HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet

TL;DR

This study investigates energetic flares and superflares on young M stars in the ultraviolet, revealing their potential impact on exoplanet atmospheres and providing new insights into stellar activity evolution.

Contribution

It presents the first detailed characterization of ultraviolet superflares on young M stars, including the discovery of the most energetic flare observed in the FUV to date.

Findings

Young M stars produce flares 100-1000 times more energetic than older stars.

The most energetic flare, 'Hazflare', emitted 10^{32.1} erg in FUV, qualifying as a superflare.

Superflares emit significant UV flux, affecting planetary atmospheric photochemistry.

Abstract

M stars are powerful emitters of far-ultraviolet light. Over long timescales, a significant, possibly dominant, fraction of this emission is produced by stellar flares. Characterizing this emission is critical to understanding the atmospheres of the stars producing it and the atmospheric evolution of the orbiting planets subjected to it. Ultraviolet emission is known to be elevated for several hundred million years after M stars form. Whether the same is true of ultraviolet flare activity is a key concern for the evolution of exoplanet atmospheres. Hubble Space Telescope (HST) observations by the HAZMAT program (HAbitable Zones and M dwarf Activity across Time) detected 18 flares on young (40 Myr) early M stars in the Tucana-Horologium association over 10 h of observations, ten having energy $>10^{30}$ erg. These imply flares on young M stars are 100--1000$\times$ more energetic than those occurring at the same rate on "inactive," field-age M dwarfs. However, when energies are are normalized by quiescent emission there is no statistical difference between the young and field-age samples. The most energetic flare observed, dubbed the "Hazflare," emitted an energy of $10^{32.1}$ erg in the FUV, 30$\times$ more energetic than any stellar flare previously observed in the FUV with HST's COS or STIS spectrographs. It was accompanied by $15,500\pm400$ K blackbody emission bright enough to designate it a superflare ($E>10^{33}$ erg), with an estimated bolometric energy of $10^{33.6_{-0.2}^{+0.1}}$ erg. This blackbody emitted 18$^{+2}_{-1}$% of its flux in the FUV (912--1700 \AA) where molecules are generally most sensitive to photolysis. Such hot superflares in young, early M stars could play an important role in the evolution of nascent planetary atmospheres.