The Classical T Tauri Spectroscopic Binary DQ Tau.I.Orbital Elements and Light Curves

Abstract

We report the discovery that the classical T Tauri star DQ Tan is a double-lined spectroscopic binary. The orbital period is 15.804 days, with 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 (≍0.5 mag in V) and bluer (≍-0.2 mag in V-I). When combined with photometry in the literature (time span ≍5000 days), a Scargle periodogram analysis reveals a highly significant periodicity of 15.80 days, essentially identical to the binary orbital period. These brightening events occur shortly before or at periastron passage. They occur during at least 65% of periastron passages, but not during all periastron passages. DQ Tau is surrounded by a circumbinary disk with mass of 0.002-0.02 Msun. The infrared spectral energy distribution resembles a power law from 1 to 60 μm. Remarkably, there is no paucity of near-infrared emission indicative of the inner disk having been cleared by the binary; there is clearly warm material within the binary orbit. We interpret the brightening events as due to a variable mass accretion rate regulated by the binary orbit. The periodic brightenings, the associated increases in emission line strength and veiling reported in an accompanying paper [Basri et al., submitted (1997)], and the circumstellar material together are consistent with a recent theoretical finding that circumbinary disk material can 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 rate is shortly before periastron, in good agreement with the phasing of the brightenings of DQ Tan. At the same time the periastron separation is smaller than the inferred stellar magnetospheric radii of classical T Tauri stars, so that such magnetospheres would interact at each periastron passage. The magnetic energies are plausibly adequate to power the brightenings. However, the strongly enhanced continuum veiling and long duration of some of the brightenings are not naturally explained in a pure flaring scenario. Nonetheless, magnetospheres likely play a role in the detailed accretion flow near the stars.