# Diagnosing FUor-like Sources: The Parameter Space of Viscously Heated Disks in the Optical and Near-IR

**Authors:** Hanpu Liu, Gregory J. Herczeg, Doug Johnstone, Carlos Contreras-Peña, Jeong-Eun Lee, Haifeng Yang, Xingyu Zhou, Sung-Yong Yoon, Ho-Gyu Lee, Masanobu Kunitomo, Jessy Jose  
**arXiv:** [2207.13324](https://arxiv.org/abs/2207.13324)  
**Date:** 2022-07-27  
**Summary page:** https://www.summarizepaper.com/en/paper/2207.13324v1/

## Key points

- The study focuses on young stars called FU Ori type objects (FUors) that undergo decades-long outbursts of accretion
- Outbursts viscously heat disks surrounding the star, causing the disk to outshine the central star
- Models for FUor objects are constructed by calculating emission components from a steady-state viscous accretion disk, a passively-heated dusty disk, magnetospheric accretion columns, and the stellar photosphere
- The models explore the parameter space of the accretion rate and stellar mass to investigate implications on the optical and near-infrared spectral energy distribution and spectral lines
- The models are validated by fitting to multi-wavelength photometry of three confirmed FUor objects: FU Ori, V883 Ori, and HBC 722
- Turning points in diagnostics occur at $\eta=1$ ("transition line") and $\eta=5$ ("sufficient dominance line"), evaluated here in the near-infrared. These turning points indicate the emergence and complete development of characteristic features such as strong CO absorption, weak metallic absorption, triangular spectral continuum shape in $H$-band, and location in color-magnitude diagrams.
- Lower stellar mass $M_*$ and higher accretion rate $\dot{M}$ lead to larger $\eta$
- The authors discuss implications of their models on extinction diagnostics, FUor brightening timescales, viscous disks during initial protostellar growth stages as well as eruptive young stellar objects (YSOs) associated with FUors.
- Observational techniques used for studying them include one dimensional spectra extracted from cross dispersed spectra obtained using high resolution spectrographs are used to derive emission line fluxes and equivalent widths while photometry can be used to complement spectroscopic observations.

## Summary

This study focuses on FU Ori type objects (FUors), which are young stars that undergo decades-long outbursts of accretion. These outbursts are strong enough to viscously heat disks surrounding the star, causing the disk to outshine the central star. The authors 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. They explore the parameter space of the accretion rate and stellar mass to investigate implications on the optical and near-infrared spectral energy distribution and spectral lines. The models are validated by fitting to multi-wavelength photometry of three confirmed FUor objects: FU Ori, V883 Ori, and HBC 722. The predicted spectrum is then compared to observed optical and infrared spectra. The brightness ratio between the viscous disk and the stellar photosphere, $\eta$, provides an important guide for identifying viscous accretion disks. Turning points in diagnostics occur at $\eta=1$ ("transition line") and $\eta=5$ ("sufficient dominance line"), evaluated here in the near-infrared. These turning points indicate the emergence and complete development of characteristic features such as strong CO absorption, weak metallic absorption, triangular spectral continuum shape in $H$-band, and location in color-magnitude diagrams. Lower stellar mass $M_*$ and higher accretion rate $\dot{M}$ lead to larger $\eta$. For $M_*=0.3~{\rm M_\odot}$, $\eta=1$ corresponds to $\dot{M}=2\times10^{-7}~{\rm M_\odot}/$yr while $\eta=5$ corresponds to $\dot{M}=6\times10^{-7}~{\rm M_\odot}/$yr. The sufficient dominance line also coincides with the expected accretion rate where accreting material directly reaches the star. The authors discuss implications of their models on extinction diagnostics, FUor brightening timescales, viscous disks during initial protostellar growth stages as well as eruptive young stellar objects (YSOs) associated with FUors. In addition to constructing models for FUor objects this study provides more context on observational techniques used for studying them; one dimensional spectra extracted from cross dispersed spectra obtained using high resolution spectrographs are used to derive emission line fluxes and equivalent widths while photometry can be used to complement spectroscopic observations. Overall this study provides a comprehensive analysis of FUor objects along with their properties; by constructing models based on physical processes driving these outbursts they provide important insights into how they can be identified through their spectral characteristics.