The Pre-Outburst Properties of the FU Ori Object HBC 722

TL;DR

This study exploits a rare pre-outburst spectrum of the FU Ori candidate HBC 722 to quantify its stellar and accretion properties before eruption. By fitting a composite model to SDSS photometry and analyzing a low-resolution red spectrum, the authors infer a photospheric temperature of about $3350\pm75$ K and a spectral type of M$3.3\pm0.4$, with modest veiling ($<0.2$) and extinction $A_V\approx1.45$ mag. The pre-outburst accretion rate is estimated at $7\times10^{-9}\,M_\odot\,\text{yr}^{-1}$, about $1.5\times10^{4}$ times lower than the outburst rate of $\sim10^{-4}\,M_\odot\,\text{yr}^{-1}$, while the stellar radius appears inflated to $\sim2.15\,R_\odot$ and the luminosity is $\sim0.53\,L_\odot$. The outburst also exhibits a higher extinction, and the Ca II infrared triplet is unusually strong, hinting at winds or inner-disk physics not captured by the simple accretion model. Overall, the work demonstrates the critical role of pre-outburst data in constraining the evolution of FU Ori events and the need to account for accretion contributions when deriving stellar parameters from photometry.

Abstract

FU Ori outbursts are thought to play an important role in stellar assembly and the evolution of protoplanetary disks. However, the progenitor young stellar objects are largely uncharacterized. We obtained a low-resolution optical spectrum of HBC 722 before its FU Ori outburst as part of a survey of young stellar objects in the North America Nebula. The spectrum yields a spectral type of M3.3$\pm$0.4, which when combined with archival photometry allows us to measure the stellar and accretion properties of a young star prior to its FU Ori outburst. The pre-outburst accretion rate of $7\times10^{-9}$ M$_\odot$ yr$^{-1}$ is high for a protoplanetary disk around an M3-M3.5 star, though about 15,000 times weaker than the accretion rate during the outburst. The pre-outburst variability, inferred from archival B-band photometry, is about a factor 5 with a standard deviation of 0.16 dex and is consistent with variable accretion onto young low-mass stars. The stellar radius is larger than the radius of accreting young stars of similar spectral type by a factor of two. The extinction to HBC 722 is $\sim 1.45\pm0.3$~mag, lower than the 2.5--3.7~mag extinction values measured during the outburst. The u-band photometry plays an especially important role in constraining the veiling at longer wavelengths and therefore also the extinction and photospheric luminosity.

Figures (5)

• Figure 1: The WFGS2 red spectrum of HBC 722 compared with spectra of non-accreting X-Shooter templates from claes24. The templates all have veiling and extinction tailored to best match the observed spectrum of HBC 722. The spectrum also includes H$\alpha$ and Ca$\;$ infrared triplet lines, indicating accretion.
• Figure 2: The synthetic photometry (red circles, calculated from the synthetic spectrum) that best reproduces the SDSS griz, 2MASS JHK$_{\rm S}$, and UKIRT JHK photometry of HBC 722 (blue asterisks). The synthetic spectrum (black line with red circles for photometry) consists of an M3.25 photosphere plus selected emission lines (green line), an accretion continuum (purple line), and a dust continuum emission (red line).
• Figure 3: Left: Contour plot of the acceptable parameters of veiling and extinction for synthetic photometry fit to SDSS photometry. The best fit (purple asterisk) is shown in Figure \ref{['fig:sedplot']}. The yellow and blue contours correspond to the acceptable fit range, with $\chi_{\rm red}^2=1.0$ and $1.5$. The broader region shows the $\chi^2$ contours for fits to griz and excluding the u-band, with acceptable parameters that extend to high veiling. Those values are ruled out because the synthetic spectrum with higher veiling and extinction produces a u-band brightness that is brighter than detected (red contours for $u=21.0$, the $1-\sigma$ limit, and 20.5, which is 0.5 mag brighter than observed). Right: contours of $\chi_{\rm red}^2=1.0$ for spectral types of M2.5 (purple), M3 (Sz 67, dark blue), M3.25 (TWA 7+TWA 15B, thick light blue), M3.5 (TWA 15B, orange), and Sz 121 (red dotted line indicating $\chi_{red}^2=1.7$, since no solutions were within $\chi_{red}^2=1.3$). In general the extinction and veiling both decrease to later spectral type, but no solutions reach $A_V>2$.
• Figure 4: The pre-outburst BVRI light curve of HBC 722 from semkov12, with vertical dashed lines showing the epoch of the SDSS photometry and the WFGS2 spectrum. The outlier $B$-band point is excluded from this Figure and Figure \ref{['fig:cmds']}.
• Figure 5: BVRI color-magnitude diagrams from the Semkov photometry, compared with synthetic photometry from our fit to SDSS spectra, varying only the accretion continuum (dashed blue line). The red stars are show the photometry from our best-fit SDSS model. Each panel also includes an extinction vector for $A_V=0.5$ mag. The purple square is single-epoch BVI photometry from guieu09.