The Most Detailed Picture Yet of an Embedded High-mass YSO

High-mass star formation is not well understood chiefly because examples are deeply embedded, relatively distant, and crowded with sources of emission. Using VLA and VLBA observations of water and SiO maser emission, we have mapped in detail the structure and proper motion of material 20-500 AU from the closest high-mass YSO, radio source-I in the Orion KL region. We observe streams of material driven in a rotating, wide angle, bipolar wind from the surface of an edge-on accretion disk. The example of source-I provides strong evidence that high-mass star formation proceeds via accretionComment: typo corrected and word added to abstract 6 pages including 4 B&W figures. To appear in the Proceeding of IAU Symposium 221, Star Formation at High Angular Resolution, Editors M. Burton, R. Jayawardhana & T. Bourke, Astronomical Society of the Pacifi

VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser

The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.Comment: 6 pages, 4 figures; to be published in A&

Soil Water Regimes.

End of Project ReportSoil moisture tension was monitored, for three years, at three sites representing different natural soil drainage classes that were defined morphologically. The soils comprised: • Gley, poorly drained, loam • Brown Earth, well drained, loam • Brown Earth, somewhat excessively drained, sandy loam. The main features of the moisture regime were: • Average annual soil water tension was analogous to the natural drainage classification and followed the sequence: somewhat excessively drained > well drained > poorly drained. • Some horizons that lacked visible evidence of reduction, in the subsoil of the Brown Earths, were saturated for long periods. • The Brown Earths were unsaturated, at 15 cm depth, throughout the three-year period. • The Gley was saturated at 15 cm depth for up to nearly four months per year. This implies that the risk of overland flow, due to saturation excess, differs among soil types. The risk is probably significant on Gleys, which occupy 25 percent of the land area; it is probably small or negligible on Brown Earths and analogous soils, which comprise over forty percent and account for virtually all of the intensive agriculture in the country.European Union Structural Funding (EAGGF

Critical self-organization of astrophysical shocks

There are two distinct regimes of the first order Fermi acceleration at shocks. The first is a linear (test particle) regime in which most of the shock energy goes into thermal and bulk motion of the plasma. The second is an efficient regime when it goes into accelerated particles. Although the transition region between them is narrow, we identify the factors that drive the system to a {\it self-organized critical state} between those two. Using an analytic solution, we determine this critical state and calculate the spectra and maximum energy of accelerated particles.Comment: To appear in ApJL, Sec.3 extensively rewritten, 4 pages, Latex, emulateapj.sty, eps

Circular Polarization of Circumstellar Water Masers around S Per

We present the first circular polarization measurements of circumstellar H2O masers. Previously the magnetic field in circumstellar envelopes has been estimated using polarization observations of SiO and OH masers. SiO masers are probes of the high temperature and density regime close to the central star. OH masers are found at much lower densities and temperatures, generally much further out in the circumstellar envelope. The detection of the circular polarization of the (6_16 - 5_23) rotational transition of the H2O maser could be attributed to Zeeman splitting due to the magnetic field in the intermediate temperature and density regime. The fields inferred here agree well with predicted values for a combination of the r^{-2} dependence of a solar-type magnetic field, and the coupling of the field to the high density masing regions. We also discuss the unexpected narrowing of the circular polarization spectrum.Comment: 4 pages, 4 figures; accepted for publication in Astronomy and Astrophysics Letter

A MERLIN Study of 6 GHz Excited-state OH & 6.7 GHz Methanol Masers in ON1

MERLIN observations of 6.668-GHz methanol and both 6.031- and 6.035-GHz hydroxyl (OH) emission from the massive star-formation region ON1 are presented. These are the first methanol observations made in full polarization using 5 antennas of MERLIN, giving high resolution and sensitivity to extended emission. Maser features are found to lie at the southern edge of the ultra-compact HII region, following the known distribution of ground-state OH masers. The masers cover a region ~1 arcsec in extent, lying perpendicular to the H13CO+ bipolar outflow. Excited-state OH emission demonstrates consistent polarization angles across the strongest linearly polarized features which are parallel to the overall distribution. The linear polarizations vary between 10.0 and 18.5 per cent, with an average polarization angle of -60 deg +/- 28 deg. The strongest 6.668-GHz methanol features provide an upper limit to linear polarization of ~1 per cent. Zeeman splitting of OH shows magnetic fields between -1.1 to -5.8 mG, and a tentative methanol magnetic field strength of -18 mG is measured.Comment: 10 Pages, 5 Figure

Polarization morphology of SiO masers in the circumstellar envelope of the AGB star R Cassiopeiae

Silicon monoxide maser emission has been detected in the circumstellar envelopes of many evolved stars in various vibrationally-excited rotational transitions. It is considered a good tracer of the wind dynamics close to the photosphere of the star. We have investigated the polarization morphology in the circumstellar envelope of an AGB star, R Cas. We mapped the linear and circular polarization of SiO masers in the v=1, J=1-0 transition. The linear polarization is typically a few tens of percent while the circular polarization is a few percent. The fractional polarization tends to be higher for emission of lower total intensity. We found that, in some isolated features the fractional linear polarization appears to exceed 100%. We found the Faraday rotation is not negligible but is ~15 deg., which could produce small scale structure in polarized emission whilst total intensity is smoother and partly resolved out. The polarization angles vary considerably from feature to feature but there is a tendency to favour the directions parallel or perpendicular to the radial direction with respect to the star. In some features, the polarization angle abruptly flips 90 deg. We found that our data are in the regime where the model of Goldreich et al (1973) can be applied and the polarization angle flip is caused when the magnetic field is at close to 55 deg. to the line of sight. The polarization angle configuration is consistent with a radial magnetic field although other configurations are not excluded.Comment: 14 pages, 15 figures. Accepted for publication in MNRA