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

We have observed the SiO J=5--4 line towards a sample of 25 low-luminosity(L* < 10^3 Lsun) protostellar outflow systems. The line was detected towards 7of the 25 sources, a detection rate of 28 per cent. The majority (5 out of 7)of sources detected were of class 0 status. We detected a higher fraction ofclass 0 sources compared with the class I and II sources, although given thesmall numbers involved the significance of this result should be regarded astentative. Most of the detected sources showed emission either at or close tothe central position, coincident with the protostar. In four cases (HH211,HH25MMS, V-380 OriNE and HH212) emission was also detected at positions awayfrom the center, and was stronger than that observed at the centre position. SiO abundances of 10^(-8) to 8x10^(-7) are derived from LTE analysis. For 2sources we have additional transitions which we use to conduct statisticalequilibrium modeling to estimate the gas density in the SiO-emitting regions.For HH25MMS these results suggest that the SiO emission arises in ahigher-density region than the methanol previously observed. We find that themost likely explanation for the preferential detection of SiO emission towardsclass 0 sources is the greater density of those environments, reinforced byhigher shock velocities. We conclude that while not all class 0 sources exhibitSiO emission, SiO emission is a good signpost for the presence of class 0sources.Comment: 21 pages, inc 10 B&W figures. Uses emulateapj5.sty. Accepted for publication in ApJ, scheduled for 1 March 2004 issu