The influence of accretion bursts on methanol and water in massive young stellar objects

Abstract

The effect of accretion bursts on massive young stellar objects (MYSOs) represents a new research field in the study of young stars and their environment. The impact of such bursts on the disk and envelope has been observed and plays the role of a "smoking gun" providing information about the properties of the burst itself. We aim to investigate the impact of an accretion burst on massive disks with different types of envelopes and to study the effects of an accretion burst on the temperature structure and the chemistry of the disk. We focus on water and methanol as chemical species for this paper. The thermochemical code of (PROtoplanetary DIsk MOdel) is used to perform simulations of high-mass protoplanetary-disk models with different types of envelopes in the presence of an accretion burst. The models in question represent different evolutionary stages of protostellar objects. We calculated and show the chemical abundances in three phases of the simulation (pre-burst, burst, and post-burst). More heavily embedded disks show higher temperatures. The impact of the accretion burst is mainly characterized by the desorption of chemical species present in the disk and envelope from the dust grains to the gas phase. When the post-burst phase starts, the sublimated species freeze out again. The degree of sublimation depends strongly on the type of envelope the disk is embedded in. An accretion burst in more massive envelopes produces stronger desorption of the chemical species. However, our models show that the timescale for the chemistry to reach the pre-burst state is independent of the type of envelope. The study shows that the disk's temperature increases with a more massive envelope enclosing it. Thus, the chemistry of MYSOs in earlier stages of their evolution reacts stronger to an accretion burst than at later stages where the envelope has lost most of its mass or has been dissipated. The study of the impact of accretion bursts could also provide helpful theoretical context to the observation of methanol masers in massive disks.