Analysis of the Stellar Occultations During the Unprecedented Long-Duration Flare

TL;DR

This study analyzes a 27-hour superflare on a young star, revealing the geometry and physical properties of cool flare loops that obscure the flare region, providing new insights into stellar flare structures.

Contribution

It presents the first detailed physical parameters and geometry of cool flare loops during a long-duration stellar flare, based on occultation analysis.

Findings

Arcade size expanded from 0.213 to 0.391 R* at ~3.5 km/s

Electron densities ranged from 10^13 to 10^14 cm^-3

Temperatures below 7,600 K, 6,400 K, and 5,077 K for successive occultations

Abstract

In strong stellar and solar flares flare loops typically appear during the decay phase, providing an additional contribution to the flare emission and, possibly, obscuring the flare emission. Super-flares, common in active, cool stars, persist mostly from minutes to several hours and alter the star's luminosity across the electromagnetic spectrum. Recent observations of a young main-sequence star reveal a distinctive cool loop arcade forming above the flaring region during a 27-hour superflare event, obscuring the region multiple times. Analysis of these occultations enables the estimation of the arcade's geometry and physical properties. The arcade's size expanded from 0.213 to 0.391 R$_*$ at a speed of approximately 3.5$\,$km/s. The covering structure exhibited a thickness below 12$\,$200$\,$km, with electron densities ranging from 10$^{13}$ to 10$^{14}\,$cm$^{-3}$ and temperatures below 7$\,$600$\,$K, 6$\,$400$\,$K, and 5$\,$077$\,$K for successive occultations. Additionally, the flare's maximum emission temperature has to exceed 12$\,$000$\,$K for the occultations to appear. Comparing these parameters with known values from other stars and the Sun suggests the structure's nature as an arcade of cool flare loops. For the first time, we present the physical parameters and the reconstructed geometry of the cool flare loops that obscure the flaring region during the gradual phase of a long-duration flare on a star other than the Sun.