[en] Observational data are presented relating to the boundary regions between dense, molecular clouds and the more diffuse atomic gas. The data set consists of H I 21 cm and CO (J = 1-0) strip maps in 14 molecular clouds which traverse 62 known CO cloud boundaries. The angular lengths of the strips range from 0.5 deg to several degrees of arc, which is generally adequate to eliminate confusion with background (and foreground) H I. Most of the crossed boundaries (49 of 62) show clear evidence for the presence of H I emission maxima, or halos, lying outside the molecular clouds' gas. There is also general evidence for limb brightening in the CO emission. 40 refs

[en] The alignment of interstellar dust grains with magnetic fields provides a key method for measuring the strength and morphology of the fields. In turn, this provides a means to study the role of magnetic fields from diffuse gas to dense star-forming regions. The physical mechanism for aligning the grains has been a long-term subject of study and debate. The theory of radiative torques, in which an anisotropic radiation field imparts sufficient torques to align the grains while simultaneously spinning them to high rotational velocities, has passed a number of observational tests. Here we use archival polarization data in dense regions of the Orion molecular cloud (OMC-1) at 100, 350, and 850 μm to test the prediction that the alignment efficiency is dependent upon the relative orientations of the magnetic field and radiation anisotropy. We find that the expected polarization signal, with a 180-degree period, exists at all wavelengths out to radii of 1.5 arcmin centered on the Becklin–Neugebauer Kleinmann-Low (BNKL) object in OMC-1. The probabilities that these signals would occur due to random noise are low (≲1%), and are lowest toward BNKL compared to the rest of the cloud. Additionally, the relative magnetic field to radiation anisotropy directions accord with theoretical predictions in that they agree to better than 15° at 100 μm and 4° at 350 μm

[en] The Southern Coalsack is located in the interior of the Upper Centaurus-Lupus (UCL) super bubble and shows many traits that point to a much more energetic environment than might be expected from a dark, starless molecular cloud. A hot, X-ray emitting envelope surrounds the cloud, it has a very strong internal magnetic field, and its darkest core seems to be on astronomical timescales 'just about' to start forming stars. In order to probe the magnetic environment of the cloud and to compare with the optical/near-infrared polarimetry-based field estimates for the cloud, we have acquired Faraday rotation measurements toward the pulsar PSR J1210-6550, probing the magnetic field in the vicinity of the cloud, and a comparison target, PSR J1435-5954, at a similar line-of-sight distance but several degrees from the cloud. Both lines of sight hence primarily probe the UCL super bubble. The earlier estimates of the magnetic field inside the Coalsack, using the Chandrasekhar-Fermi method on optical and near-infrared polarimetry, yield B_{perpendicular} = 64-93 μG. However, even though PSR J1210-6550 is located only ∼30 arcmin from the (CO) edge of the cloud, the measured field strength is only B_|| = -1.1 ± 0.2 μG. While thus yielding a very high field contrast to the cloud we argue that this might be understood as due to the effects on the cloud by the super bubble.

[en] We have obtained optical multi-band polarimetry toward sightlines through the Chamaeleon I cloud, particularly in the vicinity of the young B9/A0 star HD 97300. We show, in agreement with earlier studies, that the radiation field impinging on the cloud in the projected vicinity of the star is dominated by the flux from the star, as evidenced by a local enhancement in the grain heating. By comparing the differential grain heating with the differential change in the location of the peak of the polarization curve, we show that the grain alignment is enhanced by the increase in the radiation field. We also find a weak, but measurable, variation in the grain alignment with the relative angle between the radiation field anisotropy and the magnetic field direction. Such an anisotropy in the grain alignment is consistent with a unique prediction of modern radiative alignment torque theory and provides direct support for radiatively driven grain alignment.

[en] We present near-IR polarimetry data of background stars shining through a selection of starless cores taken in the K band, probing visual extinctions up to A_V∼48. We find that P_K/τ_K continues to decline with increasing A_V with a power law slope of roughly −0.5. Examination of published submillimeter (submm) polarimetry of starless cores suggests that by A_V≳20 the slope for P versus τ becomes ∼−1, indicating no grain alignment at greater optical depths. Combining these two data sets, we find good evidence that, in the absence of a central illuminating source, the dust grains in dense molecular cloud cores with no internal radiation source cease to become aligned with the local magnetic field at optical depths greater than A_V∼20. A simple model relating the alignment efficiency to the optical depth into the cloud reproduces the observations well.

[en] The optical and near-infrared (OIR) polarization of starlight is typically understood to arise from the dichroic extinction of that light by dust grains whose axes are aligned with respect to a local magnetic field. The size distribution of the aligned-grain population can be constrained by measurements of the wavelength dependence of the polarization. The leading physical model for producing the alignment is that of radiative alignment torques (RATs), which predicts that the most efficiently aligned grains are those with sizes larger than the wavelengths of light composing the local radiation field. Therefore, for a given grain-size distribution, the wavelength at which the polarization reaches a maximum (${\mathrm{\lambda <!--\; ??\; -->}}_{max}$) should correlate with the characteristic reddening along the line of sight between the dust grains and the illumination source. A correlation between ${\mathrm{\lambda <!--\; ??\; -->}}_{max}$ and reddening has been previously established for extinctions up to $A_V\approx <!--\; ???\; -->4$ mag. We extend the study of this relationship to a larger sample of stars in the Taurus cloud complex, including extinctions $A_V><!--\; >\; -->10$ mag. We confirm the earlier results for $A_V<<!--\; <\; -->4$ mag but find that the ${\mathrm{\lambda <!--\; ??\; -->}}_{max}$ versus A _V relationship bifurcates above $A_V\approx <!--\; ???\; -->4$ mag, with part of the sample continuing the previously observed relationship. The remaining sample exhibits a steeper rise in ${\mathrm{\lambda <!--\; ??\; -->}}_{max}$ versus A _V. We propose that the data exhibiting the steep rise represent lines of sight of high-density “clumps,” where grain coagulation has taken place. We present RAT-based modeling supporting these hypotheses. These results indicate that multiband OIR polarimetry is a powerful tool for tracing grain growth in molecular clouds, independent of uncertainties in the dust temperature and emissivity.

[en] Optical and infrared continuum polarization from the interstellar medium is driven by radiative processes aligning the grains with the magnetic field. While a quantitative, predictive theory of radiative alignment torques (RATs) exists and has been extensively tested, several parameters of the theory remain to be fully constrained. In a recent paper, Medan & Andersson showed that the polarization efficiency (and therefore grain alignment efficiency) at different locations in the wall of the Local Bubble (LB) could be modeled as proportional to the integrated light intensity from the surrounding stars and OB associations. Here we probe that relationship at high radiation field intensities by studying the extinction and polarization in the two reflection nebulae IC 59 and IC 63 in the Sh 2-185 H ii region, illuminated by the B0 IV star γ Cassiopeia. We combine archival visual polarimetry with new seven-band photometry in the Vilnius system, to derive the polarization efficiency from the material. We find that the same linear relationship seen in the LB wall also applies to the Sh 2-185 region, strengthening the conclusion from the earlier study.

[en] Interstellar dust grain alignment causes polarization from UV to mm wavelengths, allowing the study of the geometry and strength of the magnetic field. Over the last couple of decades, observations and theory have led to the establishment of the radiative alignment torque mechanism as a 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. Photodissociation regions, with their intense, anisotropic radiation fields, consequent rapid H₂ 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 H₂ formation-enhanced alignment and the first direct empirical evidence for disalignment due to gas–grain collisions using high-resolution HCO⁺(J = 1-0) observations. We find that a relative amount of polarization is marginally anticorrelated with column density of HCO⁺. However, separating the lines of sight of optical polarimetry into those behind, or in front of, a dense clump as seen from γ Cas, the distribution separates into two well defined sets, with data corresponding to “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 $R_C\propto <!--\; ???\; -->n\sqrt{T}$. Ratios of the best-fit slopes for the “illuminated” and “shaded” samples of lines of sight agrees, within the uncertainties, with the square root of the two-temperature H₂ excitation in the nebula seen by Thi et al.

[en] The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for observatory performance evaluation. These in-flight opportunities are viewed as the first comprehensive assessment of the observatory's performance and are used to guide future development activities, as well as to identify additional observatory upgrades. Pointing stability was evaluated, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an active mass damper system installed on the telescope. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have also been performed. Additional tests targeted basic observatory capabilities and requirements, including pointing accuracy, chopper evaluation, and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA observatory performance and characterization

[en] A recently discovered filament of polarized starlight that traces a coherent magnetic field is shown to have several properties that are consistent with an origin in the outer heliosheath of the heliosphere: (1) the magnetic field that provides the best fit to the polarization position angles is directed toward $\mathrm{\ell <!--\; ???\; -->}$ = 357.°3, b = 17.°0 (±11.°2); this direction is within 6.°7 ± 11.°2 of the observed upwind direction of the flow of interstellar neutral helium gas through the heliosphere. (2) The magnetic field is ordered; the component of the variation of the polarization position angles that can be attributed to magnetic turbulence is ±9.°6. (3) The axis of the elongated filament can be approximated by a line that defines an angle of 80° ± 14° with the plane that is formed by the interstellar magnetic field (ISMF) vector and the vector of the inflowing neutral gas (the $\mathit{B}\mathit{V}$ plane). We propose that this polarization feature arises from aligned interstellar dust grains in the outer heliosheath where the interstellar plasma and magnetic field are deflected around the heliosphere. This interpretation suggests that the polarization is seen where stream lines of the flow and the draped ISMF lines are approximately parallel to each other and perpendicular to the sightline. An ordered magnetic field is required so that grain alignment is not disrupted during the interaction. The filament location is consistent with dust plumes previously predicted to form around the heliosphere. The proposed outer heliosheath location of the polarizing grains can be tested with three-dimensional models that track torques on asymmetric dust grains as they propagate through the heliosheath plasma, and using these models to evaluate grain alignment and the asymmetric extinction of the grains.