[Aavso-photometry] RR Lyr surface velocity?

[arne]

Canopus56 wrote:
> 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 
> 
You can take any of the radial velocity curves, integrate
them and determine the absolute radius change in centimeters
(cgs units).  However, that doesn't tell you the fractional
change in radius/diameter - for that, you need to know the
mean radius of the star.  Unfortunately, there is no direct
measure of the diameter of an RR Lyr star - even the prototype
is too far away to have a measurable diameter with the
existing interferometers.  (They *have* done this with cepheid
variables; see, for example, Kervella et al., astro-ph/0311525)

Theoretical models do exist, of course, that indicate an RR Lyr
star changes its radius by about 15 percent during the course
of its pulsational cycle.  See Horace Smith's "RR Lyrae Stars"
textbook.
Arne