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6 H-R diagram, ages, and masses

Because we have a homogeneous and almost complete set of precise VJHK photometric data, we estimate the absorption in the line-of-sight to the stars from the observed V-J, V-H, and V-K colors and their intrinsic color indexes, which we know from their spectral types. For those few stars, for which JHK is not available, we estimate the absorption from VRI colors. Visual extinctions AV for all new TTS are listed in Table 5. Then, we can also estimate the bolometric luminosity $L_{\rm bol}$ at an assumed distance of 130 pc, also listed in Table 5.

We display the location of the new TTS in the H-R diagram in Fig. 7 together with tracks and isochrones from D'Antona & Mazzitelli (1994). They yield rough estimates for ages and masses of our new TTS, which we also list in Table 5. It can been seen in Fig. 7 that the two visual pairs of TTS, RXJ1846 and RXJ1857, are both roughly co-eval. The ages of the new TTS range from less than one million years to a few tens of million years, not surprising given the wide spatial distribution. From that wide distribution on the sky around the CrA dark cloud, we have to conclude that the distances can show a similar spread, so that the ages and masses are uncertain. For the newly found young star RXJ1901.4-3422, whose parallax was measured by the Hipparcos satellite, we estimated its luminosity using the Hipparcos distance of 65 pc; it then falls on the 30 Myrs isochrone, just above the ZAMS; because its spectral type is late F, it is difficult to classify it as either pre- or zero-age MS given its lithium. One more newly found Li-rich star, RXJ1901.1-3648, is also found near the ZAMS, using 130 pc as distance, which may of course be a wrong distance. The latter one has spectral type K4 and clearly more lithium than Pleiades K4-type stars; it is located right on the cloud and its extinction is large ( $A_{V} \simeq 3$ mag, Table 5), hence it may be deep inside the dark cloud.

Our new Li-rich ROSAT counterparts show a larger age spread than the Li-rich EO counterparts (Walter et al. 1997). This is probably due to the larger spread in distances among the ROSAT stars compared to the EO stars, because we investigated a larger area on the sky using the RASS as compared Walter et al. (1997) who used EO pointed observations centered on the CrA dark cloud.

The two new cTTS, RXJ1842.9-3532 and RXJ1852.3-3700, both appear to be $\sim 10$ Myrs old, which is relatively old for cTTS. This result only holds if they are indeed at $\sim 130$ pc as assumed for calculating their $L_{\rm bol}$, which may be unrealistic, because they are located on the line-of-sight to two small cloud-lets, both off the main CrA dark cloud. On the other hand, there are also some other cTTS with ages around 10 Myrs, namely TW Hya and HD 98800, for which the Hipparcos parallaxes are known, so that they could be placed correctly into the H-R diagram. Both two new CrA cTTS show only small IR excess emission at JHK, and one of them, RXJ1852.3-3700, is an IRAS source.

The two TTS with the latest spectral types in our sample are RXJ1857.5-3732 E and W with M5 and M6, respectively. They are both certainly young because of the strong lithium absorption. Hence, they are most certainly members of the CrA dark cloud, i.e. at 130 pc. Then, in the H-R diagram, they lie near the borderline between stars and brown dwarfs with masses around $\sim 0.1$ to $\sim 0.2~M_{\odot}$. At first glance, it appears surprising to find such low-mass objects among RASS sources. Previously, several borderline objects and even brown dwarfs were detected as X-ray sources, but only in very deep pointed observations (Neuhäuser & Comerón 1998; Neuhäuser et al. 1999; Neuhäuser & Comerón 2000). However, our two RASS detected mid- to late-M dwarfs form a close pair, i.e. only one unresolved RASS X-ray source. Given their optical/IR luminosities, spectral types, and typical $L_{\rm X}/L_{\rm bol}$ ratios, they would not have been detected individually in the RASS.


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