Resolving molecular line emission from protoplanetary disks:: Observational prospects for disks irradiated by infalling envelopes

Molecular line observations that could resolve protoplanetary disks of similar to 100 AU both spatially and kinematically would be a useful tool to identify these disks unambiguously and to determine their kinematical and physical characteristics. In this work we model the expected line emission from a protoplanetary disk irradiated by an infalling envelope, addressing the question of its detectability with subarcsecond resolution. We adopt a previously determined disk model structure that gives a continuum spectral energy distribution and a millimeter intensity spatial distribution that are consistent with observational constraints of HL Tau. An analysis of the capability of presently working and projected interferometers at millimeter and submillimeter wavelengths shows that molecular transitions of moderate opacity at these wavelengths (e.g., (CO)-O-17 lines) are good candidates for detecting disk lines at subarcsecond resolution in the near future. We suggest that, in general, disks of typical class I sources will be detectable. Higher line intensities are obtained for lower inclination angles, larger turbulent velocities, and higher temperatures, with less effect from density changes. The resulting maps show several characteristics that can be tested observationally. A clear asymmetry in the line intensity, with more intense emission in the disk area farther away from the observer, can be used to compare the geometrical relationship between disks and outflows. A decrease in intensity towards the center of the disk is also evident. Finally, the emission peaks in position velocity diagrams trace midplane Keplerian velocities.