Other
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........25n&link_type=abstract
PhD Thesis, Max-Planck-Institut für Extraterrestrische Physik, 1996.
Other
Scientific paper
This work presents results from the ROSAT All-Sky Survey (RASS) observations in the Taurus--Auriga T association. Prior to the ROSAT mission, about 150 low-mass late-type pre-main sequence stars were known in Taurus--Auriga, about as many classical T Tauri stars (cTTS) with Wλ (H α) > 10Å as weak-line T Tauri stars (wTTS) with H α either weak in emission or in absorption. Observations with the X-ray satellite EINSTEIN led to the conclusion that there might be as many as ~103 wTTS in Taurus--Auriga, many of which could be detectable with the flux-limited but spatially complete RASS. Two thirds of the well-known wTTS, but one sixth of the cTTS were detected with RASS. The energy resolution of the ROSAT PSPC allows some spectral analysis even for low S/N observations with RASS, where TTS are detected with typically 102 counts only or less. It was found that cTTS X-ray spectra appear to be harder than those of wTTS, because circumstellar material around cTTS absorbs X-rays, while wTTS are often clear of disks and envelopes. Correcting for both the X-ray emission energy and the (circumstellar, intercloud, and interstellar) absorption yield X-ray luminosities individually corrected for any detected star, so that one can study X-ray luminosity functions. Comparing cTTS and wTTS Kaplan-Meier estimators (including upper limits of undetected TTS), it was found that wTTS emit intrinsically significantly more X-rays than cTTS, consistent with significantly different RASS detection rates for wTTS and cTTS. There is a significant correlation between X-ray emission and rotation period (and rotational velocity) in TTS suggesting that a dynamo mechanism is responsible for X-ray emission in TTS. The faster a TTS rotates, the larger the stellar magnetic field, so that field line emerge out of the stellar surface forming coronal loops that contain hot plasma. It is well known that cTTS rotate slower than wTTS, thus explaining the significantly different X-ray luminosity functions. Within the cTTS sample, there is no change in X-ray emission with age, because cTTS cannot spin up while contracting along the Hayashi track, as their stellar magnetic field ankers with the disk. As soon as a TTS looses its disk, it can spin up, thus emitting more X-rays as observed (see Neuhäuser et al. 1995, A&A 297, 391). In the 103 square degree wide area studied (including the Taurus--Auriga region and its surroundings), there are almost 2 cdot 103 X-ray sources which cannot be identified with known objects (see Neuhäuser et al. 1995, A&A 295, L5). To find the most likely TTS candidates among them, a multi-parameter selection process can be applied. RASS X-ray colors, optical magnitudes, and the X-ray to optical flux ratios are known for all previously known TTS detected with RASS as well as for all unidentified sources with a near-by HST GSC counterpart. Well-known TTS can be used as training set in order to select those unidentified sources which resemble best typical TTS properties (see Sterzik et al. 1995, A&A 297, 418). Ground-based optical follow-up observations of TTS candidates (performed not only in Taurus--Auriga but also in several other star forming regions) have resulted in the discovery of hundreds of hithertoo unknown TTS, most of which are wTTS with radial velocities consistent with other TTS in their region. Additionally, 15 young stars have been found south of the Taurus--Auriga dark clouds, all of which display typical wTTS properties (H α, Lithium, v cdot sin i, and X-ray data). While some of them show radial velocities consistent with membership to the Taurus--Auriga T association (dispersed post-TTS?), others show radial velocities far off the Taurus mean. Their observed velocity dispersion is (a) much larger than typical of TTS in the dark clouds and (b) even when considering their young age (as concluded from high Lithium abundance), sufficient for the possible explanation that they have been formed in central Taurus and subsequently moved to their present location (see Neuhäuser et al. 1995, A&A 299, L13). Quantitative age estimates based on photometry and evolutionary tracks together with measurements of their surface gravities (in order to eliminate possibly large errors in age estimates based on assumptions on the distance) as well as precise proper motions should be able to test this `run--away TTS' hypothesis. (Copies of the thesis (written in german language) and papers mentioned can be obtained from the address above.)
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