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

M, X, Z

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

STELLAR STRUCTURE EQUATIONS

CHEMICAL EVOLUTION EQUATIONS XiXi + mixing

MACHINERY I  Divide star into mesh points  Differential equations become difference equations  See Meynet, Maeder, Mowlavi (2004)  Information not always available where you want it!  Choices have to be made – not always obvious what’s correct r k, m k, L k r k+1, m k+1, L k+1 r k-1, m k-1, L k-1 X k, T k, ρ k X k+1, T k+1, ρ k+1 X k-1, T k-1, ρ k-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  Slowly pulsating B star (Papics et al. 2014)  g-modes probe stellar interior  Moravveji et al. (2015) determine f =  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. (2015)

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  But which Sun?  And what parameters are included in the calibration?  And what physics do you include? Asplund 09 Grevesse & Sauval 98

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 100K  See also: VandenBerg 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?