Multiple stars: physics vs. dynamics

1. Multiple stars: physics vs. dynamics R. Zhuchkov Kazan University V. Orlov, A. Rubinov St. Petersburg University

2. Overview 1. Introduction 2. Classification of multiple stars 3. Formation of multiple stars 4. Statistics of physical and dynamical properties: • Multiplicity function • Orbital parameters • Mass ratios • Hierarchy degree 5. Possible unstable systems and their origin 6. Systems with different age components 7. Conclusions

3. Classification 1. Non-Hierarchical (Trapezium type) Trapezium of Orion Probably unstable 2. Hierarchical ( Lyrae type)  Lyr Probably stable 3. Low-Hierarchical HD 40887 (see poster of Orlov et al.) ??? Note:apparent configuration does not coincide with true configuration (projection effect)

4. Formation scenaria of multiple stars • Escape from unstable non-hierarchical small groups (Larson, 2001) or clusters • Formation as stable or unstable unit (mostly in binary and triple systems according to Goodwin & Kroupa, 2005) • Capture in galactic field or in field of common gas-star complex

5. Statistics of physical and dynamical properties Multiplicity function (Tokovinin 2001) Nnis the number of systems with n components. For small group decay scenario (about 20000 runs): f3 = 0.21; For solar neighborhood of 10 pc: f3 = 0.2  Good agreement

6. Distributions of orbital elements: Period distributionSolar type Spectroscopicbinaries binaries Duquennoy and Mayor 1991Halbwachs et al. 2004

7. Distributions of orbital elements: Period-Eccentricity diagramsSolar type Spectroscopicbinaries binaries Duquennoy and Mayor 1991Halbwachs et al. 2004

8. Distributions of mass ratio: Spectroscopic binariesHalbwachs et al. 2004For pairs withP > 50d maxima are q  0.25 and0.6.WhenP < 50d maximum is q  1 («twins»).

9. Distributions of orbital eccentricity Small group decay(Rubinov et al. 2002)Solid line – f(e)=2eWide visual binaries (Tokovinin 1998)f(e)=2e

10. Period-eccentricity diagram Small group decay(Rubinov et al. 2002)Solar type binaries (Duquennoy and Mayor 1991)

11. Distributions of mass ratiosSmall group decay (Rubinov et al. 2002)white – final binariesgray – escaping binariesfor Salpeter initial mass spectrumSpectroscopic binaries(Halbwachs et al. 2004)

12. Triple stars (comparison of simulations and observations) Hierarchy degree and eccentricities Rubinov et al. 2002 Observations Simulations

13. Triple stars (comparison of simulations and observations) Inclination of outer and inner binaries Sterzik & Tokovinin 2002

14. Sample of 18 multiple systems with measured orbital elements of subsystems 16 triple systems (close binaries withP<10d were considered as single component); 2 quadruple systems. Methods for dynamics study 1. Stability criteria for triple systems. 2. Numerical simulations for all systems. Error effect Monte Carlo approach - Variation of orbital elements and masses assuming Gaussian error distribution (1000 runs for each system).

15. Stable (black points) & unstable (red) configurations of stable (HD 198183) and probably unstable (HD 136176) systems at the Z(Z) plane

16. Results 1)13systems are probably stable (escape probability during 106 yris less than10%). 2) 5 systems are probably unstable (escape probability during 106 yris greater than90%): HD 40887 (Gliese 225.2) – probably quadruple, HD 76644 ( Uma = ADS 7114) – quadruple, HD 136176 (ADS 9578) – triple, HD 150680 (ADS 10157) – astrometric triple, HD 222326 (ADS 16904) – triple.

17. Possible reasons for appearance of unstable low-hierarchical multiple stars • Errors of observations and interpretation. • Physical youth of components. • Some additional effects are responsible for the physical stability of the system (mass loss etc.). • Some additional effects led to the formation of the unstable system (merging etc.) • Temporary capture via encounter of binary (multiple) system and single (multiple star). • Stability loss via encounter of stable multiple star with a massive object (molecular cloud, black hole etc.). • Product of dissipation of stellar group or cluster. Expectednumber of unstablesystems within a sphere of 200 pc around the Sun for scenarios 5-7 is about 110(Pout < 103 yr).

18. Physics of multiple starsMultiple stars are the best astrophysical laboratories for theory testing (formation, evolution, dynamics)• Direct measurements of parameters for components and system as a whole are possible. • Majority of stars were formed within groups (multiple stars, clusters, associations) (Larson, 2001). • “Evaporation” from such non-hierarchical groups takes place until stable configuration is formed (binary or hierarchical multiple).• Most of the stars could be formed in binary and triple stars (Goodwin & Kroupa, 2005).

19. Physics of multiple starsLet’s note thatA few wide multiple stars can have components of different ages (Popper, 1997). What is the reason? – Imperfection of star models (especially for low mass stars), capture and/or merging scenarios.

20. Conclusions 1. One can separate multiple stars into high-hierarchical, low-hierarchical, and non-hierarchical. 2. High-hierarchical systems are long-term stable,non-hierarchical systems usually disrupt, and low-hierarchical systems may be as stable, as unstable. 3.Most of OBSERVED multiple systems ARE stable, but some of them MIGHT NOT be so. 4.A few scenarios of their instability are suggested.