Observational signatures of outside-in accretion bursts in embedded protostars

TL;DR

The paper tackles the challenge of identifying infrared precursors to outside-in accretion bursts in embedded protostars by coupling time-dependent inner-disc outburst models to a dusty disc+envelope structure and solving the radiative transfer with RADMC3D. It demonstrates that infrared precursors peak in the $2-10\,\mu$m range and are detectable mainly when viewing along outflow cavities, with detectability depending on envelope infall rate $\dot{M}$ and centrifugal radius $r_c$. The study shows that Gaia~17bpi-like smaller bursts produce a precursor at $\sim 2-3\,\mu$m about two years before optical maximum, with significant dependence on inclination and $\dot{M}$; longer-wavelength monitoring (far-IR to sub-mm) can trace precursor evolution long before optical outbursts. These results provide observational guidance for identifying and characterizing protostellar accretion bursts and constrain the triggering region, emphasizing the value of multiwavelength monitoring to capture the earliest phases of protostar formation.

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 about 10^{-5} Msun/yr (rc/30 au)^{-1/2}, where rc is a characteristic inner radius for the infalling envelope, the infrared precursor is only apparent in the SED when viewed along the outflow cavity. At other inclinations, the precursor is most easily distinguished at infall rates of 10^{-6} Msun/yr (rc/30 au)^{-1/2}. We also show that far-IR 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.

Figures (17)

• Figure 1: Evolution of the "large" (FU Ori-like) outburst model, showing the surface (effective) temperatures (blue dashed curves) and the central temperatures (orange solid curves), respectively, at four times after the initial triggering.
• Figure 2: Evolution of the "small" (Gaia 17bpi-like) outburst model, showing the surface (effective) temperatures (blue dashed curve) and the central temperatures (orange solid curve), respectively, at two times after the initial triggering.
• Figure 3: A comparison between the spectral energy distributions at an inclination of $0^{\circ}$ for Model 1, with no outburst, Models and 2A - 2D of the FUor outburst sequence. The SED becomes brighter with increasing time.
• Figure 4: SEDs for Model 1 (star + disc + envelope), with an infall rate of $10^{-5} M_{\odot}\, {\rm yr^{-1}}$, with no outburst. The SEDs are fainter at higher inclination. The black dotted curve is the star and disc without envelope viewed at $i = 0^{\circ}$.
• Figure 5: SEDs for Model 2A, with infall rate of $10^{-5} M_{\odot}\, {\rm yr^{-1}}$, $r_c = 30$ au, at a time $t = 50$ yr after triggering. The infrared precursor produces a disc SED that peaks at around $8-10 \mu$m.