Study of the ¹³CO/C¹⁸O abundance ratio towards the filamentary infrared dark cloud IRDC 34.43 + 0.24

Abstract

Nowadays, there are several observational studies about the 13CO/C18O abundance ratio ( $X^{13/18}$ ) towards nearby molecular clouds. These works give observational support to the C18O selective photodissociation due to the interaction between the far ultraviolet (FUV) radiation and the molecular gas. It is necessary to increase the sample of molecular clouds located at different distances and affected in different ways by nearby or embedded H ii regions and OB associations to study the selective photodissociation. Using 12CO, 13CO, and C18O J = 1–0 data obtained from the FOREST unbiased Galactic plane imaging survey performed with the Nobeyama 45-m telescope, we analyse the filamentary infrared dark cloud IRDC $34.43+0.24$ located at the distance of about 3.9 kpc. This infrared dark cloud (IRDC) is related to several H ii regions and young stellar objects. Assuming local thermodynamic equilibrium, we obtain: $0.8 \times 10^{16} <$ N(13CO) $<4 \times 10^{17}$ cm–2 (average value $= 4.2 \times 10^{16}$ cm–2), $0.6 \times 10^{15} <$ N(C18O) $<4.4 \times 10^{16}$ cm–2 (average value $= 5.0 \times 10^{15}$ cm–2), and 3 $<$ $X^{13/18}$ $<$ 30 (average $= 8$ ) across the whole IRDC. Larger values of $X^{13/18}$ were found towards portions of the cloud related to the H ii regions associated with the N61 and N62 bubbles and with the photodissociation regions, precisely the regions in which FUV photons are strongly interacting with the molecular gas. Our result represents an observational support to the C18O selectively photodissociation phenomenon occurring in a quite distant filamentary IRDC. Additionally, based on IR data from the Hi-GAL survey, the FUV radiation field was estimated in Habing units, and the dust temperature (T $_{dust}$ ) and H2 column density (N(H2)) distribution were studied. Using the average of N(H2), values in close agreement with the ‘canonical’ abundance ratios [H2]/[13CO] and [H2]/[C18O] were derived. However, the obtained ranges in the abundance ratios show that if an accurate analysis of the molecular gas is required, the use of the ‘canonical’ values may introduce some bias. Thus, it is important to consider how the gas is irradiated by the FUV photons across the molecular cloud. The analysis of $X^{13/18}$ is a good tool to perform that. Effects of beam dilution and clumpiness were studied.

Synthetic Next Generation Very Large Array line observations of a massive star-forming cloud

“…Values 3 − 10 have been reported in dark clouds and high-mass star-forming regions alike (Paron et al 2018;Areal et al 2018;Roueff et al 2021). In our simulations, the ratio is [ 13 CO]/[C 18 O]=6.67, which is close to the average values found in IRDCs (Du & Yang 2008;Areal et al 2019). Pirogov et al (1995) reported an average value [HCO + ]/[H 13 CO + ]=29 for a sample of dense clouds.…”

High-resolution APEX/LAsMA 12CO and 13CO (3–2) observation of the G333 giant molecular cloud complex

“…T bg = 2.73 is the background temperature, and T mb dv represents the integrated intensity. In the above formulas, the correction for high optical depth was applied (Frerking et al 1982;Goldsmith et al 2008;Areal et al 2019). Assuming the 12 CO emission to be optically thick, we can estimate the excitation temperature T ex following the formula (Garden et al 1991;Pineda et al 2008)…”

Feedback from protoclusters does not significantly change the kinematic properties of the embedded dense gas structures

“…T bg = 2.73 K is the background temperature and T mb dv represents the integrated intensity. In the above formulas, the correction for high optical depth was applied (Frerking et al 1982;Goldsmith et al 2008;Areal et al 2019). Assuming optically thick emission of HCO + (1-0) emission, we can estimate the excitation temperature, T ex , following the formula (Garden et al 1991;Pineda et al 2008)…”