Variability At The Edge: Optical Near/IR Rapid-Cadence Monitoring Of Newly Outbursting FU Orionis Object HBC 722




Green, Joel D.
Robertson, Paul
Baek, Giseon
Pooley, David
Pak, Soojong
Im, Myungshin
Lee, Jeong-Eun
Jeon, Yiseul
Choi, Changsu
Meschiari, Stefano

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We present the detection of day-timescale periodic variability in the r-band lightcurve of newly outbursting FU Orionis-type object HBC 722, taken from >42 nights of observation with the CQUEAN instrument on the McDonald Observatory 2.1 m telescope. The optical/near-IR lightcurve of HBC 722 shows a complex array of periodic variability, clustering around 5.8-day (0.044 mag amplitude) and 1.28-day (0.016 mag amplitude) periods, after removal of overall baseline variation. We attribute the unusual number of comparable strength signals to a phenomenon related to the temporary increase in accretion rate associated with FUors. We consider semi-random "flickering," magnetic braking/field compression and rotational asymmetries in the disk instability region as potential sources of variability. Assuming that the 5.8-day period is due to stellar rotation and the 1.28-day period is indicative of Keplerian rotation at the inner radius of the accretion disk (at 2 R-star), we derive a B-field strength of 2.2-2.7 kG, slightly larger than typical T Tauri stars. If instead the 5.8-day signal is from a disk asymmetry, the instability region has an outer radius of 5.4 R-star, consistent with models of FUor disks. Further exploration of the time domain in this complicated source and related objects will be key to understanding accretion processes.



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Green, Joel D., Paul Robertson, Giseon Baek, David Pooley, Soojong Pak, Myungshin Im, Jeong-Eun Lee, Yiseul Jeon, Changsu Choi, and Stefano Meschiari. "Variability at the Edge: Optical near/IR Rapid-Cadence Monitoring of Newly Outbursting FU Orionis Object HBC 722." The Astrophysical Journal, Vol. 764, No. 1 (Feb., 2013): 22.