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 } \approx 0.3 ) filament of dust and gas ( the “ neck ” , \varnothing = 0.15 - 0.30 { pc } ) connecting the Horsehead western ridge , a Photon-Dominated Region illuminated by \sigma 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 \mu m. 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 ( ) 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 .