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Astron. Astrophys. 326, 249-256 (1997)

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Chromospheric models of dwarf M stars *

Pablo J.D. Mauas 1, Ambretta Falchi 2, Luca Pasquini 3 and Roberto Pallavicini 4

1 Instituto de Astronomía y Física del Espacio, CC 67, Suc. 28, 1428 Buenos Aires, R. Argentina
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50155 Firenze, Italy
3 European Southern Observatory, La Silla, Casilla 19001, Santiago 19, Chile
4 Osservatorio Astronomico, Palazzo dei Normanni, I-90134 Palermo, Italy

Received 9 October 1996 / Accepted 18 March 1997


We present chromospheric models for two dM stars considered as "basal" stars due to the low level of chromospheric activity, and compare them with the model for a very active, flare star obtained in a previous paper. These models are not based on a single spectral feature, but on the continuum in a broad wavelength range (3500 - 9000 Å), on many line profiles corresponding to three different atoms (H, Ca, Na), and on the Mg II h and k flux. We show that a marked chromosphere is present even for stars with the lowest levels of activity, and that the active stars have both a chromospheric temperature rise at larger column mass and a higher chromospheric temperature. These characteristics are responsible for the presence of the Balmer lines in emission in the dMe stars. We also show that the additional energy required to transform a dM star into an active star must be deposited in the high chromosphere, just below the transition region.

Key words: stars: atmospheres – stars: chromosphere – stars: late-type

* Tables 4 and 5 are only available in electronic form via http://cdsweb.u-strasbg.fr/abstract.html

Send offprint requests to: A. Falchi

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998