Nobeyama 45 m mapping observations toward Orion A. II. Classification of cloud structures and variation of the 13CO/C18O abundance ratio due to far-UV radiation

Abstract

We present results of the classification of cloud structures toward the Orion A Giant Molecular Cloud based on wide-field 12 CO (J = 1-0), 13 CO (J = 1-0), and C 18 O (J = 1-0) observations using the Nobeyama 45 m radio telescope. We identified 78 clouds toward Orion A by applying Spectral Clustering for Interstellar Molecular Emission Segmentation (SCIMES) to the data cube of the column density of 13 CO. Well-known subregions such as OMC-1, OMC-2/3, OMC-4, OMC-5, NGC 1977, L1641-N, and the dark lane south filament (DLSF) are naturally identified as distinct structures in Orion A. These clouds can also be classified into three groups: the integral-shaped filament, the southern regions of Orion A, and the other filamentary structures in the outer parts of Orion A and the DLSF. These groups show differences in scaling relations between the physical properties of the clouds. We derived the abundance ratio between 13 CO and C 18 O, X13 CO /X C 18 O , which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of X13 CO /X C 18 O is also seen within a cloud in both of the spatial and velocity directions and the ratio tends to be high at the edge of the cloud. The values of X13 CO /X C 18 O decrease from 17 to 10 with the median of the column densities of the clouds at the column density of N C 18 O > ∼ 1 × 10 15 cm −2 or visual extinction of A V > ∼ 3 mag under the strong far-ultraviolet (FUV) environment of G 0 > 10 3 , whereas it is almost independent of the column density in the weak FUV radiation field. These results are explained if the selective photodissociation of C 18 O is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.

Isotopic fractionation study towards massive star-forming regions across the Galaxy

“…proposed that FUV radiation can photodissociate HNC more than HCN (photodissociation cross sections larger for HNC), leading to higher HCN/HNC ratios for stronger FUV. However, Nakamura et al (2019) found a HCN/HNC ratio lower towards the region VLA 13 (a protostar nearby FIR4) than in the N 2 H + clump. Thus, since VLA 13 shows a FUV radiation strength 140 times larger than the N 2 H + clump of FIR4 (Ishii et al 2019), they concluded that the effect of a temperature higher than 24 K is the main responsible for the observed HCN/HNC ratios.…”

A Census of Protostellar Outflows in Nearby Molecular Clouds

“…The 12 CO J=1-0 and 13 CO J=1-0 observations of Orion A were carried out from 2007 to 2017 by the Nobeyama Radio Observatory 45 m telescope (NRO 45m), using two different receivers (Shimajiri et al 2015a,b;Ishii et al 2019;Nakamura et al 2019). The two maps were calibrated to the same intensity scale and combined on a common grid with a pixel scale of 7 .5, which corresponds to an effective angular resolution of 22 .…”

“…The two maps were calibrated to the same intensity scale and combined on a common grid with a pixel scale of 7 .5, which corresponds to an effective angular resolution of 22 . Ishii et al (2019) smoothed the combined map to a velocity resolution of 0.22 km/s, and further converted the intensity to the main beam temperature scale that is benchmarked with data from FCRAO and IRAM 30m. The final sensitivity for the 12 CO map is 0.35 K, while the sensitivity for the 13 CO map is 0.40 K. The velocity range of the Orion data spans -2.9 to 19.3 km s −1 .…”

“…The final sensitivity for the 12 CO map is 0.35 K, while the sensitivity for the 13 CO map is 0.40 K. The velocity range of the Orion data spans -2.9 to 19.3 km s −1 . More detailed description about the data can be found in Kong et al (2018) and Ishii et al (2019).…”

Optical Properties of Interstellar Dust around the Orion A Molecular Cloud

“…In order to reveal physical properties of the clouds and interactions between stars and ISM, there have been a number of observations in various wavelengths. Observations in the millimeter-wavelengths toward Orion A revealed the distributions and properties of the interstellar molecular gas leading to star formation (e.g., Nishimura et al 2015;Nakamura et al 2019;Ishii et al 2019), and distributions and proper motions of young stars around Orion A have been widely studied in the optical wavelengths (e.g., Kounkel et al 2018;Zari et al 2019).…”