A rotating helical filament in the L1251 dark cloud

S.A. Levshakov1,2,3, D. Reimers2, C. Henkel4,5

1Ioffe Institute, Politekhnicheskaya 26, St. Petersburg 194021, Russia
2Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029, Hamburg, Germany
3St. Petersburg Electrotechnical University "LETI", Prof. Popov Str. 5, 197376, St. Petersburg, Russia
4Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
5Astronomy Department, King Abdulaziz University, PO Box 80203, 21589, Jeddah, Saudi Arabia

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 ΩEWCO ≈ -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 ΩEWαβ ≈ 2×10-14 rad/s; (3) one third of the filament (the γ condensation) co-rotates with ΩEWγ ≈ -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 ΩSNtot ≈ 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 Rm > 600 and the Rossby number ℜ < 1, which means that dynamo action is important.

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