A Sample of [CII] Clouds Tracing Dense Clouds in Weak FUV Fields observed by Herschel

TL;DR

This study uses Herschel observations of [CII] and CO lines to analyze dense molecular clouds in the galactic plane, revealing physical conditions and star formation activity in different interstellar environments.

Contribution

It provides the first comprehensive analysis of [CII] emission in dense clouds with weak FUV fields, linking [CII], CO data, and PDR models to distinguish star-forming regions.

Findings

Most [CII] emission comes from clouds with densities 10^3.5 to 10^5.5 cm^-3 and weak FUV fields.

Identified regions with densities >10^5 cm^-3 and strong FUV fields linked to massive star formation.

[CII] combined with CO isotopes effectively differentiates star-forming regions from quiescent clouds.

Abstract

The [CII] fine--structure line at 158um 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+ (GOTC+), a Herschel Key Project devoted to study the [CII] fine structure emission in the galactic plane using the HIFI instrument. We use the [CII] 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 degrees and 20 degrees. We identify fifty-eight [CII] components that are associated with high--column density molecular clouds as traced by 13CO emission. We combine [CII], 12CO, and 13CO observations to derive the physical conditions of the [CII]--emitting regions in our sample of high--column density clouds based on comparison with results from a grid of Photon Dominated Region (PDR) models. From this unbiased sample, our results suggest that most of [CII] emission originates from clouds with H2 volume densities between 10e3.5 and 10e5.5 cm^-3 and weak FUV strength (CHI_0=1-10). We find two regions where our analysis suggests high densities >10e5 cm^-3 and strong FUV fields (CHI=10e4-10e6), likely associated with massive star formation. We suggest that [CII] emission in conjunction with CO isotopes is a good tool to differentiate between 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