Variability at the Edge: Optical Near/Ir Rapid-Cadence Monitoring of Newly Outbursting Fu Orionis Object HBC 722

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 2.1 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 semirandom "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 ), 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 , 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.

X-ray emission from an FU Orionis star in early outburst: HBC 722

“…Using R * ≈ 2 R (Green et al 2013), v ≈ 500 km s −1 (Semkov et al 2012), andṀ ≈ 10 −7 M yr −1 (assuming 10% of the mass accretion rate as for CTTS, Hartigan et al 1995), we find N H ≈ 1.7 × 10 22 cm 2 , somewhat lower than acceptable for the CXO1 observations, but our model assumptions are fairly crude.…”

“…Halfway between disk border and stellar surface, the accretion stream velocity will have reached a value of about half the freefall velocity at the stellar surface for a mass element falling from the inner-disk border. Using the inner disk radius r = 2R * , R * ≈ 2 R , and a stellar mass of M * = 0.5 M (Green et al 2013) we obtain v ≈ 126 km s −1 and therefore n H ≈ 4 × 10 12 cm −3 at r ≈ 1.5R * . Integrating over one R * (from 2R * to the stellar surface) leads to N H = 5.6 × 10 23 cm −2 , within the 90% confidence range of CXO1 results (Table 2).…”

“…The newly detected FUor HBC 722 (V2493 Cyg, LkHα 188 G4) (Semkov et al 2010) is located at a distance of 520 pc (Green et al 2013) and is only the second FUor that has been observed prior to its outburst. Its pre-outburst characteristics (Cohen & Kuhi 1979) indicate that it was an emission-line star in the spectral range K7-M0, that is most likely a classical T Tauri star (CTTS).…”

Near-Ir Photometric and Optical Spectroscopic Study of the Fu Orionis Object V582 Aurigae

“…One of the most important spectroscopic features of FUors is the P Cyg profile, which is commonly displayed in the Na I D and Hα lines, occurring due to strong winds from the center of star. Some FUors were found to exhibit periodic spectroscopic (Herbig et al, 2003;Powell et (Kenyon et al, 2000;Green et al, 2013;Siwak et al, 2013) variability on a short timescale of days.…”

FOCAL REDUCER FOR CQUEAN (Camera for QUasars in EArly uNiverse)

“…Corresponding Author : S. Pak and supernovae (Thöne et al 2011;Lee et al 2010), and young stellar objects (Green et al 2013). The focal reducer was specifically designed to be used with the 2.1 m Otto Struve telescope of McDonald observatory in Texas, US.…”