Nowadays there are several observational studies about the ^ { 13 } CO/C ^ { 18 } O abundance ratio ( X ^ { 13 / 18 } ) towards nearby molecular clouds .
These works give observational support to the C ^ { 18 } O 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 HII regions and OB associations to study the selective photodissociation .
Using ^ { 12 } CO , ^ { 13 } CO , and C ^ { 18 } O J=1–0 data obtained from the FOREST unbiased Galactic plane imaging survey performed with the Nobeyama 45 m telescope , we analyze the filamentary infrared dark cloud IRDC 34.43 + 0.24 located at the distance of about 3.9 kpc .
This IRDC is related to several HII regions and young stellar objects .
Assuming local thermodynamic equilibrium we obtain : 0.8 \times 10 ^ { 16 } < N ( ^ { 13 } CO ) < 4 \times 10 ^ { 17 } cm ^ { -2 } ( average value = 4.2 \times 10 ^ { 16 } cm ^ { -2 } ) , 0.6 \times 10 ^ { 15 } < N ( C ^ { 18 } O ) < 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 HII regions associated with the N61 and N62 bubbles and with the photodissociation regions ( PDRs ) , precisely the regions in which FUV photons are strongly interacting with the molecular gas .
Our result represents an observational support to the C ^ { 18 } O 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 _ { d } ) and H _ { 2 } column density ( N ( H _ { 2 } ) ) distribution was studied .
Using the average of N ( H _ { 2 } ) , values in close agreement with the ‘ canonical ’ abundance ratios [ H _ { 2 } ] / [ ^ { 13 } CO ] and [ H _ { 2 } ] / [ C ^ { 18 } O ] 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 far ultraviolet 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 .