Abstract
We have examined methanol emission from Orion KL with of the {\em Herschel}/HIFI instrument, and detected two methanol bands centered at 524 GHz and 1061 GHz. The 524 GHz methanol band (observed in HIFI band 1a) is dominated by the isolated $\Delta$J$=$0, K$=-4\rightarrow$-3, v$_t$$=$0 Q branch, and includes 25 E-type and 2 A-type transitions. The 1061 GHz methanol band (observed in HIFI band 4b) is dominated by the $\Delta$J$=$0, K$=7\rightarrow$6, v$_t$$=$0 Q branch transitions which are mostly blended. We have used the isolated E-type v$_t$$=$0 methanol transitions to explore the physical conditions in the molecular gas. With HIFI's high velocity resolution, the methanol emission contributed by different spatial components along the line of sight toward Orion KL (hot core, low velocity flow, and compact ridge) can be distinguished and studied separately. The isolated transitions detected in these bands cover a broad energy range (upper state energy ranging from 80 K to 900 K), which provides a unique probe of the thermal structure in each spatial component. The observations further show that the compact ridge is externally heated. These observations demonstrate the power of methanol lines as probes of the physical conditions in warm regions in close proximity to young stars.
Highlights
Methanol (CH3OH) is an abundant molecule in a wide range of interstellar conditions (e.g., Friberg et al 1988; Menten et al 1988)
We look for groups of isolated methanol transitions which are confined within small frequency range in the spectrum but which cover a wide range in upper state energy with minimal line blending
We have examined the methanol emission in HIFI bands 1a and 4b from our spectral scan observations toward Orion KL
Summary
Methanol (CH3OH) is an abundant molecule in a wide range of interstellar conditions (e.g., Friberg et al 1988; Menten et al 1988). An asymmetric-top rotor, methanol has numerous rotational transitions from far-infrared to millimeter wavelengths. Leurini et al (2004) further explore the properties of methanol emission in dense molecular clouds and illustrate how methanol transitions at sub-millimeter and millimeter wavelengths are sensitive to the density and the kinetic temperature of the gas. Orion KL, located within the Orion molecular cloud at ∼450 pc, is the nearest massive star-forming region (Genzel & Stutzki 1989). It contains several kinematic components (e.g., Blake et al 1987; Persson et al 2007), a hot core, multiple outflows, and the compact ridge, surrounding by cold quiescent gas.
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