SiO shocks in the L1157 molecular outflow

F. Gueth , S. Guilloteau , R. Bachiller
Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France
Observatorio Astronómico Nacional (I.G.N), Apartado 1143, E-28800 Alcalá de Henares, Madrid, Spain
Abstract: We report high angular resolution IRAM Plateau de Bure interferometric observations of the SiO v=0  and transitions in the southern lobe of the young L1157 molecular outflow. The resolution of these observations ( ) makes them directly comparable to available high-resolution CO maps of the flow. The known precession of the L1157 flow is fully confirmed. We find a remarkable morphological agreement between the strong SiO shocks revealed by these observations and the two CO cavities of the southern lobe of the outflow (see Figure 8): the positions, shapes and opening angles are similar in both tracers, with the SiO emission ahead of or at the edges of the CO emission. Each CO cavity is associated with a shock which is placed exactly at its apex and exhibits a linear feature pointing exactly towards the protostellar position. The CO appears in the wake of these leading shocks. These coincidences, as well as the presence of two independent shock/cavity systems, strongly support shock-entrainment models for the formation of molecular outflows.
These observations also provide detailed information on the internal structure of the shocked regions. They confirm that a strong enhancement of the SiO abundance occurs within shocks. The comparison with high-angular resolution images of other shock-tracers (NH , H ) shows that chemical and evolution effects play a crucial role in the observed brightness distributions. The SiO velocity distribution is mainly forward, but the kinematics seems to result from both the complex formation processes of SiO and the velocity field produced by bow-shocks. Finally, we briefly discuss the apparent density structure of the shocks, and especially the possible origin of the linear precursor seen downstream from the main bow-shock.
To be published in Astronomy & Astrophysics, main journal

 

 

Figure: The L1157 molecular outflow. (Left) Overlay of the integrated CO  (greyscale) and SiO  (white contours) emission. For the CO map, contours are -1.5 then 0.75 to 8.25 by 1.5 Jykms /beam, and the clean beam is at PA  . For the SiO map, contours are -0.4, 0.3, 0.8, 1.2, 2, 3, 4 Jykms /beam, and the clean beam is at PA  . (Right) Overlay of the integrated CO  and the SiO  emission at LSR velocity -2.8kms (integrated over an interval of 2.6kms \ wide). The SiO contour step is 55 mJy/beam. In both panels, the cross indicates the position of the protostar L1157-mm. For clarity, the outer regions of the SiO image, where the noise level is quite high, have been masked. Note that the northern part of the flow, near the protostar, has not been observed in the SiO line.