The L 1157 protostellar outflow imaged with the Submillimeter Array^⋆

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: arturogr@arcetri.astro.it
² Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
³ Academia Sinica, Institute of Astronomy & Astrophysics, PO Box 23–141, 106 Taipei, Taiwan

Received: 17 November 2011
Accepted: 13 August 2013

Abstract

Context. The outflow driven by the class 0 low-mass protostar L1157-mm stands out because of its peculiar chemical richness. However, its complex spatial/velocity structure makes it difficult to interpret observations of different molecular tracers.

Aims. We aim to map at high spatial resolution different molecular tracers that are important tools for studying shocks and/or thermal-density structures in outflows.

Methods. We used the Submillimeter Array at 1.4 mm to observe the blue lobe of the L1157 outflow at high spatial resolution (~3′′).

Results. We detected SiO, H₂CO, and CH₃OH lines from several molecular clumps that constitute the outflow. All three molecules were detected along the wall of the inner cavity that is thought to be related to the later ejection event. On the other hand, no emission was detected toward positions related to an old ejection episode, which is very likely due to space filtering from the interferometer. The H₂CO and CH₃OH emission is detected only at velocities close to the systemic velocity. The spatial distributions of the H₂CO and CH₃OH are similar. These emission lines trace the U-shaped structure seen in the mid-infrared image. In contrast, the SiO emission is detected in a wider velocity range with a peak at ~14 km s^-1 blueshifted from the systemic velocity. The SiO emission is brightest at the B1 position, which corresponds to the apex of the U-shaped structure. There are two compact SiO clumps along the faint arc-like feature to the east of the U-shaped structure. At the B1 position, there are two velocity components; one is a compact clump with a size of ~1500 AU seen at high velocity, the other is an extended component with lower velocities. The kinematic structure at the B1 position is different from that expected in a single bow shock. Most likely the high-velocity SiO clump at the B1 position is kinetically independent of the low-velocity gas. The line ratio between SiO (5–4) and SiO (2–1) suggests that the high-velocity SiO clumps consist of high-density gas of n ~ 10⁵−10⁶ cm^-3, which is similar to the density of the bullets in extremely high velocity (EHV) jets. The high-velocity SiO clumps in L1157 probably have the same origin as the EHV bullets.