CEP Cepheid stars

- Radial pulsating, high luminosity (classes Ib-II) variables with periods in the range ld-135d and amplitudes from several hundredths to 2"' in V (in the B band, the amplitudes are greater). Spectral type at maximum light is F; at minimum, the types are G-K and

Observation

Bright stars ^ Mixed amplitudes fl long periods <3> CCD or PEP

the periods of light variation are longer the later the spectral type. The radial-velocity curves are practically reflections of the light curves, the maximum of the surface layer expansion velocity almost coinciding with the maximum light. CEP(B) (subtype) - Cepheids displaying the presence of two or more simultaneously operating pulsation modes (usually the fundamental tone with the period P0 and the first overtone P ,). Their periods P 0 are in the range 2 to 7 days, with the ratio PJP0 « 0.71. GCVS

At the beginning of 1784, only five variable stars, apart from novae and supernovae, were known. Four of these were what we now call long period variables (Mira-type variable stars) and one was an eclipsing star, Algol. On September 10, 1784, Edward Piggot established the variability of t] (Eta) Aquilae, while his friend John Goodricke showed that ft Lyrae was variable. Shortly afterwards Goodricke found 8 Cephei to vary.

While ft Lyrae is the prototype of an important class of eclipsing variables, 8 Cep and t; Aql are what we now call Cepheids with periods of 5d4 and 7d2, respectively. Possessing visual light amplitudes of about 0n.'9, they are fairly representative of this class.

At one time, the term Cepheid meant any continuously varying star with a regular light curve and a period of less than approximately 35 days, unless it was known to be an eclipsing star. It is now recognized that the class defined in this way is heterogeneous, meaning that it contains stars in different mass ranges and evolutionary states. Stars with periods less than one day are now treated separately, mainly as RR Lyrae variables. Type II Cepheids and RV Tauri stars are also treated separately and their distinguishing features are dealt with later in this book. The remaining stars are called 8 Cephei variables, Type I Cepheids, classical Cepheids, or simply and most frequently, Cepheids. You may find examples of Cepheid variables being subdivided into long-period, short-period, very short-period, ultra short-period and pseudo-Cepheids, but this terminology has not been generally adopted.

Cepheids are strictly periodic variables with periods ranging from about 1 day to about 50 days with a few extreme examples up to 200 days. The general form of the light curve varies smoothly as one moves from shorter to longer period stars. This is known as the Hertzsprung progression, named of course after the Danish astronomer who investigated it. The shorter-

Figure 4.5. Light of the CEP-type var star, X Cyg. Cycle phase is indicated along the horizon;-axis. Data provided HIPPARCOS mission, with permission.

Figure 4.5. Light of the CEP-type var star, X Cyg. Cycle phase is indicated along the horizon;-axis. Data provided HIPPARCOS mission, with permission.

period variables have steep, narrow maxima. With increasing periods the relative widths of the maxima broaden. At periods of around 8-10 days the maxima often appear double. At shorter periods than this there are frequently bumps on the falling branch. The longer-period stars, 20-40 days, generally have very steep rising branches but the light curves of the longest-period Cepheids are more nearly sinusoidal (Figure 4.5).

The McMaster Cepheid Photometry and Radial Velocity Data Archive can be found at http:// dogwood.physics.mcmaster.ca/Cepheid//HomePage.html. This Web site contains catalog data on the Type I (classical) and Type II (BL Her and W Vir) Cepheids, and extragalactic Cepheids. You will also find the David Dunlap Observatory database of galactic classical Cepheids.

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