Jing Zhong

MULTIBAND STUDIES OF THE OPTICAL PERIODIC MODULATION IN THE X-RAY BINARY SAX J1808.4–3658 DURING ITS QUIESCENCE AND 2008 OUTBURST

We report on time-resolved optical imaging of the X-ray binary SAX J1808.4-3658 during its quiescent state and 2008 outburst. The binary, containing an accretion-powered millisecond pulsar, has a large sinusoidal-like modulation in its quiescent optical emission. We employ a Markov chain Monte Carlo technique to fit our multi-band light curve data in quiescence with an irradiated star model, and derive a tight constraint of 50(-5)(+6) deg on the inclination angle i of the binary system. The pulsar and its companion are constrained to have masses of 0.97(-0.22)(+0.31) M-circle dot and 0.04(-0.01)(+0.02) M-circle dot (both 1 sigma ranges), respectively. The dependence of these results on the measurements of the companions projected radial velocity is discussed. We also find that the accretion disk had nearly constant optical fluxes over a similar to 500 day period in the quiescent state our data covered, but started brightening 1.5 months before the 2008 outburst. Variations in modulation during the outburst were detected in our four observations made 7-12 days after the start of the outburst, and a sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller amplitude of 0.1 mag. The modulation variations are discussed. We estimate the albedo of the companion during its quiescence and the outburst, which was approximately 0 and 0.8 (for isotropic emission), respectively. This large difference probably provides additional evidence that the neutron star in the binary turns on as a radio pulsar in quiescence.

THE NEAREST HIGH-VELOCITY STARS REVEALED BY LAMOST DATA RELEASE 1

We report the discovery of 28 candidate high-velocity stars (HVSs) at heliocentric distances of less than 3 kpc, based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 1. Our sample of HVS candidates covers a much broader color range than the equivalent ranges discussed in previous studies and comprises the first and largest sample of HVSs in the immediate solar neighborhood, at heliocentric distances less than 1-3 kpc. The observed as well as the derived parameters for all candidates are sufficiently accurate to allow us to ascertain their nature as genuine HVSs, of which a subset of 12 objects represents the most promising candidates. Our results also highlight the great potential of discovering statistically large numbers of HVSs of different spectral types in LAMOST survey data. This will ultimately enable us to achieve a better understanding of the nature of Galactic HVSs and their ejection mechanisms, and to constrain the structure of the Galaxy.

THE LIKELY ORBITAL PERIOD OF THE ULTRACOMPACT LOW-MASS X-RAY BINARY 2S 0918-549

We report the discovery of the likely orbital period of the ultracompact low-mass X-ray binary (LMXB) 2S 0918-549. Using time-resolved optical photometry carried out with the 8-m Gemini South Telescope, we obtained a 2.4-hr long, Sloan r light curve of 2S 0918-549 and found a periodic, sinusoidal modulation at 17.4+/-0.1 min with a semiamplitude of 0.015+/-0.002 mag, which we identify as the binary period. In addition to 4U 0513-40 in the globular cluster NGC 1851 and the Galactic disk source 4U 1543-624, 2S 0918-549 is the third member of the ultracompact LMXBs that have orbital periods around 18 min. Our result verifies the suggestion of 2S 0918-549 as an ultracompact binary based on its X-ray and optical spectroscopic properties. Given that the donor in 2S 0918-549 has been suggested to be either a C-O or He white dwarf, its likely mass and radius are around 0.024--0.029 M_sun and 0.03--0.032 R_sun, respectively, for the former case and 0.034--0.039 M_sun and 0.033--0.035 R_sun for the latter case. If the optical modulation arises from X-ray heating of the mass donor, its sinusoidal shape suggests that the binary has a low inclination angle, probably around 10 deg.

Selecting M Giants with Infrared Photometry: Distances, Metallicities, and the Sagittarius Stream

Using a spectroscopically confirmed sample of M giants, M dwarfs, and quasars from the LAMOST survey, we assess how well Wide-field Infrared Survey Explorer (WISE) and Two Micron All Sky Survey color cuts can be used to select M giant stars. The WISE bands are very efficient at separating M giants from M dwarfs, and we present a simple classification that can produce a clean and relatively complete sample of M giants. We derive a new photometric relation to estimate the metallicity for M giants, calibrated using data from the APOGEE survey. We find a strong correlation between the (W1-W2) color and [{{M}}/{{H}}], where almost all of the scatter is due to photometric uncertainties. We show that previous photometric distance relations, which are mostly based on stellar models, may be biased and devise a new empirical distance relation, investigating trends with metallicity and star formation history. Given these relations, we investigate the properties of M giants in the Sagittarius stream. The offset in the orbital plane between the leading and trailing tails is reproduced, and by identifying distant M giants in the direction of the Galactic anticenter, we confirm that the previously detected debris in the outer halo is the apocenter of the trailing tail. We also find tentative evidence supporting an existing overdensity near the leading tail in the northern Galactic hemisphere, possibly an extension to the trailing tail (so-called Branch C). We have measured the metallicity distribution along the stream, finding a clear metallicity offset between the leading and trailing tails, in agreement with models for the stream formation. We include an online table of M giants to facilitate further studies.

A method for selecting M dwarfs with an increased likelihood of unresolved ultracool companionship

Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation models, the measurement of sub-stellar dynamical masses and radii, and for testing ultra-cool evolutionary models. We present an optimised method for identifying M dwarfs which may have unresolved ultra-cool companions. We construct a catalogue of 440,694 candidates, from WISE, 2MASS and SDSS, based on optical and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints we isolate a sub-sample of 36,898 M dwarfs and search for possible mid-infrared M dwarf + ultra-cool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). We present 1,082 M dwarf + ultra-cool dwarf candidates for follow-up. Using simulated ultra-cool dwarf companions to M dwarfs, we estimate that the occurrence of unresolved ultra-cool companions amongst our M dwarf + ultra-cool dwarf candidates should be at least four times the average for our full M dwarf catalogue. We discuss possible contamination and bias and predict yields of candidates based on our simulations.

AUTOMATED IDENTIFICATION OF 2612 LATE-K AND M DWARFS IN THE LAMOST COMMISSIONING DATA USING CLASSIFICATION TEMPLATE FITS

We develop a template-fit method to automatically identify and classify late-type K and M dwarfs in spectra from the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST). A search of the commissioning data, acquired in 2009-2010, yields the identification of 2612 late-K and M dwarfs. The template fit method also provides spectral classification to half a subtype, classifies the stars along the dwarf-subdwarf (dM/sdM/esdM/usdM) metallicity sequence, and provides improved metallicity/gravity information on a finer scale. The automated search and classification is performed using a set of cool star templates assembled from the Sloan Digital Sky Survey spectroscopic database. We show that the stars can be efficiently classified despite shortcomings in the LAMOST commissioning data which include bright sky lines in the red. In particular we find that the absolute and relative strengths of the critical TiO and CaH molecular bands around 7000 angstrom are cleanly measured, which provides accurate spectral typing from late-K to mid-M, and makes it possible to estimate metallicity classes in a way that is more efficient and reliable than with the use of spectral indices or spectral-index based parameters such as zeta(TiO/CaH). Most of the cool dwarfs observed by LAMOST are found to be metal-rich dwarfs (dM). However, we identify 52 metal-poor M subdwarfs (sdM), 5 very metal-poor extreme subdwarfs (esdM) and 1 probable ultra metal-poor subdwarf (usdM). We use a calibration of spectral type to absolute magnitude and estimate spectroscopic distances for all the stars; we also recover proper motions from the SUPERBLINK and PPMXL catalogs. Our analysis of the estimated transverse motions suggests a mean velocity and standard deviation for the UVW components of velocity to be: = -9.8 km s(-1), sigma(U) = 35.6 km s(-1); = -22.8 km s(-1), sigma(V) = 30.6 km s(-1); = -7.9 km s(-1), sigma(W) = 28.4 km s(-1). The resulting values are in general agreement with previous reported results, which yields confidence in our spectral classification and spectroscopic distance estimates, and illustrates the potential for using LAMOST spectra of K and M dwarfs for investigating the chemo-kinematics of the local Galactic disk and halo.

Low-resolution near-infrared spectroscopic signatures of unresolved ultracool companions to M dwarfs

We develop a method to identify the spectroscopic signature of unresolved L-dwarf ultracool companions, which compares the spectra of candidates and their associated control stars using spectral ratio differences and residual spectra. We present SpeX prism-mode spectra (0.7-2.5 micron) for a pilot sample of 111 mid M dwarfs, including 28 that were previously identified as candidates for unresolved ultracool companionship (a sub-sample from Cook et al. 2016; paper 1) and 83 single M dwarfs that were optically colour-similar to these candidates (which we use as `control stars). We identify four candidates with evidence for near-infrared excess. One of these (WISE J100202.50+074136.3) shows strong evidence for an unresolved L dwarf companion in both its spectral ratio difference and its residual spectra, two most likely have a different source for the near-infrared excess, and the other may be due to spectral noise. We also establish expectations for a null result (i.e. by searching for companionship signatures around the M dwarf control stars), as well as determining the expected outcome for ubiquitous companionship (as a means of comparison with our actual results), using artificially generated unresolved M+L dwarf spectra. The results of these analyses are compared to those for the candidate sample, and reasonable consistency is found. With a full follow-up programme of our candidates sample from Cook et al., we might expect to confirm up to 40 such companions in the future, adding extensively to the known desert population of M3-M5 dwarfs.

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 Hβ λ4861 and Hα λ6563 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.

M-giant star candidates identified in LAMOST DR 1

We perform a discrimination procedure with the spectral index diagram of TiO5 and CaH2+CaH3 to separate M giants from M dwarfs. Using the M giant spectra identified from LAMOST DR1 with high signal-to-noise ratio, we have successfully assembled a set of M giant templates, which show more reliable spectral features. Combining with the M dwarf/subdwarf templates in Zhong et al., we present an extended library of M-type templates which includes not only M dwarfs with a well-defined temperature and metallicity grid but also M giants with subtypes from M0 to M6. Then, the template-fitting algorithm is used to automatically identify and classify M giant stars from LAMOST DR1. The resulting catalog of M giant stars is cross-matched with 2MASS JHK s and WISE W1/W2 infrared photometry. In addition, we calculated the heliocentric radial velocity of all M giant stars by using the cross-correlation method with the template spectrum in a zero-velocity rest frame. Using the relationship between the absolute infrared magnitude M J and our classified spectroscopic subtype, we derived the spectroscopic distance of M giants with uncertainties of about 40%. A catalog of 8639 M giants is provided. As an additional result of this analysis, we also present a catalog of 101 690 M dwarfs/subdwarfs which are processed by our classification pipeline.

LAMOST DR1: Stellar Parameters and Chemical Abundances with SP_Ace

We present a new analysis of the LAMOST DR1 survey spectral database performed with the code SP_Ace, which provides the derived stellar parameters T