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Models of Blue Stragglers part I

Models of Blue Stragglers part I. A talk for Hans Zinnecker (sort of). Alison Sills, McMaster University. Collision Models. Sills (1997-2009), Glebbeek (2008-2010) head-on (S97, G08) and off-axis collisions (S01) With rotation (S05) Post-main sequence (S09)

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Models of Blue Stragglers part I

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  1. Models of Blue Stragglers part I A talk for Hans Zinnecker (sort of). Alison Sills, McMaster University

  2. Collision Models • Sills (1997-2009), Glebbeek (2008-2010) • head-on (S97, G08) and off-axis collisions (S01) • With rotation (S05) • Post-main sequence (S09) • Different compositions of parents (GS10) • Take result of collision simulations or MMAS/MMAMS • Evolve in time using detailed stellar evolution code

  3. Collision products look like normal stars (almost) Red: collision product Blue: normal star of same mass Lifetimes of collision products shortened by a factor that depends on evolutionary history of parents Glebbeek& Pols 2008

  4. Effect of Impact Parameter Same parents, three different impact parameters (0.25, 0.5, 1.0 (R1+R2) Little structural difference, some difference in internal angular velocity Sills et al 2005

  5. Binary Mass Transfer Models • Deng, Chen, Han (2004-2011) • Binary evolution with a stellar evolution code simultaneously evolving both components • Stability of mass transfer depends on structure of donor (see Natasha’s talk tomorrow) • Stable mass transfer calculated directly (mass moved from star 1 to star 2, then both stars evolved over each timestep) • Dynamical mass transfer assumed to produce a fully mixed merger product • Case A, case B, case C calculated • Solar metallicity – most compared to M67

  6. Case A mass transfer (primary on MS) Evolution is complicated, but stars spend significant time in BS region Outcome depends on masses and initial period Mass transferring binaries lie between dashed line and ~giant branch Tian et al. 2006

  7. Different locations in CMD + = case A mass transfer = case B mass transfer = merged models (case A with smaller mass ratios) Lu et al 2011

  8. Parameterized models • BSE (Hurley et al 2002) • Binary evolution followed using (semi-)analytic prescriptions for stellar evolution and interaction events • Collisions – product is fully mixed, current age set by average of parents’ time along MS • Mass transfer – if Rstar> RRoche, with timescale determined by structure of two stars • Wind accretion – Bondi-Hoyle accretion of primary’s stellar wind • All evolutionary states of stars included • Also common envelope evolution, mass loss, collisions between non-MS stars, tidal evolution, angular momentum loss mechanisms…. • Similar codes: SeBa (PortegiesZwart & Verbunt 1996), StarTrack (Belczynski et al. 2008) • Easily implemented into dynamics codes to study dynamical impacts on binary evolution

  9. Models of Individual Blue Stragglers (part II – with some repetition, some clarifications, and a side topic or two) Alison Sills

  10. (for Melvyn) Luminosity functions Observed luminosity functions (in F255W) for 6 clusters, normalized to turnoff luminosity Ferraro et al 2003

  11. Collision Models • Sills (1997-2009), Glebbeek (2008-2010) • head-on (S97, G08) and off-axis collisions (S01) • With rotation (S05) • Post-main sequence (S09) • Different compositions of parents (GS10) • Take result of collision simulations or MMAS/MMAMS • Evolve in time using detailed stellar evolution code

  12. Luminosity and temperature functions Collision tracks for variety of parent mass combinations, single-binary interactions + likelihood of collision, drawn from IMF, assumed binary fraction….. We predicted that the binary fraction in NGC 288 was much higher than in M80 Ferraro et al 2003

  13. Rotation is a problem Same collision product, but with initial angular velocity divided by factor of 5, 10, 100, and 1000. If velocity not reduced, star spins up past break-up during descent to main sequence Rotational mixing: helium to surface, hydrogen to core. Long, blue life. Sills et al. 2001

  14. A possible spin-down mechanism? • Start with off-axis collision, and evolve in YREC. Spins up as it contracts. Outer layers hit break-up and are lost. • If star has a magnetic field, then we can lock it to a disk after first 0.1 Mis lost (invoked for young low M stars in open clusters) • Still spins faster than a normal star of the same mass – but not so much mixing Sills, Adams & Davies 2005

  15. Post-MS evolution gives E-BSS Collision tracks for different combinations of parent stars Points are 107 years apart HB and AGB determined by M=0.8 M track E-BSS box determined from observations in 3 clusters (M3, M80, 47 Tuc) AGB E-BSS HB Sills et al. 2009

  16. Binary Mass Transfer Models • Deng, Chen, Han (2004-2011) • Binary evolution with a stellar evolution code simultaneously evolving both components • Stable mass transfer calculated directly (mass moved from star 1 to star 2, then both stars evolved over each timestep) • Dynamical mass transfer assumed to produce a fully mixed merger product • Case A, case B, case C calculated • Solar metallicity – most compared to M67

  17. Different locations in CMD + = case A mass transfer = case B mass transfer = merged models (case A with smaller mass ratios) Are mergers the explanation for the blue sequence in M30? Lu et al 2011

  18. Mergers from case A Monte-Carlo model for M67 (dashed line is not ZAMS?) Monte-Carlo model for NGC 2660 (1.2 Gyr) If mergers really are fully mixed, then they’ll lie ~on the ZAMS – inconsistent with NGC 188, M30, and other GCs. Environmental effect? Chen & Han 2008

  19. Parameterized models • BSE (Hurley et al 2002) • Binary evolution followed using (semi-)analytic prescriptions for stellar evolution and interaction events • Collisions – product is fully mixed, current age set by average of parents’ time along MS • Mass transfer – if Rstar> RRoche, with timescale determined by structure of two stars • Wind accretion – Bondi-Hoyle accretion of primary’s stellar wind • All evolutionary states of stars included • Also common envelope evolution, mass loss, collisions between non-MS stars, tidal evolution, angular momentum loss mechanisms…. • Similar codes: SeBa (PortegiesZwart & Verbunt 1996), StarTrack (Belczynski et al. 2008) • Easily implemented into dynamics codes to study dynamical impacts on binary evolution

  20. Do the models get things right? • Position in CMD: yes (by definition)…..in a broad sense • Mass: Well….. • Rotation rate: Basically no What else could we look at?

  21. Surface Abundances • Different formation mechanisms should produce different surface abundances • Collisions: probably remove lithium, but otherwise little/no surface abundance differences • Binary mass transfer: depends on the time of mass transfer and masses of primary/secondary • Need to be careful about effects of subsequent evolution on abundances Evert’s talk from yesterday

  22. Pulsation • Blue stragglers can be SX Phe stars (low metallicity δScuti stars (dwarf Cepheid stars)) – radial pulsators • Pulsations can give us fundamental stellar parameters such as mass, composition, etc. • Few models (Santolamazza et al. 2001, Templeton et al. 2002) concerned with location of instability strip, not individual properties • First large compilation of data for SX Phe’s in GCs (263 SX Phe stars in 46 GCs)

  23. SX Phe in Local Group They are blue stragglers. Most SX Phe stars fit the period-luminosity relation well. Cohen & Sarajedini 2012 Sub-luminous GC SX Phe stars could have higher helium abundances – from BS formation process or from 2nd generation?

  24. Side topic: Link to Multiple Populations? • Two generations of stars in globular clusters? • Second generation is enriched with the products of hot hydrogen burning (enhanced He but not C+N+O) • Any connection to blue stragglers?

  25. Population mixing changes CMD position, lifetime M=0.6 M + 0. 4 M Glebbeek, Sills & Leigh 2010

  26. Population mixing fits colour distributions better Luminosity function and colour distribution of blue stragglers in NGC 2808 best fit by mixed population of Y=0.24 and Y=0.32 parent stars (solid blue lines) Results for other clusters consistent with their inferred second generation populations Glebbeek, Sills & Leigh 2010

  27. Same radial distributions? SG giants/FG giants Plus NGC 2419 and  Cen flat in both blue stragglers and second generation Red arrows mark measured blue straggler minima Lardo et al 2011

  28. Individual models: what is needed? • Binary models: (much) more parameter space coverage • Mergers (Coalescence): are they really fully mixed? • Collisional models: do we need multiple He parent populations? Do we need them at all? • Pulsation properties look like a useful way of getting information out of blue stragglers – need specific models • Magnetic fields, anyone? (Bob?) • But we do need to deal with angular momentum redistribution/loss for both collisions and mergers

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