A stellar census in globular clusters with MUSE: A spectral catalogue of emission-line sources

Abstract

Aims. Globular clusters produce many exotic stars due to a much higher frequency of dynamical interactions in their dense stellar environments. Some of these objects were observed together with several hundred thousands other stars in our MUSE survey of 26 Galactic globular clusters. Assuming that at least a few exotic stars have exotic spectra, that means spectra that contain emission lines, we can use this large spectroscopic data set of over a million stellar spectra as a blind survey to detect stellar exotica in globular clusters. Methods. To detect emission lines in each spectrum, we model the expected shape of an emission line as a Gaussian curve. This template is used for matched filtering on the differences between each observed 1D spectrum and its fitted spectral model. The spectra with the most significant detections of Hα emission are checked visually and cross-matched with published catalogues. Results. We find 156 stars with Hα emission, including several known cataclysmic variables (CV) and two new CVs, pulsating variable stars, eclipsing binary stars, the optical counterpart of a known black hole, several probable sub-subgiants and red stragglers, and 21 background emission-line galaxies. We find possible optical counterparts to 39 X-ray sources, as we detect Hα emission in several spectra of stars that are close to known positions of Chandra X-ray sources. This spectral catalogue can be used to supplement existing or future X-ray or radio observations with spectra of potential optical counterparts to classify the sources.

Properties of Cataclysmic Variables in Globular Clusters

“…This means that the same instruments that have been successful in identifying CVs from the ground are not applicable in GCs given the spatial resolution necessary to resolve the cores (where a large populations of CVs is expected to reside) and to avoid contamination from nearby stars. These complications have led to a very limited number of CVs spectroscopically confirmed (17 CVs so far; Göttgens et al, 2019b [71]). One of the instruments that has led to half of these identifications is HST, mainly using the long-slit technique.…”

“…NGC 6397 [74]; Kamann et al 2016 [75]), NGC 2808 (Latour et al 2019 [76]), NGC 3201 (Giesers et al 2018 [77]; 2019 [78]), Terzan 9 (Ernandes et al 2019 [79]), 47 Tuc (Kamann et al 2018 [80]). More recently, MUSE has been used to observe 26 GCs to identify emission-line sources, which has led to the confirmation of five CVs and the discovery of two more (Göttgens et al 2019b [71]). This is important as previous to the MUSE era, only ten CVs had been spectroscopically confirmed in the whole Galactic GC population (Margon, Downes & Gunn 1981 [47], Grindlay et al 1995 [81], Edmonds et al 1999 [82], Deutsch et al 1999 [83], Knigge et al 2003 [84], 2008 [66], Webb & Servillat 2013 [85]).…”

Resolving stellar populations with integral field spectroscopy

“…The GC subprogram was therefore allowed to be scheduled in staggered cadence in order to address time series measurements and a search for spectroscopic variability. Results from this program have been published in a series of papers: A stellar census in GCs with MUSE: The contribution of rotation to cluster dynamics studied with 200,000 stars (Kamann et al ) A stellar census in GCs with MUSE: A spectral catalogue of emission‐line sources(Göttgens et al ) A stellar census in GCs with MUSE: Multiple populations chemistry in NGC 2808 (Latour et al ) A stellar census in GCs with MUSE: Binaries in NGC 3201 (Giesers et al ) Discovery of an old nova remnant in the Galactic GC M 22 (Göttgens et al ) A detached stellar‐mass black hole candidate in the GC NGC 3201 (Giesers et al ) …”

Massively parallel spectroscopy of sources in Galactic globular clusters

“…A.1 A Stellar Census in Globular Clusters with MUSE: A Spectral Catalogue of Emission-Line Sources (Göttgens et al, 2019b) . .…”

“…UV observations of GCs with HST resulted in the detection of several CV candidates, but spectroscopy is the gold standard of detecting and confirming CV in GCs . With MUSE data, we were able to increase the number of spectroscopically confirmed CVs in GCs from ten to 17, including two newly discovered systems (Göttgens et al, 2019b, see Chapter 3).…”