PHYSICS UNDER THE BONNET OF A STELLAR EVOLUTION

PHYSICS UNDER THE BONNET OF A STELLAR EVOLUTION CODE Richard J. Stancliffe Argelander Institut für Astronomie, Universität Bonn

M, X, Z

Microphysics: Macrophysics: • Reaction rates • Opacities • Equation of state • • Stellar structure Chemical mixing Convection Mass loss Machinery: • Solution method • Resolution

STELLAR STRUCTURE EQUATIONS

CHEMICAL EVOLUTION EQUATIONS Xi + mixing

MACHINERY I Divide star into mesh points rk-1, mk-1, Lk-1 Differential equations become difference equations See Meynet, Maeder, Mowlavi Xk-1, Tk-1, ρk-1 (2004) Information not always available where you want it! Choices have to be made – not always obvious what’s correct Xk , T k , ρ k rk, mk, Lk rk+1, mk+1, Lk+1 Xk+1, Tk+1, ρk+1

MACHINERY II Solve all equations together (simultaneous) or do some then others (non-simultaneous)? Stancliffe (2006) Have to get time and space resolution correct! And then it all has to converge… Lattanzio et al. (2015)

CONVECTION Important for two things: Energy transport Mixing of material Nearly always mixing length theory Needs calibration! Where do convective boundaries go – Schwarzchild vs. Ledoux? Magic et al. (2015)

CONVECTIVE OVERSHOOTING Most common form of ‘extra mixing’ Parameterisation of our ignorance of convective boundaries Excellent review of physics by Viallet et al. (2015) Needs calibration… and it might not be the same for all phases! Stancliffe et al. (2015)

STARS MESA Stancliffe et al. (2015)

SEISMIC CALIBRATION? KIC 10526294 Slowly pulsating B star (Papics et al. 2014) g-modes probe stellar interior Moravveji et al. (2015) determine f = 0. 017 Based on MESA + GYRE calculations Moravveji et al. (2015)

CORE HELIUM BURNING Overshooting may not be the same at all evolutionary phases Core He burning has long been a problematic phase for stellar evolution Seismology suggests that cores may be larger than models currently predict Constantino et al.

OTHER PHYSICS Gravitational settling Rotation Magnetic fields (Binaries…)

INPUT PHYSICS Equation of state Reaction rates NACRE REACLIB Opacities (Warrick Ball – Fri. ) OPAL Type I or Type II OP

SOUNDS SIMPLE, RIGHT?

Stancliffe et al. , submitted

CALIBRATION Stellar models still calibrated on one thing: the SUN Grevesse & Sauval 98 But which Sun? And what parameters are included in the calibration? And what physics do you include? Asplund 09

WARNINGS! Are your tracks properly calibrated? Are you using the correct metallicity? With an appropriate opacity table? Don’t forget the uncertainty – compare different codes where possible!

BEYOND THE MAIN SEQUENCE More problems occur beyond the main sequence Effective temperature of giant branch depends on boundary conditions May be uncertain to 100 K See also: Vanden. Berg et al. (2008), Salaris & Cassisi (2015) Dotter et al. (2007)

CONCLUSIONS Don’t treat stellar codes like black boxes! Perform resolution tests How calibrations are done is important Where are they valid? Seismology to help with free parameters?