Poster Abstracts

Name/Affiliation:  Derek Homeier (CRAL/ENS-Lyon)

Title:  The Lyon-Exeter grid of (sub)stellar evolution models

We present a new grid of evolutionary calculations for low-mass stars and brown dwarfs coupled to the latest PHOENIX atmosphere models and their synthetic spectra and colours. This represents the first full update of the Lyon evolution tracks and isochrones since the work of Baraffe et al. (1998) and its extensions (2003). The interior models are using updated 1D atmospheric structures as an outer boundary condition, which have been calibrated with an improved mixing length theory treatment of the convection in optically thin layers. The outer photosphere and deep adiabat thus succeed in matching thermal profiles extracted from 2D and 3D radiative hydrodynamic models to an unprecedented level of accuracy. This coupling between evolution and atmosphere models provides a consistent treatment of the fully convective envelopes of late-M stars and substellar objects, e.g. with matching choices for the mixing length, and compositions updated to the solar abundances of Asplund et al. (2009) and Caffau et al. (2011). Our new model grid shows notably different results both compared to the models of Baraffe et al. (1998) and calculations combining updated atmosphere models with the older evolution tracks. We find marked improvement on comparing the resulting isochrones to colour-magnitude diagrams of disk stars as well as young clusters, although some residual discrepancies remain due to incompletely known opacities in the optical and near the onset of dust formation. But the PHOENIX Settl atmosphere models paving the way for the extension of these calculations to the lowest mass brown dwarfs already uniquely reproduce the L/T transition. The colours of cooler T and Y dwarfs are also matched with improved accuracy thanks to the inclusion of low-temperature grains as additional opacity sources. All of these condensates, down to planetary cloud species like water ice and ammonium hydrosulphide, are treated in a single modelling framework.