Studying Infall in Infrared Dark Clouds with Multiple HCO+ Transitions

TL;DR

This study analyzes infall motions in 11 infrared dark clouds using multiple HCO+ transitions, revealing infall velocities, mass accretion rates, and the effectiveness of different transitions in tracing infall in star-forming regions.

Contribution

It provides new measurements of infall velocities and accretion rates in IRDCs using multi-transition HCO+ observations and applies the HILL model for detailed analysis.

Findings

Infall velocities range from 0.5 to 2.7 km/s with a median of 1.0 km/s.

Mass accretion rates are between 0.5 and 14×10^{-3} Msun/yr.

HCO+ J=3--2 and J=1--0 effectively trace infall signatures in IRDCs.

Abstract

We investigate the infall properties in a sample of 11 infrared dark clouds (IRDCs) showing blue-asymmetry signatures in HCO$^{+}$ J=1--0 line profiles. We used JCMT to conduct mapping observations in HCO$^{+}$ J=4--3 as well as single-pointing observations in HCO$^{+}$ J =3--2, towards 23 clumps in these IRDCs. We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5-2.7 km s$^{-1}$, with a median value of 1.0 km s$^{-1}$, and obtained mass accretion rates of 0.5-14$\times$10$^{-3}$ Msun yr$^{-1}$. These values are comparable to those found in massive star forming clumps in later evolutionary stages. These IRDC clumps are more likely to form star clusters. HCO$^{+}$ J =3--2 and HCO$^{+}$ J =1--0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties. HCO$^{+}$ J=4--3, on the other hand, exhibits infall signatures only in a few very massive clumps, due to smaller opacties. No obvious correlation for these clumps was found between infall velocity and the NH3/CCS ratio.