Hunting for millimeter flares from magnetic reconnection in pre-main sequence spectroscopic binaries

Abstract

Context. Recent observations of the low-mass pre-main sequence (PMS), eccentric spectroscopic binaries DQ Tau and V773 Tau A reveal that their millimeter spectrum is occasionally dominated by flares from non-thermal emission processes. The transient activity is believed to be synchrotron in nature, resulting from powerful magnetic reconnection events when the separate magnetic structures of the binary components are briefly capable of interacting and forced to reorganize, typically near periastron. Aims. We conducted the first systematic study of the millimeter variability toward a sample of 12 PMS spectroscopic binaries with the aim to characterize the proliferation of flares amongst sources likely to experience similar interbinary reconnection events. The source sample consists entirely of short-period, close-separation binaries that possess either a high orbital eccentricity (e > 0.1) or a circular orbit (e ≈ 0). Methods. Using the MAMBO2 array on the IRAM 30 m telescope, we carried out continuous monitoring at 1.25mm (240GHz) over a 4-night period during which all of the high-eccentricity binaries approached periastron. We also obtained simultaneous optical VRI measurements, since a strong link is often observed between stellar reconnection events (traced via X-rays) and optical brightenings. Results. UZ Tau E is the only source to be detected at millimeter wavelengths, and it exhibited significant variation (F_(1.25mm) = 87– 179 mJy); it is also the only source to undergo strong simultaneous optical variability (ΔR ≈ 0.9 mag). The binary possesses the largest orbital eccentricity in the current sample, a predicted factor in star-star magnetic interaction events. With orbital parameters and variable accretion activity similar to DQ Tau, the millimeter behavior of UZ Tau E draws many parallels to the DQ Tau model for colliding magnetospheres. However, on the basis of our observations alone, we cannot determine whether the variability is repetitive, or if it could also be due to variable free-free emission in an ionized wind. Conclusions. UZ Tau E brings the number of known millimeter-varying PMS sources to 3 out of a total of 14 monitored binaries now in the literature. Important factors in the non-detection of the rest of our targets are the coarse time-sampling and limited millimeter sensitivity of our survey. We recommend that future studies concentrate on close-by targets, and obtain millimeter and optical data points with better temporal resolution.

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