Discovery of Young Stellar Objects at the Edge of the Optical Disk of Our Galaxy

We report a discovery of young stellar objects associated with a molecular cloud at the edge of the optical disk of our Galaxy. This cloud is denoted as Cloud 2 in the list by Digel et al. and it is one of the most distant molecular clouds from the Galactic center known to date, with a probable distance of 15–19 kpc. We found seven red near-infrared sources associated with this cloud. Based on our near-infrared observations and far-infrared/radio data in the literature, we conclude that most sources are likely to be members of Cloud 2. The geometry of ionized gas, IRAS sources, near-infrared sources, and molecular cloud suggests that MR-1, an isolated early B-type star near Cloud 2, has triggered the star formation activity in Cloud 2. Our results show that ongoing star formation is present in Cloud 2 and that active star formation can occur in the farthest regions of the Galaxy, where the molecular gas density is extremely low, perturbation from the spiral arms is very small, and the metallicity is similar to that for irregular dwarf galaxies. Cloud 2 is an excellent laboratory in which to study the details of the star formation process in an environment that is similar to that in the early stage of the formation of the Galactic disk. Subject headings: stars: formation — Galaxy: stellar content — infrared: stars The Astrophysical Journal received 4 August 1999, accepted 16 September 1999 Visiting astronomer, Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, Hawaii 96822