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Register a new userHCT/HESP study of two carbon stars from the LAMOST survey
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Carbon stars, enhanced in carbon and neutron-capture elements, provide wealth of information about the nucleosynthesis history of the Galaxy. In this work, we present the first ever detailed abundance analysis of carbon star LAMOSTJ091608.81+230734.6 and a detailed abundance analysis of neutron-capture elements for the object LAMOSTJ151003.74+305407.3. Updates on the abundances of elements C, O, Mg, Ca, Cr, Mn and Ni for LAMOSTJ151003.74+305407.3 are also presented. Our analysis is based on high resolution spectra obtained using Hanle Echelle Spectrograph (HESP) attached to the Himalayan Chandra Telescope (HCT), IAO, Hanle. The stellar atmospheric parameters (T$_{eff}$, logg, micro-turbulance ${zeta}$, metallicity [Fe/H]) are found to be (4820, 1.43, 1.62, $-$0.89) and (4500, 1.55, 1.24, $-$1.57) for these two objects respectively. The abundance estimates of several elements, C, N, O, Na, $alpha$-elements, Fe-peak elements and neutron-capture elements Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm and Eu are presented. Our analysis shows the star LAMOSTJ151003.74+305407.3 to be a CEMP-r/s star, and LAMOSTJ091608.81+230734.6 a CH giant. We have examined if the i-process model yields ([X/Fe]) of heavy elements could explain the observed abundances of the CEMP-r/s star based on a parametric model based analysis. The negative values obtained for the neutron density dependent [Rb/Zr] ratio confirm former low-mass AGB companions for both the stars. Kinematic analysis shows that LAMOSTJ151003.74+305407.3 belongs to the Galactic halo population and LAMOSTJ091608.81+230734.6 to the disc population.
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In this work, we present the new catalog of carbon stars from the LAMOST DR2 catalog. In total, 894 carbon stars are identified from multiple line indices measured from the stellar spectra. Combining the CN bands in the red end with ctwo and other lines, we are able to identify the carbon stars. Moreover, we also classify the carbon stars into spectral sub-types of ch, CR, and cn. These sub-types approximately show distinct features in the multi-dimensional line indices, implying that in the future we can use them to identify carbon stars from larger spectroscopic datasets. Meanwhile, from the line indices space, while the cn stars are clearly separated from the others, we find no clear separation between CR and ch sub-types. The CR and ch stars seem to smoothly transition from one to another. This may hint that the CR and ch stars may not be different in their origins but look different in their spectra because of different metallicity. Due to the relatively low spectral resolution and lower signal-to-noise ratio, the ratio of $^{12}$C/$^{13}$C is not measured and thus the cj stars are not identified.
We present a study on the detailed chemical abundances of five new relatively bright $r$-process-enhanced stars that were initially observed as part of the SDSS/MARVELS pre-survey. These stars were selected, on the basis of their metallicities and carbon abundances, among a total of 60 stars, for high-resolution spectroscopic follow-up as part of the HESP-GOMPA survey (Hanle Echelle SPectrograph -- Galactic survey Of Metal Poor stArs). Here we discuss the three new $r$-I and two new $r$-II stars found in this survey. We have carried out a detailed abundance analysis for each of these stars, at a resolving power of $R sim 30,000$, and compare our results to the existing literature. We could measure three of the first $r$-process-peak elements (Sr, Y and Zr) in all five stars, while Ba, Ce, Nd, Sm, Eu, and Dy could be detected among the second $r$-process-peak elements. Thorium could also be detected in one of the targets, which is found to be an actinide-boost star. We have carried out a comparative study among the sub-populations of the $r$-process-enhanced stars and other stars of the Milky Way halo population to constrain the origin of this class of objects. These bright $r$-process-enhanced stars provide an excellent opportunity to study the nucleosynthesis history of this population in great detail, and shed light on their chemical-enrichment histories.
We present 22,901 OB spectra of 16,032 stars identified from LAMOST DR5 dataset. A larger sample of OB candidates are firstly selected from the distributions in the spectral line indices space. Then all 22,901 OB spectra are identified by manual inspection. Based on a sub-sample validation, we find that the completeness of the OB spectra reaches about $89pm22$% for the stars with spectral type earlier than B7, while around $57pm16$% B8--B9 stars are identified. The smaller completeness for late B stars is lead to the difficulty to discriminate them from A0--A1 type stars. The sub-classes of the OB samples are determined using the software package MKCLASS. With a careful validation using 646 sub-samples, we find that MKCLASS can give fairly reliable sub-types and luminosity class for most of the OB stars. The uncertainty of the spectral sub-type is around 1 sub-type and the uncertainty of the luminosity class is around 1 level. However, about 40% of the OB stars are failed to be assigned to any class by MKCLASS and a few spectra are significantly misclassified by MKCLASS. This is likely because that the template spectra of MKCLASS are selected from nearby stars in the solar neighborhood, while the OB stars in this work are mostly located in the outer disk and may have lower metallicity. The rotation of the OB stars may also be responsible for the mis-classifications. Moreover, we find that the spectral and luminosity classes of the OB stars located in the Galactic latitude larger than 20$^circ$ are substantially different with those located in latitude smaller than 20$^circ$, which may either due to the observational selection effect or hint a different origin of the high Galactic latitude OB stars.
Here we present the discovery of 895 s-process-rich candidates from 454,180 giant stars observed by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) using a data-driven approach. This sample constitutes the largest number of s-process enhanced stars ever discovered. Our sample includes 187 s-process-rich candidates that are enhanced in both barium and strontium, 49 stars with significant barium enhancement only and 659 stars that show only a strontium enhancement. Most of the stars in our sample are in the range of effective temperature and log g typical of red giant branch (RGB) populations, which is consistent with our observational selection bias towards finding RGB stars. We estimate that only a small fraction (0.5 per cent) of binary configurations are favourable for s-process enriched stars. The majority of our s-process-rich candidates (95 per cent) show strong carbon enhancements, whereas only five candidates (less than 3 percent) show evidence of sodium enhancement. Our kinematic analysis reveals that 97 percent of our sample are disc stars, with the other 3 percent showing velocities consistent with the Galactic halo. The scaleheight of the disc is estimated to be zh=0.634kpc, comparable with values in the literature. A comparison with the yields from asymptotic giant branch (AGB) models suggests that the main neutron source responsible for the Ba and Sr enhancements is the 13C(alpha,n)16O reaction. We conclude that the s-process-rich candidates may have received their overabundances via mass transfer from a previous ABG companion with an initial mass in the range 1-3Msun.
We present a catalog of stellar age and mass estimates for a sample of 640,986 red giant branch (RGB) stars of the Galactic disk from the LAMOST Galactic Spectroscopic Survey (DR4). The RGB stars are distinguished from the red clump stars utilizing period spacing derived from the spectra with a machine learning method based on kernel principal component analysis (KPCA). Cross-validation suggests our method is capable of distinguishing RC from RGB stars with only 2 per cent contamination rate for stars with signal-to-noise ratio (SNR) higher than 50. The age and mass of these RGB stars are determined from their LAMOST spectra with KPCA method by taking the LAMOST - $Kepler$ giant stars having asteroseismic parameters and the LAMOST-TGAS sub-giant stars based on isochrones as training sets. Examinations suggest that the age and mass estimates of our RGB sample stars with SNR $>$ 30 have a median error of 30 per cent and 10 per cent, respectively. Stellar ages are found to exhibit positive vertical and negative radial gradients across the disk, and the age structure of the disk is strongly flared across the whole disk of $6<R<13$,kpc. The data set demonstrates good correlations among stellar age, [Fe/H] and [$alpha$/Fe]. There are two separate sequences in the [Fe/H] -- [$alpha$/Fe] plane: a high--$alpha$ sequence with stars older than $sim$,8,Gyr and a low--$alpha$ sequence composed of stars with ages covering the whole range of possible ages of stars. We also examine relations between age and kinematic parameters derived from the Gaia DR2 parallax and proper motions. Both the median value and dispersion of the orbital eccentricity are found to increase with age. The vertical angular momentum is found to fairly smoothly decrease with age from 2 to 12,Gyr, with a rate of about $-$50,kpc,km,s$^{-1}$,Gyr$^{-1}$. A full table of the catalog is public available online.
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