The Coronal Abundance Anomalies of M Dwarfs

TL;DR

This study analyzes X-ray spectra of inactive M dwarf stars, finding evidence of an inverse FIP effect in their coronae, and explores how MHD wave models can explain this abundance anomaly.

Contribution

It provides the first evidence of inverse FIP effect in inactive M dwarfs and advances modeling of coronal abundance anomalies in these stars.

Findings

Evidence for a modest inverse FIP effect in GJ 338 binary

Supports the idea that all M dwarfs exhibit the inverse FIP effect

Models can produce inverse FIP effect through MHD wave mechanisms

Abstract

We analyze Chandra X-ray spectra of the M0 V+M0 V binary GJ 338. As quantified by X-ray surface flux, these are the most inactive M dwarfs ever observed with X-ray grating spectroscopy. We focus on measuring coronal abundances, in particular searching for evidence of abundance anomalies related to First Ionization Potential (FIP). In the solar corona and wind, low FIP elements are overabundant, which is the so-called "FIP effect." For other stars, particularly very active ones, an "inverse FIP effect" is often observed, with low FIP elements being underabundant. For both members of the GJ 338 binary, we find evidence for a modest inverse FIP effect, consistent with expectations from a previously reported correlation between spectral type and FIP bias. This amounts to strong evidence that all M dwarfs should exhibit the inverse FIP effect phenomenon, not just the active ones. We take the first step towards modeling the inverse FIP phenomenon in M dwarfs, building on past work that has demonstrated that MHD waves coursing through coronal loops can lead to a ponderomotive force that fractionates elements in a manner consistent with the FIP effect. We demonstrate that in certain circumstances this model can also lead to an inverse FIP effect, pointing the way to more detailed modeling of M dwarf coronal abundances in the future.