3D structure of HII regions in the star-forming complex S254-S258

TL;DR

This study investigates the three-dimensional structure of HII regions in the S254-258 star-forming complex using optical spectroscopy and photometry, revealing details about their density, attenuation, and shell-like structures.

Contribution

It provides the first detailed 3D analysis of these HII regions, highlighting their shell-like structures and the role of neutral material and stellar winds.

Findings

HII emission is attenuated by neutral material with A_V between 2 and 5 mag.

Electron densities are around 100 cm$^{-3}$, with peaks up to 400 cm$^{-3}$ in dense regions.

Shell-like structures can be explained by clumpy neutral shells or stellar winds.

Abstract

The S254-258 star-forming complex is a place of massive star formation where five OB-stars have created HII regions, visible as optical nebulae, and disrupted the parental molecular gas. In this work, we study the 3D structure of these \HII regions using optical spectroscopy and tunable-filter photometry with the 6-m and 1-m telescopes of the Special Astrophysical Observatory of the Russian Academy of Sciences. We construct maps of the optical extinction, and find that the HII emission is attenuated by neutral material with $2 \leq A_V \leq 5$ mag. The typical electron density in S255, and S257 is $\approx 100$ cm$^{-3}$, with enhancements up to 200 cm$^{-3}$ in their borders, and up to 400 cm$^{-3}$ toward the dense molecular cloud between them, where active star formation is taking place. We show that either a model of a clumpy dense neutral shell, where UV~photons penetrate through and ionize the gas, or a stellar wind, can explain the shell-like structure of the ionized gas. S255 is surrounded by neutral material from all sides, but S257 is situated on the border of a molecular cloud and does not have dense front and rear walls. The compact HII regions S256 and S258 are deeply embedded in the molecular clouds.