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D. J. Mullan (Bartol Research Institute, Univ. of Delaware), B. Riaz, J. E. Gizis (Univ. of Delaware)
We present Spitzer Space Telescope observations of eight active M dwarfs in the IRAC and MIPS bands. Based on their IRAC colors, we have classified these M dwarfs as Weak T Tauri Stars (WTTS). Four of our targets display small excesses at 160 μm. Over the wavelength range from a few microns to tens of μm the flux densities can be fitted well with a Rayleigh-Jeans curve. We do not detect evidence for the presence of any appreciable dust in our target stars. We argue that any small excess that we see may be due to a chromosphere/corona. Assuming that the opacity is dominated by free-free processes, we interpret our observations over the Spitzer wavelength range in terms of the rise in temperature from the photosphere into the chromosphere/corona. In order to model the free-free absorption as a function of height, we start with a model chromosphere which was derived by E. R. Houdebine (1990, Ph.D. dissertation) for the active M dwarf AU Mic. Houdebine derived a semi-empirical model based on fitting the emission line profiles of Hα and Hβ. In the model, the temperature is assumed to rise linearly with log m (where m is mass loading) from a temperature minimum of 3255 K up to T = 8000 K at log m (g cm-2) = -2.9. Above T = 8000 K, the temperature rises abruptly to T = 3 x 105 K. We consider a range of models in which the electron densities are allowed to be uniformly larger or smaller than in the Houdebine model, by factors of 0.1-100. Using these, we have obtained good fits to our Spitzer observations, suggesting that the IR excesses in our target M dwarfs are consistent with the existence of a moderately dense chromosphere/corona.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.