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We have characterized the internal structure of two close-to-round starless cores in Taurus, L1498 and L1517B, setting constraints on the initial conditions of star formation and on models of core condensation. Our analysis is based on high angular resolution observations in at least two transitions of NH3, N2H+, CS, C34S, C18O, and C17O, together with maps of the 1.2 mm continuum. For both cores, we derive radial profiles of constant temperature and constant turbulence, and density distributions close to those of non-singular isothermal spheres. Using a Monte Carlo radiative transfer model, we derive abundance profiles for all species and find a pattern of strong chemical differentiation. NH3 has a higher abundance toward the core centers while N2H+ has a constant abundance over most of the cores. Both C18O and CS (and isotopomers) are strongly depleted in the core interiors, most likely due to their freeze out onto cold dust grains. Concerning the kinematics of the dense gas, we find (in addition to constant turbulence) a pattern of internal motions at the level of 0.05 km s-1. These motions seem correlated with asymmetries in the pattern of molecular depletion, and we interpret them as residuals of core contraction. Their distribution and size suggest that core formation happens in rather irregular manner. A comparison with supersonic turbulence models of core formation shows that our observed cores are much more quiescent than allowed by these models. [ABSTRACT FROM AUTHOR]

We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field. [ABSTRACT FROM AUTHOR]

In recent years, the need to replace rate equations for studying grain-surface chemistry in the modelling of interstellar clouds has become apparent. In this article, we discuss the three new replacement methods that have been suggested to date, and contrast their relative strengths and weaknesses. [ABSTRACT FROM AUTHOR]

Discusses astrophysicist Fred Hoyle's contributions to the literature on interstellar matter and star formation. Research paper on the physical aspects of accretion by stars; Theory of the fragmentation of gas clouds into galaxies and stars; Work on the composition of interstellar grains.

The subject of interstellar grains has grown enormously in stature over the past 40 years, the duration of my collaboration with Fred Hoyle. These developments are in large measure due to Fred's prescient inputs into this subject and his encouragement. [ABSTRACT FROM AUTHOR]