Issue A&A Volume 440, Number 3, September IV 2005 Page(s) 909 - 919 Section Interstellar and circumstellar matter DOI http://dx.doi.org/10.1051/0004-6361:20041733

A&A 440, 909-919 (2005)
DOI: 10.1051/0004-6361:20041733

Velocity field and star formation in the Horsehead nebula

P. Hily-Blant^{1, 2}, D. Teyssier^{3, 4}, S. Philipp⁵ and R. Güsten<sup>5

1</sup>  LRA-LERMA, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
²  Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
    e-mail: hilyblan@iram.fr
³  Space Research Organization Netherlands, PO Box 800, 9700 AV Groningen, The Netherlands
⁴  Departamento de Astrofisica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
⁵  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 26 July 2004 / Accepted 4 May 2005 )

Abstract
Using large scale maps in C¹⁸O(2-1) and in the continuum at 1.2 mm obtained at the IRAM-30 m antenna with the Heterodyne Receiver Array (HERA) and MAMBO2, we investigated the morphology and the velocity field probed in the inner layers of the Horsehead nebula. The data reveal a non-self-gravitating ( ) filament of dust and gas (the "neck", = 0.15-0.30 pc) connecting the Horsehead western ridge, a Photon-Dominated Region illuminated by Ori, to its parental cloud L1630. Several dense cores are embedded in the ridge and the neck. One of these cores appears particularly peaked in the 1.2 mm continuum map and corresponds to a feature seen in absorption on ISO maps around 7 m. Its C¹⁸O emission drops at the continuum peak, suggestive of molecular depletion onto cold grains. The channel maps of the Horsehead exhibit an overall north-east velocity gradient whose orientation swivels east-west, showing a somewhat more complex structure than was recently reported by Pound et al. (2003) using BIMA CO(1-0) mapping. In both the neck and the western ridge, the material is rotating around an axis extending from the PDR to L1630 (angular velocity =1.5-4.0 km s^-1). Moreover, velocity gradients along the filament appear to change sign regularly (3 km s^-1 pc^-1, period = 0.30 pc) at the locations of embedded integrated intensity peaks. The nodes of this oscillation are at the same velocity. Similar transverse cuts across the filament show a sharp variation of the angular velocity in the area of the main dense core. The data also suggest that differential rotation is occurring in parts of the filament. We present a new scenario for the formation and evolution of the nebula and discuss dense core formation inside the filament.

Key words: ISM: clouds -- ISM: kinematics and dynamics -- ISM: individual objects: Horsehead nebula -- stars: formation -- radio lines: ISM

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