Extended warm gas in Orion KL as probed by methyl cyanide^⋆

¹ Centro de Astrobiología (CSIC-INTA), Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: tab@cab.inta-csic.es
² Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁴ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències, Torre C-5 parell 2, 08193 Bellaterra, Spain

Received: 10 May 2013
Accepted: 6 February 2014

Abstract

In order to study the temperature distribution of the extended gas within the Orion Kleinmann-Low nebula, we have mapped the emission by methyl cyanide (CH₃CN) in its J = 6_K−5_K, J = 12_K−11_K, J = 13_K−12_K, and J = 14_K−13_K transitions at an average angular resolution of ~10 arcsec (22 arcsec for the 6_K−5_K lines), as part of a new 2D line survey of this region using the IRAM 30 m telescope. These fully sampled maps show extended emission from warm gas to the northeast of IRc2 and the distinct kinematic signatures of the hot core and compact ridge source components. We have constructed population diagrams for the four sets of K-ladder emission lines at each position in the maps and have derived rotational excitation temperatures and total beam-averaged column densities from the fitted slopes. In addition, we have fitted LVG model spectra to the observations to determine best-fit physical parameters at each map position, yielding the distribution of kinetic temperatures across the region. The resulting temperature maps reveal a region of hot (T> 350 K) material surrounding the north-eastern edge of the hot core, whereas the column density distribution is more uniform and peaks near the position of IRc2. We attribute this region of hot gas to shock heating caused by the impact of outflowing material from active star formation in the region, as indicated by the presence of broad CH₃CN lines. This scenario is consistent with predictions from C-shock chemical models that suggest that gas-phase methyl cyanide survives in the post-shock gas and can be somewhat enhanced due to sputtering of grain mantles in the passing shock front.