The column density structure of Orion A depicted by N-PDF

Abstract

We have conducted a large-field simultaneous survey of 12 CO, 13 CO, and C 18 O J = 1 − 0 emission toward the Orion A giant molecular cloud (GMC) with a sky coverage of ∼ 4.4 deg 2 using the PMO-13.7 m millimeter-wavelength telescope. We use the probability distribution function of the column density (N-PDF) to investigate the distribution of molecular hydrogen in the Orion A GMC. The H 2 column density, derived from the 13 CO emission, of the GMC is dominated by log-normal distribution in the range from ∼4×10 21 to ∼1.5×10 23 cm −2 with excesses both at the low-density and high-density ends. The excess of the low-density end is possibly caused by an extended and low-temperature (∼10 K) component with velocities in the range of 5−8 km s −1 . Compared with the northern sub-regions, the southern sub-regions of the Orion A GMC contain less gas with column density in N H2 > 1.25 × 10 22 cm −2 . The dispersions of the N-PDFs of the sub-regions are found to correlate with the evolutionary stages of the clouds across the Orion A GMC. The structure hierarchy of Orion A GMC is explored with the DENDROGRAM algorithm, and it is found that the GMC is composed of two branches. All structures except one in the tree have virial parameters less than 2, indicating self-gravity is important on the spatial scales from ∼0.3 to ∼4 pc. Although power-laws and departures from log-normal distributions are found at the high-density end of N-PDFs of active star-forming regions, the N-PDFs of structures in the Orion A GMC are predominantly log-normal on scales from R∼0.4 to 4 pc.

Symmetry properties and widths of the filamentary structures in the Orion A giant molecular cloud

“…In this work, the calculation process for the H 2 column density is the same as Ma et al (2020). For convenience and clarity, we describe the method and the used formulae as follows.…”

“…Firstly, we calculate the excitation temperature T ex from the peak brightness temperature of the 12 CO line (e.g., Pineda et al 2010;Li et al 2018;Ma et al 2020),…”

“…where T peak is the peak brightness temperature of the 12 CO emission line. Secondly, the optical depth of the 13 CO emission can be derived from the excitation temperature T ex and the brightness temperature, T MB ( 13 CO), of the 13 CO emission line (e.g., Pineda et al 2010;Li et al 2018;Ma et al 2020) according to the following fomula,…”

“…where J(T ex ) = [e 5.29/Tex − 1] −1 . Then, the 13 CO column density can be calculated through (e.g., Pineda et al 2010;Li et al 2018;Ma et al 2020) N13 CO = 2.42 × 10 14 T ex + 0.88 1 − e −5.29/Tex τ 13 v dv.…”

“…The northern part of the Orion A GMC is exposed to the UV radiation from the Trapezium stars, which are ∼1 pc in front of the GMC (e.g., van der Werf et al 2013;Suri et al 2019). Stutz & Kainulainen (2015) and Ma et al (2020) investigated the column density structure of molecular hydrogen in the Orion A GMC using the probability distribution function (PDF) method. They found that the Orion A GMC exhibits an evolutionary trend in terms of star formation along the main ridge of the GMC from north to south, and the whole GMC is composed of two giant filaments.…”