SiO outflows in the most luminous and massive protostellar sources of the southern sky

Abstract

Context. High-mass star formation is far less understood than low-mass star formation. It entails the ejection of matter through molecular outflows, which disturbs the protostellar clump. Studying these outflows and the shocked gas caused by them is the key to a better understanding of this process. Aims. The present study aims to characterise the behaviour of molecular outflows in the most massive protostellar sources in the southern Galaxy by looking for evolutionary trends and associating the presence of shocked gas with outflow activity. Methods. We present APEX SEPIA180 (Band 5) observations (beamwidth ~36″) of SiO(4-3) molecular outflow candidates towards a well-selected sample of 32 luminous and dense clumps, which are candidates for harbouring hot molecular cores. We study the emission of the SiO(4-3) line, which is an unambiguous tracer of shocked gas, and recent and active outflow activity, as well as the HCO+(2-1) and H13CO+(2-1) lines. Results. Results show that 78% of our sample (25 sources) present SiO emission, revealing the presence of shocked gas. Nine of these sources are also found to have wings in the HCO+(2-1) line, indicating outflow activity. The SiO emission of these nine sources is generally more intense (Ta > 1 K) and wider (~61 km s−1 FWZP) than the rest of the clumps with SiO detection (~42 km s−1 FWZP), suggesting that the outflows in this group are faster and more energetic. This indicates that the shocked material gets dispersed as the core evolves and outflow activity decreases. Three positive linear correlations are found: a weak one (between the bolometric luminosity and outflow power) and two strong ones (one between the outflow power and the rate of matter expulsion and the other between the kinetic energy and outflow mass). These correlations suggest that more energetic outflows are able to mobilise more material. No correlation was found between the evolutionary stage indicator L/M and SiO outflow properties, supporting that molecular outflows happen throughout the whole high-mass star formation process. Conclusions. We conclude that sources with both SiO emission and HCO+ wings and sources with only SiO emission are in an advanced stage of evolution in the high-mass star formation process, and there is no clear evolutionary difference between them. The former present more massive and more powerful SiO outflows than the latter. Therefore, looking for more outflow signatures such as HCO+ wings could help identify more massive and active massive star-forming regions in samples of similarly evolved sources, and could also help identify sources with older outflow activity.