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

Abstract Brown dwarfs that are short-period (&lt;10 days) 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 VULCAN chemical–kinetics and 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 the 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 eff ∼ 2000 K), high-gravity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:mspace width="0.25em"/> <mml:mrow> <mml:mi>g</mml:mi> </mml:mrow> <mml:mspace width="0.25em"/> <mml:mo>∼</mml:mo> <mml:mn>5</mml:mn> </mml:math> ), cloudless brown dwarfs are the photolysis of H 2 O and an enhancement of CO 2 , which can result in part-per-thousands spectral changes in the hours after a flare.