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ROSAT and HIPPARCOS Observations of Isolated Pre-Main-Sequence Stars near HD 98800
We present new observations of the isolated young stars HD 98800 and CD-33 deg7795. Pointed ROSAT observations show that their X-rayproperties, including X-ray luminosity and variability, are consistentwith those of pre-main-sequence (PMS) stars. These observations do notreveal any additional PMS candidates in 40' fields centered on HD 98800and CD -33 deg7795. Hipparcos observations of TW Hya by Wichmann et al.and of HD 98800 by Soderblom et al. have shown that both stars areroughly 50 pc away and are PMS with ages of ~10^7 yr. We searched theHipparcos Catalogue (complete down to ~2-3 L_ȯ at this distance)for other PMS stars in the same area. In a 10 pc radius volume of spacecentered on the previously known PMS stars, we find one additionalcandidate PMS star (CD -36 deg7429), with a low space velocity, X-rayemission comparable to that of HD 98800, and Li absorption. There areeight other stars in this area that have dwarf spectral types and lieabove the main sequence, but based on their weak X-ray emission, highspace velocities, and lack of Li in low-resolution spectra [i.e., EW(Li)< 0.1 Å], these are probably misclassified subgiants or giants.The current positions and proper motions of TW Hya, HD 98800, and CD -36deg7429 are inconsistent with their having formed as a group.
| Estimation of spectral classifications for bright southern stars with interesting Stromgren indices
This paper investigates the degree of success with which uvby photometrycan be applied to predict spectral classifications for 947 A, F, and Gstars brighter than an apparent magnitude of 8.3 and with four-colorindices indicating some kind of interesting, unusual, or peculiarspectrum. One or several possible spectral classifications are estimatedfor each star from photometry alone, double stars are distinguished, andthe estimates are compared with published classifications. The resultsshow that the framework provided by uvby photometry can be extended toinclude most G and K stars, reddened stars, peculiar stars, and certaintypes of double star.
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