Milliarcsecond structure of water maser emission in two young high-mass stellar objects associated with methanol masers

The 22.2 GHz water masers are often associated with the 6.7 GHz methanol masers but owing to the different excitation conditions they likely probe independent spatial and kinematic regions around the powering young massive star. We compared the emission of these two maser species on milliarcsecond scales to determine in which structures the masers arise and to test a disc-outflow scenario where the methanol emission arises in a circumstellar disc while the water emission comes from an outflow. We obtained high-angular and spectral resolution 22.2 GHz water maser observations of the two sources G31.581+00.077 and G33.641-00.228 using the EVN. In both objects the water maser spots form complex and filamentary structures of sizes 18-160 AU. The emission towards the source G31.581+00.077 comes from two distinct regions of which one is related to the methanol maser source of ring-like shape. In both targets the main axis of methanol distribution is orthogonal to the water maser distribution. Most of water masers appear to trace shocks on a working surface between an outflow/jet and a dense envelope. Some spots are possibly related to the disc-wind interface which is as close as 100-150 AU to the regions of methanol emission.Comment: 10 pages, accepted to Astronomy and Astrophysic

Magnetic field in Cepheus A as deduced from OH maser polarimetric observations

We present the results of MERLIN polarization mapping of OH masers at 1665 and 1667 MHz towards the Cepheus A star-forming region. The maser emission is spread over a region of 6 arcsec by 10 arcsec, twice the extent previously detected. In contrast to the 22 GHz water masers, the OH masers associated with H II regions show neither clear velocity gradients nor regular structures. We identified ten Zeeman pairs which imply a magnetic field strength along the line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the arcsecond scale, pointing towards us in the west and away from us in the east side. The linearly polarized components, detected for the first time, show regularities in the polarization position angles depending on their position. The electric vectors of OH masers observed towards the outer parts of H II regions are consistent with the interstellar magnetic field orientation, while those seen towards the centres of H II regions are parallel to the radio-jets. A Zeeman quartet inside a southern H II region has now been monitored for 25 years; we confirm that the magnetic field decays monotonically over that period.Comment: 10 pages, 6 figures,accepted for publication in MNRA

Binary systems: implications for outflows & periodicities relevant to masers

Bipolar molecular outflows have been observed and studied extensively in the past, but some recent observations of periodic variations in maser intensity pose new challenges. Even quasi-periodic maser flares have been observed and reported in the literature. Motivated by these data, we have tried to study situations in binary systems with specific attention to the two observed features, i.e., the bipolar flows and the variabilities in the maser intensity. We have studied the evolution of spherically symmetric wind from one of the bodies in the binary system, in the plane of the binary. Our approach includes the analytical study of rotating flows with numerical computation of streamlines of fluid particles using PLUTO code. We present the results of our findings assuming simple configurations, and discuss the implications.Comment: 5 pages, 3 figures, Proceedings IAU Symposium No. 287, 2012, Cosmic masers - from OH to H

Instabilities in the dissolution of a porous matrix

A reactive fluid dissolving the surrounding rock matrix can trigger an instability in the dissolution front, leading to spontaneous formation of pronounced channels or wormholes. Theoretical investigations of this instability have typically focused on a steadily propagating dissolution front that separates regions of high and low porosity. In this paper we show that this is not the only possible dissolutional instability in porous rocks; there is another instability that operates instantaneously on any initial porosity field, including an entirely uniform one. The relative importance of the two mechanisms depends on the ratio of the porosity increase to the initial porosity. We show that the "inlet" instability is likely to be important in limestone formations where the initial porosity is small and there is the possibility of a large increase in permeability. In quartz-rich sandstones, where the proportion of easily soluble material (e.g. carbonate cements) is small, the instability in the steady-state equations is dominant.Comment: to be published in Geophysical Research Letter