Constraining the Evolutionary Stage of Class I Protostars: Multiwavelength Observations and Modeling

(abridged) We present new Keck images at 0.9 micron and OVRO 1.3 mm continuumimages of five Class I protostars in the Taurus star forming region. We analyzethese data in conjunction with broadband spectral energy distributions and 8-13micron spectra from the literature using a Monte Carlo radiative transfer code.By fitting models for the circumstellar dust distributions simultaneously tothe scattered light images, millimeter continuum data, and the SEDs, we attemptto distinguish between flared disks, infalling envelopes with outflow cavities,and combinations of disks and envelopes. For each of these circumstellardensity distributions, we generate grids of models for varying geometries, dustmasses, and accretion rates, and determine the best fits by minimizing theresiduals between model and data. Models incorporating both massive envelopesand massive embedded disks generally fit the imaging+SED data best. Impliedenvelope infall rates for these models are consistent with infall rates derivedby previous investigators, although they are approximately an order ofmagnitude larger than inner disk accretion rates inferred from recentspectroscopic measurements. In addition, the disk masses inferred from ourmodels are close to or larger than the limit for gravitationally stable disks,indicating that Class I disks may undergo periodic episodes of enhancedaccretion, perhaps as a result of gravitational instabilities. An importantcaveat to these results is that in some cases, no single model can fit all ofthe imaging and SED data well, suggesting that further refinements to models ofthe circumstellar dust distributions around Class I sources are necessary.Comment: 58 pages, including 18 postscript figures. Accepted for publication in ApJ (Dec 10 issue