Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Binary neutron star mergers (NSMs) have been confirmed as one source of the heaviest observable elements made by the rapid neutron-capture (<jats:italic>r</jats:italic>-) process. However, modeling NSM outflows—from the total ejecta masses to their elemental yields—depends on the unknown nuclear equation of state (EOS) that governs neutron star structure. In this work, we derive a phenomenological EOS by assuming that NSMs are the dominant sources of the heavy element material in metal-poor stars with <jats:italic>r</jats:italic>-process abundance patterns. We start with a population synthesis model to obtain a population of merging neutron star binaries and calculate their EOS-dependent elemental yields. Under the assumption that these mergers were responsible for the majority of <jats:italic>r</jats:italic>-process elements in the metal-poor stars, we find parameters representing the EOS for which the theoretical NSM yields reproduce the derived abundances from observations of metal-poor stars. For our proof-of-concept assumptions, we find an EOS that is slightly softer than, but still in agreement with, current constraints, e.g., by the Neutron Star Interior Composition Explorer, with <jats:italic>R</jats:italic> <jats:sub>1.4</jats:sub> = 12.25 ± 0.03 km and <jats:italic>M</jats:italic> <jats:sub>TOV</jats:sub> = 2.17 ± 0.03 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> (statistical uncertainties, neglecting modeling systematics).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic fields are dynamically important in the diffuse interstellar medium. Understanding how gravitationally bound, star-forming clouds form requires modeling of the fields in a self-consistent, supernova-driven, turbulent, magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to follow the formation and early evolution of clouds with final masses of 3–8 × 10<jats:sup>3</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> within such a simulation. We use the code’s adaptive mesh refinement capabilities to concentrate numerical resolution in zoom-in regions covering single clouds, allowing us to investigate the detailed dynamics and field structure of individual self-gravitating clouds in a consistent background medium. Our goal is to test the hypothesis that dense clouds are dynamically evolving objects far from magnetohydrostatic equilibrium. We find that the cloud envelopes are magnetically supported with field lines parallel to density gradients and flow velocity, as indicated by the histogram of relative orientations and other statistical measures. In contrast, the dense cores of the clouds are gravitationally dominated, with gravitational energy exceeding internal, kinetic, or magnetic energy and accelerations due to gravity exceeding those due to magnetic or thermal pressure gradients. In these regions, field directions vary strongly, with a slight preference toward being perpendicular to density gradients, as shown by three-dimensional histograms of relative orientation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Although it is accepted that perfect-merging is not a realistic outcome of collisions, some researchers state that perfect-merging simulations can still be considered as quantitatively reliable representations of the final stage of terrestrial planet formation. Citing the work of Kokubo &amp; Genda, they argue that the differences between the final planets in simulations with perfect-merging and those where collisions are resolved accurately are small, and it is justified to use perfect-merging results as an acceptable approximation to realistic simulations. In this paper, we show that this argument does not stand. We demonstrate that when the mass lost during collisions is taken into account, the final masses of the planets will be so different from those obtained from perfect-merging that the latter cannot be used as an approximation. We carried out a large number of smooth particle hydrodynamics simulations of embryo–embryo collisions and determined the amount of the mass and water lost in each impact. We applied the results to collisions in a typical perfect-merging simulation and showed that even when the mass loss in each collision is as small as 10%, perfect-merging can, on average, overestimate the masses of the final planets by ∼35% and their water content by more than 18%. Our analysis demonstrates that, while perfect-merging simulations are still a powerful tool in proving concepts, they cannot be used to make predictions, draw quantitative conclusions (especially about the past history of a planetary system), or serve as a valid approximation to the simulations in which collisions are resolved accurately.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This work analyzes the Hall magnetohydrodynamics (HMHD) and magnetohydrodynamics (MHD) numerical simulations of a flaring solar active region as a test bed while idealizing the coronal Alfvén speed to be less by two orders of magnitude. HMHD supports faster magnetic reconnection and shows richer complexity in magnetic field line evolution compared to the MHD. The magnetic reconnections triggering the flare are explored by numerical simulations augmented with relevant multiwavelength observations. The initial coronal magnetic field is constructed by non-force-free extrapolation of photospheric vector magnetic field. Magnetic structure involved in the flare is identified to be a flux rope, with its overlying magnetic field lines constituting the quasi-separatrix layers (QSLs) along with a three-dimensional null point and a null line. Compared to the MHD simulation, the HMHD simulation shows a higher and faster ascent of the rope together with the overlying field lines, which further reconnect at the QSL located higher up in the corona. The footpoints of the field lines match better with the observations for the HMHD case, with the central part of the flare ribbon located at the chromosphere. Additionally, field lines are found to rotate in a circular pattern in the HMHD, whereas no such rotation is seen in the MHD results. Interestingly, plasma is also observed to be rotating in a cospatial chromospheric region, which makes the HMHD simulation more credible. Based on the aforementioned agreements, HMHD simulation is found to agree better with observations and thus opens up a novel avenue to explore.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Plasma jets are ubiquitous in space. In geospace, jets can be generated by magnetic reconnection. These reconnection jets, typically at fluid scale, brake in the near-Earth region, dissipate their energies, and drive plasma dynamics at kinetic scales, generating field-aligned currents that are crucial to magnetospheric dynamics. Understanding of the cross-scale dynamics is fundamentally important, but observation of coupling among phenomena at various scales is highly challenging. Here we report, using unprecedentedly high-cadence data from NASA's Magnetospheric Multiscale Mission, the first observation of cross-scale dynamics driven by jet braking in geospace. We find that jet braking causes MHD-scale distortion of magnetic field lines and development of an ion-scale jet front that hosts strong Hall electric fields. Parallel electric fields arising from the ion-scale Hall potential generate intense electron-scale field-aligned currents, which drive strong Debye-scale turbulence. Debye-scale waves conversely limit intensity of the field-aligned currents, thereby coupling back to the large-scale dynamics. Our study can help in understanding how energy deposited in large-scale structures is transferred into small-scale structures in space.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The properties of the suprathermal particle distributions observed upstream of interplanetary shocks depend not only on the properties of the shocks but also on the transport conditions encountered by the particles as they propagate away from the shocks. The confinement of particles in close proximity to the shocks, as well as particle scattering processes during propagation to the spacecraft, lead to the common observation of upstream diffuse particle distributions. We present observations of a rare extended anisotropic low-energy (≲30 keV) proton beam together with a trapped ≳500 keV proton population observed in association with the arrival of an oblique interplanetary shock at the Advanced Composition Explorer, the Interplanetary Monitoring Platform-8, and the Wind spacecraft on 2001 January 31. Continuous injection of particles by the traveling shock into a smooth radial magnetic field region formed in the tail of a modest high-speed solar wind stream produced an extended foreshock region of energetic particles. The absence of enhanced magnetic field fluctuations upstream of the shock results in the observation of a prolonged anisotropic field-aligned beam of ≲30 keV protons as well as a population of higher-energy (≳500 keV) protons with small pitch-angle cosine (<jats:italic>μ</jats:italic> ∼ 0) extending far from the shock.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic holes (MHs), characterized by depressions in the magnetic field magnitude, are transient magnetic structures ubiquitous in space plasmas. The electron pitch-angle distribution inside the MHs is key to diagnosing the MH properties and has been suggested to mainly exhibit a pancake-type distribution showing pitch angles near 90°. Here, we present the first observation of electron rolling-pin distribution—showing electron pitch angles mainly at 0°, 90°, and 180°—within an electron-scale MH, by using Magnetospheric Multiscale mission high-resolution measurements. With a second-order Taylor expansion method, the magnetic field topology of the MH is reconstructed, and the characteristics of the rolling-pin distribution inside the MH are investigated. We find that the rolling-pin distribution primarily appears near the MH center and at energies ranging from 110 to 1200 eV. We interpret the rolling-pin formation as a consequence of the combination of local-scale electron trapping and global-scale Fermi acceleration. These results can improve current understanding of electron dynamics in the MHs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We introduce a simple entropy-based formalism to characterize the role of mixing in pressure-balanced multiphase clouds and demonstrate example applications using <jats:sc>enzo-e</jats:sc> (magneto)hydrodynamic simulations. Under this formalism, the high-dimensional description of the system’s state at a given time is simplified to the joint distribution of mass over pressure (<jats:italic>P</jats:italic>) and entropy (<jats:italic>K</jats:italic> = <jats:italic>P</jats:italic> <jats:italic>ρ</jats:italic> <jats:sup>−<jats:italic>γ</jats:italic> </jats:sup>). As a result, this approach provides a way to (empirically and analytically) quantify the impact of different initial conditions and sets of physics on the system evolution. We find that mixing predominantly alters the distribution along the <jats:italic>K</jats:italic> direction and illustrate how the formalism can be used to model mixing and cooling for fluid elements originating in the cloud. We further confirm and generalize a previously suggested criterion for cloud growth in the presence of radiative cooling and demonstrate that the shape of the cooling curve, particularly at the low-temperature end, can play an important role in controlling condensation. Moreover, we discuss the capacity of our approach to generalize such a criterion to apply to additional sets of physics and to build intuition for the impact of subtle higher-order effects not directly addressed by the criterion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the spin alignment of dark matter halos by considering a mechanism somewhat similar to tidal locking; we dub it tidal-locking theory (TLT). While tidal torque theory (TTT) is responsible for the initial angular momentum of dark matter halos, TLT explains the angular momentum evolution during nonlinear ages. Our previous work showed that close encounters between halos could drastically change their angular momentum. This paper argues that TLT predicts partial alignment between the speed and spin direction for large high-speed halos. To examine this prediction, we use IllustrisTNG simulations and look for the alignment of the halos’ rotation axes. We find that the excess probability of alignment between spin and speed is about 10% at <jats:italic>z</jats:italic> = 0 for the large fast halos with velocities larger than twice the median. This spin–speed alignment weakens at <jats:italic>z</jats:italic> = 1 and disappears at <jats:italic>z</jats:italic> = 4. We also show that TTT predicts that the spin of a halo tends to be aligned with the middle eigendirection of the tidal tensor. Moreover, we find that the halos at <jats:italic>z</jats:italic> = 10 are preferentially aligned with the middle eigendirection of the tidal tensor with an excess probability of 15%. We show that TTT fails to predict the correct alignment at <jats:italic>z</jats:italic> = 0, while it works almost flawlessly at <jats:italic>z</jats:italic> = 10. These findings confirm that at earlier redshifts, during which mergers and fly-bys are rare, TTT works well, but after enough time, when fly-bys have occurred, the spin of the halos tends to align with speed for high-speed halos, due to the TLT effect.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study quasi-matter bounce cosmology in light of Planck cosmic microwave background angular anisotropy measurements along with the BICEP2/Keck Array data. We propose a new primordial scalar power spectrum by considering a linear approximation of the equation of state <jats:italic>w</jats:italic> ≅ <jats:italic>w</jats:italic> <jats:sub>0</jats:sub> + <jats:italic>κ</jats:italic>(<jats:italic>η</jats:italic> − <jats:italic>η</jats:italic> <jats:sub>0</jats:sub>) for quasi-matter field in the contracting phase of the universe. Using this new primordial scalar power spectrum, we constrain the zeroth-order approximation of the equation of state <jats:italic>w</jats:italic> <jats:sub>0</jats:sub> = −0.00340 ± 0.00044 and first-order correction <jats:inline-formula> <jats:tex-math> <?CDATA ${10}^{4}\zeta =-{1.67}_{-0.83}^{+1.50}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> <mml:mi>ζ</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.67</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.83</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.50</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3ed8ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> at the 1<jats:italic>σ</jats:italic> confidence level by Planck temperature and polarization in combination with the BICEP2/Keck Array data in which <jats:italic>ζ</jats:italic> = 12<jats:italic>κ</jats:italic>/<jats:italic>k</jats:italic> <jats:sub>*</jats:sub> with pivot scale <jats:italic>k</jats:italic> <jats:sub>*</jats:sub>. The spectral index of scalar perturbations is determined to be <jats:italic>n</jats:italic> <jats:sub>Bs</jats:sub> = 0.9623 ± 0.0055, which lies 7<jats:italic>σ</jats:italic> away from the scale-invariant primordial spectrum. We find scale dependency for <jats:italic>n</jats:italic> <jats:italic> <jats:sub>s</jats:sub> </jats:italic> at the 1<jats:italic>σ</jats:italic> confidence level and a tighter constraint on the running of the spectral index compared to ΛCDM+<jats:italic>α</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> cosmology. The running of the spectral index in quasi-matter bounce cosmology is <jats:italic>α</jats:italic> <jats:sub>Bs</jats:sub> = <jats:italic>π</jats:italic> <jats:italic>ζ</jats:italic>/2<jats:italic>c</jats:italic> <jats:italic> <jats:sub>s</jats:sub> </jats:italic> = −0.0021 ± 0.0016, which is nonzero at the 1.3<jats:italic>σ</jats:italic> level, whereas in ΛCDM+<jats:italic>α</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> it is nonzero at the 0.8<jats:italic>σ</jats:italic> level for Planck temperature and polarization data. The sound speed of density fluctuations of the quasi-matter field at the crossing time is <jats:inline-formula> <jats:tex-math> <?CDATA ${c}_{{\rm{s}}}={0.097}_{-0.023}^{+0.037}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.097</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.023</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.037</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3ed8ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, which is not a very small value in the contracting phase.</jats:p>