A star cluster at the edge of the Galaxy *,**
1
INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy e-mail: brand@ira.inaf.it
2
Joint Astronomy Centre, 660 N. A'ohoku Place, University Park, Hilo, HI 96720, USA
Received:
14
April
2006
Accepted:
13
October
2006
Context.This paper is part of our ongoing study of star formation in the (far-) outer Galaxy.
Aims.Our goal in this paper is to study stars and molecular gas in the direction
of IRAS 06145+1455 (WB89-789).
The kinematic distance of the associated molecular cloud is 11.9 kpc. With a
galactocentric distance of ~
kpc, this object is at the edge of the
(molecular) disk of the Galaxy.
Methods.We use near-IR (J, H, K), molecular line-, and dust continuum observations.
Results.The near-IR data show the presence of an (embedded) cluster of about
60 stars,
with a radius ∼1.3 pc and an average stellar surface density
~
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 
molecular/dust core, from which a molecular outflow originates.
The temperature of most of the outflowing gas is 
K, and
the total mass of the swept-up material is 
. Near the center of
the flow, indications of much higher temperatures are found, probably due to
shocks.
A spectrum taken of one of the probable cluster members shows a tentative
likeness to that of a K3 III-star (with an age of at least 20 Myr). If
correct, this would confirm the kinematic distance.
Conclusions.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.
Key words: stars: formation / stars: pre-main sequence / ISM: clouds / ISM: individual objects: WB 89-789 (IRAS 06145+1455)
© ESO, 2007