High-{alpha}-Metal-Rich Stars in the LAMOST-MRS Survey and Its Connection with the Galactic Bulge. (arXiv:2102.05999v2 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_H/0/1/0/all/0/1">Haopeng Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_Y/0/1/0/all/0/1">Yuqin Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhao_G/0/1/0/all/0/1">Gang Zhao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhao_J/0/1/0/all/0/1">Jingkun Zhao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liang_X/0/1/0/all/0/1">Xilong Liang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Li_H/0/1/0/all/0/1">Haining Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wu_Y/0/1/0/all/0/1">Yaqian Wu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Luo_A/0/1/0/all/0/1">Ali Luo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_R/0/1/0/all/0/1">Rui Wang</a>
We report the detection of a large sample of high-$alpha$-metal-rich stars
on the low giant branch with $2.6<logg<3.3$ dex in the LAMOST-MRS survey. This
special group corresponds to an intermediate-age population of $5-9$ Gyr based
on the $[Fe/H]$-$[C/N]$ diagram and age-$[C/N]$ calibration. A comparison group
is selected to have solar $alpha$ ratio at super metallicity, which is young
and has a narrow age range around 3 Gyr. Both groups have thin-disk like
kinematics but the former shows slightly large velocity dispersions. The
special group shows a larger extension in vertical distance toward 1.2 kpc, a
second peak at smaller Galactic radius and a larger fraction of super metal
rich stars with $[Fe/H]>0.2$ than the comparison group. These properties
strongly indicate its connection with the outer bar/bulge region at $R=3-5$
kpc. A tentative interpretation of this special group is that its stars were
formed in the X-shaped bar/bulge region, close to its corotation radius, where
radial migration is the most intense, and brings them to present locations at 9
kpc and beyond. Low eccentricities and slightly outward radial excursions of
its stars are consistent with this scenario. Its kinematics (cold) and
chemistry ($[alpha/Fe]$ $sim 0.1$) further support the formation of the
instability-driven X-shaped bar/bulge from the thin disk.
We report the detection of a large sample of high-$alpha$-metal-rich stars
on the low giant branch with $2.6<logg<3.3$ dex in the LAMOST-MRS survey. This
special group corresponds to an intermediate-age population of $5-9$ Gyr based
on the $[Fe/H]$-$[C/N]$ diagram and age-$[C/N]$ calibration. A comparison group
is selected to have solar $alpha$ ratio at super metallicity, which is young
and has a narrow age range around 3 Gyr. Both groups have thin-disk like
kinematics but the former shows slightly large velocity dispersions. The
special group shows a larger extension in vertical distance toward 1.2 kpc, a
second peak at smaller Galactic radius and a larger fraction of super metal
rich stars with $[Fe/H]>0.2$ than the comparison group. These properties
strongly indicate its connection with the outer bar/bulge region at $R=3-5$
kpc. A tentative interpretation of this special group is that its stars were
formed in the X-shaped bar/bulge region, close to its corotation radius, where
radial migration is the most intense, and brings them to present locations at 9
kpc and beyond. Low eccentricities and slightly outward radial excursions of
its stars are consistent with this scenario. Its kinematics (cold) and
chemistry ($[alpha/Fe]$ $sim 0.1$) further support the formation of the
instability-driven X-shaped bar/bulge from the thin disk.
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