$Herschel$ SPIRE-FTS observations of RCW 120

Abstract

The expansion of Galactic HII regions can trigger the formation of a new generation of stars. However, little is know about the physical conditions that prevail in these regions. We study the physical conditions that prevail in specific zones towards expanding HII regions that trace representative media such as the photodissociation region, the ionized region, and condensations with and without ongoing star formation. We use the SPIRE Fourier Transform Spectrometer (FTS) on board $Herschel$ to observe the HII 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. For the different regions we detect the atomic lines [NII] at $205\mu$m and [CI] at $370$ and $609\mu$m, the ${}^{12}\mathrm{CO}$ ladder between the $J=4$ and $J=13$ levels and the ${}^{13}\mathrm{CO}$ 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 $10^4-10^6{\mathrm{cm}}^{-3}$, and a high column density on the order of $N_{\mathrm{H}}\sim 10^{22}{\mathrm{cm}}^{-2}$ 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 ($N_{\mathrm{H}}\sim 10^{20}{\mathrm{cm}}^{-2}$) with high temperatures ($80-200$K).

11 pages, 11 figures, accepted by A&A

References in corpus (5)

• Star formation around RCW 120, the perfect bubble
• Cloud-cloud collision as a trigger of the high-mass star formation; a molecular line study in RCW120
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• Triggered star formation on the borders of the Galactic HII region RCW 82
• Mopra CO Observations of the Bubble HII Region RCW120

Cited by in corpus (2)

• Isentropic thermal instability in atomic surface layers of photodissociation regions
• Herschel observations of the circumstellar environment of the two Herbig Be stars R Mon and PDS27