The Scuti stars are pulsating A-F stars situated on or just above the
main sequence. They display a large range in pulsational amplitude, from
the mmag level observed in the low-amplitude, multiperiodic
Scuti
stars up to almost one magnitude found in some of the
high-amplitude
Scuti stars (HADS). The HADS generally
have amplitudes exceeding 0
3 and slow rotational
velocities (
below 30 kms-1).
V1162 Ori is often considered
a HADS, although it does not qualify as such due to its full amplitude of only
0
1-0
2
and its high projected rotational velocity
(
of 46 kms-1, Solano & Fernley 1997).
It is an intermediate amplitude, up to now monoperiodic Pop I
Scuti
star with a frequency of 12.7082 d-1:
Hintz et al. (1998) claimed a secondary frequency near 16.5 d-1, but this
was later shown to arise from a variable comparison star
(Lampens & Van Cauteren 2000).
We will, however, show that V1162 Ori is not monoperiodic and that it is indeed
positioned in the narrow HADS instability strip given by McNamara (2000).
V1162 Ori has in the past shown very large amplitude changes, ranging from
half peak-to-peak values of
98 mmag observed by Poretti et al. (1990) to 50 mmag observed by Hintz
et al. (1998), who also detected a period break
using O-C analysis of times of maximum light.
Later changes observed by Arentoft & Sterken (2000, hereinafter
Paper I) could be due to period breaks or cyclic
period changes, and also these authors detected amplitude variations.
As a result it
was decided to organise a multisite campaign on V1162 Ori, spanning a full
observing season. The aims were to investigate the time scales of the changes,
how or if the amplitude and period/phase variations are related
and if possible to gain information on the underlying physical processes
causing them. Although amplitude and period variations are common
phenomena in
Scuti stars, the causes
are far from understood (see e.g. Breger & Pamyatnykh 1998; Breger 2000a).
Even the involved
time scales are very different from star to star: 4 CVn, for example, shows
amplitude
variability on time scales of years (Breger 2000b), whereas XX Pyx displays
period and amplitude variability on time scales as short as 20 d
(Handler et al. 2000).
Breger (2000a) discusses the possibility that amplitude variability
can be related to
multiperiodicity, as the monoperiodic HADS appear to have more
stable amplitudes
(e.g. Rodriguez 1999) than the multiperiodic
Scuti stars of low and
possibly also high amplitude.
The philosophy of the present multisite
campaign is different from normal campaigns on Scuti
stars: the aim was to collect as many extrema as possible over the
observing season (8 months).
Thus, the participating teams observed
V1162 Ori whenever they had sufficient time to spare to cover an extremum.
These observations,
which often covered short light curve sections - sometimes only
20 min - had the purpose
of following the evolution of the main pulsational period,
and were complemented with
dedicated time-series observations from several sites, also distributed
over the long time span.
The latter allow us to
monitor amplitude changes as well as to search for low-amplitude frequencies
- however without the usual multisite advantage of suppressed side-lobes in
the amplitude spectra.
Finding low-amplitude frequencies is very important for understanding
changes in the light curve:
low-amplitude frequencies can interfere with the
main mode and cause e.g. amplitude variations through beating or give rise to
cycle-to-cycle variations. Furthermore, detection of additional pulsation
frequencies would yield tighter constraints on stellar models.
Copyright ESO 2001