Optical and Near-Infrared Study of the Cepheus E Outflow, A Very Low-Excitation Object

We present images and spectra of the Cepheus E (Cep E) region at both optical and infrared wavelengths. Only the brightest region of the southern lobe of the Cep E outflow reveals optical emission, suggesting that the extinction close to the outflow source plays an important role in the observed difference between the optical and IR morphologies. Cep E is a unique object since it provides a link between the spectroscopic properties of the optical Herbig-Haro (HH) objects and those of deeply embedded outflows. The observed H2 infrared lines allow us to determine an excitation temperature of ∼2300 K, an Ortho-to-Para ratio of ∼3, and an H2 (1, 0)/(2, 1) S(1) line ratio of ∼9. These results are consistent with the values observed for HH objects with detected NIR emission lines, with shock excitation as the main mechanism for their formation, and also with the values observed for embedded, NIR flows. The optical spectroscopic characteristics of Cep E (HH 377) appear to be similar to the ones of low-excitation HH objects. However, the electron density determined from the [S II] λλ6731/6717 line ratio for this object (ne = 4100 cm-3), and the [O I] λ6300/Hα, [S II] λλ(6717 + 6731)/Hα ratios are higher than the values of all of the previously studied low-excitation HH objects. This result is likely to be the consequence of an anomalously high environmental density in the HH 377 outflow. The ionization fraction obtained for HH 377 is xe ∼ 1%. From this result, together with the observed [O I] λ6300/Hα line ratio, we conclude that the observed Hα line emission is collisionally excited. From a comparison with shock models, we also conclude that the extinction toward HH 377 is very low. Comparing the observed Hβ and Hα fluxes of HH 377 with model predictions, we determine a shock speed between 15 and 20 km s-1, although somewhat higher velocities also produce spectra with line ratios that qualitatively agree with the observations of HH 377.