Far–infrared spectroscopy across the asymmetric bipolar outflows from Cepheus A and L 1448 ^*

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
² Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland

Corresponding author: D. Froebrich, frobich@tls-tautenburg.de

Received: 9 August 2001
Accepted: 17 January 2002

Abstract

Bipolar outflows are driven from protostars within molecular cores. They drive into molecular clouds, generating shock waves whose molecular emission lines have been observed in the infrared with ISO. We present spectroscopic data for seven locations within two asymmetric outflows, Cepheus A and L 1448, in order to test the shock physics and shock dynamics. Here, we simultaneously interpret the CO and H₂ data sets which are generated by shocked gas, radiating at temperatures from 300 to 2000 K. We find that large-scale spatial variations in the excitation are absent across both outflows and that the excitation is low everywhere. Planar shock models are inconsistent with the data sets. Models with configurations or ensembles of shocks, in the form of bow shocks or supersonic turbulence, are consistent. This solves the previously reported problem that the CO abundances were anomalously high. Cool gas is dominant, from which we infer bow shocks with flanks more extended than for paraboloids. As a consequence, the atomic oxygen abundances must be quite low. J-type bow models require implausibly long wings. C-type physics is thus favoured. The density and the ratio of molecules to atoms are constrained by the CO/H₂ flux levels as well as the H₂ vibrational level distributions. Other C-shock parameters, such as the magnetic field strength, ion fraction and speed, are not tightly constrained. The total shock powers are derived and are comparable to the mechanical outflow luminosities for both outflows, consistent with the outflows being momentum-driven.