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  1. Abstract

    We present SCUBA-2/POL-2 850μm polarimetric observations of the circumstellar envelope (CSE) of the carbon-rich asymptotic giant branch (AGB) star IRC+10216. Both far-IR (FIR) and optical polarization data indicate grains aligned with their long axis in the radial direction relative to the central star. The 850μm polarization does not show this simple structure. The 850μm data are indicative, albeit not conclusive, of a magnetic dipole geometry. Assuming such a simple dipole geometry, the resulting 850μm polarization geometry is consistent with both Zeeman observations and small-scale structure in the CSE. While there is significant spectral-line polarization contained within the SCUBA-2 850μm passband for the source, it is unlikely that our broadband polarization results are dominated by line polarization. To explain the required grain alignment, grain mineralogy effects, due to either fossil silicate grains from the earlier oxygen-rich AGB phase of the star or due to the incorporation of ferromagnetic inclusions in the largest grains, may play a role. We argue that the most likely explanation is due to a new alignment mechanism wherein a charged grain, moving relative to the magnetic field, precesses around the induced electric field and therefore aligns with the magnetic field. This mechanism is particularly attractive as the optical, FIR, and submillimeter-wave polarization of the carbon dust can then be explained in a consistent way, differing simply due to the charge state of the grains.

     
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  2. Abstract

    We used high-resolution [Cii] 158μm mapping of two nebulae IC 59 and IC 63 from SOFIA/upGREAT in conjunction with ancillary data of the gas, dust, and polarization to probe the kinematics, structure, and magnetic properties of their photodissociation regions (PDRs). The nebulae are part of the Sh 2-185 Hiiregion that is illuminated by the B0 IVe starγCas. The velocity structure of each PDR changes with distance fromγCas, which is consistent with driving by the radiation. Based on previous far-ultraviolet (FUV) flux measurements of, and the known distance to,γCas, along with the predictions of 3D distances to the clouds, we estimated the FUV radiation field strength (G0) at the clouds. Assuming negligible extinction between the star and clouds, we find their 3D distances fromγCas. For IC 63, our results are consistent with earlier estimates of distance from Andersson et al., locating the cloud at ∼2 pc fromγCas at an angle of 58° to the plane of the sky behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70° in front of the star. We do not detect any significant correlation between the orientation of the magnetic field and the velocity gradients of [Cii] gas, which indicates a moderate magnetic field strength. The kinetic energy in IC 63 is estimated to be an order of 10 higher than the magnetic energies. This suggests that kinetic pressure in this nebula is dominant.

     
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  3. Abstract

    Photodissociation regions (PDRs), where the (far-)ultraviolet light from hot young stars interact with the gas in surrounding molecular clouds, provide laboratories for understanding the nature and role of feedback by star formation on the interstellar medium. While the general nature of PDRs is well understood—at least under simplified conditions—the detailed dynamics and chemistry of these regions, including gas clumping, evolution over time, etc., can be very complex. We present interferometric observations of the 21 cm atomic hydrogen line, combined with [Cii] 158μm observations, toward the nearby reflection nebula IC 63. We find a clumpy Histructure in the PDR, and a ring morphology for the Hiemission at the tip of IC 63. We further unveil kinematic substructure, of the order of 1 km s−1, in the PDR layers and several legs that will disperse IC 63 in <0.5 Myr. We find that the dynamics in the PDR explain the observed clumpy Hidistribution and lack of a well-defined Hi/H2transition front. However, it is currently not possible to conclude whether Hiself-absorption and nonequilibrium chemistry also contribute to this clumpy morphology and missing Hi/H2transition front.

     
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  4. Abstract

    Continuum polarization over the UV-to-microwave range is due to dichroic extinction (or emission) by asymmetric, aligned dust grains. Scattering can also be an important source of polarization, especially at short wavelengths. Because of both grain alignment and scattering physics, the wavelength dependence of the polarization, generally, traces the size of the aligned grains. Similarly because of the differing wavelength dependencies of dichroic extinction and scattering polarization, the two can generally be reliably separated. Ultraviolet (UV) polarimetry therefore provides a unique probe of the smallest dust grains (diameter$< 0.09~\upmu \text{m}$<0.09μm), their mineralogy and interaction with the environment. However, the current observational status of interstellar UV polarization is very poor with less than 30 lines of sight probed. With the modern, quantitative and well-tested, theory of interstellar grain alignment now available, we have the opportunity to advance the understanding of the interstellar medium (ISM) by executing a systematic study of the UV polarization in the ISM of the Milky Way and near-by galaxies. The Polstar mission will provide the sensitivity and observing time needed to carry out such a program (probing hundreds of stars in the Milky Way and dozens of stars in the LMC/SMC), addressing questions of dust composition as a function of size and location, radiation- and magnetic-field characteristics as well as unveiling the carrier of the 2175 Å extinction feature. In addition, using high-resolution UV line spectroscopy Polstar will search for and probe the alignment of, and polarization from, aligned atoms and ions - so called “Ground State Alignment”, a potentially powerful new probe of magnetic fields in the diffuse ISM.

     
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  5. Abstract

    The heart of the Large Magellanic Cloud, 30 Doradus, is a complex region with a clear core-halo structure. Feedback from the stellar cluster R136 has been shown to be the main source of energy creating multiple parsec-scale expanding-shells in the outer region, and carving a nebula core in the proximity of the ionization source. We present the morphology and strength of the magnetic fields (B-fields) of 30 Doradus inferred from the far-infrared polarimetric observations by SOFIA/HAWC+ at 89, 154, and 214μm. TheB-field morphology is complex, showing bending structures around R136. In addition, we use high spectral and angular resolution [Cii] observations from SOFIA/GREAT and CO(2-1) from APEX. The kinematic structure of the region correlates with theB-field morphology and shows evidence of multiple expanding-shells. OurB-field strength maps, estimated using the Davis–Chandrasekhar–Fermi method and structure-function, show variations across the cloud within a maximum of 600, 450, and 350μG at 89, 154, and 214μm, respectively. We estimated that the majority of the 30 Doradus clouds are subcritical and sub-Alfvénic. The probability distribution function of the gas density shows that the turbulence is mainly compressively driven, while the plasma beta parameter indicates supersonic turbulence. We show that theB-field is sufficient to hold the cloud structure integrity under feedback from R136. We suggest that supersonic compressive turbulence enables the local gravitational collapse and triggers a new generation of stars to form. The velocity gradient technique using [Cii] and CO(2-1) is likely to confirm these suggestions.

     
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  6. Abstract We have measured the gas temperature in the IC 63 photodissociation region (PDR) using the S(1) and S(5) pure rotation lines of molecular hydrogen with SOFIA/EXES. We divide the PDR into three regions for analysis based on the illumination from γ Cas: sunny, ridge, and shady. Constructing rotation diagrams for the different regions, we obtain temperatures of T ex = 562 − 43 + 52 K toward the ridge and T ex = 495 − 25 + 28 K in the shady side. The H 2 emission was not detected on the sunny side of the ridge, likely due to the photodissociation of H 2 in this gas. Our temperature values are lower than the value of T ex = 685 ± 68 K using the S(1), S(3), and S(5) pure rotation lines, derived by Thi et al. using lower spatial resolution ISO-SWS data at a different location of the IC 63 PDR. This difference indicates that the PDR is inhomogeneous and illustrates the need for high-resolution mapping of such regions to fully understand their physics. The detection of a temperature gradient correlated with the extinction into the cloud, points to the ability of using H 2 pure rotational line spectroscopy to map the gas temperature on small scales. We used a PDR model to estimate the FUV radiation and corresponding gas densities in IC 63. Our results shows the capability of SOFIA/EXES to resolve and provide detailed information on the temperature in such regions. 
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  7. Abstract

    Dust-induced polarization in the interstellar medium (ISM) is due to asymmetric grains aligned with an external reference direction, usually the magnetic field. For both the leading alignment theories, the alignment of the grain’s angular momentum with one of its principal axes and the coupling with the magnetic field requires the grain to be paramagnetic. Of the two main components of interstellar dust, silicates are paramagnetic, while carbon dust is diamagnetic. Hence, carbon grains are not expected to align in the ISM. To probe the physics of carbon grain alignment, we have acquired Stratospheric Observatory for Infrared Astronomy/Higch-resolution Airborne Wideband Camera-plus far-infrared photometry and polarimetry of the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10° 216. The dust in such CSEs are fully carbonaceous and thus provide unique laboratories for probing carbon grain alignment. We find a centrosymmetric, radial, polarization pattern, where the polarization fraction is well correlated with the dust temperature. Together with estimates of a low fractional polarization from optical polarization of background stars, we interpret these results to be due to a second-order, direct radiative external alignment of grains without internal alignment. Our results indicate that (pure) carbon dust does not contribute significantly to the observed ISM polarization, consistent with the nondetection of polarization in the 3.4μm feature due to aliphatic CH bonds on the grain surface.

     
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  8. Abstract Located in the Large Magellanic Cloud and mostly irradiated by the massive star cluster R136, 30 Doradus is an ideal target to test the leading theory of grain alignment and rotational disruption by RAdiative Torques (RATs). Here, we use publicly available polarized thermal dust emission observations of 30 Doradus at 89, 154, and 214 μ m using SOFIA/HAWC+. We analyze the variation of the dust polarization degree ( p ) with the total emission intensity ( I ), the dust temperature ( T d ), and the gas column density ( N H ) constructed from Herschel data. The 30 Doradus complex is divided into two main regions relative to R136, namely North and South. In the North, we find that the polarization degree first decreases and then increases before decreasing again when the dust temperature increases toward the irradiating cluster R136. The first depolarization likely arises from the decrease in grain alignment efficiency toward the dense medium due to the attenuation of the interstellar radiation field and the increase in the gas density. The second trend (the increase of p with T d ) is consistent with the RAT alignment theory. The final trend (the decrease of p with T d ) is consistent with the RAT alignment theory only when the grain rotational disruption by RATs is taken into account. In the South, we find that the polarization degree is nearly independent of the dust temperature, while the grain alignment efficiency is higher around the peak of the gas column density and decreases toward the radiation source. The latter feature is also consistent with the prediction of rotational disruption by RATs. 
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