Constraining the Envelope Structure of L1527 IRS: Infrared Scattered   Light Modeling
  We model Spitzer Space Telescope observations of the Taurus Class 0 protostar L1527 IRS (IRAS 04368+2557) to provide constraints on its protostellar envelope structure. The nearly edge-on inclination of L1527 IRS, coupled with the highly spatially-resolved near to mid-infrared images of this object and the detailed IRS spectrum, enable us to constrain the outflow cavity geometry quite well, reducing uncertainties in the other derived parameters. The mid-infrared scattered light image shows a bright central source within a dark lane; the aspect ratio of this dark lane is such that it appears highly unlikely to be a disk shadow. In modeling this dark lane, we conclude that L1527 IRS is probably not described by a standard TSC envelope with simple bipolar cavities. We find it necessary to model the dark lane and central source as a modified inner envelope structure. This structure may be due either to a complex wind-envelope interaction or induced by the central binary. To fit the overall SED, we require the central source to have a large near to mid-infrared excess, suggesting substantial disk accretion. Our model reproduces the overall morphology and surface brightness distribution of L1527 IRS fairly well, given the limitations of using axisymmetric models to fit the non-axisymmetric real object, and the derived envelope infall rates are in reasonable agreement with some other investigations. IRAC observations of L1527 IRS taken 12 months apart show variability in total flux and variability in the opposing bipolar cavities, suggesting asymmetric variations in accretion. We also provide model images at high resolution for comparison to future observations with current ground-based instrumentation and future space-based telescopes. 
John J. Tobin, Lee Hartmann, Nuria Calvet, and Paola D'Alessio