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Astron. Astrophys. 355, 629-638 (2000)

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3. Source identification

Fourteen of the 40 X-ray sources detected can be identified with previously known objects. The counterparts of the X-ray sources are listed in Tables 1 and 4. These are mainly IRAS sources, CTTS and WTTS found in previous H[FORMULA] surveys (Schwartz 1977; Whittet et al. 1991; Prusti et al. 1992; Hartigan 1993; Larson et al. 1998) and on the basis of the RASS (Alcalá et al. 1995).

Seven of the eight sources detected in the ASCA observations by Yamauchi et al. (1998) were confirmed detections in the ROSAT pointing. Their source Nr. 1, identified with the Herbig Ae candidate IRAS 12496-7650, was not detected in our deep ROSAT pointing. The source CHIIXR-4 can be identified with the ASCA source No. 2, for which Yamauchi et al. (1998) find no optical counterpart. By inspection of the Digitised Sky Survey (DSS) field, we find that there is a faint object very close to the coordinates of CHIIXR-4, which might be the optical counterpart. Since the ASCA error box is about 1', it is most likely that such object is the counterpart of the ASCA source No. 2 by Yamauchi et al. (1998) as well.

The sources CHIIXR-9, CHIIXR-27 and CHIIXR-34 have ambiguous identifications, in the sense that there is more than one possible counterpart. However, in the case of CHIIXR-9 we think that the optical counterpart is the RASS WTTS RXJ 1301.0-7654a identified by Alcalá et al. (1995) instead of Sz 49, since its offset position is about 3", while Sz 49 is located farther away at about 15" from the X-ray position. In the case of CHIIXR-27, there are two TTS candidates as possible counterparts: Hn22 and Hn23, discovered by Hartigan (1993) in a CCD objective-prism survey. These two stars are separated by 6.5" and, interestingly, both candidates are classified as CTTS on the basis of their H[FORMULA] emission line equivalent width (30Å and 24Å, respectively; Hartigan 1993). These stars are very likely the counterparts for the infrared source IRAS 13005-7633. Sz 57 and Sz 58 may be associated to the source CHIIXR-34 although Sz 57 is closer to the X-ray position.

The sources CHIIXR-2, 13 and 37 can be identified with the class-II infrared sources IRAS12535-7623, IRAS12584-7621 (CM Cha) and IRAS-F13052-7653 by Larson et al. (1998). The X-ray sources CHIIXR-10 and CHIIXR-12 might be associated with the infrared sources IRAS F12571-7657 and IRAS 12583-7634 respectively. By inspection of the ESO/SRC plates, Prusti et al. (1992) found that the IRAS error box of the former IR source is empty, but they could confirm that its IR colours are those of a class-II IR source. On the other hand, IRAS 12583-7634 has been only detected in the 100µm IRAS band (Schwartz 1991).

The X-ray source CHIIXR-3 might be associated with the infrared source IRAS 12551-7657 but, since the IR coordinates are rather uncertain, the identification should be taken with care. By inspection of the DSS field, we find that there is a faint star to the north-east of the IRAS source which practically coincides with the X-ray position. The source CHIIXR-14 is located close to the infrared source IRAS 12589-7646 but the coordinate offset is large ([FORMULA]100") and hence, it cannot be the counterpart of the IRAS source. The X-ray boxes of the sources CHIIXR-5 and CHIIXR-25 are empty and the sources CHIIXR-38 and CHIIXR-15 might be spurious, given their low ML values.

Finally, source CHIIXR-20 might be identified with the cluster of galaxies Gal 303.65-1425.

3.1. Spectroscopic observations

In order to complete the identification of the X-ray sources, the 10 sources, marked with a "[FORMULA]" in Table 1 were investigated spectroscopically during two nights in April 1995, using the Boller & Chivens spectrograph attached to the ESO 1.5m telescope. The observational method and the reduction of these spectra are described in Alcalá et al. (1995). The nominal resolution of the spectra, as measured from several isolated lines of the HeAr comparison spectrum, is 3.5 Å FWHM.

The main goal of these observations was the search for new X-ray emitting WTTS by means of the identification of the Li I 6708 Å absorption line. The identification of PMS stars by means of the lithium line in intermediate resolution spectra has been matter of controversy (Briceño et al. 1997; Favata et al. 1997). However, the effectiveness in identifying PMS stars on this basis has been proven by recent high-resolution studies (Covino et al. 1997; Wichmann et al. 1999; Alcalá et al. 1999). In any case, due to the blend of the Li I 6708 Å line with the nearby Fe lines this spectra are used only for the detection of the lithium feature and not for the measurement of line equivalent width.

Three stars, associated to the sources CHIIXR-24, 35 and 39, show the Li I 6708 Å absorption line in their optical spectra (c.f Fig. 2). The H[FORMULA] line of these stars is seen in absorption, but in the case of CHIIXR-39, a partial filling in of the line is observed, indicating some chromospheric activity. For the star associated to the source CHIIXR-11 the lithium line is found to be in blend with the Ca I 6718 Å line. Hence, this star might be either an ultra-fast rotator or an unresolved spectroscopic binary.

[FIGURE] Fig. 2. Optical spectra of new X-ray emitting WTTS candidates in Cha II. The spectral type, determined as explained in Sect. 3.2, is indicated.

Spectral types have been assigned to these stars following the procedure described in previous papers (Covino et al. 1997; Alcalá et al 2000). The spectral types are also reported in Table 1. We consider these stars as new WTTS candidates but high-resolution spectroscopic observations are needed to confirm that their lithium line is stronger than in active ZAMS stars of the same spectral type and to investigate whether their radial velocities are consistent with the mean for the Cha II cloud.

The observed stars in the error box of the other sources (CHIIXR-17, 19, 21, 28 and 36) are apparently unrelated to Cha II. However, we stress that, because of the magnitude limit (V[FORMULA]16.5) of our spectroscopic observations, only the brightest stars in the X-ray error boxes could be observed. Therefore, we cannot exclude that other faint objects are the true optical counterparts of these sources. In the case of CHIIXR-33, the X-ray coordinates match well with those of the G5V star HD113513, but lithium was not detected in this star.

The remaining X-ray boxes contain objects fainter than our limit of V[FORMULA]16.5 mag and, hence, deserve further spectroscopic studies in order to investigate whether their spectra are those of PMS stars. The notes in Table 1 give information regarding the inspection of the DSS fields.

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Online publication: March 9, 2000