1. from an evolutionary point of view Selma de Mink Utrecht University Lorentz Center Workshop “Stellar Mergers” Ines Brott (Utrecht), Matteo Cantiello (Utrecht), Joke Claeys (Utrecht) Evert Glebbeek (Hamilton), Adrian Hamers (Utrecht), Rob Izzard (Brussels), Norbert Langer (Bonn), Onno Pols (Utrecht), Sung-Chul Yoon (Bonn)

2.  Massive stars o Cosmic engines, shape the universe  Stellar winds  UV flux  SN explosions o Formation and evolution poorly understood o Very high fraction >50% in close binaries  Mergers from binaries o In contrast to mergers from collisions o Binary mergers dominate in open clusters / loose OB associations o Interaction before merging

3. Binary evolution before Binary evolution before Evolution merger after Merger Which binaries evolve into contact? What is their evolutionary status? What are the main uncertainties? Which binaries evolve into contact? What is their evolutionary status? What are the main uncertainties? Observational properties, life-time? For clusters: how many “blue stragglers”? Do they end their life, as SNe or GRBs? Observational properties, life-time? For clusters: how many “blue stragglers”? Do they end their life, as SNe or GRBs? Mass loss? Mixing? Mass loss? Mixing?

4. 1.Initial distributions 2.Evolution into contact 4. Rotationally induced mixing in (near) contact systems 1.Initial distributions 2.Evolution into contact 4. Rotationally induced mixing in (near) contact systems 3. Effects of rotationally induced mixing

6. Observed binary fraction Courtesy H. Sana Consistent with f min = 0.5

7. Single starsBinary stars  Mass  (Metallicity )  (Rotation Rate )  Mass primary  Mass ratio  Orbital period  (Eccentricity)  (Metallicity)  (Rotation rates 2x)

8. Data for six open clusters and OB associations ~50 % of the objects is detected a spectroscopic binary Proceedings paper: Sana et al. 2009 Log (Period) Mass Ratio

9. Data for six open clusters and OB associations ~50 % of the objects is detected a spectroscopic binary Proceedings paper: Sana et al. 2009 Log (Period) Mass Ratio Flat in log P? -> over abundance of systems with P<10 days Flat in log P? -> over abundance of systems with P<10 days Flat in q ? For q =0.3-1.0 Flat in q ? For q =0.3-1.0

10.  Challenges o Selection effects o Evolutionary effects  Opportunities o VLT-flames Tarantula survey o 1000 Massive stars o Designed to detect binaries

12.  Binary models tell us: o Which binaries come into contact? o When do they come into contact? o What are the properties of both stars at the moment of contact?  Chemical profile  Density / entropy profile Step 1 Step 2

14.  Case A o P orb <5 days o Donor: main sequence star  Case B o P orb = 5 - ~ 500? days o Donor: Hertzsprung gap: H shell burning  Case C o Not important for massive stars (at solar metallicity)  Stellar wind mass loss widens orbit  Massive stars never become giants

15. Z=Z , M 1 =12M  Wellstein, Langer, Braun 2001 Mass ratio M 2 /M 1 Log orbital period (d)

16. Z=Z SMC, M 1 =25M  De Mink, Pols, Hilditch 2007  From a grid of ~20.000 binary models computed for comparison with observed eclipsing binaries Conservative Mass transfer Conservative Mass transfer

17. Z=Z SMC, M 1 =25M  De Mink, Pols, Hilditch 2007  From a grid of ~20.000 binary models computed for comparison with observed eclipsing binaries Non-conservative Mass transfer Non-conservative Mass transfer

18.  Which systems come into contact? o How much mass is accreted/lost form the system o Implementation: when? Associated angular momentum loss? o Entropy accreted material!  How long can the contact configuration last? o Low mass contact systems, W Uma o What evolutionary processes play a role? Mixing?  Does contact imply a merger? o Slow contact : yes

19. Case AMergers are Main Sequence stars Slow contactShort periods M 2 ~ M 1 Equal masses -> massive mergers Equal entropy profiles -> mixing Rapid contactM 2 /M 1 < q crit Compared to “slow contact” More frequent Less massive Case B Mergers become helium burning stars Rapid contact (Early Case B) M 2 /M 1 < q crit Compared to Case A mergers More frequent Shorter life times Population synthesis of Case A mergers  Adrian Hamers

21. Convective Core Meridional circulation  Rotational “instabilities” mix rotating massive stars  Eddington-Sweet circulation most efficient process  Mixing process on t KH

22. Helium at the surface (mass fraction) Initial Yoon et al 2006

23. Fast rotator: Time Chemically Homogeneous Standard Evolution e.g. Maeder 87, Yoon & Langer 05 Slow rotator: Bifurcation :

24. R~1000 Rsun RSG R~1 Rsun WR Fast rotator Slow rotator

25. Yoon, Langer & Norman, 2006

27. Chemically Homogeneous Standard Evolution Time

28. Single star evolution track

29. 1.7 days Roche lobe overflow Z = 10 -5 M 1 ~M 2 ~100M 

30. 1.7 days Z = 10 -5 M 1 ~M 2 ~100M 

31. 1.7 days 1.4 days 1.2 days core H-burning Z = 10 -5 M 1 ~M 2 ~100M  H-shell burning

32. 1.7 days 1.4 days 1.2 days 1.15 days Start He-burning Z = 10 -5 M 1 ~M 2 ~100M  Core H-burning

34. Binary evolution before Binary evolution before Evolution merger after Merger