Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

III. The Orion molecular cloud 1^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: xdtang@mpifr-bonn.mpg.de
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
³ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁴ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁵ Department of Astronomy, Nanjing University, 210093 Nanjing, PR China
⁶ Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China

Received: 28 August 2017
Accepted: 22 September 2017

Abstract

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H₂CO (J_KaKc = 3₀₃–2₀₂, 3₂₂–2₂₁, and 3₂₁–2₂₀) using the APEX 12 m telescope. This is compared with the temperatures derived from the ratio of the NH₃ (2, 2)/(1, 1) inversion lines and the dust emission. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured averaged line ratios of para-H₂CO 3₂₂–2₂₁/3₀₃–2₀₂ and 3₂₁–2₂₀/3₀₃–2₀₂. The gas kinetic temperatures derived from the para-H₂CO line ratios are warm, ranging from 30 to >200 K with an average of 62 ± 2 K at a spatial density of 10⁵ cm^-3. These temperatures are higher than those obtained from NH₃ (2, 2)/(1, 1) and CH₃CCH (6–5) in the OMC-1 region. The gas kinetic temperatures derived from para-H₂CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H₂CO (3–2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH₃ (2, 2)/(1, 1) and the CH₃CCH (6–5) line series. With dust at MIR wavelengths and para-H₂CO (3–2) on one side, and dust at FIR wavelengths, NH₃ (2, 2)/(1, 1), and CH₃CCH (6–5) on the other, dust and gas temperatures appear to be equivalent in the dense gas (n(H₂) ≳ 10⁴ cm^-3) of the OMC-1 region, but provide a bimodal distribution, one more directly related to star formation than the other. The non-thermal velocity dispersions of para-H₂CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number ≳ 2.5) of the OMC-1, implying that the higher temperature traced by para-H₂CO is related to turbulence on a ~0.06 pc scale. Combining the temperature measurements with para-H₂CO and NH₃ (2, 2)/(1, 1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10 km s^-1 filament heated by radiation from the central Orion nebula.