A-type stars: evolution, rotation and binarity

1. A-type stars: evolution, rotation and binarity Arlette Noels, Josefina Montalban Institut d’ Astrophysique et Géophysique Université de Liège, Belgium and Carla Maceroni INAF - Rome Astronomical Observatory, Italy THE A STAR PUZZLE - IAU Symposium 224 Poprad, Slovakia July 8, 2004

2. Fundamental parameters Mass ~ 1.5 – 3 M Teff ~ 7000 – 11000 K B-V ~ 0.0 – 0.30 L ~ 10 – 50 L

3. H burning phase age = 3.12 108 yr age = 3.108 yr age = 8.107 yr

4. Convective core X profile

5. Convective core: temperature profile

6. Convective core

7. Overshooting Dt = 4. 106 tH = 2.2 108 yr tH = 3.0 108 yr Dt = 2. 107 tH = 2.9 108 yr

8. Overshooting Needed to fit CMD for open clusters and eclipsing binaries Increases with mass (Andersen et al. 1990)

9. Overshooting No isothermal core

10. Convective core: temperature profile Isothermal core

11. Overshooting same size of He core

12. Pre-main sequence 1.5 – 4 M Fully convective Fully radiative

13. Formicolaet al. 2004

14. Pre-main sequence Birthlines Behrend & Maeder 2001, dM/dt=1/3 (dM/dt)disc Palla & Stahler 1993 dM/dt = 10-5

15. Pre-main sequence FST (Canuto et al. 1996). MLT, a=1.6 Effect of treatment of convection on PMS evolutionary tracks location

16. Convective envelope HI, HeI HeII Thickness of the mixed layers Abundance anomalies Convection in A-type star envelopes is superadiabatic 1.8 M  >  >

17. Gravitational settling

18. Microscopic diffusion 2.5M 1.5M 1.7M • Radiative forces (Michaud et al. 1976, …) • Turbulent transport (Schatzman 1969, Vauclair et al. 1978) Enough but not too much Changes in the surface abundances (Richer et al. 2000) • Changes in the internal structure • Mass of the convective envelope • Fe convection zone around 200000 K

19. Rotation • A-type stars are rapid rotators: vrot up to 300 km/s. • Am and Ap: vrot < 120 km/s • Normal A0-F0 stars : vrot > 120 km/s (Abt & Morrel 1995)

20. Rotation • A-type stars are rapid rotators: vrot up to 300 km/s. • Am and Ap: vrot < 120 km/s • Normal A0-F0 stars : vrot > 120 km/s (Abt & Morrel 1995) Abt & Morrell 1995, Abt 1995: Rotation alone can explain the occurrence of abnormal or normal main-sequence A stars because of our inability to distinguish marginal Am stars from normal ones in A2-F0 and our inability to disentangle evolutionary effects BUT Debernardi & North 2001 V392 Carinae: vsini ~ 27 km/s no peculiar

21. Rotation New Catalogue by Royer et al. 2002:

22. Rotation on MS • M > 1.6M or B-V < 0.25-0.3: • Little or no stellar activity • No evidence of significant angular momentum loss • There is no trend on rotation with age (vsin i ~ cte) • M < 1.6M or B-V > 0.25-0.3: • Stellar activity does not depend on age or rotation • Very slow angular momentum loss. Braking time ~ 109yr. Rotational velocity distribution must be imposed the pre-main sequence evolution (Wolff & Simon 1997)

23. Rotation in PMS Importance of the Birthline location From vsini in 145  in Orion (1 Myr), Wolff et al. 2004: • Braking of stars with M< 2 M as they evolve down their convective tracks (disk interaction) • Conservation of angular momentum as stars evolve long their radiative traks High accretion rate birthline  at larger R Low accretion rate birthline  at radiatively low R

24. Rotation: effect on stellar evolution • Surface effects: • Photometric parameters • Anisotropic mass loss • Departure from sphericity: meridional circulation • Differential rotation and instabilities (e.g. Pinsonneault 1997) • Transport of angular momentum and chemicals Similar to overshooting in the HRD But Different internal structure?

25. Rotation: effect on stellar evolution 2.2 M 1.8 M 1.5 M 1.4 M 1.35M Palacios et al. 2003 • Maeder & Zahn (1998), Zahn (1992) Transport by meridional circulation and highly anisotropic turbulence in a rotating and non magnetic star. • Time spent on MS increases by • 20% in lower mass stars • 10% in higher mass models

26. Rotation: effect on stellar evolution Maeder 2003 No rotation Maeder & Zahn 1998 Maeder (2003): balance between horizontal turbulence and excess of energy in the differential rotation Dh Maeder 2003 >> Dh Maeder & Zahn “New prescription of Dh keeps the size of the core” (Maeder 2003)

27. Rotation: effect on stellar evolution β-viscosity prescription to determine Dh Horizontal turbulent diffusivity: Dh Vertical effective diffusivity: Deff Mathis & Zahn 2004 Mathis et al. 2004

28. Rotation: effect on stellar evolution • Differential rotation in radiative layers (Tayler instability) Magnetic field (Spruit 1999, 2002). • Magneto-rotational instability (Balbus & Hawley 1991) could transport J to the surface (Arlt et al. 2003). Timescale ~ life time for A type stars  Effect on J of Ap stars

29. Interaction rotation-convection • Convective envelope: • Reduce the size of the overshooting layer at the • bottom of the convective envelope • (Chan 1996, Julien et al. 1996) • Convective core (Browning et al. 2004): • Differential rotation • Overshooting

30. Rotation: open questions • Overshooting and/or rotatonal mixing in the internal regions? • Mixing close to the surface: • Li, Be in A-type stars and in the Sun • Am surface abundances (D ~ wD(He)0(r/ro)n) • Transport of angular momentum in the radiative regions internal rotation in A-type stars: • solid or differential rotation? • role of magnetic instabilities

31. Puzzle pieces (general trends) Binarity slowing-down of rotation Am phenomenon magnetic Ap’s: strong magnetic fields binarity

32. Perhaps ..no definite answer… A-type star binarity/non-binarity • Typically (~ not far from always) Am’s are (close) binaries • Rarely Ap’s are binaries, and anyway with an orbital P≥2.5d Questions on binarity: • is binarity a necessary and sufficient condition to be an Am ? • is binarity - through syncronization and circularization mechanisms - just an efficient brake of stellar rotation or does it affect the stellar structure in other ways?

33. ...lking for the answers The synchronization (and circularization) theories are usually compared withthe Observed (orbital) Period Distributions (OPD), the rotational data and the eccentricity - P plots. Three sorts of problems: • Limits of the available theories or in their application • Small and non homogeneous available samples with sufficiently accurate elements • Selection effects on the OPD

34. Ω • Two necessary ingredients: • tidal bulges • dissipation mechanism non-alignement torque ω a R Synchronization & circularization theories: I. Zahn’s tidal mechanisms In late-type stars it is the turbulent dissipation in the outer convection zone that retards the equilibrium tide, In early type stars the dissipation mechanism is radiative damping, which acts on the dynamical tide (forced gravity waves are emitted from a lagging convective core and damped in the outer layers).

35. Zahn tidal theory: timescales Late-type stars: related to the density profile inside the star Early –type stars: E2 is a constant strongly dependent on the size of the convective core In early type stars the timescales increase more rapidly with a (or P) and the forces have a shorter range

36. with whereis the eddy and the radiative viscosity of the outer layers (N=0 for radiative envelopes). II. Tassoul’s hydrodynamical theory Transient strong meridional currents, produced by the tidal action, transfer angular momentum between the stellar interior and the Ekman layer close to the surface. If ω>Ω the star spins down. Timescales: γtakes somehow into account the fact that the eqs are solved for ~circular and ~synchronized motions. Tassoul’s mechanism has a longer range and a much higher efficiency for early-type stars

37. Warnings! • the use of timescales cannot replace the integration of the evolutionary equations, which require as well the introduction of stellar evolutionary models (see Claret et al. 1995, Claret et Cunha 1997) • Both theories are for quasi-circular & quasi-synchronized orbits. Tassoul introduces an arbitrary factor (~10-40) in the timescales. • The strong dependence of the processes on R/a requires systems with very accurate element determination.

38. e e log (t/tcri) log (t/tcri) Application to A and early type stars(Matthews & Mathieu 1992, Claret et al. 1995, 1997) Zahn Tassoul,  = 1.6 Non-circ. Circ. t: binary age, tcrit : time for circularization. From Claret et al. 1995, 1997

39. e e log (t/tcri) log (t/tcri) Application to A and early type stars, II(Matthews & Mathieu 1992, Claret et al. 1995, 1997) Tassoul,  = 0 Tassoul,  = 1.6

40. ω=Ω M=2.0 R=3.0 q=0.2 R=2.1 q=1.0 Spin – orbit synchronization:Am (Am sample from Budaj 96 (Segewiss 93) + Updated v sin i from Royer et al. 2002) In a synchronized binary: Expectedsyncronization P: R/a≈0.25 ( North & Zahn 02)

41. ω=Ω M=2.0 R=3.0 q=0.2 R=2.1 q=1.0 Spin – orbit synchronization Am (Am sample from Budaj 96 (Segewiss 93) + Updated v sin i from Royer et al. 2002) v sin i before updating Empty region P-dependent tidal mixing Expectedsyncronization P: R/a≈0.25 ( North & Zahn 02)

42. ω=Ω M=2.0 R=3.0 q=0.2 R=2.1 q=1.0 Spin – orbit synchronization Am (Am sample from Budaj 96 (Segewiss 93). Updated v sin i from Royer et al. 2002) Expectedsyncronization P: R/a≈0.25 ( North & Zahn 02)

43. Selection effects on SB’s minimum observable radial velocity amplitude, K1≠ instr. limit maximum observable orbital Period: P=P(m1,q,e) [ sin i =1.0] if K1 =10 Km/s detailed modeling of SB8 selection effects (Hogeveen 1992) suggests for A-type stars: K1≈25 Km/s SB1 q distribution is peaked around q≈0.2. m1=2.0 Missed SB1

44. The End!