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

AuthorsHintz, 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, Pedro J. ; Quirrenbach, A.; Anglada, Guillem ; Bauer, F. F.; Béjar, V. J. S.; Cortés-Contreras, M.; Dreizler, S.; Galadí-Enríquez, D.; Guenther, E. W.; Hauschildt, P. H.; Kaminski, Adrian; Kürster, M.; Lafarga, M.; López del Fresno, M.; Montes, D.; Morales, J. C.
KeywordsStars: activity
Stars: chromospheres
Stars: late-Type
Issue Date24-Jun-2020
PublisherEDP Sciences
CitationAstronomy & Astrophysics 638: A115 (2020)
AbstractThe 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 D2, 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. © ESO 2020.
Publisher version (URL)http://dx.doi.org/10.1051/0004-6361/202037596
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