A rotating helical filament in the L1251 dark cloud^⋆

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
² Ioffe Physical-Technical Institute, Polytekhnicheskaya Str. 26, 194021 St. Petersburg, Russia
³ St. Petersburg Electrotechnical University LETI, Prof. Popov Str. 5, 197376 St. Petersburg, Russia
e-mail: lev@astro.ioffe.rssi.ru
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia

Received: 18 November 2015
Accepted: 22 December 2015

Abstract

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^′′ and a spectral resolution of 0.045 km s^-1.

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^-1 (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^-1; (3) one third of the filament (the γ condensation) co-rotates with Ω^EW_γ ≈−2 × 10^-14 rad s^-1; (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^-1.

The opposite chirality (dextral and sinistral) of the αβ and γ 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 R_m ≳ 600 and the Rossby number ℛ < 1, which means that dynamo action is important.