An HST Survey of Protostellar Outflow Cavities: Does Feedback Clear
  Envelopes?
  We study protostellar envelope and outflow evolution using Hubble Space
Telescope NICMOS or WFC3 images of 304 protostars in the Orion Molecular
clouds. These near-IR images resolve structures in the envelopes delineated by
the scattered light of the central protostars with 80 AU resolution and they
complement the 1.2-870 micron spectral energy distributions obtained with the
Herschel Orion Protostar Survey program (HOPS). Based on their 1.60 micron
morphologies, we classify the protostars into five categories: non-detections,
point sources without nebulosity, bipolar cavity sources, unipolar cavity
sources, and irregulars. We find point sources without associated nebulosity
are the most numerous, and show through monochromatic Monte Carlo radiative
transfer modeling that this morphology occurs when protostars are observed at
low inclinations or have low envelope densities. We also find that the
morphology is correlated with the SED-determined evolutionary class with Class
0 protostars more likely to be non-detections, Class I protostars to show
cavities and flat-spectrum protostars to be point sources. Using an edge
detection algorithm to trace the projected edges of the cavities, we fit
power-laws to the resulting cavity shapes, thereby measuring the cavity
half-opening angles and power-law exponents. We find no evidence for the growth
of outflow cavities as protostars evolve through the Class I protostar phase,
in contradiction with previous studies of smaller samples. We conclude that the
decline of mass infall with time cannot be explained by the progressive
clearing of envelopes by growing outflow cavities. Furthermore, the low star
formation efficiency inferred for molecular cores cannot be explained by
envelope clearing alone.
