Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Here we aim to explore the origin of the strong C<jats:sub>2</jats:sub>H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C<jats:sub>2</jats:sub>H is detected in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C<jats:sub>2</jats:sub>H is rapidly destroyed, within &lt;1000 yr. These two statements hint that the chemistry responsible for C<jats:sub>2</jats:sub>H emission must be predominantly in the gas phase and must be in equilibrium. Combining new and published chemical models, we find that elevating the total volatile (gas and ice) C/O ratio is the only natural way to create a long-lived, high C<jats:sub>2</jats:sub>H abundance. Most of the C<jats:sub>2</jats:sub>H resides in gas with an <jats:italic>F</jats:italic> <jats:sub>UV</jats:sub>/<jats:italic>n</jats:italic> <jats:sub>gas</jats:sub> ∼ 10<jats:sup>−7</jats:sup> <jats:italic>G</jats:italic> <jats:sub>0</jats:sub> cm<jats:sup>3</jats:sup>. To elevate the volatile C/O ratio, additional carbon has to be released into the gas to enable equilibrium chemistry under oxygen-poor conditions. Photoablation of carbon-rich grains seems the most straightforward way to elevate the C/O ratio above 1.5, powering a long-lived equilibrium cycle. The regions at which the conditions are optimal for the presence of high C/O ratio and elevated C<jats:sub>2</jats:sub>H abundances in the gas disk set by the <jats:italic>F</jats:italic> <jats:sub>UV</jats:sub>/<jats:italic>n</jats:italic> <jats:sub>gas</jats:sub> condition lie just outside the pebble disk as well as possibly in disk gaps. This process can thus also explain the (hints of) structure seen in C<jats:sub>2</jats:sub>H observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A complete understanding of solar radio bursts requires developing numerical techniques that can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of the magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of the dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave turbulence-particle interaction are considered using prescribed timescales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission, which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to <jats:inline-formula> <jats:tex-math> <?CDATA $\sim 2.9\times {10}^{-3}\,{E}_{{k}_{0}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd8d5ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, where <jats:inline-formula> <jats:tex-math> <?CDATA ${E}_{{k}_{0}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd8d5ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting the X2 mode in the corona during solar flares and provides new sight into how escaping radiation can be generated within a coronal loop.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In multi-messenger astronomy, rapid investigation of interesting transients is imperative. As an observatory with a 4<jats:italic>π</jats:italic> steradian field of view, and ∼99% uptime, the IceCube Neutrino Observatory is a unique facility to follow up transients, as well as to provide valuable insights for other observatories and inform their observational decisions. Since 2016, IceCube has been using low-latency data to rapidly respond to interesting astrophysical events reported by the multi-messenger observational community. Here, we describe the pipeline used to perform these followup analyses, and provide a summary of the 58 analyses performed as of July 2020. We find no significant signal in the first 58 analyses performed. The pipeline has helped inform various electromagnetic observation strategies, and has constrained neutrino emission from potential hadronic cosmic accelerators.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Because mergers of black hole–neutron star (BH–NS) binaries are widely argued to produce both gravitational and electromagnetic waves, these binaries are among the most attractive systems in the era of multi-messenger astronomy. In this paper we explore the charging processes of a moving BH in two types of charged surroundings and propose a new charging scenario differing from the elegant mechanism of Wald. During the inspiral of such a binary, the NS is strongly magnetized and the BH is moving inward. By considering this moving BH charging scenario, we find that the BH will increasingly accumulate enough net charge to light up the binary system at the inspiral stage. This charging process is universal no matter whether the BH spins or not. We show that our BH charging scenario can physically explain the BH’s unipolar inductor mechanism in a BH–NS binary system. We calculate electromagnetic emission luminosities due to various energy dissipation mechanisms and find that the electric dipole radiation of the BH makes a dominant contribution to electromagnetic emission at the final stage of inspiral if the BH spins slowly.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Massive black holes (BHs) in dwarf galaxies can provide strong constraints on BH seeds; however, reliably detecting them is notoriously difficult. High-resolution radio observations were recently used to identify accreting massive BHs in nearby dwarf galaxies, with a significant fraction found to be non-nuclear. Here we present the first results of our optical follow-up of these radio-selected active galactic nuclei (AGNs) in dwarf galaxies using integral field unit (IFU) data from Gemini-North. We focus on the dwarf galaxy J1220+3020, which shows no clear optical AGN signatures in its nuclear Sloan Digital Sky Survey spectrum covering the radio source. With our new IFU data, we confirm the presence of an active BH via the AGN coronal line [Fe <jats:sc>x</jats:sc>] and enhanced [O <jats:sc>i</jats:sc>] emission coincident with the radio source. Furthermore, we detect broad H<jats:italic>α</jats:italic> emission and estimate a BH mass of <jats:italic>M</jats:italic> <jats:sub>BH</jats:sub> = 10<jats:sup>4.9</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We compare the narrow emission line ratios to standard BPT diagnostics and shock models. Spatially resolved BPT diagrams show some AGN signatures, particularly in [O <jats:sc>i</jats:sc>]/H<jats:italic>α</jats:italic>, but overall do not unambiguously identify the AGN. A comparison of our data to shock models clearly indicates shocked emission surrounding the AGN. The physical model most consistent with the data is an active BH with a radiatively inefficient accretion flow that both photoionizes and shock-excites the surrounding gas. We conclude that feedback is important in radio-selected BHs in dwarf galaxies and that radio surveys may probe a population of low accretion rate BHs in dwarf galaxies that cannot be detected through optical surveys alone.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the discovery of possible periodic X-ray dips in a pulsating ultraluminous X-ray source, M51 ULX-7, with archival Chandra observations. With ∼20 days of monitoring in the superorbital descending state, we discovered three dips with separations of ∼2 and ∼8 days via the Bayesian block technique. A phase-dispersion minimization and a <jats:inline-formula> <jats:tex-math> <?CDATA ${\chi }^{2}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd7a5ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> test suggest that the dip is likely recurrent with a period of ∼2 days, consistent with the orbital period of M51 ULX-7. We interpret the dip as an obscuring of the emission from the pulsar by the vertical structure on the stream–disk interaction region or the atmosphere of the companion star. Both interpretations suggest the viewing angle to be ∼60°. Given that the magnetic field of M51 ULX-7 is moderately high, <jats:italic>B</jats:italic> ∼ 10<jats:sup>13</jats:sup> G, a low geometric beaming with <jats:inline-formula> <jats:tex-math> <?CDATA $b\gtrsim 1/2$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd7a5ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> is sufficient to explain the observed flux and the presence of dips. Obscuration of the stellar wind remains an alternative possible origin and further monitoring of the dips will be required.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Multiple stellar populations (MPs) are a distinct characteristic of globular clusters (GCs). Their general properties have been widely studied among main-sequence, red giant branch (RGB), and horizontal branch (HB) stars, but a common framework is still missing at later evolutionary stages. We studied the MP phenomenon along the asymptotic giant branch (AGB) sequences in 58 GCs, observed with the Hubble Space Telescope in UV and optical filters. Using UV–optical color–magnitude diagrams, we selected the AGB members of each cluster and identified the AGB candidates of the metal-enhanced population in type II GCs. We studied the photometric properties of the AGB stars and compared them to theoretical models derived from synthetic spectral analysis. We observed the following features: (i) the spread of AGB stars in photometric indices sensitive to variations of light elements and helium is typically larger than that expected from photometric errors; (ii) the fraction of metal-enhanced stars in the AGB is lower than that in the RGB in most of the type II GCs; (iii) the fraction of 1G stars derived from the chromosome map of AGB stars in 15 GCs is larger than that of RGB stars; and (v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent clear evidence of the presence of MPs along the AGB of Galactic GCs and indicate that a significant fraction of helium-enriched stars, which have lower mass in the HB, do not evolve to the AGB phase, leaving the HB sequence toward higher effective temperatures, as predicted by the AGB manqué scenario.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present timing solutions for eight binary millisecond pulsars (MSPs) discovered by searching unidentified Fermi-Large Area Telescope (LAT) source positions with the 327 MHz receiver of the Arecibo 305 m radio telescope. Five of the pulsars are “spiders” with orbital periods shorter than 8.1 hr. Three of these are in “black widow” systems (with degenerate companions of 0.02–0.03 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), one is in a “redback” system (with a non-degenerate companion of ≳0.3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), and one (J1908+2105) is an apparent middle-ground case between the two observational classes. The remaining three pulsars have white dwarf companions and longer orbital periods. With the initially derived radio timing solutions, we detected <jats:italic>γ</jats:italic>-ray pulsations from all MSPs and extended the timing solutions using photons from the full Fermi mission, thus confirming the identification of these MSPs with the Fermi-LAT sources. The radio emission of the redback is eclipsed during 50% of its orbital period, which is typical for this kind of system. Two of the black widows exhibit radio eclipses lasting for 10%–20% of the orbit, while J1908+2105 eclipses for 40% of the orbit. We investigate an apparent link between gamma-ray emission and a short orbital period among known binary MSPs in the Galactic disk, and conclude that selection effects cannot be ruled out as the cause. Based on this analysis we outline how the likelihood of new MSP discoveries can be improved in ongoing and future pulsar searches.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Parker Solar Probe (PSP) aims to explore the nascent solar wind close to the Sun. Meanwhile, PSP is also expected to encounter small objects like comets and asteroids. In this work, we survey the ephemerides to find the chance of a recent encounter and then model the interaction between released dusty plasmas and solar wind plasmas. On 2019 September 2, a comet-like object, the 322P/Solar and Heliosphere Observatory, just passed its perihelion flying to a heliocentric distance of 0.12 au and swept by PSP at a relative distance as close as 0.025 au. We present the dynamics of the dust particles released from 322P, forming a curved dust tail. Along the path of PSP in the simulated inner heliosphere, the states of plasma and magnetic field are sampled and illustrated, with the magnetic field sequences from simulation results being compared directly with the in situ measurements from PSP. Through the comparison, we suggest that 322P might be at a deficient activity level releasing limited dusty plasmas on its way to becoming a “rock comet.” We also present images of solar wind streamers as recorded by the Wide-field Imager for Solar Probe Plus, showing an indication of dust bombardment for the images superposed with messy trails. We observe from the Large Angle and Spectrometric Coronagraph that 322P was transiting from a dimming region to a relatively bright streamer during its perihelion passage, and perform a simulation to confirm that 322P was flying from relatively faster to slower solar wind streams, modifying the local plasma states of the streams.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The hot and diffuse nature of the Sun’s extended atmosphere allows it to persist in non-equilibrium states for long enough that wave–particle instabilities can arise and modify the evolution of the expanding solar wind. Determining which instabilities arise, and how significant a role they play in governing the dynamics of the solar wind, has been a decades-long process involving in situ observations at a variety of radial distances. With new measurements from the Parker Solar Probe (PSP), we can study what wave modes are driven near the Sun, and calculate what instabilities are predicted for different models of the underlying particle populations. We model two hours-long intervals of PSP/SPAN-i measurements of the proton phase-space density during the PSP’s fourth perihelion with the Sun using two commonly used descriptions for the underlying velocity distribution. The linear stability and growth rates associated with the two models are calculated and compared. We find that both selected intervals are susceptible to resonant instabilities, though the growth rates and kinds of modes driven unstable vary depending on whether the protons are modeled using one or two components. In some cases, the predicted growth rates are large enough to compete with other dynamic processes, such as the nonlinear turbulent transfer of energy, in contrast with relatively slower instabilities at larger radial distances from the Sun.</jats:p>