An Analysis of the Shapes of Interstellar Extinction Curves. VI. The Near-IR Extinction Law

We combine new HST/ACS observations and existing data to investigate the wavelength dependence of NIR extinction. Previous studies suggest a power-law form, with a "universal" value of the exponent, although some recent observations indicate that significant sight line-to-sight line variability may exist. We show that a power-law model provides an excellent fit to most NIR extinction curves, but that the value of the power, beta, varies significantly from sight line-to-sight line. Therefore, it seems that a "universal NIR extinction law" is not possible. Instead, we find that as beta decreases, R(V) [=A(V)/E(B-V)] tends to increase, suggesting that NIR extinction curves which have been considered "peculiar" may, in fact, be typical for different R(V) values. We show that the power law parameters can depend on the wavelength interval used to derive them, with the beta increasing as longer wavelengths are included. This result implies that extrapolating power law fits to determine R(V) is unreliable. To avoid this problem, we adopt a different functional form for NIR extinction. This new form mimics a power law whose exponent increases with wavelength, has only 2 free parameters, can fit all of our curves over a longer wavelength baseline and to higher precision, and produces R(V) values which are consistent with independent estimates and commonly used methods for estimating R(V). Furthermore, unlike the power law model, it gives R(V)'s that are independent of the wavelength interval used to derive them. It also suggests that the relation R(V) = -1.36 E(K-V)/E(B-V) - 0.79 can estimate R(V) to +/-0.12. Finally, we use model extinction curves to show that our extinction curves are in accord with theoretical expectations.Comment: To appear in the Astrophysical Journa

Two-fluid magnetic island dynamics in slab geometry: II - Islands interacting with resistive walls or static external resonant magnetic perturbations

The dynamics of a propagating magnetic island interacting with a resistive wall or a static external magnetic perturbation is investigated using two-fluid, drift-MHD theory in slab geometry. In both cases, the island equation of motion is found to take exactly the same form as that predicted by single-fluid MHD theory. Three separate ion polarization terms are found in the Rutherford island width evolution equation. The first is the drift-MHD polarization term for an isolated island, and is completely unaffected by interaction with a wall or magnetic perturbation. Next, there is the polarization term due to interaction with a wall or magnetic perturbation which is predicted by single-fluid MHD theory. Finally, there is a hybrid of the other two polarization terms. The sign of this term depends on many factors. However, under normal conditions, it is stabilizing if the unperturbed island propagates in the ion diamagnetic direction (in the lab. frame), and destabilizing if it propagates in the electron diamagnetic direction

State Labor Legislation Enacted in 2012

Laws concerning child labor, equal employment opportunity, human trafficking, immigration legislation, independent contractors, and prevailing wages were among the most active areas for state lawmakers in 2012

Field localization on a brane intersection in anti-de Sitter spacetime

We discuss the localization of scalar, fermion, and gauge field zero modes on a $3-$brane that resides at the intersection of two $4-$branes in six-dimensional anti-de Sitter space. This set-up has been introduced in the context of brane world models and, higher-dimensional versions of it, in string theory. In both six- and ten-dimensional cases, it has been shown that four-dimensional gravity can be reproduced at the intersection, due to the existence of a massless, localized graviton zero-mode. However, realistic scenarios require also the Standard Model to be localized on the $3-$brane. In this paper, we discuss under which conditions a higher-dimensional field theory, propagating on the above geometry, can have a zero-mode sector localized at the intersection and find that zero modes can be localized only if masses and couplings to the background curvature satisfy certain relations. We also consider the case when other 4-branes cut the bulk at some distance from the intersection and argue that, in the probe brane approximation, there is no significant effect on the localization properties at the $3-$brane. The case of bulk fermions is particularly interesting, since the properties of the geometry allow localization of chiral modes independently.Comment: 13 pages, 3 figures, the version to be published in PR