Spectroscopy of the Orion Molecular Cloud Core

Abstract

Recent infrared and radio spectroscopic data pertaining to the Orion BN-KL infrared cluster are reviewed. A new, high resolution CO map shows that the thermal structure over the central 10′(1.5 pc) in the Orion molecular cloud is dominated by energy sources in the infrared cluster and M42. Peak CO brightness temperatures of 90 K occur on KL and near the bar at the southern edge of M42. Within the central 45″ of the infrared cluster, both radio and IR data reveal a highly energetic environment. Millimeter lines of several molecules (e.g. CO, HCN, and SiO) show emission over a full velocity range of 100 km s^(−1). These supersonic flows can be modeled as a differentially expanding envelope containing a total of ~5 M⊙ of gas within an outer radius of r ≃ 1.3 × 10^(17) cm. Over the same area emission is seen from vibrationally excited molecular hydrogen at an excitation temperature of 2000 K. The high velocity mm-line emission and the NIR H_2 lines are clearly related since they exhibit similar spatial extents and line widths. Comparison of the total cooling rate for all the H_2 lines with the estimated kinetic energy and expansion time for the mm-emission region indicates that the H_2 emission probably arises from shock fronts where the expanding envelope impinges on the outer cloud. Near IR spectroscopy also probes ionized and neutral gas closely associated with BN. Br α and Br γ emission is detected from an ultracompact HII region of mass M_(HII) ≲ 10^(−4) M⊙. Full widths for the HII lines are ~400 km s^(−1). CO bandhead emission detected in BN at λ ≃ 2.3 μm is probably collisionally pumped in a high excitation zone (n_(H+H2) > 10^(10) cm^(−3) and T_K ≃ 3000 K) at only a few AU from the star. The velocity of both the HII and CO emission is V_(LSR) ≃ + 20 km s^(−1); thus BN appears to be redshifted by 11 km s^(−1) with respect to OMC-1.

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