Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...294..773h&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 294, no. 3, p. 773-791
Astronomy and Astrophysics
Astrophysics
32
Absorption Spectra, Balmer Series, Chromosphere, H Alpha Line, Lyman Spectra, Main Sequence Stars, Stellar Activity, Stellar Models, Dwarf Stars, Ionization, Paschen Series, Radiative Heat Transfer
Scientific paper
We use the constraints at the extremes in magnetic activity level to simulate the effect of varying magnetic non-thermal heating in dM and dMe chromospheres by varying the transition region pressure and temperature minimum. We built four grids of model atmospheres with temperature minimum either at 2,660 K or 3,000 K, and a range of transition region pressure. We found that when decreasing the transition region pressure (i.e. the chromospheric temperature gradient), the Balmer lines change rapidly from emission to strong absorption, then the profiles weaken and become narrower until they disappear totally (zero Halpha stars). The Paschen and Brackett series exhibit a qualitatively similar behavior, but the `emission domain' is at a higher column mass. The Brackett lines never really develop a strong absorption. In opposition with other series, the Lyman lines show a monotonous decrease and even change to absorption for very low density models. These differences are useful spectral diagnostics for M dwarf atmospheres. All hydrogen series, except the Lyman series for intermediate and high pressures, are sensitive to the temperature minimum when large changes are considered. We also investigated the effect of the temperature break zone and found it is important only for high pressure atmospheres. Our grids of models successfully reproduce all type of observed Halpha profiles: (1) high activity with strong emission and weak self-reversal, (2) filled in intermediate activity with inner wings in emission and the core in absorption, (3) intermediate activity with strong and broad absorption, (4) low activity with weak and narrow absorption, (5) `zero activity' with an undetectable profile. We discuss the line characteristics over this wide range of physical conditions. We analyze the ionization fraction and electron density for our series of chromospheres. Changes in the ionization fraction are important throughout the pressure range. Heavy elements are the main electron donor in the photosphere for all models, and in the chromosphere for low pressure models. Back-ionization by the chromospheric continuum emission and direct ionization by the photospheric radiation field are taking place at about the temperature minimum. Finally, we compare the chromospheric density regimes for main sequence stars (M, K and G dwarfs) for the quiescent and flare states.
Doyle Gerry J.
Houdebine E. R.
Koscielecki Marcin
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