Complete X-ray census of M dwarfs in the solar neighborhood

I. GJ 745 AB: Coronal-hole stars in the 10 pc sample

¹ Institut für Astronomie und Astrophysik, Eberhard-Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: caramazza@astro.uni-tuebingen.de
² INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁴ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany

Received: 21 March 2023
Accepted: 24 May 2023

Abstract

Context. X-ray emission is the most sensitive diagnostic of magnetic activity in M dwarfs and, hence, of the dynamo in low-mass stars. Moreover it is crucial for quantifying the influence of the stellar irradiation on the evolution of planet atmospheres.

Aims. We have embarked in a systematic study of the X-ray emission in a volume-limited sample of M dwarf stars to explore the full range of activity levels present in their coronae. We look to obtain a better understanding of the conditions in their outer atmospheres and their possible impact on the circumstellar environment.

Methods. Based on a recent catalog of Gaia objects within 10 pc from the Sun, we identified all its stars with spectral types between M0 and M4 and we carried out a systematic search for X-ray measurements of this sample. To this end, we used both archival data from ROSAT, XMM-Newton, and the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission, as well as our own dedicated XMM-Newton observations. To make inferences on the properties of the M dwarf corona, we compared the range of their observed X-ray emission levels to the flux radiated by the Sun from different types of magnetic structures: coronal holes, background corona, active regions, and cores of active regions. In this work, we focus on the properties of stars with the faintest X-ray emission.

Results. At the current state of our project, with more than 90% of the 10 pc M dwarf sample observed in the X-ray, there is only one star that has had no detections: GJ 745 A. With an upper limit luminosity of log L_x (erg s⁻¹) < 25.4 and an X-ray surface flux of log F_X,SURF (erg cm⁻² s⁻¹) < 3.6, GJ 745 A defines the lower boundary of the X-ray emission level for M dwarfs. Together with its proper motion companion (GJ 745 B), it is the only star in this volume-complete sample located in the range of X-ray surface flux that corresponds to the faintest solar coronal structures, namely: coronal holes. The fact that the ultra-low X-ray emission level of GJ 745 B (log L_x (erg s⁻¹) = 25.6 and log F_X,SURF (erg cm⁻² s⁻¹) = 3.8) is entirely attributed to flaring activity indicates that while its corona is dominated by “holes,” at least one magnetically active structure is present. This structure determines the total X-ray brightness and the coronal temperature of the star.