The CARMENES search for exoplanets around M dwarfs Radial-velocity variations of active stars in visual-channel spectra

L. Tal-Or, M. Zechmeister, A. Reiners, S. V. Jeffers, P. Schöfer, A. Quirrenbach, P. J. Amado, I. Ribas, J. A. Caballero, J. Aceituno, F. F. Bauer, V. J. S. Béjar, S. Czesla, S. Dreizler, B. Fuhrmeister, A. P. Hatzes, E. N. Johnson, M. Kürster, M. Lafarga, D. Montes, J. C. Morales, S. Reffert, S. Sadegi, W. Seifert, D. Shulyak. 2018. The CARMENES search for exoplanets around M dwarfs Radial-velocity variations of active stars in visual-channel spectra. Astronomy and Astrophysics 614, DOI: 10.1051/0004-6361/201732362

Previous simulations predicted the activity-induced radial-velocity (RV) variations of M dwarfs to range from similar to 1 cm s(-1) to similar 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 (0.5-1 mu m), 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 Ha 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 upsilon 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 similar to 3% for early-type M dwarfs (M0.0-2.5 V) through similar to 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 upsilon 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 H alpha 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.

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