MUSE crowded field 3D spectroscopy of over 12,000 stars in the globular cluster NGC 6397 - I. The first spectroscopic HRD of a globular cluster

Tim-Oliver Husser, Sebastian Kamann, Stefan Dreizler, Martin Wendt, Nina Wulff, Roland Bacon, Lutz Wisotzki, Jarle Brinchmann, Peter M. Weilbacher, Martin M. Roth, Ana Monreal-Ibero

Published 2016-02-04Version 1

Aims. We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using 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 PSF 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 using a full-spectrum fitting method based on a large grid of PHOENIX spectra. Results. With 18,932 spectra from 12,307 stars in NGC 6397 we have analysed the largest sample so far available for a single globular cluster. We derived a mean radial velocity of vrad=17.84+-0.07 km/s and a mean metallicity of [Fe/H]=-2.120+-0.002, with the latter seemingly varying with temperature for stars on the RGB. We determine T_eff and [Fe/H] from the spectra, and log g from HST photometry. This is the first very comprehensive HRD for a globular cluster based on the analysis of several thousands of stellar spectra. Furthermore, two interesting objects were identified with one being a post-AGB star and the other a possible millisecond-pulsar companion.