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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.