A Survey of SiO 5 → 4 Emission toward Outflows from Low‐Luminosity Protostellar Candidates

We have observed the SiO J = 5 → 4 line toward a sample of 25 low-luminosity ( L * < 10 3 L ☉ ) protostellar outflow systems. The line was detected toward seven of the 25 sources, a detection rate of 28%. The majority (5 of 7) of sources detected were of Class 0 status. We detected a higher fraction of Class 0 sources compared with the Class I and II sources, although given the small numbers involved the significance of this result should be regarded as tentative. Most of the detected sources showed emission either at or close to the central position, coincident with the protostar. In four cases (HH 211, HH 25MMS, V380 Ori NE, and HH 212) emission was also detected at positions away from the center, and it was stronger than that observed at the center position. SiO abundances of 10 -8 to 8 × 10 -7 are derived from LTE analysis. For two sources we have additional transitions, which we use to conduct statistical equilibrium modeling to estimate the gas density in the SiO-emitting regions. For HH 25MMS these results suggest that the SiO emission arises in a higher density region than the methanol previously observed. We find that the most likely explanation for the preferential detection of SiO emission toward Class 0 sources is the greater density of those environments, reinforced by higher shock velocities. We conclude that while not all Class 0 sources exhibit SiO emission, SiO emission is a good signpost for the presence of Class 0 sources.