Starburst clusters in the Galactic center

Abstract

The central region of the Galaxy is the most active site of star formation in the Milky Way, where massive stars have formed very recently and are still forming today. The rich population of massive stars in the Galactic center provide a unique opportunity to study massive stars in their birth environment and probe their initial mass function, which is the spectrum of stellar masses at their birth. The Arches cluster is the youngest among the three massive clusters in the Galactic center, providing a collection of high-mass stars and a very dense core which makes this cluster an excellent site to address questions about massive star formation, the stellar mass function and the dynamical evolution of massive clusters in the Galactic center.<br /> In this thesis, I perform an observational study of the Arches cluster using <em>K_s</em>-band imaging obtained with NAOS/CONICA at the VLT combined with Subaru/Cisco J-band data to gain a full understanding of the cluster mass distribution out to its tidal radius for the first time. Since the light from the Galactic center reaches us through the Galactic disc, the extinction correction is crucial when studying this region. I use a Bayesian method to construct a realistic extinction map of the cluster. It is shown in this study that the determination of the mass of the most massive star in the Arches cluster, which had been used in previous studies to establish an upper mass limit for the star formation process in the Milky Way, strongly depends on the assumed slope of the extinction law. Assuming the two regimes of widely used infrared extinction laws, I show that the difference can reach up to 30% for individually derived stellar masses and ∆<em>A_{K_s}</em> ∼ 1 magnitude in acquired <em>K_s</em>-band extinction, while the present-day mass function slope changes by ∼ 0.17 dex. The present-day mass function slope derived assuming the more recent extinction law, which suggests a steeper wavelength dependence for the infrared extinction law, reveals an overpopulation of massive stars in the core (<em>r</em> < 0.2 pc) with a flat slope of α Nishi = −1.50 ± 0.35 in comparison to the Salpeter slope of α = −2.3 . The slope of the mass function increases to α_{N_ishi} = −2.21 ± 0.27 in the intermediate annulus (0.2 < r < 0.4 pc). The mass function steepens to α_{N_ishi} = −3.21 ± 0.30 in the outer annulus (0.4 < r < 1.5 pc) indicating that the outer cluster region is deficient of high-mass stars. The comparison between the observed trend in the present-day mass function of the Arches cluster to existing N-body simulations of the cluster shows that this picture is consistent with mass segregation owing to the dynamical evolution of the cluster.<br /> Recently, more than 100 Wolf-Rayet and OB stars were identified in the Galactic center. About a third of these sources are not spatially associated with any of the known star clusters in this region. As the comparison of our observational study to N-body models of the cluster revealed, the clusters in the<br /> Galactic center region are dynamically evolved at younger ages due to their high cluster mass and the special Galactic center environment. Therefore, I probed the contribution of drifted sources from numerical models of the massive clusters in the Galactic center to the observed distribution of isolated massive sources in this region. This study shows that stars as massive as 100 M_⊙ drift away from the center of each cluster by up to ∼ 60 pc using the cluster models. The best analyzed model reproduces ∼ 60% of the known isolated massive stars out to 80 pc from the center of the Arches cluster. This number increases to 70 − 80% when we only consider the region that is ∼ 20 pc from the Arches cluster. Our finding shows that most of the apparently isolated high-mass stars might originate from the known star clusters. This result, together with the fact that no top-heavy mass function is required to explain the spatial variation of the mass function in the Arches cluster, implies that no evidence is seen for a deviating (top-heavy) initial mass function in the wider environment of the Galactic center.