arXiv subjects

Moshe Elitzur

Publications and source records attributed to Moshe Elitzur.

At least 55 records · Page 3Linked to original sources

arXiv2005

A New Exact Method for Line Radiative Transfer

We present a new method, the Coupled Escape Probability (CEP), for exact calculation of line emission from multi-level systems, solving only algebraic equations for the level populations. The CEP formulation of the classical two-level problem is a set of {\em linear equations}, and we uncover an exact analytic expression for the emission from two-level optically thick sources that holds as long as they are in the "effectively thin" regime. In comparative study of a number of standard problems, the CEP method outperformed the leading line transfer methods by substantial margins. The algebraic equations employed by our new method are already incorporated in numerous codes based on the escape probability approximation. All that is required for an exact solution with these existing codes is to augment the expression for the escape probability with simple zone-coupling terms. As an application, we find that standard escape probability calculations generally produce the correct cooling emission by the CII 158 mic line but not by the $^3$P lines of OI.

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arXiv2005

Near infrared and the inner regions of protoplanetary disks

We examine the ``puffed-up inner disk'' model (Dullemond, Dominik & Natta 2001), proposed for explaining the near-IR excess radiation from Herbig Ae/Be stars. Detailed model computations show that the observed near-IR excess requires more hot dust than is contained in the puffed-up disk rim. The rim can produce the observed near-IR excess only if its dust has perfectly gray opacity, but such dust is in conflict with the observed 10$μ$m spectral feature. We find that a compact ($\sim$ 10 AU) tenuous ($τ_V \la 0.4$) dusty halo around the disk inner regions contains enough dust to readily explain the observations. Furthermore, this model also resolves the puzzling relationship noted by Monnier and Millan-Gabet (2002) between luminosity and the interferometric inner radii of disks.

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arXiv2004

A Circumstellar Disc in a High-Mass Star Forming Region

We present an edge-on Keplerian disc model to explain the main component of the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N. The brightness distribution and spectrum of the line of bright masers are successfully modeled with high amplification of background radio continuum emission along velocity coherent paths through a maser disc. The bend seen in the position-velocity diagram is a characteristic signature of differentially rotating discs. For a central mass of 30 solar masses, suggested by other observations, our model fixes the masing disc to have inner and outer radii of about 350 AU and 1000 AU.

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arXiv2003

IR emission from AGNs

Unified schemes of active galactic nuclei (AGN) require an obscuring dusty torus around the central source, giving rise to type 1 line spectrum for pole-on viewing and type 2 characteristics in edge-on sources. Infrared radiation at its different wavelengths is the best probe of the dust distribution, whether the torus orientation is edge-on or pole-on. The observed IR is in broad agreement with the unified scheme but serious problems remained in all early modeling efforts. In spite of a general awareness that the dust must be concentrated in clouds, clumpiness remained the one major ingredient missing in those radiative transfer studies because of the inherent difficulties it presents. We have recently developed the formalism to handle dust clumpiness and our results indicate that its inclusion may resolve the difficulties encountered by the previous theoretical efforts. We show that these problems find a natural explanation if the dust is contained in \about\ 5--10 clouds along radial rays through the torus. The spectral energy distributions (SED) of both type 1 and type 2 sources are properly reproduced from different viewpoints of the same object if the optical depth of each cloud is \ga 40 at visual wavelengths and the clouds' mean free path increases roughly in proportion to radial distance.

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arXiv2003

Disks and Halos in Pre-Main-Sequence Stars

We study the IR emission from flared disks with and without additional optically thin halos. Flux calculations of a flared disk in vacuum can be considered a special case of the more general family of models in which the disk is imbedded in an optically thin halo. In the absence of such halo, flux measurements can never rule out its existence because the disk flaring surface defines a mathematically equivalent halo that produces the exact same flux at all IR wavelengths. When a flared disk with height H at its outer radius R is imbedded in a halo whose optical depth at visual wavelengths is τ\sub{halo}, the system IR flux is dominated by the halo whenever τ\sub{halo} > (1/4)H/R. Even when its optical depth is much smaller, the halo can still have a significant effect on the disk temperature profile. Imaging is the only way to rule out the existence of a potential halo, and we identify a decisive test that extracts a signature unique to flared disks from imaging observations.

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arXiv2002

The Structure of Winds in AGB Stars

Most dusty winds are described by a set of similarity functions of a single independent variable that can be chosen as the overall optical depth at visual. The self-similarity implies general scaling relations among the system parameters, in agreement with observations. Dust drift through the gas has a major impact on the structure of most winds.

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arXiv2001

Dusty winds: I. Self-similar solutions

We address the dusty wind problem, from the point where dust formation has been completed and outward. Given grain properties, both radiative transfer and hydrodynamics components of the problem are fully defined by four additional input parameters. The wind radiative emission and the shape of its velocity profile are both independent of the actual magnitude of the velocity and are determined by just three dimensionless free parameters. Of the three, only one is always significant---for most of phase space the solution is described by a set of similarity functions of a single independent variable, which can be chosen as the overall optical depth at visual \tV. The self-similarity implies general scaling relations among mass loss rate (\Mdot), luminosity ($L$) and terminal velocity (\vf). Systems with different \Mdot, $L$ and \vf but the same combination $\Mdot/L^{3/4}$ necessarily have also the same $\Mdot\vf/L$. ... Eliminating \tV produces $\vf^3 = A \Mdot(1 + B \Mdot^{4/3}/L)^{-1.5}$, where $A$ and $B$ are coefficients that contain the only dependence of this universal correlation on chemical composition. At a given $L$, the maximal velocity of a dusty wind is $v\sub{max} \propto L^{1/4}$ attained at $\Mdot \propto L^{3/4}$, with proportionality coefficients derived from $A$ and $B$.

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arXiv2001

Topics in Basic Maser Theory

This review covers some of the developments in basic theory of astronomical masers over the past ten years. Topics included are the effects of three dimensional geometry and polarization, with special emphasis on the differences between maser and non-maser radiation.

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arXiv2000

Water and Dust Emission from W Hydrae

We construct a self-consistent model for the wind around W Hya by solving the coupled equations describing the hydrodynamics and dust radiative transfer problems. The model matches simultaneously the observed continuum radiation and wind velocity profile. The water line emission is calculated next using the water abundance as the only free parameter, fitted from the ISO observations of Neufeld et al. (1996) and Barlow et al. (1996). The gas temperature is determined from a thermal balance calculation that includes water as one of its main components. Our model successfully fits all the observed water lines, resolving a major discrepancy between the modeling results of the two observing teams. The mass loss rate is 2.3 x 10^{-6} M_solar yr^{-1}, the water abundance is 1.0 x 10^{-4} and the ortho:para ratio is 1:1.3.

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arXiv2000

Infrared Classification of Galactic Objects

Unbiased analysis shows that IRAS data reliably differentiate between the early and late stages of stellar evolution because objects at these stages clearly segregate in infrared color-color diagrams. Structure in these diagrams is primarily controlled by the density distribution of circumstellar dust. The density profile around older objects is the steepest, declining as $r^{-2}$, while young objects have profiles that vary as $r^{-3/2}$ and flatter. The different density profiles reflect the different dynamics that govern the different environments. Our analysis also shows that high mass star formation is strongly concentrated within \about 5 kpc around the Galactic center, in support of other studies.

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arXiv1999

User Manual for DUSTY

DUSTY solves the problem of radiation transport in a dusty environment. The code can handle both spherical and planar geometries. The user specifies the properties of the radiation source and dusty region, and the code calculates the dust temperature distribution and the radiation field in it. The solution method is based on a self-consistent equation for the radiative energy density, including dust scattering, absorption and emission, and does not introduce any approximations. The solution is exact to within the specified numerical accuracy. DUSTY has built in optical properties for the most common types of astronomical dust and comes with a library for many other grains. It supports various analytical forms for the density distribution, and can perform a full dynamical calculation for radiatively driven winds around AGB stars. The spectral energy distribution of the source can be specified analytically as either Planckian or broken power-law. In addition, arbitrary dust optical properties, density distributions and external radiation can be entered in user supplied files. Furthermore, the wavelength grid can be modified to accommodate spectral features. A single DUSTY run can process an unlimited number of models, with each input set producing a run of optical depths, as specified. The user controls the detail level of the output, which can include both spectral and imaging properties as well as other quantities of interest.

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arXiv1998

Megamaser Disks in Active Galactic Nuclei

Recent spectroscopic and VLBI-imaging observations of bright extragalactic water maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity coherence length along the line of sight could account (through self-amplification) for the entire maser flux of a high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-amplification likely contributes also to the flux of the background-amplifying ``systemic'' features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2-like galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission-line region-type) clouds.

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arXiv1998

OH 1720 MHz Masers in Supernova Remnants --- C-Shock Indicators

Recent observations show that the OH 1720 MHz maser is a powerful probe of the shocked region where a supernova remnant strikes a molecular cloud. We perform a thorough study of the pumping of this maser and find tight constraints on the physical conditions needed for its production. The presence of the maser implies moderate temperatures (50 -- 125 K) and densities ($\sim 10^5 cm^{-3}$), and OH column densities of order $10^{16} cm^{-2}$. We show that these conditions can exist only if the shocks are of C-type. J-shocks fail by such a wide margin that the presence of this maser could become the most powerful indicator of C-shocks. These conditions also mean that the 1720 MHz maser will be inherently weak compared to the other ground state OH masers. All the model predictions are in good agreement with the observations.

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arXiv1998

Polarization of Astronomical Maser Radiation. IV. Circular Polarization Profiles

Profile comparison of the Stokes parameters $V$ and $I$ is a powerful tool for maser data analysis, providing the first direct methods for unambiguous determination of (1) the maser saturation stage, (2) the amplification optical depth and intrinsic Doppler width of unsaturated masers, and (3) the comparative magnitudes of Zeeman splitting and Doppler linewidth. Circular polarization recently detected in OH 1720 MHz emission from the Galactic center appears to provide the first direct evidence for maser saturation.

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arXiv1996

Self-similarity and scaling behavior of IR emission from radiatively heated dust: I. Theory

Dust infrared emission possesses scaling properties. Overall luminosity is never an input parameter of the radiative transfer problem, spectral shape is the only relevant property of the heating radiation when the inner boundary of the dusty region is controlled by dust sublimation. Similarly, the absolute scales of densities and distances are irrelevant; the geometry enters only through angles, relative thicknesses and aspect ratios, and the actual magnitudes of densities and distances enter only through one independent parameter, the overall optical depth. Dust properties enter only through dimensionless, normalized distributions that describe the spatial variation of density and the wavelength dependence of scattering and absorption efficiencies. Scaling enables a systematic approach to modeling and classification of IR spectra. We develop a new, fully scale-free method for solving radiative transfer, present exact numerical results, and derive approximate analytical solutions for spherical geometry, covering the entire range of parameter space relevant to observations. Scaling implies tight correlations among the SEDs of various members of the same class of sources such as young stellar objects, late-type stars, etc. In particular, all members of the same class occupy common, well defined regions in color-color diagrams. The observational data corroborate the existence of these correlations.

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arXiv1996

On Protostellar Disks in Herbig Ae/Be Stars

The spectral shape of IR emission from Herbig Ae/Be stars has been invoked as evidence for accretion disks around high-mass protostars. Instead, we present here models based on spherical envelopes with $r^{-1.5}$ dust density profile that successfully explain the observed spectral shapes. The spectral energy distributions (SEDs) of eight primary candidates for protostellar disks are fitted in detail for all wavelengths available, from visual to far IR. The only envelope property adjusted in individual sources is the overall visual optical depth, and it ranges from 0.3 to 3. In each case, our models properly reproduce the data for both IR excess, visual extinction and reddening. The success of our models shows that accretion disks cannot make a significant contribution to the radiation observed in these pre-main sequence stars.

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arXiv1996

IR Emission from dusty winds --- Scaling and self-similarity properties

Infrared emission from radiatively heated dust possesses general scaling properties. The spectral shape is independent of overall luminosity when the inner boundary of the dusty region is controlled by dust sublimation; the only relevant property of the heating radiation is its frequency profile. For a given type of dust grains, the emission from red giants and supergiants is essentially controlled by a single parameter --- the overall dust optical depth. This leads to tight correlations among different spectral properties that explain many available observations and enable systematic studies of large data bases.

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arXiv1996

Infrared Imaging of Late-Type Stars

Infrared imaging properties of dusty winds around late-type stars are investigated in detail, employing a self-consistent model that couples the equations of motion and radiative transfer. Because of general scaling properties, the angular profiles of surface brightness are self-similar. In any given star, the profile shape is determined essentially by overall optical depth at each wavelength and it is self-similarly scaled by the size of the dust condensation zone. We find that the mid-IR is the best wavelength range to measure directly the angular size of this zone, and from {\it IRAS} data we identify the 15 best candidates for such future observations. We also show that the visibility function at short wavelengths ($\la$ 2 \mic) directly determines the scattering optical depth, and produce theoretical visibility curves for various characteristic wavelengths and the entire parameter range relevant to late-type stars. The infrared emission should display time variability because of cyclical changes in overall optical depth, reflecting luminosity-induced movement of the dust condensation point. Calculations of the wavelength dependence of photometric amplitudes and time variability of envelope sizes are in agreement with observations; envelopes are bigger and bluer at maximum light.

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