Obscured Galactic Giant HII Regions; Discovery of Young Clusters

Extract

Giant HII (GHII) regions in our Galaxy are typically initially found by radio observations of their optically thin free-free continuum emission. Most of them are partially or totally obscured in the visible by the absorbing effect of intervening and/or local interstellar dust. We (Blum et al. 1999, 2000) have selected a list of the brightest GHII regions in our Galaxy (from Smith et al. 1978) and have begun a program of JHK imaging and K band spectroscopy to identify and classify the exciting stars. We have obtained near IR imaging of eight GHII regions (and data is available for four others). All of these, aside from W49, show the presence of a stellar cluster in the K band at the radio source position. The K,H — K diagrams are used to select the brightest stars. The J — Kvs. H — K diagrams distinguish those stars along the normal reddening line from those with K band excesses. The former group ought to have normal OB star spectra; the latter will have featureless continua in the K band due to emission from localized warm dust arising in a natal disc (Hanson et al. 1997). A disc geometry can also produce CO emission (or absorption) band features.