Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The formation of planetesimals is a challenging problem in planet formation theory. A prominent scenario for overcoming dust growth barriers is the gravitational collapse of locally over-dense regions, shown to robustly produce ∼100 km–sized objects. Still, the conditions under which planetesimal formation occurs remain unclear. For collapse to proceed, the self-gravity of an over-density must overcome stellar tidal disruption on large scales and turbulent diffusion on small scales. Here, we relate the scales of streaming and Kelvin–Helmholtz instability (KHI), which both regulate particle densities on the scales of gravitational collapse, directly to planetesimal formation. We support our analytic findings by performing 3D hydrodynamical simulations of streaming and KHI and planetesimal formation. We find that the vertical extent of the particle mid-plane layer and the radial width of streaming instability filaments are set by the same characteristic length scale, thus governing the strength of turbulent diffusion on the scales of planetesimal formation. We present and successfully test a collapse criterion, 0.1<jats:italic>Q β ϵ</jats:italic> <jats:sup>−1</jats:sup> <jats:italic>Z</jats:italic> <jats:sup>−1</jats:sup> ≲ 1, and show that even for solar metallicities, planetesimals can form in dead zones of sufficiently massive disks. For a given gas Toomre parameter <jats:italic>Q</jats:italic>, pressure gradient <jats:italic>β</jats:italic>, metallicity <jats:italic>Z</jats:italic>, and local particle enhancement <jats:italic>ϵ</jats:italic>, the collapse criterion also provides a range of unstable scales, instituting a promising path for studying initial planetesimal mass distributions. Streaming instability is not required for planetesimal collapse but, by increasing <jats:italic>ϵ</jats:italic>, can evolve a system to instability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The discrepancy between the visible mass in galaxies or galaxy clusters and that inferred from their dynamics is well known. The prevailing solution to this problem is dark matter. Here we show that a different approach, one that conforms to both the current standard model of particle physics and general relativity (GR), explains the recently observed tight correlation between the galactic baryonic mass and the measured accelerations in the galaxy. Using direct calculations based on GR’s Lagrangian and parameter-free galactic models, we show that the nonlinear effects of GR make baryonic matter alone sufficient to explain this observation. Our approach also shows that a specific acceleration scale dynamically emerges. It agrees well with the value of the MOND acceleration scale.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present results of three-dimensional (3D), radiation-magnetohydrodynamics (MHD) simulations of core-collapse supernovae in full general relativity (GR) with spectral neutrino transport. In order to study the effects of the progenitor’s rotation and magnetic fields, we compute three models, where the precollapse rotation rate and magnetic fields are included parametrically to a 20 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> star. While we find no shock revival in our two nonmagnetized models during our simulation times (∼500 ms after bounce), the magnetorotational (MR) driven shock expansion immediately initiates after bounce in our rapidly rotating and strongly magnetized model. We show that the expansion of the MR-driven flows toward the polar directions is predominantly driven by the magnetic pressure, whereas the shock expansion toward the equatorial direction is supported by neutrino heating. Our detailed analysis indicates that the growth of the so-called kink instability may hinder the collimation of jets, resulting in the formation of broader outflows. Furthermore, we find a dipole emission of lepton number, only in the MR explosion model, whose asymmetry is consistent with the explosion morphology. Although it is similar to the lepton number emission self-sustained asymmetry (LESA), our analysis shows that the dipole emission occurs not from the proto–neutron star convection zone but from above the neutrino sphere, indicating that it is not associated with the LESA. We also report several unique neutrino signatures, which are significantly dependent on both the time and the viewing angle, if observed, possibly providing rich information regarding the onset of the MR-driven explosion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off limb in the H<jats:italic>β</jats:italic> and Ca <jats:sc>ii</jats:sc> 8542 Å lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2 class solar flare SOL 2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in H<jats:italic>β</jats:italic> line-wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the H<jats:italic>β</jats:italic> wing and the nominal photospheric limb is about 300–500 km. The Ca <jats:sc>ii</jats:sc> 8542 Å line-wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, toward the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb H<jats:italic>β</jats:italic> ribbons and their significant elevation above the photosphere. The ribbons in the Ca <jats:sc>ii</jats:sc> 8542 Å line-wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the H<jats:italic>β</jats:italic> line-wing intensities can be a useful tracer of flare energy deposition in the lower solar atmosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A comprehensive understanding of gamma-ray bursts (GRBs) has been elusive due to the variety of questions surrounding the radiation mechanism at play in these events. Polarization measurements of GRBs can heavily constrain the relevant radiation mechanisms and the structure of the GRB jet; however, there is a limited number of theoretical predictions that observed GRB polarizations can be compared against. Here, we conduct radiative transfer calculations of a set of two-dimensional relativistic hydrodynamic long GRB (LGRB) jet simulations, of a constant and a variable jet, using the Monte Carlo Radiation Transport (MCRaT) code. MCRaT has been enhanced by the inclusion of polarization; it has been first verified by reproducing a variety of results in the literature and then used to obtain the time-integrated and time-resolved polarization degrees and angles of the synthetic LGRBs. While the obtained time-integrated polarization degrees (≲1%) are consistent with the constraints from the POLAR experiment, they are lower than other theoretical studies due to the lack of strong gradients in the model jet profiles that we use. The time-resolved results suggests that GRBs with wide jets observed on axis will have small polarization degrees (≲2%) and constant polarization angles, during the brightest portion of the light curve. GRBs observed off-axis will have larger polarization degrees and polarization angles that change with the temporal structure of radiating shells in the outflow. We then place our results in the context of GRB prompt emission models and future LEAP and POLAR-2 GRB polarimetry detections.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present temporal variations of the Si <jats:sc>iv</jats:sc> line profiles at the flare ribbons in three solar flares observed by the Interface Region Imaging Spectrograph. In the M1.1 flare on 2014 September 6 and the X1.6 flare on 2014 September 10, the Si <jats:sc>iv</jats:sc> line profiles evolve from wholly redshifted to red-wing enhanced with the flare development. However, in the B1.8 flare on 2016 December 2, the Si <jats:sc>iv</jats:sc> line profiles are wholly redshifted throughout the flare evolution. We fit the wholly redshifted line profiles with a single-Gaussian function, but fit the red-asymmetric ones with a double-Gaussian function to deduce the corresponding Doppler velocities. In addition, we find that hard X-ray emission above 25 keV shows up in the two large flares, implying a nonthermal electron beam heating. In the microflare, there only appears weak hard X-ray emission up to 12 keV, indicative of a mostly thermal heating. We interpret the redshifts or red asymmetries of the Si <jats:sc>iv</jats:sc> line at the ribbons in the three flares as spectral manifestations of chromospheric condensation. We propose that whether the line appears to be wholly redshifted or red-asymmetric depends on the heating mechanisms, as well as on the propagation of the condensation.</jats:p>