UPPER LIMITS TO THE DETECTION OF AMMONIA FROM PROTOPLANETARY DISKS AROUND HL TAURI AND L1551-IRS 5

We present NH3(1, 1) and (2, 2) observations of the young stellar sour ces HL Tau and L1551-IRS 5 using the VLA in its B-configuration, which provides an angular resolution of approximately 0.4'' (approximately 50 AU at 140 pc) at 1.3 cm wavelength. Our goal was to detect and reso lve circumstellar molecular disks with radius of the order of 100 AU a round these two sources. No ammonia emission was detected toward eithe r of them. The 3 sigma levels were 2.7 mJy beam-1 and 3.9 mJy beam-1 f or HL Tau and L1551-IRS 5, respectively, with a velocity resolution of approximately 5 km s-1. With this nondetection, we estimate upper lim its to the mass of the proposed protoplanetary molecular disks (within a radius of 10 AU from the central stars) on the order of 0.02 (X(NH3 )/10(-8))-1 M. for HL Tau and 0.1 (X(NH3)/10(-8))-1 M. for L1551-IRS 5 .