Stellar loci IV.red giant stars

In the fourth paper of this series,we present the metallicity-dependent Sloan Digital Sky Survey(SDSS)stellar color loci of red giant stars,using a spectroscopic sample of red giants in the SDSS Stripe82 region.The stars span a range of 0.55-1.2 mag in color g-i,-0.3--2.5 in metallicity[Fe/H],and have values of surface gravity log g smaller than 3.5 dex.As in the case of main-sequence(MS)stars,the intrinsic widths of loci of red giants are also found to be quite narrow,a few mmag at maximum.There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars.The colors of red giants are less sensitive to metallicity than those of MS stars.With good photometry,photometric metallicities of red giants can be reliably determined by fitting the u-g,g-r,r-i,and i-z colors simultaneously to an accuracy of 0.2-0.25 dex,comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10.By comparing fitting results to the stellar loci of red giants and MS stars,we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry.The technique achieves completeness of~70 per cent and efficiency of~80 per cent in selecting metal-poor red giant stars of[Fe/H]≤-1.2.It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.

LAMOST Time-Domain survey:first results of four K2 plates

From Oct.2019 to Apr.2020,LAMOST performed a time-domain(TD)spectroscopic survey of four K2 plates with both low-and medium-resolution observations.The low-resolution spectroscopic survey acquired 282 exposures(≈46.6 h)over 25 nights,yielding a total of about 767000 spectra,and the medium-resolution survey took 177 exposures(≈49.1 h)over 27 nights,collecting about 478000 spectra.More than 70%/50%of low-resolution/medium-resolution spectra have signal-to-noise ratio higher than 10.We determine stellar parameters(e.g.,Teff,log g,[Fe/H])and radial velocity(RV)with different methods,including LASP,DD-Payne and SLAM.In general,these parameter estimations from different methods show good agreement,and the stellar parameter values are consistent with those of APOGEE.We use the Gaia DR2 RV values to calculate a median RV zero point(RVZP)for each spectrograph exposure by exposure,and the RVZP-corrected RVs agree well with the APOGEE data.The stellar evolutionary and spectroscopic masses are estimated based on the stellar parameters,multi-band magnitudes,distances and extinction values.Finally,we construct a binary catalog including about 2700 candidates by analyzing their light curves,fitting the RV data,calculating the binarity parameters from medium-resolution spectra and cross-matching the spatially resolved binary catalog from Gaia EDR3.The LAMOST TD survey is expected to represent a breakthrough in various scientific topics,such as binary systems,stellar activity,stellar pulsation,etc.

Overview of the LAMOST survey in the first decade

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST),also known as the Guoshoujing Telescope,is a major national scientific facility for astronomical research located in Xinglong,China.Beginning with a pilot survey in 2011,LAMOST has been surveying the night sky for more than 10 years.The LAMOST survey covers various objects in the Universe,from normal stars to peculiar ones.