In:
The Astrophysical Journal, American Astronomical Society, Vol. 926, No. 2 ( 2022-02-01), p. 190-
Abstract:
We compare observations of H i from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO + from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse ( A V ≲ 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO + and H i absorption. These structures fall into two categories. Structures with T s 〈 40 K, mostly with N (HCO + ) ≲ 10 12 cm −2 , are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature T s 〉 40 K, mostly with N (HCO + ) ≳ 10 12 cm −2 , are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable H i . In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO + in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO + formation occurs along sight lines with N (H I) ≳ 10 21 cm −2 , the simulated data fail to explain HCO + column densities ≳ few × 10 12 cm −2 . Because a majority of our sight lines with HCO + had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight.
Type of Medium:
Online Resource
ISSN:
0004-637X
,
1538-4357
DOI:
10.3847/1538-4357/ac4160
Language:
Unknown
Publisher:
American Astronomical Society
Publication Date:
2022
detail.hit.zdb_id:
2207648-7
detail.hit.zdb_id:
1473835-1
SSG:
16,12
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