Astron. Astrophys. 331, 193-210 (1998)

1. Introduction

There is increasing evidence for strong links between X-rays and star formation. The early results of the Einstein Observatory have shown that massive stars are bright X-ray emitters (e.g., Pallavicini 1989), and that in addition diffuse X-ray emission is associated with giant HII regions (e.g., Seward & Chlebowski 1982). These massive stars could be detected at large distances because, with a typical ratio , their X-ray luminosities (likely produced by shocks in their dense winds) are sufficiently strong to be detected at distances 1 kpc. At the other end of the mass spectrum, Einstein also showed that young, low-mass stars known as T Tauri stars (TTS) are strong X-ray emitters, and helped showing that "weak" TTS (WTTS) lacking strong emission lines are severalfold more numerous than the "classical" TTS (CTTS) known previously (Herbig 1978; Walter et al. 1988; Bertout 1989; Appenzeller & Mundt 1989; Feigelson et al. 1991; Montmerle et al. 1994). It is now widely believed that CTTS are surrounded by dense circumstellar accretion disks, while WTTS have at most optically thin circumstellar disks. T Tauri stars produce X-rays more efficiently than massive stars, since , but because their are much lower than massive stars, Einstein was not sensitive enough to detect them beyond Gould's Belt (about
500 pc).

However, there was a suspicion that the unresolved X-ray emission from a large number of low-mass stars similar to those of the solar neighborhood could be visible in regions of massive star formation, such as the Carina nebula (e.g., Dorland & Montmerle 1987). But because such regions are in general at least a kiloparsec away, very little information could be gathered at any wavelength on their low-mass stellar population.

The advent of the ROSAT satellite has allowed very significant advances in our knowledge of low-mass star evolution, not only at the T Tauri stage, but also at younger and older stages. ROSAT studies include the Chamaeleon I cloud (Feigelson et al. 1993, henceforth FCMG), L1495E, and L1551 in the Taurus complex (resp. Strom & Strom 1994; Carkner et al. 1996), the embedded population in the Ophiuchi and R CrA cloud cores (resp. Casanova et al. 1995, CMFA; Neuhäuser & Preibisch 1997), the low-mass population around the Orion Trapezium (Geier et al. 1995; Gagné et al. 1995), the young clusters IC348 (Preibisch et al. 1996) and NGC1333 (Preibisch 1997), along with ROSAT All Sky Survey studies on young stars both in the clouds and far from molecular clouds (Neuhäuser et al. 1995b; Alcalá et al. 1995; Sterzik et al. 1995; see also Neuhäuser 1997). These works have conclusively shown that X-rays are a homogeneous tracer of young, low-mass stars; that is, X-ray imagery detects these stars with high efficiency in the nearest star forming regions, at ages as low as years up to years. ROSAT pointed observations are typically 10 times more sensitive than those obtained with Einstein, offering the possibility that moderately bright TTS (CTTS and WTTS alike) can be detected at distances up to 1.5-2 kpc.

In addition to low-mass stars, Einstein and ROSAT have also allowed study of higher mass young stars () known as Herbig Ae/Be stars (Palla 1991; Damiani et al. 1994; Zinnecker & Preibisch 1994). Although the dispersion in is large, these stars may have like TTS. However, since they are entirely radiative, a solar-type convective magnetic dynamo is unlikely to be active. Their X-ray emission may originate instead from wind shocks, shear dynamos, or late-type companions. Because they are intrinsically brighter than TTS, most Herbig Ae/Be stars have on average a higher (up to erg s^-1), and would then be detectable up to larger distances than TTS, unless their X-ray emission is absorbed more than TTS X-ray emission.

Building on these results, we have performed deep observations with ROSAT, with the aim of addressing in a novel way the problem of low- and intermediate-mass star formation in distant molecular clouds where massive stars have formed. The ROSAT images were obtained with the Position Sensitive Proportional Counter (PSPC); for more details about the satellite and the detector see, e.g., Trümper (1990). In this paper, we present these observations and analysis (Sect. 2 and Sect. 3), then identify, wherever possible, the X-ray sources with optical and infrared (IR) counterparts (Sect. 4). We then address collective properties of these sources such as correlations with other parameters and luminosity functions (Sect. 5). We next discuss the interpretation of several regions of extended emission (Sect. 6), ending with a summary and conclusions in Sect. 7.

© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998
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