Graduate Research Seminar I/II

Period change and stellar evolution of beta Cephei stars

Daniel Wysocki • • journal_club


“Period change and stellar evolution of beta Cephei stars” by Neilson and Ignace.


B-Cephei are pulsating variable stars (not to be confused with Cepheids). They are high mass stars (7-20 solar masses) which span from main sequence to the transition to blue supergiants. Their periods are short (on the order of a few hours) and they change over time. The rate of period change can be tied into stellar evolution models.

This paper compares published rates of period change with rates predicted by models. 8 stars are used, only 2 of which could be used to find stellar parameters from, due to the rates being too small or large.


Compare measured rates with model predictions. Model is based on period-mean density relation (P sqrt(rho) = Q). Equation 1 (page 2) gives the differential equation derived from this.


Table 1 (page 2) shows period changes from literature.

Table 2 (page 4) shows stellar parameters obtained.


Figure 1 (page 3) shows stellar evolution tracks for different stars with different assumptions made on the unknown parameters. Left plot assumes constant core overshoot varies initial rotation rates, while right plot assumes zero rotation and varies core overshoot.

Figure 2 shows that rate of period change and stellar parameters (mass, temperature, etc) cannot be used to determine rotation rates and core overshoot, as there is degeneracy (note the horizontal lines).


Rate of period change and stellar parameters do not uniquely describe the evolutionary track.

Predicted rates of change versus observed rates are only in agreement with the 2 stars with strong magnetic fields, despite neglecting those effects in the model. Possible explanations include:

  1. Evolution of B-cepheids is more complex than expected.
  2. Period-mean density relation is not constant throughout the star for all times, which was assumed by the model.
  3. Short-term period drift (separate from evolution) could be throwing off measurements. Such an effect is thought to exist in Polaris (classical Cepheid).