Of the 49 PMS stars and IRAS sources in Cha II (see the compilation in Table 4), 47 fall in the area of our ROSAT pointing. The CTTS Sz64 and the IRAS source IRAS 13047-7750 (detected only in 100µm: Schwartz 1991) lie outside the area of the pointing (c.f. Fig. 3). We report now the results of the X-ray detection rates, the spatial distribution of the X-ray sources and their X-ray luminosities.
4.1. Detection rates
There are 5 known class-I deeply embedded IRAS sources, possibly protostars, in Cha II, namely IRAS 12500-7658, IRAS 12496-7650, IRAS 12533-7632, IRAS 12553-7651 and IRAS 13036-7644 (Whittet et al. 1991; Prusti et al. 1992). The number of known class-II sources or CTTS in Cha II is about 30 (Whittet et al. 1991; Prusti et al. 1992; Hartigan 1993; Larson et al. 1998). Prior to ROSAT, only two class-III sources or WTTS were known (Hn24 and Hn26 by Hartigan 1993) in Cha II which, curiously, were not detected in our deep ROSAT pointing. Two new WTTS found on the basis of the RASS have confirmed detections in the ROSAT pointing
In a near-infrared study of 18 IRAS sources in Cha II, Larson et al. (1998) find that 14 can be classified as field class-III IR sources unrelated to Cha II; only 4 can be classified as class-II sources with infrared properties that match well those of CTTS. These four objects, namely IRAS 12535-7623, IRAS 12584-7621 (=CM Cha), IRAS 13005-7633 (= Hn22 & Hn23), and IRAS F13052-7653, are detected in X-rays (see Sect. 3). Thus, the fact that the 14 field class-III sources were not detected in X-rays gives support to the conclusion by Larson et al. (1998) that they are unrelated to the Cha II cloud.
The X-ray detection rates are summarised in Table 2. In the first column, the different IR classes of objects, namely class I IR sources, CTTS, WTTS and other IR sources (whose association with the Cha II cloud needs to be confirmed) are listed; in the second column the number of X-ray detected objects of each IR class is reported. The numbers of additional X-ray sources identified in this work, for which association to the Cha II cloud needs confirmation, are given in parenthesis; the fraction of X-ray detected objects, eg. the number of X-ray detected objects of each IR class normalized to the total of 49 known YSOs in Cha II (see also Table 4), is listed in the third column. The detection rates are as follows (see also Table 4): None of the deeply embedded class I IR sources was detected in the ROSAT deep pointing, in agreement with the result from ASCA observations by Yamauchi et al. (1998); 11 class-II sources or CTTS, were detected; finally the 2 RASS discovered class-III objects or WTTS were detected.
Table 2. Summary of X-ray detections.
The numbers above would indicate that the CTTS are the most abundant among the X-ray emitting low-mass PMS stars in the Cha II cloud. If the four new lithium stars, found in the spectroscopic identifications (see Sect. 3.1) are confirmed to be PMS stars, the number of WTTS detected would rise to 6 which is still smaller than the number of X-ray detected CTTS. Additional low-mass PMS stars are expected to be found as counterparts of the sources not yet investigated spectroscopically: from inspection of the DSS fields (see Table 1) and from the X-ray hardness ratios (see Sect. 5), we expect some 7 additional potential candidates. Therefore, the number of WTTS detected in our ROSAT pointing would rise to about 13, which would be comparable to the number of X-ray detected CTTS.
In other SFRs, it has been found that the WTTS may outnumber the CTTS by a factor running from 2 to 10 (Walter et al. 1988; Feigelson et al. 1993; Krautter et al. 1994), depending on the completeness of the surveys, in X-ray flux and spatial extent. In the Cha I cloud, the WTTS are a factor of about 2 more numerous than the CTTS (Lawson, Feigelson and Huenemoerder 1996). In Cha II the total number of class-II sources is about 30. Therefore, a remarkable result of our deep ROSAT observation in Cha II is that the number of WTTS may be at most about half of the total number of CTTS.
4.2. Spatial distribution
The spatial distribution of the ROSAT X-ray sources detected in Cha II is shown in Fig. 3. The spatial distribution of the previously known T Tauri stars and IRAS sources in Cha II is also shown for comparison. The 17 MJy sr-1 contour, which more or less defines the limits of the cloud, is represented by a thick shaded line.
Most of the optically visible PMS stars and infrared sources in Cha II are located on the north-east and south edges of the cloud (c.f. Fig. 3). The ROSAT X-ray sources tend to occupy those regions too. Interestingly, there is a concentration of X-ray sources at about & that follows the lanes of the dust emission. These sources are located where the gas density gradient is highest, between the main Cha II cloud and the clump where IRAS 13036-7644 is located. The fact that the sources are detected in such a gap might be due to an extinction effect. Note that three of the four newly identified lithium stars, marked with an asterisk in Fig. 3, are located in that area. Therefore, many of the counterparts of the X-ray sources located in the same area are expected to be PMS stars too.
The region limited by and , in which the candidate protostar IRAS 12496-7650 lies, is devoid both in X-ray sources and optically visible PMS stars. A high extinction with AV 20 mag in that part of the cloud may be expected and hence, the fact that only a few embedded IRAS sources, not detected in X-rays, are found in that part of the cloud, might also result from an extinction effect.
4.3. The X-ray luminosities
For a given source, the X-ray luminosity, assuming isotropic radiation, can be computed as , where d is the distance and is the X-ray flux at the earth in . The latter is given by , where ECF is the energy conversion factor and Z is the background and vignetting corrected broad band X-ray count rate.
The ECF was derived for each source using the "X-ray colours", or hardness ratios, and in the same way as described in Neuhäuser et al. (1995b) and Alcalá et al. (1997) 2. The relevant X-ray data are presented in Table 3. Except for three X-ray sources, discussed in Sect. 5, most of the sources have hardness ratios that are consistent with those of PMS stars (see Fig. 4 in Sect. 5).
Table 3. X-ray properties of the sources detected in the ROSAT pointed observation in the Chamaeleon II dark cloud. The columns are: (1) CHIIRX running number from Table 1; (2) Hardness ratio 1; (3) Hardness ratio 2; (4) X-ray emission energy in keV; (5) Logarithm of Hydrogen column density in ; (6) Energy conversion factor in erg sec-1 cm-2 count-1; (7) X-ray flux in erg sec-1 cm-2; (8) Logarithm of X-ray luminosity in erg sec-1 and (9) identification from Table 1.
The X-ray luminosities , listed in Column (8) of Table 3, are computed adopting a distance of 200 pc for the Cha II cloud (Hughes and Hartigan 1992) and assuming that all the X-ray sources are located at the distance of Cha II, which might not be the case for background or foreground sources. However, once the distance to those objects should be known, their X-ray luminosities can be easily scaled from those listed in Table 3.
Of the 47 PMS stars and IRAS sources that fall in the area of the ROSAT observation, 34 were not detected. For these objects, we have derived upper limits to the X-ray count rates and, using a mean ECF of 1.2 computed from the ECF values reported in Table 3, we derive upper limits for their X-ray luminosity.
In Table 4 a compilation of the 49 YSOs in the Cha II cloud is reported. The X-ray luminosities for the stars detected and the upper limits for those objects not detected are listed in Column (4) 3. The CTTS Sz64 and the IR source IRAS 13047-7750 lie outside the area of the X-ray pointing and that is why no upper limit for their X-ray luminosity is reported.
Table 4. Young stellar objects in the Chamaeleon II dark cloud and their X-ray luminosities.
The stars with the highest X-ray luminosities are RXJ 1301.0-7654a, RXJ 1303.1-7706, CM Cha and Hn22 & Hn23. We remind the reader that RXJ 1301.0-7654a is a PMS spectroscopic binary (Covino et al. 1997) and a visual double (Brandner et al. 1996). Thus, this star is probably a hierarchical triple system in which the combination of the individual X-ray luminosities of the three components results in a high level of X-ray emission. On the other hand, since the resolution of the PSPC is not sufficient to resolve Hn22 from Hn23, their X-ray luminosity should also be considered as an upper limit. For RXJ 1303.1-7706 and CM Cha there is no information on binarity neither do we find evidence of variability in their X-ray light curves. Hence, the latter two stars are intrinsically X-ray bright.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000