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Classical Cepheids

Classical Cepheids (ceph)

Classical Cepheids are young, bright (100-10,000 solar luminosities) supergiant stars that pulsate with periods of 1 to 100 days (typically several days). They are located in the main instability strip in the H-R diagram. Cepheids follow a famous pulsation period-luminosity relation, allowing the absolute magnitude of a Cepheid, and thus its distance, to be inferred from the pulsation period. Thus, Cepheids are used to measure distances to nearby galaxies.

Classification and numbers

  • Supertypes
  • variable
  • periodic
  • pulsator
  • Occurrence rate: rare, about 103 expected in ZTF data, mostly in the Galactic plane and in M31

ZTF light curves

ZTF cepheids ZTF cepheids ZTF cepheids ZTF cepheids

Description

Fundamental-mode Cepheids are easy to recognise by their distinctive sawtooth light curve shape (with a rapid rise to maximum light and a slower decline) and period range (from 1 to 100 days). A secondary bump may be seen in the light curves of some Cepheids with periods in the range 6-20 days. Cepheids pulsating in the first overtone have lower amplitudes and more symmetric light curves than fundamental-mode Cepheids. Their periods (in the Milky Way) range from 0.24 to 8 days. The lower period limit is arbitrary since there is no natural boundary between first-overtone Cepheids and delta Scuti stars. Some Cepheids pulsate both in the fundamental mode and first overtone, the period ratio ranges from 0.68 to 0.77.

Light curve characteristics

  • periodic variable
  • period range: 1-100 days (fundamental-mode pulsators), 0.24-8 days (first-overtone pulsators)
  • amplitude: moderate amplitude (~0.3 to 0.8 mag)
  • light curve shape:
    • smooth, sawtooth variations (blue/green filters show stronger sawtooth pattern); steep rise and slow decay
    • Classical Cepheids with pulsation periods 6-20 days may have a secondary bump

Other characteristics and selection methods

  • intrinsic Cepheids colors: Bp-Rp = 0.8 -- 2.0 mag, but as these stars are located near the Galactic plane, the reddening may be significant
  • absolute magnitude: -6<G<-2
  • located close to the Galactic plane (within ~10 degrees)

HR diagram of classical Cepheids

RA/Dec diagram of classical Cepheids

Period histogram of classical Cepheids

References and further reading: