Since work began on the subject (Struve & Elvey 1931), it has been observed that stellar rotation rate is directly linked to the spectral type, and A-type stars are known to be mean high rotators.
The Doppler effect allows measurement of the broadening parameter
,
the projection of the equatorial velocity v along the line of sight. From a statistically significant sample of measured
,
it is possible to derive the distribution of v assuming that the rotation axes are randomly distributed and the sample is not biased.
Projected rotational velocities can be derived in many ways. Although large surveys of
already exist, great care must be
taken when combining their data, as various calibrations were used.
The most accurate method of computing
would be the time-consuming computation of line profiles, starting from a model
atmosphere (with the introduction of other broadening mechanisms), and
their comparison with the observed lines
(see Dravins et al. 1990, for their study of Sirius). Such high precision is not justified, however, in a statistical study of high rotators like the non-peculiar A-type stars where other mechanisms (macroturbulence, instrumental) are negligible compared to rotation.
Line widths appear to be the natural indicator for measuring stellar
rotation, and most
are derived in this way, as a function of
the full-width at half-maximum (FWHM). The largest catalogue of
is by Uesugi & Fukuda (1982). It is an extremely heterogeneous
compilation of observational data mainly based on the old Slettebak
system (Slettebak 1949, 1954, 1955, 1956; Slettebak & Howard 1955). Several years ago,
Abt & Morrell (1995) measured
for 1700 A-type stars in the
northern hemisphere, calibrated with the new system from
Slettebak et al. (1975, hereafter SCBWP). More recently, Wolff & Simon (1997) measured the
of 250 stars, most of which were cooler than those in our
sample, by cross-correlation with the spectra of standard stars of similar temperature.
They found a small systematic difference with Abt & Morrell's results
(the former are larger by 
5%), and with those of Danziger & Faber (1972) (smaller by
8%). This can be explained by the difference between the "old'' and "new'' Slettebak systems.
Brown & Verschueren (1997) derived
for a sample of early-type stars in
Sco OB2 association from spectra taken with the same instrument we
used. They adopted three different techniques according to the
expected
values, which they show to be generally consistent
with each other. The
values so obtained correspond to those
defining the SCBWP scale, except for stars with
below 60
,
for which the SCBWP values are systematically lower.
The use of the Fourier technique in the determination of
remains occasional, mainly because using a calibration FWHM-
is
much easier and fitting theoretical profiles to observed ones in
wavelength space allows one to derive more parameters than simply the rotational broadening. Nevertheless, Fourier techniques are a valuable tool for investigating stellar rotation, as described by Smith & Gray (1976).
Gray (1980) compared the
obtained from Fourier transform of the Mg II 4481 line profile with the
values from Uesugi & Fukuda and SCBWP and found a reasonable agreement (deviations of 
10% with SCBWP), but his sample is quite small.
Suspecting that the small differences found with respect to standard
values could be due to an underestimation in the SCBWP calibration of
the
values, we decided to undertake a measure of
independent of any pre-existing calibration. We adopted the method described in Ramella et al. (1989).
The largest scatter in the average
distribution is
found for late B and early A stars (Gray 1992, Fig. 17.16 p. 386),
and we want to test whether this is due only to errors in measurement or if it is related to some physical effect.
Brown & Verschueren (1997), in their study of the Sco OB2 association, found
that B7-B9 stars of the Upper Scorpius subgroup rotate faster than the
B0-B6 stars. This result corresponds to Gray's result, suggesting that
the apparent scatter may disguise a physical effect. This effect has already been detected by Mouschovias (1983).
The possibility of a change on average
with evolution from zero-age to terminal-age main sequence has been studied for several decades, and the absence of any evolutionary effect for stars with a mass higher than
is confirmed by the recent study of Wolff & Simon (1997).
The fact that the colors of stars are affected by rotation was
observed for the first time by Brown & Verschueren, but only for
stars belonging to young groups, not field stars. They conclude,
moreover, that the determination of ages and mass distributions is not affected by rotation.
As a matter of fact, the effect of rotation on stellar parameters is
also known: a rapidly rotating star simulates a star with lower
and 
.
However, in this case, all quantities (line strength,
photometric colors, for example) change in the same way so that the effect is practically undetectable
(this point was already discussed by Wolff 1983, p. 159), especially when field stars are studied.
In this paper, newly determined
data, obtained with Fourier transforms, for 525 southern early-type stars are presented. The observations and the sample are described in Sect. 2. In Sect. 3 the technique used to derive
from the spectra is detailed and discussed. In Sect. 4 the results are presented and compared to data from the literature. In Sect. 5 our conclusions are summarized.
This paper is the first of a series pertaining to rotational velocities of A-type stars; data collected in the northern hemisphere and measured
will be presented in a forthcoming paper.
Copyright ESO 2001