Using the CIFIST grid of CO⁵BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours

Abstract

Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities.Aim. We aim to study, in a quantitative way, the impact of granulation on colours.Methods. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2, 3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a “3D correction” that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models.Results. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01 mag, especially for colours involving blue and UV bands. We have investigated the Gaia and HIPPARCOS photometric systems and found that the (G − Hp), (BP − RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic.Conclusions. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F, G, K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering.

Probing the existence of very massive first stars

“…Both photometric surveys can be usefully complemented by the Gaia spectrophotometry. Furthermore, when the Gaia RVS photometry becomes available it has been shown that it can provide a very powerful metallicity diagnostic for F,G and K stars (Bonifacio et al 2018a). It thus seems that a pre-selection of metal-poor giants is indeed possible.…”

Systematic investigation of chemical abundances derived using IR spectra obtained with GIANO

“…For each calibration used to derive the effective temperature, we derived the corresponding surface gravity from the Stefan-Boltzmann relation using the distances from the parallaxes provided by Gaia DR2 and the stellar absolute luminosity, derived from the Gaia G band and a bolometric correction based on three-dimensional (3D) model atmospheres (Bonifacio et al, 2018). We assumed the mass of each star according to its tem- perature, by looking at the isochrones.…”

GaiaData Release 2

“…Considerable efforts have been made in recent years to evaluate these effects, which strongly depend on the properties of the stars. The effects of the convective shift on the wavelength of all the spectral lines observed in all kinds of late-type stars will be taken into account by using synthetic spectra computed with 3D hydrodynamical models (Bonifacio et al 2018;Chiavassa et al 2018). The determination of RVs is also affected by the gravitational redshift that results from the deformation of the light path in the potential of the observed star.…”