ACYG Cygni stars

- Non-radial pulsating supergiants of Bep-Aepla spectral types. The light changes with amplitudes of the order of 0".'l often seem irregular, being caused by the superposition of many oscillations with close periods. Cycles from several days to several weeks are observed. GCVS

a Cygni4 variable stars are spectral type B and A, luminous, pulsating supergiant stars. You will notice that B- and A-type stars are located toward the upper left portion of the HR diagram where the relatively young, hot stars are positioned. The a Cyg classification now includes massive O- and late F-type stars since it has been determined that these stars also belong to the same stellar evolutionary sequence.

The star a Cyg (Deneb) is an A2Ib type star with an amplitude of lr?21—lrf29. As you can see, or Cyg is a hot supergiant itself (luminosity class lb). This should not be surprising since this star is the model star (prototype) for all stars classified as or Cyg variables.

Because some or Cyg variables display variability that is typical for other classes of variable stars, they can be confused with other types of variable stars if you are not demanding when analyzing your data. The light curve of LT CMa is shown in Figure 4.2 to illustrate a representative a Cyg variable.

You can see that LT CMa produces a well-behaved light curve: a nice even climb and decline with no remarkable features. Beware: this behavior cannot be expected with all a Cyg stars. The light curve for Rho (p) Leo is also provided for comparison in Figure 4.3.

In contrast to these two light curves, the light curves of some a Cyg stars show dramatic features or random light fluctuations and as a result of this non-strict periodicity, meaning that light curve shapes vary from cycle to cycle, the periods are in fact "quasi" or

Observation

"AT Bright stars

Small amplitude, Long periods <®> CCD or PEP

4Cygnus, the Swan, is an ancient constellation which appeared in Ptolemy's Almagest in the second century AD.

fig0re 4.2. Light curve oitheACY&type liable star, LT CMa. Julian dotes are indicated along the horizontal axis. Data provided by the HlpfARCOS mission. Used with permission

8628.00 8629.00 8630.00 8631.00 8632.00 8633.00

"pseudo-periods." Furthermore, in some cases, the variability of other types of variable stars mimics that of a Cyg type stars. One such example is the S Dor type star R71. While in its quiescent stage during 1983-85, it showed nearly the same type of optical oscillations as the normal a Cyg variables. It has even been suggested that the S Dor type stars be considered a subgroup of the a Cyg variables. To confuse the issue further, suggestions have been put forth that all S Dor type variables can be classified as P Cyg type stars since at maximum brightness all Balmer lines, some He lines and the lines of other ions show P Cyg profiles.

In recognition of this perplexity, it may help to remember that a Cyg variable stars are not: B[e] stars, ft Cephei stars, low-mass 53 Per stars that do not pulsate in a regular manner, low-mass B-type super- and hypergiants that are probably post AGB proto-planetary nebulae stars, or low-mass F-type supergiants at high galactic latitudes (high galactic latitudes meaning located far away from the galactic plane), sometimes called UU Her stars.

In any case, these are interesting variables deserving close examination but they demand the utmost care

Figure 4.3. Light curve of the ACYG-type variable star, p Leo. Cycle phose is indicated along the horizontal axis. Data provided by the HIPPAKCOS mission. Used with permission

9.200 9.250 9.300

0.000

0.250

0.500 Phase

0.750

0.000

0.250

0.500 Phase

0.750

1.000

9.200 9.250 9.300

during their examination. As a result of their intrinsic brightness, suitable comparison stars can be difficult to find when observing these stars. Because of their small amplitude, a Cyg variables are probably best studied using a CCD or stellar photometer and are not considered good candidates for visual observation. The good news is that observing and studying these stars will strengthen your ability to detect subtle differences in complex data sets.

0 0

Post a comment