ccsd-00005367, version 1 - 26 Jan 2006 Astronomy & Astrophysics manuscript no. hh˙vel˙jun05.hyper19907 January 26, 2006 (DOI: will be inserted by hand later) Velocity field and star formation in the Horsehead nebula P. Hily-Blant 1,2 , D. Teyssier 3,4 , S. Philipp 5 , and R. G ¨ usten 5 1 LRA-LERMA, ´ Ecole normale sup´ erieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris cedex 05, France 2 Institut de Radio Astronomie Millim´ etrique, 300 Rue de la Piscine, F-38406 Saint Martin d’H` eres, France 3 Space Research Organization Netherlands, P.O.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 f¨ ur Radioastronomie, Auf dem H¨ ugel 69, D-53121 Bonn, Germany Received / Accepted Abstract. Using large scale maps in C 18 O(2 − 1) and in the continuum at 1.2mm obtained at the IRAM-30m 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 (m/m vir ≈ 0.3) filament of dust and gas (the “neck”, ∅ = 0.15 − 0.30 pc) connecting the Horsehead western ridge, a Photon-Dominated Region illuminated by σOri, 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 µm. Its C 18 O 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 – kinematics and dynamics – individual objects (Horsehead nebula) – Stars: Formation – Radio lines: ISM 1. Introduction Dense molecular cores are the basic units of isolated low-mass star formation. It is now common that molecular line map- ping from dark clouds reveals clumps embedded in filamentary structures (e.g. Onishi et al. 1998, 1999; Obayashi et al. 1998; Loren 1989a; Chini et al. 1997). These filaments can be ei- ther self-gravitating or not. The clumps are mostly gravitation- ally bound, and in many cases, star formation is known to have already started. Therefore, molecular filaments are thought to play a crucial role in low-mass star formation. However, little is known about the general physical prop- erties of these filaments. For example, the density distribu- tion is very likely not uniform, but instead varies according to a power-law. Stepnik et al. (2003) have investigated this observationally in a filament of the Taurus molecular cloud, and concluded that a r −2 profile was compatible with the ob- servations. Steeper density profiles can however be observed, as is shown by Hily-Blant & Falgarone (in prep) in a non– Send offprint requests to: P. Hily-Blant e-mail: hilyblan@iram.fr self-gravitating filament connected to the low-mass dense core L1512. Theoretical models with and without magnetic fields suggest power-laws with exponents ranging from ≈−2 (Fiege & Pudritz 2000) to −4 (Ostriker 1964; Nakamura et al. 1993; Stod´ olkiewicz 1963). The importance of the velocity field in the formation of clumps in filamentary clouds has already been noted several years ago by Loren (1989b), who did a systematic study of the velocity pattern of well-known filaments in ρOph. The longi- tudinal velocity was shown to exhibit gradients near the loca- tions of the main clumps harboured in the filament. From the theoretical and numerical points of view, recent studies also stress the role of the velocity field. Dealing with self-similar rotating magnetized cylinders, Hennebelle (2003) shows that the velocity field strongly depends on the relative intensity of the toroidal component of the magnetic fields with respect to the poloidal one and to the gravitational force: the rotation can be mainly that of a rigid body or instead be differential. The importance of the velocity field has been further stressed by Tilley & Pudritz (2003) who show how it could help in