Ionized carbon in side-illuminated molecular clouds.

Area of Science:

• Astrophysics
• Interstellar Medium Studies

Background:

• Observations of ionized carbon (C II) fine-structure lines provide insights into the physical conditions of star-forming regions.
• The 2P3/2-2P1/2 transition of C II at 1900 GHz is a key tracer of ionized gas.

Purpose of the Study:

• To investigate the properties of C II emission in five star-forming regions with ionization fronts.
• To compare C II emission characteristics with molecular gas and dust properties.

Main Methods:

• Observation of the 1900 GHz C II fine-structure line using high-frequency radio telescopes.
• Analysis of line profiles, velocities, line widths, and brightness temperatures.
• Comparison with molecular line data (e.g., CO) and dust temperatures.

Main Results:

• C II emission generally agrees in LSR velocity with CO, but exhibits wider line widths (3-14 km s-1) and spatial variations.
• Wider line widths suggest C II may originate from ionized gas components or outflows.
• C II brightness temperatures are comparable to or slightly higher than dust temperatures.
• Optically thin approximations yield C II excitation temperatures >= 100 K and column densities <= 10^18 cm-2, with M17 being a notable exception.
• Column density estimates are lower than predicted by photodissociation region models for side-illuminated geometries.

Conclusions:

• The observed characteristics of C II emission, particularly its line widths, indicate complex gas dynamics and potential contributions from ionized gas or outflows.
• Discrepancies with photodissociation region models highlight uncertainties in UV flux and ionization front geometry.
• Optically thick C II emission could reconcile column density estimates with models and dust temperatures.