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Astron. Astrophys. 361, 167-174 (2000)

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1. Introduction

The first discovery of semiregular light variation for a red giant (µ Cep) dates to 1782 (see Burnham 1978for a historical review). Since that time an enormous amount of light curve data has been collected, with most of the data for large amplitude late-type variable (miras) coming from amateur astronomers. For the lower amplitude late-type variables a distinct group has been assigned, the semiregular variables (SRVs; Kholopov et al. 1985-88, GCVS4).

In this paper we will focus on the semiregular subclasses SRa and SRb only, and exclude SRc (supergiants) and SRd (yellow giants). A comprehensive discussion of the properties of SRas and SRbs has been given by Kerschbaum & Hron (1992, 1994). The SRVs are evolved stars, probably located on the AGB; however their evolutionary status on the AGB is still a matter of debate (e.g. Lebzelter & Hron 1999). The bluer SRVs, which typically also have the shorter periods, show less mass loss and no indications for thermal pulses. The redder SRVs have longer periods, and can show mass loss comparable to the miras.

The cause of the irregularities in the light variations of the SRVs is far from being understood. Recently, empirical evidence has been presented for multiple period changes (Mattei et al. 1998, Kiss et al. 1999), which may be due to many (2 or more) simultaneously excited modes of pulsation. The observed mode changes in several semiregular stars (Cadmus et al. 1991, Percy & Desjardnis 1996, Bedding et al. 1998, Kiss et al. 1999) strongly suggest a pulsational origin for the multiperiodic nature. However, other explanations, e.g. chaotic phenomena (Aikawa 1987, Icke et al. 1992) perhaps in combination with an interplay of excited modes of pulsation (Buchler & Goupil 1988) or coupling of rotation and pulsation (Barnbaum et al. 1995) have been suggested in the literature. One has to be aware that all of these mechanisms may simply be cases of `chaos' (see e.g. Buchler & Kolláth 2000). Further explanations include dust-shell dynamics (Höfner et al. 1995) or large convective cells (Antia et al. 1984). Observed spatial asymmetries in some Mira variables (Karovska 1999) could imply the possibility of non-radial oscillations. This was also discussed as an explanation for the amplitude modulation of the SRb-type variable RY UMa (Kiss et al. 2000).

A problem in understanding the nature of the SRVs is that nearly all observations have been of visual colors. To develop a physical model for the SRV changes, monitoring of time variability of other parameters like velocity or temperature variations is needed. One approach is to monitor the velocity variations of SRVs closer to the pulsation driving zone through time-series high-resolution infrared spectroscopy. Hinkle et al. (1997) and Lebzelter (1999) reported on time-series high-resolution spectroscopy of second overtone and high excitation first overtone CO lines located in an opacity minimum around 1.6 µm. However, in both papers only phase information (relative to a close light curve maximum) was used to search for regularities in the velocity variations and to derive an amplitude of the velocity curve. As the variations in SRVs are far from being regular both in the sense of period length as well as amplitude, a phase estimated in this way may significantly misrepresent the behavior of the light curve.

A direct comparison between light and velocity changes has been done for the mira variable [FORMULA] Cyg (Hinkle et al. 1982). The velocity curves of miras are all similar and discontinuous with an amplitude of 20 to 30 km s-1. Line doubling is observed around maximum light, and at that phase typically both maximum and minimum velocity occur. Around phase 0.4, i.e. shortly before light minimum, the velocity curve reaches the center-of-mass velocity. This point in the variability cycle might therefore be related to the maximum expansion of the layer producing the CO lines used to derive the velocity curve. Other miras show similar results; the velocity amplitudes of SRVs are significantly less (Hinkle et al. 1984, Hinkle et al. 1997).

This paper is dedicated to the irregularities . We will compare simultaneously measured light and velocity curves to correlate the irregularities. The velocity variations represent the movement of atmospheric layers due to stellar pulsation. In this way we will investigate whether the irregularities in the light curve are due to nonregular pulsation or due to surface structures like active regions or velocity fields introduced by convective cells.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000