Browsing by Subject "MAGNETIC-FIELD"

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  • Hietala, Heli; Partamies, N.; Laitinen, T. V.; Clausen, L. B. N.; Facsko, G.; Vaivads, A.; Koskinen, H. E. J.; Dandouras, I.; Reme, H.; Lucek, E. A. (2012)
  • Juvela, Mika; Padoan, Paolo; Ristorcelli, Isabelle; Pelkonen, Veli-Matti (2019)
    Context. The Planck Catalogue of Galactic Cold Clumps (PGCC) contains over 13 000 sources that are detected based on their cold dust signature. They are believed to consist of a mixture of quiescent, pre-stellar, and already star-forming objects within molecular clouds. Aims. We extracted PGCC-type objects from cloud simulations and examined their physical and polarisation properties. The comparison with the PGCC catalogue helps to characterise the properties of this large sample of Galactic objects and, conversely, provides valuable tests for numerical simulations of large volumes of the interstellar medium and the evolution towards pre-stellar cores. Methods. We used several magnetohydrodynamical simulation snapshots to define the density field of our model clouds. Sub-millimetre images of the surface brightness and polarised signal were obtained with radiative transfer calculations. We examined the statistics of synthetic cold clump catalogues extracted with methods similar to the PGCC. We also examined the variations of the polarisation fraction p in the clumps. Results. The clump sizes, aspect ratios, and temperatures in the synthetic catalogue are similar to the PGCC. The fluxes and column densities of synthetic clumps are smaller by a factor of a few. Rather than with an increased dust opacity, this could be explained by increasing the average column density of the model by a factor of two to three, close to N(H-2) = 10(22) cm(-2). When the line of sight is parallel to the mean magnetic field, the polarisation fraction tends to increase towards the clump centres, which is contrary to observations. When the field is perpendicular, the polarisation fraction tends to decrease towards the clumps, but the drop in p is small (e.g. from p similar to 8% to p similar to 7%). Conclusions. Magnetic field geometry reduces the polarisation fraction in the simulated clumps by only Delta p similar to 1% on average. The larger drop seen towards the actual PGCC clumps therefore suggests some loss of grain alignment in the dense medium, such as predicted by the radiative torque mechanism. The statistical study is not able to quantify dust opacity changes at the scale of the PGCC clumps.
  • Padoan, Paolo; Pan, Liubin; Juvela, Mika; Haugbolle, Troels; Nordlund, Åke (2020)
    We address the problem of the origin of massive stars, namely the origin, path, and timescale of the mass flows that create them. Based on extensive numerical simulations, we propose a scenario where massive stars are assembled by large-scale, converging, inertial flows that naturally occur in supersonic turbulence. We refer to this scenario of massive-star formation as the inertial-inflow model. This model stems directly from the idea that the mass distribution of stars is primarily the result of turbulent fragmentation. Under this hypothesis, the statistical properties of turbulence determine the formation timescale and mass of prestellar cores, posing definite constraints on the formation mechanism of massive stars. We quantify such constraints by analyzing a simulation of supernova-driven turbulence in a 250 pc region of the interstellar medium, describing the formation of hundreds of massive stars over a time of approximately 30 Myr. Due to the large size of our statistical sample, we can say with full confidence that massive stars in general do not form from the collapse of massive cores nor from competitive accretion, as both models are incompatible with the numerical results. We also compute synthetic continuum observables in the Herschel and ALMA bands. We find that, depending on the distance of the observed regions, estimates of core mass based on commonly used methods may exceed the actual core masses by up to two orders of magnitude and that there is essentially no correlation between estimated and real core masses.
  • Dimmock, A. P.; Alho, M.; Kallio, E.; Pope, S. A.; Zhang, T. L.; Kilpua, E.; Pulkkinen, T. I.; Futaana, Y.; Coates, A. J. (2018)
    Owing to the heritage of previous missions such as the Pioneer Venus Orbiter and Venus Express, the typical global plasma environment of Venus is relatively well understood. On the other hand, this is not true for more extreme driving conditions such as during passages of interplanetary coronal mass ejections (ICMEs). One of the outstanding questions is how do ICMEs, either the ejecta or sheath portions, impact (1) the Venusian magnetic topology and (2) escape rates of planetary ions? One of the main issues encountered when addressing these problems is the difficulty of inferring global dynamics from single spacecraft obits; this is where the benefits of simulations become apparent. In the present study, we present a detailed case study of an ICME interaction with Venus on 5 November 2011 in which the magnetic barrier reached over 250 nT. We use both Venus Express observations and hybrid simulation runs to study the impact on the field draping pattern and the escape rates of planetary O+ ions. The simulation showed that the magnetic field line draping pattern around Venus during the ICME is similar to that during typical solar wind conditions and that O+ ion escape rates are increased by approximately 30% due to the ICME. Moreover, the atypically large magnetic barrier appears to manifest from a number of factors such as the flux pileup, dayside compression, and the driving time from the ICME ejecta.