Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Stellar flares are characterized by sudden enhancement of electromagnetic radiation from the atmospheres of stars. Compared to their solar counterparts, our knowledge on the coronal plasma dynamics of stellar flares and their connection to coronal mass ejections remains very limited. With time-resolved high-resolution spectroscopic observations from the Chandra X-ray Observatory, we detected noticeable coronal plasma flows during several stellar flares on a nearby dMe star EV Lac. In the observed spectra of O <jats:sc>viii</jats:sc> (3 MK), Fe <jats:sc>xvii</jats:sc> (6 MK), Mg <jats:sc>xii</jats:sc> (10 MK), and Si <jats:sc>xiv</jats:sc> (16 MK) lines, these flare-induced upflows/downflows appear as significant Doppler shifts of several tens to 130 km s<jats:sup>−1</jats:sup> , and the upflow velocity generally increases with temperature. Variable line ratios of the Si <jats:sc>xiii</jats:sc> triplet reveal that this plasma flows in most flares are accompanied by an increase in the coronal plasma density and temperature. We interpret these results as X-ray evidence of chromospheric evaporation on EV Lac. In two successive flares, the plasma flow pattern and a sharp increase of the measured coronal density are highly suggestive of explosive evaporation. The transition from redshifts to blueshifts in such an explosive evaporation occurs at a temperature of at least 10 MK, much higher than that observed in solar flares (∼1 MK). However, in one flare the cool and warm upflows appear to be accompanied by a decreasing plasma density, which might be explained by a stellar filament/prominence eruption coupled to this flare. These results provide important clues to understanding the coronal plasma dynamics during flares on M dwarfs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A number of double coronal X-ray sources have been observed during solar flares by RHESSI, where the two sources reside at different sides of the inferred reconnection site. However, where and how these X-ray-emitting electrons are accelerated remains unclear. Here we present the first model of the double coronal hard X-ray (HXR) sources, where electrons are accelerated by a pair of termination shocks driven by bidirectional fast reconnection outflows. We model the acceleration and transport of electrons in the flare region by numerically solving the Parker transport equation using velocity and magnetic fields from the macroscopic magnetohydrodynamic simulation of a flux rope eruption. We show that electrons can be efficiently accelerated by the termination shocks and high-energy electrons mainly concentrate around the two shocks. The synthetic HXR emission images display two distinct sources extending to &gt;100 keV below and above the reconnection region, with the upper source much fainter than the lower one. The HXR energy spectra of the two coronal sources show similar spectral slopes, consistent with the observations. Our simulation results suggest that the flare termination shock can be a promising particle acceleration mechanism in explaining the double-source nonthermal emissions in solar flares.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the discovery of 17 double white dwarf (WD) binaries from our ongoing search for extremely low mass (ELM) &lt; 0.3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> WDs, objects that form from binary evolution. Gaia parallax provides a new means of target selection that we use to evaluate our original ELM Survey selection criteria. Cross-matching the Gaia and Sloan Digital Sky Survey (SDSS) catalogs, we identify an additional 36 ELM WD candidates with 17 &lt; <jats:italic>g</jats:italic> &lt; 19 mag and within the 3<jats:italic>σ</jats:italic> uncertainties of our original color selection. The resulting discoveries imply the ELM Survey sample was 90% complete in the color range −0.4 &lt; (<jats:italic>g</jats:italic> − <jats:italic>r</jats:italic>)<jats:sub>0</jats:sub> &lt; −0.1 mag (approximately 9000 K &lt; <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> &lt; 22,000 K). Our observations complete the sample in the SDSS footprint. Two newly discovered binaries, J123950.370−204142.28 and J232208.733+210352.81, have orbital periods of 22.5 and 32 minutes, respectively, and are future Laser Interferometer Space Antenna gravitational-wave sources.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the fine structure of the inner solar corona between 1.65 and 3.0 solar radii as revealed by the STEREO-A COR1 white-light coronagraph from 2008 June 20 to July 31. The COR1 imaging data were wavelet processed to enhance the intensity contrast of coronal features. The constructed limb synoptic maps at a range of altitudes show the evolution in time and altitude of these fine structures within the streamer belt, and equatorial and polar coronal holes during this period near the solar minimum. Distinct streamer-stalk structures are seen embedded within a diffuse background of the helmet streamer belt, which are preserved as they extend to higher heights. Pseudostreamers are also seen as multiple stalk structures, which also continue to higher heights. Various polar plume structures are seen to last from hours to days. Similar plume structures are also seen within the corona subtended by equatorial coronal holes. We compare the COR1 maps to that of the magnetic topology revealed by the modeled squashing factors, and discuss the relation between the two types of maps and its implications in the context of solar wind formation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tidal disruption of stars in dense nuclear star clusters containing supermassive central black holes (SMBH) is modeled by high-accuracy direct <jats:italic>N</jats:italic>-body simulation. Stars getting too close to the SMBH are tidally disrupted, and a tidal disruption event (TDE) happens. The TDEs probe the properties of SMBHs, their accretion disks, and the surrounding nuclear stellar cluster. In this paper, we compare the rates of full tidal disruption events (FTDEs) with partial tidal disruption events (PTDEs). Since a PTDE does not destroy the star, a leftover object emerges; we use the term “leftover star” for it. Two novel effects occur in the simulation: (1) variation of the leftover star’s mass and radius and (2) variation of the leftover star’s orbital energy. After switching on these two effects in our simulation, the number of FTDEs is reduced by roughly 28%, and the reduction is mostly due to the ejection of the leftover stars from PTDEs originally coming from a relatively large distance. The number of PTDEs is about 75% higher than the simple estimation given by Stone et al., and the enhancement is mainly due to the multiple PTDEs produced by the leftover stars residing in the diffusive regime. We compute the peak mass fallback rate for the PTDEs and FTDEs recorded in the simulation and find that 58% of the PTDEs have a peak mass fallback rate exceeding the Eddington limit, and the number of super-Eddington PTDEs is 2.3 times the number of super-Eddington FTDEs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Current sheets (CSs) are preferred sites of magnetic reconnection and energy dissipation in astrophysical plasmas. Electric currents in them may be carried by both electrons and ions. In our prior theoretical studies of the CS formation in turbulent plasmas, we utilized fully kinetic and hybrid code simulations with ions considered as particles and electrons—as a massless fluid. We found that electron-dominated CSs in which electrons become the main carriers of the electric current and contributors to energy dissipation may form inside or nearby ion-dominated CSs. These structures represent a distinguished type of CSs and should not be mixed up with so-called electron-scale CSs. Current simulations show that such CSs are characterized by the electron-to-ion bulk speed ratio (<jats:italic>u</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>/<jats:italic>u</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub>) increases that can be seen at ion scales according to theoretical predictions and high-resolution observations from the Magnetospheric Multiscale mission. Therefore, applying the <jats:italic>u</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>/<jats:italic>u</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> parameter to the solar wind data may allow locating the strongest electron-dominated CSs with an ordinary spacecraft resolution of 1−3 s. This study shows that, indeed, electron-dominated CSs observed during a period of quiet solar wind conditions at 1 au impact the surrounding plasma, which may be reflected in sharp changes of <jats:italic>u</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>/<jats:italic>u</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub>. Electron-dominated CSs are found to be localized in the vicinity of ion-dominated CSs identified via changes in the magnetic field and plasma parameters, displaying the same clustering. We conclude that <jats:italic>u</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>/<jats:italic>u</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> may be used as one of the key parameters for statistical studies of CSs in the solar wind and analyzing the role of electrons in them.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>X-shaped radio galaxies (XRGs) are those that exhibit two pairs of unaligned radio lobes (main radio lobes and wings). One of the promising models for the peculiar morphology is jet reorientation. To clarify this, we conducted a 5 GHz observation with the European VLBI Network (EVN) of XRG J0725+5835, which resembles the archetypal binary active galactic nuclei (AGNs) 0402+379 in radio morphology, but it is larger in angular size. In our observation, two milliarcsecond-scale radio components with nonthermal radio emission are detected. Each of them coincides with an optical counterpart with similar photometric redshift and (optical and infrared) magnitude, corresponding to dual active nuclei. Furthermore, with the improved Very Large Array (VLA) images, we find a bridge between the two radio cores and a jet bending in the region surrounding the companion galaxy. This further supports the interplay between the main and companion galaxies. In addition, we also report the discovery of an arcsecond-scale jet in the companion. Given the projected separation of ∼100 kpc between the main and companion galaxies, XRG J0725+5835 is likely associated with a dual jetted-AGN system. In both EVN and VLA observations, we find signatures that the jet is changing its direction, which is likely responsible for the X-shaped morphology. For the origin of jet reorientation, several scenarios are discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>LaO bands are a characteristic feature in the spectrum of cool S-type stars. La is made primarily by the <jats:italic>s</jats:italic>-process during the asymptotic giant branch phase of stellar evolution. The B<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup>–X<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup> and A<jats:sup>2</jats:sup>Π–X<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup> band systems can be used to determine the La abundances in cool S stars. The bands of the B<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup>–X<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup> with <jats:inline-formula> <jats:tex-math> <?CDATA $v^{\prime} $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac731fieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA $v^{\prime\prime} \leqslant 5$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo accent="false">′</mml:mo> <mml:mo>≤</mml:mo> <mml:mn>5</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac731fieqn2.gif" xlink:type="simple" /> </jats:inline-formula> have been rotationally analyzed from an emission spectrum from a carbon furnace. Line strengths are calculated using an ab initio transition dipole function, corrected using experimental lifetimes. We provide a line list for the B<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup>–X<jats:sup>2</jats:sup>Σ<jats:sup>+</jats:sup> band system that can be used to determine La abundances.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Giant planets have been discovered at large separations from the central star. Moreover, a striking number of young circumstellar disks have gas and/or dust gaps at large orbital separations, potentially driven by embedded planetary objects. To form massive planets at large orbital separations through core accretion within the disk lifetime, however, an early solid body to seed pebble and gas accretion is desirable. Young protoplanetary disks are likely self-gravitating, and these gravitoturbulent disks may efficiently concentrate solid material at the midplane driven by spiral waves. We run 3D local hydrodynamical simulations of gravitoturbulent disks with Lagrangian dust particles to determine whether particle and gas self-gravity can lead to the formation of dense solid bodies, seeding later planet formation. When self-gravity between dust particles is included, solids of size St = 0.1–1 concentrate within the gravitoturbulent spiral features and collapse under their own self-gravity into dense clumps up to several <jats:italic>M</jats:italic> <jats:sub>⊕</jats:sub> in mass at wide orbits. Simulations with dust that drift most efficiently, St = 1, form the most massive clouds of particles, while simulations with smaller dust particles, St = 0.1, have clumps with masses an order of magnitude lower. When the effect of dust backreaction onto the gas is included, dust clumps become smaller by a factor of a few but more numerous. The existence of large solid bodies at an early stage of the disk can accelerate the planet formation process, particularly at wide orbital separations, and potentially explain planets distant from the central stars and young protoplanetary disks with substructures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent high-resolution X-ray spectroscopy revealed the possible presence of charge exchange (CX) X-ray emission in supernova remnants (SNRs). Although CX is expected to take place at the outermost edges of SNR shells, no significant measurement has been reported so far due to the lack of nearby SNR samples. Here we present an X-ray study of SNR G296.1−0.5, which has a complicated multiple-shell structure, with the Reflection Grating Spectrometer on board XMM-Newton. We select two shells in different regions and find that in both regions the O <jats:sc>vii</jats:sc> line shows a high forbidden-to-resonance (<jats:italic>f</jats:italic>/<jats:italic>r</jats:italic>) ratio that cannot be reproduced by a simple thermal model. Our spectral analysis suggests a presence of CX and the result is also supported by our new radio observation, where we discover evidence of molecular clouds associated with these shells. Assuming G296.1−0.5 has a spherical shock, we estimate that CX is dominant in a thin layer with a thickness of 0.2%–0.3% of the shock radius. The result is consistent with a previous theoretical expectation and we therefore conclude that CX occurs in G296.1−0.5.</jats:p>