TESTING MODELS OF LOW-MASS STAR FORMATION - HIGH-RESOLUTION FAR-INFRARED OBSERVATIONS OF L1551 IRS-5

We report the results of a high-resolution study of L1551 IRS 5 at 50 and 100-mu-m, using a far-infrared scanning photometer aboard the Kuiper Airborne Observatory. The 100-mu-m emission from L1551 IRS 5 is extended, with a beam-deconvolved diameter of approximately 11", but the source is unresolved at 50-mu-m, with a diameter less than 6". The data agree well with the predictions of theoretical models by Adams, Lada, & Shu. More generally, we are able to constrain the range of possible density gradients through modeling the source emission, using a spherially symmetric radiative transfer program developed by Egan, Leung, & Spagna and comparing beam-convolved model profiles with the observed scans at 50 and 100-mu-m. The data are consistent with a density gradient of the form n(r) poportional r-1.5; this result is expected for an infalling envelope, such as predicted by Adams, Lada, & Shu. We explore the effects of varying the dust grain properties on the spectral energy distribution of the source. The amount of mid-infrared emission is very sensitive to the dust opacity and, given the uncertainty in our understanding of dust properties, is not a sensitive test for the presence of disks. The strong emission seen in the millimeter-wave interferometer observations of Keene & Masson appears to be the strongest evidence for a compact continuum structure, presumably a disk, surrounding L1551 IRS 5 because models without disks cannot produce the observed ratio of the fluxes in small and large beams at 2.73 mm. However, the mass of the compact millimeter continuum source in L1551 IRS 5 is quite uncertain, because of uncertainties in the opacities of dust grains at these long wavelengths.