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Register a new userNew light on an old problem of the cores of solar resonance lines
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We re-examine a 50+ year-old problem of deep central reversals predicted for strong solar spectral lines, in contrast to the smaller reversals seen in observations. We examine data and calculations for the resonance lines of H I, Mg II and Ca II, the self-reversed cores of which form in the upper chromosphere. Based on 3D simulations as well as data for the Mg II lines from IRIS, we argue that the resolution lies not in velocity fields on scales in either of the micro- or macro-turbulent limits. Macro-turbulence is ruled out using observations of optically thin lines formed in the upper chromosphere, and by showing that it would need to have unreasonably special properties to account for critical observations of the Mg II resonance lines from the IRIS mission. The power in turbulence in the upper chromosphere may therefore be substantially lower than earlier analyses have inferred. Instead, in 3D calculations horizontal radiative transfer produces smoother source functions, smoothing out intensity gradients in wavelength and in space. These effects increase in stronger lines. Our work will have consequences for understanding the onset of the transition region, the energy in motions available for heating the corona, and for the interpretation of polarization data in terms of the Hanle effect applied to resonance line profiles.
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Sulfur appears to be depleted by an order of magnitude or more from its elemental abundance in star-forming regions. In the last few years, numerous observations and experiments have been performed in order to to understand the reasons behind this depletion without providing a satisfactory explanation of the sulfur chemistry towards high-mass star-forming cores. Several sulfur-bearing molecules have been observed in these regions, and yet none are abundant enough to make up the gas-phase deficit. Where, then, does this hidden sulfur reside? This paper represents a step forward in our understanding of the interactions among the various S-bearing species. We have incorporated recent experimental and theoretical data into a chemical model of a hot molecular core in order to see whether they give any indication of the identity of the sulfur sink in these dense regions. Despite our model producing reasonable agreement with both solid-phase and gas-phase abundances of many sulfur-bearing species, we find that the sulfur residue detected in recent experiments takes up only ~6 per cent of the available sulfur in our simulations, rather than dominating the sulfur budget.
The $Kepler$ $problem$ studies the planar motion of a point mass subject to a central force whose strength varies as the inverse square of the distance to a fixed attracting center. The orbits form a 3-parameter family of unparametrized plane curves, consisting of all conics sharing a focus at the attracting center. We study the geometry and symmetry properties of this family, as well as natural 2-parameter subfamilies, such as those of fixed energy or angular momentum. Our main result is that Kepler orbits form a `flat family, that is, the local diffeomorphisms of the plane preserving this family form a 7-dimensional local group, the maximum dimension possible for the symmetry group of a 3-parameter family of plane curves (a result of S. Lie). The new symmetries are different from the well-studied `hidden symmetries of the Kepler problem, acting on energy levels in the 4-dimensional phase space of the problem. Furthermore, each 2-parameter family of Kepler orbits with fixed non-zero energy admits $mathrm{PSL}_2(mathbb{R})$ as its symmetry group and coincides with one of the items of a classification due to A. Tresse (1896) of 2nd order ODEs admitting a 3-dimensional group of point symmetries. Other items on Tresse list also appear in Keplers problem by considering repulsive instead of attractive force or motion on a surface with (non-zero) constant curvature. Underlying these newly found symmetries is a duality between Keplers plane and Minkowskis 3-space parametrizing the space of Kepler orbits.
Understanding how light interacts at the nanoscale with metals, semiconductors, or ordinary dielectrics is pivotal if one is to properly engineer nano-antennas, filters and, more generally, devices that aim to harness the effects of new physical phenomena that manifest themselves at the nanoscale. We presently report experimental results on second and third harmonic generation from 20nm- and 70nm-thick gold layers, for TE- and TM-polarized incident light pulses. We highlight and discuss for the first time the relative roles bound electrons and an intensity dependent free electron density (hot electrons) play in third harmonic generation. While planar structures are generally the simplest to fabricate, metal layers that are only a few nanometers thick and partially transparent are almost never studied. Yet, transmission offers an additional reference point for comparison, which through relatively simple experimental measurements affords the opportunity to test the accuracy of available theoretical models. Our experimental results are explained well within the context of the microscopic hydrodynamic model that we employ to simulate second and third harmonic conversion efficiencies, and to simultaneously and uniquely predict the nonlinear dispersive properties of a gold nanolayer under pulsed illumination. Using our experimental observations and our model, based solely on the measured third harmonic power conversion efficiencies we predict |chi3|~10^(-18)-10^(-17)(m/V)^2, triggered mostly by hot electrons, without resorting to the implementation of a z-scan set-up.
We construct updated solar models with different sets of solar abundances, including the most recent determinations by Asplund et al. (2009). The latter work predicts a larger ($sim 10%$) solar metallicity compared to previous measurements by the same authors but significantly lower ($sim 25%$) than the recommended value from a decade ago by Grevesse & Sauval (1998). We compare the results of our models with determinations of the solar structure inferred through helioseismology measurements. The model that uses the most recent solar abundance determinations predicts the base of the solar convective envelope to be located at $R_{rm CZ}= 0.724{rm R_odot}$ and a surface helium mass fraction of $Y_{rm surf}=0.231$. These results are in conflict with helioseismology data ($R_{rm CZ}= 0.713pm0.001{rm R_odot}$ and $Y_{rm surf}=0.2485pm0.0035$) at 5$-sigma$ and 11$-sigma$ levels respectively. Using the new solar abundances, we calculate the magnitude by which radiative opacities should be modified in order to restore agreement with helioseismology. We find that a maximum change of $sim 15%$ at the base of the convective zone is required with a smooth decrease towards the core, where the change needed is $sim 5%$. The required change at the base of the convective envelope is about half the value estimated previously. We also present the solar neutrino fluxes predicted by the new models. The most important changes brought about by the new solar abundances are the increase by $sim 10%$ in the predicted $^{13}$N and $^{15}$O fluxes that arise mostly due to the increase in the C and N abundances in the newly determined solar composition.
We discuss the level of agreement of a new generation of standard solar models (SSMs), Barcelona 2016 or B16 for short, with helioseismic and solar neutrino data, confirming that models implementing the AGSS09met surface abundances, based on refined three-dimensional hydrodynamical simulations of the solar atmosphere, do not not reproduce helioseismic constraints. We clarify that this solar abundance problem can be equally solved by a change of the composition and/or of the opacity of the solar plasma, since effects produced by variations of metal abundances are equivalent to those produced by suitable modifications of the solar opacity profile. We discuss the importance of neutrinos produced in the CNO cycle for removing the composition-opacity degeneracy and the perspectives for their future detection.
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