X-rays and star formation: ROSAT observations of the Monoceros and Rosette molecular clouds

We report ROSAT observations of two molecular clouds previously unstudied in the X-ray band, examining the possibility that X-rays can serve as tracers of star formation at distances >∼ 1 kpc. The targets are the molecular clouds associated with the Monoceros R2 reflection nebula (d = 0.8 kpc) and with the Rosette giant HII region (d = 1.5 kpc). In both cases, the PSPC images show point sources and extended emission. Most of the 41 distinct ROSAT sources in the Monoceros cloud and 21 sources in the Rosette cloud have optical counterparts on digitized plates, but a significant minority (20 30%) do not and are probably embedded in the clouds. Comparison with recent near-infrared surveys of theMonoceros and Rosette clouds supports the broad correspondence, previously found in nearby clouds, betweenX-ray andnear-infrared sources associatedwith YoungStellarObjects. X-ray luminosities areLX ∼ 1030−1032 erg s−1. The X-ray hardness ratios and LX/Lbol ratios of the Monoceros and Rosette X-ray sources are comparable to those found for lowand intermediate-mass pre-main sequence stars in nearby star forming clouds. We conclude that most of the point-like ROSAT sources are T Tauri and Herbig Ae/Be stars. Extended X-ray “hot spots” are seen in both clouds, which likely result from the partially resolved emission of embedded young star clusters, similar to the ρ Oph cluster. The detection of individual stars in the high-luminosity tail of the low-mass stellar population, and of embedded young star clusters in starforming molecular cores, confirm that X-rays can efficiently trace lowand intermediate-mass star formation at significant distances across the Galaxy.