Coronal and chromospheric activity of Teegarden’s star

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 Tenerife, Spain
⁶ Centro de Astrobiología (CSIC-INTA), ESAC campus, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Institut d’Estudis Espacials de Catalunya, 08034 Barcelona, Spain
⁹ Institut de Ciències de l’Espai (CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹⁰ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany

^★ Corresponding author; bfuhrmeister@tls-tautenburg.de

Received: 29 July 2024
Accepted: 24 September 2024

Abstract

Teegarden’s star is a late-type M-dwarf planet host, typically showing only rather low levels of activity. In this paper we present an extensive characterisation of this activity at photospheric, chromospheric, and coronal levels. We specifically investigated TESS observations of Teegarden’s star, which showed two very large flares with an estimated flare fluence between 10²⁹ and 10³² erg comparable to the largest solar flares. We furthermore analysed nearly 300 CARMENES spectra and 11 ESPRESSO spectra covering all the usually used chromospheric lines in the optical from the Ca II H & K lines at 3930 Å to the He I infrared triplet at 10 830 Å. These lines show different behaviour: The He I infrared triplet is the only one absent in all spectra, some lines show up only during flares, and others are always present and highly variable. Specifically, the Hα line is more or less filled in during quiescence; however, the higher Balmer lines are still observed in emission. Many chromospheric lines show a correlation with Hα variability, which, in addition to stochastic behaviour, shows systematic behaviour on different timescales including the rotation period. Moreover, we found several flares and also report hints of an erupting prominence, which may have led to a coronal mass ejection. Finally, we present X-ray observations of Teegarden’s star (i.e. a discovery pointing obtained with the Chandra observatory) and an extensive study with the XMM-Newton observatory, which observed two large flares. One of these showed clear signatures of the Neupert effect, suggesting the production of hard X-rays in the system.