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Tom Mazanec
2004-Aug-16, 04:02 PM
What decides the "period" and "amplitude" of an accreting white dwarf variable star? And what determines the amplitude of a pulsating variable like a Cepheid?

RobinSky
2011-Mar-31, 06:59 AM
I know the post is old but, no one has answered it yet.

The amplitude of a pulsating variable cepheid is determined from the following min. value and max. value. Let's say we observe a cepheid for a couple of months. Observations shows that we have measured the brightest value as +8.0, and the lowest value as +9.0. Then the amplitude is 1 magnitude, and the magnitude range is magn.V 8 - magn.V 9. (8-9V).
The star should be observed for at least 2 period before any amplitude can be determined, because there can be rare brightenings or vice versa.

Robin

Tom Mazanec
2011-Apr-05, 05:28 PM
No, that was not what I asked (or meant to ask). WHY does Cepheid A have an amplitude of 1 magnitude, while Cepheid B has an amplitude of 2.5 magnitude and Cepheid C has an amplitude of 0.3 magnitude? What is different about the three stars that their amplitudes are so different?

Hornblower
2011-Apr-05, 06:10 PM
No, that was not what I asked (or meant to ask). WHY does Cepheid A have an amplitude of 1 magnitude, while Cepheid B has an amplitude of 2.5 magnitude and Cepheid C has an amplitude of 0.3 magnitude? What is different about the three stars that their amplitudes are so different?

My educated guess would be differences in their metallicity and/or how far along they are in their evolution.

StupendousMan
2011-Apr-05, 07:51 PM
The short answer is mass, which determines size. Big stars have a longer period because the physical changes inside the star's atmosphere which lead to pulsations just take longer to occur.

The long answer would involve calculating the change in level populations of the H- ion -- that's a hydrogen atom with one extra electron -- in the atmosphere. You could look up phrases involving "opacity" and "hydrostatic equilibrium" and "Cepheid".

Hornblower
2011-Apr-05, 08:14 PM
The short answer is mass, which determines size. Big stars have a longer period because the physical changes inside the star's atmosphere which lead to pulsations just take longer to occur.

The long answer would involve calculating the change in level populations of the H- ion -- that's a hydrogen atom with one extra electron -- in the atmosphere. You could look up phrases involving "opacity" and "hydrostatic equilibrium" and "Cepheid".

Tom was asking about differences in amplitude, not period. Delta Cephei and Polaris have similar periods of a few days, but Delta has a vastly larger amplitude. I think it is more complicated than just differences in mass.

StupendousMan
2011-Apr-05, 09:59 PM
The period-amplitude relationship has much the same form as the period-luminosity relationship: long-period variables have larger amplitudes. The basic explanation for that is again a simple matter of size (I believe).

There is quite a large scatter in this relationship, however; some stars with similar periods can have amplitudes which differ by up to 0.3 or 0.4 magnitudes. Some of this difference can be attributed to different modes of oscillation: the stars with larger amplitudes tend to oscillate in the fundamental mode, while stars with smaller amplitudes tend to oscillate in an overtone. This is a simplified view of a complex situation that I don't understand.

You could learn a bit from the observational side by reading

http://arxiv.org/abs/0908.3561

I'll leave it to someone else to suggest a good reference for a paper which approaches the problem from the theoretical side.

neilzero
2011-Apr-07, 05:18 PM
I'm only mildly stressed by being proved wrong, so, I'll say the elliptical orbit of the mass donor star means it gives mass to the white dwarf once per orbit. Neil