Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Parker Solar Probe (PSP) observed predominately Alfvénic fluctuations in the solar wind near the Sun where the magnetic field tends to be radially aligned. In this paper, two magnetic-field-aligned solar wind flow intervals during PSP’s first two orbits are analyzed. Observations of these intervals indicate strong signatures of parallel/antiparallel-propagating waves. We utilize multiple analysis techniques to extract the properties of the observed waves in both magnetohydrodynamic (MHD) and kinetic scales. At the MHD scale, outward-propagating Alfvén waves dominate both intervals, and outward-propagating fast magnetosonic waves present the second-largest contribution in the spectral energy density. At kinetic scales, we identify the circularly polarized plasma waves propagating near the proton gyrofrequency in both intervals. However, the sense of magnetic polarization in the spacecraft frame is observed to be opposite in the two intervals, although they both possess a sunward background magnetic field. The ion-scale plasma wave observed in the first interval can be either an inward-propagating ion cyclotron wave (ICW) or an outward-propagating fast-mode/whistler wave in the plasma frame, while in the second interval it can be explained as an outward ICW or inward fast-mode/whistler wave. The identification of the exact kinetic wave mode is more difficult to confirm owing to the limited plasma data resolution. The presence of ion-scale waves near the Sun suggests that ion cyclotron resonance may be one of the ubiquitous kinetic physical processes associated with small-scale magnetic fluctuations and kinetic instabilities in the inner heliosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Line-intensity mapping observations will find fluctuations of integrated line emission are attenuated by varying degrees at small scales due to the width of the line emission profiles. This attenuation may significantly impact estimates of astrophysical or cosmological quantities derived from measurements. We consider a theoretical treatment of the effect of line broadening on both the clustering and shot-noise components of the power spectrum of a generic line-intensity power spectrum using a halo model. We then consider possible simplifications to allow easier application in analysis, particularly in the context of inferences that require numerous, repeated, fast computations of model line-intensity signals across a large parameter space. For the CO Mapping Array Project and the CO(1–0) line-intensity field at <jats:italic>z</jats:italic> ∼ 3 serving as our primary case study, we expect a ∼10% attenuation of the spherically averaged power spectrum on average at relevant scales of <jats:italic>k</jats:italic> ≈ 0.2–0.3 Mpc<jats:sup>−1</jats:sup> compared to ∼25% for the interferometric Millimetre-wave Intensity Mapping Experiment targeting shot noise from CO lines at <jats:italic>z</jats:italic> ∼ 1–5 at scales of <jats:italic>k</jats:italic> ≳ 1 Mpc<jats:sup>−1</jats:sup>. We also consider the nature and amplitude of errors introduced by simplified treatments of line broadening and find that while an approximation using a single effective velocity scale is sufficient for spherically averaged power spectra, a more careful treatment is necessary when considering other statistics such as higher multipoles of the anisotropic power spectrum or the voxel intensity distribution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We calculate the ages, orbits and phase-space coordinates for a sample of ∼4 million LAMOST and Gaia stars. The ages are cross-matched and compared with values from two other popular age catalogs, which derive the ages using different methods. Of these ∼4 million stars, we select a sample of 1.3 million stars and investigate their radial metallicity gradients (as determined by orbital radii) as a function of their ages. This analysis is performed on various subsets of the data split by chemistry and orbital parameters. We find that commonly used selections for “thin disk” stars (such as low-<jats:italic>α</jats:italic> chemistry or vertically thin orbits) yield radial metallicity gradients that generally grow shallower for the oldest stars. We interpret this as a hallmark feature of radial migration (churning). Constraining our sample to very small orbital <jats:italic>Z</jats:italic> <jats:sub>max</jats:sub> (the maximal height of a star’s integrated orbit) makes this trend most pronounced. A chemistry-based “thin disk” selection of <jats:italic>α</jats:italic>-poor stars displays the same trend, but to a lesser extent. Intriguingly, we find that “thick disk” selections in chemistry and <jats:italic>Z</jats:italic> <jats:sub>max</jats:sub> reveal slightly positive radial metallicity gradients, which seem similar in magnitude at all ages. This may imply that the thick disk population is well mixed in age, but not in radius. This finding could help constrain conditions during the early epochs of Milky Way formation and shed light on processes such as the accretion and reaccretion of gases of different metallicities.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the results of the Cosmic Origins Spectrograph-Intragroup Medium (COS-IGrM) Survey that used the COS on the Hubble Space Telescope to observe a sample of 18 UV bright quasars, each probing the IGrM of a galaxy group. We detect Ly<jats:italic>α</jats:italic>, C <jats:sc>ii</jats:sc>, N <jats:sc>v</jats:sc>, Si <jats:sc>ii</jats:sc>, Si <jats:sc>iii</jats:sc>, and O <jats:sc>vi</jats:sc> in multiple sightlines. The highest ionization species detected in our data is O <jats:sc>vi</jats:sc>, which was detected in eight out of 18 quasar sightlines. The wide range of ionization states observed provide evidence that the IGrM is patchy and multiphase. We find that the O <jats:sc>vi</jats:sc> detections generally align with radiatively cooling gas between 10<jats:sup>5.8</jats:sup> and 10<jats:sup>6</jats:sup> K. The lack of O <jats:sc>vi</jats:sc> detections in 10 of the 18 groups illustrates that O <jats:sc>vi</jats:sc> may not be the ideal tracer of the volume filling component of the IGrM. Instead, it either exists at trace levels in a hot IGrM or is generated in the boundary between the hotter IGrM and cooler gas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Galaxy clusters are considered to be gigantic reservoirs of cosmic rays (CRs). Some of the clusters are found with extended radio emission, which provides evidence for the existence of magnetic fields and CR electrons in the intra-cluster medium. The mechanism of radio halo (RH) emission is still under debate, and it has been believed that turbulent reacceleration plays an important role. In this paper, we study the reacceleration of CR protons and electrons in detail by numerically solving the Fokker–Planck equation, and show how radio and gamma-ray observations can be used to constrain CR distributions and resulting high-energy emission for the Coma cluster. We take into account the radial diffusion of CRs and follow the time evolution of their one-dimensional distribution, by which we investigate the radial profile of the CR injection that is consistent with the observed RH surface brightness. We find that the required injection profile is nontrivial, depending on whether CR electrons have a primary or secondary origin. Although the secondary CR electron scenario predicts larger gamma-ray and neutrino fluxes, it is in tension with the observed RH spectrum for hard injection indexes, <jats:italic>α</jats:italic> &lt; 2.45. This tension is relaxed if the turbulent diffusion of CRs is much less efficient than the fiducial model, or the reacceleration is more efficient for lower-energy CRs. In both the secondary and primary scenario, we find that galaxy clusters can make a sizable contribution to the all-sky neutrino intensity if the CR energy spectrum is nearly flat.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Exoplanets orbiting M dwarfs within habitable zones are exposed to stellar environments more extreme than that terrestrial planets experience in our solar system, which can significantly impact the atmospheres of the exoplanets and affect their habitability and sustainability. This study provides the first prediction of hot oxygen corona structure and the associated photochemical loss from a 1 bar CO<jats:sub>2</jats:sub>-dominated atmosphere of a Venus-like rocky exoplanet, where dissociative recombination of O<jats:sub>2</jats:sub> <jats:sup>+</jats:sup> ions is assumed to be the major source reaction for the escape of neutral O atoms and formation of the hot O corona (or exospheres) as on Mars and Venus. We employ a 3D Monte Carlo code to simulate the exosphere of Proxima Centauri b (PCb) based on the ionosphere simulated by a 3D magnetohydrodynamic model. Our simulation results show that variability of the stellar wind dynamic pressure over one orbital period of PCb does not affect the overall spatial structure of the hot O corona but contributes to the change in the global hot O escape rate that varies by an order of magnitude. The escape increases dramatically when the planet possesses its intrinsic magnetic fields as the ionosphere becomes more extended with the presence of a global magnetic field. The extended hot O corona may lead to a more extended H exosphere through collisions between thermal H and hot O, which exemplifies the importance of considering nonthermal populations in exospheres to interpret future observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Metal-poor massive stars dominate the light we observe from star-forming dwarf galaxies and may have produced the bulk of energetic photons that reionized the universe at high redshift. Yet, the rarity of observations of individual O stars below the 20% solar metallicity (<jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>) of the Small Magellanic Cloud (SMC) hampers our ability to model the ionizing fluxes of metal-poor stellar populations. We present new Hubble Space Telescope far-ultraviolet (FUV) spectra of three O-dwarf stars in the galaxies Leo P (3% <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>), Sextans A (6% <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>), and WLM (14% <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>). We quantify equivalent widths of photospheric metal lines and strengths of wind-sensitive features, confirming that both correlate with metallicity. We infer the stars’ fundamental properties by modeling their FUV through near-infrared spectral energy distributions and identify stars in the SMC with similar properties to each of our targets. Comparing to the FUV spectra of the SMC analogs suggests that (1) the star in WLM has an SMC-like metallicity, and (2) the most metal-poor star in Leo P is driving a much weaker stellar wind than its SMC counterparts. We measure projected rotation speeds and find that the two most metal-poor stars have high <jats:inline-formula> <jats:tex-math> <?CDATA $v\,\sin \,(i)$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mspace width="0.25em" /> <mml:mi>sin</mml:mi> <mml:mspace width="0.25em" /> <mml:mo stretchy="false">(</mml:mo> <mml:mi>i</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac1ce2ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> ≥ 290 km s<jats:sup>−1</jats:sup>, and estimate just a 3%–6% probability of finding two fast rotators if the metal-poor stars are drawn from the same <jats:inline-formula> <jats:tex-math> <?CDATA $v\,\sin \,(i)$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mspace width="0.25em" /> <mml:mi>sin</mml:mi> <mml:mspace width="0.25em" /> <mml:mo stretchy="false">(</mml:mo> <mml:mi>i</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac1ce2ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> distribution observed for O dwarfs in the SMC. These observations suggest that models should include the impact of rotation and weak winds on ionizing flux to accurately interpret observations of metal-poor galaxies in both the near and distant universe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The long-period, highly eccentric Wolf-Rayet star binary system WR 140 has exceptionally well-determined orbital and stellar parameters. Bright, variable X-ray emission is generated in shocks produced by the collision of the winds of the WC7pd+O5.5fc component stars. We discuss the variations in the context of the colliding-wind model using broadband spectrometry from the RXTE, Swift, and NICER observatories obtained over 20 yr and nearly 1000 observations through three consecutive 7.94 yr orbits, including three periastron passages. The X-ray luminosity varies as expected with the inverse of the stellar separation over most of the orbit; departures near periastron are produced when cooling shifts to excess optical emission in C <jats:sc>iii</jats:sc> <jats:italic>λ</jats:italic>5696 in particular. We use X-ray absorption to estimate mass-loss rates for both stars and to constrain the system morphology. The absorption maximum coincides closely with the inferior conjunction of the WC star and provides evidence of the ion-reflection mechanism that underlies the formation of collisionless shocks governed by magnetic fields probably generated by the Weibel instability. Comparisons with <jats:italic>K</jats:italic>-band emission and He <jats:sc>i</jats:sc> <jats:italic>λ</jats:italic>10830 absorption show that both are correlated after periastron with the asymmetric X-ray absorption. Dust appears within a few days of periastron, suggesting formation within shocked gas near the stagnation point. The X-ray flares seen in <jats:italic>η</jats:italic> Car have not occurred in WR 140, suggesting the absence of large-scale wind inhomogeneities. Relatively constant soft emission revealed during the X-ray minimum is probably not from recombining plasma entrained in outflowing shocked gas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using numerical simulations of helical inflationary magnetogenesis in a low reheating temperature scenario, we show that the magnetic energy spectrum is strongly peaked at a particular wavenumber that depends on the reheating temperature. Gravitational waves (GWs) are produced at frequencies between 3 nHz and 50 mHz for reheating temperatures between 150 MeV and 3 × 10<jats:sup>5</jats:sup> GeV, respectively. At and below the peak frequency, the stress spectrum is always found to be that of white noise. This implies a linear increase of GW energy per logarithmic wavenumber interval, instead of a cubic one. Both in the helical and nonhelical cases, the GW spectrum is followed by a sharp drop for frequencies above the respective peak frequency. In this magnetogenesis scenario, the presence of a helical term extends the peak of the GW spectrum and therefore also the position of the aforementioned drop toward larger frequencies compared to the case without helicity. This might make a difference in it being detectable with space interferometers. The efficiency of GW production is found to be almost the same as in the nonhelical case, and independent of the reheating temperature, provided the electromagnetic energy at the end of reheating is fixed to be a certain fraction of the radiation energy density. Also, contrary to the case without helicity, the electric energy is now less than the magnetic energy during reheating. The fractional circular polarization is found to be nearly 100% in a certain range below the peak frequency range.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A joint X-ray (0.2–2 keV) and ultraviolet (1150–3000 Å) time-domain study has been carried out on three nearby bright late-type stars, bracketing the Sun in properties. Alpha Cen A (HD 128620: G2 V) is a near twin to the Sun, although slightly more massive and luminous, slightly metal-rich, but older. Alpha Cen B (HD 128621: K1 V) is cooler than the Sun, somewhat less massive and lower in luminosity. Procyon (HD 61421: F5 IV–V) is hotter, more massive and more luminous than the Sun, half the age, but more evolved. Stellar observations were from Chandra X-ray Observatory and Hubble Space Telescope (HST). The Sun provided a benchmark through high-energy spectral scans from solar irradiance satellites and novel high-dispersion full-disk profiles of key UV species—Mg <jats:sc>ii</jats:sc>, C <jats:sc>ii</jats:sc>, and Si <jats:sc>iv</jats:sc>—from the Interface Region Imaging Spectrograph. Procyon’s flux history was strikingly constant at all wavelengths, in contrast to the other three cycling-dynamo stars. Procyon also displays a strong subcoronal (<jats:italic>T</jats:italic> ∼ 1 × 10<jats:sup>5</jats:sup> K) emission excess, relative to chromospheric Mg <jats:sc>ii</jats:sc> (<jats:italic>T</jats:italic> ≲ 10<jats:sup>4</jats:sup> K), although its X-rays (<jats:italic>T</jats:italic> ∼ 2 MK) appear to be more normal. At the same time, the odd sub-Gaussian shapes, and redshifts, of the subgiant’s “hot lines” (such as Si <jats:sc>iv</jats:sc> and C <jats:sc>iv</jats:sc>) are remarkably similar to the solar counterparts (and <jats:italic>α</jats:italic> Cen AB). This suggests a Sun-like origin, namely a supergranulation network supplied by magnetic flux from a noncycling “local dynamo.”</jats:p>