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Cepheids & RR Lyrae Nancy, Chris, Raina, Kate,Anthony Presentation based on reference: “The period change of the Cepheid Polaris suggests enhanced mass loss” The Astronomical Journal Letters, February 2012, 745:L32 (4pp), Authors: Hilding R. Neilson, Scott G. Engle, Ed Guinan, Norbert Langer, Richard P. Wasatonic, and David B. Williams Variable Stars: Cepheids and RR Lyrae • Variable stars lack the balance between gravity and internal heat • Cepheid variables are a type of radially pulsating giant stars named after the star delta Cephei • Cepheids cross the instability strip on the HertzsprungRussel diagram multiple times as they use a He-II ionization zone pulsation mechanism • All RR Lyrae stars have about the same intrinsic brightness, so differences in apparent magnitude result only from differences in their distances from us • Cepheid periods range from 2-60 days and RR Lyrae variables have periods of less than a day • Provide a good method of measuring distances Polaris Radial Velocity Curve Nielson et al. used Ṗ/P = 6/7*(Ŀ/L) – 24/7*(Ṫeff/Teff) *note mass loss is negligible here for their models of stellar evolution. But the rate at which it's period is changing is inconsistent with our models of stellar evolution. One suggestion is that it is loosing mass at a significantly larger rate than previously predicted. Taking mass loss in to account they got Ṗ/P = -4/7*(Ṁ/M) + 6/7*(Ŀ/L) – 24/7*(Ṫeff/Teff) which helped resolve the difference between the observed and theoretical rates of period change. Unsolved Questions/Problems Is significant mass loss common to Classic Cepheids or is Polaris Unique? o Theories predict mass loss to be much less than 10-6 solar masses per year, whereas models using those same theories pace mass loss at the order of 10-7 solar masses per year There was degeneracy in the models between different masses and levels of convective core overshooting Where is Polaris on the Stellar Evolution path o Observed rates of period change are consistent with Classic Cepheids during their first crossing of the instability gap, whereas models have the rate of change for Polaris at four times what is observed. Possible Solutions • T o find if mass loss is a phenomena common to other Cepheids, observations must be made which constrain relevant properties to see if they also conform to the model • Degeneracy between the models could be dealt with through additional measurements in more wavelengths • In order to determine where Polaris is on its evolutionary path, further investigation is needed into its mass loss history and the mechanism by which it loses mass. However, it appears to agree with what updated models of the blue loop stage of evolution predict.