[Ne II] Observations of Gas Motions in Compact and Ultracompact H II Regions

TL;DR

This study uses high-resolution [Ne II] line observations to reveal that gas motions in compact and ultracompact H II regions are dominated by surface flows along cavity edges, challenging previous models of their dynamics.

Contribution

It provides new high-resolution kinematic data showing surface flows dominate H II region dynamics, refining the understanding of their structure and motion.

Findings

Most sources show significant velocity gradients (5-20 km/s).

Gas motions are mainly tangential along dense ionized shells.

H II regions are confined to cavity edges created by stellar winds.

Abstract

We present high spatial and spectral resolution observations of sixteen Galactic compact and ultracompact H II regions in the [Ne II] 12.8 microns fine structure line. The small thermal width of the neon line and the high dynamic range of the maps provide an unprecedented view of the kinematics of compact and ultracompact H II regions. These observations solidify an emerging picture of the structure of ultracompact H II regions suggested in our earlier studies of G29.96-0.02 and Mon R2 IRS1; systematic surface flows, rather than turbulence or bulk expansion, dominate the gas motions in the H II regions. The observations show that almost all of the sources have significant (5-20 km/s) velocity gradients and that most of the sources are limb-brightened. In many cases, the velocity pattern implies tangential flow along a dense shell of ionized gas. None of the observed sources clearly fits into the categories of filled expanding spheres, expanding shells, filled blister flows, or cometary H II regions formed by rapidly moving stars. Instead, the kinematics and morphologies of most of the sources lead to a picture of H II regions confined to the edges of cavities created by stellar wind ram pressure and flowing along the cavity surfaces. In sources where the radio continuum and [Ne II] morphologies agree, the majority of the ionic emission is blue-shifted relative to nearby molecular gas. This is consistent with sources lying on the near side of their natal clouds being less affected by extinction and with gas motions being predominantly outward, as is expected for pressure-driven flows.