A spectroscopic survey of subarcsecond binaries in the Taurus-Auriga dark cloud with the Hubble Space Telescope

We report the results of a spectroscopic survey of 20 close T Tauri binaries in the Taurus-Auriga dark cloud where the separations between primaries and their secondaries are less than the typical size of a circumstellar disk around a young star. Analysis of low- and medium-resolution Space Telescope Imaging Spectrograph spectra yields the stellar luminosities, reddenings, ages, masses, mass accretion rates, IR excesses, and emission-line luminosities for each star in each pair. We examine the ability of IR color excesses, H equivalent widths, [O I] emission, and veiling to distinguish between weak emission and classical T Tauri stars. Four pairs have one classical T Tauri star ( CTTS) and one weak-lined T Tauri star (WTTS); the CTTS is the primary in three of these systems. This frequency of mixed pairs among the close T Tauri binaries is similar to the frequency of mixed pairs in wider young binaries. Extinctions within pairs are usually similar; however, the secondary is more heavily reddened than the primary in some systems, where it may be viewed through the primary's disk. Mass accretion rates of primaries and secondaries are strongly correlated, and H luminosities, IR excesses, and ages also correlate within pairs. Primaries tend to have somewhat larger accretion rates than their secondaries do and are typically slightly older than their secondaries according to three different sets of modern pre main-sequence evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in reflection nebulae, are striking: the secondary in each pair is less massive but more luminous than the primary. The stellar masses of the UY Aur and GG Tau binaries measured from their rotating molecular disks are about 30% larger than the masses inferred from the spectra and evolutionary tracks. This discrepancy can be resolved in several ways, among them a 10% closer distance for the Taurus-Auriga dark cloud.