5. Conclusions and outlook
Velocity variations were clearly detected in all short period semiregular variables of our sample. These variations can reach an amplitude of up to 4 km s-1. Periodicity is detected in several cases, but did not always follow the GCVS4 period from the visual light change. Some objects seem even to vary on two time scales. Following the classification by Kerschbaum & Hron (1992) we find that `red' SRVs on average have a larger velocity amplitude than the `blue' SRVs. Combining these results with previous investigations on SRVs with longer periods we conclude that this group of variables follows a rough period-velocity amplitude relation, with long period objects having larger velocity amplitudes. This relation will be discussed in more detail in a forthcoming paper. Like in Hinkle et al. (1997) we note that semiregular and mira variables can be clearly separated on their velocity amplitude.
The difference between center of mass velocities and the measurements presented in this paper remains puzzling. Any attempt to derive accurate center of mass velocities at high accuracy from the velocity variations at 1.6 µm is problematic unless the origin of this velocity shift has been understood. Not enough data are available, both at 1.6 µm and in the radio regime, to derive any kind of statistical correction factor, if this were possible at all.
Data for a significantly larger sample would be necessary to investigate both, the velocity displacement and the possibility of subtypes among the SRVs according to the regularity, periodicity and shape of their velocity variations. Such an extension would have to include both long period and short period SRVs. However, obtaining time series of these variables consumes enormous amounts of telescope time. Especially the scheduling of such an observing program is difficult. While data for long period objects like miras, with only small changes in the behavior from cycle to cycle, can be collected over a long time span requiring only a small number of observing nights in each season, short period objects like the SRVs would need continuous observations over a long time to be able to establish systematics in the velocity changes.
Spectroscopy of the investigated CO lines with a significantly higher spectral resolution would be necessary for a detailed investigation of the profiles of individual lines to derive further conclusions on possible surface structures influencing the observed velocity variations. This could lead to an improved understanding of the nature of semiregular variables in general.
An interesting aspect would be a comparison with an extended sample of CO radio velocities for these SRVs, which is unfortunately not yet available in the literature. Finally, new concepts to understand this behavior will only be provided by improved models of the stellar atmosphere in AGB stars.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999