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Crash Course in Stellar Pulsation

Crash Course in Stellar Pulsation. Ryan Maderak A540 April 27, 2005. Mechanisms. k mechanism Compression of partial ionization zones -> ionization -> small change in T k ~ r / T 3.5 , increase r -> increase k g mechanism

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Crash Course in Stellar Pulsation

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  1. Crash Course in Stellar Pulsation Ryan Maderak A540 April 27, 2005

  2. Mechanisms • k mechanism • Compression of partial ionization zones -> ionization -> small change in T • k ~ r / T3.5, increase r -> increase k • g mechanism • Heat flow into partial ionization zone from higher temperature layers • So, compression -> higher k -> energy buildup -> energy release -> expansion

  3. Mechanisms • e mechanism • Compression -> higher T -> higher energy production rate -> expansion • stochastic excitation • convective turbulence -> acoustic noise -> solar-type oscillations • oscillatory convection • convective + g-mode in rotating stars -> oscillatory modes • tidal interaction • periodic fluid motion -> non-radial modes

  4. HR Diagram Gautschy & Saio, 1995

  5. Main Sequence • Solar-type stars • solar-type oscillations expected • more precise photometry needed • ~mmag • greatest amp. at ~1.5 MSun

  6. Main Sequence • roAp = rapidly oscillating Ap stars • P = 5-15 min, multi-periodic, ~50 mmag • ~2 MSun • magnetically modulated rotational splitting • overlap with d Scuti instability strip, but excitation mechanism uncertain • kg in He II zone suppressed by diffusion of He • convection + B ? kg in Si IV zone?

  7. Main Sequence Gautschy & Saio, 1996

  8. Main Sequence • d Scuti • P = 0.01-0.2 days, 0.003 to 0.9 mag, multi-periodic (up to 12 modes observed) • 1.5 – 2.5 Msun, A0 – F5 IV - V, disk population • non-radial p-modes, driven by kg in He II zone • amp. limited by coupling between p and g modes • “stable” stars observed within d Scuti instability strip • suspected to be very low amplitude variables • more precise photometry needed

  9. Main Sequence • d Scuti http://users.skynet.be/bho/deltascutis.htm

  10. Main Sequence • Slowly Pulsating B Stars (SPB) • P = 1 – 3 days, low amp., multi-periodic • 2.5 – 5 Msun, B3 – B8 IV • kg driven g-modes • can be thought of as an extension of the b Cephei instability to longer periods

  11. Main Sequence • b Cephei • P = 0.1 – 0.6 days, 0.01 – 0.3 mag • majority multi-periodic, a few non-radial • 7 – 8 Msun, O8 – O6 • p-modes, driven by kg in the “z-bump” • metalicity dependent pulsational stability • b Cep strip extends farther blue-ward for higher metalicity stars • b Cep-type variability appears in at least a few cases to be transient • Spica exhibited b Cep variability from ~1890 to 1972

  12. Main Sequence • bCephei http://www.aavso.org/vstar/vsots/winter05.shtml

  13. Main Sequence • Be stars • exhibit photometric and line profile variability with periods of <1 day • found within the b Cep/SPB instability region -> “z-bump” driving • MS 60 – 120 Msun • models suggest e driving from CNO burning • e driving may be one of the factors which determines the high mass cutoff of the MS

  14. Horizontal Branch • RR Lyrae • P = 0.3 – 1.2 days, 0.2 – 2 mag • < 0.75 Msun, A – F, prominent in globular clusters • kg driven, but convective flux is thought to be important • important standard candles for clusters, but the P-L relationship is metalicity dependent • the period decreases as cluster metalicity increases (for fixed Teff) • careful calibration and stellar evolution models needed

  15. Horizontal Branch • RR Lyrae http://www.dur.ac.uk/john.lucey/astrolab/pulsating.html

  16. Horizontal Branch • RR Lyrae • RRab: asymmetric light curves, longer periods, higher amp. • RRc: nearly sinusoidal light curves, shorter periods, lower amp. • RRd: bi-periodic • RRab’s exhibit a periodic change in light curve shape and amp. -> “Blazhko” effect • coupling between B and rotation?

  17. Horizontal Branch • P-L Relation http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html

  18. Horizontal Branch • “Classical” Cepheids • P = 1 – 135 days, ~0.01 – 2 mag • > 4 – 5 MSun, F at maximum light, G - K at minimum light • stars above 4 – 5 MSun pass through the instability strip during each of one or more blue loops • for ~4 MSun -> bi-periodic cepheid

  19. Horizontal Branch • Classical Cepheid http://www.astronomynotes.com/ismnotes/s5.htm

  20. Horizontal Branch • “Classical” Cepheids • masses from evolution versus pulsation theories did not agree historically, but improved opacities solved the problem • but pulsational models using the improved values give periods that are metalicity dependent • careful abundance measurements are needed to use the P-L relationship accurately

  21. AGB • W Virginis (Population II Cepheids) • P = 0.8 – 35 days, 0.3 – 1.2 mag • M ~ 0.5 MSun • cross instability strip in late HB or early AGB evolution • fundamental or 1st harmonic, driven by He II and H/He I zones • instability strip is wider for metal poor stars

  22. AGB • W Virginis http://www.astronomynotes.com/ismnotes/s5.htm

  23. AGB • RV Tau • P = 30 – 150 days, 1.5 – 2 mag • M = 0.5 – 0.7 MSun, F – G at maximum light, K – M at minimum light • driven by H and He I zones • characteristic “double peak” pattern • resonances between fundamental and 1st harmonic • chaotic motion of multiple atmospheric layers • low-dimensional chaotic attractors

  24. AGB • RV Tauri

  25. AGB • RV Tau • various irregularities • change in depth of primary and secondary minima • changes in period • relatively few known ~130 (GCVS) • duration of phase only ~500yr • believed to be post-AGB/proto-planetary • have experienced significant mass loss • RVb: long term (600 – 1500 day) variation in mean brightness • eclipsing binary? episodic mass loss? dust shell eclipse?

  26. AGB • Mira • P = 80 – 1000 days, 2.5 – 11 mag • low-mass, Me – Se • First variable discovered: 1595 • fundamental, driven by H and He I zones • coupling between pulsation and convection

  27. AGB • Mira

  28. AGB • Semi-Regular • P = 20 – 2000+, ~0.01 – 2 mag, multi-periodic • occupy same part of HR diagram as Mira’s – physically similar • distinguished by amplitude • difference due to mass, composition, age • SRb: power spectra exhibit broadened mode-envelopes • stochastic excitation?

  29. AGB • Semi-Regular

  30. Planetary Nebula • PG1159 (variable planetary nebula nuclei = PNNV) • P = 7 – 30 min • g-modes, driven by C and/or O K-shell ionization • Teff = 70000 – 170000, strong C, He, and O features

  31. Cooling Track • DB-type variable WD (DBV) • P = 140 – 1000 seconds, non-radial • M ~ 0.6 MSun, Teff = 21500 – 24000 • g-modes, driven by He II zone • complicated power spectra • need high time resolution and long data sets to resolve peaks -> WET

  32. Cooling Track • ZZ Ceti (DA-type variable WD) • Similar to DBV • g-modes may be driven by ionization of a surface H layer • lower Teff -> blue edge of instability ~13000K • H rich, with almost no He or metals

  33. Future Work • Larger samples of Cepheids and RR Lyrae’s ---> more accurate determination of metalicity dependence of P-L • Continued high time resolution, long duration astroseismology -> better understanding of interior structure and excitation mechanisms • Better theory of convection -> better understanding of coupling between convection and pulsation

  34. References • Carrol, B.W., & Ostlie, D.A. 1996, “An Introduction to Modern Astrophysics,” Addison-Wesley, Reading, MA. • Gautschy, A., & Saio, H. 1995, ARA&A, 34, 551. • Gautschy, A., & Saio, H. 1996, ARA&A, 33, 75. • “GCVS Variability Types.” http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt

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