Mergers in Massive Binaries

from an evolutionary point of view

Selma de MinkUtrecht 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)

Massive starso Cosmic engines, shape the universe

Stellar winds UV flux SN explosions

o Formation and evolution poorly understoodo Very high fraction >50% in close binaries

Mergers from binarieso In contrast to mergers from collisionso Binary mergers dominate in open clusters / loose OB associationso Interaction before merging

Motivation

Evolutionary point of view

Binary evolutionbefore

Evolution merger after

Merger

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?

Mass loss?

Mixing?

Outline

1. Initial distributions

2. Evolution into contact

4. Rotationally induced mixing in (near) contact systems

3. Effects of rotationally induced mixing

Initial properties of massive binaries

Binary fraction among massive stars

Obse

rved

bin

ary

fra

ctio

n

Courtesy H. Sana

Consistent with fmin = 0.5

Key parameters

Single stars Binary stars

Mass

(Metallicity ) (Rotation Rate )

Mass primary Mass ratio Orbital period

(Eccentricity) (Metallicity) (Rotation rates 2x)

Data for six open clusters and OB associations~50 % of the objects is detected a spectroscopic binary

Cumulative distribution functions

Proceedings paper: Sana et al. 2009

Log (Period) Mass Ratio

Data for six open clusters and OB associations~50 % of the objects is detected a spectroscopic binary

Cumulative distribution functions

Proceedings paper: Sana et al. 2009

Log (Period) Mass Ratio

Flat in log P?

-> over abundance of systems with P<10 days

Flat in q ?

For q =0.3-1.0

Summary: initial binary properties Challenges

o Selection effectso Evolutionary effects

Opportunitieso VLT-flames Tarantula surveyo 1000 Massive stars o Designed to detect binaries

Evolution into contact

Binary evolution

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

Step 1: Evolution into Roche-lobe overflow

Case Ao Porb <5 dayso Donor: main sequence star

Case Bo Porb = 5 - ~ 500? dayso Donor: Hertzsprung gap: H shell burning

Case Co Not important for massive stars (at solar metallicity)

Stellar wind mass loss widens orbit Massive stars never become giants

Step 1

Step 2: Evolution into contact

Z=Z, M1=12M

Wellstein, Langer, Braun 2001

Mass ratio M2/M1

Log

orb

ital peri

od

(d)

Z=ZSMC, M1=25M

De Mink, Pols, Hilditch 2007 From a grid of ~20.000 binary models computedfor comparison with observed eclipsing binaries

Step 2: Case A contact

ConservativeMass transfer

Z=ZSMC, M1=25M

De Mink, Pols, Hilditch 2007 From a grid of ~20.000 binary models computedfor comparison with observed eclipsing binaries

Step 2: Case A contact

Non-conservativeMass transfer

Which systems come into contact?o How much mass is accreted/lost form the systemo Implementation: when? Associated angular momentum loss?

o Entropy accreted material!

How long can the contact configuration last?o Low mass contact systems, W Umao What evolutionary processes play a role? Mixing?

Does contact imply a merger?o Slow contact : yes

Uncertainties

Summary evolution into contactCase A Mergers are Main Sequence stars

Slow contact Short periodsM2 ~ M1

Equal masses -> massive mergersEqual entropy profiles -> mixing

Rapid contact

M2/M1 < qcrit Compared to “slow contact”• More frequent• Less massive

Case B Mergers become helium burning stars

Rapid contact (Early Case B)

M2/M1 < qcrit Compared to Case A mergers• More frequent• Shorter life times

Population synthesis of Case A mergers Adrian Hamers

Mergers and rotational mixing

Fast rotating stars

Convective Core

Meridional circulation

Rotational “instabilities” mix rotating massive stars

Eddington-Sweet circulation most efficient process

Mixing process on tKH

Rotational mixing

Helium at the surface(mass fraction)

Initial

Yoon et al 2006

Slow rotator - fast rotator

Fast rotator:

Time

Chemically Homogeneous

Standard Evolution

e.g. Maeder 87, Yoon & Langer 05

Slow rotator:

Bifurcation :

Bifurcation

R~1000 Rsun

RSG

R~1 Rsun

WR

Fast rotator

Slow rotator

Yoon, Langer & Norman, 2006

Effects of rotational mixing in (near) contact

systems

Binary contextChemically HomogeneousStandard Evolution

Time

Z = 10-5

M1~100M

Single star evolution track

Binary models1.7 days

Roche lobe overflow

Z = 10-5

M1~M2~100M

Zoom in1.7 days

Z = 10-5

M1~M2~100M

Binary models1.7 days

1.4 days

1.2 days

core H-burning

Z = 10-5

M1~M2~100M

H-shell burning

Binary models1.7 days

1.4 days

1.2 days

1.15 days

Start He-burning

Z = 10-5

M1~M2~100M

Core H-burning

Summary

Summary

Binary evolutionbefore

Evolution merger after

Merger