THE CLASSICAL T-TAURI SPECTROSCOPIC BINARY DQ-TAU .1. ORBITAL ELEMENTS AND LIGHT CURVES

We report the discovery that the classical T Tauri star DQ Tau is a do uble-lined spectroscopic binary. The orbital period is 15.804 days, wi th a large orbital eccentricity e=0.556. The mass ratio is 0.97+/-0.15 . We have monitored DQ Tau photometrically over two observing seasons and observed recurring episodes during which the stars get brighter (a pproximate to 0.5 mag in V) and bluer (approximate to-0.2 mag in V-I). When combined with photometry in the literature (time span approximat e to 5000 days), a Scargle periodogram analysis reveals a highly signi ficant periodicity of 15.80 days, essentially identical to the binary orbital period. These brightening events occur shortly before or at pe riastron passage. They occur during at least 65% of periastron passage s, but not during all periastron passages. DQ Tau is surrounded by a c ircumbinary disk with mass of 0.002-0.02 M-0. The infrared spectral en ergy distribution resembles a power law from 1 to 60 mu m. Remarkably, there is no paucity of near-infrared emission indicative of the inner disk having been cleared by the binary; there is clearly warm materia l within the binary orbit. We interpret the brightening events as due to a variable mass accretion rate regulated by the binary orbit. The p eriodic brightenings, the associated increases in emission line streng th and veiling reported in an accompanying paper [Basri et al., Al, su bmitted (1997)], and the circumstellar material together are consisten t with a recent theoretical finding that circumbinary disk material ca n stream across a binary orbit at certain orbital phases, resulting in a pulsed accretion flow onto the stars [Artymowicz & Lubow, ApJ, 467, L77 (1996)]. The theoretically predicted phase of maximum accretion r ate is shortly before periastron, in good agreement with the phasing o f the brightenings of DQ Tau. At the same time the periastron separati on is smaller than the inferred stellar magnetospheric radii of classi cal T Tauri stars, so that such magnetospheres would interact at each periastron passage. The magnetic energies are plausibly adequate to po wer the brightenings. However, the strongly enhanced continuum veiling and long duration of some of the brightenings are not naturally expla ined in a pure flaring scenario. Nonetheless, magnetospheres likely pl ay a role in the detailed accretion flow near the stars. (C) 1997 Amer ican Astronomical Society.