Submillimeter imaging spectroscopy of the Horsehead nebula

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Ism: Individual: Objects: Horsehead Nebula, Ism: Molecules, Ism: Structure, Ism: Abundances, Ism: Clouds, Stars: Formation

Scientific paper

We present 15 arcsecond resolution single-dish imaging of the Horsehead nebula in the Ci (1-0) and CO (4-3) lines, carried out using the CHAMP array at the Caltech Submillimeter Observatory (CSO). The data are used together with supporting observations of the (2-1) transitions of the CO isotopologues to determine the physical conditions in the atomic and molecular gas via Photon Dominated Region (PDR) modeling. The CO (4-3)/(2-1) line ratio, which is an excellent tracer of the direction of the incoming UV photons, increases at the western and northern edges of the nebula, confirming that the illumination is provided mostly by the stars σ and ζ Orionis. The observed line intensities are consistent with PDR models with an H nuclei volume density of ˜ 3{ -} 7 × 104 cm-3. The models predict a kinetic temperature of 12 K and a C18O fractional abundance with respect to H atoms of 2.4 × 10-7 in the shielded region, which in turn imply a total molecular mass of 24 M&sun; in the C18O filament. The outer halo, devoid of C18O, but traced by the Ci emission has a comparable density and contributes additional 13 M&sun; of material, resulting in an upper limit of 37 M&sun; for the total molecular mass of the nebula.

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