Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>To undergo diffusive shock acceleration, electrons need to be preaccelerated to increase their energies by several orders of magnitude, else their gyroradii will be smaller than the finite width of the shock. In oblique shocks, where the upstream magnetic field orientation is neither parallel nor perpendicular to the shock normal, electrons can escape to the shock upstream, modifying the shock foot to a region called the electron foreshock. To determine the preacceleration in this region, we undertake particle-in-cell simulations of oblique shocks while varying the obliquity and in-plane angles. We show that while the proportion of reflected electrons is negligible for <jats:italic>θ</jats:italic> <jats:sub>Bn</jats:sub> = 74.°3, it increases to <jats:italic>R</jats:italic> ∼ 5% for <jats:italic>θ</jats:italic> <jats:sub>Bn</jats:sub> = 30°, and that, via the electron acoustic instability, these electrons power electrostatic waves upstream with energy density proportional to <jats:italic>R</jats:italic> <jats:sup>0.6</jats:sup> and a wavelength ≈2<jats:italic>λ</jats:italic> <jats:sub>se</jats:sub>, where <jats:italic>λ</jats:italic> <jats:sub>se</jats:sub> is the electron skin length. While the initial reflection mechanism is typically a combination of shock-surfing acceleration and magnetic mirroring, we show that once the electrostatic waves have been generated upstream, they themselves can increase the momenta of upstream electrons parallel to the magnetic field. In ≲1% of cases, upstream electrons are prematurely turned away from the shock and never injected downstream. In contrast, a similar fraction is rescattered back toward the shock after reflection, reinteracts with the shock with energies much greater than thermal, and crosses into the downstream.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Multi-Order Solar Extreme Ultraviolet Spectrograph (MOSES) sounding rocket was launched from White Sands Missile Range on 2006 February 8th, to capture images of the Sun in the He <jats:sc>ii</jats:sc> 303.8 Å emission line. MOSES is a slitless spectrograph that forms images in multiple spectral orders simultaneously using a concave diffraction grating in an effort to measure line profiles over a wide field of view from a single exposure. Early work on MOSES data showed evidence of solar features composed of neither He <jats:sc>ii</jats:sc> 303.8 Å nor the nearby Si <jats:sc>xi</jats:sc> 303.3 Å spectral lines. We have built a forward model that uses cotemporal EIT images and the Chianti atomic database to fit synthetic images with known spectra to the MOSES data in order to quantify this additional spectral content. Our fit reveals a host of dim lines that alone are insignificant but combined contribute a comparable intensity to MOSES images as Si <jats:sc>xi</jats:sc> 303.3 Å. In total, lines other than He <jats:sc>ii</jats:sc> 303.8 Å and Si <jats:sc>xi</jats:sc> 303.3 Å contribute approximately 10% of the total intensity in the MOSES zero order image. This additional content, if not properly accounted for, could significantly impact the analysis of MOSES and similar slitless spectrograph data, especially those using a zero-order (undispersed) image. More broadly, this serves as a reminder that multilayer EUV imagers are sensitive to a host of weak contaminant lines.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Protostellar outflows and jets play a vital role in star formation as they carry away excess angular momentum from the inner disk surface, allowing the material to be transferred toward the central protostar. Theoretically, low-velocity and poorly collimated outflows appear from the beginning of the collapse at the first hydrostatic core (FHSC) stage. With growing protostellar core mass, high-density jets are launched, entraining an outflow from the infalling envelope. Until now, molecular jets have been observed at high velocity (≳100 km s<jats:sup>−1</jats:sup>) in early Class 0 protostars. We, for the first time, detect a dense molecular jet in SiO emission with low velocity (∼4.2 km s<jats:sup>−1</jats:sup>, deprojected ∼24 km s<jats:sup>−1</jats:sup>) from source G208.89–20.04Walma (hereafter G208Walma) using ALMA Band 6 observations. This object has some characteristics of FHSCs, such as a small outflow/jet velocity, extended 1.3 mm continuum emission, and <jats:italic>N</jats:italic> <jats:sub>2</jats:sub>D<jats:sup>+</jats:sup> line emission. Additional characteristics, however, are typical of early protostars: collimated outflow and SiO jet. The full extent of the outflow corresponds to a dynamical timescale of ∼<jats:inline-formula> <jats:tex-math> <?CDATA ${930}_{-100}^{+200}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>930</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>100</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>200</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac67a1ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> yr. The spectral energy distribution also suggests a very young source having an upper limit of <jats:italic>T</jats:italic> <jats:sub>bol</jats:sub> ∼ 31 K and <jats:italic>L</jats:italic> <jats:sub>bol</jats:sub> ∼ 0.8 <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub>. We conclude that G208Walma is likely in the transition phase from FHSC to protostar, and the molecular jet has been launched within a few hundred years of initial collapse. Therefore, G208Walma may be the earliest object discovered in the protostellar phase with a molecular jet.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We consider the effective field theory formulation of torsional gravity in a cosmological framework to alter the background evolution. Then we use the latest <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> measurement from the SH0ES Team, as well as observational Hubble data from cosmic chronometer and radial baryon acoustic oscillations, and we reconstruct the <jats:italic>f</jats:italic>(<jats:italic>T</jats:italic>) form in a model-independent way by applying Gaussian processes. Since the special square-root term does not affect the evolution at the background level, we finally summarize a family of functions that can produce the background evolution required by the data. Lastly, performing a fitting using polynomial functions and implementing the Bayesian information criterion, we find an analytic expression that may describe the cosmological evolution in great agreement with observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present new period-<jats:italic>ϕ</jats:italic> <jats:sub>31</jats:sub>-[Fe/H] relations for first-overtone RRL stars (RRc), calibrated over a broad range of metallicities (−2.5 ≲ [Fe/H] ≲ 0.0) using the largest currently available set of Galactic halo field RRL with homogeneous spectroscopic metallicities. Our relations are defined in the optical (ASAS-SN <jats:italic>V</jats:italic> band) and, inaugurally, in the infrared (WISE <jats:italic>W1</jats:italic> and <jats:italic>W2 </jats:italic>bands). Our <jats:italic>V</jats:italic>-band relation can reproduce individual RRc spectroscopic metallicities with a dispersion of 0.30 dex over the entire metallicity range of our calibrator sample (an rms smaller than what we found for other relations in literature including nonlinear terms). Our infrared relation has a similar dispersion in the low- and intermediate-metallicity range ([Fe/H] ≲ −0.5), but tends to underestimate the [Fe/H] abundance around solar metallicity. We tested our relations by measuring both the metallicity of the Sculptor dSph and a sample of Galactic globular clusters, rich in both RRc and RRab stars. The average metallicity we obtain for the combined RRL sample in each cluster is within ±0.08 dex of their spectroscopic metallicities. The infrared and optical relations presented in this work will enable deriving reliable photometric RRL metallicities in conditions where spectroscopic measurements are not feasible; e.g., in distant galaxies or reddened regions (observed with upcoming Extremely Large Telescopes and the James Webb Space Telescope), or in the large sample of new RRL that will be discovered in large-area time-domain photometric surveys (such as the LSST and the Roman space telescope).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this work we conduct a thorough investigation of the X-ray and ultraviolet (UV) properties of Y Gem based on six archival XMM-Newton and Chandra observations to explore the nature of the system. The results show that Y Gem has strong (10<jats:sup>32–34</jats:sup> erg s<jats:sup>−1</jats:sup>) X-ray emission, including a hard (with a maximum emission temperature of 8–16 keV) and a soft (with emission temperatures of 0.02–0.2 and 0.2–0.9 keV) component. The integrated UV luminosity of Y Gem reaches ∼10<jats:sup>35</jats:sup> erg s<jats:sup>−1</jats:sup>. We show that the previous asymptotic giant branch-main-sequence (AGB-MS) Roche-lobe overflow (RLOF) scenario is dynamically unstable and can hardly explain the ∼10 keV X-ray emission temperature. We propose Y Gem as a symbiotic star, where a white dwarf (WD) accretes from its AGB companion based on its X-ray and UV properties. We make numerical simulations to examine the evolutionary history of this system. The simulations can produce the observed properties of Y Gem in the wind WRLOF scenario. An ∼0.8<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> WD with a ∼1.0–1.8<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> companion in a ∼2000–32,000 day initial orbit may evolve to a Y Gem-like system. Our finding implies a potential population of symbiotic stars that may have been misclassified as AGB-MS binaries. What is more, their high mass accretion rates may enable mass accumulation to the WD and makes them candidates of Type Ia supernovae progenitors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Since the day of its explosion, supernova (SN) 1987A has been closely monitored to study its evolution and to detect its central compact relic. In fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the SN. However, besides the detection in the Atacama Large Millimeter/submillimeter Array (ALMA) data of a feature that is compatible with the emission arising from a protopulsar wind nebula (PWN), the only hint of the existence of such an elusive compact object is provided by the detection of hard emission in NuSTAR data up to ∼20 keV. We report on the simultaneous analysis of multiepoch observations of SN 1987A performed with Chandra, XMM-Newton, and NuSTAR. We also compare the observations with a state-of-the-art three-dimensional magnetohydrodynamic simulation of SN 1987A. A heavily absorbed power law, consistent with the emission from a PWN embedded in the heart of SN 1987A, is needed to properly describe the high-energy part of the observed spectra. The spectral parameters of the best-fit power law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed nonthermal emission. The information extracted from our analysis is used to infer the physical characteristics of the pulsar and the broadband emission from its nebula, in agreement with the ALMA data. Analysis of the synthetic spectra also shows that, in the near future, the main contribution to the Fe K emission line will originate in the outermost shocked ejecta of SN 1987A.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Isolated neutron stars that are asymmetric with respect to their spin axis are possible sources of detectable continuous gravitational waves. This paper presents a fully coherent search for such signals from eighteen pulsars in data from LIGO and Virgo’s third observing run (O3). For known pulsars, efficient and sensitive matched-filter searches can be carried out if one assumes the gravitational radiation is phase-locked to the electromagnetic emission. In the search presented here, we relax this assumption and allow both the frequency and the time derivative of the frequency of the gravitational waves to vary in a small range around those inferred from electromagnetic observations. We find no evidence for continuous gravitational waves, and set upper limits on the strain amplitude for each target. These limits are more constraining for seven of the targets than the spin-down limit defined by ascribing all rotational energy loss to gravitational radiation. In an additional search, we look in O3 data for long-duration (hours–months) transient gravitational waves in the aftermath of pulsar glitches for six targets with a total of nine glitches. We report two marginal outliers from this search, but find no clear evidence for such emission either. The resulting duration-dependent strain upper limits do not surpass indirect energy constraints for any of these targets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper, we report on 606 S-type stars identified from Data Release 9 of the LAMOST medium-resolution spectroscopic (MRS) survey; 539 of them are reported for the first time. The discovery of these stars is a three-step process, i.e., selecting with ZrO-band indices greater than 0.25, excluding non-S-type stars with the iterative Support Vector Machine method, and finally retaining stars with absolute bolometric magnitude larger than −7.1. The 606 stars are consistent with the distribution of known S-type stars in the color–magnitude diagram. We estimated the C/Os using the [C/Fe] and [O/Fe] provided by APOGEE and the MARCS model for S-type stars, respectively, and the results of the two methods show that the C/Os of all stars are larger than 0.5. Both the locations on the color–magnitude diagram and C/Os further verify the nature of our S-type sample. Investigating the effect of TiO and atmospheric parameters on ZrO with the sample, we found that log <jats:italic>g</jats:italic> has a more significant impact on ZrO than <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> and [Fe/H], and both TiO and log <jats:italic>g</jats:italic> may negatively correlate with ZrO. According to the criterion of Tian et al., a total of 238 binary candidates were found by the zero-point-calibrated radial velocities from the officially released catalog of LAMOST MRS and the catalog of Zhang et al. A catalog of these 606 S-type stars is available from the following link: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://doi.org/10.12149/101097" xlink:type="simple">doi.org/10.12149/101097</jats:ext-link>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Relativistic spacecraft, like those proposed by the NASA Starlight program and the Breakthrough Starshot Initiative, will have to survive radiation production that is unique when compared to that experienced by conventional spacecraft. In a relativistic interstellar spacecraft’s reference frame, the interstellar medium (ISM) will look like a nearly monoenergetic beam of charged particles which impinges upon the leading edge of the spacecraft. Upon impact, ISM protons and electrons will travel characteristic lengths through the spacecraft shield and come to a stop via electronic and nuclear stopping mechanisms. As a result, bremsstrahlung photons will be produced within the spacecraft shield. In this work, we discuss the interstellar environment and its implications for radiation damage on relativistic spacecraft. We also explore expected radiation doses in terms of onboard device radiation tolerance.</jats:p>