A star on the horizontal branch has luminosity and effective temperature

PHYSICS · Multi-part · Fermi University Physics · Astrophysics & Cosmology · Stellar structure, energy generation and evolution · Stellar structure, energy generation and evolution

Stars from the Inside, Ch Pulsating stars: the kappa mechanism, the Leavitt law and asteroseismology

Problem

A star on the horizontal branch has luminosity L=45 L⊙L = 45\,L_\odot and effective temperature Teff=8300 KT_{\rm eff} = 8300\,\mathrm{K}. At this luminosity the classical instability strip runs from about 7500 K7500\,\mathrm{K} (blue edge) to about 6500 K6500\,\mathrm{K} (red edge), the range occupied by RR Lyrae stars. ((a)) State whether this star is expected to pulsate, and identify which edge of the strip is relevant. ((b)) Explain the physical reason, in terms of the He II ionisation zone and the kappa mechanism.

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