On the excitation of the infrared knots in the HH99 outflow

TL;DR

This study analyzes near-infrared spectra of HH99 knots to understand shock processes and emission origins, revealing that different shock types explain the observed molecular and atomic emissions, supporting episodic outflow models.

Contribution

It combines C- and J-type shock models to reproduce observed IR emission lines, providing detailed shock parameters and supporting episodic outflow theories.

Findings

H2 emission explained by J-type shocks with magnetic precursors

Atomic and ionic emissions arise from higher ionization regions

Fe is eroded from grains by earlier shocks

Abstract

We present near infrared (IR) spectra (0.98--2.5 mu m) from the group of Herbig-Haro (HH) objects comprising HH99: a series of knots forming a bow, HH99B, and a separate knot, HH99A. Observations of H2, [Fe II] and [C I] are used to constrain shock model parameters and determine the origin of the emission. Previous work has shown that it is likely that the atomic and ionic emission arises in regions of higher ionization than the molecular emission. On the basis of observations, it has often been suggested that the [Fe II] and [C I] emission could arise, for example, at the apex of a bow shock, with the H2 emission produced in the wings. Accordingly, we have combined models of C-component and J-type shocks in order to reproduce the observed H2, [Fe II] and [C I] line spectra. We can account for the H2 emission towards the HH99B complex by means of J-type shocks with magnetic precursors. We derive shock velocities of about 30 km s-1 and ages of order 100 yr; the pre-shock gas has a density of approximately 10^4 cm-3 and the magnetic field is approximately 100 mu G. The J-type shocks required to reproduce the intensities of the observed [Fe II] and [C I] lines have velocities of 50 km s-1. It is necessary to assume that Fe has been previously eroded from grains, probably by the earlier passage of a C-type shock wave. Thus, our analysis supports the view that molecular outflows are episodic phenomena whose observed emission arises in shock waves.