Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Using an exact law for incompressible Hall magnetohydrodynamics (HMHD) turbulence, the energy cascade rate is computed from three-dimensional HMHD-CGL (biadiabatic ions and isothermal electrons) and Landau-fluid numerical simulations that feature different intensities of Landau damping over a broad range of wavenumbers, typically 0.05 ≲ <jats:italic>k</jats:italic> <jats:sub>⊥</jats:sub> <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> ≲ 100. Using three sets of cross-scale simulations where turbulence is initiated at large, medium, and small scales, the ability of the fluid energy cascade to “sense” the kinetic Landau damping at different scales is tested. The cascade rate estimated from the exact law and the dissipation calculated directly from the simulation are shown to reflect the role of Landau damping in dissipating energy at all scales, with an emphasis on the kinetic ones. This result provides new prospects on using exact laws for simplified fluid models to analyze dissipation in kinetic simulations and spacecraft observations, and new insights into theoretical description of collisionless magnetized plasmas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The cutoff mass ratio is under debate for contact binaries. In this paper, we present the investigation of two contact binaries with mass ratios close to the low mass ratio limit. It is found that the mass ratios of VSX J082700.8+462850 (hereafter J082700) and 1SWASP J132829.37+555246.1 (hereafter J132829) are both less than 0.1 (<jats:italic>q</jats:italic> ∼ 0.055 for J082700 and <jats:italic>q</jats:italic> ∼ 0.089 for J132829). J082700 is a shallow contact binary with a contact degree of ∼19%, and J132829 is a deep contact system with a fill-out factor of ∼70%. The <jats:italic>O</jats:italic> − <jats:italic>C</jats:italic> diagram analysis indicated that the two systems manifested long-term period decreases. In addition, J082700 exhibits a cyclic modulation which is more likely resulting from the Applegate mechanism. In order to explore the properties of extremely low mass ratio contact binaries (ELMRCBs), we carried out a statistical analysis on contact binaries with mass ratios of <jats:italic>q</jats:italic> ≲ 0.1 and discovered that the values of <jats:italic>J</jats:italic> <jats:sub>spin</jats:sub>/<jats:italic>J</jats:italic> <jats:sub>orb</jats:sub> of three systems are greater than 1/3. Two possible explanations can interpret this phenomenon. One explanation is that some physical processes, unknown to date, are not considered when Hut presented the dynamic stability criterion. The other explanation is that the dimensionless gyration radius (<jats:italic>k</jats:italic>) should be smaller than the value we used (<jats:italic>k</jats:italic> <jats:sup>2</jats:sup> = 0.06). We also found that the formation of ELMRCBs possibly has two channels. The study of evolutionary states of ELMRCBs reveals that their evolutionary states are similar with those of normal W UMa contact binaries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The temperature in most parts of a protoplanetary disk is determined by irradiation from the central star. Numerical experiments of Watanabe and Lin suggested that such disks, also called “passive disks,” suffer from a thermal instability. Here we use analytical and numerical tools to elucidate the nature of this instability. We find that it is related to the flaring of the optical surface, the layer at which starlight is intercepted by the disk. Whenever a disk annulus is perturbed thermally and acquires a larger scale height, disk flaring becomes steeper in the inner part and flatter in the outer part. Starlight now shines more overhead for the inner part and so can penetrate into deeper layers; conversely, it is absorbed more shallowly in the outer part. These geometric changes allow the annulus to intercept more starlight, and the perturbation grows. We call this the irradiation instability. It requires only ingredients known to exist in realistic disks and operates best in parts that are both optically thick and geometrically thin (inside 30 au, but can extend to further reaches when, e.g., dust settling is considered). An unstable disk develops traveling thermal waves that reach order unity in amplitude. In thermal radiation, such a disk should appear as a series of bright rings interleaved with dark shadowed gaps, while in scattered light it resembles a moving staircase. Depending on the gas and dust responses, this instability could lead to a wide range of consequences, such as ALMA rings and gaps, dust traps, vertical circulation, vortices, and turbulence.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The behavior of current density accumulation around the sharp gradient of magnetic field structure or a 3D magnetic null point and with the presence of finite plasma pressure is investigated. It has to be stated that in this setup, the fan plane locates at the <jats:italic>xy</jats:italic> plane and the spine axis aligns along the <jats:italic>z</jats:italic>-axis. Current density generation in presence of the plasma pressure that acts as a barrier for developing current density is less well understood. The shock-capturing Godunov-type PLUTO code is used to solve the magnetohydrodynamic set of equations in the context of wave-plasma energy transfer. It is shown that propagation of Alfvén waves in the vicinity of a 3D magnetic null point leads to current density excitations along the spine axis and also around the magnetic null point. Besides, it is pointed out the <jats:italic>x</jats:italic> component of current density has oscillatory behavior while the <jats:italic>y</jats:italic> and <jats:italic>z</jats:italic> components do not show this property. It is plausible that it happens because the fan plane encompasses separating unique topological regions, while the spine axis does not have this characteristic and is just a line without separate topological regions. Besides, current density generation results in plasma flow. It is found that the <jats:italic>y</jats:italic> component of the current density defines the <jats:italic>x</jats:italic> component of the plasma flow behavior, and the <jats:italic>x</jats:italic> component of the current density prescribes the behavior of the <jats:italic>y</jats:italic> component of the plasma flow.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Fitting parameterized models to images of galaxies has become the standard for measuring galaxy morphology. This forward-modeling technique allows one to account for the point-spread function to effectively study semi-resolved galaxies. However, using a specific parameterization for a galaxy’s surface brightness profile can bias measurements if it is not an accurate representation. Furthermore, it can be difficult to assess systematic errors in parameterized profiles. To overcome these issues we employ the Multi-Gaussian expansion (MGE) method of representing a galaxy’s profile together with a Bayesian framework for fitting images. MGE flexibly represents a galaxy’s profile using a series of Gaussians. We introduce a novel Bayesian inference approach that uses pre-rendered Gaussian components, which greatly speeds up computation time and makes it feasible to run the fitting code on large samples of galaxies. We demonstrate our method with a series of validation tests. By injecting galaxies, with properties similar to those observed at <jats:italic>z</jats:italic> ∼ 1.5, into deep Hubble Space Telescope observations we show that it can accurately recover total fluxes and effective radii of realistic galaxies. Additionally we use degraded images of local galaxies to show that our method can recover realistic galaxy surface brightness and color profiles. Our implementation is available in an open source python package <jats:monospace>imcascade</jats:monospace>, which contains all methods needed for the preparation of images, fitting, and analysis of results.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The transport of energetic particles in the heliosphere is reviewed regarding the treatment of their drifts over an entire solar cycle including the periods around solar maximum, when the tilt angles of the heliospheric current sheet increase to large values and the sign of the magnetic polarity changes. While gradient and curvature drifts are well-established elements of the propagation of cosmic rays in the heliospheric magnetic field, their perturbation by the solar-activity-induced large-scale distortions of dipole-like field configurations and by magnetic turbulence is an open problem. Various empirical or phenomenological approaches have been suggested, but either lack a theory-based motivation or have been shown to be incompatible with measurements. We propose a new approach of more closely investigating solar magnetograms obtained from GONG maps, leading to a new definition of (i) tilt angles that may exceed those provided by the Wilcox Solar Observatory during high activity and of (ii) a “noninteger sign” that can be used to reduce the drifts during these periods as well as to provide a refinement of the magnetic field polarity. The change of sign from <jats:italic>A</jats:italic> &lt; 0 to <jats:italic>A</jats:italic> &gt; 0 of solar cycle 24 can be in this way localized to occur between Carrington Rotations 2139 and 2140 in mid 2013. This treatment is fully consistent in the sense that the transport modeling uses the same input data to formulate the boundary conditions at the heliobase as do the magnetohydrodynamic models of the solar wind and the embedded heliospheric magnetic field that exploit solar magnetograms as inner boundary conditions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We calculate the stellar evolution of both white dwarfs (WDs) in AM CVn binaries with orbital periods of <jats:italic>P</jats:italic> <jats:sub>orb</jats:sub> ≈ 5–70 minutes. We focus on the cases where the donor starts as a <jats:italic>M</jats:italic> <jats:sub>He</jats:sub> &lt; 0.2<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> helium WD and the accretor is a <jats:italic>M</jats:italic> <jats:sub>WD</jats:sub> &gt; 0.6 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> WD. Using Modules for Experiments in Stellar Astrophysics, we simultaneously evolve both WDs assuming conservative mass transfer and angular momentum loss from gravitational radiation. This self-consistent evolution yields important feedback of the properties of the donor on the mass-transfer rate, <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2b2aieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, as well as the thermal evolution of the accreting WD. Consistent with earlier work, we find that the high <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2b2aieqn2.gif" xlink:type="simple" /> </jats:inline-formula>'s at early times forces an adiabatic evolution of the donor for <jats:italic>P</jats:italic> <jats:sub>orb</jats:sub> &lt; 30 minutes so that its mass–radius relation depends primarily on its initial entropy. As the donor reaches <jats:italic>M</jats:italic> <jats:sub>He</jats:sub> ≈ 0.02–0.03 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> at <jats:italic>P</jats:italic> <jats:sub>orb</jats:sub> ≃ 30 minutes, it becomes fully convective and could lose entropy and expand much less than expected under further mass loss. However, we show that the lack of reliable opacities for the donor’s surface inhibit a secure prediction for this possible cooling. Our calculations capture the core heating that occurs during the first ≈10<jats:sup>7</jats:sup> yr of accretion and continue the evolution into the phase of WD cooling that follows. When compared to existing data for accreting WDs, as seen by Cheng and collaborators for isolated WDs, we also find that the accreting WDs are not as cool as we would expect given the amount of time they have had to cool.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using the Giant Metrewave Radio Telescope, we report the detailed spectral measurements over a wide frequency range of three pulsars (J1741−3016, J1757−2223, and J1845−0743), which allow us to identify them as new gigahertz-peaked spectra pulsars. Our results indicate that their spectra show turnovers at the frequencies of 620 MHz, 640 MHz, and 650 MHz, respectively. Our analysis proves that wideband observations improve estimations of spectral nature using a free–free thermal absorption model, and thus allow for a more accurate approximation of the maximum energy in the spectrum. While there is no evidence as yet that these objects are associated with a supernova remnant or pulsar wind nebula, they will make good targets when looking for interesting environments in the future, or when conducting more sensitive sky surveys.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Theory and observations suggest that single-star evolution is not able to produce black holes with masses in the range 3–5<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and above ∼45<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, referred to as the lower mass gap and the upper mass gap, respectively. However, it is possible to form black holes in these gaps through mergers of compact objects in, e.g., dense clusters. This implies that if binary mergers are observed in gravitational waves with at least one mass-gap object, then either clusters are effective in assembling binary mergers, or our single-star models have to be revised. Understanding how effective clusters are at populating both mass gaps have therefore major implications for both stellar and gravitational wave astrophysics. In this paper we present a systematic study of how efficient stellar clusters are at populating both mass gaps through in-cluster mergers. For this, we derive a set of closed form relations for describing the evolution of compact object binaries undergoing dynamical interactions and mergers inside their cluster. By considering both static and time-evolving populations, we find in particular that globular clusters are clearly inefficient at populating the lower mass gap in contrast to the upper mass gap. We further describe how these results relate to the characteristic mass, time, and length scales associated with the problem.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nonthermal desorption of ices on interstellar grains is required to explain observations of molecules that are not synthesized efficiently in the gas phase in cold dense clouds. Perhaps the most important nonthermal desorption mechanism is one induced by cosmic rays (CRs), which, when passing through a grain, heat it transiently to a high temperature—the grain cools back to its original equilibrium temperature via the (partial) sublimation of the ice. Current cosmic ray induced desorption (CRD) models assume a fixed grain cooling time. In this work, we present a revised description of CRD in which the desorption efficiency depends dynamically on the ice content. We apply the revised desorption scheme to two-phase and three-phase chemical models in physical conditions corresponding to starless and prestellar cores, and to molecular cloud envelopes. We find that, inside starless and prestellar cores, introducing dynamic CRD can decrease gas-phase abundances by up to an order of magnitude in two-phase chemical models. In three-phase chemical models, our model produces results very similar to those of the static cooling scheme—when only one monolayer of ice is considered active. Ice abundances are generally insensitive to variations in the grain cooling time. Further improved CRD models need to take into account additional effects in the transient heating of the grains—introduced, for example, by the adoption of a spectrum of CR energies.</jats:p>