Astron. Astrophys. 359, 113-130 (2000)

7. Summary and conclusions

7.1. Main observational results

We have obtained two deep exposures of the Oph cloud core region (d=140 pc) with the ROSAT   High Resolution Imager (core A: 51 ks, core F: 77 ks, in three partial exposures). The improved position accuracy (1"-6") with respect to previous recent X-ray observations (ROSAT   PSPC , Casanova et al. 1995; and ASCA, Koyama et al. 1994 and Kamata et al. 1997) have allowed us to remove a number of positional ambiguities for the detected sources. We have cross-correlated the X-ray positions with IR sources found in the ISOCAM survey of the same region at 6.7 and 14.3 µm, in addition to sources known in the optical, IR, and radio from ground-based observations. We thus have now at our disposal the best-studied sample of X-ray emitting YSO in a star-forming region. We first summarize the main observational results of this article.

1. We detect 63 HRI X-ray sources, and 55 are identified. Of the 55 identified X-ray sources 40 are PSPC sources, and 9 are ASCA sources.

2. The IR classification (ground-based and ISOCAM survey) for the 55 identified X-ray sources yields: one Class I protostar (YLW15=IRS43); 23 Class II sources, including 4 new ISOCAM Class II sources; 21 Class III sources, including 13 new Class III sources; 8 new Class II or Class III source candidates; one early-type Class III source (the young magnetic B3 star S1), and one field star (the F2V star HD148352). The contamination of the sample of new X-ray sources by field stars is negligible.

3. There is no statistically significant difference between the X-ray luminosity functions of HRI -detected Class II and Class III sources, i.e. T Tauri stars with and without disks, confirming that the contribution of these disks to X-ray absorption, or emission (for instance by magnetic reconnection between the star and the disk), must be small.

4. X-ray variability of HRI -detected T Tauri stars has been studied by comparing the HRI data with the previously obtained PSPC data, and using HRI observations done at three different epochs. The resulting statistics show that most of the sources are variable, and that their X-ray variability is consistent with a solar-like (hence magnetic) flare origin.

5. We use the information given both by the ISOCAM survey and by our HRI deep exposure to study the T Tauri star population of the Oph dense cores. We confirm that essentially all Class II and Class III YSO are X-ray emitters, and that a strong correlation () exists between the X-ray luminosity and the stellar luminosity of T Tauri stars, likely down to low luminosities ( ). We confirm that the characteristic for T Tauri stars is 10^-4 in the  Oph cloud, albeit with a large dispersion. There is no evidence for a magnetic "saturation" seen at a level of 10^-3 in late-type main sequence stars.

6. However, most of the new ISOCAM Class II sources are not detected by the HRI . We show that this is consistent with their intrinsic X-ray luminosities being too faint if "predicted" using the above - correlation.

7.2. What have we learned?

1. The first general conclusion we can draw from the HRI results presented above is a complete confirmation of the PSPC results obtained by CMFA. This was not a priori obvious, since the CMFA population (PSPC and near-IR) overlaps, but is different from, the HRI /near-IR/ISOCAM population presented in this paper: many PSPC sources are not detected by the HRI (see Appendix C), and some HRI sources are Class II and Class III newly classified thanks to a combined identification with ISOCAM . This shows that the - correlation is robust for the  Oph TTS.

2. The second, and perhaps most important, conclusion is the probable existence of unknown X-ray YSO down to a limit of erg s^-1 in the HRI/ISOCAM overlapping area, which should be mainly low- to very low-mass (-0.6 ) diskless, "Class III TTS". This prediction is based both on the use of the - correlation, legitimated by its robustness, and on the discovery of a large number of faint new IR sources by ISOCAM . As shown below, it may be soon verified by the next generation of X-ray satellites, namely XMM-Newton and Chandra . In this respect, the present paper can be taken as a "transition" paper between two generations of X-ray satellites.

Why is the detection of these "unknown TTS" important? Because they are diskless, they are unlikely to be recognized as YSO by IR observations alone; and because they are likely to be as numerous as the YSO with IR excess, they have to be included in any reliable census of YSO, with an impact on such basic quantities as the initial mass function, or the star formation efficiency, especially if considered from an evolutionary point of view. For instance, from the results in this paper it is impossible to study the real connection between the distributions of the Class II and Class III sources in the densest regions, in particular to see whether the distribution of the Class III sources is also centered on the same DCO⁺ cores as the Class II sources. The number of Class III sources embedded in the densest regions may, or may not, be comparable to that of the Class II sources, depending on the timescale for disk dispersal, especially among low-mass YSO. An X-ray improved census of Class III sources may also be crucial in determining whether a burst of star formation is presently going on in the  Oph cores, as some recent indications suggest (see Martín et al. 1998). It will also allow to study the - correlation for Class III and Class II sources seperately, which was not possible in this paper (x) due to insufficient statistics.

7.3. The potential of XMM-Newton and Chandra

To quantify the prospects for improvement in the X-ray domain, we have computed the detection threshold for the X-ray camera EPIC aboard XMM-Newton , which was successfully launched in December 1999 (see Fig. 7). The improved sensitivity and enlarged energy range (0.5-12 keV) of EPIC will allow to detect the weak ISOCAM Class II sources, and also to discover numerous unknown faint or embedded Class III sources, in particular if they have high plasma temperatures (several keV) reached during flares, and extend the census of this population towards the low-mass end. In the best case, the XMM-Newton sensitivity will reach erg s^-1 for , for long exposures (75 ksec). This is nearly two orders of magnitude more sensitive than ROSAT . In case the faint Class III sources turn out to be so crowed that confusion problems arise, the excellent angular resolution of Chandra will be critical.

In Oph, there are already several identified bona fide and candidate brown dwarfs (see review in Neuhäuser et al. 1999, and references therein), and four of them have been recently detected in X-ray using the ROSAT   PSPC archive (Neuhäuser et al. 1999). Neuhäuser et al. have also shown that brown dwarfs could be X-ray emitters with the same ratio than for T Tauri stars. Thus Chandra and XMM-Newton should be able to detect many more of these objects with low stellar luminosity and masses, shedding a new light on their nature and early evolution.

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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