MUSE crowded field 3D spectroscopy of over 12 000 stars in the globular cluster NGC 6397

I. The first comprehensive HRD of a globular cluster^⋆,⋆⋆

¹ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: husser@astro.physik.uni-goettingen.de
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Institut für Physik und Astronomie, Universität Potsdam, 14476 Potsdam, Germany
⁴ CRAL, Observatoire de Lyon, CNRS, Université Lyon 1, 9 avenue Ch. André, 69561 Saint-Genis-Laval Cedex, France
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ GEPI, Observatoire de Paris, CNRS, Université Paris-Diderot, Place Jules Janssen, 92190 Meudon, France

Received: 13 July 2015
Accepted: 13 February 2016

Abstract

Aims. We demonstrate the high multiplex advantage of crowded field 3D spectroscopy with the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12 000 individual stars in the globular cluster NGC 6397.

Methods. The stars are deblended with a point spread function fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis via a full-spectrum fitting method based on a large grid of PHOENIX spectra.

Results. We analysed the largest sample so far available for a single globular cluster of 18 932 spectra from 12 307 stars in NGC 6397. We derived a mean radial velocity of v_rad = 17.84 ± 0.07 km s^-1 and a mean metallicity of [Fe/H] = −2.120 ± 0.002, with the latter seemingly varying with temperature for stars on the red giant branch (RGB). We determine T_eff and [Fe/H] from the spectra, and log g from HST photometry. This is the first very comprehensive Hertzsprung-Russell diagram (HRD) for a globular cluster based on the analysis of several thousands of stellar spectra, ranging from the main sequence to the tip of the RGB. Furthermore, two interesting objects were identified; one is a post-AGB star and the other is a possible millisecond-pulsar companion.