GCAS y Cassiopeiae stars

- These are rapidly rotating B III-IVe stars with mass outflow from their equatorial zones. The formation of

'Cassiopeia, a queen, was married to King Cepheus.

equatorial rings or disks is often accompanied by temporary fading. Light amplitudes may reach ยก".'5 in V. GCVS

y Cas was the first star in which the B[e] phenomenon, the appearance of one emission line at one epoch within the spectrum, was observed in 1866.s An emission line arises from hot gas in an equatorial ring, shell or cloud surrounding the star. The B(e] stars should not be confused with, or mistaken for, the y Cas group of stars and so care must be taken when distinguishing between the two types. As you probably suspect, distinguishing between B[e] and y Cas stars is not easy.

Essentially, the difference between B[e] stars and y Cas stars is that light variability is attributed to a shell or disk surrounding the y Cas stars (Figure 3.2), whereas for B[e] stars, light variations are not necessarily related to shell events and in some cases the variations are quasi-periodic. In y Cas stars, this shell of ejected material produces a P Cygni feature that is the hallmark of the circumstellar envelope.

The P Cygni characteristic, a spectral line having a redward displaced emission component with one or

Observation

Key lAr Bright stars

Small amplitudes Short periods 0> CCD or PEP

Figure 3.2. Artist's impression of a ( type variable star showing the disk of material surrounding star. Copyright: Gerry Good.

8By Pietro Angelo Secchi (1818-78), Italian Jesuit priest and astrophysicist.

more blueward displaced absorption components, is the result of one or more expanding gas envelopes surrounding the star and is named for the first star observed to display this particular line structure. The intensity of the P Cygni characteristic varies inversely with the rate of mass flow from the star. This distinction is beyond the abilities of most amateurs to detect, so a search of the literature will be necessary when compiling a list of observation targets. Be forewarned, no elaborated system of classification for B[e] variables exists and when a B[e] variable cannot be readily described as a y Cas star, it is simply classed as a B[e] star. As a result, pay particular attention to the study of individual stars since their classification may change as new data is made available.

These variable stars can be interesting to observe but they do require a patient, long-term approach if you wish to extract any valuable information from them. As with B[e] stars, y Cas stars are receiving increased interest from within the amateur community. There are almost 160 y Cas stars found within the GCVS.

Observation Key

Mixed stars Mixed amplitudes Mixed periods <S> Visuol, CCD/PEP

0 0

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