Radiation Transfer of Models of Massive Star Formation. IV. The Model Grid and Spectral Energy Distribution Fitting

Yichen Zhang, Jonathan C. Tan

Published 2017-08-29Version 1

We present a continuum radiative transfer model grid for fitting observed spectral energy distributions (SEDs) of massive protostars. The model grid is based on the paradigm of core accretion theory for massive star formation, that is, initial conditions are pre-assembled gravitationally bound cores. In particular, following the Turbulent Core Model of McKee & Tan, initial core properties are set primarily by their mass and the pressure of their ambient clump environment. We then model the evolution of the protostar, infalling envelope, accretion disk, and outflow cavity in a self-consistent way. The current model grid contains about 9000 SEDs with 4 free parameters: initial core mass; the mean mass surface density of the star forming environment that sets the bounding pressure on the core; the protostellar mass which describes the evolutionary stage of a particular core model; and the inclination angle. Then, typically adopting source distance as an input constraint, the model grid is used to fit observed SEDs via chi-square minimization, with the method additionally estimating the level of foreground extinction. We demonstrate the fitting process and results using the massive protostar G35.20-0.74 as an example. Compared with other SED model grids used for massive star formation studies, in our model grid, the properties and evolution of the protostar and its surrounding structures are more physically connected, which reduces the dimensionality of the parameter spaces and excludes possible fitting of models that are physically unrealistic or that are not internally self-consistent in the context of the Turbulent Core Model. Thus, this model grid serves not only as a fitting tool to estimate properties of massive protostars from observed SEDs, but also as a test of core accretion theory. The SED model grid and fitting tool is publicly released with this paper.