Opening the Treasure Chest: A Newborn Star Cluster Emerges from Its Dust Pillar in Carina

We present detailed observations of the Treasure Chest, a compact nebula atthe head of a dust pillar in the southern Carina nebula. This object is ofinterest because it is an example of a dense young cluster containing at leastone massive star, the formation of which may have been triggered by feedbackfrom the very massive stars in the Carina nebula, and possibly Eta Car itself.We investigate both the nebular and stellar content of the object. Near-IRemission-line images reveal a cavity inside the head of the dust pillar, whichcontains a dense cluster of young stars, while the observed spectrum of thenebula is consistent with an H II region ionized by an O9.5 V star. Aftersubtracting contamination of field stars within the Carina nebula itself, wecompare the cluster's color magnitude diagram with pre-main-sequence isochronesto derive a likely cluster age less than about 0.1 Myr. This is in reasonableagreement with the dynamical age of a few times 10^4 yr for the expandingnebular cavity, indicating extreme youth. Stars in the Treasure Chest clusterare highly reddened, with extinction values as large as A_V=50. Two-thirds ofcluster members show strong infrared excess colors indicative of circumstellardisks, which may prove to be among the highest fraction yet seen for a youngcluster. All evidence suggests that the Treaure Chest is an extremely youngcluster that is just now breaking out of its natal cloud into the surroundingmassive star forming region, and is a good target for more detailed study.Comment: 23 pages, 10 figs, 4 color fig