Gravitationally bound cores in a molecular cirrus cloud

Using MAMBO at the IRAM 30 m telescope we have observed a dense core in the cirrus cloud MCLD 123.5+24.9 in the dust continuum emission at 250 GHz. The core is detected as an elongated filament with an extent of $4.\!'2\times 0.\!'7$, corresponding to 0.18 pc$\times$0.03 pc at an adopted distance of 150 pc. We find a close correlation between the continuum emission and previously observed C18O (1$\to$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 0.66 $M_{\odot}$. We also present observations of the HC3N (3$\to$2), (4$\to$3), and (10$\to$9) emission lines obtained with the MPIfR 100 m and the IRAM 30 m telescopes. The distribution is very different from the dust continuum and the C18O (1$\to$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 $N({\rm HC_3N})/\Delta v\approx10^{13}$cm-2/kms-1and volume densities of $n({\rm H}_2)\approx10^5$cm-3. We find that the HC3N clumps have masses of 0.13 and 0.19 $M_{\odot}$. Our data demonstrate that the cirrus cloud cores are gravitationally bound, and that they show chemical structure indicating different evolutionary stages within the cloud.