Variability at the Edge: Optical Near/IR Rapid Cadence Monitoring of Newly Outbursting FU Orionis Object HBC 722

TL;DR

This study reports rapid, day-scale optical/near-IR variability in the FU Orionis object HBC 722, revealing complex periodic signals likely linked to accretion processes and magnetic activity, with implications for understanding FUor disk dynamics.

Contribution

First detection of multi-periodic rapid variability in a FU Orionis object, providing insights into accretion and magnetic phenomena during outburst phases.

Findings

Detected 5.8 and 1.28 day periodic signals in HBC 722.

Estimated magnetic field strength of 2.2-2.7 kG.

Suggested disk asymmetry or stellar rotation as variability sources.

Abstract

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 2.1m 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 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.