Aims. To derive physical properties of a filament discovered in a 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 (α, β, and γ) of elongated morphology, which are combined in a long and narrow structure covering a 38' × 3' angular range (∼ 3.3 pc × 0.3 pc). Comparing the kinematics with the more extended envelope (∼ 61' × 33') emitting in ¹³CO we find that: (1) the angular velocity of the envelope around the horizontal axis E → W is Ω^EW_CO ≈ -2×10^-14 rad/s (the line-of-sight velocity is more negative to the north); (2) approximately one half of the filament (combined α and β condensations) exhibits counter-rotation with Ω^EW_αβ ≈ 2×10^-14 rad/s; (3) one third of the filament (the γ condensation) co-rotates with Ω^EW_γ ≈ -2×10^-14 rad/s; (4) the central part of the filament between these two kinematically distinct regions does not show any rotation around this axis; (5) the whole filament revolves slowly around the vertical axis S → N with Ω^SN_tot ≈ 7×10^-15 rad/s. The opposite chirality (dextral and sinistral) of the αβ and γ condensations indicates magnetic field helicities of two types, negative and positive, caused, most probably, by dynamo mechanisms. We estimate the magnetic Reynolds number R_m > 600 and the Rossby number ℜ < 1, which means that dynamo action is important.

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