An Expanding Accretion Disk and a Warm Disk Wind As Seen In the Spectral Evolution of HBC 722

TL;DR

This paper analyzes the spectral evolution of HBC 722, revealing an expanding accretion disk and a warm disk wind, with detailed modeling of the physical parameters and wind components during the FU Ori outburst.

Contribution

It introduces a modified viscous accretion disk model and detailed wind analysis to explain the spectral evolution of HBC 722 during its outburst.

Findings

Accretion rate of 10^{-4.0} M_sun/yr during outburst

Outer disk radius expands outward over time

A warm disk wind with 4000-6000 K explains observed line profiles

Abstract

We present a comprehensive analysis of the post-outburst evolution of the FU Ori object HBC 722 in optical/near-infrared (NIR) photometry and spectroscopy. Using a modified viscous accretion disk model, we fit the outburst epoch SED to determine the physical parameters of the disk, including $\dot{M}_\mathrm{acc} = 10^{-4.0} \ M_\odot$ yr$^{-1}$, $R_\mathrm{inner} = 3.65 \ R_\odot$, $i = 79^\circ$, and a maximum disk temperature of $T_\mathrm{max} = 5700$ K. We then use a decade of optical/NIR spectra to demonstrate a changing accretion rate drives the visible-range photometric variation, while the NIR shows the outer radius of the active accretion disk expands outward as the outburst progresses. We also identify the major components of the disk system: a plane-parallel disk atmosphere in Keplerian rotation and a 2-part warm disk wind that is collimated near the star and wide-angle at larger radii. The wind is traced by classic wind lines, and appears as a narrow, low-velocity, deep absorption component in several atomic lines spanning the visible spectrum and in the CO 2.29$\mu$m band. We compare the wind lines to those computed from wind models for other FU Ori systems and rapidly accreting young stellar disks and find a 4000-6000 K wind can explain the observed line profiles. Fitting the progenitor spectrum, we find $M_* = 0.2 \ M_\odot$ and $\dot{M}_\mathrm{progenitor} = 7.8 \times 10^{-8} \ M_\odot \ \mathrm{yr}^{-1}$. Finally, we discuss HBC 722 relative to V960 Mon, another FU Ori object we have previously studied in detail.