Our current understanding of the chemistry and mass-loss processes in solar-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere (1). Threedimensional hydrodynamical stellar atmosphere models provide observational predictions (2), but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimeter continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time larger than commonly assumed. A shocked hot layer will be an important ingredient in the models of stellar convection, pulsation and chemistry that underlie our current understanding of the late stages of stellar evolution.Asymptotic giant branch (AGB) stars are among the most important sources of enrichment of the Galactic interstellar medium (ISM). Molecules and dust formed in the warm extended atmospheres and the cool and dense circumstellar envelopes (CSEs) around AGB stars are injected into the ISM by a stellar wind that has overcome stellar gravity (1). It is generally assumed that the stellar wind is driven by radiation pressure on dust that forms at a few stellar radii, where the temperature in the CSE has dropped so that dust condensation can occur (3). In order for the gas in the extended stellar atmosphere to reach the dust formation region, the most recent AGB mass-loss models typically invoke stellar pulsations and convective motions (2,(4)(5)(6).Both convective motions and pulsations induce outward moving shocks that critically affect the upper layers of the AGB atmosphere where the stellar mass loss is determined. The propagation of shocks also strongly affects the chemistry in the stellar atmosphere (7-9). In early AGB atmosphere models, the outward propagation of strong shocks is responsible for the creation of a chromosphere (6, 10), from which ultraviolet line and continuum emissions originate. Such emissions have been observed from AGB stars (11,12). However, the observations of molecules and dust close to the star are not consistent with the extended chromosphere produced by the models. Observations have so far not been able to resolve this ambiguity. High angular resolution images of the stellar disks of AGB stars have revealed asymmetries of which the source is not yet clear, but convective motions are believed to play a role (13)(14)(15)(16). Since at most wavelengths, the observations are probing distinct molecular opacity sources (16), or averages over the stellar disk (17), the dynamics and temperature structures in the atmosphere closest to the stellar photosphere have not yet been observed in detail.We present observations of the AGB star W Hya that reveal evidence for the presence of shocks and map the distribution of molecular g...

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Despite their importance in the star formation process, measurements of magnetic field strength in proto-planetary discs remain rare. While linear polarisation of dust and molecular lines can give insight into the magnetic field structure, only observations of the circular polarisation produced by Zeeman splitting provide a direct measurement of magnetic field strenghts. One of the most promising probes of magnetic field strengths is the paramagnetic radical CN. Here we present the first Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Zeeman splitting of CN in the disc of TW Hya. The observations indicate an excellent polarisation performance of ALMA, but fail to detect significant polarisation. An analysis of eight individual CN hyperfine components as well as a stacking analysis of the strongest (non-blended) hyperfine components yields the most stringent limits obtained so far on the magnetic field strength in a proto-planetary disc. We find that the vertical component of the magnetic field |B z | < 0.8 mG (1σ limit). We also provide a 1σ toroidal field strength limit of < 30 mG. These limits rule out some of the earlier accretion disc models, but remain consistent with the most recent detailed models with efficient advection. We detect marginal linear polarisation from the dust continuum, but the almost purely toroidal geometry of the polarisation vectors implies that his is due to radiatively aligned grains.

We present a rigorous derivation of the nuclear spin-rotation and spin-torsion coupling terms in the hyperfine Hamiltonian for molecules with internal rotation. Our formulas differ from the expressions derived by Heuvel and Dymanus [J. Mol. Spectrosc. 47, 363 (1973)], which these authors used and which were also applied recently by others to interpret experimental hyperfine spectra of such molecules. In the present work, our theoretical results are applied to methanol. We calculate the nuclear spin-spin magnetic dipole-dipole interactions and the nuclear contribution to the spin-torsion coupling vectors from the nuclear coordinates as functions of the internal rotation angle γ, compute the spin-rotation coupling tensors by ab initio electronic structure methods also as functions of γ, and obtain the missing parameters for the electronic contribution to the spin-torsion coupling from a fit to measured spectra. The resulting hyperfine Hamiltonian is then used to compute hyperfine transition frequencies and intensities for twelve torsion-rotation transitions in methanol. With the use of the ab initio calculated spin-rotation coupling parameters without any modification, and physically reasonable values for the spin-torsion coupling parameters from the fit, we find good agreement with all of the measured spectra.