Herschel(star) SPIRE-FTS observations of RCW 120
Résumé
Context. The expansion of Galactic H II regions can trigger the formation of a new generation of stars. However, little is know about the physical conditions that prevail in these regions. Aims. We study the physical conditions that prevail in specific zones towards expanding H II regions that trace representative media such as the photodissociation region, the ionized region, and condensations with and without ongoing star formation. Methods. We use the SPIRE Fourier Transform Spectrometer (FTS) on board Herschel to observe the H II region RCW 120. Continuum and lines are observed in the 190-670 mu m range. Line intensities and line ratios are obtained and used as physical diagnostics of the gas. We used the Meudon PDR code and the RADEX code to derive the gas density and the radiation field at nine distinct positions including the PDR surface and regions with and without star-formation activity. Results. For the different regions we detect the atomic lines [NII] at 205 mu m and [CI] at 370 and 609 mu m, the (CO)-C-12 ladder between the J = 4 and J = 13 levels and the (CO)-C-13 ladder between the J = 5 and J = 14 levels, as well as CH+ in absorption. We find gas temperatures in the range 45-250 K for densities of 10(4) - 10(6) cm(-3), and a high column density on the order of N-H similar to 10(22) cm(-2) that is in agreement with dust analysis. The ubiquitousness of the atomic and CH+ emission suggests the presence of a low-density PDR throughout RCW 120. High-excitation lines of CO indicate the presence of irradiated dense structures or small dense clumps containing young stellar objects, while we also find a less dense medium (N-H similar to 10(20) cm(-2)) with high temperatures (80-200 K).