Odin CO and $\mathsf{^{13}}$CO J = 5–4 mapping of Orion KL – a step towards accurate water abundances

Abstract

Aims. The very high main beam efficiency (90%) of the telescope on the sub-millimetre wave satellite Odin, in combination with the small calibration errors in the absence of atmospheric attenuation, assures that observed line brightness temperatures are very accurately determined. Based on this, we attempt to determine the column density distribution of H 2 , and the ortho-water abundance, in the Orion KL region. Methods. We have, for the first time, mapped the 12 CO J = 5−4 emission in a 7 × 7 region covering Orion KL, observed simultaneously with a 13 CO J = 5−4 map. Also presented are C 18 O J = 5−4 emission data at four different positions and a C 17 O J = 5−4 emission spectrum detected towards the Orion KL position. The Odin mapping was performed at 1 spacing (beam full width at half maximum 126 at 557 GHz). Results. The CO J = 5−4 narrow line emission from this region mainly arises in the warm, dense gas at the interface (the photondominated region) between the M 42 H ii region and the Orion A molecular cloud, the Orion PDR. The 12 CO and 13 CO J = 5−4 emission maps have been used to determine the column density distribution of H 2 gas across the Orion KL region. The results have been verified by comparing to column densities obtained using the decidedly optically thin C 18 O emission as input to the RADEX radiative transfer code. We find H 2 column densities ranging from 5 × 10 21 cm −2 at map edges to 7× 10 22 cm −2 at the molecular ridge. The mass of the gas in the mapped region is estimated to be 480 M , of which 320 M is situated towards the molecular ridge. We estimate that about half of this mass belongs to the warm Orion PDR interface layer. Finally, based on data from the positions where C 18 O J = 5−4 has been observed, we estimate the ortho-water abundance in the Orion PDR layer to be ≥8 × 10 −8 , higher than previously estimated.

The APEX-CHAMP+view of the Orion Molecular Cloud 1 core

“…Therefore, the dense gas mass is ∼140 M in an area of 0.13 pc 2 in the OMC-1 core. The H 2 column density agrees with the density of 5×10 21 −7×10 22 cm −2 derived from the mid-J CO isotopologue observations by Wirström et al (2006). Our results agree with the recent measurement by Wilson et al (2011) using the 13 CO J = 6−5 line with a lower [ 12 CO]/[H 2 ] ratio of 2 × 10 −5 .…”

“…If we include the warm diffuse gas (10 4 cm −3 ) in the same volume, the total warm gas mass is 86−285 M in the OMC-1 core. This mass estimate for the OMC-1 core is close to that derived from the Odin 12 CO and 13 CO J = 5−4 observations (Wirström et al 2006), i.e., 320 M in the molecular ridge for a farther distance of 500 pc. The result of Wilson et al (2001) also suggests a similar estimate of a warm gas mass of 310−430 M from the observations of 12 CO J = 7−6 and J = 4−3, again for a distance of 500 pc.…”

A spectral line survey of Orion KL in the bands 486-492 and 541-577 GHz with the Odin satellite

“…The range of temperatures obtained from inversion transitions of NH 3 is 165−400 K (Wilson et al 2000). -Photo Dissociation Region (PDR): the extended interface region between the molecular cloud and the foreground M42 Hii region (Rodríguez-Franco et al 1998Wirström et al 2006, and references therein) at velocities 8−10 km s −1 .…”

A spectral line survey of Orion KL in the bands 486–492 and 541–577 GHz with the Odin satellite

“…By employing the LTE assumption with full beamfilling, an excitation temperature of 100 K, and a dipole moment of 0.273 D (adopted from the ab initio calculations of Senekowitsch et al 1985), we arrive at a figure of N SH − = 2 × 10 13 cm −2 . The scenario used would be consistent with SH − residing in the extended OMC-1/M42 face-on PDR (discussed in Wirström et al 2006). Should the origin of the line (if real) be any of the smaller Orion KL components -as indeed the observed line width (∆v FWHM = 11 km s −1 ) seems to indicate -the column density rises by a factor of ∼200.…”

“…A very useful review has been written by Genzel & Stutzki (1989); for reference updates see e.g., Olofsson et al (2003), and Wirström et al (2006). Here we summarise some source component designations and dynamical properties particularly relevant to the molecular line identification work in the current presentation of our Odin spectral scan data, which includes our molecular line assignments (in the Online Table 6).…”