The observations reported here were performed with the ESO 3.6 m telescope and its Cassegrain Echelle Spectrograph (CASPEC) on the following dates: May 21-25, 1991; June 7-9, 1992; August 10-13, 1992; February 7-8, 1993; and July 24-27, 1993. The Zeeman analyzer built into CASPEC was inserted in the light path to separate incoming light of right and left circular polarization (RCP and LCP, resp.), and the spectra corresponding to both polarizations were recorded simultaneously, interleaved as described in Paper I.
The configuration of the Zeeman analyzer used in 1991 and 1992 is identical to the one that had been employed between 1985 and 1988 to record the spectra from which the magnetic results presented in Papers I to V had been derived. A detailed description of this configuration has been given in Paper I. It appears useful to summarize it here for understandability of the differences introduced in the instrumental setup after 1992. Namely, the analyzer was originally a self-contained unit mounted behind the entrance slit of the spectrograph on the wheel also bearing the "rear slit viewer'' used to center the target in the slit at acquisition time. This wheel is remotely controlled via the instrument computer, so that the analyzer can be called into the light beam at exposure definition time through the observation interface. Within the analyzer, the light beam passes consecutively through:
From 1993, the Zeeman analyzer of CASPEC has been modified as followed (the February 1993 observations reported here were the first test of this modified configuration):
There are a number of additional differences of instrumental setup between the various observing runs reported in this paper, and with respect to the homogeneous set of observations discussed in Papers I to V. They will now be described.
In May 1991 and in June 1992, we used the same camera ("short
camera'', f/1.6) and echelle grating (52 lines/mm) as between
1985 and 1988. But the CCD RCA #3 ( pixels of
m2) that we had previously employed was replaced
by the CCD Tektronix #16 (
pixels of
m2). As a result, a slightly higher spectral
resolution was achieved (
), and
a significantly longer spectral range was recorded overall (from
5400 Å to 6800 Å), although at the expense of somewhat larger
wavelength gaps between consecutive echelle orders.
In August 1992, the "long camera'' (f/3, Pasquini 1993) of CASPEC
was for the first time used in combination with the Zeeman analyzer
(still in its old configuration). The larger interorder separation
then achieved on the CCD allowed the use of the 31.6 lines/mm echelle
grating without overlapping of opposite polarizations of consecutive
orders (see Paper I). The efficiency of this grating in the spectral
region of interest is higher than that of the 52 lines/mm grating,
which compensates to some extent for the increased slit losses (for full
exploitation of the resolution achievable with the long camera,
, the slit
width was set to 105, compared to 21 with the short
camera). Also, all other things being equal, the use of the
31.6 lines/mm grating would reduce the interorder gaps in the
wavelength coverage, compared to the 52 lines/mm. However, with the small
size of CCD #16 (which was mounted for the considered run), the increase
in the geometrical scale between the short and the long camera more
than cancels this advantage, since a large fraction of the whole
cross-dispersed echelle pattern falls outside the detector boundaries.
As a result, the spectra range only from 5710 Å to 6450 Å, with gaps of
17 Å to 27 Å between consecutive orders.
The first tests of the modified configuration of the Zeeman analyzer in February 1993 were carried out with essentially the same setup as used in May 1991 and June 1992: 52 lines/mm echelle grating and short camera. This time, the CCD was #32, a Tektronix chip having the same format as #16 but a higher efficiency in the blue-violet part of the spectrum: that is, this detector was essentially equivalent to #16 for our purpose.
CCD #32 was also used for the observations of July 1993, together with the configuration of the Zeeman analyzer introduced in Februray 1993, the 31.6 lines/mm echelle grating, and the long camera of CASPEC.
This latter configuration is closest to the present standard setup of
CASPEC for observations with the Zeeman analyzer. The only difference
is that CCD #32 has been replaced by #37, a Tektronix of
pixels of
m2. With this setup,
spectra can now be recorded at a resolving power
over the range
,
without gaps in the wavelength coverage. This configuration is
mentioned here only for completeness of the record. Using it,
we are currently
regularly obtaining observations for a new programme, the
results of which will be presented in a future paper.
The various configurations of CASPEC and of its Zeeman analyzer that we have employed for studies of stellar magnetic fields are summarized in Table 1 (click here). The dates at which each configuration was used are given in Col. 1. The two configurations of the Zeeman analyzer (ZA) itself, which have been described above, are called "old'' and "new'' in Col. 2. The echelle grating is referred to in Col. 3 by its number of lines per millimeter. Since 1991, CASPEC users have been offered the choice between two cross-dispersers (X-disp.), a "blue'' one and a "red'' one, which are optimized to give a better efficiency resp. at blue or at red wavelengths. Although the red cross-disperser should be slightly more efficient than the blue one in the spectral range covered by our observations, it is unsuitable for our purpose: when using it, the echelle orders are too packed, and opposite polarisations of consecutive orders would overlap when the Zeeman analyzer is inserted in the light beam. Accordingly, we have always used the blue cross-disperser, as indicated in Col. 4 of Table 1 (click here). Columns 5 and 6 of this table give the camera used (short or long), and the CCD (referred to by its internal ESO number).
Table 1: Configurations of CASPEC and of its Zeeman analyzer used to
study stellar magnetic fields
The data reduction steps have been described in Paper II. The procedure applied to the present observations differed from the one used for the 1985-1988 data only in two (minor) aspects:
Table 2: Programme stars
References: 1: Mathys et al. (1996) (MHLLM);
2: Manfroid & Renson (1994);
3: Kurtz et al. (1996); 4: North (1990) (unpublished; cited by Catalano et al.
1991); 5: Mathys (1991) (Paper II);
6: Pedersen (1979); 7: Waelkens (1985); 8: Kurtz et al. (1992);
9: Hensberge (1993); 10: this paper; 11: Babcock (1960);
12: Kurtz et al. (1994); 13: Mathys & Lanz (1996); 14: Kurtz (1989);
15: Catalano & Leone (1993); 16: Leroy et al. (1994);
17: Wolff (1975); 18: Kurtz & Marang (1987);
19: Preston (1970); 20: Lanz & Mathys (1991);
21: Babcock (1958);
22: Bohlender et al. (1987); 23: Landstreet (unpublished; cited by Mathys
1991); 24: Wolff (1969).
The reduced spectra have signal-to-noise ratios ranging from 100 to 300 (most of them between 150 and 200). The rms deviation of the lines of the ThAr arc spectra used for wavelength calibration with respect to the fitted dispersion law is of the order of 3 to 3.5 mÅ with the short camera, and of 1.5 to 1.8 mÅ with the long camera.
The stars for which magnetic field measurements are reported here are listed in Table 2 (click here). The presentation is identical to Table 1 (click here) of Paper II (or Table 2 (click here) of Paper III): the HD (or HDE) number and another identifier in Cols. 1 and 2, the V magnitude and the spectral type (both from the catalog of Renson et al. 1991) in Cols. 3 and 4, the adopted value of the rotation period P and the reference from which it is retrieved in Cols. 5 and 6, the heliocentric Julian date taken as the origin of the phases, the particular property (e.g. a magnetic field extremum) from which this origin is defined, and the corresponding reference in Cols. 7 to 9.