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Astron. Astrophys. 361, 581-593 (2000)

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5. Conclusions

We have searched the ROSAT All-Sky Survey for X-ray emission from potential members of the recently identified Tucanae association. 59% of the stars labeled `probable' members by Zuckerman & Webb (2000) are detected, but only 7% of the `improbable' members. The RASS XLDF of the probable members is very similar to the XLDF for the Taurus-Auriga star forming region, and the young open cluster IC 2602. The XLDF of the TW Hydrae association has the same [FORMULA], but shows a somewhat smaller spread of luminosities. This is presumably due to the narrow range of spectral types among the TW Hydrae stars and/or the assumed uniform distance of 55 pc for all stars without measured parallax. The similarity of the XLDF for the above mentioned regions indicates that the X-ray emission does not change significantly with age from the early PMS stage (Taurus-Auriga) until the phase when the main-sequence (MS) is reached (IC 2602). However, the XLDF of the 100 Myr old Pleiades cluster is characterized by significantly weaker X-ray emitters than all other samples and suggests that once on the MS the stellar X-ray luminosity decreases. From this comparison we infer an age between 10-30 Myr for the Tucanae association.

Most of the RASS detected Tucanae members have highly variable lightcurves. The only star observed in a long PSPC pointed exposure shows strong variations there, but was not significantly variable during the RASS. This strengthens the hypothesis that probably all Tucanae stars are variable given long enough observing time. The strong X-ray variability observed can be considered another indicator for the youth of these systems.

The youth and close distance of the Tucanae stars makes them good candidates for direct imaging of substellar companions, both brown dwarfs and even giant planets, because substellar objects are hot and bright when young (Burrows et al. 1997) and well separated when nearby. I.e. they are detectable with the current technology (e.g. Neuhäuser et al. 2000a). The Tucanae members are as well suited for this purpose as the TW Hya members and the MBM 12 T Tauri stars (Hearty et al. 2000a, Hearty et al. 2000b).

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000