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Up: V1162 Ori: A multiperiodic  Scuti amplitude


   
1 Introduction

The $\delta$ 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 $\delta$ Scuti stars up to almost one magnitude found in some of the high-amplitude $\delta$ Scuti stars (HADS). The HADS generally have amplitudes exceeding 0 $.\!\!^{\rm m}$3 and slow rotational velocities ($v\sin i$ 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 $.\!\!^{\rm m}$1-0 $.\!\!^{\rm m}$2 and its high projected rotational velocity ($v\sin i$ of 46 kms-1, Solano & Fernley 1997). It is an intermediate amplitude, up to now monoperiodic Pop I $\delta$ 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 $\delta$ 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 $\delta$ Scuti stars of low and possibly also high amplitude.

The philosophy of the present multisite campaign is different from normal campaigns on $\delta$ 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.

  \begin{figure}
{\includegraphics[width=12cm]{1425f1n.eps} }\end{figure} Figure 1: Examples of light curves obtained in two of the cases of overlapping data. Dots are data from SAAO, open circles data from Athens University Observatory. We also show the difference between the datapoints from Athens and interpolated values of the SAAO datapoints (triangles, shifted by 120 mmag).


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Up: V1162 Ori: A multiperiodic  Scuti amplitude

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