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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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An early transition to magnetic supercriticality in star formation
Abstract Magnetic fields have an important role in the evolution of interstellar medium and star formation 1,2 . As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas 3 . Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA) 4,5 towards L1544 6,7 —a well-studied prototypical prestellar core in an early transition between starless and protostellar phases 8–10 characterized by a high central number density 11 and a low central temperature 12 . A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes 13,14 .
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- PAR ID:
- 10345815
- Date Published:
- Journal Name:
- Nature
- Volume:
- 601
- Issue:
- 7891
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 49 to 52
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41586-021-04159-x
