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

ABSTRACT The S254-258 star-forming complex is a place of massive star formation where five OB-stars have created H ii regions, visible as optical nebulae, and disrupted the parental molecular gas. In this work, we study the 3D structure of these H ii regions using optical spectroscopy and tunable-filter photometry with the 6- 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 H ii emission is attenuated by neutral material with 2 ≤ AV ≤ 5 mag. The typical electron density in S255, and S257 is ≈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 H ii regions S256 and S258 are deeply embedded in the molecular clouds.