Surface convection and red-giant radius measurements

Abstract

Phenomenological models of convection adopt characteristic length scales that are chosen to fit solar or stellar observations. We investigate whether changes in these length scales are required between the Sun and low mass stars on the red giant branch (RGB). The question is addressed jointly in the frameworks of the mixing length theory and the full spectrum of turbulence model. For both models, the convective length scale is assumed to be a fixed fraction of the local pressure scale height. We use constraints provided by the observed effective temperatures and linear radii independently. We consider a sample of 38 nearby giants and subgiants for which surface temperatures and luminosities are known accurately and the radii are determined by interferometry to better than 10%. We computed dedicated models for the few cases where the stellar masses were determined by asteroseismological measurements. First we calibrated the solar models. With the same physics, we then computed RGB models for masses between 0.9 M and 2.5 M and metallicities ranging from [Fe/H] = −0.34 to solar. The evolution is followed up to 10 3 L . Special attention is given to the opacities and the non-grey atmosphere models used as boundary conditions for which the model of convection is the same as in the interior. For both the mixing length theory and the full spectrum of turbulence models, the characteristic solar length scale for convection has to be slightly reduced to fit the lower edge of the observed RGB. The corresponding models also agree more closely with the expected mass distribution on the RGB and the seismic constraints. These results are robust regardless of effective temperatures determined spectroscopically or radii determined interferometrically are used.

FUNDAMENTAL PROPERTIES OF STARS USING ASTEROSEISMOLOGY FROMKEPLERANDCoRoTAND INTERFEROMETRY FROM THE CHARA ARRAY

“…We find good agreement for all stars for both methods, with a residual mean of −2 ± 2% and +2 ± 2% for (V −K) and IRFM, respectively, both with a scatter of 5%. Our results therefore seem to confirm that the relation of Kervella et al (2004) is also valid for red giants, as suggested by Piau et al (2011), and that combining the IRFM method with asteroseismic constraints, as done by Silva Aguirre et al (2012), yields accurate diameters for both evolved and unevolved stars.…”

“…Examples include the improvement of model physics of stellar structure and evolution such as convection (see, e.g., Demarque et al 1986;Monteiro et al 1996;Deheuvels & Michel 2011;Trampedach & Stein 2011;Piau et al 2011), the calibration of empirical relations such as the color-temperature scale for cool stars (see, e.g., Flower 1996;Ramírez & Meléndez 2005;Casagrande et al 2010), and the characterization of physical Notes. Stars are separated into main-sequence and subgiant stars (top) and red giants (bottom).…”

Seismic sensitivity to sub-surface solar activity from 18 yr of GOLF/SoHO observations

“…The introduction of a magnetic field strength of 1200 G at the bottom of the simulation compatible with the strength of the toroidal field at 0.996 R extracted by Baldner et al (2009) from the seismic observations collected by the Michelson Doppler Imager (MDI; Scherrer et al 1995) on board SoHO, slightly reduces the turbulence and leads to a small decrease in the solar radius of dozens of kilometers at the surface (Piau et al 2011). The l = 0 radial differences at the high-frequency range extracted from the GOLF observations (Fig.…”

The Pisa pre-main sequence tracks and isochrones

“…We also note that there is no reason throughout the whole evolution of the star to assume a constant value of α (see e.g., Siess & Livio 1997;Piau et al 2011, and references therein). In this respect, the progresses in 2D and 3D radiative hydrodynamical simulations of convection in the past decade (Ludwig et al 1999;Trampedach et al 1999;Trampedach 2007) provided evidence of both variations in α, which can depends on both T eff and log g, and the possibility of theoretically calibrating the mixing length parameter (see also .…”