# Dynamical response of a protoplanetary disk to an FU Ori-type outburst **Authors:** Roman Zolotarev, Yaroslav Pavlyuchenkov, Vitaly Akimkin **arXiv:** [2610.25435](https://arxiv.org/abs/2610.25435) **Date:** 2026-10-18 **Summary page:** https://www.summarizepaper.com/en/arxiv-id/2610.25435v1/ ## Key points - The study finds that FU Orionis-type outbursts alter protoplanetary disk geometry and motion, causing apparent brightness variations independently of changes in accretion. - Radiation-hydrodynamic simulations model a 0.01-solar-mass disk around a 0.6-solar-mass star, with an imposed accretion luminosity rising to 450 solar luminosities and declining over 110 years. - Irradiation heats the disk within months; subsequent vertical expansion roughly doubles its aspect ratio and creates an outward-moving thickened region that shadows and cools the outer disk. - The thickened region advances approximately as time to the two-thirds power because vertical adjustment takes longer at larger radii, not because matter is simply transported outward. - Structural changes produce radial velocities of several kilometers per second and rotation approximately 5% below Keplerian speeds; recovery continues for roughly a century after the outburst ends. - Observational effects depend strongly on viewing angle: extinction changes are small below approximately 40° from the disk axis but can become extreme at inclinations of approximately 40–60°. - Modeled delayed brightness dips reach factors of 2–6 around 1.5 years after onset, with a longer-lasting factor-of-20 decline at 60°; such dimming need not indicate declining accretion. - Evolving geometry changes the 10-micrometer silicate feature between emission and absorption and alters resolved disk images through shadowing. - Similar delayed dips occur in some observed FU Orionis-type light curves, but the study does not establish matches to individual objects. - Key limitations include prescribed outburst behavior, simplified inner-disk treatment, and omitted envelopes, winds, outflows, and dust evolution; time-resolved optical and mid-infrared observations are needed to distinguish structural effects from accretion variability. ## Summary This paper investigates the dynamical response of the outer regions of a protoplanetary disk to an FU Orionis-type accretion outburst and the observational consequences of the resulting structural changes. These outbursts dramatically increase the luminosity of a young star and its innermost disk. Although their thermal and chemical effects have been studied extensively, their influence on disk geometry and motion remains less well understood. The study examines whether these structural changes can produce apparent brightness variations independently of intrinsic accretion variability. The authors use the HURAKAN radiation-hydrodynamics code to evolve disk temperature and density self-consistently. Frequency-dependent ray tracing models irradiation from the central source, while flux-limited diffusion describes thermal radiation transport. The model consists of a 0.01-solar-mass disk surrounding a 0.6-solar-mass star, with a computational domain spanning 2–300 astronomical units (au). The actively accreting inner disk is excluded from the domain and represented by a prescribed luminosity. Its accretion luminosity rises from 0.05 to 450 solar luminosities over four months, then declines linearly to its initial value over 110 years. The star contributes a constant three solar luminosities. Synthetic fluxes, spectra, and images are subsequently calculated using RADMC-3D and compared with results from models in which the disk density remains fixed. Irradiation initially heats the disk within months, before its density structure can respond substantially. Vertical expansion then develops on the local dynamical timescale, approximately 0.6 years at 2 au. The disk aspect ratio increases by roughly a factor of two, creating an outward-moving thickened region, or bump. The bump’s radial position increases approximately as time to the two-thirds power. This behavior reflects progressively slower vertical adjustment at larger radii rather than simple outward transport of matter. The optically thick bump casts a shadow over the disk beyond it, reducing temperatures in those regions. Associated radial velocities reach several kilometers per second, while rotation can become approximately 5% slower than Keplerian motion. After the outburst ends, cooling launches additional relaxation waves. By 125 years after onset, the original bump lies near 30 au, and structural recovery requires approximately another century. The strongest observational effects depend on viewing angle. At inclinations below approximately 40° from the disk axis, changes in extinction toward the star are small. At intermediate inclinations of approximately 40–60°, vertical expansion can increase visual extinction from less than one magnitude to around 100 magnitudes. Delayed optical dimming therefore does not necessarily indicate declining accretion. Several modeled viewing orientations exhibit brightness dips by factors of 2–6 approximately 1.5 years after onset, equivalent to about 0.8–2 magnitudes. At 60°, a longer-lasting decline reaches a factor of 20, or approximately three magnitudes. The evolving geometry also shifts the 10-micrometer silicate feature among emission, a nearly flat profile, and absorption, while shadowing alters resolved disk images. The authors conclude that outward-moving relaxation waves may be a general response to strong irradiation, producing observable signatures beyond the initial luminosity increase. Some observed FU Orionis-type light curves exhibit potentially similar delayed dips, but the study does not establish matches to individual objects. Important limitations include the prescribed outburst, the simplified treatment of the inner disk, and the omission of realistic envelopes, winds, outflows, dust redistribution, and changes in dust composition or crystallinity. Tests incorporating attenuation by unresolved inner material preserve the overall dynamics, although the bump’s shape changes. Time-resolved optical and mid-infrared observations are needed to distinguish these structural effects from genuine accretion variability.