We have used the high sensitivity and resolution of the IRAM interferometer to produce subarcsecond 12CO J \ 2È1 images of nine protoplanetary disks surrounding T Tauri stars in the Taurus-Auriga cloud (seven singles and two binaries). The images demonstrate the disks are in Keplerian rotation around their central stars. Using the least-square Ðt method described in the 1998 work by Guilloteau & Dutrey, we derive the diskÏs properties, in particular its inclination angle and rotation velocity, hence the dynamical mass. Since the disk mass is usually small, this is a direct measurement of the stellar mass. Typically, we reach an internal precision of 10% in the determinations of stellar mass. The overall accuracy is limited by the uncertainty in the distance to a speciÐc star. In a distance-independent way, we compare the derived masses with theoretical tracks of preÈmain-sequence evolution. Combined with the mean distance to the Taurus region (140 pc), for stars with mass close to 1 our results tend to favor M _ , the tracks with cooler photospheres (higher masses for a given spectral type). We Ðnd that in UZ Tau E, the disk and the spectroscopic binary orbit appear to have di †erent inclinations.

We present an IR lunar occultation and direct imaging search for companions in the Ophiuchus starforming region and update a similar search of the Taurus region. The search is sensitive to companions in the angular separation range 0.005-10". In Ophiuchus we surveyed 35 young star targets; this sample contains at least 10 binaries, two triples, and one quadruple. Ten of the companion stars are newly discovered. In Taurus, the survey now includes 47 systems among which there are at least 22 binaries and four triples. Only two companion stars are newly identified because there is strong overlap with prior work. All the triples and quadruple are hierarchical. The observed binary frequency in Ophiuchus, in the 3-1400 AU range of separations, is at least 1.1 ± 0.3 that of the nearby solar-like stars. This value is a lower bound because we make no corrections for incompleteness. In Taurus, in the same range of separations, the observed binary frequency is at least 1.6 ± 0.3 that of the nearby solar-like stars. This value extends Ghez et al.'s (1993) and Leinert's et al.'s (1993) determination of an excess binary frequency to 3 AU separation. We used the WT/TT type and the K-L color index to distinguish between systems with and without inner disks. We find no convincing difference in the binary frequency or distribution of separations of the systems with and without inner disks. The 1.3 mm continuum emission of the single systems exceeds that of the multiples suggesting that their extensive outer disks are more massive. The specific angular momenta of the binaries overlap those of molecular cloud cores measured by Goodman et al. (1993). Subject headings: binaries: visual-occultations-open clusters and associations: generalstars: pre-main-sequence a coauthor. David Allen died on 1994 July 26. We dedicate the paper to his memory.

This paper describes our study of the astrophysics of individual components in close pre-main-sequence binaries. We observed both stars in 17 systems, located in 4 nearby star forming regions, using low-resolution (R=760), infrared spectroscopy and photometry. For 29 components we detected photospheric absorption lines and were able to determine spectral type, extinction, K-band excess, and luminosity. The other 5 objects displayed featureless or pure emission line spectra. In ~50 % of the systems, the extinction and K-band excess of the primary stars dominate those of the secondaries. Masses and ages were determined for these 29 objects by placing them on the H-R diagram, overlaid with theoretical pre-main-sequence tracks. Most of the binaries appear to be coeval. The ages span 5x10^5 to 1x10^7 years. The derived masses range from the substellar, 0.06 M_sun, to 2.5 M_sun, and the mass ratios from M_2/M_1=0.04 to 1.0. Fourteen stars show evidence of circumstellar disks. The K-band excess is well correlated with the K-L color for stars with circumstellar material.Comment: 63 pages, 14 figures, accepted for publication in the Ap

Abstract. We present new single-dish (30-m) observations and an improved analysis of our earlier interferometric observations of BP Tau . Our analysis yields a detailed description of the properties of the circumstellar disk. The disk is small (outer radius 120 AU) and marginally optically thick in the 12 CO J = 2→1 line. It is also relatively hot, about 50 K at 100 AU. The mm continuum emission is consistent with a thermal emission from circumstellar dust with an emissivity index β 0.7, and a disk mass about 1.2 × 10 −3 M . The anomalously low 12 CO to dust emission ratio implies that either CO is depleted by a factor of the order of 150 with respect to H 2 , or has a very low gas to dust ratio, or highly anomalous dust properties. The disk exhibits direct evidence of Keplerian rotation, but because of a combination of insufficient resolution and optically thin CO emission, the mass derived for the star remains inaccurate (between 0.6 and 1.3 M for 140 pc). The unusual properties of the circumstellar disk suggest that BP Tau may be a transient object in the process of clearing its disk.