Simulations of Flare Chemistry in Brown Dwarf Companions to Active M Dwarfs

Abstract

Brown dwarfs that are short period ($<10$day) companions to actively flaring M dwarfs may provide a context to directly observe flare-driven photochemistry and structural changes in an extrasolar planet-like atmosphere. To assess the viability of directly observing flare impacts in the atmosphere of a brown dwarf, we perform self-consistent temperature-chemistry modeling of the atmospheric response to individual energetic superflares. We modified the existing open-source \texttt{VULCAN} chemical-kinetics and \texttt{HELIOS} radiative-transfer codes for this purpose. Similar to previous studies of flare impacts on hydrogen dominated atmospheres, we find flares are capable of orders-of-magnitude changes in the mixing abundances of many chemical species, including important opacity sources like CH${}_4$ and CO${}_2$. However, due to fast chemical timescales resulting from high temperatures and densities in brown dwarf atmospheres, these changes last for a short-period of time, generally less than a day, and are only plausibly observable via high resolution emission spectroscopy. We find that the most observable, short-term spectral changes in hot (T${}_{\text{eff}}\sim 2000$K), high-gravity ($\log \text{g}\sim 5$), cloudless brown dwarfs are the photolysis of H${}_2$O and enhancement of CO${}_2$, which can result in part-per-thousands spectral changes in the hours after a flare.

16 pages, 6 figures, accepted for publication in ApJ