L1448

1 Introduction

Bipolar outflows plough into the environments of their driving protostars, generating bow-shaped shock structures. On scales comparable to the parental molecular cloud, they can be studied in the infrared even when the protostar itself is still hidden deep within a dense core. In order to learn about the energetics and evolution of the protostar and the star forming region, we first need to determine the shock physics and shock morphology. Ground-based telescopes have provided quality near-infrared spectral data for many outflows (e.g. Fernandes & Brand 1998). The ISO satellite, however, offered a unique opportunity to gain spectral data on the shocked molecular gas which radiates profusely from the near- to the far-infrared. Here we evaluate two outflows which were observed at several locations, CepheusA and L1448.

The two opposite lobes of each of these outflows are highly asymmetric, one appearing quite spatially turbulent and the other containing ordered shock structures. The CepheusA outflow has at least one early B star at its source, with a high mass outflow and mechanical luminosity of $\sim$ $4\times 10^{-4}~M_{\odot}$ and $\sim$60 $L_{\odot}$(e.g. Narayanan & Walker 1996). The L1448 outflow, on the contrary, has an order of magnitude lower estimated mass outflow rate and a mechanical power of $\sim$ $2\times 10^{-5}~M_{\odot}$ and $\sim$0.3 $L_{\odot}$ (O` Linger et al. 1999). Apart from the L1448C outflow, which originates from a Class0 protostar, several other flows are observed to its north, which also emanate from deeply embedded young stellar objects (e.g. Eislöffel 2000).

Figure 1: Positions of apertures of SWS and LWS observations in CepheusA superimposed on an image in the 1-0S(1) line of H2 at 2.12$\mu$m.

Specific sets of ISO data for CepheusA East and West have been investigated by van den Ancker et al. (2000) and Wright et al. (1996), respectively. Uniform components and planar shocks were tested and temperatures for the low-lying H₂ rotational levels of $\sim$700K were found. A full set of ISO data has been analysed for L1448 by Nisini et al. (1999) and Nisini et al. (2000), who concluded a best fit temperature to both the CO and H₂ of $\sim$1300K. Here, we introduce and apply detailed shock models to the full ISO data sets. We thus find that we can differentiate between several shock models, involving the chemistry (e.g. oxygen abundance), the physics (ambipolar diffusion) and the geometry (degree of curvature).

We present the observational results in Sect.3. We detect gas at temperatures of 300-2000K and summarise the models which can generate this excitation range in Sect.4. Strong non-dissociative J-shocks heat gas to above 10000K. Hence, we `see' these shocks only when the gas has cooled, usually making them inefficient infrared radiators. C-shocks heat the gas directly to temperatures under 5000K and emit strongly in the infrared. Bow shocks heat gas to a whole range of temperatures according to the location on the bow front. We use the accumulated set of models in Sect.5 to interpret CepheusA East. We have updated the old codes for C-shocks, J-shocks and both types of bow shock (e.g. Smith & Brand 1990; Smith 1994b) by employing the H₂ collisional rates according to Le Bourlot et al. (1999) and added vibrational H₂O and CO cooling (Neufeld & Kaufman 1993). In fact, we find no significant differences with the previous predictions such as used for ISO SWS observations of CepheusA West (Smith 2000). Having established plausible shock models, we apply and adjust these to the other seven locations in CepheusA West (Sect.6) and L1448 (Sect.7). These results are then summarised and discussed in Sect.8.

L1448