Astron. Astrophys. 318, 819-834 (1997)
1. Introduction
Early-B hypergiants belong to the most luminous stars
in the Universe. Their optical spectra are
distinguished by strong P Cygni-type profiles of HI and
HeI lines indicating strong mass loss.
The B hypergiants represent a short-lived evolutionary phase and
are of particular interest in connection with current evolutionary
theories of massive stars with mass loss (Meynet et al. 1994; Schaerer
et al. 1996a, b).
Quite recently, Kudritzki (1995) found an empirical relationship
between the expression (the product of
mass-loss rate, terminal wind velocity, and photospheric radius) and
the luminosity of O, B, and A supergiants. This relation makes
luminous hot stars potentially useful for measuring the cosmological
distance scale. Since the B hypergiants represent extreme cases of
supergiants, they are especially important to test the validity of
this relation over a wide luminosity range. Since these stars are also
visually bright enough to be easily observed in extragalactic systems,
they can be used to test the metallicity dependence of this
relation.
The best-investigated early-B hypergiant is the prototype
Sco (HD 152236, B1.5
), which is a member of the Sco OB1 association.
Radio (Bieging et al. 1989) and infrared observations (Leitherer &
Wolf 1984; Bertout et al. 1985) have shown that
Sco loses mass at a very high rate of the order of
. A similar mass-loss rate was derived by
Leitherer (1988) from the P Cygni-type profile.
The satellite-UV spectrum of Sco was first
investigated in detail by Wolf & Appenzeller (1979) and
Appenzeller & Wolf (1979) using high-resolution IUE observations.
Sco has a high mass-loss rate and a low
terminal velocity of only 400 50
(cf. Prinja et al. 1990; Lamers et al. 1995).
Therefore the wind density is high and the UV spectrum shows in
addition to the typical wind indicators (strong UV resonance lines)
also P Cygni profiles in various lines from transitions of
metastable lower states (mainly of FeIII).
Lamers et al. (1995) found that the terminal velocities of stellar
winds show a transition from fast to slow winds at the temperatures of
early-B stars. The ratio drops from a mean
value of about 2.6 at to 1.3 at
within one spectral type. The stars
Sco and HD 190603 are, with
=1.1 and 1.2, respectively, at the low side of
this "bi-stability jump". Lamers et al. did not study the third
star in our sample, HD 169454, but the similarity of the three
stars suggests that this star is on the low side of the jump, too. (A
selection of the UV wind profiles of Sco
and HD 190603 is published in "The Atlas of Ultraviolet P Cygni
profiles" by Snow et al. (1994)).
Among the most interesting characteristics of early-B hypergiants
are the various kinds of photometric and spectroscopic variations.
Sterken (1977) and Burki et al. (1982) discovered photometric
variations of Sco of about 0.1 mag on
typical time scales of 11 to 16 days. Similar results were obtained
for HD 169454.
Extensive high-dispersion spectroscopic observations of
Sco in the optical range were performed
during several campaigns between 1972 and 1975 with the Coudé
spectrograph at ESO (Sterken & Wolf 1978, 1979). These authors
found pronounced line-profile variations of the P Cygni-type lines and
radial-velocity variations with peak-to-peak values of about 20
. These variations of photospheric lines indicate
considerable mass-loss variations and the presence of time- and
depth-dependent velocity fields in the atmosphere. Again, similar
variations were found for HD 169454. Moreover it should be noted
that all lines of Sco are asymmetric with
blue wings extending to to
. This indicates that practically no line is
formed in a pure hydrostatic photosphere (cf. Wolf & Appenzeller
1979).
Burki et al. (1982) investigated the variability of the UV
resonance lines of Sco from several
high-resolution IUE spectra. They found profile-variable discrete
absorption components, which they ascribed to a discontinuous mass
loss in addition to the continuous global flow.
These snap-shot observations clearly showed that the mass loss of
early-B hypergiants is variable. On the other hand it became evident
that a better understanding of the physical processes in the
atmosphere and wind of these extreme stars requires long-term
spectroscopic monitoring campaigns, with good resolution both in
wavelength and time, and covering a wide wavelength range. Using our
fiber-linked echelle spectrograph (Mandel 1988)
such observing campaigns were carried out very successfully by our
group in connection with LBVs (Wolf et al. 1993), P Cygni (Stahl et
al. 1993, 1994), and with B and A supergiants (Kaufer et al.
1996a, b).
We therefore decided to monitor the early-B hypergiants
Sco (B1.5 ),
HD 169454 (B1 ), and HD 190603 (B1.5
) during several years. Since the typical time
scales determined from photometric variability are of the order of
weeks we observed the objects during several months to cover typically
more than three cycles. Our experience with BA supergiants showed that
the repetition of such campaigns over several years is important to
provide information on the stability of such cycles and to investigate
the variability on still longer time scales.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998
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