2. The target clouds
In order to study with adequate spatial resolution and sensitivity the X-ray emitting young population associated with clouds forming massive stars, we have chosen distant clouds (d 1.5 kpc) showing evidence for OB associations, CO molecular cores, and emission or reflection nebulae. As part of a larger ROSAT program, the targets studied in the present paper are the Monoceros R2 (simply "Monoceros" hereafter) and Rosette clouds, which fulfill these conditions. These regions were not observed with Einstein, hence were unknown in X-rays prior to the present paper. They are otherwise well-known star-forming regions and many results concerning their structure have been published. We will summarize here only the literature relevant to our ROSAT observations.
2.1. The Monoceros cloud
The central region of the Monoceros cloud is a dense, active star forming region as testified by the presence of B stars, a cluster of IR sources close to a small reflection nebula (Mon R2), a compact HII region, OH and masers and a CO bipolar outflow (Torrelles et al. 1990; Gonatas et al. 1992). The cloud is located at a distance of 830 pc according to Herbst & Racine (1976), but a value of 950 pc has also been quoted by some authors (van der Bergh 1966, Racine 1968).
The IR population has been studied by polarimetric and photometric imaging (Aspin & Walther 1990; Howard et al. 1994). More recent photometric data have been obtained in the J, H, and K bands by Carpenter et al. (1997). These latter observations cover only the central 15' 15' region, which is much smaller than the ROSAT diameter PSPC field.
The distribution of the gas is found from the observation of molecules such as (Torrelles et al. 1990), (Gonatas et al. 1992), , and CS (Montalbán et al. 1990, Wolf et al. 1990, Plume et al. 1992). The dust content of the central core of Monoceros is known from high resolution continuum maps obtained at 0.87 mm and 1.3 mm, by Walker et al. (1990) and Henning et al. (1992). All these observations span different fields, at most in size. The center of our ROSAT image is located at the dense core of the cloud, coincident with the IR cluster.
2.2. The Rosette cloud
The famous Rosette emission nebula is an HII region excavated by the strong winds of several O stars, located at a distance of 1.5 kpc at the tip of a giant molecular complex 100 pc in extent (e.g., Dorland & Montmerle 1987). The distribution of molecular gas has been studied in CO by Blitz & Thaddeus (1980) with a 1' resolution, and a recent re-analysis using the CLUMPFIND algorithm (Williams et al. 1995; see also Blitz & Stark 1986) confirms the presence of several gravitationally bound clumps surrounded by unbound diffuse gas. The molecular gas appears to be embedded in an HI cloud.
Our ROSAT PSPC image is centered on the densest concentration of molecular gas appearing on the original CO maps of Blitz & Thaddeus (1980), southeast of the HII region. There is little astronomical information on the stellar content of this region. A number of stars are listed in the Hubble Space Telescope Guide Star Catalog (GSC), but only 14 are coincident with X-ray sources, and stellar parameters are known for only 3 of them. A recent near-IR survey (Phelps & Lada 1997), covering deg2 of the densest molecular gas, has revealed seven embedded clusters, five of them new, all associated with IRAS sources.
© European Southern Observatory (ESO) 1998
Online publication: February 4, 1998