The Quintuplet cluster

Abstract

Young massive clusters define the high mass range of current clustered star formation and are frequently found in starburst and interacting galaxies. As - with the exception of the nearest galaxies within the local group - extragalactic clusters can not be resolved into individual stars, the few young massive clusters in the Milky Way and the Magellanic Clouds might serve as templates for unresolved young massive clusters in more distant galaxies. Due to their high masses, these clusters sample the full range of stellar masses. In combination with the small or negligible spreads in age or metallicity of their stellar populations, this makes these object unique laboratories to study stellar evolution, especially in the high mass range.Furthermore, they allow to probe the initial mass function, which describes the distribution of masses of a stellar population at its birth, in its entirety.<br /> The Quintuplet cluster is one of three known young massive clusters residing in the central molecular zone and is located at a projected distance of 30 pc from the Galactic centre. Because of the rather extreme conditions in this region, a potential dependence of the outcome of the star formation process on the environmental conditions under which the star formation event takes place might leave its imprint in the stellar mass function. As the Quintuplet cluster is lacking a dense core and shows a somewhat dispersed appearance, it is crucial to effectively distinguish between cluster stars and the rich population of stars from the Galactic field along the line of sight to the Galactic centre in order to measure its present-day mass function.<br /> In this thesis, a clean sample of cluster stars is derived based on the common bulk proper motion of the cluster with respect to the Galactic field and a subsequent colour selection. The diffraction limited resolution of multi-epoch near-infrared imaging observations obtained at the ESO Very Large Telescope with adaptive optics correction provided by the NAOS-CONICA instrument allowed to determine individual stellar proper motions even at the Galactic centre distance of 8 kpc. The required colour information was provided by additional near-infrared data from the Very Large Telescope and the WFC3 camera onboard the Hubble Space Telescope. The knowledge of both, the individual proper motions and stellar colours, was found to be essential in order to derive the cleanest possible cluster sample. The clean cluster sample allowed to derive the present-day mass function of the Quintuplet cluster for the first time in the approximate mass range from 4 < m < 40 M_sun and out to a distance of 2.1 pc from the cluster centre. While the mass function in the central part of the cluster (r < 0.5 pc) is found to be top-heavy, i.e. overabundant in high mass stars compared to the standard initial mass function, its slope steepens towards larger radii and is consistent with the standard initial mass function in the outermost covered annulus (1.2 < r < 2.1 pc). The observed outward steepening of the mass function is indicative of mass segregation which is a common finding in young massive clusters. The determined mass function is discussed and compared to the findings in other young massive clusters with special regard to the Arches cluster which is also located in the Central Molecular Zone. The extrapolated total present-day mass of the cluster is found to be on the order of 2×10⁴ M_sun. Based on their position in the J_s-K_s , K_s-L' colour-colour diagram, a fraction of 2.5±0.8 % of proper motion members (K_s < 17.5 m) were found to show an excess in the near-infrared. The excess sources cover the mass range from 2 to 10 M_sun.This excess fraction is compared to the fraction of circumstellar discs in young clusters from the literature and, as the survival of primordial circumstellar discs around intermediate mass stars to the age of the Quintuplet cluster is surprising, alternative origins of the near-infrared excess are discussed.<br /> Future work based on the presented study might involve the inference of the initial mass function and other initial properties of the Quintuplet cluster by numerical models, customized to the observed properties of the cluster. The nature of the detected excess sources as potential circumstellar discs could be supported or disproved by the presence or absence of rotation signatures in near-infrared spectra covering the wavelength range of first overtone CO bandhead emission.