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Astron. Astrophys. 319, 578-588 (1997)

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5. Conclusions

In this paper we have demonstrated that the observed power-law flux distributions of a number of dMe stars in the radio - IR frequency range cannot be reconciled with that of a stellar wind. Although the radio data suggest mass loss rates of the order of [FORMULA], the resulting flux distribution is not a power-law. The reason is that this mass loss rate is much lower than the rates of hot stars leading to a reduction of the optical depths in the wind and a corresponding modification of the spectrum. A second difference is caused by the fact that for dMe stars the temperatures of the winds are higher than the effective temperatures of the stars. If the radio, JCMT and IRAS are to be fitted simultaneously, it is necessary to invoke the presence of a wind acceleration region. This increases the emission measure of the wind so strongly that at high frequencies [FORMULA] a strong excess is present which has not been observed. This excess is caused by the fact that the required emission measure is so high and that [FORMULA].

Reliable upper limits for the mass loss rate from dMe stars can be obtained by considering the fact that the flux by the wind must not exceed the observed fluxes by instruments like IUE and ROSAT. Also, at radio wavelengths the size of the wind region cannot exceed the upper limits for the radio source size as follow from intercontinental VLBI. Furthermore, if the winds are cool ([FORMULA]), the contribution to the interstellar absorption must not be larger than [FORMULA]. By applying these constraints we arrive at an upper limit for the mass loss rate of [FORMULA]. At higher temperatures, e.g. [FORMULA] a safe upper limit is [FORMULA]. Additional support for this upper limit comes from the fact that at higher mass loss rates EUV and X-ray line photons would be subject to considerable scattering in the wind (and possibly subsequent photon destruction). This has not been observed.

Even if the mass loss from dMe stars would amount to [FORMULA] the observed excess fluxes at mm wavelengths and in the IR cannot be explained by a stellar wind. This implies that if instruments like ISO and SCUBA would find evidence of excess emission, alternative explanations, like e.g. emission from circumstellar dust, are required.

If the mass loss is to proceed in a clumpy way, in the form of coronal mass ejections, then our arguments still apply. If there are only a few remnants of ejecta around the star the approach we followed in this paper is not valid but then the contribution to a possible stellar wind is small anyway. If there are many ejecta near the star, then one has to consider the way this affects the optical depth [FORMULA]. In general it will be reduced compared to a homogeneous wind. This will cause the turn-over frequency [FORMULA] to shift to lower frequencies resulting in lower fluxes at mm and infrared wavelengths. If the number of ejecta becomes very high then one approaches the situation of a homogeneous wind as we considered in this paper.

Given the reduction of the maximum allowable mass loss rate from dMe stars by a factor 100, when compared to the estimates by MDRM, the winds from dMe stars become less important a mass donors for the interstellar medium. At most they contribute [FORMULA] but it is likely that as additional constraints from EUVE data and VLBI observations become available, this number will be reduced. Finally we note that our derived upper limits for the mass loss rate are in agreement with the (independently) derived upper limits by Lim and White (1996).

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998