Astron. Astrophys. 320, 525-539 (1997)

2. Sample stars and X-ray data analysis

For this study we have used archive data from 24 deep ROSAT PSPC observations of young clusters or individual stars. We extracted X-ray spectra of TTS from ROSAT observations of the Orion Nebula, the Oph star forming region, and the Cha I dark cloud. X-ray spectra of young main-sequence stars were obtained from archive data of the young cluster IC2391, the Pleiades, and the Hyades. In order to fill the gap between the age of the Hyades and the Sun, we also analyzed archive data on 8 field stars, originally collected by Dorren & Guinan (1994) as a sample of very good solar proxies with different ages and rotational velocities (see also Dorren et al. 1995; Güdel et al. 1996a, b). Basic information on the data sets used in our study is summarized in Table 1. In Table 2 we give spectral type, stellar radius, and rotational properties of the sample stars. The entry "bin" in the last column means that the star is known as a binary, "bin?" means that binarity is suspected. "C" is the abbreviation for "classical T Tauri star", defined as showing H in emission with Å, "W" that for "weak line T Tauri star" ( Å). The data have been collected from the references given in Table 1 and additionally from Gagné & Caillault (1994), Gagné et al. (1995b), Hempelmann et al. (1995), Herbig & Bell (1988), Huenemoerder et al. (1994), Micela et al. (1990), and Prosser et al. (1995).

Table 1. Basic information on the stellar sample and the ROSAT data sets (identified by their observation numbers ROR) used for this study.

Table 2. Properties of the sample stars (for details see text).

All X-ray data were retrieved from the ROSAT data archive at MPE. The data analysis was performed with the EXSAS software package (Zimmermann et al. 1993). We extracted X-ray spectra for all sources that could be properly identified with late type cluster members and had at least about 1000 source counts. The identification of the X-ray sources was taken from the literature (mainly from the prime reference given in Table 1) and checked by comparing optical and X-ray positions. Background spectra were extracted from nearby source free regions. We only used spectra of unblended sources, where no problems due to other nearby sources occurred. Furthermore, we investigated the X-ray light curves of all sources and excluded objects showing strong variability or flares. This resulted in 62 X-ray spectra for 56 different stars. The background subtraction and the rebinning of the spectra was performed with the corresponding EXSAS routines. We used a flexible binning scheme, in which channels are added to a bin until a signal to noise ratio of is reached inside this bin. This binning scheme makes optimal use of the available spectral information and avoids bins with nearly zero source counts.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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