The evolution of young HII regions I. Continuum emission and internal dynamics

Context. High-mass stars form in much richer environments than those associated with isolated low-mass stars, and once they reach a certain mass, produce ionised (HII) regions. The formation of these pockets of ionised gas are unique to the formation of high-mass stars (M > 8 M-circle dot), and present an excellent opportunity to study the final stages of accretion, which could include accretion through the HII region itself.

Aims. This study of the dynamics of the gas on both sides of these ionisation boundaries in very young HII regions aims to quantify the relationship between the HII regions and their immediate environments.

Methods. We present high-resolution (similar to 0.5 '') ALMA observations of nine HII regions selected from the red MSX source survey with compact radio emission and bolometric luminosities greater than 10(4) L-circle dot. We focus on the initial presentation of the data, including initial results from the radio recombination line H29 alpha, some complementary molecules, and the 256 GHz continuum emission.

Results. Of the six (out of nine) regions with H29 alpha detections, two appear to have cometary morphologies with velocity gradients across them, and two appear more spherical with velocity gradients suggestive of infalling ionised gas. The remaining two were either observed at low resolution or had signals that were too weak to draw robust conclusions. We also present a description of the interactions between the ionised and molecular gas (as traced by CS (J = 5-4)), often (but not always) finding the HII region had cleared its immediate vicinity of molecules.

Conclusions. Of our sample of nine, the observations of the two clusters expected to have the youngest HII regions (from previous radio observations) are suggestive of having infalling motions in the H29 alpha emission, which could be indicative of late stage accretion onto the stars despite the presence of an HII region.