A rotating helical filament in the L1251 dark cloud
  (Abridged) Aims. We derive the physical properties of a filament discovered
in the dark cometary-shaped cloud L1251. Methods. Mapping observations in the
NH3(1,1) and (2,2) inversion lines, encompassing 300 positions toward L1251,
were performed with the Effelsberg 100-m telescope at a spatial resolution of
40 arcsec and a spectral resolution of 0.045 km/s. Results. The filament L1251A
consists of three condensations (alpha, beta, and gamma) of elongated
morphology, which are combined in a long and narrow structure covering a 38
arcmin by 3 arcmin angular range. The opposite chirality (dextral and
sinistral) of the alpha+beta and gamma condensations indicates magnetic field
helicities of two types, negative and positive, which were most probably caused
by dynamo mechanisms. We estimated the magnetic Reynolds number Rm > 600 and
the Rossby number R < 1, which means that dynamo action is important.
