X-ray surveys in star forming regions (SFR) have been very successful on finding X-ray emitting pre-main sequence stars (Montmerle et al. 1983; Walter et al. 1988; Feigelson and Kriss 1989; Krautter et al. 1994; Walter et al. 1999). However, the X-ray detection rate of strong infrared (IR) sources (or class-I IR sources), normally associated with embedded young stellar objects (YSOs), is much lower than the detection rate of classical T Tauri stars (CTTS or class-II IR sources) and weak T Tauri stars (WTTS or class-III IR sources). Casanova et al. (1995) report a deep (33.4 ksec) ROSAT pointed observation in the central part of the Ophiuchi SFR. Most of their X-ray sources could be identified with class-II and class-III IR sources, but between two and four sources might be class-I proto-stars. They predict that many (up to 200) low-mass X-ray emitting YSOs may exist in the Ophiuchi SFR. They claim that the X-ray emission of these stars may have a very important impact which probably provides a negative feedback on star formation in the denser parts of the Ophiuchi cloud. This effect may play an important role in the self-regulation of low-mass star formation in molecular clouds. Other authors also claim the X-ray detection of class-I protostars in the Cr A dark clouds and NGC 1333 (Koyama et al. 1994, 1996; Grosso et al. 1997; Kamata et al. 1997; Neuhäuser & Preibisch 1997).
The southern star forming region (SFR) in Chamaeleon consists of three main dark clouds named Cha I, Cha II and Cha III (Hoffmeister 1963). Another relatively small cloud, located between Cha I and Cha II is also part of the complex. Because of its proximity, the Chamaeleon SFR is an ideal place to study low-mass star formation. The X-ray properties of the Cha I members have been investigated by Feigelson et al. (1993) and by Alcalá et al. (1997).
At a distance of about 200 pc (Hughes and Hartigan 1992), the Cha II dark cloud is well characterised by the presence of H emission line objects (Schwartz 1991; Hartigan 1993) as well as of embedded class-I and class-II sources (Whittet et al. 1991; Prusti et al. 1992; Larson et al. 1998). Therefore, Cha II is an ideal laboratory to study the X-ray emission from protostars and low-mass young stellar objects, and the interaction of the X-ray photons with the interstellar gas. The PMS population in Cha II has been characterised by Gauvin and Strom (1992) and Hughes and Hartigan (1992).
The first X-ray observations in the Cha II dark cloud were carried out during the ROSAT all-sky survey (RASS). Ten X-ray sources were detected in the general direction of Cha II during the RASS, with two of them being identified with new class-III, PMS stars (Alcalá et al. 1995; Covino et al. 1997). In an X-ray observation of Cha II carried out by the ASCA satellite, eight X-ray sources were detected with high confidence level but, none of the class I IR sources or protostar candidates was detected (Yamauchi et al. 1998).
In order to further study and characterise the X-ray emitting population of the Cha II dark cloud, we have carried out a 13.5 ksec PSPC ROSAT pointed observation. In this paper, we report the X-ray sources detected, their optical identification with YSOs in Cha II as well the X-ray properties of the PMS population. The analysis of X-ray spectra and light curves are deferred to a forthcoming paper.
In Sect. 2, the X-ray pointed observation as well as the detection of the X-ray sources are discussed. The identification of the X-ray sources is reported in Sect. 3 and the results regarding detection rates, spatial distribution and X-ray properties are reported in Sect. 4. Finally, in Sect. 5 the discussion and conclusions are presented.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000