Other
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt.......105a&link_type=abstract
Ph.D. Thesis, Ruprecht-Karls-Universität, Heidelberg, (1994)
Other
11
Scientific paper
Weak--line T Tauri stars (WTTS) are low--mass pre--main sequence (PMS) stars which lack both the strong emission lines and the infra--red (IR) excesses typical of classical T Tauri stars (CTTS). Most of them have been discovered on the basis of their solar--like X--ray emission. In this work, X--ray data from the ROSAT all-sky survey in the Chamaeleon (Cha) and the Orion star forming regions (SFR's), and from one ROSAT pointed observation in the Cha I dark cloud, were used to determine the number of WTTS, to investigate their spatial distribution, and to study their physical properties and their X--ray emission. The total number of X-ray sources detected in the ROSAT survey is 820 in Orion (~450 square degrees), and 181 in Chamaeleon (~200 square degrees). Fifty four X--ray sources were detected in the ROSAT pointed observation in Cha I. Cross correlation of the X-ray positions with those of catalogued objects in the Simbad database gives about 30% of coincidences in the two SFR's. These counterparts are mainly extragalactic objects, HD, HR or SAO stars, CTTS, and a few WTTS known from previous EINSTEIN observations. For the remaining sources, a programme of spectroscopic observations has been conducted. These observations led to the identification of 85 and 58 new WTTS in the Chamaeleon and Orion SFR's respectively. For the Chamaeleon SFR, the spectroscopic identification of the survey sources is complete, while for the Orion SFR there are about 470 sources still to be investigated. Eighty percent of the sources from the pointed observation in Cha I have been identified. In addition, optical and near IR photometric observations of a sub--sample of the new WTTS were carried out. The spectral energy distributions of these stars are very similar to those of normal stars of the same spectral type. The effective temperature and the stellar luminosity were derived in order to place the sub--sample of WTTS in the H--R diagram. By comparison with the theoretical evolutionary tracks from D'Antona and Mazzitelli (1993), the sub--sample of WTTS in Chamaeleon are estimated to have masses in the range 0.3--2.5Modot, and ages in the range 105yr to 5 × 107yr, with about 70% of the stars having ages less than 3 × 106yr. On the other hand, the sub--sample of WTTS in Orion have masses in the range 1--3Modot, and ages in the range 105yr to 107yr. A comparison of the bulk properties of WTTS and of CTTS gives the following results: i) the WTTS are distributed throughout the studied areas, while the CTTS are mainly concentrated in the cloud cores; ii) no substantial difference between the ages of WTTS and CTTS is found; iii) WTTS have, on average, earlier spectral types than CTTS; iv) on the basis of the survey data, the ratio of the number of WTTS to CTTS is >= 1.5 in the Chamaeleon complex. However, a larger ratio (up to 7) can not yet be ruled out. This ratio will be derived for the Orion complex when sufficient spectroscopic observations are complete. The X--ray luminosity function (XLF) for the identified sources was derived. WTTS with very low levels of X--ray emission (logLx(erg/s) <= 28.7) were found. These stars are the main contribution to this XLF at lower X--ray luminosities. A correlation between the bolometric luminosity and the X--ray luminosity of the new WTTS was found. This may indicate that a mass--X--ray luminosity relation exists for low--mass PMS stars. A comparison of the strength of the Hα emission line of WTTS to that of other magnetically--active stars shows that the emission in WTTS represents the maximum level which can be attributed to stellar chromospheric emission. No anticorrelation is found between the strength of the Hα emission line and the level of X--ray emission, as would be expected for smothered coronae of T Tauri stars. However, a strong trend for the K -- L index, a diagnostic of accretion activity, to increase when the level of X-ray emission decreases is found. High accretion rates in CTTS may produce high hydrogen column densities between the disk and the star. Matter at these densities efficiently absorbs the coronal X--ray photons, leading to a smothering of the coronal emission in CTTS. First results of spectroscopic and photometric monitoring show that strong variability can be found among WTTS. Spectacular variability exhibited by the Hα line of the star T Chamaeleontis has been found. The line can change from absorption to strong emission on timescales of one day, and sometimes an inverse P Cygni profile is seen. The analysis of the X--ray light curves of the ROSAT survey sources demonstrated that flare activity may enhance the X--ray emission in T Tauri stars, but this is not the main mechanism for the X-ray emission. Finally, it is concluded that the initial physical conditions during the star formation process may determine the stellar characteristics, the evolution of angular momentum towards the main-sequence, and consequently the X--ray properties.
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