We present analytical approximations for calculating the scattering, absorption and escape of nonionizing photons from a spherically symmetric two-phase clumpy medium, with either a central point ...source of isotropic radiation, a uniform distribution of isotropic emitters, or uniformly illuminated by external sources. The analytical approximations are based on the mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman, combined with escape and absorption probability formulae for homogeneous media. The accuracy of the approximations is examined by comparison with 3D Monte Carlo simulations of radiative transfer, including multiple scattering. Our studies show that the combined mega-grains and escape/absorption probability formulae provide a good approximation of the escaping and absorbed radiation fractions for a wide range of parameters characterizing the medium. A realistic test is performed by modeling the absorption of a starlike source of radiation by interstellar dust in a clumpy medium, and by calculating the resulting equilibrium dust temperatures and infrared emission spectrum of both the clumps and the interclump medium. In particular, we find that the temperature of dust in clumps is lower than in the interclump medium if clumps are optically thick. Comparison with Monte Carlo simulations of radiative transfer in the same environment shows that the analytic model yields a good approximation of dust temperatures and the emerging UV to FIR spectrum of radiation for all three types of source distributions mentioned above. Our analytical model provides a numerically expedient way to estimate radiative transfer in a variety of interstellar conditions and can be applied to a wide range of astrophysical environments, from star forming regions to starburst galaxies.
A Monte Carlo model of radiative transfer in multi-phase dusty media is applied to the situation of stars and clumpy dust in a sphere or a disk. The distribution of escaping and absorbed photons are ...shown for various filling factors and densities. Analytical methods of approximating the escaping fraction of radiation, based on the Mega-Grains approach, are discussed. Comparison with the Monte Carlo results shows that the escape probability formulae provide a reasonable approximation of the escaping/absorbed fractions, for a wide range of parameters characterizing a clumpy dusty medium. A possibly more realistic model of the interstellar medium is one in which clouds have a self-similar hierarchical structure of denser and denser clumps within clumps, resulting in a fractal distribution of gas and dust. Monte Carlo simulations of radiative transfer in such multi-phase fractal media are compared with the two-phase clumpy case.
A new infrared array camera system using a Hughes/SBRC 58x62 pixel hybrid Si: Ga array detector has been successfully applied to high-background 5-18 µm astronomical imaging observations. The ...off-axis reflective optical system minimizes thermal background loading and produces diffraction-limited images with negligible spatial distortion. The noise equivalent flux density (NEFD) of the camera at 10 µm on the 3.0-m NASA/Infrared Telescope Facility with broadband (Δλ/λ= 0.1) interference filters and 0.26 arcsec pixel is NEFD= 0.01 Jy min~ 1/2 pixel" 1 (1σ), and it operates at a frame rate of 30 Hz with no compromise in observational efficiency. The electronic and optical design of the camera, its photometric characteristics, examples of observational results, and techniques for successful array imaging in a high-background astronomical application are discussed.
A new infrared array camera system using a Hughes/SBRC 58 x 62 pixel hybrid Si:Ga array detector has been successfully applied to high-background 5-18-micron astronomical imaging observations. The ...off-axis reflective optical system minimizes thermal background loading and produces diffraction-limited images with negligible spatial distortion. The noise equivalent flux density (NEFD) of the camera at 10 microns on the 3.0-m NASA/Infrared Telescope Facility with broadband interference filters and 0.26 arcsec pixel is NEFD = 0.01 Jy/sq rt min per pixel (1sigma), and it operates at a frame rate of 30 Hz with no compromise in observational efficiency. The electronic and optical design of the camera, its photometric characteristics, examples of observational results, and techniques for successful array imaging in a high- background astronomical application are discussed.
We present a new short-period brown dwarf candidate around the star TYC 1240-00945-1. This candidate was discovered in the first year of the Multi-object APO Radial Velocity Exoplanets Large-area ...Survey (MARVELS), which is part of the third phase of the Sloan Digital Sky Survey (SDSS-III), and we designate the brown dwarf as MARVELS-1b. MARVELS uses the technique of dispersed fixed-delay interferometery to simultaneously obtain radial velocity measurements for 60 objects per field using a single, custom-built instrument that is fiber fed from the SDSS 2.5-m telescope. From our 20 radial velocity measurements spread over a ~370 d time baseline, we derive a Keplerian orbital fit with semi-amplitude K=2.533+/-0.025 km/s, period P=5.8953+/-0.0004 d, and eccentricity consistent with circular. Independent follow-up radial velocity data confirm the orbit. Adopting a mass of 1.37+/-0.11 M_Sun for the slightly evolved F9 host star, we infer that the companion has a minimum mass of 28.0+/-1.5 M_Jup, a semimajor axis 0.071+/-0.002 AU assuming an edge-on orbit, and is probably tidally synchronized. We find no evidence for coherent instrinsic variability of the host star at the period of the companion at levels greater than a few millimagnitudes. The companion has an a priori transit probability of ~14%. Although we find no evidence for transits, we cannot definitively rule them out for companion radii ~<1 R_Jup.
Using the IRAS Infrared Sky Survey Atlas, we have made 60 x 60 deg mosaics of
the far-infrared emission in the Milky Way. By applying a median normalizing
spatial filter, we were able to eliminate ...the strong gradient in brightness
towards the Galactic midplane. The resulting images reveal a "froth" of
superposed filaments, voids, and shells. This fine-scale structure extends all
the way down to the Galactic midplane. Moreover, it scales in intensity with
the smoothly varying background, independent of latitude, thus indicating that
the fine-scale residual emission is co-extensive with the smooth background. We
conclude that the fine-scale structure is not merely of local origin, but
consists of both nearby and more distant features in the disk. Although we had
expected to find morphological evidence for supernova-driven "worms" or
"chimneys" rooted in the Galactic plane, our processing shows the FIR
fine-scale structure to be more complex (e.g. less coherent and less rooted) as
viewed in projection. Analysis of the spatial statistics shows that the FIR
fine-scale structure is self-similar with a spatial power-law exponent of -3
and a fractal dimension of 2.5 --- similar behavior to that found in isolated
cirrus and molecular clouds. On scales larger than 1.5 deg, the power-law
exponent flattens to -2.5, perhaps indicating a change in the characteristic
structure. This could be due to different dynamical inputs organizing the small
and large-scale structures (e.g. turbulence and diffusion on small scales vs.
macroscopic winds and shock fronts on larger scales).
Using the IRAS Infrared Sky Survey Atlas, we have made 60 x 60 deg mosaics of the far-infrared emission in the Milky Way. By applying a median normalizing spatial filter, we were able to eliminate ...the strong gradient in brightness towards the Galactic midplane. The resulting images reveal a "froth" of superposed filaments, voids, and shells. This fine-scale structure extends all the way down to the Galactic midplane. Moreover, it scales in intensity with the smoothly varying background, independent of latitude, thus indicating that the fine-scale residual emission is co-extensive with the smooth background. We conclude that the fine-scale structure is not merely of local origin, but consists of both nearby and more distant features in the disk. Although we had expected to find morphological evidence for supernova-driven "worms" or "chimneys" rooted in the Galactic plane, our processing shows the FIR fine-scale structure to be more complex (e.g. less coherent and less rooted) as viewed in projection. Analysis of the spatial statistics shows that the FIR fine-scale structure is self-similar with a spatial power-law exponent of -3 and a fractal dimension of 2.5 --- similar behavior to that found in isolated cirrus and molecular clouds. On scales larger than 1.5 deg, the power-law exponent flattens to -2.5, perhaps indicating a change in the characteristic structure. This could be due to different dynamical inputs organizing the small and large-scale structures (e.g. turbulence and diffusion on small scales vs. macroscopic winds and shock fronts on larger scales).
