The hydrodynamical models of the cometary compact H II region

TL;DR

This paper presents a numerical study of gas dynamics in cometary ultra-compact H II regions, comparing bow-shock and champagne-flow models to understand their distinct kinematic signatures.

Contribution

The authors develop a comprehensive numerical method to simulate the gas dynamics and line profiles in cometary H II regions, distinguishing between bow-shock and champagne-flow models.

Findings

Champagne-flow models show ionized gas acceleration due to density gradients.

Bow-shock models exhibit ionized gas movement aligned with stellar motion.

Line profiles can differentiate between the two models.

Abstract

We have developed a full numerical method to study the gas dynamics of cometary ultra-compact (UC) H II regions, and associated photodissociation regions (PDRs). The bow-shock and champagne-flow models with a $40.9/21.9 M_\odot$ star are simulated. In the bow-shock models, the massive star is assumed to move through dense ($n=8000~cm^{-3}$) molecular material with a stellar velocity of $15~km~s^{-1}$. In the champagne-flow models, an exponential distribution of density with a scale height of 0.2 pc is assumed. The profiles of the [Ne II] 12.81\mum and $H_2~S(2)$ lines from the ionized regions and PDRs are compared for two sets of models. In champagne-flow models, emission lines from the ionized gas clearly show the effect of acceleration along the direction toward the tail due to the density gradient. The kinematics of the molecular gas inside the dense shell is mainly due to the expansion of the H II region. However, in bow-shock models the ionized gas mainly moves in the same direction as the stellar motion. The kinematics of the molecular gas inside the dense shell simply reflects the motion of the dense shell with respect to the star. These differences can be used to distinguish two sets of models.