* Astronomy

Blobrana · L

RE: Arches cluster

Blobrana · L

Title: The Arches cluster out to its tidal radius: dynamical mass segregation and the effect of the extinction law on the stellar mass function
Authors: Maryam Habibi, Andrea Stolte, Wolfgang Brandner, Benjamin HuBmann, Kentaro Motohara

The Galactic Center is the most active site of star formation in the Milky Way Galaxy, where particularly high-mass stars have formed very recently and are still forming today. However, since we are looking at the Galactic Center through the galactic disk, knowledge of extinction is crucial to study the region. The Arches cluster is a young, massive starburst cluster, near the Galactic Center. We observed the Arches cluster out to its tidal radius using Ks band imaging obtained with NAOS/CONICA at the VLT combined with Subaro/Cisco J-band data to gain a full understanding of the cluster mass distribution. We show 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 we show that the difference can reach up to 30% in extracted initial mass and ~1 magnitude in acquired Ks-band extinction while the present mass function slope changes by ~ 0.17 dex. The present-day mass function slopes derived assuming the Nishiyama et al. (2009) extinction law are increasing from a flat slope of alpha_{Nishi}=-1.76 ± 0.22 in the core (r<0.2 pc) to alpha_{Nishi}=-2.23 ± 0.27 in the intermediate annulus 0.2 <r<0.4 and become high mass depleted, alpha_{Nishi}=-2.95 ± 0.26, in the outer annulus (0.4<r<1.5 pc). This picture is consistent with mass segregation due to the dynamical evolution of the cluster.

Read more (1047kb, PDF)

Blobrana · L

Title: Proper motions of the Arches cluster with Keck-LGS Adaptive Optics: the first kinematic mass measurement of the Arches
Authors: Will Clarkson, Andrea Ghez, Mark Morris, Jessica Lu, Andrea Stolte, Nate McCrady, Tuan Do, Sylvana Yelda

We report the first detection of the intrinsic velocity dispersion of the Arches cluster - a young (~2 Myr), massive (~10,000 Solar Mass) starburst cluster located near the Galactic center. This was accomplished using proper motion measurements within the central region of the cluster, obtained with the laser guide star adaptive optics system at Keck Observatory over a 3 year time baseline (2006-2009). This uniform dataset results in proper motion measurements that are improved by a factor ~5 over previous measurements from heterogeneous instruments, yielding internal velocity dispersion estimates 0.15 ±0.01 mas/yr, which corresponds to 5.4 ±0.4 km/s at a distance of 8.4 kpc.
Projecting a simple model for the cluster onto the sky to compare with our proper motion dataset, in conjunction with surface density data, we estimate the total present-day mass of the cluster to be 15,000 (+7400 -6000) Solar masses. The mass in stars observed within a cylinder of radius R=0.4 pc is found to be 9000 (+4000 -3500) Solar Masses at formal 3-sigma confidence. This mass measurement is free from assumptions about the mass function of the cluster, and thus may be used to check mass estimates from photometry and simulation. When we conduct this check, we find that the present-day mass function of the Arches cluster is likely either top-heavy or truncated at low-mass, or both.
Collateral benefits of our data and analysis include: 1. cluster membership probabilities, which may be used to extract a clean cluster sample for future photometric work; 2. a refined estimate of the bulk motion of the Arches cluster with respect to the field, which we find to be 172 ±15 km/s, which is slightly slower than suggested by previous VLT-Keck measurements; and 3. a velocity dispersion estimate for the field itself, which is likely dominated by the inner galactic bulge and the nuclear disk.

Read more (2202kb, PDF)