1. The extend of convective cores in massive stars
Convective core overshooting
Dany Vanbeveren
Vrije Universiteit Brussel + K. U. Leuven
Leiden, 2015

2. A problem that exists already 3-4 decades
2500 stars (Observational data from Humphreys & McElroy, 1984)
The same is visible in LMC data

3. C. Doom, 1985
No overshooting
Large overshooting
Conclusion (1985): the observed width of the main sequence of massive stars and the location of turn off points of some well defined clusters seem to indicate large convective core overshooting (much larger than the standard a = 0.2 formalism promoted by the Geneva team, however the data are rather old.

4. Blue line: the observed width of the main sequence proposed by Castro et al., 2014
based on ~ 600 stars collected from different sources ( not homogeneous)

5. Overshooting distance calibrated by comparing the theoretical width of the main sequence with the observed one of Castro et al. (2014)
10 Mo a = 0,25 (similar to Ekstrom et al. 2012)
15 Mo a = 0,35 (similar to Brott et al. 2011)
30 Mo a = 0,40
but…

6. The evolution of a 20 Mo star (small compared to large overshooting)
Warning:
The evolution of a 20 Mo star (small compared to large overshooting)
Time (Myr) Xc log Teff
Conclusion
The observed width of the main sequence may not be a very good overshooting calibrator
Log Teff

7. Binaries as test beds to calibrate the amount of overshooting
The eclipsing HMXB Vela X-1 : optical component = B0.5Ib
Joss and Rappaport (1984)
M = 21,5 – 26,5 Mo (binary dynamics)
R = 28 – 35 Ro (X-ray eclipse)
vrot ≈ 125 km/s
Vanbeveren et al. (1994) (Hg, HeI 4388, HeII 4200, using the NLTE model atmospheres of Herrero et al., 1992)
Teff = K
= NHe/(NH+NHe) = 0,3

8. Vanbeveren et al., 1998
accretion induced full mixing +
large convective core overshooting

9. Discussion point
The observed width of the main sequence may not be a very good convective core overshooting calibrator, binary test beds may shed some light, of course astro-seismology but still very few results.