HCN emission from the HII regions G75.78+0.34 and G75.77+0.34

TL;DR

This study uses high-resolution interferometric imaging to analyze HCN emission around two HII regions, revealing high-density clouds and outflow dynamics, with implications for understanding star-forming environments.

Contribution

First detailed interferometric imaging of HCN hyperfine lines in G75.78+0.34 and G75.77+0.34, showing complex emission structures and deviations from LTE predictions.

Findings

HCN traces high-density clouds not seen in CO

HCN emission follows bipolar outflows in G75.78+0.34

Hyperfine line ratios deviate from LTE predictions

Abstract

We present images for the 3.5 mm continuum and HCN(J=1-0) hyperfine line emission from the surroundings of the HII regions G75.78+0.34 and G75.77+0.34 obtained with the Berkeley Illinois Maryland Association (BIMA) interferometer using the D configuration at a spatial resolution of 18 arcsec and spectral sampling of 0.34 km/s. The continuum emission of both objects is dominated by free-free emission from the ionized gas surrounding the exciting stars. Dust emission may contribute only a small fraction of the 3.5 mm continuum from G75.78+0.34 and is negligible for G75.77+0.34. The high spectral resolution reached by BIMA allowed us to separate the emission from each hyperfine transition (F=1-1, F=2-1 and F=0-1), as well as to construct velocity channel maps along each emission-line profile. The HCN flux distributions are similar to those observed for the CO emission, but with some knots of high intensities indicating that the HCN traces high density clouds not seen in CO. The HCN hyperfine line ratios for both HII regions differ from those predicted theoretically for Local Thermodynamic Equilibrium (LTE), probably due to scattering of radiation processes. The velocity channels shows that the HCN emission of G75.78+0.34 follows the bipolar molecular outflows previously observed in CO. For G75.77+0.34, the outflowing gas contributes only a small fraction of the HCN emission.