An early transition to magnetic supercriticality in star formation

Magnetic fields have an important role in the evolution of interstellar medium and star formation1,2. 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 gas3. 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 L15446,7—a well-studied prototypical prestellar core in an early transition between starless and protostellar phases8–10 characterized by a high central number density11 and a low central temperature12. 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 envelopes13,14.

Multi-Scale Coherent Magnetic Field from Atomic Medium to L1544 Molecular Core

Magnetic fields play an important role in the evolution of interstellar medium and star formation. As the only direct tracer of interstellar field strength, credible Zeeman measurements remain sparse due to rather limited number of spectral lines with discernible Zeeman effect, particularly for cold, molecular gas. Here we report the detection of a magnetic field of 3.8 ± 0.3 μG through a new tracer, the HI narrow self-absorption (HINSA), toward the prestellar core L1544 of the Taurus molecular cloud using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). A combined analysis of the Zeeman measurements of quasar HI absorption, HI emission, OH emission, and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope of the L1544. We find that the molecular envelope traced by HINSA is already magnetically supercritical, with a field strength comparable to that in the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, necessary for star formation, thus seems to happen during the transition from the diffuse CNM to the molecular gas traced by HINSA, earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes. The HINSA Zeeman effect opens up a new window on the interstellar magnetic field that is poised for rapid growth in the era of Square Kilometer Array and its precursors.

Macromolecular phasing using diffraction from multiple crystal forms

A phasing algorithm for macromolecular crystallography is proposed that utilizes diffraction data from multiple crystal forms – crystals of the same molecule with different unit-cell packings (different unit-cell parameters or space-group symmetries). The approach is based on the method of iterated projections, starting with no initial phase information. The practicality of the method is demonstrated by simulation using known structures that exist in multiple crystal forms, assuming some information on the molecular envelope and positional relationships between the molecules in the different unit cells. With incorporation of new or existing methods for determination of these parameters, the approach has potential as a method for ab initio phasing.

Depth of the dynamo region and zonal circulation of the molecular layer in Saturn inferred from its equatorially symmetric gravitational field

ABSTRACT The high-precision equatorially symmetric gravitational field of Saturn (the even gravitational coefficients J2, J4, …, J12) measured by the Cassini Grand Finale reflects its internal structure, its non-spherical shape caused by rotation and its strong zonal circulation whose location is controlled by the depth of its dynamo. We construct a four-layer, non-spheroidal (i.e. its shape is irregular) model of Saturn comprised of an inner core, a metallic dynamo region, an outer molecular envelope and a thin transition layer between the metallic and molecular regions. The model produces the even zonal gravitational coefficients that are in agreement with those measured by the Cassini Grand Finale within the error bars. Our Saturnian model reveals that (i) the observed cloud-top winds extending to any depth on cylinders cannot explain the measured coefficients J2, J4, …, J12; (ii) a deep zonal flow confined in the 20 000 km thick molecular layer is required to interpret them; (iii) the profile of the zonal flow – whose direction is sufficiently alternating with several retrograde peaks – significantly differs from that of the surface winds, implying that the observed winds are confined to a shallow layer, do not extend deeply into Saturn and do not contribute to the observed gravity; and (iv) the Saturnian dynamo can substantially affect the structure of its equatorially symmetric gravitational field by stopping the zonal-flow penetration and by changing, because of the boundary condition at the metallic and molecular interface, the distribution of the dynamic density anomalies.

Molecular envelope around the HII region RCW 120

ABSTRACT The H ii region RCW 120 is a well-known object, which is often considered as a target to verify theoretical models of gas and dust dynamics in the interstellar medium. However, the exact geometry of RCW 120 is still a matter of debate. In this work, we analyse observational data on molecular emission in RCW 120 and show that 13CO(2–1) and C18O(2–1) lines are fitted by a 2D model representing a ring-like face-on structure. The changing of the C18O(3–2) line profile from double-peaked to single-peaked from the dense molecular Condensation 1 might be a signature of stalled expansion in this direction. In order to explain a self-absorption dip of the 13CO(2–1) and 13CO(3–2) lines, we suggest that RCW 120 is surrounded by a diffuse molecular cloud, and find confirmation of this cloud on a map of interstellar extinction. Optically thick 13CO(2–1) emission and the infrared 8 $\mu$m PAH band form a neutral envelope of the H ii region resembling a ring, while the envelope breaks into separate clumps on images made with optically thin C18O(2–1) line and far-infrared dust emission.

Direct Phasing of Protein Crystals with Non-Crystallographic Symmetry

An iterative projection algorithm proposed previously for direct phasing of high-solvent-content protein crystals is extended to include non-crystallographic symmetry (NCS) averaging. For proper NCS, when the NCS axis is positioned, the molecular envelope can be automatically rebuilt. For improper NCS, when the NCS axis and the translation vector are known, the molecular envelope can also be automatically reconstructed. Some structures with a solvent content of around 50% could be directly solved using this ab initio phasing method. Trial calculations are described to illustrate the methodology. Real diffraction data are used and the calculated phases are good for automatic model building. The refinement of approximate NCS parameters is discussed.

The phase problem for two-dimensional crystals. II. Simulations

Phasing of diffraction data from two-dimensional crystals using only minimal molecular envelope information is investigated by simulation. Two-dimensional crystals are an attractive target for studying membrane proteins using X-ray free-electron lasers, particularly for dynamic studies at room temperature. Simulations using an iterative projection algorithm show that phasing is feasible with fairly minimal molecular envelope information, supporting recent uniqueness results for this problem [Arnal & Millane (2017). Acta Cryst. A73, 438–448]. The effects of noise and likely requirements for structure determination using X-ray free-electron laser sources are investigated.

The phase problem for two-dimensional crystals. I. Theory

Properties of the phase problem for two-dimensional crystals are examined. This problem is relevant to protein structure determination using diffraction from two-dimensional crystals that has been proposed using new X-ray free-electron laser sources. The problem is shown to be better determined than for conventional three-dimensional crystallography, but there are still a large number of solutions in the absence of additionala prioriinformation. Molecular envelope information reduces the size of the solution set, and for an envelope that deviates sufficiently from the unit cell a unique solution is possible. The effects of various molecular surface features and incomplete data on uniqueness and prospects forab initiophasing are assessed. Simulations of phase retrieval for two-dimensional crystal data are described in the second paper in this series.