Large Scale Observations of [CI] $^3$P$_1$ $\rightarrow$ $^3$P$_0$ in Photon\\ Dominated Regions
Session 26 -- Molecular Cloud Structure and Energetics
Oral presentation, Wednesday, January 12, 10:15-11:45, Crystal Forum Room (Crystal City Marriott)

## [26.07] Large Scale Observations of [CI] $^3$P$_1$ $\rightarrow$ $^3$P$_0$ in Photon\\ Dominated Regions

R. Plume, D. Jaffe, K. Tatematsu (U. Texas), J. Keene (Caltech)

Since the $^3$P$_1$ $\rightarrow$ $^3$P$_0$ (492 GHz) fine--structure transition of neutral carbon arises from a state only 23 K above ground, and has a critical density of $\sim$10$^3$ cm$^{-3}$, it should be readily observable over a large fraction of the dense interstellar medium. Observations of our Galaxy show that over large scales the 492 GHz line has a larger intensity than any individual $^{12}$CO rotational line. The ratio of the 492 GHz [CI] line flux to the flux in the $^{12}$CO J=2$\rightarrow$1 line in the Milky Way (2.3$\pm$0.6), is also comparable to the inner regions of other galaxies (1.5$\pm$0.4 in IC 342; 1.3--1.6 in M82). This implies that the ground state fine--structure transitions of C$^o$ are important coolants of the dense neutral interstellar medium.

Theories of photon dominated regions (PDRs) predict that [CI] emission should arise from a thin layer at the surfaces of UV illuminated molecular clouds. The large--scale [CI] observations show that the line emission must come from extended regions of molecular clouds, and cannot arise strictly from the immediate vicinities of massive star formation. The distribution of [CI] emission has been studied, on smaller scales, in a number of interstellar clouds. However, these observations are limited in area and are usually restricted to regions near ionization fronts. Therefore, we have little knowledge of the typical C$^o$ distribution and abundance over extended regions in molecular clouds.

We will present results from the first large--scale (typically 30\arcmin\ $\times\$ 30\arcmin ) mapping of the 492 GHz $^3$P$_1$ $\rightarrow$ $^3$P$_0$ transition of neutral carbon toward CepA, M17, NGC 2024, S140, and W3. We will also present observations of the same regions in the $^{13}$CO J=2$\rightarrow$1 and C$^{18}$O J=2$\rightarrow$1 lines for comparison. The observations were made with the Texas Gaussian Focal Reducer" which reduces the effective aperture of the Caltech Submillimeter Observatory from 10.4m to $\sim$ 60cm. The resulting beamsize at 492 GHz was 3\arcmin\ and allowed us to rapidly survey [CI] emission over large areas in 5 molecular clouds.