Application of a Steady-State Accretion Disk Model to Spectrophotometry and High-Resolution Spectra of Two Recent FU Ori Outbursts

TL;DR

This study applies a steady-state accretion disk model to spectrophotometric data of two FU Ori outbursts, estimating accretion rates and disk temperatures, and explores model variations and parameter degeneracies.

Contribution

It introduces a detailed modeling approach for FU Ori objects using MCMC to estimate accretion rates and disk properties, including boundary region effects.

Findings

HBC 722 has an accretion rate of approximately 10^{-4.9} M_sun/yr.

Gaia 17bpi has a lower accretion rate of about 10^{-6.7} M_sun/yr.

HBC 722's accretion rate aligns with typical FU Ori objects, while Gaia 17bpi's is lower.

Abstract

We apply a conventional accretion disk model to the FU Ori-type objects HBC 722 and Gaia 17bpi. Our base model is a steady-state, thin Keplerian disk featuring a modified Shakura-Sunyaev temperature profile, with each annulus radiating as an area-weighted spectrum given by a NextGen atmosphere at the appropriate temperature. We explore departures from the standard model by altering the temperature distribution in the innermost region of the disk to account for "boundary region"-like effects. We consider the overall spectral energy distribution (SED) as well as medium- and high-resolution spectra in evaluating best-fit models to the data. Parameter degeneracies are studied via a Markov Chain Monte Carlo (MCMC) parameter estimation technique. Allowing all parameters to vary, we find accretion rates for HBC 722 of $\dot{M} = 10^{-4.90} M_\odot \textrm{ yr}^{-1}\; {}^{+0.99}_{-0.40} \textrm{ dex}$ and for Gaia 17bpi of $\dot{M} = 10^{-6.70} M_\odot \textrm{ yr}^{-1}\; {}^{+0.46}_{-0.36} \textrm{ dex}$; the corresponding maximum disk temperatures are $7100_{-500}^{+300}$ K and $7900_{-400}^{+900}$ K, respectively. While the accretion rate of HBC 722 is on the same order as other FU Ori-type objects, Gaia 17bpi has a lower rate than previously reported as typical, commensurate with its lower luminosity. Alternate models that fix some disk or stellar parameters are also presented, with tighter confidence intervals on the remaining fitted parameters. In order to improve upon the somewhat large credible intervals for the $\dot{M}$ values, and make progress on boundary layer characterization, flux-calibrated ultraviolet spectroscopy is needed.