Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Star-forming galaxies are considered the likeliest source of the H <jats:sc>i</jats:sc> ionizing Lyman continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above <jats:italic>z</jats:italic> ≳ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of <jats:italic>indirect</jats:italic> indicators for LyC that can be calibrated at lower redshifts and applied in the epoch of reionization. Emission from the Mg <jats:sc>ii</jats:sc> <jats:italic>λλ</jats:italic>2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for eight LyC emitter candidates, selected to have strong Mg <jats:sc>ii</jats:sc> emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2<jats:italic>σ</jats:italic> significance. This high detection rate suggests that strong Mg <jats:sc>ii</jats:sc> emitters might be more likely to leak LyC than similar galaxies without strong Mg <jats:sc>ii</jats:sc>. Using photoionization models, we constrain the escape fraction of Mg <jats:sc>ii</jats:sc> as ∼15%–60%. We confirm that the escape fraction of Mg <jats:sc>ii</jats:sc> correlates tightly with that of Ly<jats:italic>α</jats:italic>, which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg <jats:sc>ii</jats:sc> emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg <jats:sc>ii</jats:sc> emission can be adopted to estimate the escape fraction of Ly<jats:italic>α</jats:italic> and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent analyses have shown that close encounters between stars and stellar black holes occur frequently in dense star clusters. Depending upon the distance at closest approach, these interactions can lead to dissipating encounters such as tidal captures and disruptions, or direct physical collisions, all of which may be accompanied by bright electromagnetic transients. In this study, we perform a wide range of hydrodynamic simulations of close encounters between black holes and main-sequence stars that collectively cover the parameter space of interest, and we identify and classify the various possible outcomes. In the case of nearly head-on collisions, the star is completely disrupted with roughly half of the stellar material becoming bound to the black hole. For more distant encounters near the classical tidal-disruption radius, the star is only partially disrupted on the first pericenter passage. Depending upon the interaction details, the partially disrupted stellar remnant may be tidally captured by the black hole or become unbound (in some cases, receiving a sufficiently large impulsive kick from asymmetric mass loss to be ejected from its host cluster). In the former case, the star will undergo additional pericenter passages before ultimately being disrupted fully. Based on the properties of the material bound to the black hole at the end of our simulations (in particular, the total bound mass and angular momentum), we comment upon the expected accretion process and associated electromagnetic signatures that are likely to result.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Cosmic rays are mostly composed of protons accelerated to relativistic speeds. When those protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma-rays. This offers a compelling way to identify the acceleration sites of protons. A characteristic hadronic spectrum, with a low-energy break around 200 MeV, was detected in the gamma-ray spectra of four supernova remnants (SNRs), IC 443, W44, W49B, and W51C, with the Fermi Large Area Telescope. This detection provided direct evidence that cosmic-ray protons are (re-)accelerated in SNRs. Here, we present a comprehensive search for low-energy spectral breaks among 311 4FGL catalog sources located within 5° from the Galactic plane. Using 8 yr of data from the Fermi Large Area Telescope between 50 MeV and 1 GeV, we find and present the spectral characteristics of 56 sources with a spectral break confirmed by a thorough study of systematic uncertainty. Our population of sources includes 13 SNRs for which the proton–proton interaction is enhanced by the dense target material; the high-mass gamma-ray binary LS I+61 303; the colliding wind binary <jats:italic>η</jats:italic> Carinae; and the Cygnus star-forming region. This analysis better constrains the origin of the gamma-ray emission and enlarges our view to potential new cosmic-ray acceleration sites.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This work aims to provide an accurate description and calculations of collision frequencies in conditions relevant to the solar atmosphere. To do so, we focus on the detailed description of the collision frequency in the solar atmosphere based on a classical formalism with Chapman–Cowling collision integrals, as described by Zhdanov. These collision integrals allow linking the macroscopic transport fluxes of multifluid models to the kinetic scales involved in the Boltzmann equations. In this context, the collision frequencies are computed accurately while being consistent at the kinetic level. We calculate the collision frequencies based on this formalism and compare them with approaches commonly used in the literature for conditions typical of the solar atmosphere. To calculate the collision frequencies, we focus on the collision integral data provided by Bruno et al., which is based on a multicomponent hydrogen–helium mixture used for conditions typical for the atmosphere of Jupiter. We perform a comparison with the classical formalism of Vranjes &amp; Krstic and Leake &amp; Linton. We highlight the differences obtained in the distribution of the cross sections as functions of the temperature. Then, we quantify the disparities obtained in numerical simulations of a 2.5D solar atmosphere by calculating collision frequencies and ambipolar diffusion. This strategy allows us to validate and assess the accuracy of these collision frequencies for conditions typical of the solar atmosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>HCN is among the most commonly detected molecules in star- and planet-forming regions. It is of broad interest as a tracer of star formation physics, a probe of nitrogen astrochemistry, and an ingredient in prebiotic chemical schemes. Despite this, one of the most fundamental astrochemical properties of HCN remains poorly characterized: its thermal desorption behavior. Here, we present a series of experiments to characterize the thermal desorption of HCN in astrophysically relevant conditions, with a focus on predicting the HCN sublimation fronts in protoplanetary disks. We derive HCN–HCN and HCN–H<jats:sub>2</jats:sub>O binding energies of 3207 ± 197 and 4192 ± 68 K, which translate to disk midplane sublimation temperatures around 85 and 103 K. For a typical midplane temperature profile, HCN should only begin to sublimate ∼1–2 au exterior to the H<jats:sub>2</jats:sub>O snow line. Additionally, in H<jats:sub>2</jats:sub>O-dominated mixtures (20:1 H<jats:sub>2</jats:sub>O:HCN), we find that the majority of HCN remains trapped in the ice until H<jats:sub>2</jats:sub>O crystallizes. Thus, HCN may be retained in disk ices at almost all radii where H<jats:sub>2</jats:sub>O-rich planetesimals form. This implies that icy body impacts to planetary surfaces should commonly deliver this potential prebiotic ingredient. A remaining unknown is the extent to which HCN is pure or mixed with H<jats:sub>2</jats:sub>O in astrophysical ices, which impacts the HCN desorption behavior as well as the outcomes of ice-phase chemistry. Pure HCN and HCN:H<jats:sub>2</jats:sub>O mixtures exhibit distinct IR bands, raising the possibility that the James Webb Space Telescope will elucidate the mixing environment of HCN in star- and planet-forming regions and address these open questions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The state-resolved capture cross sections for principal <jats:italic>n</jats:italic> and orbital angular momentum <jats:italic>l</jats:italic> play an important role in modeling soft X-ray emissions induced by charge exchange for many astrophysical environments. However, the empirical and semiclassical theories used to produce these data of <jats:italic>n</jats:italic>- and <jats:italic>l</jats:italic>-resolved state-selective capture have not been well tested. Using the cold target recoil ion momentum spectroscopy apparatus at Fudan University, we perform a series of measurements of Ar<jats:sup>8+</jats:sup> ion charge exchange with He in the collision energy range from 1.4 to 20 keV u<jats:sup>−1</jats:sup>. We find that electrons are mainly captured in the <jats:italic>n</jats:italic> = 4 state of Ar<jats:sup>7+</jats:sup> ions. This agrees with the prediction of the scaling law for <jats:italic>n </jats:italic>capture. Moreover, the relative cross sections are reported for 4<jats:italic>s-</jats:italic>, 4<jats:italic>p-</jats:italic>, 4<jats:italic>d-</jats:italic>, and 4<jats:italic>f</jats:italic>-resolved state-selective capture. The often used analytical <jats:italic>l</jats:italic> distributions in the astrophysical literature are evaluated by comparing to the measurements.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Utilizing high-resolution data from the Magnetospheric Multiscale mission, we present new observations of lower-hybrid drift waves (LHDWs) in terrestrial magnetotail reconnection with guide field levels of ∼70% and asymmetric plasma density (<jats:italic>N</jats:italic> <jats:sub>high</jats:sub>/<jats:italic>N</jats:italic> <jats:sub>low</jats:sub> ∼ 2.5). The LHDWs, driven by lower-hybrid drift instability, were observed in correlation with magnetic field and density gradients at separatrices on both sides of the reconnection current sheet. The properties of the LHDWs at both sides of the separatrices are different: (1) At high-density side separatrices, the LHDWs with wavelength <jats:italic>kρ</jats:italic> <jats:sub>e</jats:sub> ∼ 0.41 propagated away from the X-line mainly in the L–M plane; (2) at the low-density side separatrices, the LHDWs with wavelengths <jats:italic>kρ</jats:italic> <jats:sub>e</jats:sub> ∼ 0.76 and <jats:italic>kρ</jats:italic> <jats:sub>e</jats:sub> ∼ 0.35 propagated mainly along the outflow direction and current sheet normal. It is also found that the perpendicular magnetic field fluctuations were comparable to the parallel component. Wave potential of the LHDWs was 20% ∼ 35% of the electron temperature. The LHDWs contributed to electron demagnetization and energy dissipation. Our study can promote understanding of properties of LHDWs during magnetic reconnection.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stellar flares sometimes show red/blue asymmetries of the H<jats:italic>α</jats:italic> line, which can indicate chromospheric dynamics and prominence activations. However, the origin of asymmetries is not completely understood. For a deeper understanding of stellar data, we performed a Sun-as-a-star analysis of H<jats:italic>α</jats:italic> line profiles of an M4.2-class solar flare showing dominant emissions from flare ribbons by using the data of the Solar Dynamics Doppler Imager on board the Solar Magnetic Activity Research Telescope at the Hida Observatory. Sun-as-a-star H<jats:italic>α</jats:italic> spectra of the flare show red asymmetry of up to ∼95 km s<jats:sup>−1</jats:sup> and line broadening of up to ∼7.5 Å. The Sun-as-a-star H<jats:italic>α</jats:italic> profiles are consistent with spectra from flare regions with weak intensity, but they take smaller redshift velocities and line widths by a factor of ∼2 than those with strong intensity. The redshift velocities, as well as line widths, peak out and decay more rapidly than the H<jats:italic>α</jats:italic> equivalent widths, which is consistent with the chromospheric condensation model and spatially resolved flare spectra. This suggests that as a result of superposition, the nature of chromospheric condensation is observable even from stellar flare spectra. The time evolution of redshift velocities is found to be similar to that of luminosities of near-ultraviolet rays (1600 Å), while the time evolution of line broadening is similar to that of optical white lights. These H<jats:italic>α</jats:italic> spectral behaviors in Sun-as-a-star view could be helpful to distinguish whether the origin of H<jats:italic>α</jats:italic> red asymmetry of stellar flares is a flare ribbon or other phenomena.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the independent discovery of PSR J0026-1955 with the Murchison Widefield Array (MWA) in the ongoing Southern-sky MWA Rapid Two-metre pulsar survey. J0026-1955 has a period of ∼1.306 s, a dispersion measure of ∼20.869 pc cm<jats:sup>−3</jats:sup>, and a nulling fraction of ∼77%. This pulsar highlights the advantages of the survey's long dwell times (∼80 minutes), which, when fully searched, will be sensitive to the expected population of similarly bright, intermittent pulsars with long nulls. A single-pulse analysis in the MWA's 140–170 MHz band also reveals a complex subpulse drifting behavior, including both rapid changes of the drift rate characteristic of mode switching pulsars, as well as a slow, consistent evolution of the drift rate within modes. In some longer drift sequences, interruptions in the otherwise smooth drift rate evolution occur preferentially at a particular phase, typically lasting a few pulses. These properties make this pulsar an ideal test bed for prevailing models of drifting behavior such as the carousel model.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an analysis of oscillation mode variability in the hot B subdwarf star EPIC 220422705, a new pulsator discovered from ∼78 days of K2 photometry. The high-quality light curves provide a detection of 66 significant independent frequencies, from which we identified nine incomplete potential triplets and three quintuplets. Those <jats:italic>g-</jats:italic> and <jats:italic>p-</jats:italic>multiplets give rotation periods of ∼36 and 29 days in the core and at the surface, respectively, potentially suggesting a slightly differential rotation. We derived a period spacing of 268.5 s and 159.4 s for the sequence of dipole and quadrupole modes, respectively. We characterized the precise patterns of amplitude and frequency modulations (AM and FM) of 22 frequencies with high enough amplitude for our science. Many of them exhibit intrinsic and periodic patterns of AM and FM, with periods on a timescale of months as derived by the best fitting and Markov Chain Monte Carlo test. The nonlinear resonant mode interactions could be a natural interpretation for such AMs and FMs after other mechanisms are ruled out. Our results are the first step to building a bridge between mode variability from K2 photometry and the nonlinear perturbation theory of stellar oscillation.</jats:p>