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arXiv · 1911.00246

The CARMENES search for exoplanets around M dwarfs. The He I triplet at 10830 $\mathrm{\AA}\,$ across the M dwarf sequence

B. Fuhrmeister·S. Czesla·L. Hildebrandt·E. Nagel·J. H. M. M. Schmitt·D. Hintz·E. N. Johnson·J. Sanz-Forcada·P. Schöfer·S. V. Jeffers·J. A. Caballero·M. Zechmeister·A. Reiners·I. Ribas·P. J. Amado·A. Quirrenbach·F. F. Bauer·V. J. S. Béjar·M. Cortés-Contreras·E. Díez-Alonso·S. Dreizler·D. Galadí-Enríquez·E. W. Guenther·A. Kaminski·M. Kürster·M. Lafarga·D. Montes
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Abstract

The He I infrared (IR) triplet at 10830 AA is an important activity indicator for the Sun and in solar-type stars, however, it has rarely been studied in relation to M dwarfs to date. In this study, we use the time-averaged spectra of 319 single stars with spectral types ranging from M0.0 V to M9.0 V obtained with the CARMENES high resolution optical and near-infrared spectrograph at Calar Alto to study the properties of the He I IR triplet lines. In quiescence, we find the triplet in absorption with a decrease of the measured pseudo equivalent width (pEW) towards later sub-types. For stars later than M5.0 V, the He I triplet becomes undetectable in our study. This dependence on effective temperature may be related to a change in chromospheric conditions along the M dwarf sequence. When an emission in the triplet is observed, we attribute it to flaring. The absence of emission during quiescence is consistent with line formation by photo-ionisation and recombination, while flare emission may be caused by collisions within dense material. The He I triplet tends to increase in depth according to increasing activity levels, ultimately becoming filled in; however, we do not find a correlation between the pEW(He IR) and X-ray properties. This behaviour may be attributed to the absence of very inactive stars ($L_{\rm X}/L_{\rm bol}$ < -5.5) in our sample or to the complex behaviour with regard to increasing depth and filling in.

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B. Fuhrmeister, S. Czesla, L. Hildebrandt, E. Nagel, J. H. M. M. Schmitt, D. Hintz, E. N. Johnson, J. Sanz-Forcada, P. Schöfer, S. V. Jeffers, J. A. Caballero, M. Zechmeister, A. Reiners, I. Ribas, P. J. Amado, A. Quirrenbach, F. F. Bauer, V. J. S. Béjar, M. Cortés-Contreras, E. Díez-Alonso, S. Dreizler, D. Galadí-Enríquez, E. W. Guenther, A. Kaminski, M. Kürster, M. Lafarga, D. Montes. 2019-11-01. The CARMENES search for exoplanets around M dwarfs. The He I triplet at 10830 $\mathrm{\AA}\,$ across the M dwarf sequence. https://doi.org/10.1051/0004-6361%2F201936193

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