On the relations between infrared colors and mass loss rates for Mira stars

T. Le Bertre and J.M. Winters

Astronomy & Astrophysics, 334, 173-180 (1998)

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Abstract:

A relation between the near-infrared color K-L" and the mass loss rate, dot{M}, is found for nearby (d<2kpc) oxygen-rich Mira stars. This relation is qualitatively similar to the one for carbon stars, but quantitatively different.

Red supergiants, and maybe long-period OH/IR stars, do not follow this relation. This could point to a different process for mass loss in these more luminous sources.

Relations between the J-K and K-12 indices and dot{M} are also found, but present more scatter. Nevertheless, they could be useful in the context of the near-infrared surveys, DENIS and 2MASS, which are in progress. Correlations between the IRAS colors and dot{M} are found to be loose. These relations are investigated on the basis of hydrodynamical wind models. For carbon-rich Miras, there is a partial agreement (within a factor 2) between the time dependent models and the observations. For the same infrared color, the stationary models require a much higher luminosity (typically a factor 3) and give larger mass loss rates. Although no oxygen-rich consistent model is presently available, it seems that the stationary case could apply to M supergiants. Our results support the idea that the mass loss from Mira stars results from the combined actions of i) dissipation of shock wave momentum leading to a density enhancement in the atmosphere and triggering the onset of dust
condensation, and ii) radiation pressure on the dust grains which actually initiates the outflow and accelerates the wind to its terminal velocity. We suggest that some supergiant winds, by contrast, could be purely dust-driven.