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Abstract We investigate the conditions for the Hi-to-H2transition in the solar neighborhood by analyzing Hiemission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with12CO(1–0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases, and show that the observed LOSs are mostly Hi-dominated. In addition, we find that the CO-dark H2, not the optically thick Hi, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between Hiand CO, and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature < 200 K, peak optical depth > 0.1, CNM fraction of ∼0.6, andV-band dust extinction > 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H2formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the Hi-to-H2transition.
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A New Constraint on the Relative Disorder of Magnetic Fields between Neutral Interstellar Medium Phases
Abstract Utilizing Planck polarized dust emission maps at 353 GHz and large-area maps of the neutral hydrogen (Hi) cold neutral medium (CNM) fraction (fCNM), we investigate the relationship between dust polarization fraction (p353) andfCNMin the diffuse high latitude ( ) sky. We find that the correlation betweenp353andfCNMis qualitatively distinct from thep353–Hicolumn density (NHi) relationship. At low column densities (NHi< 4 × 1020cm−2) wherep353andNHiare uncorrelated, there is a strong positivep353–fCNMcorrelation. We fit thep353–fCNMcorrelation with data-driven models to constrain the degree of magnetic field disorder between phases along the line of sight. We argue that an increased magnetic field disorder in the warm neutral medium (WNM) relative to the CNM best explains the positivep353–fCNMcorrelation in diffuse regions. Modeling the CNM-associated dust column as being maximally polarized, with a polarization fractionpCNM∼ 0.2, we find that the best-fit mean polarization fraction in the WNM-associated dust column is 0.22pCNM. The model further suggests that a significantfCNM-correlated fraction of the non-CNM column (an additional 18.4% of the Himass on average) is also more magnetically ordered, and we speculate that the additional column is associated with the unstable medium. Our results constitute a new large-area constraint on the average relative disorder of magnetic fields between the neutral phases of the interstellar medium, and are consistent with the physical picture of a more magnetically aligned CNM column forming out of a disordered WNM.
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- Award ID(s):
- 2106607
- PAR ID:
- 10536220
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 972
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 66
- Size(s):
- Article No. 66
- Sponsoring Org:
- National Science Foundation
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