Infrared Search for Young Stars in HI High-velocity Clouds

Abstract

We have searched the IRAS Point Source Catalog and HIRES maps for young stellar objects (YSOs) in the direction of five \HI high-velocity clouds (HVCs). In agreement with optical searches in the halo, no evidence was found for extensive star-forming activity inside the high-latitude HVCs. Specifically, we have found no signs of star formation or YSOs in the direction of the A IV cloud or in the very-high-velocity clouds HVC~110-7-465 and HVC~114-10-440. We have identified only one young star in the direction of the M~I.1 cloud, which shows almost perfect alignment with a knot of \HI emission. Because of the small number of early-type stars observed in the halo, the probability for such a positional coincidence is low; thus, this young star appears to be physically associated with the M~I.1 cloud. We have also identified a good YSO candidate in the \HI shell-like structure observed in the core region of the low-latitude cloud complex H (HVC~131+1-200). This region could be a supernova remnant with several other YSO candidates formed along the shock front produced by the explosion. In agreement with recent theoretical estimates, these results point to a low but significant star-formation rate in intermediate and high Galactic latitude HVCs. For M~I.1 in particular, we estimate that the efficiency of the star-formation process is $M(YSO)/M(\HI)\ga 10^{-4}-10^{-3}$ by mass. Such efficiency is sufficient to account for (a) the existence of the few young blue stars whose ages imply that they were born in the Galactic halo, and (b) the nonprimordial metallicities inferred for some HVCs if their metal content proves to be low.