[Aavso-photometry] RR Lyr surface velocity?

[Donn Starkey]

Kurt:

Horace Smith's book, "RR Lyrae Stars" gives some insight on the surface
velocity of these stars.  In the 'Survey' section of the book, there is a
good review of RV measurement using Hydrogen lines vs. metallic element
lines in the spectra and why each gives a different velocity value.  There
is also a nice explanation of the calculation for the Rosseland mean opacity
calculation.  This leads to a determination of the density of the star which
I suppose could be used to calculate the diameter of the star at any point
in the phase curve if you determine the surface temperature.  Since only the
surface of the RR Lyr stares is changing in diameter [double ionized surface
helium creates the 'valve' mechanism] I'm not sure how you would equate that
mass change to a diameter change since you could not infer the diameter of
the star by its type or color.  

You might also want to look at Jones et al. on his treatment of the diameter
changes for RS Boo as a function of the phase.  Here, they are using the
Baade-Wesselink method for calculating the distances to RR Lyr stars.

Donn


Ref:
Smith, H, A., "RR Lyrae Stars". Cambridge University Press; New Ed edition
(September 16, 2004) ISBN-10: 0521548179

Jones, R. V., Carney, B. W., Latham, D. W., "The Baade-Wesselink method and
the distances to RR Lyrae stars. VI - The field stars RS Bootis, TW
Herculis, VY Serpentis, and UU Virginis, and the absolute magnitudes of RR
Lyrae stars".1988ApJ 332, 206.

==============================================
Donn Starkey
starkey73 at mchsi.com
http://www.starkey.ws
AAVSO Observer SDB  -  CBA Indiana 
VSNET Observing Team  -  MPC Code H63
============================================== 


> Date: Fri, 7 Sep 2007 13:31:26 -0700 (PDT)
> From: Canopus56 <canopus56 at yahoo.com>
> Subject: [Aavso-photometry] RR Lyr surface velocity?
> 
> I was typing up some notes on the archetype
> short-period (13 hours) variable RR Lyr.  Assuming
> that the variable does expand and contract across the
> period, is there a stellar model estimate or an
> empirical estimate of how much the diameter of the
> star changes across its cycle and the velocity at
> which the star's surface moves? - Kurt
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