Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>We consider a natural generalization of the Kazantsev–Kraichnan model for a small-scale turbulent dynamo. This generalization takes into account the statistical time asymmetry of a turbulent flow and thus allows one to describe velocity fields with energy cascade. For three-dimensional velocity fields, a generalized Kazantsev equation is derived, and the evolution of the second-order magnetic field correlator is investigated for large but finite magnetic Prandtl numbers. It is shown that as Pr<jats:sub> <jats:italic>m</jats:italic> </jats:sub> → ∞, the growth increment tends to the limit known from the T-exponential (Lagrangian deformation) method. Magnetic field generation is shown to be weaker than that in the Gaussian velocity field for any direction of the energy cascade and essentially depends on the Prandtl number.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper, we simulate the prompt emission light curve, spectrum, and <jats:italic>E</jats:italic> <jats:sub>p</jats:sub> evolution patterns of gamma-ray bursts (GRBs) within the framework of the Internal-collision-induced Magnetic Reconnection and Turbulence (ICMART) model. We show that this model can produce a Band-shaped spectrum, whose parameters (<jats:italic>E</jats:italic> <jats:sub>p</jats:sub>, <jats:italic>α</jats:italic>, <jats:italic>β</jats:italic>) could follow the typical distribution of GRB observations, as long as the magnetic field and the electron acceleration process in the emission region are under appropriate conditions. On the other hand, we show that for one ICMART event, <jats:italic>E</jats:italic> <jats:sub>p</jats:sub> evolution is always a hard-to-soft pattern. However, a GRB light curve is usually composed of multiple ICMART events that are fundamentally driven by the erratic activity of the GRB central engine. In this case, we find that if one individual broad pulse in the GRB light curve is composed of multiple ICMART events, the overall <jats:italic>E</jats:italic> <jats:sub>p</jats:sub> evolution could be disguised as an intensity-tracking pattern. Therefore, mixed <jats:italic>E</jats:italic> <jats:sub>p</jats:sub> evolution patterns can coexist in the same burst, with a variety of combined patterns. Our results support the ICMART model as a competitive model to explain the main properties of GRB prompt emission. The possible challenges faced by the ICMART model are also discussed in detail.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first linear polarization measurements from the 2015 long-duration balloon flight of <jats:sc>Spider</jats:sc>, which is an experiment that is designed to map the polarization of the cosmic microwave background (CMB) on degree angular scales. The results from these measurements include maps and angular power spectra from observations of 4.8% of the sky at 95 and 150 GHz, along with the results of internal consistency tests on these data. While the polarized CMB anisotropy from primordial density perturbations is the dominant signal in this region of sky, Galactic dust emission is also detected with high significance. Galactic synchrotron emission is found to be negligible in the <jats:sc>Spider</jats:sc> bands. We employ two independent foreground-removal techniques to explore the sensitivity of the cosmological result to the assumptions made by each. The primary method uses a dust template derived from <jats:italic>Planck </jats:italic>data to subtract the Galactic dust signal. A second approach, which constitutes a joint analysis of <jats:sc>Spider</jats:sc> and <jats:italic>Planck </jats:italic>data in the harmonic domain, assumes a modified-blackbody model for the spectral energy distribution of the dust with no constraint on its spatial morphology. Using a likelihood that jointly samples the template amplitude and <jats:italic>r</jats:italic> parameter space, we derive 95% upper limits on the primordial tensor-to-scalar ratio from Feldman–Cousins and Bayesian constructions, finding <jats:italic>r</jats:italic> &lt; 0.11 and <jats:italic>r</jats:italic> &lt; 0.19, respectively. Roughly half the uncertainty in <jats:italic>r</jats:italic> derives from noise associated with the template subtraction. New data at 280 GHz from <jats:sc>Spider</jats:sc>’s second flight will complement the <jats:italic>Planck </jats:italic>polarization maps, providing powerful measurements of the polarized Galactic dust emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present statistics of He <jats:sc>ii</jats:sc> Ly<jats:italic>α</jats:italic> transmission spikes and large-scale absorption troughs using archival high-resolution (<jats:italic>R</jats:italic> = <jats:italic>λ</jats:italic>/Δ<jats:italic>λ</jats:italic> ≃ 12,500–18,000) far-UV spectra of eight He <jats:sc>ii</jats:sc>-transparent quasars obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers the redshift range 2.5 ≲ <jats:italic>z</jats:italic> ≲ 3.8, thereby probing the rapidly evolving He <jats:sc>ii</jats:sc> absorption at the end of He <jats:sc>ii</jats:sc> reionization epoch. The measured lengths of the troughs decrease dramatically from <jats:italic>L</jats:italic> ≳ 100 cMpc at <jats:italic>z</jats:italic> &gt; 3 to <jats:italic>L</jats:italic> ≃ 30 cMpc at <jats:italic>z</jats:italic> ∼ 2.7, signaling a significant progression of He <jats:sc>ii</jats:sc> reionization at these redshifts. Furthermore, unexpectedly long <jats:italic>L</jats:italic> ∼ 65 cMpc troughs detected at <jats:italic>z</jats:italic> ≃ 2.9 suggest that the UV background fluctuates at larger scales than predicted by current models. By comparing the measured incidence of transmission spikes to predictions from forward-modeled mock spectra created from the outputs of a (146 cMpc)<jats:sup>3</jats:sup> optically thin <jats:monospace>Nyx</jats:monospace> hydrodynamical simulation employing different UV background models, we infer the redshift evolution of the He <jats:sc>ii</jats:sc> photoionization rate Γ<jats:sub>He <jats:sc>II</jats:sc> </jats:sub>(<jats:italic>z</jats:italic>). The photoionization rate decreases with increasing redshift from ≃ 4.6 × 10<jats:sup>−15</jats:sup> s<jats:sup>−1</jats:sup> at <jats:italic>z</jats:italic> ≃ 2.6 to ≃ 1.2 × 10<jats:sup>−15</jats:sup> s<jats:sup>−1</jats:sup> at <jats:italic>z</jats:italic> ≃ 3.2, in agreement with previous inferences from the He <jats:sc>ii</jats:sc> effective optical depth, and following expected trends of current models of a fluctuating He <jats:sc>ii</jats:sc>-ionizing background.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze Very Large Telescope/UVES observations of the quasar SDSS J024221.87+004912.6. We identify four absorption outflow systems: a C <jats:sc>iv</jats:sc> broad absorption line (BAL) at <jats:italic>v</jats:italic> ≈ −18,000 km s<jats:sup>−1</jats:sup> and three narrower low-ionization systems with centroid velocities ranging from –1200 to –3500 km s<jats:sup>−1</jats:sup>. These outflows show similar physical attributes to the [O <jats:sc>iii</jats:sc>] outflows studied by Liu et al. (2013). We find that two of the systems are energetic enough to contribute to active galactic nucleus feedback, with one system reaching above 5% of the quasar’s Eddington luminosity. We also find that this system is at a distance of 67 kpc away from the quasar, the farthest detected mini-BAL absorption outflow from its central source to date. In addition, we examine the time-variability of the BAL and find that its velocity monotonically increases, while the trough itself becomes shallower over time.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the ion acceleration in a mesoscale, spontaneously fragmenting flare current sheet (SFCS) characterized by the presence of a plasmoid cascade. The main subject of our investigation is to determine whether and how plasmoid cascades at intermediate scales in a fragmented current sheet of a solar flare can impact the (preferential) acceleration of specific ions. The time evolution of the SFCS is obtained from high-resolution 2.5D MHD simulations. The ion trajectories (in the background fields resulting from the MHD model), energies, and pitch angles are calculated using a relativistic test-particle code based on the half-acceleration–rotation–half-acceleration method. For light ions, the main acceleration effects of electromagnetic fields within the SFCS are analyzed using the guiding center approximation. We identify regions with the most-efficient ion acceleration within the SFCS, the accelerator efficiency, and spectra of the accelerated ions. The influence of the charge-to-mass ratio on ion behavior is also studied and resulting ion abundances are compared with observational data. The main ion acceleration takes place in the regions with a strong polarization term, which is part of the first-order Fermi acceleration. Because the term is mass dependent, heavier ions undergo preferential acceleration. The ion energy spectra, abundance-enhancement factors, and differential fluxes, obtained from the model, exhibit power-law profiles, in agreement with observed solar energetic particle events. Nonetheless, the obtained slopes for the abundance-enhancement factor do not exactly match the observed data. The computed slopes and profiles are not sensitive to changes in the initial plasma temperature.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The interest in the structure of ice-rich planetary bodies, in particular the differentiation between ice and rock, has grown due to the discovery of Kuiper Belt objects and exoplanets. We thus carry out a parameter study for a range of planetary masses <jats:italic>M</jats:italic>, yielding radii 50 ≲ <jats:italic>R</jats:italic> ≲ 3000 km, and for rock to ice mass ratios between 0.25 and 4, evolving them for 4.5 Gyr in a cold environment, to obtain the present structure. We use a thermal evolution model that allows for liquid and vapor flow in a porous medium, solving mass and energy conservation equations under hydrostatic equilibrium for a spherical body in orbit around a central star. The model includes the effect of pressure on porosity and on the melting temperature, heating by long-lived radioactive isotopes, and temperature-dependent serpentinization and dehydration. We obtain the boundary in parameter space (size, rock content) between bodies that differentiate, forming a rocky core, and those which remain undifferentiated: small bodies, bodies with a low rock content, and the largest bodies considered, which develop high internal pressures and barely attain the melting temperature. The final differentiated structure comprises a rocky core, an ice-rich mantle, and a thin dense crust below the surface. We obtain and discuss the bulk density–radius relationship. The effect of a very cold environment is investigated, and we find that at an ambient temperature of ∼20 K, small bodies preserve the ice in amorphous form to the present.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Sun and Sun-like stars commonly host multimillion-kelvin coronae and 10,000 K chromospheres. These extremely hot gases generate X-ray and extreme ultraviolet emissions that may impact the erosion and chemistry of (exo)planetary atmospheres, influencing the climate and conditions for habitability. However, the mechanism of coronal and chromospheric heating is still poorly understood. While the magnetic field most probably plays a key role in driving and transporting energy from the stellar surface upwards, it is not clear whether the atmospheric heating mechanisms of the Sun and active Sun-like stars can be described in a unified manner. To this end, we report on a systematic survey of the responses of solar and stellar atmospheres to surface magnetic flux over a wide range of temperatures. By analyzing 10 years of multiwavelength synoptic observations of the Sun, we reveal that the irradiance and magnetic flux show power-law relations with an exponent decreasing from above unity to below as the temperature decreases from the corona to the chromosphere. Moreover, this trend indicating the efficiency of atmospheric heating can be extended to Sun-like stars. We also discover that the power-law exponent depends on the solar cycle, becoming smallest at maximum activity, probably due to the saturation of atmospheric heating. Our study provides observational evidence that the mechanism of atmospheric heating is universal among the Sun and Sun-like stars, regardless of age or activity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present observations of SN 2021csp, the second example of a newly identified type of supernova (SN) hallmarked by strong, narrow, P Cygni carbon features at early times (Type Icn). The SN appears as a fast and luminous blue transient at early times, reaching a peak absolute magnitude of −20 within 3 days due to strong interaction between fast SN ejecta (<jats:italic>v</jats:italic> ≈ 30,000 km s<jats:sup>−1</jats:sup>) and a massive, dense, fast-moving C/O wind shed by the WC-like progenitor months before explosion. The narrow-line features disappear from the spectrum 10–20 days after explosion and are replaced by a blue continuum dominated by broad Fe features, reminiscent of Type Ibn and IIn supernovae and indicative of weaker interaction with more extended H/He-poor material. The transient then abruptly fades ∼60 days post-explosion when interaction ceases. Deep limits at later phases suggest minimal heavy-element nucleosynthesis, a low ejecta mass, or both, and imply an origin distinct from that of classical Type Ic SNe. We place SN 2021csp in context with other fast-evolving interacting transients, and discuss various progenitor scenarios: an ultrastripped progenitor star, a pulsational pair-instability eruption, or a jet-driven fallback SN from a Wolf–Rayet (W-R) star. The fallback scenario would naturally explain the similarity between these events and radio-loud fast transients, and suggests a picture in which most stars massive enough to undergo a W-R phase collapse directly to black holes at the end of their lives.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present 3D velocity measurements and acceleration limits for stars within a few parsec of the Galactic Center (GC) black hole, Sgr A*, based on observations of 43 and 86 GHz circumstellar maser emission. Observations were taken with the Very Large Array in 2013, 2014, and 2020 and with the Atacama Large Millimeter/submillimeter Array in 2015 and 2017. We detect 28 masers in total, of which four are new detections. Combining these data with extant maser astrometry, we calculate stellar proper motions and accelerations with uncertainties as low as ∼10 <jats:italic>μ</jats:italic>as yr<jats:sup>−1</jats:sup> and 0.5 <jats:italic>μ</jats:italic>as yr<jats:sup>−2</jats:sup>, respectively, corresponding to approximately 0.5 km s<jats:sup>−1</jats:sup> and 0.04 km s<jats:sup>−1</jats:sup> yr<jats:sup>−1</jats:sup> at a distance of 8 kpc. We measure radial velocities from maser spectra with ∼0.5 km s<jats:sup>−1</jats:sup> uncertainties, though the precision and accuracy of such measurements for deducing the underlying stellar velocities are limited by the complex spectral profiles of some masers. We therefore measure radial acceleration limits with typical uncertainties of ∼0.1 km s<jats:sup>−1</jats:sup> yr<jats:sup>−1</jats:sup>. We analyze the resulting 3D velocities and accelerations with respect to expected motions resulting from models of the mass distribution in the GC.</jats:p>