2002A&A...383..591H

Using MAMBO at the IRAM 30m telescope we have observed a dense core in the cirrus cloud MCLD 123.5+24.9 in the dust continuum emission at 250GHz. The core is detected as an elongated filament with an extent of 4.2'x0.7', corresponding to 0.18pcx0.03pc at an adopted distance of 150pc. We find a close correlation between the continuum emission and previously observed C¹⁸O (1-0) line emission. Using standard dust models we derive hydrogen column densities of up to 10²²cm^–2. The total mass for the filament is about 0.66M_☉. We also present observations of the HC₃N (3-2), (4-3), and (10-9) emission lines obtained with the MPIfR 100m and the IRAM 30m telescopes. The distribution is very different from the dust continuum and the C¹⁸O (1-0) line emission. HC₃N is concentrated in two distinct clumps located at the ends of the filament seen in the other tracers. Based on a LVG analysis of the HC₃N transitions we derive column densities of N(HC₃N)/Δv≃10¹³cm^–2/km/s and volume densities of n(H₂)≃10⁵cm^–3. We find that the HC₃N clumps have masses of 0.13 and 0.19M_☉. Our data demonstrate that the cirrus cloud cores are gravitationally bound, and that they show chemical structure indicating different evolutionary stages within the cloud.