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

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–heating 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⊙. The static RADMC-3D code was used to model synthetic spectral energy distributions (SEDs) of the disk based on the temperatures calculated with HURAKAN. 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, an unusual temperature gradient occurs in the vertical direction with the 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.