Study of Lithium-rich giants with the GALAH spectroscopic survey. (2019)
Keywords :
nuclear reactions, nucleosynthesis, abundances - surveys - stars abundances - stars: evolution - stars: kinematics and dynamics - Hertzsprung-Russell and colour-magnitude diagrams
Abstract:In this article, we speculate on the possible mechanisms for Li enhancement origin in RGB stars based on a large data set of around 340299 stars collected from the GALAH survey combined with the Gaia astrometry. Data has 51982 low mass (M=<2M_{sun}_) RGB stars with reliable atmospheric parameters. The data set shows a well-populated RGB with well-defined luminosity bump and red clump with significant number of stars at each of these two key phases. We found 335 new Li-rich RGB stars with Li abundance, A(Li)>=1.80+/-0.14dex, of which 20 are super Li-rich with A(Li)>=3.20dex. Most of them appear to be in the red clump region which, when combined with stellar evolutionary time-scales on RGB, indicates that the Li enhancement origin may lie
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The GALAH spectrograph is a large-scale spectroscopic survey of
spectral resolution of R(={Delta}{lambda}/{lambda})~28,000 with
spectrograph attached to 4-m Anglo-Australian Telescope (AAT). In the
GALAH Data Release 2 (GALAH DR2), the GALAH team has provided
quantitatively derived abundances of about 23 elements including the
light element Li, which is the focus of this article, for a sample of
342682 stars (see Buder et al. 2018MNRAS.478.4513B, Cat.
J/MNRAS/478/4513). The sample is searched for corresponding
astrometric and photometric data from the Gaia Data Release 2
(hereafter Gaia DR2) of 1.7 billion stars. Source matching is done
using the Gaia DR2 source identifier provided in the GALAH DR2 data
set (see Buder et al. 2018MNRAS.478.4513B, Cat. J/MNRAS/478/4513; Gaia
Collaboration et al. 2018A&A...616A...1G, Cat. I/345). This resulted
in a sample of 340299 common stars among the two data sets.
For this study, we adopt a single limit of A(Li)=1.8dex, instead of
using two upper limits, as most of the Li-rich giants in the sample
are in the higher metallicity group (i.e [Fe/H]>-0.6 dex). This is
also the same upper limit set by standard models for low-mass
(1M_{sun}_) Li-normal giants. In addition, given the relatively larger
uncertainties (~+/-0.14) involved in the derivation of Li abundances
from the spectra, adoption of a slightly higher limit to avoid
contamination with Li-normal giants is justified. With this
definition, we found 335 giants with A(Li)>=1.8dex. This is about 0.64
per cent of total RGB giants considered in this study and confirms the
previous survey results (Brown et al. 1989ApJS...71..293B; Kumar et
al. 2011ApJ...730L..12K; Kirby et al. 2012ApJ...752L..16K) that
Li-rich giants are a rare group.
