• The Acceleration of Charged Particles at a Spherical Shock Moving through an Irregular Magnetic Field

      Giacalone, J.; Univ Arizona, Dept Planetary Sci (IOP PUBLISHING LTD, 2017-10-23)
      We investigate the physics of charged-particle acceleration at spherical shocks moving into a uniform plasma containing a turbulent magnetic field with a uniform mean. This has applications to particle acceleration at astrophysical shocks, most notably, to supernovae blast waves. We numerically integrate the equations of motion of a large number of test protons moving under the influence of electric and magnetic fields determined from a kinematically defined plasma flow associated with a radially propagating blast wave. Distribution functions are determined from the positions and velocities of the protons. The unshocked plasma contains a magnetic field with a uniform mean and an irregular component having a Kolmogorov-like power spectrum. The field inside the blast wave is determined from Maxwell's equations. The angle between the average magnetic field and unit normal to the shock varies with position along its surface. It is quasi-perpendicular to the unit normal near the sphere's equator, and quasi-parallel to it near the poles. We find that the highest intensities of particles, accelerated by the shock, are at the poles of the blast wave. The particles "collect" at the poles as they approximately adhere to magnetic field lines that move poleward from their initial encounter with the shock at the equator, as the shock expands. The field lines at the poles have been connected to the shock the longest. We also find that the highest-energy protons are initially accelerated near the equator or near the quasi-perpendicular portion of the shock, where the acceleration is more rapid.
    • Dust Transport and Processing in Centrifugally Driven Protoplanetary Disk Winds

      Giacalone, Steven; Teitler, Seth; Königl, Arieh; Krijt, Sebastiaan; Ciesla, Fred J.; Univ Arizona, Dept Astron & Steward Observ (IOP PUBLISHING LTD, 2019-08-29)
      There is evidence that protoplanetary disks including the protosolar one-contain crystalline dust grains on spatial scales where the dust temperature is lower than the threshold value for their formation through thermal annealing of amorphous interstellar silicates. We interpret these observations in terms of an extended, magnetocentrifugally driven disk wind that transports grains from the inner disk-where they are thermally processed by the stellar radiation after being uplifted from the disk surfaces-to the outer disk regions. For any disk radius r, there is a maximum grain size a(max)(r) that can be uplifted from that location: grains of size a << a(max) are carried away by the wind, whereas those with a less than or similar to a(max) reenter the disk at larger radii. A significant portion of the reentering grains converge to-and subsequently accumulate in-a narrow region just beyond r(max)(a), the maximum radius from which grains of size a can be uplifted. We show that this model can account for the inferred crystallinity fractions in classical T Tauri and Herbig Ae disks and for their indicated near constancy after being established early in the disk evolution. It is also consistent with the reported radial gradients in the mean grain size, crystallinity, and crystal composition. In addition, this model yields the properties of the grains that remain embedded in the outflows from protoplanetary disks and naturally explains the inferred persistence of small grains in the surface layers of these disks.
    • Relative alignment between dense molecular cores and ambient magnetic field: the synergy of numerical models and observations

      Chen, Che-Yu; Behrens, Erica A; Washington, Jasmin E; Fissel, Laura M; Friesen, Rachel K; Li, Zhi-Yun; Pineda, Jaime E; Ginsburg, Adam; Kirk, Helen; Scibelli, Samantha; et al. (OXFORD UNIV PRESS, 2020-03-28)
      The role played by magnetic field during star formation is an important topic in astrophysics. We investigate the correlation between the orientation of star-forming cores (as defined by the core major axes) and ambient magnetic field directions in (i) a 3D magnetohydrodynamic simulation, (ii) synthetic observations generated from the simulation at different viewing angles, and (iii) observations of nearby molecular clouds. We find that the results on relative alignment between cores and background magnetic field in synthetic observations slightly disagree with those measured in fully 3D simulation data, which is partly because cores identified in projected 2D maps tend to coexist within filamentary structures, while 3D cores are generally more rounded. In addition, we examine the progression of magnetic field from pc to core scale in the simulation, which is consistent with the anisotropic core formation model that gas preferably flows along the magnetic field towards dense cores. When comparing the observed cores identified from the Green Bank Ammonia Survey and Planck polarizationinferred magnetic field orientations, we find that the relative core-field alignment has a regional dependence among different clouds. More specifically, we find that dense cores in the Taurus molecular cloud tend to align perpendicular to the background magnetic field, while those in Perseus and Ophiuchus tend to have random (Perseus) or slightly parallel (Ophiuchus) orientations with respect to the field. We argue that this feature of relative core-field orientation could be used to probe the relative significance of the magnetic field within the cloud.
    • Relative Alignment between the Magnetic Field and Molecular Gas Structure in the Vela C Giant Molecular Cloud Using Low- and High-density Tracers

      Fissel, Laura M.; Ade, Peter A. R.; Angilè, Francesco E.; Ashton, Peter; Benton, Steven J.; Chen, Che-Yu; Cunningham, Maria; Devlin, Mark J.; Dober, Bradley; Friesen, Rachel; et al. (IOP PUBLISHING LTD, 2019-06-19)
      We compare the magnetic field orientation for the young giant molecular cloud Vela C inferred from 500 mu m polarization maps made with the BLASTPol balloon-borne polarimeter to the orientation of structures in the integrated line emission maps from Mopra observations. Averaging over the entire cloud we find that elongated structures in integrated line-intensity or zeroth-moment maps, for low-density tracers such as (CO)-C-12 and (CO)-C-13 J -> 1 - 0, are statistically more likely to align parallel to the magnetic field, while intermediate- or high-density tracers show (on average) a tendency for alignment perpendicular to the magnetic field. This observation agrees with previous studies of the change in relative orientation with column density in Vela C, and supports a model where the magnetic field is strong enough to have influenced the formation of dense gas structures within Vela C. The transition from parallel to no preferred/perpendicular orientation appears to occur between the densities traced by (CO)-C-13 and by (CO)-O-18 J -> 1 - 0. Using RADEX radiative transfer models to estimate the characteristic number density traced by each molecular line, we find that the transition occurs at a molecular hydrogen number density of approximately 10(3) cm(-3). We also see that the Centre Ridge (the highest column density and most active star-forming region within Vela C) appears to have a transition at a lower number density, suggesting that this may depend on the evolutionary state of the cloud.