Letter to the Editor

A sample of [C II] clouds tracing dense clouds in weak FUV fields observed by Herschel ^*

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA e-mail: Jorge.Pineda@jpl.nasa.gov

Received: 31 May 2010
Accepted: 27 July 2010

Abstract

The [C ii] fine-structure line at 158 μm is an excellent tracer of the warm diffuse gas in the ISM and the interfaces between molecular clouds and their surrounding atomic and ionized envelopes. Here we present the initial results from Galactic observations of terahertz C⁺ (GOT C+), a Herschel key project devoted to studying the [C ii] emission in the Galactic plane using the HIFI instrument. We used the [C ii] emission, together with observations of CO, as a probe to understand the effects of newly formed stars on their interstellar environment and characterize the physical and chemical state of the star-forming gas. We collected data along 16 lines-of-sight passing near star-forming regions in the inner Galaxy near longitudes 330° and 20°. We identified fifty-eight [C ii] components that are associated with high-column density molecular clouds as traced by ¹³CO emission. We combined [C ii], ¹²CO, and ¹³CO observations to derive the physical conditions of the [C ii]-emitting regions in our sample of high-column density clouds based on comparing results from a grid of photon dominated region (PDR) models. From this unbiased sample, our results suggest that most of the [C ii] emission originates in clouds with H₂ volume densities between 10^3.5 and 10^5.5 cm^-3 and weak FUV strength (χ₀ = 1–10). We find two regions where our analysis suggest high densities >10⁵ cm^-3 and strong FUV fields (χ₀ = 10⁴–10⁶), likely associated with massive star formation. We suggest that [C ii] emission in conjunction with CO isotopes is a good tool for differentiating regions of massive star formation (high densities/strong FUV fields) and regions that are distant from massive stars (lower densities/weaker FUV fields) along the line-of-sight.