Asteroseismic study of helium and heavy element diffusion in solar-type stars

Abstract

Aims. Element diffusion is a basic physical element transport mechanism which induces the redistribution of chemical elements. Using the asteroseismic method, we study the effects of helium and heavy element diffusion on the internal structure and stellar evolution of solar-type stars. We also provide asteroseismic parameters for a grid of models which would be useful for direct comparison with the Kepler mission observations. Methods. We construct a grid of solar-type stellar models with various masses (from 0.8 M to 1.2 M ) and metallicities (Z i = 0.03, 0.025, 0.02, 0.015, 0.01, 0.005) with and without helium and heavy element diffusion. We compute "second differences" and "small separations" of the solar-metallicity models (Z i = 0.02) to analyze the effects of diffusion on the convection zone, helium abundance and the evolutionary sequence of the star. In order to study the asteroseismic property of models with and without diffusion, we compute the p-mode oscillation frequencies of low-degree modes for a grid of models and construct the ( Δν 0 , d 02 ) asteroseismic diagram. Results. We find that the element diffusion could speed up the evolution of the star, especially in the main sequence. The results show that it could enlarge the convective core and change the base of the convection envelope. In addition, the helium and heavy element diffusion make the models evolve to lower large and small separations in the asteroseismic diagram. This effect is more efficient at lower metallicity.

Calculating Asteroseismic Diagrams for Solar-Like Oscillations

“…Several studies have already investigated the impact of the choice of these parameters, including composition, mixing, overshooting and diffusion (e.g. Monteiro et al 2002;Mazumdar 2005;Gai et al 2009), so we have not investigated the breadth of this parameter space with our model grid. Our aim is to investigate the bulk behavior of asteroseismic observables across a wide range of evolutionary states, from the ZAMS to the tip of the RGB.…”

“…Previous studies of the C-D diagram and its variations have determined the expected evolution of stars with varying mass and metallicity (Ulrich 1986;Gough 1987;Christensen-Dalsgaard 1988), and assessed the feasibility of applying the diagram to real data (Monteiro et al 2002;Otí Floranes et al 2005;Mazumdar 2005;Gai et al 2009). However, none of these studies followed the evolution beyond the end of the main sequence.…”

ASTEROSEISMIC DIAGRAMS FROM A SURVEY OF SOLAR-LIKE OSCILLATIONS WITH KEPLER

“…The different dependencies of the large and small separations have led to the idea of plotting one against the other in the so-called C-D diagram (Christensen-Dalsgaard 1984). Theoretical calculations for the C-D diagram have been carried out for main-sequence stars (e.g., Monteiro et al 2002;Otí Floranes et al 2005;Mazumdar 2005;Gai et al 2009;White et al 2011) but prior to Kepler, observations were only available for a small number of stars. An observational C-D diagram has now been produced for red giants using Kepler observations (Bedding et al 2010a;Huber et al 2010).…”

Recent advances in modeling stellar interiors

“…Radiative levitation produces abundance anomalies in main sequence A and F stars (Turcotte 1998b), and concentration of Fe due to levitation + settling is proposed as the mechanism for opacity-induced pulsation driving in the newly discovered classes of helium-burning subdwarf O and B variable stars (see, e.g., Randall et al 2009;Rodríguez-López et al 2010;Hu et al 2009Hu et al , 2010. Diffusion affects stellar age determinations and the location of the main sequence turnoff used for cluster dating, and is included in model grids for the lower main sequence (Dotter et al 2008;Gai et al 2009). Gravitational settling stratifies white dwarf star envelopes into H, He, and C/O layers, and removes all of the metals from visibility on short timescales.…”