Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The Zeeman effect is of limited utility for probing the magnetism of the quiet solar chromosphere. The Hanle effect in some spectral lines is sensitive to such magnetism, but the interpretation of the scattering polarization signals requires taking into account that the chromospheric plasma is highly inhomogeneous and dynamic (i.e., that the magnetic field is not the only cause of symmetry breaking). Here we investigate the reliability of a well-known formula for mapping the azimuth of chromospheric magnetic fields directly from the scattering polarization observed in the Ca <jats:sc>ii</jats:sc> 8542 Å line, which is typically in the saturation regime of the Hanle effect. To this end, we use the Stokes profiles of the Ca <jats:sc>ii</jats:sc> 8542 Å line computed with the PORTA radiative transfer code in a three-dimensional (3D) model of the solar chromosphere, degrading them to mimic spectropolarimetric observations for a range of telescope apertures and noise levels. The simulated observations are used to obtain the magnetic field azimuth at each point of the field of view, which we compare with the actual values within the 3D model. We show that, apart from intrinsic ambiguities, the method provides solid results. Their accuracy depends more on the noise level than on the telescope diameter. Large-aperture solar telescopes, like DKIST and EST, are needed to achieve the required polarimetric sensitivity using reasonable exposure times.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>While gas-phase astrochemical reaction networks nicely replicate the abundance of hydrogen-deficient organics like linear cyanopolyynes, pathways to complex organic molecules (COMs)—organic molecules with six or more atoms—have not been completely understood, with gas-phase models often significantly underestimating fractional abundances of the astronomically observed organics by orders of magnitude. Here, by exploiting cyclopropenone (<jats:italic>c</jats:italic>-C<jats:sub>3</jats:sub>H<jats:sub>2</jats:sub>O) as a tracer, laboratory experiments on the processing of an ice mixture of acetylene(C<jats:sub>2</jats:sub>H<jats:sub>2</jats:sub>) and carbon monoxide (CO) by energetic electrons coupled with astrochemical model simulations expose a previously poorly explored reaction class leading to COMs via galactic cosmic-ray-mediated nonequilibrium chemistry. These processes occur within interstellar ices at ultralow temperatures, but not through traditional radical–radical pathways on grain surfaces in the warm-up phase of the ices as hypothesized for the last decades, but more likely through barrierless excited state reactions during the irradiation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Between 30% and 50% of white dwarfs (WDs) show heavy elements in their atmospheres. This pollution is thought to arise from the accretion of planetesimals perturbed by outer planet(s) to within the WD’s tidal disruption radius. A small fraction of these WDs show either emission or absorption from circumstellar (C-S) gas. The abundances of metals in the photospheres of WDs with C-S gas are mostly similar to the bulk composition of the Earth. The C-S component arises from gas produced through collisions and/or the sublimation of disintegrating planetesimals. High-resolution spectroscopic observations of WD 1124−293 reveal photospheric and C-S absorption of Ca in multiple transitions. Here, we present high signal-to-noise ratio spectra, an updated WD atmosphere analysis, and a self-consistent model of its C-S gas. We constrain the abundances of Ca, Mg, and Fe in the photosphere of WD 1124−293, and find agreement with the abundances of these three species in the C-S gas. We find the location of the C-S gas is ∼100 white dwarf radii, the C-S and photospheric compositions are thus far consistent, the gas is not isothermal, and the amount of C-S Ca has not changed in two decades. We also demonstrate how to use Cloudy to model C-S gas viewed in absorption around polluted WDs. Modeling the abundances of gas around polluted WDs with Cloudy provides a new method to measure the composition of exoplanetesimals and will allow a direct comparison to the composition of rocky bodies in the solar system.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Biermann battery is a promising generation mechanism of a seed magnetic field in the universe. We propose a new driving mechanism for the Biermann battery in an inhomogeneous plasma with a beam component. An inhomogeneous electron flow induced by the return current can make an electron pressure gradient that is not parallel to the density gradient. As a result, the magnetic field is generated by the Biermann battery. We demonstrate the new generation mechanism of the magnetic field by conducting three-fluid plasma simulations. By this mechanism, the first cosmic rays can generate the seed magnetic field with astrophysical scales at redshift around 20.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We estimate the intensity of interstellar pickup protons accelerated to ∼50 keV at various locations along the solar-wind termination shock, using two-dimensional hybrid simulations. Parameters for the solar wind, interstellar pickup ions (PUIs), and magnetic field just upstream of the termination shock at one flank of the heliosphere, and at the location in the downwind (or tail-ward) direction are based on a solar-wind/pickup-ion/turbulence model. The parameters upstream of the shock where Voyager 2 crossed are based on observations. The simulation is limited in size, and therefore cannot accurately model the distribution to energies much beyond ∼50 keV. This is sufficient to study the origin of the high-energy tail of the distribution, which is the low-energy portion of the anomalous cosmic-ray spectrum. We also extrapolate our results to other locations along the termination shock, such as the other flank, and the poles of the heliosphere. We find that the intensity of ∼10–50 keV accelerated pickup protons is remarkably similar at all three locations we simulated, suggesting that particles in this energy range are relatively uniformly distributed along the termination shock, and are likely quite uniform throughout the entire heliosheath. In addition, we find significant differences in the distribution in the 0.5–1 keV energy range for energetic neutral atoms coming from the tail region of the heliosphere compared to that at the nose or flank look directions. This is because the peak in the PUI distribution is at a higher energy there.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Yellowballs (YBs) were first discovered during the Milky Way Project (MWP) citizen science initiative. The MWP users noticed compact, yellow regions in Spitzer Space Telescope mid-infrared (MIR) images of the Milky Way plane and asked professional astronomers to explain these “yellow balls.” Follow-up work by Kerton et al. determined that YBs likely trace compact photodissociation regions associated with massive and intermediate-mass star formation. The YBs were included as target objects in a version of the MWP launched in 2016, which produced a listing of over 6000 YB locations. We have measured distances, cross-match associations, physical properties, and MIR colors of ∼500 YBs within a pilot region covering the <jats:italic>l</jats:italic> = 30°–40°, <jats:italic>b</jats:italic> = ±1° region of the Galactic plane. We find that ∼20%–30% of YBs in our pilot region contain high-mass star formation capable of becoming expanding H <jats:sc>ii</jats:sc> regions that produce MIR bubbles. A majority of YBs represent intermediate-mass star-forming regions whose placement in evolutionary diagrams suggest they are still actively accreting and may be precursors to optically revealed Herbig Ae/Be nebulae. Many of these intermediate-mass YBs were missed by surveys of massive star formation tracers; thus, this catalog provides information for many new sites of star formation. Future work will expand this pilot region analysis to the entire YB catalog.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Connecting the composition of planet-forming disks with that of gas giant exoplanet atmospheres, in particular through C/O ratios, is one of the key goals of disk chemistry. Small hydrocarbons like C<jats:sub>2</jats:sub>H and C<jats:sub>3</jats:sub>H<jats:sub>2</jats:sub> have been identified as tracers of C/O, as they form abundantly under high C/O conditions. We present resolved <jats:italic>c</jats:italic>–C<jats:sub>3</jats:sub>H<jats:sub>2</jats:sub> observations from the TW Hya Rosetta Stone Project, a program designed to map the chemistry of common molecules at 15–20 au resolution in the TW Hya disk. Augmented by archival data, these observations comprise the most extensive multi-line set for disks of both ortho and para spin isomers spanning a wide range of energies, <jats:italic>E</jats:italic> <jats:sub> <jats:italic>u</jats:italic> </jats:sub> = 29–97 K. We find the ortho-to-para ratio of <jats:italic>c</jats:italic>–C<jats:sub>3</jats:sub>H<jats:sub>2</jats:sub> is consistent with 3 throughout extent of the emission, and the total abundance of both <jats:italic>c</jats:italic>–C<jats:sub>3</jats:sub>H<jats:sub>2</jats:sub> isomers is (7.5–10) × 10<jats:sup>−11</jats:sup> per H atom, or 1%–10% of the previously published C<jats:sub>2</jats:sub>H abundance in the same source. We find <jats:italic>c</jats:italic>–C<jats:sub>3</jats:sub>H<jats:sub>2</jats:sub> comes from a layer near the surface that extends no deeper than <jats:italic>z</jats:italic>/<jats:italic>r</jats:italic> = 0.25. Our observations are consistent with substantial radial variation in gas-phase C/O in TW Hya, with a sharp increase outside ∼30 au. Even if we are not directly tracing the midplane, if planets accrete from the surface via, e.g., meridional flows, then such a change should be imprinted on forming planets. Perhaps interestingly, the HR 8799 planetary system also shows an increasing gradient in its giant planets’ atmospheric C/O ratios. While these stars are quite different, hydrocarbon rings in disks are common, and therefore our results are consistent with the young planets of HR 8799 still bearing the imprint of their parent disk’s volatile chemistry.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Symbiotic stars show emission across the electromagnetic spectrum from a wide array of physical processes. At centimeter waves, both synchrotron and thermal emissions are seen, often highly variable and associated with outbursts in the optical and X-rays. Most models of the radio emission include an ionized region within the dense wind of the red giant star that is kept ionized by activity on the white dwarf companion or its accretion disk. In some cases, there is ongoing shell burning on the white dwarf due to its high mass accretion rate or a prior nova eruption; in other cases, nuclear fusion occurs only occasionally as recurrent nova events. In this study, we measure the spectral indices of a sample of symbiotic systems in the Southern Hemisphere using the Australia Telescope Compact Array. Putting our data together with results from other surveys, we derive the optical depths and brightness temperatures of some well-known symbiotic stars. Using parallax distances from Gaia Data Release 3, we determine the sizes and characteristic electron densities in the radio emission regions. The results show a range of a factor of 10<jats:sup>4</jats:sup> in radio luminosity and a factor of 100 in linear size. These numbers are consistent with a picture where the rate of shell burning on the white dwarf determines the radio luminosity. Therefore, our findings also suggest that radio luminosity can be used to determine whether a symbiotic star is powered by accretion alone or also by shell burning.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The past decade has experienced an explosive increase of optically discovered tidal disruption events (TDEs) with the advent of modern time-domain surveys. However, we still lack a comprehensive observational view of their infrared (IR) echoes in spite of individual detections. To this end, we have conducted a statistical study of the IR variability of the 23 optical TDEs discovered between 2009 and 2018 using the full public data set of the Wide-field Infrared Survey Explorer. The detection of variability is performed on the difference images, yielding 11 objects with significant (&gt;3<jats:italic>σ</jats:italic>) variability in at least one band, while dust emission can be only fit in 8 objects. Their peak dust luminosity is around 10<jats:sup>41</jats:sup>–10<jats:sup>42</jats:sup> erg s<jats:sup>−1</jats:sup>, corresponding to a dust-covering factor <jats:italic>f</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> ∼ 0.01 at a subparsec scale. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity (∼10<jats:sup>43.5</jats:sup> erg s<jats:sup>−1</jats:sup>), and this makes its nature even more elusive. Other nondetected objects show even lower <jats:italic>f</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub>, which could be lower by one more order of magnitude. The derived <jats:italic>f</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> is generally much lower than those of dusty tori in active galactic nuclei, suggesting either a dearth of dust or a geometrically thin and flat disk in the vicinity of supermassive black holes. Our results also indicate that the optical TDE sample (post-starburst galaxies overrepresented) is seriously biased to events with little dust at subparsec scale, while TDEs in dusty star-forming systems could be more efficiently unveiled by IR echoes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Multiwavelength bright points (BPs) are taken to be cross sections of magnetic flux tubes extending from the surface of the photosphere upward to the higher photosphere. We aim to study the characteristics of isolated multiwavelength BPs using the cotemporal and cospatial TiO band and H<jats:italic>α</jats:italic> line wings from the Goode Solar Telescope at Big Bear Solar Observatory. A deep-learning method, based on Track Region-based Convolutional Neural Networks, is proposed to detect, segment, and match the BPs across multiple wavelength observations, including the TiO, H<jats:italic>α</jats:italic> + 1 Å, H<jats:italic>α</jats:italic> − 1 Å, H<jats:italic>α</jats:italic> + 0.8 Å, and H<jats:italic>α</jats:italic> − 0.8 Å line wings. Based on the efficient detection and matching result with a precision of 0.98, 1283 groups of BPs matched in all five wavelengths are selected for statistics analysis. The characteristic values of the BPs observed at the same red and blue line wings are averaged. For the BPs of the TiO, averaged H<jats:italic>α</jats:italic> ± 1 Å, and averaged H<jats:italic>α</jats:italic> ± 0.8 Å line wings, the mean equivalent diameters are 162 ± 32, 254 ± 33, and 284 ± 28 km, respectively. The maximum intensity contrasts are 1.11 ± 0.09, 1.05 ± 0.03, and 1.05 ± 0.02<jats:inline-formula> <jats:tex-math> <?CDATA $\left\langle {I}_{\mathrm{QS}}\right\rangle $?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe705ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, respectively. The mean eccentricities are 0.65 ± 0.14, 0.63 ± 0.11, and 0.65 ± 0.11, respectively. Moreover, the characteristic ratios of each H<jats:italic>α</jats:italic> ± 1 Å and H<jats:italic>α</jats:italic> ± 0.8 Å BP to its corresponding TiO BP are derived. H<jats:italic>α</jats:italic> ± 1 Å and H<jats:italic>α</jats:italic> ± 0.8 Å line wings BPs show 60% and 80% increases compared to TiO BPs, respectively. With increasing height, most BPs almost keep their shapes. This work is helpful for modeling the three-dimensional structure of flux tubes.</jats:p>