Astron. Astrophys. 326, 647-654 (1997)

2. The observed sample

Our sample consists of a number of late-type stars which we have previously investigated in the context of our study of X-ray selected stars from the EMSS and the Einstein Slew Survey (ESS). For all these stars we had available both low- and high-resolution spectra. The stars were selected to span a range of spectral types, as well as of lithium abundance. The sample stars have spectral types ranging from G0 to M0, and have lithium abundances (as determined on the basis on high-resolution spectroscopy by Favata et al. 1995) ranging from below the detection limit in high-resolution spectra (implying equivalent widths of the Li I 6707.8 Å doublet smaller than mÅ) up to cosmic lithium abundance ( ). Their spectra are representative of the larger EMSS and ESS samples. The characteristics of the sample stars are listed in Table 1. In addition, we have studied three M stars which are listed in Table 2 and are discussed in detail in Sect. 3.1. This small sample of M stars includes a main-sequence, very active star as well as two bona fide PMS stars, and, while not homogeneous in selection criteria with the first sample, it supplements it at the cooler end.

Table 1. The stars for which we have measured the equivalent width of the Li I 6707.8 A doublet in both high-resolution (0.05 A per pixel) and low-resolution (2 A per pixel) spectra. The equivalent width of the real Li I doublet measured in the high-resolution spectra is taken from Favata et al. (1995) while the equivalent width of the feature at A visible in all the low-resolution spectra has been measured in the present work. The projected rotational velocity of each source (which gives a measure of the eventual rotational broadening of the lines), the lithium abundance and the are also from Favata et al. (1995). The last column gives the difference between the true and measured (on the low-resolution spectra) distance between the 6708 A Li I feature and the 6717 A Ca I feature. In all cases this difference is significantly smaller than the spectral resolution of A , indicating the the identification of the measured feature with the lithium feature at 6708 A appears to be reliable.

The low-resolution spectra used here have all been acquired using the ESO 1.5 m spectroscopic telescope with the Boller & Chivens spectrograph. The combination of grating and CCD chip used yielded a resolution of 1.9 Å per pixel, or a two-pixel resolution of Å, very similar to the higher resolution employed by Alcala et al. (1995). The high-resolution spectra were acquired using the ESO CAT 1.4 m telescope with the Coudé Echelle Spectrometer (CES). The short camera with the RCA CCD (ESO #9) was used, yielding an effective resolution of about 50 000 (or Å per pixel). The spectra are centered on the Li I 6707.8 Å doublet, and cover the range -6730 Å. The data reduction procedure has been described in detail in Favata et al. (1993), to which the reader is referred for details. These spectra allow a measurement of the equivalent width of the Li I 6707.8 Å doublet to a precision of a few mÅ (depending on the rotational velocity), and thus form an excellent benchmark for checking the possibility of measuring the equivalent width of the same line at lower resolutions.

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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