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 thousand other stars in our MUSE survey of 26 Galactic globular clusters. Assuming that at least a few exotic stars have exotic spectra (i.e. 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 modelled the expected shape of an emission line as a Gaussian curve. This template was 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 detected 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.
AbstractIn dense stellar systems the frequent dynamical interactions between stars play a crucial role in the formation and evolution of compact binaries. We study these processes using a novel approach combining a state-of-the- art binary population synthesis code with a simple treatment of dynamical interactions in dense star cluster cores. Here we focus on the dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We demonstrate that dynamics can increase by factors ~ 2 – 100 the production rates of interesting binaries such as cataclysmic variables, “nonflickerers” (He white dwarfs with a heavier dark companion), merging white dwarf binaries with total masses above the Chandrasekhar limit, and white dwarf binaries emitting gravitational waves in the LISA band.
AbstractWe present some results from the DRAGON simulations, a set of four direct N-body simulations of globular clusters (GCs) with a million stars and five percent initial (primordial) binaries. These simulations were undertaken with the NBODY6++GPU code, which allowed us to follow dynamical and stellar evolution of individual stars and binaries, formation and evolution of white dwarfs, neutron stars, and black holes, and the effect of a galactic tidal field. The simulations are the largest existing models of a realistic globular cluster over its full lifetime of 12 billion years. In particular we will show here an investigation of the population of binaries including compact objects (such as white dwarfs - cataclysmic variables and merging black hole binaries in the model as counterparts of LIGO/Virgo sources); their distribution in the cluster and evolution with time.
The study of cluster white dwarfs (WDs)
has been invigorated recently bythe Hubble Space
Telescope (HST). Recent WD studies have been
motivated by the new and independent cluster
distance (Renzini et al. 1996), age (von Hippel et
al. 1995; Richer et al. 1997), and stellar
evolution (Koester & Reimers 1996) information
that cluster WDs can provide. An important
byproduct of these studies has been an estimate of
the WD mass contribution in open and globular
clusters. The cluster WD mass fraction is of
importance for understanding the dynamical state
and history of star clusters. It also bears an
important connection to the WD mass fractions of
the Galactic disk and halo. Current evidence
indicates that the open clusters (e.g. von Hippel
et al. 1996; Reid this volume) have essentially
the same luminosity function (LF) as the solar
neighborhood population. The case for the halo is
less clear, despite the number of very good
globular cluster LFs down to nearly 0.1 solar
masses (e.g. Cool et al. 1996; Piotto, this
volume), as the field halo LF is poorly known. For
most clusters dynamical evolution should cause
evaporation of the lowest mass members, biasing
clusters to have flatter present-day mass
functions (PDMFs) than the disk and halo field
populations. Dynamical evolution should also allow
cluster WDs to escape, though not in the same
numbers as the much lower mass main sequence
stars. The detailed connection between cluster
PDMFs and the field IMF awaits elucidation from
observations and the new combined N-body and
stellar evolution models (Tout, this volume).
Nevertheless, the WD mass fraction of clusters
already provides an estimate for the WD mass
fraction of the disk and halo field populations. A
literature search to collect cluster WDs and a
simple interpretive model follow. This is a work
in progress and the full details of the literature
search and the model will be published
elsewhere.
AbstractThe features and make up of the population of X-ray sources in Galactic star clusters reflect the properties of the underlying stellar environment. Cluster age, mass, stellar encounter rate, binary frequency, metallicity, and maybe other properties as well, determine to what extent we can expect a contribution to the cluster X-ray emission from low-mass X-ray binaries, millisecond pulsars, cataclysmic variables, and magnetically active binaries. Sensitive X-ray observations withXMM-Newton and certainlyChandra have yielded new insights into the nature of individual sources and the effects of dynamical encounters. They have also provided a new perspective on the collective X-ray properties of clusters, in which the X-ray emissivities of globular clusters and old open clusters can be compared to each other and to those of other environments. I will review our current understanding of cluster X-ray sources, focusing on star clusters older than about 1 Gyr, illustrated with recent results.
AbstractCCD photometry of standard areas of the seven-color Strömvil photometric system in some open and globular clusters is described. The primary standards are being measured with a two-channel photoelectric photometer on the 1.5 meter telescope on Mt. Lemmon. CCD photometry of standard areas is in progress with the Vatican Advanced Technology Telescope on Mt. Graham and with the CASLEO 2.1 m telescope in Argentina. Two additional Strömvil projects are described.
A stellar census in globular clusters with MUSE: A spectral catalogue of emission-line sources