Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Photon–axion mixing can create observable signatures in the thermal spectra of isolated, cooling neutron stars. Their shape depends on the polarization properties of the radiation, which, in turn, are determined by the structure of the stellar outermost layers. Here we investigate the effect of mixing on the spectrum and polarimetric observables, polarization fraction and polarization angle, using realistic models of surface emission. We focus on RX J1856.5–3754, the only source among the X-ray-dim isolated neutron stars for which polarimetric measurements in the optical band were performed. Our results show that in the case of a condensed surface in both fixed and free-ion limits, the mixing can significantly limit the geometric configurations that reproduce the observed linear polarization fraction of 16.43%. In the case of an atmosphere, the mixing does not create any noticeable signatures. Complementing our approach with the data from upcoming soft X-ray polarimetry missions will allow one to obtain constraints on <jats:italic>g</jats:italic> <jats:sub> <jats:italic>γ</jats:italic> <jats:italic>a</jats:italic> </jats:sub> ∼ 10<jats:sup>−11</jats:sup> GeV<jats:sup>−1</jats:sup> and <jats:italic>m</jats:italic> <jats:sub> <jats:italic>a</jats:italic> </jats:sub> ≲ 10<jats:sup>−6</jats:sup> eV, improving the present experimental and astrophysical limits.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a novel approach to the riddle of star cluster multiple populations. Stars form from molecular cores. But not all cores form stars. Following their initial compression, such “failed” cores re-expand, rather than collapsing. We propose that their formation and subsequent dispersal regulate the gas density of cluster-forming clumps and, therefore, their core and star formation rates. Clumps for which failed cores are the dominant core type experience star formation histories with peaks and troughs (i.e., discrete star formation episodes). In contrast, too few failed cores results in smoothly decreasing star formation rates. We identify three main parameters shaping the star formation history of a clump: the star and core formation efficiencies per free-fall time, and the timescale on which failed cores return to the clump gas. The clump mass acts as a scaling factor. We use our model to constrain the density and mass of the Orion Nebula Cluster progenitor clump, and to caution that the star formation histories of starburst clusters may contain close-by peaks concealed by stellar age uncertainties. Our model generates a great variety of star formation histories. Intriguingly, the chromosome maps and O–Na anticorrelations of old globular clusters also present diverse morphologies. This prompts us to discuss our model in the context of globular cluster multiple stellar populations. More massive globular clusters exhibit stronger multiple stellar population patterns, which our model can explain if the formation of the polluting stars requires a given stellar mass threshold.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Kodaikanal Observatory has provided long-term synoptic observations of chromospheric activities in the Ca <jats:sc>ii</jats:sc> K line (393.34 nm) since 1907. This article investigates temporal and periodic variations of the hemispheric Ca–K-index time series in the low-latitude zone (±40°), utilizing the recently digitized photographic plates of Ca–K images from the Kodaikanal Observatory for the period of 1907–1980. We find that the temporal evolution of the Ca–K index differs from one hemisphere to another, with the solar cycle peaking at different times in the opposite hemisphere, except for cycles 14, 15, and 21, when the phase difference between the two hemispheres was not significant. The monthly averaged data show a higher activity in the northern hemisphere during solar cycles 15, 16, 18, 19, and 20, and in the southern hemisphere during cycles 14, 17, and 21. We notice an exponentially decaying distribution for each hemisphere’s Ca–K index and the whole solar disk. We explored different midterm periodicities of the measured Ca–K index using the wavelet technique, including Rieger-type and quasi-biennial oscillations on different timescales present in the time series. We find a clear manifestation of the Waldmeier effect (stronger cycles rise faster than the weaker ones) in both the hemispheres separately and the whole disk in the data. Finally, we have found the presence of the Gnevyshev gap (time interval between two cycle maxmima) in both the hemispheric data during cycles 15 to 20. Possible interpretations of our findings are discussed with the help of existing theoretical models and observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a novel analytic framework to model the steady-state structure of multiphase galactic winds comprised of a hot, volume-filling component and a cold, clumpy component. We first derive general expressions for the structure of the hot phase for arbitrary mass, momentum, and energy source terms. Next, informed by recent simulations, we parameterize the cloud–wind mass transfer rates, which are set by the competition between turbulent mixing and radiative cooling. This enables us to cast the cloud–wind interaction as a source term for the hot phase and thereby simultaneously solve for the evolution of both phases, fully accounting for their bidirectional influence. With this model, we explore the nature of galactic winds over a broad range of conditions. We find that (i) with realistic parameter choices, we naturally produce a hot, low-density wind that transports energy while entraining a significant flux of cold clouds, (ii) mixing dominates the cold cloud acceleration and decelerates the hot wind, (iii) during mixing thermalization of relative kinetic energy provides significant heating, (iv) systems with low hot phase mass loading factors and/or star formation rates can sustain higher initial cold phase mass loading factors, but the clouds are quickly shredded, and (v) systems with large hot phase mass loading factors and/or high star formation rates cannot sustain large initial cold phase mass loading factors, but the clouds tend to grow with distance from the galaxy. Our results highlight the necessity of accounting for the multiphase structure of galactic winds, both physically and observationally, and have important implications for feedback in galactic systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We measure the relationship between stellar mass and stellar metallicity for 1336 star-forming galaxies at 1.6 ≤ <jats:italic>z</jats:italic> ≤ 3.0 using rest-frame far-ultraviolet spectra from the zCOSMOS-deep survey. High signal-to-noise ratio composite spectra containing stellar absorption features are fit with stellar population synthesis model spectra of a range of stellar metallicity. We find stellar metallicities, which mostly reflect instantaneous iron abundances, scaling as <jats:inline-formula> <jats:tex-math> <?CDATA $[\mathrm{Fe}/{\rm{H}}]=-(0.81\pm 0.01)+(0.32+0.03)\mathrm{log}({M}_{* }/{10}^{10}{M}_{\odot })$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">[</mml:mo> <mml:mi>Fe</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0.81</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.01</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0.32</mml:mn> <mml:mo>+</mml:mo> <mml:mn>0.03</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac399eieqn1.gif" xlink:type="simple" /> </jats:inline-formula> across the stellar mass range of 10<jats:sup>9</jats:sup> ≲ <jats:italic>M</jats:italic> <jats:sub>*</jats:sub>/<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> ≲ 10<jats:sup>11</jats:sup>. The instantaneous oxygen-to-iron ratio (<jats:italic>α</jats:italic>-enhancement) inferred using the gas-phase mass–metallicity relation is on average found to be <jats:inline-formula> <jats:tex-math> <?CDATA $\left[{\rm{O}}/\mathrm{Fe}\right]\approx 0.47$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="]" open="["> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>≈</mml:mo> <mml:mn>0.47</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac399eieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, being higher than the local <jats:inline-formula> <jats:tex-math> <?CDATA $\left[{\rm{O}}/\mathrm{Fe}\right]\approx 0$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="]" open="["> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>≈</mml:mo> <mml:mn>0</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac399eieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. The observed changes in [O/Fe] and [Fe/H] are reproduced in simple gas-regulator models with steady star formation histories. Our models show that the [O/Fe] is determined almost entirely by the instantaneous specific star formation rate alone while being independent of the mass and the characteristic of the gas regulation. We also find that the locations of ∼ 10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> galaxies at <jats:italic>z</jats:italic> ∼ 2 in the [O/Fe]–metallicity planes are in remarkable agreement with the sequence of low-metallicity thick-disk stars in our own Galaxy. This manifests a beautiful concordance between the results of Galactic archeology and observations of high-redshift Milky Way progenitors. There remains, however, a question of how and when the old metal-rich, low <jats:italic>α</jats:italic>/Fe stars seen in the bulge had formed by <jats:italic>z</jats:italic> ∼ 2 because such a stellar population is not seen in our data and is difficult to explain in the context of our models.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>By plotting empirical chemical element abundances on Earth relative to the Sun and normalized to silicon versus their first ionization potentials, we confirm the existence of a correlation reported earlier. To explain this, we develop a model based on principles of statistical physics that predicts differentiated relative abundances for any planetary body in a solar system as a function of its orbital distance. This simple model is successfully tested against available chemical composition data from CI chondrites and surface compositional data of Mars, Earth, the Moon, Venus, and Mercury. We show, moreover, that deviations from the proposed law for a given planet correspond to later surface segregation of elements driven both by gravity and chemical reactions. We thus provide a new picture for the distribution of elements in the solar system and inside planets, with important consequences for their chemical composition. Particularly, a 4 wt% initial hydrogen content is predicted for bulk early Earth. This converges with other works suggesting that the interior of the Earth could be enriched with hydrogen.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The optical light curves of quiescent black hole low-mass X-ray binaries often exhibit significant nonellipsoidal variabilities, showing the photospheric radiation of the companion star is veiled by other sources of optical emission. Assessing this “veiling” effect is critical to the black hole mass measurement. Here in this work, we carry out a strictly simultaneous spectroscopic and photometric campaign on the prototype of black hole low-mass X-ray binary A0620-00. We find that for each observation epoch, the extra optical flux beyond a pure ellipsoidal modulation is positively correlated with the fraction of veiling emission, indicating the accretion disk contributes most of the nonellipsoidal variations. Meanwhile, we also obtain a K2V spectral classification of the companion, as well as the measurements of the companion’s rotational velocity <jats:inline-formula> <jats:tex-math> <?CDATA $v\sin i=83.8\pm 1.9$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>83.8</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1.9</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac4332ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> and the mass ratio between the companion and the black hole <jats:italic>q</jats:italic> = 0.063 ± 0.004.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper we investigate the robustness of age measurements, age spreads, and stellar models in young pre-main-sequence stars. For this effort, we study a young cluster, <jats:italic>λ</jats:italic> Orionis, within the Orion star-forming complex. We use Gaia data to derive a sample of 357 targets with spectroscopic temperatures from spectral types or from the automated spectroscopic pipeline in APOGEE Net. After accounting for systematic offsets between the spectral type and APOGEE temperature systems, the derived properties of stars on both systems are consistent. The complex interstellar medium, with variable local extinction, motivates a star-by-star dereddening approach. We use a spectral energy distribution fitting method calibrated on open clusters for the Class III stars. For the Class II population, we use a Gaia G-RP dereddening method, minimizing systematics from disks, accretion, and other physics associated with youth. The cluster age is systematically different in models incorporating the structural impact of starspots or magnetic fields than in nonmagnetic models. Our mean ages range from 2–3 Myr (nonmagnetic models) to 3.9 ± 0.2 Myr in the SPOTS model (<jats:italic>f</jats:italic> = 0.34). We find that star-by-star dereddening methods distinguishing between pre-main-sequence classes provide a smaller age spread than techniques using a uniform extinction, and we infer a minimum age spread of 0.19 dex and a typical age spread of 0.35 dex after modeling age distributions convolved with observed errors. This suggests that the <jats:italic>λ</jats:italic> Ori cluster may have a long star formation timescale and that spotted stellar models significantly change age estimates for young clusters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Seyfert-1 galaxy NGC 3516 has undergone major spectral changes in recent years. In 2017 we obtained Chandra, NuSTAR, and Swift observations during its new low-flux state. Using these observations, we model the spectral energy distribution (SED) and the intrinsic X-ray absorption, and compare the results with those from historical observations taken in 2006. We thereby investigate the effects of the changing-look phenomenon on the accretion-powered radiation and the ionized outflows. Compared to its normal high-flux state in 2006, the intrinsic bolometric luminosity of NGC 3516 was lower by a factor of 4–8 during 2017. Our SED modeling shows a significant decline in the luminosity of all the continuum components from the accretion disk and the X-ray source. As a consequence, the reprocessed X-ray emission lines have also become fainter. The Swift monitoring of NGC 3516 shows remarkable X-ray spectral variability on short (weeks) and long (years) timescales. We investigate whether this variability is driven by obscuration or the intrinsic continuum. We find that the new low-flux spectrum of NGC 3516, and its variability, do not require any new or variable obscuration, and instead can be explained by changes in the ionizing SED that result in the lowering of the ionization of the warm-absorber outflows. This in turn induces enhanced X-ray absorption by the warm-absorber outflows, mimicking the presence of new obscuring gas. Using the response of the ionized regions to the SED changes, we place constraints on their densities and locations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We describe the validation of the HERA Phase I software pipeline by a series of modular tests, building up to an end-to-end simulation. The philosophy of this approach is to validate the software and algorithms used in the Phase I upper-limit analysis on wholly synthetic data satisfying the assumptions of that analysis, not addressing whether the actual data meet these assumptions. We discuss the organization of this validation approach, the specific modular tests performed, and the construction of the end-to-end simulations. We explicitly discuss the limitations in scope of the current simulation effort. With mock visibility data generated from a known analytic power spectrum and a wide range of realistic instrumental effects and foregrounds, we demonstrate that the current pipeline produces power spectrum estimates that are consistent with known analytic inputs to within thermal noise levels (at the 2<jats:italic>σ</jats:italic> level) for <jats:italic>k</jats:italic> &gt; 0.2<jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> for both bands and fields considered. Our input spectrum is intentionally amplified to enable a strong “detection” at <jats:italic>k</jats:italic> ∼ 0.2 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup>—at the level of ∼25<jats:italic>σ</jats:italic>—with foregrounds dominating on larger scales and thermal noise dominating at smaller scales. Our pipeline is able to detect this amplified input signal after suppressing foregrounds with a dynamic range (foreground to noise ratio) of ≳10<jats:sup>7</jats:sup>. Our validation test suite uncovered several sources of scale-independent signal loss throughout the pipeline, whose amplitude is well-characterized and accounted for in the final estimates. We conclude with a discussion of the steps required for the next round of data analysis.</jats:p>