Models of Blue Stragglers part IA 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) Different compositions of parents (GS10)

Take result of collision simulations or MMAS/MMAMS

Evolve in time using detailed stellar evolution code

Glebbeek & Pols 2008

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

Effect of Impact Parameter

Sills et al 2005

Same parents, three different impact parameters (0.25, 0.5, 1.0 (R1+R2)

Little structural difference, some difference in internal angular velocity

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

Case A mass transfer (primary on MS)

Tian et al. 2006

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

Different locations in CMD

Lu et al 2011

+ = case A mass transfer

= case B mass transfer

= merged models (case A with smaller mass ratios)

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 (Portegies Zwart & Verbunt 1996), StarTrack (Belczynski et al. 2008)

Easily implemented into dynamics codes to study dynamical impacts on binary evolution

Models of Individual Blue Stragglers

(part II – with some repetition, some clarifications, and a side topic or two)

Alison Sills

(for Melvyn) Luminosity functions

Ferraro et al 2003

Observed luminosity functions (in F255W) for 6 clusters, normalized to turnoff luminosity

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

Luminosity and temperature functions

Ferraro et al 2003

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

Rotation is a problem

Sills et al. 2001

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.

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

Post-MS evolution gives E-BSS

Sills et al. 2009

HB E-BSS

AGB

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)

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

Different locations in CMD

Lu et al 2011

+ = 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?

Mergers from case A

Chen & Han 2008

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?

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 (Portegies Zwart & Verbunt 1996), StarTrack (Belczynski et al. 2008)

Easily implemented into dynamics codes to study dynamical impacts on binary evolution

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?

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 t

alk from yest

erday

Pulsation

Blue stragglers can be SX Phe stars (low metallicity δ Scuti 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)

SX Phe in Local Group

Cohen & Sarajedini 2012

Sub-luminous GC SX Phe stars could have higher helium abundances – from BS formation process or from 2nd generation?

They are blue stragglers.

Most SX Phe stars fit the period-luminosity relation well.

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?

Population mixing changes CMD position, lifetime

Glebbeek, Sills & Leigh 2010M=0.6 M + 0. 4 M

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

Population mixing fits colour distributions better

Same radial distributions?

Lardo et al 2011

Plus NGC 2419 and Cen flat in both blue stragglers and second generation

Red arrows mark measured blue straggler minima

SG

gia

nts

/FG

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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