Detection of a Disk Surrounding the Variably Accreting Young Star HBC722

FU Orionis objects (hereafter FUors) are a group of pre-main-sequence stars that abruptly increase in brightness by several magnitudes, lasting decades or longer. There are about $10-20$ FUors confirmed by direct observation of their bursts, along with an approximately equal number of candidates found to display similar spectral characteristics (e.g., Reipurth & Aspin, 2010). The FUors’ large amplitude bursts are credited to enhanced mass accretion from the surrounding circumstellar disk, with various triggering mechanisms being proposed over the years, e.g. gravitational and/or magnetorotational instabilities, thermal instabilities, or interactions with binary companions (e.g., Hartmann & Kenyon, 1985, 1996; Audard et al., 2014). FUors may represent the late stages of a cycle of episodic accretion bursts and luminosity flares during the embedded phase (Dunham et al., 2014b). Thus, studying and characterizing each FUor is important for understanding how they fit into the general star formation process.

In this paper, we present new 1 mm continuum observations of the FUor HBC722 obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA). HBC722, located in the North American/Pelican Nebula Complex ($d\sim 800$ pc; Zucker et al., 2020), was undetected in previous 1 mm continuum data from the Submillimeter Array (SMA), setting an upper limit for the disk mass of 0.05 M_⊙ (after rescaling to the current distance; Dunham et al., 2012). With our new ALMA data we present the first millimeter continuum detection of this object, and use our data to calculate the mass of its circumstellar disk.

We obtained ALMA Cycle 2, Band 6, 12-m array observations of HBC722 on 2014 May 01 and 2015 May 03, with 34 – 36 operational antennas. The array configuration provided projected baseline lengths of 12$-$450 m and a synthesized beam size (assuming natural weighting) of 1.67$"$ $\times$ 0.89$"$ at a position angle of 5.82^∘ (measured east of north). To measure the continuum with the widest possible bandwidth while avoiding contamination from bright lines, the four spectral windows were centered at 224, 226, 240, and 242 GHz, and were configured to provide 128 channels over 1.875 GHz bandwidth. The resulting continuum image is centered at 233 GHz (1.29 mm) and has a total bandwidth of 7.5 GHz.

Calibration was performed using the Common Astronomy Software Applications (CASA) package¹¹1Available at http://casa.nrao.edu, following the standard techniques described in Petry et al. (2014) and Schnee et al. (2014). We then applied two rounds of phase self-calibration, followed by one round of amplitude self-calibration. The effective solutions intervals used for self-calibration were 390 s (first round of phase calibration), 30 s (second round of phase calibration), and 900 s (amplitude calibration). After applying the amplitude self-calibration we verified that the noise in the image decreased while the total fluxes of the detected objects did not change. The self-calibrated image has a 1$\sigma$ rms of 0.057 mJy beam^-1.

In Figure 1, which displays the self-calibrated ALMA 233 GHz continuum image, HBC722 is detected with a peak signal-to-noise ratio of 51. With a peak intensity of 2.9 mJy beam^-1, the ALMA detection of HBC722 is consistent with the previous SMA non-detection from Dunham et al. (2012) at approximately the same frequency.

We used the CASA tool gaussfit to fit an elliptical Gaussian in the image plane to all of the detected sources in the ALMA continuum image. The resulting integrated flux densities are included as annotations in Figure 1. For HBC722, we then calculated the total circumstellar disk mass $M$ as:

where $d=800$ pc, $S_{\nu}$ is the measured integrated flux density, $B_{\nu}(T_{D})$ is the Planck function at the isothermal dust temperature $T_{D}$, $\kappa_{\nu}$ is the opacity of the dust, and the factor of 100 is the assumed gas-to-dust ratio. We adopt the dust opacities of Ossenkopf & Henning (1994) for thin ice mantles after 10⁵ yrs of coagulation at a gas density of 10⁶ cm^-3, giving $\kappa_{\nu}=0.901$ cm² g^-1 at 233 GHz. Assuming $T_{D}=30$ K, Equation 1 gives a total circumstellar disk mass for HBC722 of 0.024 M_⊙. With typical uncertainties of a factor of a few (Dunham et al., 2014a), this result is in good agreement with the total gas mass of 0.03 M_⊙ found by Kóspál et al. (2016).

With a burst accretion rate of 10^-6 M_⊙ yr^-1 (Kóspál et al., 2011), it would take 24,000 yr to drain a 0.024 M_⊙ disk. Thus this disk mass is sufficient to power the current accretion burst. With a known stellar mass of approximately 0.5 M_⊙ (Cohen & Kuhi, 1979), our calculated disk mass implies that HBC722 has a disk-to-star mass ratio of approximately 5%. While this is a factor of ten higher than the median disk-to-star mass ratio for T Tauri stars (Andrews & Williams, 2005), it is still marginally too low for gravitational instabilities to serve as the burst triggering mechanism, as such instabilities likely require disk-to-star mass ratios of 10% or higher. However, given the uncertainties in the calculated mass, and the possibility that the ALMA continuum detection is optically thick, gravitational instabilities remain a possible triggering mechanism for this object. Future analysis using radiative transfer modeling is required to better determine the true mass of the HBC722 disk.