Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The calcium abundance in flare plasmas is estimated using X-ray spectra from the Solar Maximum Mission Bent Crystal Spectrometer (BCS) during the decays of 194 flares (Geostationary Operational Environmental Satellite, GOES, classifications from B6.4 to X13) occurring between 1980 and 1989. Previous work by Sylwester et al. found that the abundance varied from flare to flare. That analysis is improved on here using updated instrument parameters and by including all calcium lines viewed by the BCS instead of only the resonance line, so greatly enhancing the photon count statistics. The abundance variations are confirmed with the average abundance, <jats:italic>A</jats:italic>(Ca) (expressed logarithmically with <jats:italic>A</jats:italic>(H) = 12), equal to 6.77 ± 0.20 for 194 flares (141 of which are new in this study). This range corresponds to factors of between 1.7 and 7.2 larger than the photospheric abundance, and so our results are in line with a “first ionization potential” (FIP) effect whereby low-FIP elements like Ca (FIP = 6.11 eV) have enhanced coronal abundances. The Ca flare abundance is uncorrelated with solar activity indices, but weak correlations are suggested with GOES flare class and duration (larger <jats:italic>A</jats:italic>(Ca) for smaller and shorter flares). The ponderomotive force theory of Laming explaining the FIP effect gives a range of parameters within which our estimates of <jats:italic>A</jats:italic>(Ca) agree with the theory. However, this then gives rise to disagreements with previous estimates of the flare silicon and sulfur abundances, although those of argon and iron are in good agreement. Small adjustments of the theory may thus be necessary.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Knowledge of a star’s evolutionary history combined with estimates of planet occurrence rates allows one to infer whether a star would be a good target in a search for biosignatures, and to quantify this intuition using long-term habitability metrics. In this study, we analyze the sensitivity of the biosignature yield metrics formulated by Tuchow &amp; Wright to uncertainties in observable stellar properties and to model uncertainties. We characterize the uncertainties present in fitting models to stellar observations by generating a stellar model with known properties and adding synthetic uncertainties in the observable properties. We scale the uncertainty in individual observables and observe the effects on the precision of properties such as stellar mass, age, and our metrics. To determine model uncertainties, we compare four well-accepted stellar models using different model physics and see how they vary in terms of the values of our metrics. We determine the ability of future missions to rank target stars according to these metrics, given the current precision to which host star properties can be measured. We show that obtaining independent age constraints decreases both the model and systematic uncertainties in determining these metrics and is the most powerful way to improve assessments of the long-term habitability of planets around low-mass stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Considerable observational efforts are being dedicated to measuring the sky-averaged (global) 21 cm signal of neutral hydrogen from Cosmic Dawn and the Epoch of Reionization. Deriving observational constraints on the astrophysics of this era requires modeling tools that can quickly and accurately generate theoretical signals across the wide astrophysical parameter space. For this purpose artificial neural networks were used to create the only two existing global signal emulators, 21<jats:sc>cm</jats:sc>GEM and <jats:sc>globalemu</jats:sc>. In this paper we introduce 21<jats:sc>cm</jats:sc>VAE, a neural-network-based global signal emulator, trained on the same data set of ∼30,000 global signals as the other two emulators, but with a more direct prediction algorithm that prioritizes accuracy and simplicity. Using neural networks, we compute derivatives of the signals with respect to the astrophysical parameters and establish the most important astrophysical processes that drive the global 21 cm signal at different epochs. 21<jats:sc>cm</jats:sc>VAE has a relative rms error of only 0.34%—equivalently 0.54 mK—on average, which is a significant improvement compared to the existing emulators, and a run time of 0.04 s per parameter set. The emulator, the code, and the processed data sets are publicly available at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://github.com/christianhbye/21cmVAE" xlink:type="simple">https://github.com/christianhbye/21cmVAE</jats:ext-link> and through <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://zenodo.org/record/5904939" xlink:type="simple">https://zenodo.org/record/5904939</jats:ext-link>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Azimuthally asymmetric structures have been discovered in millimeter continuum emission from many protoplanetary disks. One hypothesis is that they are vortices produced by the Rossby wave instability, for example at the edges of planet-opened gaps or dead zones. Confirming the vortex nature of these structures will have profound implications to planet formation. One way to test the hypothesis is to compare the observed morphology of vortex candidates in near-infrared scattered light with theoretical expectations. To this end, we synthesize the appearance of vortices in <jats:italic>H</jats:italic>-band polarized light by combining hydrodynamic and radiative transfer simulations of the Rossby wave instability at a dead-zone edge. In a disk at 140 pc, at the peak in its evolution a vortex at 65 au may appear as a radially narrow arc 50%–70% brighter compared with an axisymmetric disk model. The contrast depends on the inclination of the disk and the position angle of the vortex only weakly. Such contrast levels are well detectable in imaging observations of bright disks using instruments such as the Very Large Telescope/SPHERE, Subaru/SCExAO, and Gemini/GPI. A vortex also casts a shadow in the outer disk, which may aid its identification. Finally, at modest-to-high inclinations (e.g., 60°) a vortex may mimic a one-armed spiral. In the HD 34282 disk, such a one-armed spiral with a shadowed region on the outside has been found in scattered light. This feature roughly coincides with an azimuthal asymmetry in millimeter continuum emission, signifying the presence of a vortex.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the discovery of an exceptional dimming event in a cool supergiant star in the Local Volume spiral M51. The star, dubbed M51-DS1, was found as part of a Hubble Space Telescope (HST) search for failed supernovae (SNe). The supergiant, which is plausibly associated with a very young (≲6 Myr) stellar population, showed clear variability (amplitude Δ<jats:italic>F</jats:italic>814<jats:italic>W</jats:italic> ≈ 0.7 mag) in numerous HST images obtained between 1995 and 2016, before suddenly dimming by &gt;2 mag in <jats:italic>F</jats:italic>814<jats:italic>W</jats:italic> sometime between late 2017 and mid-2019. In follow-up data from 2021, the star rebrightened, ruling out a failed supernova. Prior to its near-disappearance, the star was luminous and red (<jats:italic>M</jats:italic> <jats:sub> <jats:italic>F</jats:italic>814<jats:italic>W</jats:italic> </jats:sub> ≲ − 7.6 mag, <jats:italic>F</jats:italic>606<jats:italic>W</jats:italic> − <jats:italic>F</jats:italic>814<jats:italic>W</jats:italic> = 1.9–2.2 mag). Modeling of the pre-dimming spectral energy distribution of the star favors a highly reddened, very luminous (<jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}[L/{L}_{\odot }]=5.4$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">[</mml:mo> <mml:mi>L</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mn>5.4</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac626cieqn1.gif" xlink:type="simple" /> </jats:inline-formula>–5.7) star with <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> ≈ 3700–4700 K, indicative of a cool yellow or post-red supergiant (RSG) with an initial mass of ≈26–40 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. However, the local interstellar extinction and circumstellar extinction are uncertain, and could be lower: the near-IR colors are consistent with an RSG, which would be cooler (<jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> ≲ 3700 K) and slightly less luminous (<jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}[L/{L}_{\odot }]=5.2$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">[</mml:mo> <mml:mi>L</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mn>5.2</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac626cieqn2.gif" xlink:type="simple" /> </jats:inline-formula>–5.3), giving an inferred initial mass of ≈19–22 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. In either case, the dimming may be explained by a rare episode of enhanced mass loss that temporarily obscures the star, potentially a more extreme counterpart to the 2019–2020 “Great Dimming” of Betelgeuse. Given the emerging evidence that massive evolved stars commonly exhibit variability that can mimic a disappearing star, our work highlights a substantial challenge in identifying true failed SNe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Local sources, such as the Geminga supernova remnant (SNR), may have played an important role in the anomaly of protons, electrons, and anisotropy in past works. In fact, there exist 12 SNRs around the solar system within 1 kpc. One question is whether other SNRs also possibly contribute to the spectra of nuclei and electrons, and explain the special structure of the anisotropy. In this work, under spatial-dependent propagation, we systematically study the contribution of all local SNRs, within 1 kpc around the solar system, to the spectra of nuclei and electrons, as well as the energy dependence of the anisotropy. As a result, only the Geminga, the Monogem, and the Vela SNRs have quantitative contributions to the nuclei and electron spectra, and the anisotropy. Here, the Geminga SNR is the sole optimal candidate and the Monogem SNR is controversial due to the tension of the anisotropy between the model calculation and the observations. The Vela SNR contributes to a new spectral structure beyond TeV energy, hinted by the HESS, the VERITAS, the DAMPE, and the CALET measurements. More interestingly, the electron anisotropy satisfies the Fermi-LAT limit below TeV energy, but rises greatly and reaches 10% at several TeV. This novel structure will shed new light on verifying our model. We hope that the new structure of the electron spectrum and anisotropy can be observed by the spaceborne DAMPE and HERD, and the ground-based HAWC and LHAASO experiments in the near future.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Galaxy clusters are the largest virialized objects in the universe. Their merger dynamics and their interactions with the cosmic filaments that connect them are important for our understanding of the formation of large-scale structure. In addition, cosmic filaments are thought to possess the missing baryons in the universe. Studying the interaction of galaxy clusters and filaments therefore has the potential to unveil the origin of the baryons and the physical processes that occur during merger stages of galaxy clusters. In this paper, we study the connection between A3395 and the intercluster filament with NuSTAR, XMM-Newton, and Suzaku data. Since the NuSTAR observation is moderately contaminated by scattered light, we present a novel technique developed for disentangling this background from the emission from the intracluster medium. We find that the interface of the cluster and the intercluster filament connecting A3395 and A3391 does not show any signs of heated plasma, as was previously thought. This interface has low temperature, high density, and low entropy, thus we suggest that the gas is cooling, being enhanced by the turbulent or tidal “weather” driven during the early stage of the merger. Furthermore, our temperature results from the NuSTAR data are in agreement with those from XMM-Newton and from joint NuSTAR and XMM-Newton analysis for a region with ∼25% scattered light contamination within 1<jats:italic>σ</jats:italic>. We show that the temperature constraint of the intracluster medium is valid even when the data are contaminated up to ∼25% for ∼5 keV cluster emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The poleward migration of the active regions’ magnetic flux on the solar surface plays an important role in the development of the large-scale field development, especially the polar-field reversal, which is a key process in the Babcock–Leighton-type solar dynamos. The poleward flux transport is nonuniform, centered around poleward surges as suggested by previous observations. The strong, long-lasting surges are related to activity complexes, and often result in violent polar-field reversal. However, the nonuniformity of poleward flux transport has not been evaluated quantitatively. We propose a statistical method to analyze the poleward flux transport during solar cycles 21–24 by considering the frequency distributions of the magnetic field at latitudes of poleward surges occurring during solar cycles. The nonuniformity is quantified as the kurtosis statistics representing the tailedness of the distributions. We test the method on results of surface flux transport simulations, and apply it to WSO, National Solar Observatory, MWO, and HMI data. We confirm that the poleward surges are of significance during solar cycles 21–24 in general. The kurtosis within a solar cycle is affected by different latitudes of the magnetic field and different data sources. The southern hemisphere of cycle 24 exhibits the largest kurtosis, agreeing with the super-surge concept from previous work. The significant nonuniformity of poleward flux transport originates from the nonrandomness of active regions, which favors the activity complexes as the origin of poleward surges.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We use a sample of 13,511 nearby galaxies from the ALFALFA and Sloan Digital Sky Survey spectroscopic surveys to study the relation between the spatial distribution of H <jats:sc>i</jats:sc> 21 cm emission and star formation rate (SFR). We introduce a new nonparametric quantity <jats:italic>K</jats:italic>, measured from the curve of growth of the line, to describe the shape of the integrated H <jats:sc>i</jats:sc> profile. The value of <jats:italic>K</jats:italic> increases from double-horned to single-peaked profiles, depending on projection effects and the spatial and velocity distribution of the gas. Using carefully chosen samples to control for the competing factors that influence the integrated line profile, we argue that useful inferences can be made on the spatial distribution of the gas. We find that galaxies with a high value of <jats:italic>K</jats:italic> tend to have more centrally concentrated H <jats:sc>i</jats:sc> distribution within the optical disk of the galaxy at fixed conditions, and that larger values of <jats:italic>K</jats:italic> are associated with higher levels of total and central SFR. The results suggest that the global concentration of H <jats:sc>i</jats:sc> plays an important role in facilitating the conversion of neutral atomic hydrogen to molecular hydrogen gas, which, in turn, affects the star formation activity throughout the optical disk. Our sample is biased against quiescent galaxies, and thus the conclusions may not hold for galaxies with low SFR or low H <jats:sc>i</jats:sc> content.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The reconstruction of the differential emission measure (DEM) from observations of spectral line intensities provides information on the temperature distribution of the emission measure in the region observed. The inversion process is known to be highly unstable, and it has been necessary to provide additional constraints, such as requiring that the DEM should be smooth. However, this is a nonphysical constraint. The goal of this analysis is to make an empirical determination of the ability of a set of emission-line intensities to constrain the reconstruction. Here, a simple model is used, by means of a Markov Chain Monte Carlo process, to arrive at solutions that reproduce the observed intensities in a region of the quiet Sun and a solar active region. These solutions are compared by means of the reduced chi-squared. The conclusion from this analysis is that the observations are only capable of constraining a model consisting of four temperature–emission measure pairs plus a determination of the standard deviation of the model from the observed line intensities. A more complex model with five temperature–emission measure pairs does not improve the fit and leads to parameters that are irrelevant. A more general conclusion is that the information content of a set of observed emission lines is limited with respect to the determination of the emission measure distribution.</jats:p>