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 sources HL Tau and L1551-IRS 5 using the VLA in its B-configuration which provides an angular resolution of ∼0″.'4 (∼50 AU at 140 pc) at 1.3 cm wavelength. Our goal was to detect and resolve circumstellar molecular disks with radius of the order of 100 AU around these two sources. No ammonia emission was detected toward either of them. The 3 σ levels were 2.7 mJy beam-1 and 3.9 mJy beam-1 for HL Tau and L1551-IRS 5, respectively, with a velocity solution of ∼5 km s-1. With this nondetection, we estimate upper limits 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 (XNH3/10-8)-1 M⊙ for HL Tau and 0.1 (XNH3/10-8)-1 M⊙ for L1551-IRS 5.