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Astron. Astrophys. 333, 619-628 (1998) 3. Optical spectroscopy with FLAIR and data analysisFor our observations, we used the wide-field multi-object
spectrograph FLAIR II (fiber-linked array-image reformatter; see
Parker & Watson 1995) on the UK Schmidt Telescope of the
Anglo-Australian Observatory. This instrument has 91 fibers which are
terminated with right-angled prisms. Each fiber has to be positioned
in exact alignment with the selected target star visible on a film
copy of the corresponding Schmidt plate and glued onto the film. The
core diameter of each fiber is 100 µm, corresponding to
FLAIR allows to observe up to 91 objects simultaneously in the
While FLAIR is a powerful facility in terms of multiplex advantage
and area coverage, it also imposes some observational restrictions
which influence the selection of target stars. One factor is that the
magnitude difference between the brightest and faintest stars observed
simultaneously has to be kept relatively small ( It has recently been shown that the determination of the equivalent
width of the 6708 Å Li line can be problematic if the spectral
resolution is too low. Low resolution (4 - 8 Å) spectroscopy can
lead to a serious overestimation of the Li line width (Covino et
al. 1997). Intermediate resolution of at least 2 Å is
necessary to measure the equivalent width reliably
(cf. Neuhäuser et al. 1997). We therefore used a 1200
line/mm grating, the highest dispersion grating available for use with
FLAIR, which gives a spectral resolution of Our observing procedure was as follows: First we took bias and
dome-flat frames and arc spectra with a Ne and a Hg-Cd calibration
lamp. We started with the 1200 line/mm grating and took a series of
During our first observing run in June 1996 we could only obtain the intermediate resolution spectra for the western-most field due to very bad weather conditions. During our second observing run in June 1997 we could obtain intermediate and low resolution spectra for the other 5 fields. During two nights up to three of the individual exposures contained only very low signal due to moving clouds. However, for each series of exposures we had at least 9 usable frames. In total we took spectra for 88 X-ray selected candidates, 136 proper motion candidates, and additionally for 13 known PMS stars from the list of W94 and K98 in order to use them as spectral standards. Bias subtraction and flat-fielding was done in the usual way with the corresponding standard IRAF 1 routines. Then the individual frames of each series were summed and cosmic rays removed. We used the IRAF task dohydra for subtraction of scattered light, extraction of the spectra, and for wavelength calibration. The dome-flat frames were used for the throughput correction during the sky subtraction. Nearly all spectra have at least 10 000 counts per pixel in the continuum around 6700 Å. As an example for our data we show parts of several intermediate resolution spectra in Fig. 2.
From our FLAIR data we could obtain usable spectra
( The equivalent width was measured in two ways, first by integrating
over the line profile, and second by fitting a Gaussian to the line
profile. The mean of both values was used as the final equivalent
width. For nearly all spectra the two values agree to within
For all stars with strong Li lines we determined the spectral type
from the low resolution spectra by comparison with the standard stars.
We assume our spectral types to be accurate to ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: April 20, 1998 ![]() |