The Class 0 protostar BHR71: Herschel observations and dust continuum models

Abstract

We performed a comprehensive analysis of the Herschel spectra of BHR71, an embedded Class 0 protostar. We recovered 66 lines in the central spaxel. Counting detections in all spaxels in PACS and SPIRE, more than 700 lines were detected. A CO rotational diagram analysis shows four excitation temperature components, 51 K, 153 K, 409 K, and 1053 K. Low-J CO lines trace the outflow while the high-J CO lines are centered on the infrared source. The low-excitation emission lines of water trace the large-scale outflow, while the high-excitation emission lines trace a small scale distribution around the equatorial plane. We model the structure of the envelope using the dust radiative transfer code, Hyperion, to fit the spectral energy distribution (SED) observed by Spitzer and Herschel. The model incorporates rotational collapse and an outer static envelope as well as an outflow cavity and disk. Our exploration of parameter space shows that the evolution of a collapsing envelope can be constrained by the Herschel SED and that the structure of the outflow cavity plays a critical role at shorter wavelengths. A cavity with a constant-density inner region and a power-law density outer region can reproduce the observations. The best fit model has a mass of 22 solar mass inside a radius of 0.2 pc and a central luminosity of 15.18 solar luminosity. The time since collapse began is 1.2x10^4 year with considerable uncertainty. The central luminosity in the best-fit model is greater than the observed luminosity because radiation is channeled out the outflow cavity. Even with this correction, the current mass accretion rate determined from the luminosity is about a factor of three less than the mass infall rate, suggestive of episodic accretion.