Diagnosing FU Ori-like Sources: The Parameter Space of Viscously Heated Disks in the Optical and Near-infrared

Abstract

FU Ori-type objects (FUors) are decades-long outbursts of accretion onto young stars that are strong enough to viscously heat disks so that the disk outshines the central star. We construct models for FUor objects by calculating emission components from a steady-state viscous accretion disk, a passively-heated dusty disk, magnetospheric accretion columns, and the stellar photosphere. We explore the parameter space of the accretion rate M ̇ and stellar mass M * to investigate implications on the optical and near-infrared spectral energy distribution and spectral lines. The models are validated by fitting to multiwavelength photometry of three confirmed FUor objects, FU Ori, V883 Ori, and HBC 722, and then comparing the predicted spectra to the observed optical and infrared spectra. The brightness ratio between the viscous disk and the stellar photosphere, η, provides an important guide for identifying viscous accretion disks, with η = 1 (“transition line”) and η = 5 (“sufficient dominance line”) marking turning points in diagnostics, evaluated here in the near-infrared. These turning points indicate the emergence and complete development of FUor-characteristic strong CO absorption, weak metallic absorption, the triangular spectral continuum shape in the H band, and location in color–magnitude diagrams. Lower M * and higher M ̇ imply larger η; for M * = 0.3 M ⊙, η = 1 corresponds to M ̇ = 2 × 10 − 7 M ⊙ yr−1 and η = 5 to M ̇ = 6 × 10 − 7 M ⊙ yr−1. The “sufficient dominance line” also coincides with the expected accretion rate where accreting material directly reaches the star. We discuss implications of the models on extinction diagnostics, FUor brightening timescales, viscous disks during initial protostellar growth, and eruptive young stellar objects associated with FUors.

Post-outburst Evolution of Bona Fide FU Ori-type V2493 Cygnus: A Spectro-photometric Monitoring

“…Comparing our absolute V versus (V − I C ) CMD, 21 (Figure 4) with Figure 15 of Liu et al (2022), we find after its first outburst V2493 Cyg has mostly resided over η > 5 region in the CMD plane. The η > 5 region signifies the region in the optical CMD plane where the circumstellar disk outshines the central PMS star, and all the observed spectral features originate from the disk itself.…”

“…The color evolution of V2493 Cyg can be further analyzed based on the theoretical model developed by Liu et al (2022). According to their model, the PMS stars accreting in the "FU Ori" regime occupy a certain locus in the colormagnitude diagram (CMD) plane as compared to the stars still accreting via magnetospheric regime.…”

“…Based on the values of parameter η, the CMD plane can be divided into regions of magnetospheric accretion regime and boundary layer accretion regime for a given M * . For η = 0, the model, developed by Liu et al (2022), degenerates into classical T Tauri star magnetospheric accretion regime.…”

“…The η > 5 region signifies the region in the optical CMD plane where the circumstellar disk outshines the central PMS star, and all the observed spectral features originate from the disk itself. It is to be noted that, in η > 5 region, the magnetospheric accretion ceases, and the central PMS star accretes via the boundary layer accretion Liu et al (2022). Therefore, the reddening of the optical colors likely points toward the expansion of the emitting region around V2493 Cyg rather than the PMS star exiting the "FUor stage.…”

“…Thus, the NIR color evolution also reinforces our assumptions based on our reddening-invariant color analysis in the optical regime. 21 We have obtained the values of isomass curves of Liu et al (2022) using the online available plot digitizer tool at https://apps.automeris.io/wpd/. The obtained values are in the Gaia photometric system.…”