Magnetic Inflation and Stellar Mass. V. Intensification and saturation of M dwarf absorption lines with Rossby number

TL;DR

This study investigates how magnetic field indicators in M dwarfs, using photospheric absorption lines, increase with faster rotation and then saturate at low Rossby numbers, suggesting saturation occurs beneath the stellar surface.

Contribution

It provides observational evidence that magnetic saturation in M dwarfs occurs at or below the photosphere, using spectral line analysis across a range of Rossby numbers.

Findings

Equivalent widths increase with decreasing Rossby number.

Saturation of absorption lines occurs at Rossby number ≈ 0.1.

Magnetic saturation likely occurs beneath the stellar photosphere.

Abstract

In young sun-like stars and field M dwarf stars, chromospheric and coronal magnetic activity indicators such as H$\alpha$, X-ray and radio emission are known to saturate with low Rossby number ($Ro \lesssim 0.1$), 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 $Ro \lesssim 0.1$. 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.