Longterm Light Curves of Cepheid Variables

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Long-term Light Curves of Cepheid Variables
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Visual data for Cepheids reputation

• Imprecise rms scatter 0.2 mag.
• Low amplitude often lt 1 mag., even ltlt
• Resolution 0.1 mag too grainy
• Each observer defines a different scale
• Severe aging of the optics
• Reputation just not good enough for serious
  study of Cepheids

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Compensating factors

• Large number of data
• delta Cep online photometry 160 data points
• AAVSO gt 26,000 data points
• Isolate data from individual observers
• Timing is still very precise

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Sample

• 65 stars with gt 500 data points
• Well-observed sample 28 stars with gt 2000
  data points
• One double-mode Cepheid TU Cas

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Period changes

• Spike temporary increase
• Dip temporary decrease
• Jitter irregular changes
• Rise steady increase

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TU Cas

• Double-mode Cepheid (two independent periods)
• Excellent data by LX (Wayne Lowder) and SSW
  (Stanislaw Swierczynski)
• Is it mode switching?

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Fourier decomposition

• Amplitude and phase for each harmonic
  term
• Shape is defined by relative amplitudes
• and relative phases

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General light curve shape

• Rough approximation sawtooth wave
• two straight line segments (rising branch and
  descending branch)
• Better approximation bent-sawtooth wave
• three straight line segments
• Best approximation bent-sawtoothbumps wave

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Mean light curves

• Are they good enough?
• Compare visual mean light curves to published
  photometry

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Hertzsprung progression

• Around P7d, bumps appear on the descending
  branch
• Around P10d, bumps are near maximum
• Around P12d, bumps recede to ascending branch
• By P20d, bumps no more

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Other bumps

• Some exists beyond P20d
• Some stars show bumps on both ascending and
  descending branches

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Conclusions

• 1. Periods more unstable than often thought
• 2. TU Cas not mode-switching
• 3. Light curves well approximated by bent
  sawtooth bumps model
• 4. Bumps outside Hertzsprung progression, and
  sometimes on both ascending and descending
  branches

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Main conclusion

• Visual data are more than adequate for serious
  study of Cepheid (and other low-amplitude)
  variables