paper

A star cluster at the edge of the Galaxy

arXiv:astro-ph/0702541 · doi:10.1051/0004-6361:20065437

Abstract

We study stars and molecular gas in the direction of IRAS06145+1455 (WB89-789) through NIR (JHK), molecular line-, and dust continuum observations. The kinematic distance of the associated molecular cloud is 11.9 kpc. With a galactocentric distance of about 20.2 kpc, this object is at the edge of the (molecular) disk of the Galaxy. The near-IR data show the presence of an (embedded) cluster of about 60 stars, with a radius ca. 1.3 pc and an average stellar surface density of ca. 12 pc^{-2}. We find at least 14 stars with NIR-excess, 3 of which are possibly Class I objects. The cluster is embedded in a 1000 Mo molecular/dust core, from which a molecular outflow originates. The temperature of most of the outflowing gas is < 40 K, and the total mass of the swept-up material is < 10 Mo. Near the center of the flow, indications of much higher temperatures are found, probably due to shocks. A spectrum of one of the probable cluster members shows a tentative likeness to that of a K3III-star (with an age of at least 20 Myr). If correct, this would confirm the kinematic distance. This cluster is the furthest one from the Galactic center yet detected. The combination of old and recent activity implies that star formation has been going on for at least 20 Myr, which is difficult to understand considering the location of this object, where external triggers are either absent or weak, compared to the inner Galaxy. This suggests that once star formation is occurring, later generations of stars may form through the effect of the first generation of stars on the (remnants of) the original molecular cloud.

Accepted for publication in A&A; 14 pages, 12 figures, 4 tables

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