astroseismology of a -cephei star nick cowan april 2006 nick cowan april 2006
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Astroseismology of a -Cephei starAstroseismology of a -Cephei star
Nick Cowan
April 2006
Nick Cowan
April 2006
Motivation: Blowing up StarsMotivation: 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.
• 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.
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-Cephei Stars-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.
• 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.
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Fe-Peak -MechanismFe-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.
• 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.
More on the -MechanismMore 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
• 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
Pulsation ModesPulsation Modes
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HD 203664HD 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.
• 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.
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Critical RotationCritical 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.
• 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.
Aliases & PrewhiteningAliases & 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.
• 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.
Theoretical CodesTheoretical 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.
• 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.
SummarySummary• 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!
• 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!