Bright hot impacts by erupted fragments falling back on the Sun: UV redshifts in stellar accretion

TL;DR

This study uses solar eruption observations and hydrodynamic simulations to explain broad redshifted UV lines in young stars, linking stellar accretion flows to impact dynamics of falling fragments.

Contribution

It demonstrates that fragmented accretion impacts can produce broad redshifted UV lines, providing a new explanation for stellar accretion signatures.

Findings

UV emission from impacts is mainly from shocked fragment shells.

Impacts produce initial upflows followed by shock interactions.

Fragmented impacts can reach velocities up to 400 km/s, matching stellar observations.

Abstract

A solar eruption after a flare on 7 Jun 2011 produced EUV-bright impacts of fallbacks far from the eruption site, observed with the Solar Dynamics Observatory. These impacts can be taken as a template for the impact of stellar accretion flows. Broad red-shifted UV lines have been commonly observed in young accreting stars. Here we study the emission from the impacts in the Atmospheric Imaging Assembly's UV channels and compare the inferred velocity distribution to stellar observations. We model the impacts with 2D hydrodynamic simulations. We find that the localised UV 1600A emission and its timing with respect to the EUV emission can be explained by the impact of a cloud of fragments. The first impacts produce strong initial upflows. The following fragments are hit and shocked by these upflows. The UV emission comes mostly from the shocked front shell of the fragments while they are still falling, and is therefore redshifted when observed from above. The EUV emission instead continues from the hot surface layer that is fed by the impacts. Fragmented accretion can therefore explain broad redshifted UV lines (e.g. C IV 1550A) to speeds around 400 km/s observed in accreting young stellar objects.