Astron. Astrophys. 359, 181-190 (2000)

2. Observations and data reduction

High-resolution echelle spectra were obtained at the Observatoire de Haute Provence, using the spectrograph ELODIE at the 193 cm telescope (for a detailed description of the instrument see Baranne et al. 1996). Observations took place on 14-15 Nov 1994 (observer: P. Corporon) and on 21-27 Nov 1994 (observers: S. Allain and R. Wichmann). The run was split in two because of the unfavorable position of the moon in between.

The sample selected for these observations comprised all stars with magnitudes mag from Wichmann et al. 1996. However, due to technical problems (readout failures), not for all observed stars spectra are available. As the nature of this problem is independent of the stellar magnitude, we regard the sample as statistically representative for the full original, magnitude-limited sample.

The data were reduced by C. Melo at the Observatoire de Geneve, using dedicated software developed for ELODIE. Radial and rotational velocities were determined by cross-correlation with a numerical mask.

For the determination of lithium equivalent widths, all spectra of the relevant order were averaged to find regions in the continuum unaffected by metal lines. These regions then were used to fit the continuum by a straight line. The equivalent width of the Li line () then was determined by fitting a gaussian curve. The derivation of was performed completely independently by two of us (C. Melo and R. Wichmann). The results, which in general were consistent within 20-40 mÅ, were averaged to produce the final . The uncertainty of is due to the low S/N of the spectra (typically about 10-15).

Additional high-resolution spectroscopic observations for nearly 2/3 of the sample were obtained at the Harvard-Smithsonian Center for Astrophysics (CfA). We used nearly identical echelle spectrographs on the 1.5m Wyeth reflector at the Oak Ridge Observatory (Harvard, Massachusetts), the 1.5m Tillinghast reflector at the F. L. Whipple Observatory (Mt. Hopkins, Arizona), and on the Multiple Mirror Telescope (also Mt. Hopkins, Arizona).

The observations were obtained on a number of runs over the period 1996-1998. The instrument, setup, and reduction procedures are described in detail by Neuhäuser et al. (1997). We obtained multiple observations of most of the objects to check for binarity, with an average of about 4 exposures per star. In addition to the radial velocities, derived with standard cross-correlation techniques, we determined also effective temperatures and projected rotational velocities by comparison with a large grid of synthetic spectra based on Kurucz model atmospheres. Details on these reductions are given also by Neuhäuser et al. (1997).

The results from these spectroscopic observations at OHP and CfA are shown in Table 1.

Table 1. Results from echelle spectroscopy. We list seqence number Seq from Wichmann et al. (1996), designation, spectral type SpT (Wichmann et al. 1996), equivalent widths , (both in mÅ), effective temperatures from cross-correlation and from SpT (in K), from CfA data and from ELODIE data, from CfA data and from ELODIE data, as well as the weighted mean (all in km s^-1). We also add a note on spectroscopic binaries (SB1=single-lined, SB2=double-lined). Stars with are those regarded as PMS (designation in italics), where is the offset to the Pleiades upper limit (see Sect. 3). RX J0424.8+2643A/B: in Wichmann et al. (1996), incorrectly the (fainter) NW star is named BD+26 718, and the SE star BD+26 718B. The SE star (=BD+26 718) is RX J0424.8+2643B in SIMBAD. The two stars annotated with `(?)SB2' have recently been reobserved (once each) at the Swiss 1.2 m-telescope on La Silla by one of us (C. Melo). A preliminary analysis of the data could not confirm their SB2 nature.

Table 1. (continued)

Photometric data for our sample of ROSAT LRSs in Taurus-Auriga are partly taken from Bouvier et al. 1997. Additional data were obtained at the Calar Alto Observatory, Spain, with the 1.23 m telescope and the CCD camera. The detector was a Tektronics 1024x1024 CCD with 24 pixels. Observations were done during several runs, on Dec 2, 1996, Nov 4-7, 1994, Dec 16-19, 1993, and Dec 29, 1993 to Jan 2, 1994. Typically six standard star fields from the list of Landolt 1983were observed each night to transform the instrumental system into the standard BVRI system. Data reduction was performed at Landessternwarte Heidelberg using standard IRAF/apphot routines.

Assuming a distance of 140 pc, i.e. the HIPPARCOS distance of the Taurus-Auriga clouds (Wichmann et al. 1998), we have computed masses and ages using D'Antona & Mazzitelli (1994) evolutionary tracks with Canuto & Mazzitelli (1991) convection and Alexander et al. (1991) opacities. (For details on the determination of luminosities and effective temperature, we refer to Wichmann et al. 1997a). The absolute masses and ages provided by these tracks might be doubtful (cf. Forestini 1994, Wuchterl 1999 for alternative tracks). However, they have been used frequently in the literature, and thus are well suited for comparison. Results from photometry are displayed in Table 2.

Table 2. Results from photometry. We list seqence number Seq from Wichmann et al. (1996), designation, spectral type SpT (Wichmann et al. 1996), V magnitude, B-V, V-R, and R-I. Luminosities and radii are computed for an assumed distance of 140 pc. Masses and ages are from comparison with evolutionary tracks from D'Antona & Mazzitelli 1994 (CM convection, Alexander opacities). RX J0435.9+2352 and RX J0441.4+2715 would fall above the birthline and below the ZAMS, respectively, for the assumed distance. Designation in italics indicates stars regarded as PMS.

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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