Kavli Affiliate: Ke Wang
| First 5 Authors: Fengwei Xu, Ke Wang, Yuxin He, Jingwen Wu, Lei Zhu
| Summary:
Gas infall motions play a crucial role in high-mass star formation and are
characterized by observable signatures in the form of blue-shifted asymmetric
spectral line profiles ("blue profiles"). However, the connection between blue
profiles and infall motions is unclear due to complex gas motions at parsec
scales. In this study, we present the results of an HCN (4-3) mapping survey
conducted with the JCMT, towards 38 massive clumps exhibiting blue profiles in
HCO+ (3-2). We extract 34 HCN cores from the 38 observed fields. The
core-averaged spectra show various line profiles, indicating that blue-profile
HCO+ (3-2) does not guarantee the same in HCN (4-3). Through non-LTE radiation
transfer calculations, we attribute the low detection rate of high-$J$ blue
profiles to a combination of insufficient HCN (4-3) opacity and intricate gas
motion across different density layers. The comparison between the MALT90 and
BGPS line surveys highlights the importance of appropriate tracers, high
spectral resolution, and column density thresholds when searching for blue
profiles. We select 11 reliable infall candidates and adopt the Hill5 model to
fit the infall velocity of 0.2-1.9 km/s, corresponding to 5% to 74% of
free-fall velocity. Assuming a spherically collapsing model, we estimate the
median and mean mass infall rates to be 4.5E-3 and 7.6E-3 Msun/year,
respectively. The consistency of the mass infall rates among different
transitions suggests a steady accretion process from the clump gas envelope to
the inner region.
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