APOGEE reveals the coolest stars in the galactic center

The APOGEE survey, part of the SDSS-IV, is a near infrared high-resolution spectroscopic survey with the aim to get about half a million stars with high S/N spectra in the Milky Way, covering all galactic populations. Using the latest data observed from the southern hemisphere (Las Campanas, Chile), APOGEE is now able to penetrate the highly obscured regions: the galactic center with its surrounding stellar nuclear stellar disc (NSD). The NSD is a dense stellar structure in the center of the Milky Way which cohabits with the nuclear star cluster and its central massive black hole.

In this publication, APOGEE reveals for the first time high-resolution spectra of red supergiants and asymptotic giant branch stars (AGB), tracers of a young and intermediate age population in the galactic center. The figure below shows the APOGEE spectrum of the red supergiant star VR 5-7 (black line), located at the center of our galaxy, together with the best fit from synthetic spectra of the APOGEE (ASPCAP) pipeline (red line). Despite its low effective temperature of 3200 K and the presence of strong molecular bands such as CN, CO and OH, APOGEE can now derive stellar parameters (temperature, surface gravity and metallicity) for these extreme cool objects.

In addition, thanks to the precise chemical abundances of Mg, Si, Ca and O (so-called alpha-elements), stars in the NSD show a distinct chemical signature compared to stars embedded in the nuclear star cluster. While these stars show evidence of a recent star-burst, stars in the NSD, in contrary, do not show this behavior as their formation history is different.

For the first time, the rotation of the nuclear stellar disc reveals that the metal-rich stars with super solar metallicities rotate with a much higher rotation velocity compared to metal-poor stars. This could be sign of a different origin of the metal-poor stars which could have formed from disrupted stellar clusters inspiralling to the center of the Milky Way.