Revisiting the shocks in BHR71: new observational constraints and H2O predictions for Herschel

TL;DR

This study combines new SiO observations with existing data to model shocks in the BHR71 outflow, constraining shock parameters and predicting water emission for Herschel observations, advancing understanding of star formation processes.

Contribution

It introduces combined modeling of H$_2$ and SiO emissions in BHR71, providing new constraints on shock properties and predicting water emission for Herschel instruments.

Findings

Best-fit models are non-stationary shocks with low velocities.

Pre-shock densities of 10^4 and 10^5 cm$^{-3}$ fit observations.

Predicted water emission lines for Herschel observations.

Abstract

During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow phenomenon plays an important role in the regulation of star formation. Understanding the associated shocks and their energetic effects is therefore essential to the study of star formation. We present comparisons of shock models with observations of H$_2$ and SiO emission in the bipolar outflow BHR71, and predict water emission, under the basic assumption that the emission regions of the considered species coincide, at the resolution of currently available observations. New SiO observations from APEX are presented, and combined with \textit{Spitzer} and ground-based observations of H$_2$ to constrain shock models. The shock regions associated with targeted positions in the molecular outflow are studied by means of a 1D code that generates models of the propagation of stationary shock waves, and approximations to non-stationary ones. The SiO emission in the inner part of the outflow is concentrated near the apex of the corresponding bow-shock that is also seen in the pure rotational transitions of H$_2$. Simultaneous modelling is possible for H$_2$ and SiO and leads to constraints on the silicon pre-shock distribution on the grain mantles and/or cores. The best-fitting models are found to be of the non-stationary type, but the degeneracy of the solutions is still large. Pre-shock densities of 10$^4$ and 10$^5$ cm$^{-3}$ are investigated, and the associated best-model candidates have rather low velocity (respectively, 20-30 and 10-15 km s$^{-1}$) and are not older than 1000 years. We provide emission predictions for water, focusing on the brightest transitions, to be observed with the PACS and HIFI instruments of the \textit{Herschel} Telescope.