Time-dependent response of protoplanetary disk temperature to an FU Ori-type luminosity outburst

TL;DR

This study models the time-dependent thermal response of protoplanetary disks to FU Ori-type outbursts, revealing that optically thick regions heat and cool over years, while outer layers respond almost instantly, affecting observable spectra.

Contribution

It provides the first detailed time-dependent radiation transfer simulations of disk thermal evolution during FU Ori outbursts, considering different outburst scenarios.

Findings

Optically thick regions take years to heat and cool.

Outer disk layers respond almost instantaneously.

Infrared flux rises rapidly, millimeter flux changes slowly.

Abstract

Context. The most prominent cases of young star variability are accretion outbursts in FU Ori-type systems. The high power of such outbursts causes dramatic changes in the physical and chemical structure of a surrounding protoplanetary disk. As characteristic thermal timescales in the disk are comparable to the duration of the outburst, the response of its thermal structure is inherently time dependent. Aims. We analyzed how the disk thermal structure evolves under the substantial-yet transient-eating of the outburst. To cover different possible physical mechanisms driving the outburst, we examined two scenarios: one in which the increased accretion rate is confined to a compact sub-au inner region and the other where it affects the entire disk. Methods. To model the disk temperature response to the outburst we performed time-dependent radiation transfer using the HURAKAN code. The disk structure and the luminosity profile roughly correspond to those of the FU Ori system itself, which went into outburst about 90 years ago and reached a luminosity of 450 L_Sun. Results. We find that optically thick disk regions require several years to become fully heated during the outburst and a decade to cool after it. The upper layers and outer parts of the disk, which are optically thin to thermal radiation, are heated and cooled almost instantaneously. This creates an unusual radial temperature profile during the early heating phase with minima at several au both for the fully active and compact active disk scenarios. At the cooling phase, upper layers being colder than the midplane for both scenarios. Near- and mid-infrared SEDs demonstrate a significant and almost instantaneous rise by 1 - 2 orders of magnitude during the outburst, while the millimeter flux shows a change of only a factor of a few, and is slightly delayed with respect to the central region luminosity profile.