1. Stars Collide…… …..and then what? An overview of the advancements in modelling of stellar collisions, filtered through the biases of Alison Sills MODEST-6, Northwestern, August 2005

2. What can you get? MSRGWDNS MSBSRG sdB RG CV LMXB MSP RGRG He WD CV sdB He WD TZ LMXB MSP WDCV He WD SN Ia GRB NS Table stolen from talks by Charles Bailyn, 1995-2000

3. Added a new kind of star PMSMSRGWDNS PMSPMSMSRG TZ MSBSRGsdBRGCVLMXBMSP RGRG He WD CVsdB TZLMXBMSP WDCV GRB NSGRB Simulations done by Laycock & Sills 2005

4. Everything you wanted to know about MS-MS collisions Freitag & Benz 2005 Most masses Most velocities Most impact parameters SPH simulations with ~few thousand particles Results: mass loss, energy loss, deflection angle, angular momentum loss, merger or disruption

5. 0.1 74.2 M 2 in M  M 1 in M  0.174.2 Freitag & Benz 2005 d min V rel

6. What happens after the collision? We already knew that MS-MS collision products retain chemical info about parents, are puffed up and take a ~ thermal time to settle to main sequence (e.g. Sills et al. 1997, 2001) We already knew that MS-MS collision products retain chemical info about parents, are puffed up and take a ~ thermal time to settle to main sequence (e.g. Sills et al. 1997, 2001) Evolution after the collision is like truncated main sequence evolution, and otherwise normal Evolution after the collision is like truncated main sequence evolution, and otherwise normal Angular momentum problem solvable with disks and/or winds (Sills, Adams & Davies 2005) Angular momentum problem solvable with disks and/or winds (Sills, Adams & Davies 2005)

7. What happens with populations of collisions? M67 from Hurley et al: very nice fit to data; no blue stragglers from single-single collision M67 from Hurley et al: very nice fit to data; no blue stragglers from single-single collision (Ultra-)simple cluster models + (slightly more) detailed collision models  distributions of blue stragglers in CMD (Ultra-)simple cluster models + (slightly more) detailed collision models  distributions of blue stragglers in CMD (yes, I’ve been presenting these for a long time, but they’re still really interesting and they still tell us something) (yes, I’ve been presenting these for a long time, but they’re still really interesting and they still tell us something)

8. Core: 0 – 23” Mass function x = -8 Formation time: 7 Gyr ago to now KS test: 93% Outside: 130 – 1200” Mass function x = 1.35 Formation time: 7 – 1.2 Gyr ago KS test: 35% Mid: 23 – 130” Mass function x = -3 Formation time: 7 – 0.6 Gyr ago KS test: 44% Monkman & Sills 2006 young, massive ! older, less massive ? old, light ?? 47 Tuc data from Ferraro et al & Piotto et al HST + ground-based

9. Beyond Blue Stragglers Stripping of giants (Adams, Davies & Sills 2004) – not the dominant way of getting rid of bright giants Stripping of giants (Adams, Davies & Sills 2004) – not the dominant way of getting rid of bright giants Increased interest in mergers/collisions in context of massive star formation (Bally & Zinnecker 0502485, Bonnell & Bate 0506689) Increased interest in mergers/collisions in context of massive star formation (Bally & Zinnecker 0502485, Bonnell & Bate 0506689) Runaway collisions  IMBH (SM & SPZ, MF & FR) – depends on knowing radii of products after collision (Fregeau et al 2004, see Marc’s talk) Runaway collisions  IMBH (SM & SPZ, MF & FR) – depends on knowing radii of products after collision (Fregeau et al 2004, see Marc’s talk) Ultra-compact X-Ray Binaries (see Jamie’s talk) Ultra-compact X-Ray Binaries (see Jamie’s talk)

10. Final parameters for 32 collisions between neutron stars & (sub)giants Common Envelope Mass  Neutron Star Mass  Semimajor axis  Eccentricity  Periastron separation of initial orbit  Lombardi et al 2005

11. The Story So Far We basically understand blue stragglers if they are collision products (and perhaps some of them are….) We basically understand blue stragglers if they are collision products (and perhaps some of them are….) We can model and evolve collisions of non-main sequence stars, and we will if you make us We can model and evolve collisions of non-main sequence stars, and we will if you make us But we’d rather be modeling binary mergers (if we could only figure out how) But we’d rather be modeling binary mergers (if we could only figure out how)