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Dust extinction is one of the fundamental measurements of dust grain sizes, compositions, and shapes. Most of the wavelength-dependent variations seen in Milky Way extinction are strongly correlated with the single parameter R (V) = A (V)/ E (B − V). Existing R (V)-dependent extinction relationships use a mixture of spectroscopic and photometry observations, and hence do not fully capture all the important dust features or continuum variations. Using four existing samples of spectroscopically measured dust extinction curves, we consistently measure the R (V)-dependent extinction relationship spectroscopically from the far-ultraviolet (FUV) to mid-infrared for the first time. Linear fits of A (λ)/ A (V) dependent on R (V) are done using a method that fully accounts for their significant and correlated uncertainties. These linear parameters are fit with analytic wavelength-dependent functions to determine the smooth R (V) (2.3–5.6) and wavelength (912 Å–32 μ m) dependent extinction relationship. This relationship shows that the FUV rise, 2175 Å bump, and the three broad optical features are dependent on R (V), but the 10 and 20 μ m features are not. Existing literature relationships show significant deviations compared to this relationship especially in the FUV and infrared (IR). Extinction curves that clearly deviate from this relationship illustrate that this relationship only describes the average behavior versus R (V). We find tentative evidence that the relationship may not be linear with R (V)−1 especially in the ultraviolet (UV). For the first time, this relationship provides measurements of dust extinction that spectroscopically resolve the continuum and features in the UV, optical, and IR as a function of R (V), enabling detailed studies of dust grain properties and full spectroscopic accounting for the effects of dust extinction on astrophysical objects. [ABSTRACT FROM AUTHOR]

Interstellar dust extinction curves provide valuable information about dust properties, including the composition and size of the dust grains, and are essential to correct observations for the effects of interstellar dust. In this work, we measure a representative sample of near-infrared (NIR; 0.8â€"5.5 ÎĽ m) spectroscopic extinction curves for the first time, enabling us to investigate the extinction at wavelengths where it is usually only measured in broad photometric bands. We use IRTF/SpeX spectra of a sample of reddened and comparison stars to measure 15 extinction curves with the pair method. Our sample spans A (V) values from 0.78 to 5.65 and R (V) values from 2.43 to 5.33. We confirm that the NIR extinction curves are well fit by a power law, with indices and amplitudes differing from sight line to sight line. Our average diffuse NIR extinction curve can be represented by a single power law with index α = 1.7, but because of the sight line-to-sight line variations, the shape of any average curve will depend on the parental sample. We find that most of the variation in our sample can be linked to the ratio of total-to-selective extinction R (V), a rough measurement of the average dust grain size. Two sight lines in our sample clearly show the ice extinction feature at 3 ÎĽ m, which can be fitted by a modified Drude profile. We find tentative ice detections with slightly over 3 σ significance in two other sight lines. In our average diffuse extinction curve, we measure a 3 σ upper limit of A (ice)/ A (V) = 0.0021 for this ice feature. [ABSTRACT FROM AUTHOR]

Context. Thermal emission by stochastically heated dust grains (SHGs) plays an important role in the radiative transfer (RT) problem for a dusty medium. It is therefore essential to verify that RT codes properly calculate the dust emission before studying the effects of spatial distribution and other model parameters on the simulated observables. Aims. We define an appropriate problem for benchmarking dust emissivity calculations in the context of RT simulations, specifically including the emission from SHGs. Our aim is to provide a self-contained guide for implementors of such functionality and to offer insight into the effects of the various approximations and heuristics implemented by the participating codes to accelerate the calculations. Methods. The benchmark problem definition includes the optical and calorimetric material properties and the grain size distributions for a typical astronomical dust mixture with silicate, graphite, and PAH components. It also includes a series of analytically defined radiation fields to which the dust population is to be exposed and instructions for the desired output. We processed this problem using six RT codes participating in this benchmark effort and compared the results to a reference solution computed with the publicly available dust emission code DustEM. Results. The participating codes implement different heuristics to keep the calculation time at an acceptable level. We study the effects of these mechanisms on the calculated solutions and report on the level of (dis)agreement between the participating codes. For all but the most extreme input fields, we find agreement within 10% across the important wavelength range 3 μm < λ < 1000 μm. Conclusions. We conclude that the relevant modules in RT codes can and do produce fairly consistent results for the emissivity spectra of SHGs. This work can serve as a reference for implementors of dust RT codes, and it will pave the way for a more extensive benchmark effort focusing on the RT aspects of the various codes. [ABSTRACT FROM AUTHOR]