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

Abstract

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H 2 CO (J KaKc = 3 03 -2 02 , 3 22 -2 21 , and 3 21 -2 20 ) using the APEX 12 m telescope. This is compared with the temperatures derived from the ratio of the NH 3 (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-H 2 CO 3 22 -2 21 /3 03 -2 02 and 3 21 -2 20 /3 03 -2 02 . The gas kinetic temperatures derived from the para-H 2 CO line ratios are warm, ranging from 30 to >200 K with an average of 62 ± 2 K at a spatial density of 10 5 cm −3 . These temperatures are higher than those obtained from NH 3 (2,2)/(1,1) and CH 3 CCH (6-5) in the OMC-1 region. The gas kinetic temperatures derived from para-H 2 CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H 2 CO (3-2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH 3 (2,2)/(1,1) and the CH 3 CCH (6-5) line series. With dust at MIR wavelengths and para-H 2 CO (3-2) on one side and dust at FIR wavelengths, NH 3 (2,2)/(1,1), and CH 3 CCH (6-5) on the other, dust and gas temperatures appear to be equivalent in the dense gas (n(H 2 ) 10 4 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-H 2 CO 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-H 2 CO is related to turbulence on a ∼0.06 pc scale. Combining the temperature measurements with para-H 2 CO and NH 3 (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.

ATLASGAL-selected massive clumps in the inner Galaxy

“…The obtained T turb values are slightly lower than the averaged gas kinetic temperature (T kin ∼ 91 K) derived from the para-H 2 CO line ratios. This indicates that turbulent heating significantly contributes to gas temperature in these massive clumps on scales of ∼0.1-1.8 pc, which agrees with previous observational results with H 2 CO in the Orion molecular cloud 1 (OMC-1; Tang et al 2017c). Apparently, turbulent heating plays an important role in heating the dense gas in massive star-forming clumps (Pan & Padoan 2009).…”

“…Apparently dense gas traced by H 2 CO is associated well with the dust traced by 870 µm emission in the massive star-forming clumps. Mapping observations of para-H 2 CO (3 03 -2 02 , 3 22 -2 21 , and 3 21 -2 20 ) towards the Orion molecular cloud 1 (OMC1) with the APEX telescope also show that para-H 2 CO integrated intensity distributions agree well with the dust emission observed at 850 µm (Johnstone & Bally 1999;Tang et al 2017c). Previous observations of H 2 CO (4 04 -3 03 , 4 23 -3 22 , 4 22 -3 21 , 4 32 -3 31 , and 4 31 -3 30 ) towards massive clumps in the W33 region with the APEX telescope (Immer et al 2014) also indicate that H 2 CO distributions are consistent with the dust emission traced by 870 µm.…”

“…Wouterloot et al 1988;Molinari et al 1996;Jijina et al 1999;Wu et al 2006;Urquhart et al 2011Urquhart et al , 2015Wienen et al 2012;Lu et al 2014;Immer et al 2016;Tang et al 2017c) suggest that the line width is correlated with kinetic temperature. It is suggested that the correlation between kinetic temperature and line width is due to a conversion of turbulent energy into heat in the Galactic central clouds (e.g.…”

“…The straight lines are results from unweighed linear fits. Urquhart et al 2011Urquhart et al , 2015Wienen et al 2012;Lu et al 2014;Tang et al 2017c). The gas is heated by turbulent energy according to the approximate relation T kin ∝ ∆v 0.8−1.0 (gas kinetic temperature measured with NH 3 and H 2 CO) in molecular clouds of the Galactic centre (Güsten et al 1985;Mauersberger et al 1987;Immer et al 2016), which is consistent with our result (only in terms of slope, not of intercept and absolute value).…”

“…Following the method applied by Tang et al (2017c) in their Equation (2), and the cloud size L is in units of pc; we determined the gas kinetic temperature caused by turbulent energy. We computed the gas kinetic temperature assuming turbulent heating dominates the heating process.…”