The CARMENES search for exoplanets around M dwarfs. The He I infrared triplet lines in PHOENIX models of M 2-3 V stars

Publication date: 
Main author: 
Hintz, D.
IAA authors: 
Amado, P. J.;Anglada-Escudé, G.;Bauer, F. F.
Hintz, D.;Fuhrmeister, B.;Czesla, S.;Schmitt, J. H. M. M.;Schweitzer, A.;Nagel, E.;Johnson, E. N.;Caballero, J. A.;Zechmeister, M.;Jeffers, S. V.;Reiners, A.;Ribas, I.;Amado, P. J.;Quirrenbach, A.;Anglada-Escudé, G.;Bauer, F. F.;Béjar, V. J. S.;Cortés-Contreras, M.;Dreizler, S.;Galadí-Enríquez, D.;Guenther, E. W.;Hauschildt, P. H.;Kaminski, A.;Kürster, M.;Lafarga, M.;López del Fresno, M.;Montes, D.;Morales, J. C.
Astronomy and Astrophysics
Publication type: 
The He I infrared (IR) line at a vacuum wavelength of 10 833 Å is a diagnostic for the investigation of atmospheres of stars and planets orbiting them. For the first time, we study the behavior of the He I IR line in a set of chromospheric models for M-dwarf stars, whose much denser chromospheres may favor collisions for the level population over photoionization and recombination, which are believed to be dominant in solar-type stars. For this purpose, we use published PHOENIX models for stars of spectral types M2 V and M3 V and also compute new series of models with different levels of activity following an ansatz developed for the case of the Sun. We perform a detailed analysis of the behavior of the He I IR line within these models. We evaluate the line in relation to other chromospheric lines and also the influence of the extreme ultraviolet (EUV) radiation field. The analysis of the He I IR line strengths as a function of the respective EUV radiation field strengths suggests that the mechanism of photoionization and recombination is necessary to form the line for inactive models, while collisions start to play a role in our most active models. Moreover, the published model set, which is optimized in the ranges of the Na I D<SUB>2</SUB>, Hα, and the bluest Ca II IR triplet line, gives an adequate prediction of the He I IR line for most stars of the stellar sample. Because especially the most inactive stars with weak He I IR lines are fit worst by our models, it seems that our assumption of a 100% filling factor of a single inactive component no longer holds for these stars.
ADS Bibcode: 
stars: activity;stars: chromospheres;stars: late-type