Magnetic Inflation and Stellar Mass. V. Intensification and Saturation of M-dwarf Absorption Lines with Rossby Number

Abstract

In young sun-like stars and field M dwarf stars, chromospheric and coronal magnetic activity indicators such as Hα, X-ray and radio emission are known to saturate with low Rossby number (Ro 0.1), defined as the ratio of rotation period to convective turnover time. The mechanism for the saturation is unclear. In this paper, we use photospheric Ti I and Ca I absorption lines in Y band to investigate magnetic field strength in M dwarfs for Rossby numbers between 0.01 and 1.0. The equivalent widths of the lines are magnetically enhanced by photospheric spots, a global field or a combination of the two. The equivalent widths behave qualitatively similar to the chromospheric and coronal indicators: we see increasing equivalent widths (increasing absorption) with decreasing Ro and saturation of the equivalent widths for Ro 0.1. The majority of M dwarfs in this study are fully convective. The results add to mounting evidence that the magnetic saturation mechanism occurs at or beneath the stellar photosphere.

Magnetic fields of M dwarfs

“…Indirect magnetic activity proxies, such as chromospheric and coronal emission and photospheric absorption line strengths, are known to exhibit a prominent dependence on stellar rotation (e.g., Reiners et al 2014;Newton et al 2017;Wright et al 2011Wright et al , 2018Muirhead et al 2020). This dependence is best represented as a function of Rossby number (Ro), which is defined as the ratio of stellar rotation period P rot and convective turnover time-scale s. The typical behaviour found for cool active stars, including M dwarfs, is an increase of activity proxies and magnetic field strength with decreasing Rossby number until a saturation at Ro % 0.1 (e.g., Vidotto et al 2014a;Wright et al 2018;Kochukhov et al 2020).…”

Suppression of lithium depletion in young low-mass stars from fast rotation

“…Discussing their study of M35, Jeffries et al (2021) note that stars currently with moderately slow 10 day periods would have had Rossby numbers low enough for the magnetic activity indicators to be saturated during the rapid lithium depletion phase (e.g. Vilhu 1984;Pizzolato et al 2003;Reiners et al 2009;Marsden et al 2009;Wright et al 2011;Jeffries et al 2011;Muirhead et al 2020). This suggests that even if magnetic effects tightly correlate with rotation, they alone cannot explain the observed lithium-rotation trend.…”

Magnetic fields of M dwarfs

“…Indirect magnetic activity proxies, such as chromospheric and coronal emission and photospheric absorption line strengths, are known to exhibit a prominent dependence on stellar rotation (e.g. Reiners et al 2014;Newton et al 2017;Wright et al 2011Wright et al , 2018Muirhead et al 2020). This dependence is best represented as a function of Rossby number (Ro), which is defined as the ratio of stellar rotation period 𝑃 rot and convective turnover time-scale 𝜏.…”