Kavli Affiliate: Huawei Zhang
| First 5 Authors: Weixiang Sun, Yang Huang, Haifeng Wang, Chun Wang, Meng Zhang
| Summary:
Using a sample of nearly 140,000 primary red clump stars selected from the
LAMOST and $Gaia$ surveys, we have identified a large sample of "young"
[$alpha$/Fe]-enhanced stars with stellar ages younger than 6.0 Gyr and
[$alpha$/Fe] ratios greater than 0.15 dex. The stellar ages and [$alpha$/Fe]
ratios are measured from LAMOST spectra, using a machine learning method
trained with common stars in the LAMOST-APOGEE fields (for [$alpha$/Fe]) and
in the LAMOST-$Kepler$ fields (for stellar age). The existence of these "young"
[$alpha$/Fe]-enhanced stars is not expected from the classical Galactic
chemical evolution models. To explore their possible origins, we have analyzed
the spatial distribution, and the chemical and kinematic properties of those
stars and compared the results with those of the chemically thin and thick disk
populations. We find that those "young" [$alpha$/Fe]-enhanced stars have
distributions in number density, metallicity, [C/N] abundance ratio, velocity
dispersion and orbital eccentricity that are essentially the same as those of
the chemically thick disk population. Our results clearly show those so-called
"young" [$alpha$/Fe]-enhanced stars are not really young but $genuinely$
$old$. Although other alternative explanations can not be fully ruled out, our
results suggest that the most possible origin of these old stars is the result
of stellar mergers or mass transfer.
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