Um estudo sobre a atividade magnética e a rotação de estrelas anãs M das missões Kepler, K2 e TESS

Abstract

Despite being the most abundant stars in the Galaxy, M dwarfs are one of the least known objects in terms of their physical properties, especially regarding its magnetic activity and the underlying dynamo mechanism. Thus, a broad study on different activity diagnostics and their relationship with stellar rotation is necessary. In this work we performed a spectroscopic survey of 122 M dwarfs present in the Southern Continuous Viewing Zone (CVZ) of the TESS space mission, based on observations with the Gemini and SOAR telescopes. We obtained at least one optical spectrum for each star, with R ≈ 2000, from which we measured the chromospheric emission in the Hα line. Our sample has 21 stars with measured rotation periods (Prot), of which 12 were determined in this work from TESS light curves. The active fraction of the stars in our sample is consistent with that expected for field M dwarfs, confirming that late spectral-type stars generally remain active longer than early-type stars. Two stars presented enough rotation to be considered magnetically saturated, although they are at activity levels below the saturation region in the rotation-activity diagram, indicating the possibility that M dwarfs with short periods also present inactivity in Hα. We also analyzed a sample of 1788 field M dwarfs from Kepler and K2 missions, for which we investigated the distributions of Prot and the photometric activity index Sph, associated with the standard deviation of the light curve, using color-magnitude diagrams with accurate distance and magnitude data from Gaia mission. We note that the stars with the highest values of Sph in the sample (≳ 10000 ppm) are in general very young, with isochronal ages ≤ 50 Myr. We compared the photometric index with canonical chromospheric and coronal activity indicators in order to validate it as a proxy for magnetic activity in low-mass stars. Based on this index, we also studied the rotation-activity diagram, taking into account the transition to fully convective stars (≈ 0.35 M⊙), where we noticed the same distribution between these stars and those partially convective in both saturated and unsaturated regimes, suggesting a solar-like dynamo behavior, even for stars without the presence of a tacocline.