Spitzer observations of the HH 1/2 system. The discovery of the counterjet

TL;DR

This study uses high-resolution Spitzer IRAC images to discover and analyze the counterjet in the HH 1/2 system, revealing its properties and the nature of its emission and extinction.

Contribution

First detection of the optically invisible counterjet in the HH 1/2 system using high-resolution mid-infrared imaging.

Findings

Counterjet is about half as bright as the jet at 4.5 microns.

Counterjet length is approximately 10 arcseconds.

Jet and counterjet emission dominated by collisionally excited H2 lines.

Abstract

We present unpublished Spitzer IRAC observations of the HH 1/2 young stellar outflow processed with a high angular resolution deconvolution algorithm that produces sub-arcsecond (approx. 0.6" - 0.8") images. In the resulting mid-infrared images the optically invisible counterjet is detected for the first time. The counterjet is approximately half as bright as the jet at 4.5 micron (the IRAC band that best traces young stellar outflows) and has a length of approx. 10". The NW optical jet itself can be followed back in the mid-IR to the position of the exciting VLA 1 source. An analysis of the IRAC colors indicates that the jet/counterjet emission is dominated by collisionally excited H2 pure rotational lines arising from a medium with a neutral Hydrogen gas density of 1000-2000 per cubic cm and a temperature of 1500 K. The observed jet/counterjet brightness asymmetry is consistent with an intrinsically symmetric outflow with extinction from a dense, circumstellar structure of 6" size (along the outflow axis), and with a mean visual extinction of Av=11 mag.