Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>During solar flares, plasma is typically heated to very high temperatures, and the resulting redistribution of energy via thermal conduction is a primary mechanism transporting energy throughout the flaring solar atmosphere. The thermal flux is usually modeled using Spitzer’s theory, which is based on local Coulomb collisions between the electrons carrying the thermal flux and those in the background. However, often during flares, temperature gradients become sufficiently steep that the collisional mean free path exceeds the temperature-gradient scale size, so that thermal conduction becomes inherently nonlocal. Further, turbulent angular scattering, which is detectable in nonthermal widths of atomic emission lines, can also act to increase the collision frequency and thus suppress the heat flux. Recent work by Emslie &amp; Bian extended Spitzer’s theory of thermal conduction to account for both nonlocality and turbulent suppression. We have implemented their theoretical expression for the heat flux (which is a convolution of the Spitzer flux with a kernel function) into the RADYN flare-modeling code and performed a parameter study to understand how the resulting changes in thermal conduction affect the flare dynamics and hence the radiation produced. We find that models with reduced heat fluxes predict slower bulk flows, less intense line emission, and longer cooling times. By comparing the features of atomic emission lines predicted by the models with Doppler velocities and nonthermal line widths deduced from a particular flare observation, we find that models with suppression factors between 0.3 and 0.5 relative to the Spitzer value best reproduce the observed Doppler velocities across emission lines forming over a wide range of temperatures. Interestingly, the model that best matches the observed nonthermal line widths has a kappa-type velocity distribution function.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present joint Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) and Interface Region Imaging Spectrograph (IRIS) observations of Fe <jats:sc>xii</jats:sc> lines in active regions, both on-disk and off-limb. We use an improved calibration for the EIS data, and find that the 192.4 Å/1349 Å observed ratio is consistent with the values predicted by CHIANTI and the coronal approximation in quiescent areas, but not in all active-region observations, where the ratio is often lower than expected by up to a factor of about two. We investigate a number of physical mechanisms that could affect this ratio, such as opacity and absorption from cooler material. We find significant opacity in the EIS Fe <jats:sc>xii</jats:sc> 193 and 195 Å lines, but not in the 192.4 Å line, in agreement with previous findings. As we cannot rule out possible EUV absorption by H, He, and He <jats:sc>ii</jats:sc> in the on-disk observations, we focus on an off-limb observation where such absorption is minimal. After considering these, as well as possible nonequilibrium effects, we suggest that the most likely explanation for the observed low Fe <jats:sc>xii</jats:sc> 192.4 Å/1349 Å ratio is the presence of non-Maxwellian electron distributions in the active regions. This is in agreement with previous findings based on EIS and IRIS observations independently.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first systematic study of lithium abundance in a chemically homogeneous sample of 27 red supergiants (RSGs) in the young Perseus complex. For these stars, accurate stellar parameters and detailed chemical abundances of iron and iron peak, CNO, alpha, light, and neutron capture elements have already been obtained by means of high-resolution optical and near-infrared spectroscopy. The observed RSGs have half-solar metallicity, 10–30 Myr ages, bolometric luminosities in the 10<jats:sup>4</jats:sup>–10<jats:sup>5</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub> range, and likely mass progenitors in the 9–14 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> range. We detected the optical Li <jats:sc>i</jats:sc> doublet in eight out of the 27 observed K- and M-type RSGs, finding relatively low A(Li) &lt; 1.0 dex abundances, while for the remaining 19 RSGs upper limits of A(Li) &lt; –0.2 dex have been set. Warmer and less luminous (i.e., likely less massive) as well as less mixed (i.e., with lower [C/N] and <jats:sup>12</jats:sup>C/<jats:sup>13</jats:sup>C depletion) RSGs with Li detection show somewhat higher Li abundances. In order to explain the Li detection in ∼30% of the observed RSGs, we speculate that some stochasticity should be at work, in a scenario where the Li was not completely destroyed in the convective atmospheres and/or a secondary production took place during the post-main-sequence evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The search for signs of life on other worlds has largely focused on terrestrial planets. Recent work, however, argues that life could exist in the atmospheres of temperate sub-Neptunes. Here we evaluate the usefulness of carbon dioxide isotopologues as evidence of aerial life. Carbon isotopes are of particular interest, as metabolic processes preferentially use the lighter <jats:sup>12</jats:sup>C over <jats:sup>13</jats:sup>C. In principle, the upcoming James Webb Space Telescope (JWST) will be able to spectrally resolve the <jats:sup>12</jats:sup>C and <jats:sup>13</jats:sup>C isotopologues of CO<jats:sub>2</jats:sub>, but not CO and CH<jats:sub>4</jats:sub>. We simulated observations of CO<jats:sub>2</jats:sub> isotopologues in the H<jats:sub>2</jats:sub>-dominated atmospheres of our nearest (&lt;40 pc), temperate (equilibrium temperature of 250–350 K) sub-Neptunes with M-dwarf host stars. We find <jats:sup>13</jats:sup>CO<jats:sub>2</jats:sub> and <jats:sup>12</jats:sup>CO<jats:sub>2</jats:sub> distinguishable if the atmosphere is H<jats:sub>2</jats:sub> dominated with a few percentage points of CO<jats:sub>2</jats:sub> for the most idealized target with an Earth-like composition of the two most abundant isotopologues, <jats:sup>12</jats:sup>CO<jats:sub>2</jats:sub> and <jats:sup>13</jats:sup>CO<jats:sub>2</jats:sub>. With a Neptune-like metallicity of 100× solar and a C/O of 0.55, we are unable to distinguish between <jats:sup>13</jats:sup>CO<jats:sub>2</jats:sub> and <jats:sup>12</jats:sup>CO<jats:sub>2</jats:sub> in the atmospheres of temperate sub-Neptunes. If atmospheric composition largely follows metallicity scaling, the concentration of CO<jats:sub>2</jats:sub> in a H<jats:sub>2</jats:sub>-dominated atmosphere will be too low to distinguish CO<jats:sub>2</jats:sub> isotopologues with JWST. In contrast, at higher metallicities, there will be more CO<jats:sub>2</jats:sub>, but the smaller atmospheric scale height makes the measurement impossible. Carbon dioxide isotopologues are unlikely to be useful biosignature gases for the JWST era. Instead, isotopologue measurements should be used to evaluate formation mechanisms of planets and exosystems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Traditional large-scale models of reionization usually employ simple deterministic relations between halo mass and luminosity to predict how reionization proceeds. We here examine the impact on modeling reionization of using more detailed models for the ionizing sources as identified within the 100 <jats:italic>h</jats:italic> <jats:sup>−1</jats:sup> Mpc cosmological hydrodynamic simulation S<jats:sc>imba</jats:sc>, coupled with postprocessed radiative transfer. Comparing with simple (one-to-one) models, the main difference with using S<jats:sc>imba</jats:sc> sources is the scatter in the relation between dark matter halos and star formation, and hence ionizing emissivity. We find that, at the power spectrum level, the ionization morphology remains mostly unchanged, regardless of the variability in the number of sources or escape fraction. In particular, the power spectrum shape remains unaffected and its amplitude changes slightly by less than 5%–10%, throughout reionization, depending on the scale and neutral fraction. Our results show that simplified models of ionizing sources remain viable to efficiently model the structure of reionization on cosmological scales, although the precise progress of reionization requires accounting for the scatter induced by astrophysical effects.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We perform a statistical investigation of the geometric features of interplanetary discontinuities (IDs) in the near-Earth solar wind and magnetosheath, by utilizing 14 months of Magnetospheric Multiscale mission data. 117,669 IDs are collected, including 108,049 events in the solar wind and 6399 events in the magnetosheath, with the remnant in the magnetosphere or near the bow shock/magnetopause. We find the following: (1) the ID occurrence rate is 17.0 events hr<jats:sup>−1</jats:sup> in the solar wind and 5.5 events hr<jats:sup>−1</jats:sup> in the magnetosheath, (2) the field rotation angles during ID crossings in the magnetosheath exhibit a two-exponential distribution with a breakpoint at 50°, which is not observed for IDs in the solar wind, (3) the magnetosheath IDs with small field rotation angles tend to be clustered, (4) by classifying the IDs into rotational discontinuities (RDs), tangential discontinuities (TDs), either TDs or RDs (EDs), and neither TDs nor RDs (NDs), we estimate RD:TD:ED:ND = 68%:5%:20%:7% in the solar wind, and RD:TD:ED:ND = 15%:44%:18%:23% in the magnetosheath, (5) the occurrence rates of RDs and TDs are, respectively 7.95 and 0.58 events hr<jats:sup>−1</jats:sup> in the solar wind, and 0.57 and 1.60 events hr<jats:sup>−1</jats:sup> in the magnetosheath, (6) RDs are more likely to propagate antisunward in the plasma rest frame, especially in the magnetosheath, and (7) the average thicknesses of the RDs and TDs are estimated, respectively, as 10.4 and 8.1 proton gyroradii (<jats:italic>r</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub>) in the solar wind, and 17.4 and 5.0 <jats:italic>r</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> in the magnetosheath. This work can improve our understanding of IDs’ interaction with the terrestrial bow shock.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Slow magnetoacoustic oscillations in stellar coronal loops with gravitational stratification are analyzed with a numerical solution of the boundary value problem for eigenvalues and eigenfunctions. In this study, we only focus on the resonant periods. The effects of the gravitational stratification, star mass, loop temperature, and loop length on the properties of slow magnetoacoustic oscillations are investigated. It is shown that the discrepancy between stratified and nonstratified loops is higher in density perturbations than in velocity perturbations. When the star has a larger mass, higher coronal temperature, and longer loop, the density perturbations in the stratified loop are significantly different from the harmonic functions. The periods in the stratified loop are slightly longer than in the nonstratified loop. The periods calculated in our model (14–644 minutes) are consistent with the periods of stellar quasi-periodic pulsations observed in both soft X-rays (2–70 minutes) and white lights (8–390 minutes).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We can learn about the formation and evolution of compact objects, such as neutron stars and black holes (BHs), by studying the X-ray emission from accreting systems in nearby star-forming galaxies. The hard (<jats:italic>E</jats:italic> &gt; 10 keV) X-ray emission in particular allows strong discrimination among the accretion states and compact object types. We conducted a NuSTAR survey (∼600 ks) of the Local Group spiral galaxy M33 to study the distribution of X-ray binary (XRB) accretors in an actively star-forming environment. We constructed color–intensity and color–color diagrams to infer XRB accretion states. Using these diagrams, we have classified 28 X-ray sources in M33 by comparing their hard X-ray colors to those of known systems. Four sources lie in the parameter space occupied by X-ray pulsars, while 8, 10, and 4 sources lie in the parameter space occupied by BHs in the hard, intermediate, and soft states, respectively. The known ultraluminous X-ray source M33 X-8 is also found to be consistent with that source type. Some sources overlap within the Z/Atoll sources due to the overlap of the two categories of BHs and Z/Atoll sources. In contrast to a similar NuSTAR survey of M31 (with a low-mass XRB-dominant population), the source population in M33 is dominated by high-mass XRBs (HMXBs), allowing the study of a very different population with similar sensitivity due to the galaxy's similar distance. This characterization of a population of HMXB accretion states will provide valuable constraints for theoretical XRB population synthesis studies to their formation and evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Determining the ages and helium core sizes of red giants is a challenging problem. To estimate the age and helium core size precisely requires a good understanding of the internal structure of the red giant. The properties of the <jats:italic>g</jats:italic>-dominated mixed modes of red giants are closely related to their inner radiative cores, especially the central helium core. Thus, the <jats:italic>g</jats:italic>-dominated mixed modes are useful indicators for probing the properties of the helium core and constraining the age of red giants. In our previous work, we have estimated the helium core sizes of the red giants KIC 9145955 and KIC 9970396 by asteroseismic models. In this work, we take a further step to calibrate the ages and core overshooting parameters for these two red giants. We find that the ages of these two stars are 4.61 ± 0.23 and 6.13 ± 0.19 Gyr, respectively. From a comparative study, we find that, for a single red giant, the age estimated by the asteroseismology of <jats:italic>g</jats:italic>-dominated mixed modes is likely to be more precise than that estimated by the combination of the asteroseismic (Δ<jats:italic>ν</jats:italic> and Δ<jats:italic>P</jats:italic> <jats:sub>obs</jats:sub>) and spectroscopic (<jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> and [Fe/H]) observations. In addition, we estimate the core overshooting parameters of these two stars. We find that the overshooting parameter <jats:italic>f</jats:italic> <jats:sub>ov</jats:sub> of KIC 9145955 and KIC 9970396 was probably overestimated in previous works.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The majority of classical Cepheids are binary stars, yet the contribution of companions’ light to the total brightness of the system has been assumed negligible and lacked a thorough, quantitative evaluation. We present an extensive study of synthetic populations of binary Cepheids, which aims to characterize Cepheids’ companions (e.g., masses, evolutionary, and spectral types), quantify their contribution to the brightness and color of Cepheid binaries, and assess the relevance of input parameters on the results. We introduce a collection of synthetic populations, which vary in metal content, initial parameter distribution, location of the instability strip edges, and star formation history. Our synthetic populations are free from the selection bias, while the percentage of Cepheid binaries is controlled by the binarity parameter. We successfully reproduce recent theoretical and empirical results: the percentage of binary Cepheids with main-sequence (MS) companions, the contrast–mass ratio relation for binary Cepheids with MS companions, the appearance of binary Cepheids with giant, evolved companions as outlier data points above the period–luminosity relation. Moreover, we present the first estimation of the percentage of binary Cepheids in the Large Magellanic Cloud and announce the quantification of the effect of binarity on the slope and zero-point of multiband period–luminosity relations, which will be reported in the next paper of this series.</jats:p>