Cool stars in the Galactic center as seen by APOGEE

M giants, AGB stars, and supergiant stars and candidates

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, Laboratoire Lagrange, CNRS, Blvd de l’Observatoire, 06304 Nice, France
e-mail: mathias.schultheis@oca.eu
² Instituto de Astrofísica, Facultad de Física, Pontificia, Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
³ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ University of Arizona, Tucson, AZ 85719, USA
⁵ Observatório Nacional, Sao Cristóvao, Rio de Janeiro, Brazil
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Laboratório Interinstitucional de e-Astronomia, – LIneA, Rua Gal. Joseé Cristino 77, Rio de Janeiro, RJ 20921-400, Brazil
⁸ Department of Physics & Astronomy, University of Utah, Salt Lake City, UT 84112, USA
⁹ Departamento de Ciencias Fisicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile
¹⁰ Vatican Observatory, V00120 Vatican City State, Italy
¹¹ Instituto de Astrofísica de Canarias, Calle Via Láctea s/n, 38206 La Laguna, Tenerife, Spain
¹² Universidad de La Laguna (ULL), Departamento de Astrofísica, 30206 La Laguna, Tenerife, Spain
¹³ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta 1270300, Chile
¹⁴ Instituto de Astronomía y Ciencias Planterias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
¹⁵ Departamento de Astronomia, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile

Received: 3 May 2020
Accepted: 22 August 2020

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 v_gal >  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.