Polarized Infrared Emission by Polycyclic Aromatic Hydrocarbons resulting from Anisotropic Illumination

We study the polarized infrared emission by Polycyclic Aromatic Hydrocarbons (PAHs), when anisotropically illuminated by UV photons. PAH molecules are modeled as planar disks with in-plane and out-of-plane vibrational dipoles. As first pointed out by Leger (1988), infrared emission features resulting from in-plane and out-of-plane modes should have orthogonal polarization directions. We show analytically how the degree of polarization depends on the viewing geometry and the molecule's internal alignment between principal axis of inertia and angular momentum, which gets worse after photon absorption. Longer wavelength features, emitted after better internal alignment is recovered, should be more strongly polarized. The degree of polarization for uni-directional illumination (e.g., by a star) is larger than for diffuse illumination (e.g., by a disk galaxy), all else being equal. For PAHs in the Cold Neutral Medium, the predicted polarization is probably too small to distinguish from the contribution of linear dichroism by aligned foreground dust. The level of polarization predicted for PAH emission from the Orion Bar is only ~0.06% at 3.3 microns; Sellgren et al. (1988) report a much larger value, 0.86+-0.28%, which suggests that the smallest PAHs may have moderately suprathermal rotation rates. Future observations of (or upper limits on) the degree of polarization for the Orion Bar or for dust above edge-on galaxies (e.g., NGC 891 or M82) may constrain the internal alignment of emitting PAHs, thus providing clues to their rotational dynamics.Comment: 9 pages, 4 figures, 1 table, submitted to Ap

The Detection of Anomalous Dust Emission in the Nearby Galaxy NGC 6946

We report on the Ka-band (26-40 GHz) emission properties for 10 star-forming regions in the nearby galaxy NGC 6946. From a radio spectral decomposition, we find that the 33 GHz flux densities are typically dominated by thermal (free-free) radiation. However, we also detect excess Ka-band emission for an outer-disk star-forming region relative to what is expected given existing radio, submillimeter, and infrared data. Among the 10 targeted regions, measurable excess emission at 33 GHz is detected for half of them, but in only one region is the excess found to be statistically significant ($\approx7\sigma$). We interpret this as the first likely detection of so called `anomalous' dust emission outside of the Milky Way. We find that models explaining this feature as the result of dipole emission from rapidly rotating ultrasmall grains are able to reproduce the observations for reasonable interstellar medium conditions. While these results suggest that the use of Ka-band data as a measure of star formation activity in external galaxies may be complicated by the presence of anomalous dust, it is unclear how significant a factor this will be for globally integrated measurements as the excess emission accounts for \la10% of the total Ka-band flux density from all 10 regions.Comment: 6 pages; Accepted to ApJ Letter

Radiative torques alignment in the presence of pinwheel torques

We study the alignment of grains subject to both radiative torques and pinwheel torques while accounting for thermal flipping of grains. By pinwheel torques we refer to all systematic torques that are fixed in grain body axes, including the radiative torques arising from scattering and absorption of isotropic radiation. We discuss new types of pinwheel torques, which are systematic torques arising from infrared emission and torques arising from the interaction of grains with ions and electrons in hot plasma. We show that both types of torques are long-lived, i.e. may exist longer than gaseous damping time. We compare these torques with the torques introduced by E. Purcell, namely, torques due to H$_2$ formation, the variation of accommodation coefficient for gaseous collisions and photoelectric emission. Furthermore, we revise the Lazarian & Draine model for grain thermal flipping. We calculate mean flipping timescale induced by Barnett and nuclear relaxation for both paramagnetic and superparamagnetic grains, in the presence of stochastic torques associated with pinwheel torques, e.g. the stochastic torques arising from H$_2$ formation, and gas bombardment. We show that the combined effect of internal relaxation and stochastic torques can result in fast flipping for sufficiently small grains and, because of this, they get thermally trapped, i.e. rotate thermally in spite of the presence of pinwheel torques. For sufficiently large grains, we show that the pinwheel torques can increase the degree of grain alignment achievable with the radiative torques by increasing the magnitude of the angular momentum of low attractor points and/or by driving grains to new high attractor points.Comment: 23 pages and 15 figures emulated ApJ style. Thermal flipping and trapping revised; paper accepted to Ap

Direct evidence of dust growth in L183 from MIR light scattering

Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission). We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree agree with the AV map and compare them to the Spitzer data. The Spitzer data of L183 show emission in the 3.6 and 4.5 micron bands, while the 5.8 micron band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 micron, which is not seen. Hence, we interpret this emission to be MIR cloudshine. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 micron.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and Astrophysic

Circumstellar grain formation

Dust formation around cool giant and supergiant stars is examined in terms of grain formulation. Optical properties of small clusters, molecular physics of cluster nucleation and growth, circumstellar mass flows, and their application to alpha Ori are discussed

Constraining Spinning Dust Parameters with the WMAP Five-Year Data

We characterize spinning dust emission in the warm ionized medium by comparing templates of Galactic dust and Halpha with the 5-year maps from the Wilkinson Microwave Anisotropy Probe. The Halpha-correlated microwave emission deviates from the thermal bremsstrahlung (free-free) spectrum expected for ionized gas, exhibiting an additional broad bump peaked at ~40 GHz which provides ~20% of the peak intensity. We confirm that the bump is consistent with a modified Draine & Lazarian (1998) spinning dust model, though the peak frequency of the emission is somewhat lower than the 50 GHz previously claimed. This frequency shift results from systematic errors in the large-scale modes of the 3-year WMAP data which have been corrected in the 5-year data release. We show that the bump is not the result of errors in the Halpha template by analyzing regions of high free-free intensity, where the WMAP K-band map may be used as the free-free template. We rule out a pure free-free spectrum for the Halpha-correlated emission at high confidence: ~27sigma for the nearly full-sky fit, even after marginalizing over the CMB cross-correlation bias. We also extend the previous analysis by searching the parameter space of the Draine & Lazarian model but letting the amplitude float. The best fit for reasonable values of the characteristic electric dipole moment and density requires an amplitude factor of ~0.3. This suggests that small PAHs in the warm ionized medium are depleted by a factor of ~3.Comment: 17 pages, 13 figures; submitted to Ap

The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 and 6.85 micrometer emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be <15%. We conclude that the UIE emitters are predominantly aromatic with aliphatic material at most a minor part of the UIE carriers. The PAH model is consistent with astronomical observations and PAHs dominate the strong UIE bands.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter