OBSERVATIONS OF [C-I] AND CO ABSORPTION IN COLD, LOW-DENSITY CLOUD MATERIAL TOWARD THE GALACTIC-CENTER BROAD-LINE EMISSION

We report the detection of a deep P-3(1) --> P-3(0) (C I) absorption f eature at upsilon(LSR) = 12 km s(-1) with a line width of 6 km s(-1) t oward extended line emission at a distance of 11' from Sgr C. The 492 GHz observations were made with the Antarctic Submillimeter Telescope and Remote Observatory. The absorption feature allows the derivation o f lower limits for the C I column density in the cold foreground mater ial. The feature is unlikely to be caused by self-absorption within th e [C I]-emitting cloud because it is observed over a region at least 4 ' across and is also seen in emission 22' north of the Galactic plane in (CO)-C-12 J = 2-1. In order to determine the temperature and the ab undance ratio of C I to CO in the foreground gas, we compare the obser vations with (CO)-C-12 and (CO)-C-13 J = 1-0 observations obtained wit h the Bell Labs 7 m antenna and with (CO)-C-12 and (CO)-C-13 J = 2-1 o bservations made with the Kolner Observatorium fur Submillimeter und M illimeter Astronomie 3 m telescope. All these observations have about the same beam size. On the assumption that the background emission is not spatially associated with the absorbing cloud(s), a consistent mod el for the observed line intensities yields an excitation temperature of 3.5 K for (CO)-C-13 and 5 K for [C I], which implies low volume den sities, n(H-2) less than or similar to 10(3) cm(-3). The measured abun dance ratio of C I to (CO)-C-13 is similar to 34. This value is consis tent with photochemical model calculations that predict an abundance r atio of C I to (CO)-C-12 of similar to 1 and a (CO)-C-12 to (CO)-C-13 ratio of similar to 30 (reduced in comparison to the intrinsic C-12 to C-13 isotopic ratio of 60 by fractionation). The observed (CO)-C-13 c olumn density corresponds to an A(v) of 4.6 mag, i.e., the hydrogen co lumn density N(H) is similar to 9 x 10(21) Cm-2. This, together with t he observed [C I] line width, indicates that the absorption is likely due to several translucent clouds. We compare our results with line fl uxes derived from the large-scale, low-resolution COBE FIRAS spectral line survey of [C I] P-3(1) --> P-3(0) and [C I] P-3(2) --> P-3(1) emi ssion in the Galactic plane. Taking into account beam filling, the low er limit for the column density of cold (T-ex less than or equal to 10 K) C I that is traced by our absorption observations is at least a fa ctor of 2 higher than the column density of the warmer C I(T-ex greate r than or equal to 20 K) detected in emission by COBE. Our results sug gest that a substantial fraction of atomic carbon in the interstellar medium may be difficult to detect in [C I] emission, owing to its low excitation.