Dynamical state of Westerlund 1
Abstract
Using high resolution optical spectroscopy (R = 53.000) we constrain the dynamical state of Westerlund 1, perhaps the most massive Population I cluster in the Milky Way. Our goal is to understand whether massive clusters such as Westerlund 1 emerge bound or whether they will disperse and contribute significantly to the field star population. The cluster also serves as a template for young extragalactic star clusters. We have obtained 64 optical spectra of a sample of 22 stars in Westerlund 1 using the MIKE spectrograph on the Magellan Clay telescope. All of the stars were observed for two or three epochs to allow us to check for the presence of short period equal mass binaries which can substantially affect the observed single epoch velocity dispersion. We are left with a sample of ten stars, after removing stars showing large variability and early-type stars for which we do not obtain the necessary precision in the radial velocity. We find that these stars are consistent with being drawn from a velocity distribution characterized by a sigma of 3.5+1.8-0.5 km/s. After correcting for possible mass segregation and the effect of undetected binaries this corresponds to a dynamic mass of Westerlund I of 3+5-3 × 10^4 Msun, compared to a NIR photometric mass estimate of 4.9+1.8-0.5 × 10^4 Msun. This dynamic mass implies that Westerlund 1 is not strongly supervirial and is thus not expanding into the field.
- Publication:
-
Stellar Clusters & Associations: A RIA Workshop on Gaia
- Pub Date:
- 2011
- Bibcode:
- 2011sca..conf..113C