Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Subsurface convection zones are ubiquitous in early-type stars. Driven by narrow opacity peaks, these thin convective regions transport little heat but play an important role in setting the magnetic properties and surface variability of stars. Here we demonstrate that these convection zones are <jats:italic>not</jats:italic> present in as wide a range of stars as previously believed. In particular, there are regions which 1D stellar evolution models report to be convectively unstable but which fall below the critical Rayleigh number for onset of convection. For sub-solar metallicity this opens up a <jats:italic>stability window</jats:italic> in which there are no subsurface convection zones. For Large Magellanic Cloud metallicity this surface stability region extends roughly between 8 and 16<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, increasing to 8–35<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> for Small Magellanic Cloud metallicity. Such windows are then an excellent target for probing the relative influence of subsurface convection and other sources of photometric variability in massive stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report upper limits on the Epoch of Reionization 21 cm power spectrum at redshifts 7.9 and 10.4 with 18 nights of data (∼36 hr of integration) from Phase I of the Hydrogen Epoch of Reionization Array (HERA). The Phase I data show evidence for systematics that can be largely suppressed with systematic models down to a dynamic range of ∼10<jats:sup>9</jats:sup> with respect to the peak foreground power. This yields a 95% confidence upper limit on the 21 cm power spectrum of <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{\Delta }}}_{21}^{2}\leqslant {(30.76)}^{2}\ {\mathrm{mK}}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>21</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>≤</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mn>30.76</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.33em" /> <mml:msup> <mml:mrow> <mml:mi>mK</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac1c78ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> at <jats:italic>k</jats:italic> = 0.192 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> at <jats:italic>z</jats:italic> = 7.9, and also <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{\Delta }}}_{21}^{2}\leqslant {(95.74)}^{2}\ {\mathrm{mK}}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>21</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>≤</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mn>95.74</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.33em" /> <mml:msup> <mml:mrow> <mml:mi>mK</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac1c78ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> at <jats:italic>k</jats:italic> = 0.256 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> at <jats:italic>z</jats:italic> = 10.4. At <jats:italic>z</jats:italic> = 7.9, these limits are the most sensitive to date by over an order of magnitude. While we find evidence for residual systematics at low line-of-sight Fourier <jats:italic>k</jats:italic> <jats:sub>∥</jats:sub> modes, at high <jats:italic>k</jats:italic> <jats:sub>∥</jats:sub> modes we find our data to be largely consistent with thermal noise, an indicator that the system could benefit from deeper integrations. The observed systematics could be due to radio frequency interference, cable subreflections, or residual instrumental cross-coupling, and warrant further study. This analysis emphasizes algorithms that have minimal inherent signal loss, although we do perform a careful accounting in a companion paper of the small forms of loss or bias associated with the pipeline. Overall, these results are a promising first step in the development of a tuned, instrument-specific analysis pipeline for HERA, particularly as Phase II construction is completed en route to reaching the full sensitivity of the experiment.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the existence and properties of fast magnetosonic modes in 3D compressible MHD turbulence by carrying out a number of simulations with compressible and incompressible driving conditions. We use two approaches to determine the presence of fast modes: mode decomposition based on spatial variations only and spatio-temporal 4D fast Fourier transform (4D FFT) analysis of all fluctuations. The latter method enables us to quantify fluctuations that satisfy the dispersion relation of fast modes with finite frequency. Overall, we find that the fraction of fast modes identified via the spatio-temporal 4D FFT approach in total fluctuation power is either tiny with nearly incompressible driving or ∼2% with highly compressible driving. We discuss the implications of our results for understanding the compressible fluctuations in space and astrophysical plasmas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the optically thin regime, the intensity ratio of the two Si <jats:sc>iv</jats:sc> resonance lines (1394 and 1403 Å) are theoretically the same as the ratio of their oscillator strengths, which is exactly 2. Here, we study the ratio of the integrated intensity of the Si <jats:sc>iv</jats:sc> lines (<jats:italic>R</jats:italic> = ∫<jats:italic>I</jats:italic> <jats:sub>1394</jats:sub>(<jats:italic>λ</jats:italic>)d<jats:italic>λ</jats:italic>/∫<jats:italic>I</jats:italic> <jats:sub>1403</jats:sub>(<jats:italic>λ</jats:italic>)d<jats:italic>λ</jats:italic>) and the ratio of intensity at each wavelength point (<jats:italic>r</jats:italic>(Δ<jats:italic>λ</jats:italic>) = <jats:italic>I</jats:italic> <jats:sub>1394</jats:sub>(Δ<jats:italic>λ</jats:italic>)/<jats:italic>I</jats:italic> <jats:sub>1403</jats:sub>(Δ<jats:italic>λ</jats:italic>)) in two solar flares observed by the Interface Region Imaging Spectrograph. We find that at flare ribbons, the ratio <jats:italic>R</jats:italic> ranges from 1.8 to 2.3 and would generally decrease when the ribbons sweep across the slit position. In addition, the distribution of <jats:italic>r</jats:italic>(Δ<jats:italic>λ</jats:italic>) shows a descending trend from the blue wing to the red wing. In loop cases, the Si <jats:sc>iv</jats:sc> line presents a wide profile with a central reversal. The ratio <jats:italic>R</jats:italic> deviates little from 2, but the ratio <jats:italic>r</jats:italic>(Δ<jats:italic>λ</jats:italic>) can vary from 1.3 near the line center to greater than 2 in the line wings. Hence we conclude that in flare conditions, the ratio <jats:italic>r</jats:italic>(Δ<jats:italic>λ</jats:italic>) varies across the line, due to the variation of the opacity at the line center and line wings. We notice that, although the ratio <jats:italic>r</jats:italic>(Δ<jats:italic>λ</jats:italic>) could present a value that deviates from 2 as a result of the opacity effect near the line center, the ratio <jats:italic>R</jats:italic> is still close to 2. Therefore, caution should be taken when using the ratio of the integrated intensity of the Si <jats:sc>iv</jats:sc> lines to diagnose the opacity effect.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the local universe, disk galaxies are generally well evolved and Toomre stable. Their collisions with satellite galaxies naturally produce ring structures, which have been observed and extensively studied. By contrast, at high redshifts, disk galaxies are still developing and clumpy. These young galaxies interact with each other more frequently. However, the products of their collisions remain elusive. Here, we systematically study the minor collisions between a clumpy galaxy and a satellite on orbits with different initial conditions, and find a new structure that is different from the local collisional ring galaxies. The clumpiness of the target galaxy is fine-tuned by the values of Toomre parameter, <jats:italic>Q</jats:italic>. Interestingly, a thick and knotty ring structure is formed without any sign of a central nucleus in the target galaxy. Our results provide a promising explanation of the empty ring galaxy recently observed in R5519 at redshift <jats:italic>z</jats:italic> = 2.19. Moreover, we show that the clumpy state of the collided galaxy exists for a much longer timescale compared to isolated self-evolved clumpy galaxies that have been widely investigated.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present accretion-disk structure measurements from UV–optical reverberation mapping (RM) observations of a sample of eight quasars at 0.24 &lt; <jats:italic>z</jats:italic> &lt; 0.85. Ultraviolet photometry comes from two cycles of Hubble Space Telescope monitoring, accompanied by multiband optical monitoring by the Las Cumbres Observatory network and Liverpool Telescopes. The targets were selected from the Sloan Digital Sky Survey Reverberation Mapping project sample with reliable black hole mass measurements from H<jats:italic>β</jats:italic> RM results. We measure significant lags between the UV and various optical <jats:italic>griz</jats:italic> bands using <jats:monospace>JAVELIN</jats:monospace> and <jats:monospace>CREAM</jats:monospace> methods. We use the significant lag results from both methods to fit the accretion-disk structure using a Markov Chain Monte Carlo approach. We study the accretion disk as a function of disk normalization, temperature scaling, and efficiency. We find direct evidence for diffuse nebular emission from Balmer and Fe <jats:sc>ii</jats:sc> lines over discrete wavelength ranges. We also find that our best-fit disk color profile is broadly consistent with the Shakura &amp; Sunyaev disk model. We compare our UV–optical lags to the disk sizes inferred from optical–optical lags of the same quasars and find that our results are consistent with these quasars being drawn from a limited high-lag subset of the broader population. Our results are therefore broadly consistent with models that suggest longer disk lags in a subset of quasars, for example, due to a nonzero size of the ionizing corona and/or magnetic heating contributing to the disk response.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Atmospheric escape from close-in exoplanets is thought to be crucial in shaping observed planetary populations. Recently, significant progress has been made in observing this process in action through excess absorption in-transit spectra and narrowband light curves. We model the escape of initially homogeneous planetary winds interacting with a stellar wind. The ram pressure balance of the two winds governs this interaction. When the impingement of the stellar wind on the planetary outflow is mild or moderate, the planetary outflow expands nearly spherically through its sonic surface before forming a shocked boundary layer. When the confinement is strong, the planetary outflow is redirected into a cometary tail before it expands to its sonic radius. The resultant transmission spectra at the He 1083 nm line are accurately represented by a 1D spherical wind solution in cases of mild to moderate stellar wind interaction. In cases of strong stellar wind interaction, the degree of absorption is enhanced and the cometary tail leads to an extended egress from transit. The crucial features of the wind–wind interaction are, therefore, encapsulated in the light curve of He 1083 nm equivalent width as a function of time. The possibility of extended He 1083 nm absorption well beyond the optical transit carries important implications for planning out-of-transit observations that serve as a baseline for in-transit data.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>On 2013 June 21, a solar prominence eruption was observed, accompanied by an M2.9 class flare, a fast coronal mass ejection, and a type II radio burst. The concomitant emission of solar energetic particles (SEPs) produced a significant proton flux increase, in the energy range 4–100 MeV, measured by the Low and High Energy Telescopes on board the Solar TErrestrial RElations Observatory (STEREO)-B spacecraft. Only small enhancements, at lower energies, were observed at the STEREO-A and Geostationary Operational Environmental Satellite (GOES) spacecraft. This work investigates the relationship between the expanding front, coronal streamers, and the SEP fluxes observed at different locations. Extreme-ultraviolet data, acquired by the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO), were used to study the expanding front and its interaction with streamer structures in the low corona. The 3D shape of the expanding front was reconstructed and extrapolated at different times by using SDO/AIA, STEREO/Sun Earth Connection Coronal and Heliospheric Investigation, and Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph observations with a spheroidal model. By adopting a potential field source surface approximation and estimating the magnetic connection of the Parker spiral, below and above 2.5 <jats:italic>R</jats:italic> <jats:sub>⊙</jats:sub>, we found that during the early expansion of the eruption, the front had a strong magnetic connection with STEREO-B (between the nose and flank of the eruption front) while having a weak connection with STEREO-A and GOES. The obtained results provide evidence, for the first time, that the interaction between an expanding front and streamer structures can be responsible for the acceleration of high-energy SEPs up to at least 100 MeV, as it favors particle trapping and hence increases the shock acceleration efficiency.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Detection of low-frequency (≤1.4 GHz) radio emission from stellar and planetary systems can lead to new insights into stellar activity, extrasolar space weather, and planetary magnetic fields. In this work, we investigate three large field-of-view surveys at 74 MHz, 150 MHz, and 1.4 GHz, as well as a myriad of multiwavelength ancillary data, to search for radio emission from about 2600 stellar objects, including about 800 exoplanetary systems, 600 nearby low-mass stars, and 1200 young stellar objects located in the Taurus and Upper Scorpius star-forming regions. The selected sample encompasses stellar spectral types from B to L and distances between 5 and 300 pc. We report the redetection of five stars at 1.4 GHz, one of which also shows emission at 150 MHz. Four of these are low- and intermediate-mass young stars, and one is the evolved star <jats:italic>α</jats:italic> Sco. We also observe radio emission at the position of a young brown dwarf at 1.4 GHz and 150 MHz. However, due to the large astrometric uncertainty of radio observations, a follow-up study at higher angular resolution would be required to confirm whether the observed emission originates from the brown dwarf itself or a background object. Notably, all of the selected radio sources are located in nearby star-forming regions. Furthermore, we use image stacking and statistical methods to derive upper limits on the average quiescent radio luminosity of the families of objects under investigation. These analyses provide observational constraints for large-scale searches for current and ongoing low-frequency radio emissions from stars and planets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accretion plays an important role in protoplanetary disk evolution, and it is thought that the accretion mechanism changes between low- and high-mass stars. Here we characterize accretion in intermediate-mass, pre-main-sequence Herbig Ae/Be (HAeBe) stars to search for correlations between accretion and system properties. We present new high-resolution, near-infrared spectra from the Immersion GRating INfrared Spectrograph for 102 HAeBes and analyze the accretion-tracing Br<jats:italic>γ</jats:italic> line at 2.166 <jats:italic>μ</jats:italic>m. We also include the samples of Fairlamb et al. and Donehew &amp; Brittain, for a total of 155 targets. We find a positive correlation between the Br<jats:italic>γ</jats:italic> and stellar luminosity, with a change in the slope between the Herbig Aes and Bes. We use <jats:italic>L</jats:italic> <jats:sub>Br<jats:italic>γ</jats:italic> </jats:sub> to determine the accretion luminosity and rate. We find that the accretion luminosity and rate depend on stellar mass and age; however, the trend disappears when normalizing the accretion luminosity by the stellar luminosity. We classify the objects into flared (group I) or flat (group II) disks and find that there is no trend with accretion luminosity or rate, indicating that the disk dust structure is not impacting accretion. We test for Br<jats:italic>γ</jats:italic> variability in objects that are common to our sample and previous studies. We find that the Br<jats:italic>γ</jats:italic> line equivalent width is largely consistent between the literature observations and those that we present here, except in a few cases where we may be seeing changes in the accretion rate.</jats:p>