1. ARC Meeting Multiplicity of O-type stars in the young open cluster NGC 2244 Laurent Mahy (GAPHE)

2. Why study the multiplicity of O-stars in clusters? They have same age, same distance, and same chemical composition The knowledge of formation mechanisms of high masses stars are still fragmented (stellar collisions and mergers, accretion disk, accretion by radiative pressure,...) A further complication to any model of massive star formation is that most massive stars are in binary systems Study the multiplicity is interesting for understanding the physical properties of these objects

3. Multiplicity in NGC 2244 and Mon OB2 A first study was made by Garcia & Mermilliod (2001) concerning rich O-star clusters from HETEROGENOUS DATA 1. IC 1805 2. NGC 6231 3. NGC 2244 4. IC 2944 5. NGC 6611 6.... 80% 79% 50% 44% 42%... What are our conclusions in NGC 2244 and the other clusters from homogenous data?

4. Multiplicity in NGC 2244 and Mon OB2 In NGC 2244 and Mon OB2 (Mahy et al. 2009): Our dataset: 207 spectra over 9 years coming from: The 1.52m telescope @ OHP (France) with Aurelie The 1.93m telescope @ OHP (France) with Elodie The 2.1m telescope @ SPM (Mexico) with Espresso The 2.2m telescope @ La Silla (Chile) with FEROS The 1.98m telescope @ Asiago (Italie) Covers long and short timescales !!!

5. Multiplicity in NGC 2244 HD 46149 : From RVs: M1/M2 ~ 1.6 From Equivalent widths: L1/L2 ~ 2.3 Early B-type for the secondary while the primary is estimated to be O8V

6. Multiplicity in NGC 2244 HD 46150: TVS (Temporal Variance Spectrum, Fullerton 1996) revealed variations but no secondary component was found Binary candidate V sin i ~ 100 km/s Classified as O5.5 V ((f))

7. Multiplicity in NGC 2244 HD 46202 & HD 46223: HD 46202 O9V V sin i ~ 50 km/s HD 46223O4V ((f+)) V sin i ~ 100 km/s km/s TVS Probably single stars

8. Multiplicity in NGC 2244 HD 46056 & HD 46485: Classified as O8V V sin i ~ 350 km/s Classified as O8V V sin i ~ 300 km/s Probably single stars

9. Multiplicity in Mon OB2 What is the situation? 2 Binary systems have been already detected: HD 47129 (Plaskett's star, Linder et al. 2008) RLOF with secondary (giant/Main-Sequence) = fast rotator ~ 300 km/s and primary (supergiant/giant) is overabundant in Nitrogen Orbital period ~ 14.3 days Brightness ration ~ 1.9 HD 48099 (Mahy et al. 2010) Primary is a fast rotator and enhanced in Nitrogen Both stars are on Main-Sequence band Orbital period ~ 3.078 days Brightness ration ~ 4.0 HD 46966 and HD 48279 do not show any variation in their spectrum

10. Multiplicity in Mon OB2 HD 46573 : SB 1 system with V sin i ~ 100 km/s And the primary is classified as O7.5V ((f)) Orbital period = 10.67 days Amplitudes are really small !!! (TVS)

11. 1. IC 1805 2. NGC 6231 3. NGC 2244 4. IC 2944 5. NGC 6611 6.... 80% 79% 50% 44% 42%... Remind you the results of Garcia & Mermilliod (2001) What about our results : 1. IC 1805 20-60% (4 stars not studied,1 short,1 long period binary) Rauw & De Becker 2004; De Becker et al. 2006 2. NGC 6231 63-81% (Sana et al. 2008) 3. NGC 2244 17-33% (Mahy et al. 2009) 4. IC 2944 ??? 5. NGC 6611 44-67% (Sana et al. 2009) What about the binary fraction in young clusters?

12. Link between different clusters? Anticorrelation between the number of binaries and the density of clusters was proposed by Garcia & Mermillod (2001) But, after our study, the correlation has appeared to be more probable However, the recent investigation made by Sana et al (2009) on the multiplicity of NGC 6611 does not confirm the last assumption

13. Future works Investigation of binary fraction of O-type stars in OB associations which are less dense that open clusters. Determination of the stellar and wind parameters of all these stars by using the atmosphere code CMFGEN (Hillier & Miller 1998). Investigation of the CoRoT light curves of 4 stars of this sample to find periodic variations.