BCEP [ft Cephei stars

- Pulsating 08-B6 I-V stars with periods of light and radial-velocity variations in the range of 01'I-ff!6 and light amplitudes from 0'!'Oi-OT3 in V. The light curves are similar in shape to average radial-velocity curves but lag in phase a quarter of the period, so that maximum brightness corresponds to maximum contraction, i.e. to minimum stellar radius. The majority of these stars probably shows radial pulsations, but some display non-radial pulsations; multiperiodicity is characteristic of these stars. BCEPS (subtype) A short-period group of ft Cep variables. The spectral types are B2-B3 IVV; periods and light amplitudes are in the ranges ff.'02-fff04 and ff!015-ff!025 respectively, i.e. an order of magnitude smaller than the normally observed ones. GCVS

f) Cephei5 variable stars, occasionally called ff Canis Majoris stars, are a group of apparently normal early B giant and subgiant stars that display short-period light variations. The periods, lasting between two and seven hours, are too short to be explained by purely geometric effects, such as rotation and/or binary motion. Astronomers have recognized that the only remaining explanation requires stellar pulsation (Figure 4.4).

The interest in these variables for theoretical astrophysics was because theorists were not able to find a consistent explanation for the pulsational behavior of these stars and so the unknown driving mechanism for f) Cep star pulsation remained one of the outstanding problems of stellar pulsation theory.

Observation

Bright stars Small amplitudes ■f* long periods <S> CCD or PEP

sCepheus, the King of )oppa and one of the Argonauts who journeyed with Jason, was Cassiopeia's husband and Andromeda's father in ancient mythology.

Figure 4.4. Light curve

0| the BCEP-type variable star, fi Cep.

Cycle phase is indicated ak^g the horizontal

0Xis. Data provided by the

HIPPARCOS mission. Used with permission.

Figure 4.4. Light curve

0| the BCEP-type variable star, fi Cep.

Cycle phase is indicated ak^g the horizontal

0Xis. Data provided by the

HIPPARCOS mission. Used with permission.

The variability of the radial velocity of ft Cep was discovered at the beginning of the twentieth century at Yerkes Observatory. The star's period of variability was also determined to be 4h34m. In 1908 astronomers at Lick Observatory found that ft CMa showed similar variation, and it turned out that this star became the first well-studied member of the group of ft Cep variables. As a result, for several decades these stars were labeled ft Canis Majoris stars.

The range of spectral and luminosity types among the ft Cep stars force them into a small region within the HR diagram. This region is commonly labeled the ft Cephei instability strip. However, it should be mentioned that some B[e] stars share this area in the HR diagram with the ft Cep stars. Also, it should also be stressed that stars seen as ft Cep stars at one time may later become B[e] stars. A most notorious example is ft Cep itself, showing unprecedented strong emission in the core of the Ha_line in 1990, and conversely, one case is know of ft Cep pulsation appearing in a well-observed B[e] star: 27 EW CMa developed a pulsation somewhere between 1987 and 1990.

Because of their small amplitudes, these stars are best studied using instruments; however, their relatively short periods allow one or more full cycles to be observed during an evening.

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