Other
Scientific paper
Dec 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.3204k&link_type=abstract
American Astronomical Society, 195th AAS Meeting, #32.04; Bulletin of the American Astronomical Society, Vol. 31, p.1416
Other
Scientific paper
The X-ray emission from TW Hya, HD 98800, and the three other ``original'' members of the TW Hya Association (TWA) is central to the argument that this group likely represents an association of ``old'' T Tauri stars (Kastner et al. 1997, Science, 277, 67), and X-ray emission has pointed the way to the subsequent identification of additional candidate TWA star systems (Webb et al. 1999, ApJ, 512, L63; Sterzik et al. 1999, A&A 346, L41). We are now beginning to take advantage of the proximity and, hence, large X-ray fluxes of the TWA stars to explore the mechanisms underlying X-ray emission from and variability of T Tauri stars. Observations of TW Hya with the ASCA satellite indicate that its X-ray spectrum is well modeled by coronal emission (Kastner et al. 1999, ApJ, in press). The ASCA data show fluctuations in X-ray flux on timescales 1 hr, while comparison of ROSAT and ASCA spectra indicate variations in the column depth of X-ray absorbing material on the order of years, suggesting that a combination of short-lived flares and longer-term changes in obscuration are responsible for the optical variability of TW Hya. Pending observations of TW Hya by the Chandra X-ray Observatory and the X-ray Multimirror Mission satellite telescopes will shed further light on the nature and variability of its X-ray spectrum. The rich spectrum of radio molecular line emission from TW Hya (Kastner et al. 1997), combined with its bright submillimeter continuum emission, offers compelling evidence that TW Hya is surrounded by a dusty molecular disk that is viewed nearly pole-on. Although several other TWA systems (including certain recently identified members) are sources of dust continuum emission in the radio and infrared, TW Hya remains the only known molecular line source among the TWA stars. Given our rapidly improving knowledge of the distance, age, and membership of the TWA, the apparent lack of molecules around these stars provides ever more stringent constraints on the survival timescales of gaseous circumstellar disks and, hence, the formation timescales of Jovian planets (Zuckerman et al. 1995, Nature, 373, 494). We highlight future radio and infrared observations that should provide improved estimates of (or upper limits for) the gas masses of circumstellar disks surrounding TWA stars.
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