arXiv subjects

B. T. Draine

Publications and source records attributed to B. T. Draine.

At least 127 records · Page 7Linked to original sources

arXiv2004

Propagation of Electromagnetic Waves on a Rectangular Lattice of Polarizable Points

We discuss the propagation of electromagnetic waves on a rectangular lattice of polarizable point dipoles. For wavelengths long compared to the lattice spacing, we obtain the dispersion relation in terms of the lattice spacing and the dipole polarizabilities. We also obtain the polarizabilities required for the lattice to have the same dispersion relation as a continuum medium of given refractive index m.; our result differs slightly from previous work by Draine & Goodman (1993). Our new prescription can be used to assign dipole polarizabilities when the discrete dipole approximation is used to study scattering by finite targets. Results are shown for selected cases.

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arXiv2004

X-ray Scattering Halos from the Galactic Center: Implications for Diffuse Emission Around SgrA*

We consider the absorption and scattering of X-rays observed from the Galactic center. One objective is to characterize the intrinsic X-ray emission from the central black hole, SgrA*, in its quiescent and flaring states. We correct the fluxes observed by the Chandra and XMM telescopes for absorption and scattering, using realistic models for the dust grain size distribution, gas and dust abundances and spatial distributions. However, there are still large uncertainties. Since much of the intervening dust is close to SgrA*, the scattered halo of X-ray photons is very concentrated: its intensity can dominate the Chandra PSF inside 1", and so affects estimates of the point source flux. It also broadens the radial intensity profiles of Galactic center sources - observations of this effect can help constrain models of the spatial distribution of the dust. We estimate that the combined scattering halos from observed Galactic center sources within 10" of SgrA* contribute up to ~10% of the observed diffuse emission in this region. Dust-scattered photons suffer a time delay relative to the photons that arrive directly. For dust that is 100 pc towards us from the Galactic center, this delay is about 1000 s at angles of 1", and 100 ks at 10". We illustrate how the evolution of the scattering halo following X-ray flares from SgrA* or other sources can also help to constrain the dust's line of sight distribution. We discuss the implications of X-ray scattering halos for the intensity of diffuse emission that has been reported within a few arcseconds of SgrA*: in the most extreme, yet viable, model we consider, ~1/3 of it is due to dust scattering of an unresolved source. The remainder results from an extended source of emission.

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arXiv2003

Interstellar Dust

In the interstellar medium of the Milky Way, certain elements -- e.g., Mg, Si, Al, Ca, Ti, Fe -- reside predominantly in interstellar dust grains. These grains absorb, scatter, and emit electromagnetic radiation, heat the interstellar medium by photoelectric emission, play a role in the ionization balance of the gas, and catalyze the formation of molecules, particularly H2. I review the state of our knowledge of the composition and sizes of interstellar grains, including what we can learn from spectral features, luminescence, scattering, infrared emission, and observed gas-phase depletions. The total grain volume in dust models which reproduce interstellar extinction is significantly greater than estimated from observed depletions. Dust grains might reduce the gas-phase D/H ratio, providing an alternative mechanism to explain observed variations in the gas-phase D/H ratio in the local interstellar medium. Transport in dust grains could cause elemental abundances in newly-formed stars to differ from interstellar abundances.

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arXiv2003

Discrete-dipole approximation with polarizabilities that account for both finite wavelength and target geometry

The discrete-dipole approximation (DDA) is a powerful method for calculating absorption and scattering by targets that have sizes smaller than or comparable to the wavelength of the incident radiation. We present a new prescription -- the Surface-Corrected Lattice Dispersion Relation (SCLDR) -- for assigning the dipole polarizabilities that takes into account both target geometry and finite wavelength. We test the SCLDR in DDA calculations using spherical and ellipsoidal targets and show that for a fixed number of dipoles, the SCLDR prescription results in increased accuracy in the calculated cross sections for absorption and scattering. We discuss extension of the SCLDR prescription to irregular targets.

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arXiv2003

User Guide for the Discrete Dipole Approximation Code DDSCAT.6.0

DDSCAT.6.0 is a freely available software package (http://www.astro.princeton.edu/~draine/DDSCAT.6.0.html) which applies the "discrete dipole approximation" (DDA) to calculate scattering and absorption of electromagnetic waves by targets with arbitrary geometries and complex refractive index. DDSCAT.6.0 allows accurate calculations of electromagnetic scattering from targets with ``size parameters'' 2*pi*a/lambda < 15 provided the refractive index m is not large compared to unity (|m-1| < 1). DDSCAT.6.0 includes the option of using the FFTW (Fastest Fourier Transform in the West) package. DDSCAT.6.0 also includes MPI support, permitting parallel calculations on multiprocessor systems. DDSCAT package is written in Fortran and is highly portable. The program supports calculations for a variety of target geometries (e.g., ellipsoids, regular tetrahedra, rectangular solids, finite cylinders, hexagonal prisms, etc.). Target materials may be both inhomogeneous and anisotropic. It is straightforward for the user to ``import'' arbitrary target geometries into the code, and relatively straightforward to add new target generation capability to the package. DDSCAT automatically calculates total cross sections for absorption and scattering and selected elements of the Mueller scattering intensity matrix for specified orientation of the target relative to the incident wave, and for specified scattering directions. This User Guide explains how to use DDSCAT.6.0 to carry out electromagnetic scattering calculations. CPU and memory requirements are described.

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arXiv2003

Scattering by Interstellar Dust Grains: Optical and Ultraviolet

Scattering and absorption properties at optical and ultraviolet wavelengths are calculated for an interstellar dust model consisting of carbonaceous grains and amorphous silicate grains. Polarization as a function of scattering angle is calculated for selected wavelengths from the IR to the vacuum UV. The widely-used Henyey-Greenstein phase function provides a good approximation for the scattering phase function between ~0.4 and 1 micron, but fails to fit the calculated phase functions at shorter wavelengths and longer wavelengths. A new analytic phase function is presented. It is exact at long wavelengths, and provides a good fit to the numerically-calculated phase function for lambda > 0.27um. Observational determinations of the scattering albedo and show considerable disagreement, especially in the ultraviolet. Possible reasons for this are discussed.

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arXiv2003

Scattering by Interstellar Dust Grains. II. X-Rays

Scattering and absorption of X-rays by interstellar dust is calculated for a model consisting of carbonaceous grains and amorphous silicate grains. The calculations employ realistic dielectric functions with structure near X-ray absorption edges, with resulting features in absorption, scattering, and extinction. Differential scattering cross sections are calculated for energies between 0.3 and 10 keV. The median scattering angle is given as a function of energy, and simple but accurate approximations are found for the X-ray scattering properties of the dust mixture, as well as for the angular distribution of the scattered X-ray halo for dust with simple spatial distributions. Observational estimates of the X-ray scattering optical depth are compared to model predictions. Observations of X-ray halos to test interstellar dust grain models are best carried out using extragalactic point sources.

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arXiv2003

SINGS: The SIRTF Nearby Galaxies Survey

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies, and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multi-wavelength library of ancillary and complementary observations, including radio continuum, HI, CO, submillimeter, BVRIJHK, H-alpha, Paschen-alpha, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

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arXiv2003

Astrophysics of Dust in Cold Clouds

Nine lectures reviewing the astrophysics of dust in interstellar clouds. Topics include: (1) Summary of observational evidence concerning interstellar dust: broadband extinction, scattering of starlight, polarization of starlight, spectroscopy of dust, IR and FIR emission, and depletions of grain-forming elements. (2) Optics of interstellar dust grains: dielectric functions of nonconducting and conducting materials, calculational techniques, formulae valid in the Rayleigh limit, Kramers-Kronig relations, microwave emission mechanisms, and X-ray scattering. (3) IR and FIR emission: heating of interstellar dust, including single-photon heating, and resulting IR emission spectrum. (4) Charging of dust grains: collisional charging, photoelectric emission, and resulting charge distribution functions. (5) Dynamics: gas drag, Lorentz force, forces due to anisotropic radiation, and resulting drift velocities. (6) Rotational dynamics: brownian rotation, suprathermal rotation, and effects of starlight torques. (7) Alignment of interstellar dust: observations and theories. (8) Evolution of the grain population: dust formation in outflows, grain growth in the ISM, photodesorption, and grain destruction in shock waves. (9) Effects of dust grains: photoelectric heating, H2 formation, ion recombination, coupling of gas to magnetic fields, and dust grains as indicators of magnetic field direction.

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arXiv2003

Interstellar Dust Grains

This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to IR; optical luminescence; IR emission; microwave emission; optical, UV, and X-ray scattering by dust; and polarization of starlight and of infrared emission. The relationship between presolar grains in meteorites and the interstellar grain population is discussed. Candidate grain materials and abundance constraints are considered. A dust model consisting of amorphous silicate grains, graphite grains, and PAHs is compared with observed emission and scattering. Some issues concerning evolution of interstellar dust are discussed.

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arXiv2003

The X-Ray Scattering Halo Around Nova Cygni 1992: Testing a Model for Interstellar Dust

We use published ROSAT observations of the X-ray Nova V1974 Cygni 1992 to test a model for interstellar dust, consisting of a mixture of carbonaceous grains and silicate grains. The time-dependent X-ray emission from the nova is modelled, and X-ray scattering is calculated for a dust mixture with a realistic size distribution. Model results are compared with the scattered X-ray halos measured by ROSAT at 9 epochs. The observed X-ray halos are consistent with the halos calculated for the size distribution of Weingartner & Draine which reproduces the Milky Way extinction with R_V=3.1, provided that the reddening E(B-V) approx 0.20, consistent with E(B-V) = 0.19 inferred from the late-time Balmer decrement. The time delay of the scattered halo relative to the direct flux from the nova is clearly detected. Models with smoothly-distributed dust give good overall agreement with the observed scattering halo, but tend to produce somewhat more scattering than observed at 200-300", and insufficient scattering at 50-100". While an additional population of large grains can increase the scattered intensity at 50-100", this could also be achieved by having ~30% of the dust in a cloud at a distance from us equal to ~95% of the distance to the nova. Such a model also improves agreement with the data at larger angles, and illustrates the sensitivity of X-ray scattering halos to the location of the dust. The observations therefore do not require a population of micron-sized dust grains.

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arXiv2002

Radiative Torques on Interstellar Grains. III. Dynamics with Thermal Relaxation

In the previous papers in this series, we found that radiative torques can play a major role in the alignment of grains with the interstellar magnetic field. Since the radiative torques can drive the grains to suprathermal rotational speeds, in previous work we made the simplifying assumption that the grain principal axis of greatest moment of inertia is always parallel to the grain angular momentum. This enabled us to describe many of the features of the grain dynamics. However, this assumption fails when the grains enter periods of thermal rotation, which occur naturally in the radiative torque alignment scenario. In the present paper, we relax this assumption and explore the consequences for the grain dynamics. We develop a treatment to follow the grain dynamics including thermal fluctuations and ``thermal flipping'', and show results for one illustrative example. By comparing with a treatment without thermal fluctuations, we see that inclusion of thermal fluctuations can lead to qualitative changes in the grain dynamics. In a future installment in this series, we will use the more complete dynamical treatment developed here to perform a systematic study of grain alignment by radiative torques.

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arXiv2002

User Guide for the Discrete Dipole Approximation Code DDSCAT (Version 5a10)

DDSCAT.5a is a freely available software package which applies the "discrete dipole approximation" (DDA) to calculate scattering and absorption of electromagnetic waves by targets with arbitrary geometries and complex refractive index. The DDA approximates the target by an array of polarizable points. DDSCAT.5a requires that these polarizable points be located on a cubic lattice. DDSCAT.5a10 allows accurate calculations of electromagnetic scattering from targets with "size parameters" 2 pi a/lambda < 15 provided the refractive index m is not large compared to unity (|m-1| < 1). The DDSCAT package is written in Fortran and is highly portable. The program supports calculations for a variety of target geometries (e.g., ellipsoids, regular tetrahedra, rectangular solids, finite cylinders, hexagonal prisms, etc.). Target materials may be both inhomogeneous and anisotropic. It is straightforward for the user to import arbitrary target geometries into the code, and relatively straightforward to add new target generation capability to the package. DDSCAT automatically calculates total cross sections for absorption and scattering and selected elements of the Mueller scattering intensity matrix for specified orientation of the target relative to the incident wave, and for specified scattering directions. This User Guide explains how to use DDSCAT.5a10 to carry out EM scattering calculations. CPU and memory requirements are described.

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arXiv2002

Infrared Emission from Interstellar Dust. III. The Small Magellanic Cloud

The infrared (IR) emission from interstellar dust in the Small Magellanic Cloud (SMC) is modelled using a mixture of amorphous silicate and carbonaceous grains, including a population of polycyclic aromatic hydrocarbon (PAH) molecules. (1) It is shown that this dust model is able to reproduce the spectral energy distribution from near-IR to far-IR for the entire SMC Bar region, provided the PAH abundance in the SMC Bar region is very low. (2) The IR spectrum of the SMCB1#1 molecular cloud can also be reproduced by our dust model provided the PAH abundance is increased relative to the overall SMC Bar. The PAHs in SMCB1#1 incorporate ~3% of the SMC C abundance, compared to <0.4% in the SMC Bar. (3) The spectrum of SMCB1#1 is best reproduced if the PAH mixture has intrinsic IR band strengths which differ from the band strengths which best fit Milky Way PAH mixtures. The variation in the PAH IR band strengths would imply different PAH mixtures, presumably a consequence of differing metallicity or environmental conditions. Other possibilities such as super-hydrogenation of PAHs and softening of the starlight spectrum are also discussed.

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arXiv2002

Do the Infrared Emission Features Need Ultraviolet Excitation? The PAH Model in UV-Poor Reflection Nebulae

One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6, and 11.3um interstellar IR emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the recent detection of these bands in regions with little UV illumination, since small, neutral PAH molecules have little or no absorption at visible wavelengths and therefore require UV photons for excitation. We show here that our "astronomical" PAH model, incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases, can closely reproduce the observed infrared emission bands of vdB 133, a UV-poor reflection nebula. It is also shown that single-photon heating of ``astronomical'' PAHs in reflection nebulae near stars as cool as T_eff=3000K can result in observable emission at 6.2, 7.7, 8.6, and 11.3um. Illustrative mid-IR emission spectra are also calculated for reflection nebulae illuminated by cool stars with T_eff=3500, 4500, 5000K. These will allow comparison with future Space Infrared Telescope Facility (SIRTF) observations of vdB 135 (T_eff=3600K), vdB 47 (T_eff=4500K), and vdB 101 (T_eff=5000K). It is also shown that the dependence of the 12um IRAS emission relative to the total far-IR emission on the effective temperature of the exciting star is consistent with the PAH model expectation for 3000K < T_eff < 30000K.

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arXiv2001

Gamma-Ray Burst in a Molecular Cloud: Destruction of Dust and H2, and Emergent Spectrum

A gamma ray burst with strong optical-UV emission occuring in a molecular cloud will photodissociate H2, photoionize H2, H, and He, and destroy dust grains. We model these processes, including time-dependent radiative transfer in both continuum radiation and the resonance lines of H2. The UV will pump H2 into vibrationally-excited levels. We calculate the absorption spectrum imprinted on radiation from the GRB at various times. In addition to the strong absorption lines of H2(v=0) at lambda < 1110 Angstrom, due to cold ambient gas, we find that radiation reaching us from the GRB and its afterglow will show strong absorption lines due to vibrationally-excited H2 at 1110 < lambda < 1705 Angstrom. These absorption lines, if observed, would provide unequivocal evidence for association of the GRB with molecular gas. Low-resolution spectra will exhibit conspicuous features due to clustering of individual lines; a list of the strongest such absorption features is given for spectral resolution R approx 350 characteristic of the grism on the Swift UV-Optical Telescope.

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arXiv2001

Are Silicon Nanoparticles an Interstellar Dust Component?

Silicon nanoparticles (SNPs) have been proposed as the source of the observed "extended red emission" (ERE) from interstellar dust. We calculate the thermal emission expected from such particles in a reflection nebula such as NGC 2023, and in the diffuse interstellar medium (ISM). Pure neutral Si SNPs would emit at 16.4 micron (um), while Si/SiO2 SNPs (both neutral and charged) produce a feature at 20 um. Observational upper limits on the 16.4 um and 20 um features in NGC 2023 impose upper limits of < 1.5 ppm in pure Si SNPs, and or < 0.2 ppm in Si/SiO2 SNPs. The observed ERE intensity from NGC 2023 then gives a lower bound on the required photoluminescence efficiency eta. For foreground extinction A(0.68um) = 1.2, we find eta > 5% for Si SNPs, or eta > 24% for Si/SiO2 SNPs in NGC 2023. Measurement of the R band extinction toward the ERE-emitting region could strengthen these lower limits. The ERE emissivity of the diffuse ISM appears to require > 42% (> 33%) of solar Si abundance in Si/SiO2 (Si) SNPs. We predict IR emission spectra and show that DIRBE photometry appears to rule out such high abundances of free-flying SNPs in the diffuse ISM. If the ERE is due to SNPs, they must be either in clusters or attached to larger grains. Future observations by SIRTF will be even more sensitive to the presence of free-flying SNPs.

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arXiv2001

Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium

We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains > 1 um in diameter. We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties for radii a > 50 A. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross sections for use in modeling neutral and ionized PAHs from the far ultraviolet to the far infrared. We also propose modifications to the far-infrared emissivity of "astronomical silicate". We calculate energy distribution functions for small grains undergoing "temperature spikes" due to stochastic absorption of starlight photons, using realistic heat capacities and optical properties. Using a grain size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3um. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by COBE and IRTS. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, and we provide tabulated dust opacities (extended tables available at http://www.astro.princeton.edu/~draine/dust/dustmix.html)

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