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

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5. Discussion

Except for two objects, practically all YSOs in Cha II lie in the area of the ROSAT pointing. This provides us with a spatially complete sample and a good census of the X-ray emitting population of Cha II.

Class-III IR sources or WTTS show-up preferentially in X-rays. From previous ROSAT studies in SFRs, it was found that YSOs have hardness ratios typically in the ranges [FORMULA] and [FORMULA] (Neuhäuser et al. 1995a). This, of course, does not exclude that other active field stars may contaminate samples of X-ray selected sources with hardness ratios that fall in the box defined above. However, it is a starting point to pre-select objects with X-ray characteristics similar to those of active stars and in particular to those of YSOs.

In Fig. 4 the hardness ratio diagram, HR2 versus HR1 is shown. The objects identified with previously known YSOs are indicated with shaded circles, while the remaining X-ray sources are marked with crosses. Many of the X-ray sources identified with YSOs in the Cha II pointing have hardness ratios that fall in the box defined above. Note, however, that some of them have [FORMULA], but this also happens in the sample of YSOs by Neuhäuser et al. (1995a; see their Fig. 1). Thus some YSOs may still have [FORMULA] (but [FORMULA]). Also three of the newly identified lithium stars (indicated with a dot in Fig. 4) satisfy the hardness ratio criterion.

Three X-ray sources with [FORMULA] have hardness ratios inconsistent with those of YSOs. Two of them, CHIIXR-33 and CHIIXR-36, are identified with the stars HD113513 and HD113696 respectively, which are not related to the Cha II cloud. In the error box of the other source (CHIIXR-26) there are faint objects in the centre of the field which may be foreground or background objects.

From the similarities between the X-ray properties of the sources not yet optically identified (represented with crosses in Fig. 4) and those of the YSOs in Cha II and in other SFRs, it is expected that several of them will have a YSO counterpart. In any case, because of the small number of sources left, the result that in Cha II the CTTS may be more numerous than the WTTS prevails.

An alternative hypothesis is that WTTS in Cha II may be more numerous than those detected in this study, but they are embedded or lie behind the dark cloud with their X-ray emission being screened by several magnitudes of extinction. Note that most of the known YSOs in Cha II lie on the western side of the dark cloud, which leads to the idea that we are mainly detecting the less "obscured edge" of the entire population of YSOs. However, since in hard X-rays the absorption is lower than in the optical (for the hard band AV = 5 means AX [FORMULA] 1), 1-2 keV photons can reach us even from embedded objects with up to 10 - 15 mags. of visual extinction, which makes possible the X-ray detection of embedded IR sources in hard X-rays (Casanova et al. 1995; Montmerle 1996). Therefore, if there were many embedded or screened X-ray emitting YSOs in Cha II, at least some of them would have been detected in the deep ASCA observation by Yamauchi et al. (1998). In fact, the Herbig Ae star IRAS 12496-7650 was detected by ASCA despite more than 100 magnitudes of extinction 4.

Another possibility is that the X-ray fluxes of many WTTS fall below the detection limit of the ASCA and ROSAT observations. There may be two possible explanations for this: first, there is clear evidence that the Cha II cloud is located farther away than 180 pc. X-ray emitting YSOs with X-ray fluxes below [FORMULA], which is the limiting flux of our ROSAT pointing at a distance of 200 pc of the Cha II cloud (Hughes and Hartigan 1992), would escape from detection; second, the mass function of the Cha II cloud indicates that there is a larger fraction of low-mass stars in Cha II than in the Cha I SFR (Hughes and Hartigan 1992). Therefore, the strong dependence of the X-ray luminosity on the mass (Feigelson et al. 1993; Walter 1996), would make difficult the X-ray detection of PMS stars with masses lower than about 0.15 [FORMULA]. In fact, the X-ray luminosities of the Cha II objects are apparently consistent with low masses 5. In order for these two effects to explain the paucity of WTTS and the low WTTS/CTTS ratio, the WTTS would have to be intrinsically less X-ray bright than CTTS. However, this is not generally the case. Therefore, we conclude that the low WTTS/CTTS ratio in Cha II is real.

We have attempted to determine an X-ray luminosity function (XLF), using the data listed in Table 4, but the fact that 70% of the X-ray luminosities are upper-limits leads to a bias in the XLF.

The comparison with the XLF of the Cha I sample, derived using the X-ray luminosities reported by Feigelson et al. (1993) and including the lithium stars identified by Lawson, Feigelson and Huenemoerder (1996), shows that there is a slight trend for the Cha I sample to have a higher level of X-ray emission (c.f. Fig. 5). The mean X-ray luminosity of the Cha II members ([FORMULA] = 28.86 [FORMULA] 0.10) is slightly lower than that for the Cha I members ([FORMULA]=29.20 [FORMULA] 0.05). This difference may again reflect the strong dependence of X-ray luminosity on mass (Feigelson et al. 1993; Walter et al. 1996) as a consequence of the fact that there is a larger fraction of low-mass stars in Cha II than there is in Cha I.

[FIGURE] Fig. 5. Kaplan-Meier X-ray luminosity functions of PMS stars in the Cha II cloud (thick line) and Cha I cloud (thin line).

On the other hand, the comparison of the age distributions of the Cha I and Cha II stars, shows that the Cha II stars are, on average, younger than those of Cha I (see Fig. 7 by Alcalá et al. 1997) and, hence, it is likely that Cha II is in an earlier evolutionary stage than Cha I (Gauvin and Strom 1992 and Prusti et al. 1992). Assuming the scenario in which CTTS eventually evolve into WTTS by the dispersion of their circumstellar disk or the formation of planets, then in Cha II one would expect the ratio No.(WTTS)/No.(CTTS) to be less than in Cha I, as a consequence of the different evolutionary status of the two regions. Such scenario would be consistent with the low detection rate of WTTS in the ROSAT pointed observation.

The non detection of any of the protostar candidates in Cha II in the ROSAT pointing, which is in line with the ASCA results by Yamauchi et al. (1998), might in principle be attributed to their X-ray variability but, it is hard to believe that X-ray variability is the reason for the non detection of all five protostar candidates in two X-ray observations. We think that the probability to detect X-ray emission from a protostar, where the "circumprotostellar" matter should be distributed more in a torus-like structure than in a disk, is intrinsically low because a small angle of the polar axis with respect to the line of sight would be sufficient for the protostellar matter to screen the X-ray emission. In this sense, the non detection of any of the protostar candidates might be consistent with the possibility that the class-I IR sources in Cha II are seen most likely edge-on, as proposed by Yamauchi et al. (1998).

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© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000