arXiv subjects

Moshe Elitzur

Publications and source records attributed to Moshe Elitzur.

58 records · Page 4Linked to original sources

arXiv1996

Dust Emission from IRC+10216

Infrared emission from the dust shell around IRC+10216 is analysed in detail, employing a self-consistent model for radiatively driven winds around late-type stars that couples the equations of motion and radiative transfer in the dust. The resulting model provides agreement with the wealth of available data, including the spectral energy distribution in the range 0.5--1000 \mic, and visibility and array observations. Previous conclusions about two dust shells, derived from modelling the data with a few single-temperature components of different radii, are not supported by our results. The extended, continuous temperature and density distributions derived from our model obviate the need for such discrete shells. The IR properties vary with the stellar phase, reflecting changes in both the dust condensation radius $r_1$ and the overall optical depth $τ$ -- as the luminosity increases from minimum to maximum, $r_1$ increases while $τ$ decreases. We find that the angular size of the dust condensation zone varies from 0.3 arcsec at minimum light to 0.5 arcsec at maximum. The shortage of flux at short wavelengths encountered in previous studies is resolved by employing a grain size distribution that includes grains larger than \about\ 0.1 \mic, required also for the visibility fits. This distribution is in agreement with the one recently proposed by Jura in a study that probed the outer regions of the envelope. Since our constraints on the size distribution mostly reflect the envelope's inner regions, the agreement of these independent studies is evidence against significant changes in grain sizes through effects like sputtering or grain growth after the initial formation at the dust condensation zone.

astro-ph↗

arXiv1995

Shell Masers

We present the analytical solution of a maser shaped like a spherical shell. We determine the general condition on the size of the central cavity at which a sphere becomes a shell maser, and derive the intensity, beaming angle and observed size of both unsaturated and saturated shells.

astro-ph↗

arXiv1995

Polarization of Astronomical Maser Radiation. III. Arbitrary Zeeman Splitting and Anisotropic Pumping

General solutions of the maser polarization problem are presented for arbitrary absorption coefficients. The results are used to calculate polarization for masers permeated by magnetic fields with arbitrary values of \xB, the ratio of Zeeman splitting to Doppler linewidth, and for anisotropic pumping. The $\xb \to 0$ limit of the magnetic solution reproduces the linear polarization derived in previous studies, which were always conducted at this unphysical limit. While terms of higher order in \xb\ have a negligible effect on the magnitude of $q$, they produce some major new results. In particular, the linear polarization is accompanied by circular polarization, proportional to \xb. Because \xb\ is proportional to the transition wavelength, the circular polarization of SiO masers should decrease with rotation quantum number, as observed. In the absence of theory for $\xb < 1$, previous estimates of magnetic fields from detected maser circular polarization had to rely on conjectures in this case and generally need to be revised downward. The fields in SiO masers are \about\ 2--10 G and were overestimated by a factor of 8. The OH maser regions around supergiants have fields of \about\ 0.1--0.5 mG, which were overestimated by factors of 10--100. The fields were properly estimated for OH/IR masers ($\la$ 0.1 mG) and \H2O masers in star-forming regions (\about\ 15--50 mG). Spurious solutions that required stability analysis for their removal in all previous studies are never reproduced here; in particular, there are no stationary physical solutions for propagation at $\sin^2θ< \third$, where $θ$ is the angle from the direction of the magnetic field, so such radiation is unpolarized. These spurious solutions can be identified as the \xb\ = 0 limits of non-physical solutions and they never arise at finite

astro-ph↗

arXiv1995

INFRARED EMISSION AND DYNAMICS OF OUTFLOWS IN LATE-TYPE STARS

The dynamical structure and infrared emission of winds around late-type stars are studied in a self-consistent model that couples the equations of motion and radiative transfer. Both the dynamics and IR spectrum of the solution are fully characterized by the flux averaged optical depth. Five types of dust grains are considered: astronomical silicate, SiC, crystalline olivine, graphite and amorphous carbon as well as mixtures. Analysis of infrared signatures provides constraints on the grain chemical composition and indications for the simultaneous existence of silicate and carbon grains. The region of parameter space where radiation pressure can support a given mass-loss rate is identified and a new method to derive mass loss rates from IR data is presented. IRAS LRS classes are associated with optical depth for various grain materials and the regions of color-color diagrams expected to be populated by late-type stars are identified. We show that cirrus emission can severely affect point source measurements to the extent that their listed IRAS long wavelength fluxes are unreliable. After accounting for cirrus contamination, essentially all IRAS point sources located in the relevant regions of the color-color diagrams can be explained as late-type stars.

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