The Circumstellar Structure of the Class I Protostar TMC‐1 (IRAS 04381+2540) from<i>Hubble Space Telescope</i>NICMOS Data

The class I protostar TMC-1 (IRAS 04381+2540) is oriented favorably for determining the properties of its circumstellar envelope and outflow cavity. Deep, high spatial resolution Hubble Space Telescope (HST) NICMOS images at 1.6 μm exhibit both a narrow jet and a wide-angle conical outflow cavity. Model images of the scattered-light distribution fit the data well, reproducing the intensity level, cavity width, and observed limb brightening. The best-fit geometry for TMC-1 has a 45° ± 5° source inclination and an 80° ± 5° deprojected wind opening angle (full width). The age, normally a poorly known quantity, is well constrained; the protostar age, i.e., time since the onset of cloud collapse, is 1 × 105 yr to within a factor of 2. We offer a possible resolution to the well-known luminosity problem. By considering the efficiency of infall onto the protostar, we find that plausible parameters can give an efficiency, and hence accretion luminosity, as low as 10% of the value derived from the collapsing cloud core. The efficiency, together with a luminosity constraint, leads to a mass estimate that ranges from about 0.1 M☉ for high efficiency to 0.2 M☉ for low accretion efficiency onto the protostar. Similarly, the estimated mass accretion rate onto the protostar ranges over roughly (0.9-1.4) × 10-6 M☉ yr-1, which is smaller than the (1.6-3.5) × 10-6 M☉ yr-1 infall rate of the cloud. If low efficiency rates are prevalent for protostars, one important consequence is that it will take longer to assemble the central star than the time t = Min/in, a time that assumes all of the infalling material lands on the protostar.