New Metal-poor Star Candidates from Guo Shou-jing Telescope (LAMOST) Commissioning Observations

Since metal-deficient stars preserve the chemical abundance formation and evolution information from the Galactic origin,they are regarded as fossils of the early generation of stars. Searching for them in large samples of stellar spectra is an important task related to stellar astrophysics. Guo Shou-jing Telescope(LAMOST),a 4-meter telescope with 4 000 fibers mounted on the large focal plane,has been already executing its commissioning observation and accumulating its observing dataset.The telescope can acquire 4 000 spectra in a 5 degree field at each exposure, and there might be a lot of metal-poor stars(here after MPs) hidden in the flood of data. To pick out MPs from LAMOST observations,the key process is to precisely measure the metallicity for each star.In the paper,three independent methods are applied to determine the metallicity of stars in an observation field of LAMOST,which were observed on Feb 13,2010.The resolution of the spectra is R~2 000.All targets are selected from UCAC3 with R magnitude brighter than 16.5 mag.The integration time of this field is 30 min for one exposure and 20 min for another.Seeing is about 3.3 arcsecond in that observation night. All raw data is reduced by standard LAMOST pipeline,including bias,flat,cosmic ray remove,sky subtraction,trace and extraction,and wavelength calibration etc.The observed spectrum of each target is assembled by two parts,blue range and red range,within 370-590 nm and 570-900 nm respectively.Lacking of flux standard stars in the observing plan,connection of the two parts of the individual spectrum is not smooth enough due to the approximate flux calibration.For the purpose on MPs searching,using the blue arm is enough,because most of the important abundance features are included in this wavelength range.Hence,in this work,we adopt this wavelength range for the metallicity measuring methods. Three stellar atmospheric parameters measuring methods mentioned above are ULySS,Non- Parametric Regression(NPR),and SSPP.ULySS is a newly developed tool based on matching with the ELODIE stellar library,and NPR is a method based on interpolation in the spectra grid constructed by Kurucz NEWODF,while SSPP is the SDSS stellar parameter pipeline.Metal- licities([Fe/H]) and the radial velocities(RV,see Table 2) determined by the three methods are statistically consistent. With the derived stellar atmospheric parameters,8 MP candidates with[Fe/H]—1.00 dex are found.Among them,there is one very metal-poor(VMP) candidate star with[Fe/H]= -2.73 dex.Table 3 displays all the ULySS determined atmospheric parameters(effective temperature, surface gravity,[Fe/H]and RV) for these MP candidates.The result of the 8 MPs is a prelude to the feasibility of LAMOST's capability on searching and enlarging the sample of metal-poor stars in the Milky Way.Besides of the 8 MPs,stars in this observing field could also be used for future calibrations of the LAMOST standard stellar parameter pipeline.