Sh 2 - 205 : I . The surrounding interstellar medium

We investigate the distribution of the interstellar matter in the environs of the H ii region Sh2-205, based on neutral hydrogen 21–cm line data and radio continuum images at 408 and 1420 MHz obtained from the CGPS, 12 CO (J=1→0) observations, high resolution IRAS data (HIRES), and MSX data. Sh2-205 can be separated in three independent optical structures: SH 149.25–0.0, SH 148.83–0.67, and LBN 148.11–0.45. The three regions are detected both at 408 and 1420 MHz. The derived spectral indices show the thermal nature of SH 148.83–0.67 and LBN 148.11–0.45. LBN 148.11–0.45 is a classical H ii region surrounded by an H i shell. The associated neutral atomic and ionized masses are 65 M ⊙ and 70 M ⊙ , respectively. Dust and molecular gas (≈ 6 × 10 4 M ⊙) are found related to this ionized region. Particularly, a photodissociation region is detected at the interface between the ionized and molecular regions. If the proposed exciting star HD 24094 were an O8–O9 type star, as suggested by its near-infrared colors, its UV photon flux would be enough to explain the ionization of the nebula. The optical, radio continuum, and 21–cm line data allow us to conclude that SH 148.83–0.67 is an interstellar bubble powered by the energetic action of HD 24431. The associated neutral atomic and ionized masses are 180 M ⊙ and 300 M ⊙ , respectively. The emission of SH 149.25–0.0 is too faint to derive the dust and gas parameters.