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Astron. Astrophys. 351, 644-656 (1999)

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1. Introduction

Variability is a primary characteristic of stars on the asymptotic giant branch. Brightness variations occur on time scales of 30 to 1000 days and these variables are therefore called long period variables. They can be further divided into miras and semiregular variables (SRVs) depending on the amplitude and regularity of the variation.

The variability is thought to be caused by a pulsation of the atmosphere. This pulsation has been successfully monitored in miras and a few long period semiregular variables with the help of high excitation [FORMULA]v[FORMULA]2 and [FORMULA]v[FORMULA]3 vibration rotation lines of CO by Hinkle and collaborators (see Lebzelter et al. 1999 for a complete list of references). These lines are located in the near infrared (2.3 and 1.6 µm, respectively), where the flux maximum of cool stars coincides with an opacity minimum allowing a much deeper look into the atmosphere than in the visual region.

The result of these investigations was that all miras exhibit similar velocity variations clearly correlated with the visual light cycle. The velocity curve is s-shaped and discontinuous with line doubling around maximum phase. This is an indicator of a shock front running through the atmosphere. The amplitude of the velocity variations is similar for all miras and lies between 20 and 30 km s-1. On the other hand, the few SRVs investigated up to now show much smaller velocity amplitudes. The irregularity known from the light curves of SRVs is also found in the shape of the velocity curve. While the variations are quite periodic for miras, non-periodic behavior seems common among the SRVs (Hinkle et al. 1997). The characteristics of the variations are independent from metallicity or chemistry both for miras and SRVs.

The pulsational properties revealed by the monitoring of near infrared CO lines provide an essential input for the understanding and modelling of the atmospheres of long period variables. The pulsation leads to a largely extended atmosphere and is an important factor for the extensive mass loss observed in these stars.

The properties of the SRVs and the relation of this class of variables to the miras are still a matter of debate. Kerschbaum & Hron (1992) showed that miras and SRVs have similar number densities and scale heights (except the short period miras and the `blue' SRVs), suggesting that both groups of variables are in a similar evolutionary status. At a given period SRVs are cooler and more luminous than miras 1, suggesting a higher mode of pulsation of the SRVs. However, on average SRVs are hotter than miras and show a less expressed mass loss.

Miras form a much more homogeneous group of objects in several aspects (visual amplitude, velocity amplitude, period and periodicity) than SRVs do. The detection of a set of parameters typical for miras was supported by the much richer observational data base existing for this group of variables. For SRVs data are still lacking, also concerning velocity variations of these objects. In this paper we present new results in the latter field for a sample of short period SRVs with periods [FORMULA] 150 days. In a forthcoming paper (Lebzelter, Kiss & Hinkle, in preparation) we will compare the velocity variations of semiregular variables with the light change in the visual. A third paper will deal with the relation between the velocity variations and other stellar properties for the whole sample of long period variables (Lebzelter, Hinkle & Hron, in preparation).

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999