The CARMENES search for exoplanets around M dwarfs
Radial-velocity variations of active stars in visual-channel spectra★
Institut für Astrophysik, Georg-August-Universität,
2 Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
3 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
4 Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
5 Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
6 Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
7 Centro Astronómico Hispano-Alemán (CSIC-MPG), Observatorio Astronómico de Calar Alto, Sierra de los Filabres-04550 Gérgal, Almería, Spain
8 Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
9 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
10 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
11 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
12 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
13 Departamento de Física de la Tierra y Astrofísica & UPARCOS-UCM (Unidad de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
Accepted: 28 February 2018
Context. Previous simulations predicted the activity-induced radial-velocity (RV) variations of M dwarfs to range from ~1 cm s−1 to ~1 km s−1, depending on various stellar and activity parameters.
Aims. We investigate the observed relations between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing CARMENES high-resolution visual-channel spectra (0.5–1μm), which were taken within the CARMENES RV planet survey during its first 20 months of operation.
Methods. During this time, 287 of the CARMENES-sample stars were observed at least five times. From each spectrum we derived a relative RV and a measure of chromospheric Hα emission. In addition, we estimated the chromatic index (CRX) of each spectrum, which is a measure of the RV wavelength dependence.
Results. Despite having a median number of only 11 measurements per star, we show that the RV variations of the stars with RV scatter of >10 m s−1 and a projected rotation velocity v sin i > 2 km s−1 are caused mainly by activity. We name these stars “active RV-loud stars” and find their occurrence to increase with spectral type: from ~3% for early-type M dwarfs (M0.0–2.5 V) through ~30% for mid-type M dwarfs (M3.0–5.5 V) to >50% for late-type M dwarfs (M6.0–9.0 V). Their RV-scatter amplitude is found to be correlated mainly with v sin i. For about half of the stars, we also find a linear RV–CRX anticorrelation, which indicates that their activity-induced RV scatter is lower at longer wavelengths. For most of them we can exclude a linear correlation between RV and Hα emission.
Conclusions. Our results are in agreement with simulated activity-induced RV variations in M dwarfs. The RV variations of most active RV-loud M dwarfs are likely to be caused by dark spots on their surfaces, which move in and out of view as the stars rotate.
Key words: stars: late-type / stars: activity / stars: rotation / techniques: radial velocities
The data presented in Figs. 5 and A.1 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz- bin/qcat?J/A+A/614/A122
© ESO 2018