × 10-6 days. The present CCD photometric observations reveal that the light curves are obviously asymmetrical, and show a positive O'Connell effect, while the light curves obtained in 1982 exhibit a negative O'Connell effect. The present light curves were analyzed by means of the latest version of the Wilson-Devinney code, which was also used to correct the photometric effects, including the distortion on the radial-velocity curves obtained by Rucinski et al. The following absolute dimensions have been derived: MI -1.28 ±0.08 M⊙, M2 = 0.48 ±0.06 M⊙, R1 = 1.28 ±0.06 R⊙,     

Orbital period correction and light curve modeling of the W-subtype shallow contact binary OW Leo

Orbital period and multi-color light curves’investigation of OW Leo are presented for the first time.The orbital period of OW Leo is corrected from P=0.325545 days to P=0.32554052 days in our work,and the observational data from the All-Sky Automated Survey for Super Novae(ASAS-SN)are used to test the newly determined orbital period.Then,the phased light curves are calculated with the new period and the Wilson-Devinney program is applied to model the light curves,which reveal that OW Leo is a W-subtype shallow contact binary system(q=3.05,f=12.8%).The absolute physical parameters of the two component stars are estimated to be M₁=0.31(1)M_⊙,M₂=0.95(3)M_⊙,R₁=0.63(1)R⊙,R₂=1.04(1)R_⊙,L₁=0.43(1)L_⊙and L₂=1.01(2)L_⊙.The evolutionary status shows that the more massive star is less evolved than the less massive star.OW Leo has very low metal abundance,which means its formation and evolution are hardly influenced by any additional component.It is formed from an initially detached binary system through nuclear evolution and angular momentum loss via magnetic braking,and has passed a very long time of main sequence evolution.     

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.     

On the Red Edge of the δ Scuti Instability Strip

The S Scuti star catalogue is used to derive the observational locations of such stars on the HR diagram. The theoretical and observational instability strips are compared to check the theoretical red edge obtained by considering nonlocal time-dependent convection theory. The observational instability strip almost overlaps with the theoretical one, but the observed blue and red envelopes are hotter than the theoretical edges. The distribution of S Scuti stars in the pulsation strip is not uniform.     

Photometric Monitoring of ROSAT-Selected Weak Line T Tauri Stars

We monitored the light curves of 22 weak-line T Tauri stars (WTTSs) discovered among the X-ray sources in the field of the Taurus-Auriga cloud. For 12 of the 22 WTTSs photometric periodic variability is confirmed and their rational periods are determined using Phase Dispersion Minimization (PDM) and Fourier analysis. Most of them are found to have periods shorter than one day. This gives further evidence for the spin up of solar-type stars predicted by the models of angular momentum evolution of pre-main sequence stars.     

V2551 Cyg:a pulsating star with enigmatic peculiarities

Intensive photometric and spectral observations of the variable star V2551 Cyg are presented.The light curve shape reveals that the target is a pulsating star, contrary to its previous classification as an eclipsing binary. The period and amplitude of the light curve, the amplitudes of color changes and the radial velocity curve of V2551 Cyg are similar to those of a high-amplitude δ Scuti variable. The target seems to pulsate with the fundamental mode. However, V2551 Cyg exhibits several important peculiarities:(i) the decreasing branch of its light curve is steeper than the increasing one;(ii) the radial velocity curve has a flat section in the phase range 0.7–1.2 and short increase of the negative radial velocity at phase 0.7;(iii) the rotational velocity is quite big for a HADS star;(iv) the Fourier coefficients of V2551 Cyg are quite different from those of HADS stars. The target classification is difficult due to these peculiarities.     

The YNEV stellar evolution and oscillation code

We have developed a new stellar evolution and oscillation code YNEV,which calculates the structures and evolutions of stars,taking into account hydrogen and helium burning.A nonlocal turbulent convection theory and an updated overshoot mixing model are optional in this code.The YNEV code can evolve low-and intermediate-mass stars from the pre-main sequence to a thermally pulsing asymptotic branch giant or white dwarf.The YNEV oscillation code calculates the eigenfrequencies and eigenfunctions of the adiabatic oscillations for a given stellar structure.The input physics and numerical scheme adopted in the code are introduced.Examples of solar models,stellar evolutionary tracks of low-and intermediate-mass stars with different convection theories(i.e.mixing-length theory and nonlocal turbulent convection theory),and stellar oscillations are shown.     

变星HR8851

1. Discovory The star HR 8851 was discovered occasionally to be variable star which has somethinglike δ Scutl stars on December 23 and 25, 1992. These observations were made in Ⅴ bandon the 60cm reflector located at Xinglong Station, Beijing Astronomical Observatory. The light curves are shown in Fig.1. Using periodogram method, we estimate the variation peri-     

Identifying Carbon stars from the LAMOST pilot survey with the efficient manifold ranking algorithm

Carbon stars are excellent kinematic tracers of galaxies and can serve as a viable standard candle,so it is worthwhile to automatically search for them in a large amount of spectra.In this paper,we apply the efficient manifold ranking algorithm to search for carbon stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) pilot survey,whose performance and robustness are verified comprehensively with four test experiments.Using this algorithm,we find a total of183 carbon stars,and 158 of them are new findings.According to different spectral features,our carbon stars are classified as 58 C-H stars,11 C-H star candidates,56C-R stars,ten C-R star candidates,30 C-N stars,three C-N star candidates,and four C-J stars.There are also ten objects which have no spectral type because of low spectral quality,and a composite spectrum consisting of a white dwarf and a carbon star.Applying the support vector machine algorithm,we obtain the linear optimum classification plane in the J — H versus H — K_s color diagram which can be used to distinguish C-H from C-N stars with their J — H and H — K_s colors.In addition,we identify 18 dwarf carbon stars with their relatively high proper motions,and find three carbon stars with FUV detections likely have optical invisible companions by cross matching with data from the Galaxy Evolution Explorer.In the end,we detect four variable carbon stars with the Northern Sky Variability Survey,the Catalina Sky Survey and the LINEAR variability databases.According to their periods and amplitudes derived by fitting light curves with a sinusoidal function,three of them are likely semiregular variable stars and one is likely a Mira variable star.     

Rotation curve of the Galactic outer disk derived from radial velocities and UCAC3 proper motions of carbon stars

The availability of astrometric data and radial velocities of carbon stars near the Galactic plane enables us to investigate the kinematics of the Milky Way,especially the rotation curve.The recently published Third U.S.Naval Observatory CCD Astrograph Catalog (UCAC3) provides the opportunity to test this problem using three-dimensional velocity in order to obtain more reliable rotation curves.We intend to study the Galactic rotation curve up to 15 kpc using the radial velocities and proper motions of carbon stars.The motivation for using UCAC3 is to provide high precision proper motions which have hardly been used in determining the rotation velocity of tracers.Seventy-four carbon stars and carbon-rich Mira variables toward the anti-center direction (90°＜(e)＜ 270°,|b| ＜ 6°) are picked up from the literature then matched with UCAC3 carbon star candidates to obtain their proper motions.A rigorous geometrical method is employed to compute the rotation velocity of each object.Taking carbon stars as tracers,we find a fiat rotation curve of 210 ± 12kms-1assuming/to = 8.0 kpc for the gaiactocentric distance and V0 = 220 km s-1 for the rotation velocity of the Sun.Due to the uncertainties of distances,the rotation velocities are more dispersed if tangential velocities enter the calculation,compared to those derived from radial velocities only.However,the whole rotation curve shows coherence with previous results.Increasing observation and study of carbon stars would be desirable in order to provide more homogeneous data for the kinematical study of the Galactic disk.     

On the Rd Edge of the δ Scuti Instability Strip

The δ Scuti star ctalogue is used to derive the observational locations of such stars on the HR diagram.The theoretical and observational instability strips are compared to check the theoretical red edge obtained by considering nonlocal time-dependent convection theory.The observational instability strip almost overlaps with the theoretical one,but the observed blue and red envelopes are hotter overlaps with the theoretical one,but the observed blue and red envelopes are hotter than the theoretical edges.The distribution of δ Scuti stars in the pulsation strip is not uniform.     

A multiwavelength view of a classical T Tauri star CV Cha

Using long-term optical, ultraviolet(UV) and X-ray data, we present a study of a classical T Tauri star CV Cha. The V-band light curve obtained from the All Sky Automated Survey(ASAS) shows short as well as long-term variability. The short-term variability could be due to rotational modulation of CV Cha. We derive the rotational period of 3.714 ± 0.001 d for CV Cha. UV light curves obtained from Swift also show the variations. X-ray light curves from XMM-Newton and Swift do not show any significant short as well as long-term variability. However, the light curve from Chandra appears to be variable, which could be due to the emergence of flaring activities. X-ray spectra from all observations are explained well by the single temperature plasma of 0.95 keV with X-ray luminosity of 1030.4erg s-1in the 0.5–7.5 keV energy band. It appears that variability in optical and UV bands could be due to the presence of both hot and cool spots on the surface, while X-ray emission is dominated by magnetic processes.     

Contribution of dust produced by binary merger ejecta

By means of a population synthesis code and by constructing a simple toy model of dust produced by asymptotic giant branch(AGB) stars, common envelope(CE) ejecta and binary merger ejecta, we estimate the dust product rates(DPRs) of these processes in the Milky Way. The total DPR from AGB stars is～ 6.7 × 10-4M yr-1, in which about 73% of dust grains are carbon, 24% are silicates and 3% are iron. The total DPR from CE ejecta is ～ 4.2 × 10-4M yr-1, in which about 83% of dust grains are silicates, about 12% are carbon and 5% are iron.The DPR from binary merger ejecta is less than 1/3 that from AGB stars or CE ejecta,and it could even be negligible under certain circumstances. Therefore, compared with AGB stars and CE ejecta, the contribution of dust produced by binary merger ejecta to total dust grains in the Milky Way is smaller or can be negligible.