Observational signatures of outside-in accretion bursts in embedded protostars

ABSTRACT Optical and infrared surveys have detected increasing numbers of disc accretion outbursts in young stars. Some models of these FU Ori-type events predict that the outburst should start at near- to mid-infrared wavelengths before an optical rise is detected, and this lag between infrared and optical bursts has been observed in at least two systems. Detecting and characterizing infrared precursors can constrain the outburst trigger region, and thus help identify the mechanism producing the outburst. However, because FU Ori objects are generally young and usually embedded in dusty protostellar envelopes, it is not clear whether or how well such infrared precursors can be detected in the presence of strong envelope extinction. To explore this question, we combine time-dependent outburst models of the inner disc with an outer dusty disc and protostellar envelope, and calculate the resulting spectral energy distributions (SEDs) using the radiative transfer code radmc3d. We find that, for envelope mass infall rates $\gtrsim 10^{-5}\,{\rm M}_{\odot }\, {\rm yr^{-1}}\, (r_c/{\rm 30\, au})^{-1/2}$, where $r_c$ is a characteristic inner radius for the infalling envelope, the infrared precursor is only apparent in the SED when viewed along an outflow cavity. At other inclinations, the precursor is most easily distinguished with limited envelope extinction at infall rates $\lesssim 10^{-6} \,{\rm M}_{\odot }\, {\rm yr^{-1}}\, (r_c/{\rm 30 \, au})^{-1/2}$. We also show that far-infrared and submm/mm monitoring can enable the indirect detection of precursor evolution long before the optical outburst, emphasizing the potential of long-wavelength monitoring for studying the earliest stages of protostar formation.