Expansion Signatures in 35 HII Regions traced by SOFIA [CII] Emission

TL;DR

This study uses SOFIA [CII] emission data to identify and analyze expansion signatures in 35 HII regions, revealing that about one-third show clear expansion, with implications for understanding their feedback and evolution.

Contribution

First comprehensive analysis of HII region expansion using [CII] emission, providing new insights into expansion velocities, geometries, and driving mechanisms.

Findings

Approximately 34% of regions exhibit clear expansion signatures.

Expansion velocities average around 12.2 km/s.

Most expansion candidates are bubble-shaped and blueshifted.

Abstract

We analyze the expansion signatures of 35 HII regions mapped in [CII] 158 micron emission by the Stratospheric Observatory for Infrared Astronomy (SOFIA). The [CII] emission primarily traces photodissociation regions (PDRs) at the transition between ionized and neutral gas. The brightness and narrow linewidth of [C II] allow us to measure PDR expansion. Bubble-shaped regions often exhibit expansion, while irregular-shaped ones are less likely to. Of the 35 HII regions, 12 (~34%) exhibit clear expansion in position-velocity (PV) diagrams, making them expansion candidates (ECs), with an average expansion velocity of ~12.2 km/s. The remaining 23 regions show no clear expansion signatures, though they may still be expanding below detection limits. Blueshifted expansion is more common (eight ECs solely blueshifted; one redshifted; three both), with mean velocities of ~10.9 km/s (blueshifted) and ~13.2 km/s (redshifted). A comparison of our observations to spherical expansion models supports expansion in eight of 12 ECs. Estimated dynamical ages are 10 to 100 times shorter than the ionizing star lifetimes, in agreement with the results of previous studies. Of the 35 regions, 14 (~40%) appear as [CII] bubbles; nine of the 12 ECs are bubble-shaped. Thermal pressure likely drives expansion in M43, while stellar winds dominate in M17, M42, RCW 120, and RCW 79. For other ECs, available data do not allow a definitive conclusion. Larger samples and more information about ionizing sources are needed to refine our understanding of HII region feedback and evolution.