Stellar Interactions in Young Star Clusters

Transcription

1 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 Stellar Interactions in Young Star Clusters Environmental Effects on the Evolution of Protoplanetary Discs Christoph Olczak (1,,4) Rainer Spurzem (1,,3), Susanne Pfalzner (4), Andreas Eckart (4,5) (1) National Astronomical Observatories of China (NAOC), CAS, Beijing, China () The Kavli Institute for Astronomy and Astrophysics (KIAA), PKU, China (3) Astronomisches Rechen-Institut (ARI), University of Heidelberg, Germany (4) I. Physics Institute, University of Cologne, Germany (5) ax-planck-institut für Radioastronomie (PIfR), Bonn, Germany

2 Outline Outline 1. Introduction Star and planet formation Stellar Interactions and the ass Function. Numerical Simulations: Principles and Results Numerical method Encounter-induced disc-mass loss in the ONC Encounter-induced angular-momentum loss in the ONC Capture-induced binarity in the ONC Star-disc encounters in dense and sparse clusters 3. Summary Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 /11

3 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 3/11 Introduction Star and planet formation Some facts about star and planet formation Planets and their hosts: stars form with dusty discs protoplanetary discs protoplanetary discs serve as hosts of planet formation protoplanetary discs last for ~10 yr planets Stars and their hosts: more than 80% of all stars form in clusters (Lada et al., 003) more than 50% of all stars form in massive clusters (N > 1000) star clusters last for 10 yr coronograph Ori HR 8799 IC 348 NGC 3603 O Dell & star Beckwith and (1997) planet formation arois et al. (008) is affected uench by et the al. (003) cluster environment Brandl et al. (001) HR Ori NGC IC O Dell arois & investigation et Beckwith al. (008) (1997) of the effect of stellar uench Brandl. encounters et al. (001) (003) on protopl. discs

4 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 4/11 Introduction Stellar Interactions and the ass Function The dynamically outstanding role of massive stars The effect of stellar encounters is dominated by massive stars twofold: 1. Gravitational focusing b G r enc 1 renc 1 rencv 0 Two different perturbers in the ONC (r enc = 100 e.g. AU, Clarke m & 1 = 0:5 Pringle סּ, v(1993) 0 = 16 / π σ ): Heller (1993, 1995) m θ b/r enc e.g. Pfalzner et al. (005) oeckel & Bally (006) A 50 סּ star focuses a 0.5 סּ star from b = 550 AU to r enc = 100 AU!. ass-ratio dependent perturbation Perturbations (loss of mass, angular momentum, etc.) are strongly massdependent: Equivalent disc-mass loss: 50 סּ perturber at r enc = 500 AU 0.5 סּ perturber at r enc = 100 AU Total disc destruction: 50 סּ perturber at r enc = 100 AU causes 97% disc-mass loss.

5 Numerical Simulations Numerical ethod Realization of the numerical simulations simulations of star cluster dynamics (pure particle model, ~1000 simulations, JUP) direct N-body code (NBODY6++) tracking of encounters record of encounter parameters parameterized fit formula for effect of encounters: - disc mass - angular momentum encounter-induced evolution of star-disc systems in a cluster environment parameter study of star-disc encounters (pure particle model, ~000 simulations, local) tree code Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 5/11

6 Hernandez et al. (008) Numerical Simulations Encounter-induced disc-mass loss in the ONC Encounter-induced disc destruction in the ONC Numerical evolution of the dynamical model of the ONC (age of the ONC 1 yr). Stellar encounters lead to significant disc destruction (> 90% mass loss): - 5% discs destroyed in entire cluster (R =.5 pc) - 0-5% discs destroyed in cluster core (R = 0.3 pc, Trapezium Cluster ) High-mass stars dominate interactions: gravitational foci. Olczak et al. (006), Pfalzner et al. (006) Trapezium Cluster Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 6/11

7 Numerical Simulations Encounter-induced tidal tails in the ONC Encounter-induced angular momentum loss in the ONC Encounter-induced angular momentum loss (AL) in the ONC (at about 1 yr). Stellar encounters lead to 3-5% average AL in the entire cluster Pronounced spiral arm structure triggered in most of the cluster stars. Planet formation via gravitational instabilities might be common. Pfalzner et al. (007a) (see also Rice et al. 004, 006; Clarke & Lodato 009) Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 7/11

8 Numerical Simulations Formation of Transient Binaries Capture-induced binarity in the ONC Three-body encounters lead to the formation of Transient Bound Systems (TBS). assive stars are affected the most (number and duration of bound states). Formation of TBS affects significantly the (apparent/observed) binary rate. - 15% of the 13 OB stars are in a bound state, - 30% of the five most massive stars are in a bound state. Pfalzner & Olczak 007 (see also oeckel & Bally 007a,b) Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 8/11

9 Numerical Simulations Disc destruction in density-scaled models Star-disc encounters in dense and sparse clusters Evolution of cluster disc fraction (CDF) in density-scaled ONC models (ONC = D): Trend as expected: CDF decreases with higher density. Critical density of ONC: -4 times denser system shows prominent effects. In agreement with observations. (Olczak et al. 009) below critical density: high-mass stars dominate disc-mass loss focusing NGC 04 ρ 10 3 סּ pc -3 CDF 85% ONC ρ 10 4 סּ pc -3 CDF 80% decreasing CDF increasing density NGC 3603 ρ 10 5 סּ pc -3 CDF 40% above critical density: low-mass stars dominate disc-mass loss chance encounters Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 9/11

10 Numerical Simulations The precursors of leaky clusters Star-disc encounters in dense and sparse clusters Pfalzner (009): Star clusters are organized in a bimodal sequence in the density-radius plane. I. Starburst clusters II. Leaky clusters Cluster models cover the parameter space of embedded leaky clusters. Star-disc encounters are an important process in this evolutionary phase. Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11

11 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11 Summary Star and planet formation Summary Planet formation and evolution occurs in star clusters. Stellar encounters in a cluster environment Various dynamical effects: - disc destruction probability of planetary systems - formation of disc structure probability of giant planets - formation of transient binaries models of primordial binary population Critical density : - ONC: assive stars act as gravitational foci. - NGC 3603: Low-mass stars dominate disc destruction via chance encounters. The precursors of leaky clusters Density-scaled ONC models represent leaky clusters in their embedded phase. Star-disc encounters are an important process in this evolutionary phase.

12 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, 009 1/11

13 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11 Outlook Outlook Extension of the cluster models: gas potential/expulsion, star formation, primordial binaries optimum: fully realistic cluster model Extension of the star-disc encounter simulations repeated encounters, arbitrary inclinations and eccentricities, optimum: simulation of full disc evolution in cluster environment Project: Heidelberg ARI-ZAH + NAOC + KIAA (R. Spurzem) Aim: Realistic simulation of cluster of star-disc systems Code: NBODY6++ + GPU + gas dynamics + star formation odelling: observations of real star clusters Equipment: 170 GPU cluster Simulations: direct N-body integration of ~10 million particles

14 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11 A New Cluster odel of the ONC A nice side effect: the new model fits better to the observational data. Comparing the surface density distribution of both models: old model new model

15 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11 Results for practical usage in the cluster simulations fit functions have been applied: - D disc / disc : A = 1. B = DL 100 /L 100 : A = 0.55 B = 0.7 these results are similar to the formula of E. Ostriker who calculated the energy and angular momentum transfer using linear perturbation theory (Ostriker 94): for r peri /r disc D 3/ 1 1 5/ 1 3 exp disc peri r r L D exp.8 log 0.5 3/ disc peri disc peri r r B r r A X X

16 Numerical Simulations Numerical models of ONC-like star clusters Disc destruction in different cluster environments Using standard ONC-model for construction of additional models. variation of size, density, and particle number (R, ρ, N) Two families of models: 1. Size-scaled: R ~ N (ρ = const). Density-scaled: ρ ~ N (R = const) In total 11 cluster models with 1k, k, 4k, 8k, 16k, and 3k particles: size-scaled S0 S1 S/D S3 S4 S5 density-scaled D0 D1 (ONC) D3 D4 D5 Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11

17 Numerical Simulations Disc destruction in different cluster environments The density-dependent interaction of low-mass stars How does the interaction character of low-mass stars change with higher density? Comparison of low-mass star dynamics in 1000, 4000, and particle models: low-mass stars become dominant interaction partners low-mass stars interact via (strongly) hyperbolic encounters low-mass stars interact via chance encounters (no focusing!) Christoph Olczak Stellar Interactions in Young Star Clusters December 13, /11