In the updated APOGEE- Kepler catalog , we have asteroseismic and spectroscopic data for over 3000 first ascent red giants .
Given the size and accuracy of this sample , these data offer an unprecedented test of the accuracy of stellar models on the post-main-sequence .
When we compare these data to theoretical predictions , we find a metallicity dependent temperature offset with a slope of around 100 K per dex in metallicity .
We find that this effect is present in all model grids tested and that theoretical uncertainties in the models , correlated spectroscopic errors , and shifts in the asteroseismic mass scale are insufficient to explain this effect .
Stellar models can be brought into agreement with the data if a metallicity dependent convective mixing length is used , with \Delta \alpha _ { ML,YREC } \sim 0.2 per dex in metallicity , a trend inconsistent with the predictions of three dimensional stellar convection simulations .
If this effect is not taken into account , isochrone ages for red giants from the Gaia data will be off by as much as a factor of 2 even at modest deviations from solar metallicity ( [ Fe/H ] = - 0.5 ) .