Bibliographic Details
| Title: |
GMF G214.5-1.8 as traced by CO: I – cloud-scale CO freeze-out as a result of a low cosmic-ray ionization rate. |
| Authors: |
Clarke, S D1 (AUTHOR) sclarke@asiaa.sinica.edu.tw, Makeev, V A2,3 (AUTHOR), Sánchez-Monge, Á4,5 (AUTHOR), Williams, G M6,7 (AUTHOR), Tang, Y -W1 (AUTHOR), Walch, S8 (AUTHOR), Higgins, R8 (AUTHOR), Nürnberger, P C8 (AUTHOR), Suri, S9 (AUTHOR) |
| Source: |
Monthly Notices of the Royal Astronomical Society. Feb2024, Vol. 528 Issue 2, p1555-1572. 18p. |
| Subject Terms: |
*CO-combustion, *STAR formation, *COSMIC rays, *SPINE, *TIME management |
| Abstract: |
We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5-1.8 (G214.5) using IRAM 30m data of 12CO, 13CO, and C18O. We find that the 12CO (1-0) and (2-1) derived excitation temperatures are near identical and are very low, with a median of 8.2 K, showing that the gas is extremely cold across the whole cloud. Investigating the abundance of 13CO across G214.5, we find that there is a significantly lower abundance along the entire 13 pc spine of the filament, extending out to a radius of ∼0.8 pc, corresponding to Av ≳ 2 mag and Tdust ≲ 13.5 K. Due to this, we attribute the decrease in abundance to CO freeze-out, making G214.5 the largest scale example of freeze-out yet. We construct an axisymmetric model of G214.5's 13CO volume density considering freeze-out and find that to reproduce the observed profile significant depletion is required beginning at low volume densities, n ≳ 2000 cm−3. Freeze-out at this low number density is possible only if the cosmic-ray ionization rate is ∼1.9 × 10−18 s−1, an order of magnitude below the typical value. Using time scale arguments, we posit that such a low ionization rate may lead to ambipolar diffusion being an important physical process along G214.5's entire spine. We suggest that if low cosmic-ray ionization rates are more common in the outer Galaxy, and other quiescent regions, cloud-scale CO freeze-out occurring at low column and number densities may also be more prevalent, having consequences for CO observations and their interpretation. [ABSTRACT FROM AUTHOR] |
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