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Perez-Montero E.,Institute Astrofisica Of Andalucia
Monthly Notices of the Royal Astronomical Society | Year: 2014

The derivation of abundances in gaseous nebulae ionized by massive stars using optical collisionally excited emission lines is studied in this work, comparing the direct or Te method with updated grids of photoionization models covering a wide range of input conditions of O/H and N/O abundances and ionization parameter. The abundances in a large sample of compiled objects with at least one auroral line are re-derived and later compared with the χ2-weighted-mean abundances from the models. The agreement between the abundances using the two methods both for O/H and N/O is excellent with no additional assumptions about the geometry or physics governing the HII regions. Although very inaccurate model-based O/H are obtained when no auroral lines are considered, this can be overcome assuming empirical laws between O/H, log U, and N/O to constrain the considered models. In this way, for 12+log(O/H) > 8.0, a precision better than 0.1 dex consistent with the direct method is attained. For very low Z, models give higher O/H values and a high dispersion, possibly owing to the contamination of the low-excitation emission lines. However, in this regime, the auroral lines are usually well detected. The use of this procedure, in a publicly available script, HII-CHI-MISTRY, leads to the derivation of abundances in faint-/high-redshift objects consistent with the direct method based on collisionally excited lines. © 2014 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Claret A.,Institute Astrofisica Of Andalucia
Astronomy and Astrophysics | Year: 2012

Aims. In this paper we investigate the deviations from von Zeipel's theorem at the upper layers of a distorted star. In addition, we derive an analytic expression for the gravity-darkening exponent β 1. Methods. We introduce two different methods to derive the theoretical gravity-darkening exponents. In the first one we use a perturbation theory to derive an analytical expression for the gravity-darkening exponent for slow rotating stars as a function of the rotation law, colatitude, and opacity derivatives. In the second procedure we explore the validity of the mentioned theorem in the upper layers of a distorted star by adapting grey and non-grey atmosphere models to our numerical method, designed originally for stellar envelopes. Results. We have found significant deviations from von Zeipel's theorem when we compute gravity-darkening exponents at the upper layers of a distorted star using our modified numerical method. This is a consequence of using a transfer equation that is more elaborated than the diffusion approach, therefore such a theorem is not strictly valid at lower optical depths. The shifts depend on the optical depth, on the effective temperature, and on the adopted atmosphere models. For large depths, we restore the classical value of β 1, say, 1.0 for hotter stars. Taking such deviations into account, it may be possible to explain the scattering around the theoretical predictions for double-lined eclipsing binaries, as well as the low value of β 1 (0.75), recently inferred for the very fast rotating star α Leo for which the classical von Zeipel's theorem predicts β 1 = 1.0. © 2012 ESO.

Toala J.A.,Institute Astrofisica Of Andalucia | Arthur S.J.,National Autonomous University of Mexico
Monthly Notices of the Royal Astronomical Society | Year: 2014

We carry out high-resolution two-dimensional radiation-hydrodynamic numerical simulations to study the formation and evolution of hot bubbles inside planetary nebulae. We take into account the evolution of the stellar parameters, wind velocity and mass-loss rate from the final thermal pulses during the asymptotic giant branch (AGB) through to the post-AGB stage for a range of initial stellarmasses. The instabilities that form at the interface between the hot bubble and the swept-up AGB wind shell lead to hydrodynamical interactions, photoevaporation flows and opacity variations.We explore the effects of hydrodynamical mixing combined with thermal conduction at this interface on the dynamics, photoionization, and emissivity of our models.We find that even models without thermal conduction mix significant amounts of mass into the hot bubble. When thermal conduction is not included, hot gas can leak through the gaps between clumps and filaments in the broken swept-up AGB shell and this depressurises the bubble. The inclusion of thermal conduction evaporates and heats material from the clumpy shell, which expands to seal the gaps, preventing a loss in bubble pressure. The dynamics of bubbles without conduction is dominated by the thermal pressure of the thick photoionized shell, while for bubbles with thermal conduction it is dominated by the hot, shocked wind. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Luque A.,Institute Astrofisica Of Andalucia
Physical Review Letters | Year: 2014

We investigate the first example of self-consistent impact ionization fronts propagating at relativistic speeds and involving interacting, high-energy electrons. These fronts, which we name relativistic runaway ionization fronts, show remarkable features such as a bulk speed within less than one percent of the speed of light and the stochastic selection of high-energy electrons for further acceleration, which leads to a power-law distribution of particle energies. A simplified model explains this selection in terms of the overrun of Coulomb-scattered electrons. Appearing as the electromagnetic interaction between electrons saturates the exponential growth of a relativistic runaway electron avalanche, relativistic runaway ionization fronts may occur in conjunction with terrestrial gamma-ray flashes and thus explain recent observations of long, power-law tails in the terrestrial gamma-ray flash energy spectrum. © 2014 American Physical Society.

Gordillo-Vazquez F.J.,Institute Astrofisica Of Andalucia
Journal of Geophysical Research: Space Physics | Year: 2010

The vibrational kinetics of air plasmas produced by the presence of sprites in the mesosphere of the Earth is studied in detail. The present model solves the coupled electron Boltzmann equation and rate equations for electrons, neutrals (ground and excited), and ions. Special attention is paid to the vibrational kinetics of N2 (X1 Σg +) and to that of the N2 triplet states (A3 Σu +, B3∏g, C 3∏u, W3Δu, and B' 3Σu +). The results presented are for an altitude of 78 km over sea level where the model-predicted vibrational distribution function (VDF) of N2 (B3∏g) is in agreement with available VDF derived from sprite spectra (640- 820 nm) obtained using spectrographs coupled to high-speed video cameras (300 frames per second (fps)). In addition, the model predictions indicate that the sprite plasma VDFs of N2 (B3∏g) and N2 (C 3∏u) are almost in thermal equilibrium and exhibit a maximum at v = 0 followed by lower values for v = 1 and v = 2. Finally, it is also predicted that the N2 (B3∏g) and N 2(C3∏u) VDFs recorded with very high speed video cameras (up to 10000 fps) should not differ much from the N2 (3∏g) and N2 (C3∏ u) VDFs obtained with cameras working at lower speeds (30 and 300 fps). Copyright 2010 by the American Geophysical Union.

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