Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The collapse of degenerate oxygen–neon cores (i.e., electron-capture supernovae or accretion-induced collapse) proceeds through a phase in which a deflagration wave (“flame”) forms at or near the center and propagates through the star. In models, the assumed speed of this flame influences whether this process leads to an explosion or to the formation of a neutron star. We calculate the laminar flame speeds in degenerate oxygen–neon mixtures with compositions motivated by detailed stellar evolution models. These mixtures include trace amounts of carbon and have a lower electron fraction than those considered in previous work. We find that trace carbon has little effect on the flame speeds, but that material with electron fraction <jats:inline-formula> <jats:tex-math> <?CDATA ${Y}_{{\rm{e}}}\approx 0.48-0.49$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab6f03ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> has laminar flame speeds that are <jats:inline-formula> <jats:tex-math> <?CDATA $\approx 2$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab6f03ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> times faster than those at <jats:inline-formula> <jats:tex-math> <?CDATA ${Y}_{{\rm{e}}}=0.5$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab6f03ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. We provide tabulated flame speeds and a corresponding fitting function so that the impact of this difference can be assessed via full star hydrodynamical simulations of the collapse process.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Space weather forecasting is very important, and the prediction of space weather, especially for solar flares, has increasingly attracted research interests with the numerous recent breakthroughs in machine learning. In this study, we propose a novel convolutional neural network (CNN) model to make binary class prediction for both ≥C-class and ≥M-class flares within 24 hr. We collect magnetogram samples of solar active regions (ARs) provided by the Space-weather Helioseismic and Magnetic Imager Active Region Patches (SHARP) data from 2010 May to 2018 September. These samples are used to construct 10 separate data sets. Then, after training, validating, and testing our model, we compare the results of our model with previous studies in several metrics, with a focus on the true skill statistic (TSS). The major results are summarized as follows. (1) We propose a method of shuffle and split cross-validation (CV) based on AR segregation, which is the first attempt to verify the validity and stability of the model in flare prediction. (2) The proposed CNN model achieves a relatively high score of TSS = 0.749 ± 0.079 for ≥M-class prediction, and TSS = 0.679 ± 0.045 for ≥C-class prediction, which is greatly improved compared with previous studies. (3) The model trained on 10 CV data sets is considerably robust and stable in making flare prediction for both ≥C class and ≥M class. Our experimental results indicate that our proposed CNN model is a highly effective method for flare forecasting, with quite excellent prediction performance.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We develop empirical relationships for the accretion and erosion of colliding gravity-dominated bodies of various compositions under conditions expected in late-stage solar system formation. These are fast, easily coded relationships based on a large database of smoothed particle hydrodynamics (SPH) simulations of collisions between bodies of different compositions, including those that are water rich. The accuracy of these relations is also comparable to the deviations of results between different SPH codes and initial thermal/rotational conditions. We illustrate the paucity of disruptive collisions between major bodies, as compared to collisions between less massive planetesimals in late-stage planet formation, and thus focus on more probable, low-velocity collisions, though our relations remain relevant to disruptive collisions as well. We also pay particular attention to the transition zone between merging collisions and those where the impactor does not merge with the target, but continues downrange, a “hit-and-run” collision. We find that hit-and-run collisions likely occur more often in density-stratified bodies and across a wider range of impact angles than suggested by the most commonly used analytic approximation. We also identify a possible transitional zone in gravity-dominated collisions where larger bodies may undergo more disruptive collisions when the impact velocity exceeds the sound speed, though understanding this transition warrants further study. Our results are contrary to the commonly assumed invariance of total mass (scale), density structure, and material composition on the largest remnants of giant impacts. We provide an algorithm for adopting our model into <jats:italic>N</jats:italic>-body planet formation simulations, so that the mass of growing planets and debris can be tracked.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The formation of shocks within the solar atmosphere remains one of the few observable signatures of energy dissipation arising from the plethora of magnetohydrodynamic waves generated close to the solar surface. Active region observations offer exceptional views of wave behavior and its impact on the surrounding atmosphere. The stratified plasma gradients present in the lower solar atmosphere allow for the potential formation of many theorized shock phenomena. In this study, using chromospheric Ca <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>8542 line spectropolarimetric data of a large sunspot, we examine fluctuations in the plasma parameters in the aftermath of powerful shock events that demonstrate polarimetric reversals during their evolution. Modern inversion techniques are employed to uncover perturbations in the temperatures, line-of-sight velocities, and vector magnetic fields occurring across a range of optical depths synonymous with the shock formation. Classification of these nonlinear signatures is carried out by comparing the observationally derived slow, fast, and Alfvén shock solutions with the theoretical Rankine–Hugoniot relations. Employing over 200,000 independent measurements, we reveal that the Alfvén (intermediate) shock solution provides the closest match between theory and observations at optical depths of <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{log}}_{10}\tau =-4$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>log</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> <mml:mi>τ</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7a90ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, consistent with a geometric height at the boundary between the upper photosphere and lower chromosphere. This work uncovers first-time evidence of the manifestation of chromospheric intermediate shocks in sunspot umbrae, providing a new method for the potential thermalization of wave energy in a range of magnetic structures, including pores, magnetic flux ropes, and magnetic bright points.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this study, the detailed magnetic field structure of the dense protostellar core Barnard 335 (B335) was revealed, based on near-infrared polarimetric observations of background stars to measure dichroically polarized light produced by magnetically aligned dust grains in the core. Magnetic fields pervading B335 were mapped using 24 stars after subtracting unrelated ambient polarization components, revealing that they have an axisymmetrically distorted hourglass-shaped structure toward the protostellar core. On the basis of simple two- and three-dimensional magnetic field modeling, magnetic inclination angles in the plane-of-sky and line-of-sight directions were determined to be 90° ± 7° and 50° ± 10°, respectively. The total magnetic field strength of B335 was determined to be 30.2 ± 17.7 <jats:italic>μ</jats:italic>G. The critical mass of B335, evaluated using both magnetic and thermal/turbulent support against collapse, was determined to be <jats:italic>M</jats:italic> <jats:sub>cr</jats:sub> = 3.37 ± 0.94 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which is identical to the observed core mass of <jats:italic>M</jats:italic> <jats:sub>core</jats:sub> = 3.67 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We thus concluded that B335 started its contraction from a condition near equilibrium. We found a linear relationship in the polarization versus extinction diagram, up to <jats:italic>A</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> ∼ 15 mag toward the stars with the greatest obscuration, which verified that our observations and analysis provide an accurate depiction of the core.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The tip of the red giant branch (TRGB) method provides one of the most accurate and precise means of measuring the distances to nearby galaxies. Here we present a multi-wavelength, <jats:italic>VIJHK</jats:italic> absolute calibration of the TRGB based on observations of TRGB stars in the Large Magellanic Cloud (LMC), grounded on a geometric distance, determined by detached eclipsing binaries (DEBs). This paper presents a more detailed description of the method first presented by Freedman et al. for measuring corrections for the total line-of-sight extinction and reddening to the LMC. In this method, we use a differential comparison of the red giant population in the LMC, first with red giants in the Local Group galaxy IC 1613, and then with those in the Small Magellanic Cloud (SMC). As a consistency check, we derive an independent calibration of the TRGB sequence using the SMC alone, invoking its geometric distance also calibrated by DEBs. An additional consistency check comes from near-infrared observations of Galactic globular clusters covering a wide range of metallicities. In all cases we find excellent agreement in the zero-point calibration. We then examine the recent claims by Yuan et al., demonstrating that, in the case of the SMC, they corrected for extinction alone while neglecting the essential correction for reddening. In the case of IC 1613, we show that their analysis contains an incorrect treatment of (over-correction for) metallicity. Using our revised (and direct) measurement of the LMC TRGB extinction, we find a value of <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 69.6 ± 0.8 (±1.1% stat) ± 1.7 (±2.4% sys) km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent estimates point to abundances of <jats:italic>z</jats:italic> &gt; 4 submillimeter galaxies far above model predictions. The matter is still debated. According to some analyses the excess may be substantially lower than initially thought and perhaps accounted for by flux boosting and source blending. However, there is no general agreement on this conclusion. An excess of <jats:italic>z</jats:italic> &gt; 6 dusty galaxies has also been reported albeit with poor statistics. On the other hand, evidence of a top-heavy initial mass function (IMF) in high-<jats:italic>z</jats:italic> starburst galaxies has been reported in the past decades. This would translate into a higher submillimeter luminosity of dusty galaxies at fixed star formation rate, i.e., into a higher abundance of bright high-<jats:italic>z</jats:italic> submillimeter galaxies than expected for a universal Chabrier IMF. Exploiting our physical model for high-<jats:italic>z</jats:italic> protospheroidal galaxies, we find that part of the excess can be understood in terms of an IMF somewhat top-heavier than Chabrier. Such an IMF is consistent with that recently proposed to account for the low <jats:sup>13</jats:sup>C/<jats:sup>18</jats:sup>O abundance ratio in four dusty starburst galaxies at <jats:italic>z</jats:italic> = 2–3. However, extreme top-heavy IMFs are inconsistent with the submillimeter counts at <jats:italic>z</jats:italic> &gt; 4.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Interplanetary coronal mass ejections (ICMEs) could be classified into magnetic clouds (MCs) and non-MCs according to their magnetic field signatures, and into prominence-inside ICMEs (PIs) and non-PIs based on whether they contain colder and higher helium abundance plasmas than the solar wind. It is known that the MCs often lead to magnetic storms. However, whether or not the PIs have significant geoeffectiveness is unclear. This statistical work studies the southward interplanetary magnetic field (IMF) magnitude of the PIs, and the related magnetic storms’ level. The data include the IMF and plasma moments measured by <jats:italic>ACE</jats:italic> and <jats:italic>WIND</jats:italic>, and the Dst index from 1998 to 2011. The hypothesis test based on the proportions of two groups is used to analyze 95 ICMEs related to single storms (SSs). The results show that the magnetic storms caused by the PIs mostly distribute at a strong level, while that caused by the non-PIs and by all the 95 ICMEs mostly distribute at a moderate level. The PIs have a significantly higher probability of generating SSs than the non-PIs. Moreover, the MCs containing carbon-cold and helium-enhanced materials  (MC&amp;PIs) have the highest fraction of minimum <jats:italic>B</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>, less than −11 nT. Since the MC&amp;PIs have large-scale magnetic flux rope and prominence material, the stronger southward IMF is probably provided by the prominence. It is in accordance with the observed injection of enhanced twisted flux ropes to prominence. Therefore, the detailed eruption and propagation processes of the three-part coronal mass ejections deserve more concern from a space weather perspective.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the third catalog of active galactic nuclei detected by the <jats:italic>Fermi</jats:italic> Large Area Telescope Clean (3LAC) sample, there are 402 blazar candidates of uncertain type (BCU). The proposed analysis will help to evaluate the potential optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae (BL Lac) objects of BCUs, which can help to understand which is the most elusive class of blazar hidden in the <jats:italic>Fermi</jats:italic> sample. By studying the 3LAC sample, we found some critical values of <jats:italic>γ</jats:italic>-ray photon spectral index (Γ<jats:sub>ph</jats:sub>), variability index (VI), and radio flux (<jats:inline-formula> <jats:tex-math> <?CDATA ${F}_{{\rm{R}}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab722dieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) of the sources separate known FSRQs and BL Lac objects. We further utilize those values to defined an empirical “high-confidence” candidate zone that can be classified as BCUs. Within such a zone (Γ<jats:sub>ph</jats:sub> &lt; 2.187, log <jats:italic>F</jats:italic> <jats:sub>R</jats:sub> &lt; 2.258, and log VI &lt; 1.702), we found that 120 BCUs can be classified as BL Lac object candidates with a higher degree of confidence (with a misjudged rate &lt;1%). Our results suggest that an empirical “high-confidence” diagnosis is possible to distinguish the BL Lac objects from the <jats:italic>Fermi</jats:italic> observations based on only the direct observational data of Γ<jats:sub>ph</jats:sub>, VI, and <jats:italic>F</jats:italic> <jats:sub>R</jats:sub>.</jats:p>