Abstract
The Galactic center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), allow the measurement of metallicities in the inner region of our Galaxy. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool Asymptotic Giant branch (AGB) stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the red giant branch. We studied metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ∼ +0.3 dex and a metal-poor peak around {Fe/H] = −0.5 dex, which is 0.2 dex poorer than Baade’s Window. The α-elements Mg, Si, Ca, and O show a similar trend to the Galactic bulge. The metal-poor component is enhanced in the α-elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high-velocity stars with vgal > 300 km s−1; the metal-rich stars show a much higher rotation velocity (∼200 km s−1) with respect to the metal-poor stars (∼140 km s−1). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disk and the nuclear star cluster show distinct chemical signatures and might be formed differently.
Highlights
The Milky Way bulge is such a complex system that its formation and evolution are still poorly understood
Using the latest DR16 Apache Point Observatory Galactic Evolution Experiment (APOGEE) data release, we studied cool M giants in the GALCEN sample going down to temperatures of 3200 K
We confirm that a photometric H–K color cut combined with a dereddened magnitude cut of K0 is a very powerful criterion to select Asymptotic Giant branch (AGB)/supergiant stars
Summary
The Milky Way bulge is such a complex system that its formation and evolution are still poorly understood. Schultheis et al (2015) (hereafter referred to as S15) studied 33 M giant stars of APOGEE in the so-called GALCEN field to study the metallicity distribution function and chemical abundances using the DR12 data release (Alam et al 2015) They had to limit their analysis to stars with Teff > 3700 K, due to problems modeling the coolest stars (Holtzman et al 2015), excluding a large number of cool stars (including asymptotic giant branch stars (AGB) and supergiants) from their analysis. We present results for the metallicity distribution function and chemical abundances of 157 M giants, which constitutes one of the largest high-resolution spectroscopic samples in the inner degree of the Milky Way to date This is the first study of cool AGB stars and supergiants in the GALCEN field using results from the APOGEE survey. These data were reduced with the same pipeline as the DR16 stars
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