ARCSAT ID NUMBER: AS02

DESCRIPTIVE TITLE:  
Accretion vs Magnetic Reconnection: Determining the Source of Periodic Variability in the Pre-Main Sequence Binary DQ Tau

PI: Ben Tofflemire

OBSERVER(S): Ben Tofflemire

UNCERTIFIED/UNTRAINED OBSERVERS: N/A

COLLABORATORS: Robert Mathieu
 
CONTACT INFORMATION: tofflemi@astro.wisc.edu

TIME REQUESTED: 
Monday, Oct 27th - Sunday, Nov 3rd (1 week, request use of all 7 full nights)
Monday, Jan 12th - Sunday, Jan 25th (2 weeks, request use of only 7 full nights from 1/13 - 1/19)

INSTRUMENT: FlareCam

FILTERS: u, g, r, and H-alpha if available.

COMMENTS: 
This proposal requests two, 7 night runs centered on periastron passages of the pre-main sequence binary DQ Tau. The first periastron passage proposed here falls on the week of Monday, Oct 27th. The second falls between the weeks of Monday, Jan 12th and Monday, Jan 19th. These are the only available observing windows we have during Q4. 

BRIEF SCIENCE JUSTIFICATION: (restrict yourself to 1-2 paragraphs)
Pre-main sequence binary stars can have up to three accretion disks: two circumstellar disks and a circumbinary disk separated by a dynamically cleared gap. Theory suggests that mass may periodically flow in an accretion stream from a circumbinary disk across the gap to circumstellar disks or stellar surfaces. Thus, accretion rates are predicted to be dynamically modulated on the orbital period. For our target, DQ Tau (P=15.8 days), numerical simulations predict a near order of magnitude increase in the accretion rate that rises gradually, peaking at periastron passage (Artymowicz & Lubow 1996; AL96). Time-series photometric observations of DQ Tau with ~1 day cadence have shown periodic increases in broad-band luminosities on the binary orbital period at most periastron passages as predicted by AL96 (Mathieu et al. 1997). While this observed behavior supports the accretion stream theory, it is not exclusive to variable accretion rates. Magnetic reconnection (flares) during the collisions of stellar magnetospheres at periastron (when the stars are separated by only 8 stellar radii) could produce the same periodic, broadband variability at a 1 day cadence (Getman et al. 2011). Indeed, gyrosynchrotron, millimeter wavelength flares have also been seen near periastron passages of DQ Tau (typical of stellar flares; Salter et al. 2010).

Our proposal requests two, 7-night runs centered on periastron passages of DQ Tau to trace the transition from the quiescent to elevated states with high-cadence photometry. Detailed lightcurve morphologies will be able to distinguish between the gradual rise and fall on multi-day time-scales predicted by the accretion stream theory and the hour time-scale rapid-rise and exponential-decay typical of stellar flares. Observations in u-band are especially crucial as they provide the best tracer of accretion and flares in cool stars. Due to the stochastic nature of accretion, flares, and the Mathieu et al. lightcurves, we require observations over multiple periastron passages, two of which will be obtained in Nov and Dec with the WIYN 0.9m telescope. ARCSAT/FlareCam observations will provide higher-cadence lightcurves compared to the WIYN 0.9m, better suited to characterizing stellar flares, and will be essential in determining the repeatability and the relative significance of accretion and magnetic reconnection in the optical variability.