We have presented 95 new determinations of the mean longitudinal
magnetic field and of the crossover in 44 Ap stars, which complement
those already published in Papers III and IV. For the mean quadratic
magnetic field, only 79 measurements are reported: the attempt to
diagnose this field moment yielded a result devoid of physical meaning
(a negative value of the mean square magnetic field) for
16 observations. As
already noted in Sect. 3, the quadratic field diagnosis proved more
difficult than in Paper V, since the changes of instrumental
configuration between the various observations of a given star
prevented us from combining them to derive a single contribution for the
combination of the non-magnetic part of the line profile and of the
instrumental profile. The reduced wavelength coverage achieved with the
long camera of CASPEC used with a "small'' ( pixels)
CCD, and the resulting small number of lines available in some cases
to diagnose the field, also severely hampered our ability to derive a
meaningful quadratic field.
As indicated in Sect. 1, one of the purposes of the observations
reported here was to monitor the evolution of the performance of the
Zeeman analyzer of CASPEC throughout the modifications progressively
undergone by the instrument. To this effect, we reobserved several
of the stars that had already been studied in detail in our previous
work. The present determinations of the longitudinal
field and of the crossover for these stars (e.g., HD 125248,
HD 137909, HD 147010, ) are consistent with those of
Papers III and IV, indicating that the polarimetric performance of
CASPEC has remained mostly unaltered through the configuration
changes. With the long camera of CASPEC, the uncertainty of the best
measurements of
(achieved for stars with rich, sharp-lined spectra
such as HD 137909 and HD 201601) has gone down to
. This
improvement with respect to the smallest errors of
obtained in
Paper III reflects well the increase of resolving power by a factor of
about 2 resulting from the change from short to long camera. Similar
progress is achieved in the accuracy of the crossover determinations
(again, see e.g. HD 137909), where the smallest errors are in the
range. By contrast, the accuracy of quadratic field
data is often worse than, and at best similar to that achieved in
Paper V, due to the already repeatedly stressed
impossibility to combine observations obtained at different phases for
the derivation of this field moment. As mentioned in the previous
papers of this series, the formal errors that we determine from the rms
of the least-squares fits performed to derive the field moments appear
as good estimates of the actual measurement uncertainties. This is
supported, in particular, by the fact that, for stars repeatedly
observed throughout their rotation cycle, a reduced
close to
1 is generally obtained when fitting the data by
functions of the types given by Eqs. (4) or (5). This indicates that
the uncertainties assigned to the measurements are consistent with their
scatter about a smooth variation curve.
For several of the stars, the new measurements obtained here allowed significant improvement to be achieved in the definition of the variation curves. This happens, in particular, for HD 137509 (see Sect. 4.2.10), whose magnetic field is as a result shown to be predominantly quadrupolar, a structure which had been observed so far in only a couple of stars (Thompson & Landstreet 1985; Landstreet 1990). HD 137509 had already been pointed out in Paper V as a star of particular interest, for having the strongest quadratic field measured so far. The present finding of a strong quadrupolar component in its magnetic field still increases this interest.
Our new quadratic field data for HD 153882 bring full confirmation of
the suspicion expressed in Paper V that the variation of that field moment
is best represented by a sinusoid with twice the stellar rotation
frequency, with no significant contribution of a term with the rotation
frequency. This does not imply any peculiar structure of the magnetic
field: depending on the geometry of the observation,
such behaviour of might be seen for a centred dipole (very much
like in the case of the mean field modulus, which has been discussed in
some detail by MHLLM).
The knowledge of the rotation period of several of the studied stars
was improved as a result of the measurements reported here. A refined
value of the period of HD 119419 has been derived, thanks to the longer
timebase now covered by the avaible data. New measurements of the
longitudinal field of HD 24712 almost certainly solve the ambiguity
between the values of the
period proposed by Kurtz & Marang (1987) and in Paper II, providing
strong evidence that the latter is indeed the correct one. As shown
already by MHLLM, combining our determinations with those of
other authors for HD 137949 gives clear indications that the rotation
period of that star must be at least of the order of 75 years, making
it one of the two Ap stars with the longest period currently known.
In the course of this study, 14 roAp stars (out of 28 such stars currently known) have been observed. For 6 of them, no attempt to detect a magnetic field had ever been made. Magnetic fields are believed to play an essential rôle in roAp stars. According to the most popular model for those stars, the oblique pulsator model (Kurtz 1982), the pulsation modes are aligned with the magnetic axis of the star. Even more fundamentally, most assumptions made about the excitation mechanism of the pulsation require the presence of a magnetic field. This stresses the interest of studies of magnetic fields in roAp stars: even the most basic question whether all those stars do have a magnetic field is crucial. The following elements of answer are brought by the present study:
In summary, more investigations of the magnetic field of the roAp stars would be most desirable, since at present, it is not even clear whether some of them are magnetic at all. We are planning to pursue such a study.
The sample considered here also contains 21 Ap stars with resolved magnetically split spectral lines, out of the 42 such stars currently known (MHLLM). For 8 of them, we report the first determination of the longitudinal field. Our two attempts to measure the longitudinal field of HD 134214 were the first ever made; that both of them yielded null results may possibly be purely coincidental and result from an unfortunate phasing of the observations (see Sect. 4.3.14), as it definitely is for HD 165474 (see Sect. 4.2.14). A long-term programme of systematic study of the longitudinal field of the Ap stars with magnetically resolved lines is in progress, with a view to modelling the field structure from the simultaneous consideration of longitudinal field and field modulus (from MHLLM) data well distributed throughout the stellar rotation cycles.
Generally, the results presented here are intended for future use in determinations of the geometric structure of the magnetic fields of the studied stars. This will be the subject of future works.
Acknowledgements
This research has made use of the Simbad database, operated at CDS, Strasbourg, France.