The CARMENES search for exoplanets around M dwarfs

Abstract

Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) survey is searching for Earth-like planets orbiting M dwarfs using the radial velocity method. Studying the stellar activity of the target stars is important to avoid false planet detections and to improve our understanding of the atmospheres of late-type stars. Aims. In this work we present measurements of activity indicators at visible and near-infrared wavelengths for 331 M dwarfs observed with CARMENES. Our aim is to identify the activity indicators that are most sensitive and easiest to measure, and the correlations among these indicators. We also wish to characterise their variability. Methods. Using a spectral subtraction technique, we measured pseudo-equivalent widths of the He I D3, Hα, He I λ10833 Å, and Pa β lines, the Na I D doublet, and the Ca II infrared triplet, which have a chromospheric component in active M dwarfs. In addition, we measured an index of the strength of two TiO and two VO bands, which are formed in the photosphere. We also searched for periodicities in these activity indicators for all sample stars using generalised Lomb-Scargle periodograms. Results. We find that the most slowly rotating stars of each spectral subtype have the strongest Hα absorption. Hα is correlated most strongly with He I D3, whereas Na I D and the Ca II infrared triplet are also correlated with Hα. He I λ10833 Å and Paβ show no clear correlations with the other indicators. The TiO bands show an activity effect that does not appear in the VO bands. We find that the relative variations of Hα and He I D3 are smaller for stars with higher activity levels, while this anti-correlation is weaker for Na I D and the Ca II infrared triplet, and is absent for He I λ10833 Å and Paβ. Periodic variation with the rotation period most commonly appears in the TiO bands, Hα, and in the Ca II infrared triplet.

The CARMENES search for exoplanets around M dwarfs

“…11 and 12). These, however, are active stars that have, for instance, log L Hα /L bol > −4 (Schöfer et al 2019). The latter two are also in the CARMENES radial-velocity-loud sample discussed by Tal-Or et al (2018).…”

“…11. However, as an X-ray source (Haakonsen & Rutledge 2009) this is an active star (Ansdell et al 2015;Schöfer et al 2019), but we attribute its deviation in the diagrams instead to a wrong parallactic distance (d = 26 ± 3 pc). In the literature, there have been at least four additional distance determinations from photometry and spectroscopy (Lépine & Gaidos 2011;Lépine et al 2013;Cortés-Contreras 2016), and they all point toward a closer distance in the range d = 14.0−19.6 pc, which would lead to a luminosity and mass consistent with its measured effective temperature.…”

“…A.2 we now plot the ratio M M−K s over M M−R against the metallicity from Sect. 3.2 and also take into account the activity of the stars using the pseudo equivalent width pEW Hα of Schöfer et al (2019) for the color coding. We can now identify that all Hα emittiers have slightly higher M M−R masses (and by Eqs.…”

“…We also repeat the HRD of Fig. 10 but using the pseudo equivalent width pEW Hα of Schöfer et al (2019) for the color coding in Fig. A.3.…”

“…Three of these 1 were included in the sample of Díez Alonso et al (2019), who studied the rotation periods of CARMENES targets. All of these eight objects except one 2 were included in the study of Schöfer et al (2019), who investigated the activity of the CARMENES targets. We, finally, included six 3 of the eight new targets.…”