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Astron. Astrophys. 336, 654-661 (1998)

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3. Gas expansion velocities

We will compare stellar and circumstellar characteristics of the IRVs, SRVs, and Miras in this and the next section. For a comparison with SRVs and Miras additional data from the literature were used (Young 1995, Loup et al. 1993, Kahane & Jura 1994, SRIV), as well as new observations of SRVs presented in a catalogue paper (Kerschbaum & Olofsson in preparation). According to the classification scheme for SRVs presented in Kerschbaum & Hron (1992) the SRVs are split into `blue'-, `red'-, and `Mira'-SRVs with the first ones being ignored here since they show no mass-loss. For the plots the border between `red'- and `Mira'-SRVs is defined by period (P[FORMULA]200d for `red', and P[FORMULA]200d for `Mira').

3.1. Distribution

Whereas mass-loss rates taken from the literature often differ substantially, expansion velocities can easily be derived without too many assumptions even from lower S/N-ratio spectra. Consequently, using [FORMULA] larger comparison samples are available. If for an individual object more than one independent measurement of [FORMULA] are available the best was chosen, or, if they are of comparable quality, they were averaged.

The majority of the detected IRVs and SRVs have, with few exceptions, envelopes with small expansion velocities, Fig. 2. They range from 3 to 15 km[FORMULA]s with a mean value around 8 km[FORMULA]s for both groups. Except for a few objects with expansion velocities above 15 km[FORMULA]s the Miras cover the same velocity range and have roughly the same distribution. In particular, all three groups of variables show a number of very low expansion velocity objects (below 6 km[FORMULA]s) like the IRVs V584 Aql and AZ UMa shown in Fig. 1.

[FIGURE] Fig. 2. Distribution of expansion velocities

The measured expansion velocities are comparable to the terminal velocities derived by Netzer & Elitzur (1993) in models of O-rich objects with mass-loss rates between a few [FORMULA] to [FORMULA] [FORMULA].

3.2. Stellar parameters

3.2.1. Visual amplitude

In Fig. 3 we have plotted the gas expansion velocity as a function of the V-amplitude listed in the new Tycho-Catalogue (ESA 1997). For all three groups of variables there appears to be no trend in expansion velocity with amplitude. The distributions of the IRVs and the `red'-SRVs are indistinguishable from each other and well separated from that of the `Mira'-SRVs and the Miras.

[FIGURE] Fig. 3. Expansion velocity as a function of Tycho-amplitude

3.2.2. M-subclass

The comparison with the averaged M-subclass (GCVS4), which mainly reflects the stellar effective temperature, shows some trends, Fig. 4. The lowest expansion velocities are found between M6.5 and M8 (but this could be an effect of the larger number of stars in this range). The intermediate expansion velocities (6-12 km[FORMULA]s) are found for a wide range of spectral classes (M4-M8), with the `hotter' stars being mainly IRVs and `red'-SRVs. High expansion velocities are only found for late-type Miras.

[FIGURE] Fig. 4. Expansion velocity as a function of mean M-subclass

3.2.3. IRAS LRS-class

An independent quantity measuring dust mass-loss is the IRAS LRS-class. In Fig. 5 we plot [FORMULA] versus the IRAS LRS-class. A clear trend is visible. The objects with the strongest silicate emission feature (high 2n class) show the highest expansion velocities. Low [FORMULA] is accompanied by a low 2n class. The IRVs and the `red'-SRVs mostly show only a weak silicate emission feature indicating low mass-loss rates. For all groups of variables also objects without a clear emission feature (only a `flat' continuum; LRS-classes 14-16) are found, and they have with only one exception expansion velocities below [FORMULA]10 km[FORMULA]s. Thus, it appears that the expansion velocity increases with the efficiency of the mass loss process.

[FIGURE] Fig. 5. Expansion velocity as a function of IRAS LRS-class

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998