Star-forming complex S254-S258 HII regions

In this work, we study the 3D structure of these HII 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 HII emission is attenuated by neutral material with 2<=AV<=5mag.

The typical electron density in S255, and S257 is ~100cm^-3^, with enhancements up to 200cm^-3^ in their borders, and up to 400cm^-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.