Infall through the evolution of high-mass star-forming clumps

TL;DR

This study used SOFIA to detect ammonia absorption lines in nine massive star-forming clumps, revealing widespread infall signatures across various evolutionary stages and providing insights into the infall rates and dynamics of high-mass star formation.

Contribution

First detection of ammonia infall signatures in multiple high-mass star-forming clumps using THz observations, demonstrating infall ubiquity and comparing with other molecular tracers.

Findings

Ammonia absorption detected in all nine clumps, including two in infrared dark clouds.

Infall velocities are 3-30% of free-fall velocities, indicating active accretion.

Infall rates range from 0.3 to 16 x 10^{-3} solar masses per year, correlating with virial parameters.

Abstract

With the GREAT receiver at the Stratospheric Observatory for Infrared Astronomy (SOFIA), nine massive molecular clumps have been observed in the ammonia $3_{2+}- 2_{2-}$ line at 1.8~THz in a search for signatures of infall. The sources were selected from the ATLASGAL submillimeter dust continuum survey of our Galaxy. Clumps with high masses covering a range of evolutionary stages based on their infrared properties were chosen. The ammonia line was detected in all sources, leading to five new detections and one confirmation of a previous detection of redshifted absorption in front of their strong THz continuum as a probe of infall in the clumps. These detections include two clumps embedded in infrared dark clouds. The measured velocity shifts of the absorptions compared to optically thin \CSEO\ (3--2) emission are 0.3--2.8~km/s, corresponding to fractions of 3\%\ to 30\% of the free-fall velocities of the clumps. The ammonia infall signature is compared with complementary data of different transitions of HCN, HNC, CS, and HCO$^+$, which are often used to probe infall via their blue-skewed line profiles. The best agreement with the ammonia results is found for the HCO$^+$ (4--3) transitions, but the latter is still strongly blended with emission from associated outflows. This outflow signature is far less prominent in the THz ammonia lines, which confirms it as a powerful probe of infall in molecular clumps. Infall rates in the range from 0.3 to 16~$10^{-3}\,M_\odot/$yr were derived with a tentative correlation with the virial parameters of the clumps. The new observations show that infall on clump scales is ubiquitous through a wide range of evolutionary stages, from $L/M$ covering about ten to several hundreds.