Correlation between activity indicators: Halpha and Ca II lines in M dwarfs stars

IBAÑEZ BUSTOS, R.V.; BUCCINO, A P; FLORES, M.; MARTÍNEZ, C.; MAUAS, P. J. D.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2023

0004-6361

Context. Different approaches have been adopted to study stellar magnetic activity, either short- or long-term, and although themechanisms by which low-mass stars generate large-scale magnetic fields are not well understood, it is known that stellar rotationplays a key role.Aims. There are stars that show cyclical behaviour in their activity studied on the blue side of the visible spectrum, which can beexplained by solar dynamo or αΩ dynamo models. However, when studying late stars, they become redder and it is necessary toimplement other indicators to analyse their magnetic activity. In the present work we perform a comparative study between thebest-known activity indicators so far defined from the Ca II and Hα lines to analyse M dwarfs stars.Methods. We studied a sample of 29 M stars with different chromospheric activity levels and spectral classes ranging from dM0 todM6. To do so, we employed 1796 spectra from different instruments with a median timespan of observations of 21 years. The spectrahave a wide spectral range that allow us to compute the chrosmospheric activity indicators based on Ca II and Hα. In addition, wecomplement our data with photometric observations for better stellar characterisation and short-term analysis.Results. We found a good and significant correlation (ρ = 0.91) between the indexes defined from the two lines for the whole set ofstars in the sample. However, we found that there is a deviation for faster rotators (with Prot < 4 days) and higher flare activity (at leastone flare per day). For the individual analysis, we found that the indexes computed individually for each star correlate independentlyof the level of chromospheric emission and the rotation period.Conclusions. There is an overall positive correlation between Ca II and Hα emission in dM stars, except during flare events.In particular, we found that low energy high-frequency flares could be responsible for the deviation in the linear trend infast rotators M dwarfs. This imply the rotation period could be a fundamental parameter to study the stellar activity andthe rotation could be drive the magnetic dynamo in low-mass active stars.Key words. techniques: spectroscopic – stars: late-type – stars: activityM dwarfs, with masses between 0.1 and 0.5 M⊙, constitute∼ 75% of the stars in the solar neighbourhood. Because of theirlow mass, M stars represent an important laboratory for detect-ing Earth-like planets orbiting around them. On the other hand,they have a high occurrence rate of extrasolar planets orbitingin the habitable zone1 surrounding the star (Bonfils et al. 2013;Dressing & Charbonneau 2015). However, several of these Mstars present high chromospheric activity that can exceed the so-⋆ Contact e-mail: ribanez@iafe.uba.ar⋆⋆ Based on data obtained at Complejo Astronómico El Leoncito, op-erated under agreement between the Consejo Nacional de Investiga-ciones Científicas y Técnicas de la República Argentina and the Na-tional Universities of La Plata, Córdoba and San Juan.1 The habitable zone is defined as the region in the equatorial plane ofthe star where the water in the planet, if any, can be found in its liquidstate.lar magnetic activity. They are mostly flare stars due to the highfrequency of these high-energy transient events. Thus, an Earth-type planet orbiting a M star could be continuously affected bysmall short flares and eventually by long high-energy flares and itcould constrain the exoplanet’s habitability (Buccino et al. 2007;Vida et al. 2017) or even play an important role in the atmo-spheric chemistry of an orbiting planets (Miguel et al. 2015).On the other hand, the phenomena associated with stellar ac-tivity also can be studied from several spectral lines that are sen-sitive to chromospheric activity (e.g., Mg II, Ca II, Na II, Hα).The mean integrated line-core fluxes are used as indicators ofthe total activity level of the stars, and their variability with timeare related to stellar rotation and the evolution of the stellar ac-tive regions. They provide information on the different regionsat different height of the stellar atmosphere.In 2007, Livingston et al. confirmed the correlation betweenthe H&K lines of the Ca II and Hα emissions during the 11-yearactivity cycle of the Sun. Several authors have suggested thatArticle number, page 1 of 11