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Mar del Plata, Argentina

Deal M.,Toulouse 1 University Capitole | Deal M.,French National Center for Scientific Research | Deheuvels S.,Toulouse 1 University Capitole | Deheuvels S.,French National Center for Scientific Research | And 5 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Recent observations of a large number of DA and DB white dwarfs show evidence of debris disks, which are the remnants of old planetary systems. The infrared excess detected with Spitzer and the lines of heavy elements observed in their atmospheres with high-resolution spectroscopy converge on the idea that planetary material accretes onto these stars. Accretion rates have been derived by several authors with the assumption of a steady state between accretion and gravitational settling. The results are unrealistically different for DA and DB white dwarfs. Aims. When heavy matter is accreted onto stars, it induces an inverse μ-gradient that leads to fingering (thermohaline) convection. The aim of this letter is to study the impact of this specific process on the derived accretion rates in white dwarfs and on the difference between DA and DB. Methods. We solve the diffusion equation for the accreted heavy elements with a time-dependent method. The models we use have been obtained both with the IRAP code, which computes static models, and the La Plata code, which computes evolutionary sequences. Computations with pure gravitational settling are compared with computations that include fingering convection. Results. The most important result is that fingering convection has very important effects on DAs but is inefficient in DBs. When only gravitational settling is taken into account, the time-dependent computations lead to a steady state, as postulated by previous authors. When fingering convection is added, this steady state occurs much later. Conclusions. The surprising difference found in the past for the accretion rates derived for DA and DB white dwarfs disappears. The derived accretion rates for DAs are increased when fingering convection is taken into account, whereas those for DBs are not modified. More precise and developed results will be given in a forthcoming paper © ESO, 2013. Source

Forte J.C.,National University of La Plata | Forte J.C.,CONICET | Vega E.I.,National University of La Plata | Vega E.I.,CONICET | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

As shown in previous works, globular clusters (GCs) can be used to trace the overall structure of the diffuse stellar populations in early-type galaxies if the number of clusters per unit stellar mass depends on metallicity. In this paper we further test this assumption in the galaxy NGC 4486 (M87) by combining several data sources. The results show that GCs allow the mapping of the galaxy in terms of the surface brightness profile, integrated colour gradient, chemical abundance and mass-to-luminosity ratios up to 1000arcsec (or 80.4kpc) from its centre (i.e. some 10 effective radii). The analysis indicates the presence of a dominant high-metallicity bulge associated with the red globulars, whose ellipticity increases outwards, and of a more flattened low-metallicity halo connected with the blue globulars. The chemical abundance gradient of the composite stellar population is remarkably similar to that inferred from X-ray observations of hot gas. The mass-metallicity spectrum of the stellar population can, in principle, be understood in terms of inhomogeneous enrichment models. In turn, the distribution of the bluest GCs, and lowest metallicity halo stars, has an intriguing similarity with that of dark matter, a feature shared with NGC 1399. Also, in these two galaxies, the number of blue GCs per dark mass unit is identical within the errors, ≈1.0(±0.3) × 10 -9. The total stellar mass derived for NGC 4486 is 6.8(±1.1) × 10 11M ⊙ with a baryonic mass fraction f b= 0.08(±0.01). © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

Tecce T.E.,Institute Astronomia y Fisica del Espacio | Tecce T.E.,CONICET | Cora S.A.,CONICET | Cora S.A.,National University of La Plata | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

Using a hybrid method that combines non-radiative hydrodynamical simulations with a semi-analytic model of galaxy formation, we determine the ram pressure experienced by galaxies in haloes with virial masses 12.5 ≤ log(M 200/h -1M ⊙) < 15.35 as a function of the halocentric distance, for redshifts 0 ≤z≤ 3. The ram pressure is calculated with a self-consistent method that uses the simulation gas particles to obtain the properties of the intergalactic medium. The ram pressure profiles obtained can be well described by beta profile models, with parameters that depend on redshift and halo virial mass in a simple fashion. The fitting formulae provided here will prove useful to include ram pressure effects into semi-analytic models based on methods that lack gas physics, such as dark-matter-only simulations or the Press-Schechter formalism. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Bravo E.,Polytechnic University of Catalonia | Althaus L.G.,Polytechnic University of Catalonia | Althaus L.G.,National University of La Plata | Althaus L.G.,Institute Astrofisica Of La Plata | And 3 more authors.
Astronomy and Astrophysics | Year: 2011

Aims. Gravitational settling of 22Ne in cooling white dwarfs can affect the outcome of thermonuclear supernovae. We investigate how the supernova energetics and nucleosynthesis are in turn influenced by this process. We use realistic chemical profiles derived from state-of-the-art white dwarf cooling sequences. The cooling sequences provide a link between the white dwarf chemical structure and the age of the supernova progenitor system. Methods. The cooling sequence of a 1 M⊙ white dwarf was computed until freezing using an up-to-date stellar evolutionary code. We computed explosions of both Chandrasekhar mass and sub-Chandrasekhar mass white dwarfs, assuming spherical symmetry and neglecting convective mixing during the pre-supernova carbon simmering phase to maximize the effects of chemical separation. Results. Neither gravitational settling of 22Ne nor chemical differentiation of 12C and 16O have an appreciable impact on the properties of type Ia supernovae, unless there is a direct dependence of the flame properties (density of transition from deflagration to detonation) on the chemical composition. At a fixed transition density, the maximum variation in the supernova magnitude obtained from progenitors of different ages is ∼0.06 mag, and even assuming an unrealistically large diffusion coefficient of 22Ne it would be less than ∼0.09 mag. However, if the transition density depends on the chemical composition (all other things being equal) the oldest SNIa can be as much as 0.4 mag brighter than the youngest ones (in our models the age difference is 7.4 Gyr). In addition, our results show that 22Ne sedimentation cannot be invoked to account for the formation of a central core of stable neutron-rich Fe-group nuclei in the ejecta of sub-Chandrasekhar models, as required by observations of type Ia supernovae. © 2010 ESO. Source

Folatelli G.,Institute Astrofisica Of La Plata | Folatelli G.,National University of La Plata | Folatelli G.,University of Tokyo | Bersten M.C.,Institute Astrofisica Of La Plata | And 9 more authors.
Astrophysical Journal | Year: 2015

Hubble Space Telescope observations of the site of the supernova (SN) SN 2008ax obtained in 2011 and 2013 reveal that the possible progenitor object detected in pre-explosion images was in fact multiple. Four point sources are resolved in the new, higher-resolution images. We identify one of the sources with the fading SN. The other three objects are consistent with single supergiant stars. We conclude that their light contaminated the previously identified progenitor candidate. After subtraction of these stars, the progenitor appears to be significantly fainter and bluer than previously measured. Post-explosion photometry at the SN location indicates that the progenitor object has disappeared. If single, the progenitor is compatible with a supergiant star of B to mid-A spectral type, while a Wolf-Rayet (W-R) star would be too luminous in the ultraviolet to account for the observations. Moreover, our hydrodynamical modeling shows that the pre-explosion mass was 4-5 Mo and the radius was 30-50 Ro, which is incompatible with a W-R progenitor. We present a possible interacting binary progenitor computed with our evolutionary models that reproduces all the observational evidence. A companion star as luminous as an O9-B0 main-sequence star may have remained after the explosion. © 2015. The American Astronomical Society. All rights reserved. Source

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