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Register a new userElectric dipole moments and disalignment of interstellar dust grains
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The degree to which interstellar grains align with respect to the interstellar magnetic field depends on disaligning as well as aligning mechanisms. For decades, it was assumed that disalignment was due primarily to the random angular impulses a grain receives when colliding with gas-phase atoms. Recently, a new disalignment mechanism has been considered, which may be very potent for a grain that has a time-varying electric dipole moment and drifts across the magnetic field. We provide quantitative estimates of the disalignment times for silicate grains with size > approximately 0.1 micron. These appear to be shorter than the time-scale for alignment by radiative torques, unless the grains contain superparamagnetic inclusions.
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Several mechanisms have been proposed to explain the alignment of grains with the interstellar magnetic field, including paramagnetic dissipation, radiative torques, and supersonic gas-grain streaming. These must compete with disaligning processes, including randomly directed torques arising from collisions with gas atoms. I describe a novel disalignment mechanism for grains that have a time-varying electric dipole moment and that drift across the magnetic field. Depending on the drift speed, this mechanism may yield a much shorter disalignment timescale than that associated with random gas atom impacts. For suprathermally rotating grains, the new disaligning process may be more potent for carbonaceous dust than for silicate dust. This could result in efficient alignment for silicate grains but poor alignment for carbonaceous grains.
Interstellar dust grain alignment causes polarization from UV to mm wavelengths, allowing the study of the geometry and strength of the magnetic field. Over last couple of decades observations and theory have led to the establishment of the Radiative Alignment Torque (RAT) mechanism as leading candidate to explain the effect. With a quantitatively well constrained theory, polarization can be used not only to study the interstellar magnetic field, but also the dust and other environmental parameters. Photo-dissociation Regions (PDRs), with their intense, anisotropic radiation fields, consequent rapid $rm H_{2}$ formation, and high spatial density-contrast provide a rich environment for such studies. Here we discuss an expanded optical, NIR, and mm-wave study of the IC,63 nebula, showing strong $rm H_{2}$ formation-enhanced alignment and the first direct empirical evidence for disalignment due to gas-grain collisions using high-resolution $rm HCO^{+}$(J=1-0) observations. We find that relative amount of polarization is marginally anti-correlated with column density of $rm HCO^{+}$. However, separating the lines of sight of optical polarimetry into those behind, or in front of, a dense clump as seen from $gamma$ Cas, the distribution separates into two well defined sets, with data corresponding to enquote{shaded} gas having a shallower slope. This is expected if the decrease in polarization is caused by collisions since collisional disalignment rate is proportional to R$_Cpropto nsqrt{T}$. Ratios of the best-fit slopes for the enquote{illuminated} and enquote{shaded} samples of lines of sight agrees, within the uncertainties, with the square-root of the two-temperature H$_2$ excitation in the nebula seen by Thi et al. (2009).
Interstellar dust plays a central role in shaping the detailed structure of the interstellar medium, thus strongly influencing star formation and galaxy evolution. Dust extinction provides one of the main pillars of our understanding of interstellar dust while also often being one of the limiting factors when interpreting observations of distant objects, including resolved and unresolved galaxies. The ultraviolet (UV) and mid-infrared (MIR) wavelength regimes exhibit features of the main components of dust, carbonaceous and silicate materials, and therefore provide the most fruitful avenue for detailed extinction curve studies. Our current picture of extinction curves is strongly biased to nearby regions in the Milky Way. The small number of UV extinction curves measured in the Local Group (mainly Magellanic Clouds) clearly indicates that the range of dust properties is significantly broader than those inferred from the UV extinction characteristics of local regions of the Milky Way. Obtaining statistically significant samples of UV and MIR extinction measurements for all the dusty Local Group galaxies will provide, for the first time, a basis for understanding dust grains over a wide range of environments. Obtaining such observations requires sensitive medium-band UV, blue-optical, and mid-IR imaging and followup R ~ 1000 spectroscopy of thousands of sources. Such a census will revolutionize our understanding of the dependence of dust properties on local environment providing both an empirical description of the effects of dust on observations as well as strong constraints on dust grain and evolution models.
The electric and magnetic dipole moments of dyon fermions are calculated within N=2 supersymmetric Yang-Mills theory including the theta-term. It is found, in particular, that the gyroelectric ratio deviates from the canonical value of 2 for the monopole fermion (n_m=1,n_e=0) in the case theta ot=0. Then, applying the S-duality transformation to the result for the dyon fermions, we obtain an explicit prediction for the electric dipole moment (EDM) of the charged fermion (`electron). It is thus seen that the approach presented here provides a novel method for computing the EDM induced by the theta-term.
Interstellar dust grains are non-spherical and, in some environments, partially aligned along the direction of the interstellar magnetic field. Numerous alignment theories have been proposed, all of which examine the grain rotational dynamics. In 1999, Lazarian & Draine introduced the important concept of thermal flipping, in which internal relaxation processes induce the grain body to flip while its angular momentum remains fixed. Through detailed numerical simulations, we study the role of thermal flipping on the grain dynamics during periods of relatively slow rotation, known as `crossovers, for the special case of a spheroidal grain with a non-uniform mass distribution. Lazarian & Draine proposed that rapid flipping during a crossover would lead to `thermal trapping, in which a systematic torque, fixed relative to the grain body, would time average to zero, delaying spin-up to larger rotational speeds. We find that the time-averaged systematic torque is not zero during the crossover and that thermal trapping is not prevalent. As an application, we examine whether the classic Davis-Greenstein alignment mechanism is viable, for grains residing in the cold neutral medium and lacking superparamagnetic inclusions. We find that Davis-Greenstein alignment is not hindered by thermal trapping, but argue that it is, nevertheless, too inefficient to yield the alignment of large grains responsible for optical and infrared starlight polarization. Davis-Greenstein alignment of small grains could potentially contribute to the observed ultraviolet polarization. The theoretical and computational tools developed here can also be applied to analyses of alignment via radiative torques and rotational disruption of grains.
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