Variability of a stellar corona on a time scale of days

TL;DR

This study monitors the chemical and thermal evolution of a stellar corona over six days, revealing that active regions exhibit a consistent inverse FIP effect and significant element enrichment, providing insights into coronal element fractionation.

Contribution

It presents the first detailed time-resolved analysis of elemental abundance variations in a stellar corona over days, linking active region evolution to FIP bias changes.

Findings

Coronal temperature distribution remains steady over six days.

FIP bias and emission measure vary and are correlated.

High FIP elements increase by approximately 75% in active regions.

Abstract

Elemental abundance effects in active coronae have eluded our understanding for almost three decades, since the discovery of the First Ionization Potential (FIP) effect on the sun. The goal of this paper is to monitor the same coronal structures over a time interval of six days and resolve active regions on a stellar corona through rotational modulation. We report on four iso-phase X-ray spectroscopic observations of the RS CVn binary EI Eri with XMM-Newton, carried out approximately every two days, to match the rotation period of EI Eri. We present an analysis of the thermal and chemical structure of the Ei Eri corona as it evolves over the six days. Although the corona is rather steady in its temperature distribution, the emission measure and FIP bias both vary and seem to be correlated. An active region, predating the beginning of the campaign, repeatedly enters into our view at the same phase as it rotates from beyond the stellar limb. As a result, the abundances tend slightly, but consistently, to increase for high FIP elements (an inverse FIP effect) with phase. We estimate the abundance increase of high FIP elements in the active region to be of ~75% over the coronal mean. This observed fractionation of elements in an active region on time scales of days provides circumstantial clues regarding the element enrichment mechanism of non-flaring stellar coronae.