A catalog of early-type emission-line stars and Hα line profiles from LAMOST DR2

We present a catalog including 11 204 spectra of 10436 early-type emission-line stars from LAMOST DR2, among which 9752 early-type emission-line spectra are newly discovered. For these early- type emissionqine stars, we discuss the morphological and physical properties of their low-resolution spectra. In this spectral sample, the Hα emission profiles display a wide variety of shapes. Based on the Hα line profiles, these spectra are categorized into five distinct classes： single-peak emission, single-peak emission in absorption, double-peak emission, double-peak emission in absorption, and P-Cygni profiles. To better understand what causes the Hc~ line profiles, we divide these objects into four types from the perspective of physical classification, which include classical Be stars, Herbig Ae/Be stars, close binaries and spectra contaminated by H n regions. The majority of Herbig Ae/Be stars and classical Be stars are identified and separated using a （H-K, K-W1） color-color diagram. We also discuss 31 binary systems that are listed in the SIMBAD on-line catalog and identify 3600 spectra contaminated by HⅡ regions after cross-matching with positions in the Dubout-Crillon catalog. A statistical analysis of line profiles versus classifications is then conducted in order to understand the distribution of Hc~ profiles for each type in our sample. Finally, we also provide a table of 172 spectra with Fen emission lines and roughly calculate stellar wind velocities for seven spectra with P-Cygni profiles.

New CCD photometric investigation of the early-type overcontact binary BH Cen in the young star-forming Galactic cluster IC 2944

BH Cen is a short-period early-type binary with a period of 0.792^d in the extremely young star-forming cluster IC 2944. New multi-color CCD photometric light curves in U, B, V, R and I bands are presented and are analyzed by using the Wilson-Devinney code. It is detected that BH Cen is a high-mass-ratio overcontact binary with a fill-out factor of 46.4% and a mass ratio of 0.89. The derived orbital inclination i is 88.9 degrees, indicating that it is a totally eclipsing binary and the photometric parameters can be determined reliably. By adding new eclipse times, the orbital period changes in the binary are analyzed. It is confirmed that the period of BH Cen shows a long-term increase while it undergoes a cyclic oscillation with an amplitude of A3 = 0.024 d and a period of P3 = 50.3 yr. The high mass ratio, overcontact configuration and long-term continuous increase in the orbital period all suggest that BH Cen is in the evolutionary state after the shortest-period stage of Case A mass transfer.The continuous increase in period can be explained by mass transfer from the secondary component to the primary one at a rate of˙M2 = 2.8×10^-6 M⊙per year. The cyclic change can be plausibly explained by the presence of a third body because both components in the BH Cen system are early-type stars. Its mass is determined to be no less than 2.2 M⊙ at an orbital separation of about 32.5 AU. Since no third light was found during the photometric solution, it is possible that the third body may be a candidate for a compact object.

Surface photometry and radial color gradients of nearby luminous early-type galaxies in SDSS Stripe 82

We make use of the images from the Sloan Digital Sky Survey Stripe 82（Stripe 82） to present an analysis of r band surface brightness profiles and radial color gradients（g-r,u-r） in our sample of 111 nearby early-type galaxies（ETGs） .Thanks to the Stripe 82 images,each of which is co-added from about 50 single frames,we are able to pay special attention to the low-surface-brightness areas（LSB areas） of the galaxies.The LSB areas affect the Sérsic fittings and concentration indices by making both of the indices less than the typical values for ETGs.In the Sérsic fits to all the surface brightness profiles,we found some Sérsic indices that range from 1.5 to 2.5,much smaller than those of typical de Vaucouleur profiles and relatively close to those of exponential disks,and some others much larger than four but still with accurate fitting.Two galaxies cannot be fitted with a single Sérsic profile,but once we try double Sérsic profiles,the fittings are improved：one with a profile relatively close to the de Vaucouleur law in the inner area and a profile relatively close to an exponential law in the LSB area,the other with a nice fitting in the inner area but still having a failed fitting in the outer area.About 60%of the sample has negative color gradients（red-core） within 1.5Re,much more than the approximately 10%positive ones（blue-core） within the same radius.However,taking into account the LSB areas,we find that the color gradients are not necessarily monotonic：about one third of the red-core（or blue-core） galaxies have positive（or negative） color gradients in the outer areas. So LSB areas not only make ETGs’Sérsic profiles deviate from de Vaucouleur ones and shift to the disk end,but also reveal that quite a number of ETGs have opposite color gradients in inner and outer areas.These outcomes remind us of the necessity of double-Sérsic fitting.These LSB phenomena may be interpreted by mergers and thus have different metallicity in the outer areas.Isophotal parameters are also discussed briefly in this paper with the following conclusion：there are more disky nearby ETGs that are identified than boxy ones.

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.

Formation of a Rapidly Rotating Classical Be-star in a Massive Close Binary System

This paper investigates the spin-up of a mass-accreting star in a close binary system passing through the first stage of mass exchange in the Hertzsprung gap. Inside an accreting star, angular momentum is carried by meridional circulation and shear turbulence. The circulation carries part of the angular momentum of the accreted layers to the accretor's surface. The greater the rate of arrival of angular momentum in the accretor is, the greater this part. It is assumed that this part of the angular momentum can be removed by the disk further from the accretor. If the angular momentum in the matter entering the accretor is more than half the Keplerian value, then the angular momentum obtained by the accretor during mass exchange stage does not depend on the rate of arrival of angular momentum. The accretor may have the characteristics of a Be-star immediately after the end of mass exchange.

An Asymmetric Galactic Stellar Disk Traced by OB-type Stars from LAMOST DR7

Using 9943 OB-type stars from LAMOST DR7 in the solar neighborhood,we fit the vertical stellar density profile with the model including a single exponential distribution at different positions(R,Φ).The distributions of the scale heights and scale length show that the young disk traced by the OB-type stars is not axisymmetric.The scale length decreases versus the azimuthal angleΦ,i.e.,from.■kpc withΦ=-3°to■kpc withΦ=9°.Meanwhile we find signal of non-symmetry in the distribution of the scale height of the north and south of the disk plane.The scale height in the north side shows signal of flaring of the disk,while that of the south disk stays almost constant around h_(s)=130 pc.The distribution of the displaceeent of the disk plane Z_(0)also shows variance versus the azimuthal angleΦ,which displays significant differences with the warp model constrained by the Cepheid stars.We also test different values for the position of the Sun,and the distance between the Sun and the Galactic center affects the scale heights and the displacement of the disk significantly,but that does not change our conclusion that the disk is not axisymmetric.

Large-scale CO(J=1-0)Observations toward the M120.1+3.0 Molecular Cloud:A Filament with a Chain of Starburst Clusters

We present large-scale(2°×2°)observations toward the molecular cloud M120.1+3.0,using ^(12)CO,^(13)CO and C^(18)O(J=1-0)data from the Purple Mountain Observatory 13.7 m millimeter telescope.The distance of the cloud is measured to be~1.1 kpc.Using the ^(13)CO data,we identify a main filament F1 and two sub-filaments F2 and F3 in the cloud,which together show a"hub-filament"structure.Filaments F1 and F2 are thermally supercritical.Furthermore,F1 displays clear localized systematic motions in the ^(13)CO position-velocity diagram,which could be explained by accretion along the filament.The mean estimated accretion rate is~132M_(⊙)Myr^(-1).Approximately 150 ^(13)CO clumps are identified in the cloud,of which 39 are gravitationally bound.Most of these virialized clumps are well distributed along the supercritical filaments F1 and F2.Based on the complementary infrared and optical data,we identify~186 young stellar objects in the observed area and extract five clusters within the dense ridge of F1.The calculated star formation rate(SFR)surface densities(∑_(SFR))in the clusters range from 1.4 to 2.5 M_(⊙)Myr^(-1)pc^(-2),with a mean value of~2.0M_(⊙)Myr^(-1)pc^(-2).We therefore regard them as mini-starburst cluster candidates.The comparison between ∑_(SFR) and column density N_(gas) along the skeleton of F1 suggests that star formation is closely related to the dense gas in the cloud.Along the main filament F1,five bipolar outflows are also found.All these results indicate intense star-forming activities in the M120.1+3.0 molecular cloud.

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.

Searching for classical Be stars in LAMOST DR1

We report on searching for Classical B-type emission-line（CBe） stars in the first data release of the Large Sky Area Multi-Object fiber Spectroscopic Telescope（LAMOST; also called the Guo Shou Jing Telescope）. A total of 192 objects（including 12 previously known CBes） were identified as CBe candidates with prominent He I λ4387, He I λ4471 and Mg II λ4481 absorption lines, as well as Hβλ4861and Hαλ6563 emission lines. These candidates significantly increase the currently known sample of CBes by about 8%. Most of the CBe candidates are distributed near the Galactic Anti-Center due to the observing strategy used for LAMOST. Only two CBes are in star clusters. These two CBes have ages of 15.8 and 398 Myr, respectively.

The Spin-up of a Star Gaining Mass in a Close Binary System on the Thermal Time Scale

We investigate the exchange of mass in a binary system as a channel through which a Be star can receive a rapid rotation.The mass-transfer phase in a massive close binary system in the Hertzsprung-gap is accompanied by the spinning up of the accreting component.We consider a case when the mass of the accreting component increases by 1.5 times.The component acquires mass and angular momentum while in a state of critical rotation.The angular momentum of the component increases by 50 times.Meridional circulation effectively transports angular momentum inside the component during the mass-transfer phase and during the thermal timescale after the end of the mass-transfer phase.As a result of mass transfer,the component acquires the rotation typical of classical Be stars.

Application of Random Forest Regressions on Stellar Parameters of A-type Stars and Feature Extraction

Measuring the stellar parameters of A-type stars is more difficult than FGK stars because of the sparse features in their spectra and the degeneracy between effective temperature(T_(eff))and gravity(log g).Modeling the relationship between fundamental stellar parameters and features through machine learning is possible because we can employ the advantage of big data rather than sparse known features.As soon as the model is successfully trained,it can be an efficient approach for predicting Teffand log g for A-type stars especially when there is large uncertainty in the continuum caused by flux calibration or extinction.In this paper,A-type stars are selected from LAMOST DR7 with a signal-to-noise ratio greater than 50 and the Teffranging within 7000 to 10,000 K.We perform the Random Forest(RF)algorithm,one of the most widely used machine learning algorithms to establish the regression relationship between the flux of all wavelengths and their corresponding stellar parameters(T_(eff))and(log g)respectively.The trained RF model not only can regress the stellar parameters but also can obtain the rank of the wavelength based on their sensibility to parameters.According to the rankings,we define line indices by merging adjacent wavelengths.The objectively defined line indices in this work are amendments to Lick indices including some weak lines.We use the Support Vector Regression algorithm based on our new defined line indices to measure the temperature and gravity and use some common stars from Simbad to evaluate our result.In addition,the Gaia Hertzsprung-Russell diagram is used for checking the accuracy of Teffand log g.

Analysis of the X-ray emission of OB stars:O stars

We investigate the global properties of X-ray emission from O stars, analyzing the X-ray spec- tra of 32 O stars from archival data of the XMM-Newton space observatory. We examine two hypotheses about of the origin of X-ray emission from O stars. The first is a paradigm proposed by Pollock, that was revealed from an analysis of the （ Ori X-ray observation. The second is the magnetically confined wind-shock （MCWS） model. For checking Pollock＇s hypothesis, we determine the distribution of the ratio of half width at half maximum （HWHM） to the wind terminal velocity for lines in spectra of all examined stars. In addition, we check three probable consequences from the MCWS model. We analyze if a correlation exists between the spectral hardness and such stellar parameters as the wind terminal velocity, stellar magnetic field and mass loss rate. The result showed that Pollock＇s hypothesis is not correct. We also established that not all consequences of the MCWS model considered by us are con- firmed. In addition, our spectral analysis method indicated that O stars probably have clumped stellar winds with spherical clumps.

The search for roAp stars: null results and new candidates from Strmgren-Crawford photometry

The rapidly oscillating Ap(roAp) stars exhibit pulsational photometric and/or radial velocity variations on time scales of several minutes, which are essential to test current pulsation models as well as our assumptions of atmospheric structure characteristics. In addition, their chemical peculiarity makes them very interesting for probing stellar formation and evolution in the presence of a global magnetic field. To date, a limited number of only 61 roAp stars is known to show photometric variability. On the other hand, a literature survey yields 619 unique stars that have unsuccessfully been searched for variability of this kind. Str o¨mgren-Crawford uvbyβ photometry of stars from both subgroups was used to investigate whether there is a selection bias for the investigated stars. We also present new photometric measurements(202 hours on 59 different nights) of 55 roAp candidates. We did not detect any new roAp star. Although our detection limits are comparable to other surveys, we also did not find pulsations in the known roAp star HD 12098, which may be a consequence of temporal amplitude changes. On the other hand, we do find some evidence for photometric variability of beta CrB at its spectroscopically derived pulsation period. From the uvbyβ photometry we conclude that the blue border of the roAp instability strip appears observationally well defined, whereas the red border is rather poorly known and studied.Within these boundaries, a total of 4646 candidates were identified which appear worth investigating for short-term pulsational variability.

Discovery of 2716 hot emission-line stars from LAMOST DR5

We present a catalog of 3339 hot emission-line stars(ELSs)identified from 451695 O,B and A type spectra,provided by LAMOST Data Release 5(DR5).We developed an automated Python routine that identified 5437 spectra having a peak between 6561 and 6568.False detections and bad spectra were removed,leaving 4138 good emission-line spectra of 3339 unique ELSs.We re-estimated the spectral types of 3307 spectra as the LAMOST Stellar Parameter Pipeline(LASP)did not provide accurate spectral types for these emission-line spectra.As Herbig Ae/Be stars exhibit higher excess in near-infrared and mid-infrared wavelengths than classical Ae/Be stars,we relied on 2 MASS and WISE photometry to distinguish them.Finally,we report 1089 classical Be,233 classical Ae and 56 Herbig Ae/Be stars identified from LAMOST DR5.In addition,928 B[em]/A[em]stars and 240 CAe/CBe potential candidates are identified.From our sample of 3339 hot ELSs,2716 ELSs identified in this work do not have any record in the SIMBAD database and they can be considered as new detections.Identification of such a large homogeneous set of emission-line spectra will help the community study the emission phenomenon in detail without worrying about the inherent biases when compiling from various sources.

Photometric and Spectroscopic Analysis of LBV Candidate J004341.84+411112.0 in M31

We study Luminous Blue Variable(LBV)candidate J004341.84+411112.0 in the Andromeda galaxy.We present optical spectra of the object obtained with the 6 m telescope of BTA SAO RAS.The candidate shows typical LBV features in its spectra:broad and strong hydrogen lines and the He I lines with P Cygni profiles.Its remarkable spectral resemblance to the well known LBV P Cygni suggests a common nature of the objects and supports LBV classification of J004341.84+411112.0.We estimate the temperature,reddening,radius and luminosity of the star using its spectral energy distribution.Obtained bolometric luminosity of the candidate(M_(bol)=-10.41±0.12 mag)is quite similar to those of known LBV stars in the Andromeda galaxy.We analyzed a ten year light curve of the object in R filter.The candidate demonstrates photometric variations of the order of 0.4 mag,with an overall brightness increasing trendΔR>0.1 mag.Therewith,the corresponding color variations of the object are fully consistent with LBV behavior when a star become cooler and brighter in the optical spectral range with a nearly constant bolometric luminosity.LBV-type variability of the object,similarity of its spectrum and estimated luminosity to those of known LBVs allow us to classify J004341.84+411112.0 as an LBV.

Toward an Understanding of the Periastron Puzzle of PSR B1259-63

Efforts are made to understand the timing behaviors （e.g., the jumps in the projected pulsar semimajor axis at the periastron passages） observed in the 13-year monitoring of PSR B1259-63. Planet-like objects are suggested to orbit around the Be star, which may gravitationally perturb the （probably low mass） pulsar when it passes through periastron. An accretion disk should exist outside the pulsar＇s light cylinder, which creates a spindown torque on the pulsar due to the propeller effect. The observed negative braking index and the discrepant timing residuals close to periastron could be related to the existence of a disk with a varying accretion rate. A speculation is presented that the accretion rate may increase on a long timescale in order to explain the negative braking index.

New Massive Contact Twin Binary in a Radio-quiet HⅡRegion Associated with the M17 Complex

Early-B stars,much less energetic than O stars,may create an HⅡregion that appears as radio-quiet.We report the identification of new early-B stars associated with the radio-quiet HⅡregion G014.645–00.606 in the M17 complex.The radio-quiet HⅡregion G014.645–00.606 is adjacent to three radio-quiet WISE HⅡregion candidates.The ionizing sources of the radio-quiet HⅡregions are expected to be later than B1 V,given the sensitivity about 1–2 m Jy of the MAGPIS 20 cm survey.The stars were first selected if their parallaxes of Gaia EDR3 match that of the 22 GHz H2 O maser source within the same region.We used the color–magnitude diagram made from the Zwicky Transient Facility photometric catalog to select the candidates for massive stars because the intrinsic g-r colors of massive stars change little from B-type to O-type stars.Five stars lie in the areas of the color–magnitude diagram where either reddened massive stars or evolved post-main sequence stars of lower masses are commonly found.Three of the five stars,sources 1,2,and 3,are located at the cavities of the three IR bubbles,and extended Hαemission is detected around the three IR bubbles.We suggest that sources 1,2,and 3 are candidates for early-B stars associated with the radio-quiet region G014.645–00.606.Particularly,source 1 is an EW type eclipsing binary with a short period of 0.825 day,while source 2 is an EA type eclipsing binary with a short period of 0.919 day.The physical parameters of the two binary systems have been derived through the PHOEBE model.Source 1 is a twin binary of two stars with Teff≈23,500 K,and source 2 contains a hotter component(T_(eff)≈20,100 K)and a cooler one(T_(eff)≈15,500 K).The O-C values of source 1 show a trend of decline,implying that the period of the source is deceasing.Source 1 is likely a contact early-B twin binary,for which mass transfer might cause its orbit to shrink.

Stellar X-ray Sources in the Rosette Nebula

We present optical photometric and spectroscopic studies of ROSAT X-ray stellar sources in the Rosette Nebula star-forming region. The brightest X-ray sources are either massive stars or active T Tauri stars associated with the open cluster NGC 2244, or are foreground stars. Some of the spectra of the young stars newly identified in the region are presented.

Absolute parameters of young stars:PU Pup

We present combined photometric and spectroscopic analyses of the southern binary star PU Pup.High-resolution spectra of this system were taken at the University of Canterbury Mt.John Observatory in the years 2008 and again in 2014-2015.We find the light contribution of the secondary component to be only 2%of the total light of the system in optical wavelengths,resulting in a single-lined spectroscopic binary.Recent TESS data revealed grazing eclipses within the light minima,though the tidal distortion,examined also from Hipparcos data,remains the predominating light curve effect.Our model shows PU Pup to have the more massive primary relatively close to filling its Roche lobe.PU Pup is thus approaching the rare‘fast phase’of interactive(Case B)evolution.Our adopted absolute parameters are as follows:M_(1)=4.10(±0.20)M_(⊙),M_(2)=0.65(±0.05)M_(⊙),R_(1)=6.60(±0.30)R_(⊙),R_(2)=0.90(±0.10)R_(⊙);T_(1)=11500(±500)K,T_(2)=5000(±350)K;photometric distance=186(±20)pc,age=170(±20)Myr.The less-massive secondary component is found to be significantly oversized and overluminous compared to standard main sequence models.We discuss this discrepancy referring to heating from the reflection effect.

Binary fraction of O and B-type stars from LAMOST data

Abstract:Binary stars play an important role in the evolution of stellar populations.The intrinsic binary fraction(fbin)of O and B-type(OB)stars in LAMOST DR5 was investigated in this work.We employed a cross-correlation approach to estimate relative radial velocities for each of the stellar spectra.The algorithm described by Sana et al.(2013)was implemented and several simulations were made to assess the performance of the approach.The binary fraction of the OB stars is estimated through comparing the unidistribution between observations and simulations with the Kolmogorov-Smirnov tests.Simulations show that it is reliable for stars most of whom have six,seven and eight repeated observations.The uncertainty of orbital parameters of binarity becomes larger when observational frequencies decrease.By adopting the fixed power exponents ofπ=-0.45 andκ=-1 for period and mass ratio distributions,respectively,we obtain that fbin=0.4-0.06+0.05 for the samples with more than three observations.When we consider the full samples with at least two observations,the binary fraction turns out to be 0.37-0.03+0.03.These two results are consistent with each other in 1σ.