The CARMENES search for exoplanets around M dwarfs

Abstract

Stellar activity is ubiquitously encountered in M dwarfs and often characterised by the Hα line. In the most active M dwarfs, Hα is found in emission, sometimes with a complex line profile. Previous studies have reported extended wings and asymmetries in the Hα line during flares. We used a total of 473 high-resolution spectra of 28 active M dwarfs obtained by the CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-Earths with Near-infrared and optical Echelle Spectrographs) spectrograph to study the occurrence of broadened and asymmetric Hα line profiles and their association with flares, and examine possible physical explanations. We detected a total of 41 flares and 67 broad, potentially asymmetric, wings in Hα. The broadened Hα lines display a variety of profiles with symmetric cases and both red and blue asymmetries. Although some of these line profiles are found during flares, the majority are at least not obviously associated with flaring. We propose a mechanism similar to coronal rain or chromospheric downward condensations as a cause for the observed red asymmetries; the symmetric cases may also be caused by Stark broadening. We suggest that blue asymmetries are associated with rising material, and our results are consistent with a prevalence of blue asymmetries during the flare onset. Besides the Hα asymmetries, we find some cases of additional line asymmetries in He I D3, Na I D lines, and the He I line at 10 830 Å taken all simultaneously thanks to the large wavelength coverage of CARMENES. Our study shows that asymmetric Hα lines are a rather common phenomenon in M dwarfs and need to be studied in more detail to obtain a better understanding of the atmospheric dynamics in these objects.

The CARMENES search for exoplanets around M dwarfs

“…The first was listed as a candidate by Schlieder et al (2012b), and the second by Kastner et al (2017). The second is also an active star discussed in Fuhrmeister et al (2018).…”

“…We imposed the same requirements and only used those CARMENES spectra with a signal-to-noise ratio (S/N) of S /N > 75. Furthermore, as in Passegger et al (2018, hereafter Pass18) we excluded targets with spectral line profiles that cannot be reproduced by a purely photospheric model, i.e., stars with signs of activity that significantly affect the profiles of the lines used by Pass18 for determining stellar parameters; see Tal-Or et al (2018) and Fuhrmeister et al (2018) for the impact of activity on CARMENES target stars. Finally, as in Pass18 we excluded fast rotators.…”

“…10), although not always an obvious one. It is an active (Fuhrmeister et al 2018) and young star belonging to the Local Association (Tetzlaff et al 2011). -33 (J16102−193).…”

CARMENES input catalogue of M dwarfs

“…They presented a list of 31 RV-loud stars: CARMENES GTO M-dwarf targets that displayed largeamplitude radial-velocity variations due to activity. Most of these stars, close to activity saturation, were also part of the investigation on wing asymmetries of Hα, Na i D, and He i lines in CARMENES spectra by Fuhrmeister et al (2018). We found rotation periods from photometric time series for all of them except for nine stars 18 , J12156+526 = StKM 2-809, J14173+454 = RX J1417.3+4525, J15499+796 = LP 022-420, J16555-083 = vB 8, J18189+661 = LP 071-165, J19255+096 = LSPM J1925+0938, and J20093-012 = 2MASS J20091824-0113377, which are either extremely active (e.g.…”

A census of coronal mass ejections on solar-like stars

“…Leitzinger et al (2011); Argiroffi et al (2019) found Doppler-shifted emission features at FUV and X-ray wavelengths, respectively, both being well below the escape velocities of the investigated stars. Several dedicated search programs for stellar CMEs in the optical did not show convincing results (Leitzinger et al 2014; Korhonen et al 2017;Fuhrmeister et al 2018). Beside the method of Doppler shifted emission/absorption of spectral lines in optical, FUV, and X-ray domains (as described above) there were also several attempts to interpret detected continuous X-ray absorptions as CMEs temporarily obscuring the star (Haisch et al 1983;Ottmann & Schmitt 1996;Tsuboi et al 1998;Favata & Schmitt 1999;Franciosini et al 2001;Covino et al 2001;Pandey & Singh 2012).…”