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Astroseismology of a -Cephei star

Astroseismology of a -Cephei star. Nick Cowan April 2006. Motivation: Blowing up Stars. O & B stars die in spectacular explosions known as Type II SN. Try as they may, theoreticians are unable to properly model these explosions.

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Astroseismology of a -Cephei star

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  1. Astroseismology of a -Cephei star Nick Cowan April 2006

  2. Motivation: Blowing up Stars • O & B stars die in spectacular explosions known as Type II SN. • Try as they may, theoreticians are unable to properly model these explosions. • Astroseismology allows us to map out the interior of such stars and understand how they die.

  3. -Cephei Stars • Late O to early B variable stars at the end of their MS life. • Periods of a few hours. • Mostly have radial modes, though some have more complex vibrations.

  4. Fe-Peak -Mechanism • The opacity of stellar gas typically decreases as the temperature increases. • The trend reverses at temperatures where H or He ionize, and the later is thought to cause the oscillations in classical Cepheids and RR-Lyrae. • In the case of -Cephei stars, the ionization of Fe-peak elements (Sc through Cu) results in an opacity reversal for T > 105 K.

  5. More on the -Mechanism • If energy is being transported radiatively at one of these reversal layers, the star may be unstable to pulsations. • If a layer is compressed, its temperature increases and so does its opacity. This leads to an increase in the restoring radiative pressure and an overshoot in the decompression. • The resulting pulsations have periods which scale roughly with the star’s density: P (G)-1/2

  6. Pulsation Modes

  7. HD 203664 • B0.5 III • B = 8.33, V = 8.53 • log Teff = 4.47 • log g = 3.9 • Vsini = 200 km/s • M = 14 Msun • Parallax is inconclusive (0.5-3 kpc) • 6.02 cd-1 frequency (4 hr period) based on HIPPARCOS photometry.

  8. Critical Rotation • If a star spins sufficiently rapidly, material at its equator is simply in orbit around the star. • Even below the critical velocity, serious deformation of the star may occur. • HD 203664 rotates at about 40% vcrit.

  9. Periodogram of HD 203664

  10. Aliases & Prewhitening • Aliases • If the frequency of variations are integer or half-integer multiples of the frequency of your observations, you may not be observing the true frequency, but merely its alias. • HIPPARCOS was capable of continuous observations and so was impervious to aliasing. • Prewhitening • Removes any previously known correlations from data. • Essentially subtract the frequencies you know so that you may better see other frequencies.

  11. Phase Diagrams for HD 203664

  12. Theoretical Codes • Code Liegeois d’Evolution Stellaire (CLES) • Modeled the MS evolutionary track for B stars with a variety of masses • MAD • Modeled the non-adiabatic oscillations • Did not take into account rotation, even though second-order rotational effects shift the frequency of pulsations.

  13. Amplitude Ratios of HD 203664

  14. Frequency Spectra of HD 203664

  15. Summary • HD 203664 exhibits at least two non-radial pulsation modes. • The primary mode has a frequency of 6 cd-1 and is unambiguously identified as a dipole oscillation. It is responsible for the ~0.5 mag variations of the star in the visible. • At least one weaker mode is present but the nature of the oscillation could not be determined from the data. • What this means about the star’s interior is completely beyond me!

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