The effects of stellar rotation and magnetism on oscillation frequencies

Transcription

1 The effects of stellar rotation and magnetism on oscillation frequencies Daniel Reese Joint HELAS and CoRoT/ESTA Workshop November 2006, CAUP, Porto - Portugal

2 Introduction traditionally, stellar pulsations are calculated assuming spherical symmetry however, neither stellar rotation nor stellar magnetism respect this symmetry Oscillation modes are no longer described by a single spherical harmonic No longer 1D calculations, but 2D or 3D 1/28

3 Outline 1. The effects of stellar rotation 2. The effects of stellar magnetism 3. Conclusion 2/28

4 Incidence of stellar rotation A few statistics : β Cephei (Stankov & Breger, 2005) δ Scuti (Rodríguez et al., 2000) ζ Oph stars(based on Balona & Dziembowski, 1999) 3/28

5 Targets for space missions Identification Name Type v sin i (in km s 1 ) Mission HD Altair δ Scuti 230 WIRE HD ζ Oph ζ Oph 380 MOST HD δ Scuti 170 CoRoT HD γ Doradus 90 CoRoT HD γ Doradus 72 CoRoT HD δ Scuti 198 CoRoT HD δ Scuti > 200 CoRoT HD δ Scuti > 200 CoRoT 4/28

6 Effects of rotation Two forces appear because of rotation centrifugal force : stellar deformation and modification of equilibrium quantities Coriolis force : intervenes in all dynamical processes Fig. 2. V Domiciano de Souza et al. (2003) 5/28

7 Models of rapidly rotating stars A few references : Meynet and Maeder ( ) Roxburgh (2004, 2006) Jackson et al. (2005), MacGregor et al. (2006) ESTER (Rieutord et al., 2005, Rieutord, 2006) 1032 M. Rieutord: The dynamics of the radiative envelope of rapidly rotating stars. I. Fig. 4. Left: the meridional circulation of the baroclinic flow generated by the Brunt-Väisälä frequency profile in Fig. 1b (solid curve) for a fluid with constant viscosity. Right: the associated differential rotation showing the fast rotating pole and slow equator (solid lines are for positive contours, dotted lines for negative ones). Roxburgh (2004) Rieutord (2006) 6/28

8 Rotation and oscillations Two basic approaches to take the effects of rotation into account : Perturbative approach the rotation rate Ω is considered to be small equilibrium model and oscillation modes : v = v 0 + v 1 Ω + v 2 Ω O ( Ω n+1) ω = ω 0 + ω 1 Ω + ω 2 Ω O ( Ω n+1) Complete approach the rotation rate Ω is not considered small equilibrium model and oscillation modes = a solution to a 2D problem which fully includes the effects of rotation 7/28

9 A few references... Perturbative approach 2 nd order methods : Saio (1981) Gough & Thompson (1990) Dziembowski & Goode (1992) 3 rd order methods : Soufi et al. (1998) Karami et al. (2005) Complete approach Clement ( ) Dintrans et al. (1999), Dintrans & Rieutord (2000) Espinosa et al. (2004) Lignières et al. (2006), Reese et al. (2006) 8/28

10 Slow rotation rates Perturbative expression of pulsation frequencies : ω = ω 0 m(1 C)Ω + ( D 1 + D 2 m 2) Ω 2 + m ( T 1 + T 2 m 2) Ω 3 + O ( Ω 4) 9/28

11 Slow rotation rates Perturbative expression of pulsation frequencies : ω = ω 0 m(1 C)Ω + ( D 1 + D 2 m 2) Ω 2 + m ( T 1 + T 2 m 2) Ω 3 + O ( Ω 4) 9/28

12 Slow rotation rates Perturbative expression of pulsation frequencies : ω = ω 0 m(1 C)Ω + ( D 1 + D 2 m 2) Ω 2 + m ( T 1 + T 2 m 2) Ω 3 + O ( Ω 4) 9/28

13 Solar rotation profile use of 1 st order methods inversion techniques Schou et al. (1998), Thompson et al. (2003) 10/28

14 High rotation rates A multiplet : 11/28

15 High rotation rates Modes : n = 1 to 6 l = 0 to 3 m = l to l 12/28

16 High rotation rates Validity domain for 150 days of observation ( ω = 0.08 µhz) - 1 st order - 2 nd order - 3 rd order (see Reese et al., 2006) 13/28

17 Organisation of frequency spectrum f n l m = f 0 n l m + f 1 n l mω + f 2 n l mω 2 + f 3 n l mω 3 + O(Ω 4 ) 14/28

18 Organisation of frequency spectrum f n l m n n + l l + m m + α ± 14/28 (see Lignières et al., 2006, and Reese, 2006)

19 Avoided crossings 15/28

20 Mode identification n =? l =? 16/28

21 Outline 1. The effects of stellar rotation 2. The effects of stellar magnetism 3. Conclusion 17/28

22 roap stars Discovered by Kurtz in 1978 Characteristics : peculiar chemical composition, strong dipolar magnetic field, Pulsation modes : luminosity variations with periods ranging from 5 to 15 min. well described by the oblique pulsator model (e.g. Kurtz, 1990) 18/28

23 Magnetism and oscillations A few references : Roberts & Soward (1983), Campbell & Papaloizou (1986) Dziembowski & Goode (1996), Bigot et al. (2000), Bigot & Dziembowski (2002) Cunha & Gough (2000), Cunha (2006) Balmforth et al. (2001), Théado et al. (2005) Rincon & Rieutord (2003), Reese et al. (2004) Saio & Gautschy (2004), Saio (2005) Effects of magnetism : suppression of convection near magnetic poles diffusion cyclic behaviour of frequency shifts self-similar structure in frequency spectrum magnetic shear layers magnetic oscillations and different frequency spectrum structure 19/28

24 Magnetism, convection and diffusion Balmforth et al. (2001) : convection suppressed in polar regions due to vertical B chemical diffusion in polar regions enable κ mechanism in the hydrogen ionisation zone operating in polar regions 20/28

25 Trapping of magnetic waves coupling of acoustic and magnetic waves in outer region, and decoupling below v A c dissipation of slow magnetic waves below high damping rate when wave has an antinode near v A c low damping rate when wave has a node near v A c Saio & Gautschy (2004), see also Cunha & Gough (2000) 21/28

26 Self similarity of frequency shifts ω = f(ω 0 B α p ) (e.g. Cunha & Gough, 2000, Saio & Gautschy, 2004) α = 1/(1 + N) for polytropes Saio (2004) 22/28

27 Axis of pulsation Bigot & Dziembowski (2002) predict that the pulsation axis is located somewhere between the magnetic axis and the rotation axis. z Ω B Mode plane Z n β Dipole axis ξ r δ Extrema of pulsation and magnetic field in phase. y X 23/28

28 Magnetic shear layers include viscosity and/or magnetic diffusivity magnetic shear layers may intervene in mode selection (Rincon & Rieutord, 2003) 24/28

29 Alfvén waves different frequency spectrum different structure to pulsation modes certain types become singular in the ideal (inviscid) limit (Reese et al., 2004) 25/28

30 Latitudinal structure and quantification 26/28

31 Outline 1. The effects of stellar rotation 2. The effects of stellar magnetism 3. Conclusion 27/28

32 Conclusion stellar rotation and magnetism introduce many new phenomena increased difficulty for calculating pulsation modes need for powerful numerical and theoretical methods in order to interpret observed pulsations exciting prospects for stellar physics 28/28

33 Radial structure 29/28

34 Radial structure 29/28

35 Radial structure 29/28

36 Radial structure 29/28

37 Latitudinal structure 30/28

38 Latitudinal structure 30/28

39 Latitudinal structure 30/28

40 Latitudinal structure 30/28

41 Effects of viscosity and magnetic diffusivity Empirical law (for E = E m ) : 31/28 position E 1/4 thickness E 1/4

42 Asymptotic formulas Analytical solutions for small diffusivities : Form of solutions : E = Kε, E m = K m ε, ε 0 b(r, ν) = b n (r)f n,q (ε 1/4 ν) + O(ε 1/2 ) v(r, ν) = v n (r)f n,q (ε 1/4 ν) + O(ε 1/2 ) λ n,q = λ 0 n + ε 1/2 λ 1 n,q + O(ε) Zeroth order : radial structure (b n, v n ) and mode quantification (n) Next order : latitudinal structure (f n,q ) and mode quantification (q) use of adjoint system to obtain f n,q 32/28

43 Non-axisymmetric modes poloidal and toroidal components are now coupled 33/28

44 Comparison with axisymmetric modes Poloidal m = 1 34/28

45 Comparison with axisymmetric modes Toroidal m = 1 35/28

46 Conclusion Toroidal modes : singular Non-axisymmetric modes : poloidal or toroidal characteristics Prospects study of magneto-acoustic waves study of magneto-inertial waves understanding/constraining the interior of planets such as Jupiter 36/28

47 Asymptotic developments Change of variables (r, ν = sin θ r, ϕ) Scale change E 1/4 = (K ε) 1/4 and E 1/4 m = (K m ε) 1/4 where ε 0 λb = λv = where Θ[b] = 2 b ˆν 2 + 1ˆν b ˆν bˆν 2 ( ε1/2 rˆν 2 )[ 1 r 3 v ( ε1/2 rˆν 2 )[ 1 r 3 b r + 3b 2r 4 r 3v ] 2r 4 + ε1/2 K m r 3 Θ[b] ] + ε1/2 K r 3 Θ[v] 37/28

48 Asymptotic developments at zeroth order, we have : λ 0 b = 1 v r 3 r 3 2r 4v, λ 0 v = 1 b r 3 r + 3 2r 4b, v(η) = 0, b(1) = 0. at next order, we get : b(r, ν) = b n (r)f(ˆν) + O(ε 1/2 ) v(r, ν) = v n (r)f(ˆν) + O(ε 1/2 ) λ = λ 0 n + O(ε 1/2 ) λ 0 nb 1 1 v 1 r 3 r + 3v1 2r 4 = λ 1 b 0 nf λ 0 n rˆν2 b 0 n f λ 0 nv 1 1 b 1 r 3 r 3b1 2r 4 = λ 1 vnf 0 λ 0 n rˆν2 vn 0 f b 1 (r = 1, ˆν) = 0, v 1 (r = η, ˆν) = b0 n Θ[f], r v0 n Θ[f], r 3 This is of the form L 0 Ψ 1 = L 1 Ψ 0 : solution of adjoint problem 38/28 { f(ˆν) = fn,q (ˆν) λ = λ 0 n + ε 1/2 λ 1 n + O(ε 1/2 )