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Take 2: Rarity of RR Lyrae Binaries



 
 
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  #1  
Old November 17th 14, 07:45 PM posted to sci.astro.research
Robert L. Oldershaw
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Default Take 2: Rarity of RR Lyrae Binaries

This month it was noted that the number of observed RR Lyrae binaries
is "astoundingly low", especially given the commonality of these
variable stars and the usual incidences for binary systems.

Is there any conventional explanation for this dearth of RR Lyrae
binaries?
  #2  
Old November 18th 14, 11:27 AM posted to sci.astro.research
wlandsman
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Default Take 2: Rarity of RR Lyrae Binaries

On Monday, November 17, 2014 1:45:40 PM UTC-5, Robert L. Oldershaw wrote:

Is there any conventional explanation for this dearth of RR Lyrae
binaries?


I suspect that the main cause is that RR Lyrae stars are an evolved
evolutionary stage, and most close-in binaries (the easiest to detect
by radial velocity observations) are disrupted before they can become
RR Lyrae stars. RR Lyrae are in a core helium-burning stage and have
already expanded to their maximum dimensions at the tip of the giant
branch. If any mass transfer to a companion occurred during the red
giant expansion, then either the normal evolution of the primary is
disrupted or the binary expands to a larger orbit.

RR Lyrae are also about 30 times more luminous than the Sun, which is
why they can be used in Galactic structure and Hubble constant
studies. But this also makes them likely to be much brighter than any
companion, which causes difficulties for photometric searches for a
binarity.

My first thought was that RR Lyrae stars are always low-metallicity
systems and are seen only in the halo and globular clusters.
(Solar -type stars go through the core helium burning stage redward of
the RR Lyrae instability strip.) So one should compare the binary
frequency of RR Lyraes to low-metaliciity systems and *not* to the
Solar neighborhood. I knew that the binary frequency in globular
clusters was quite low (~10%) but I was surprised to find that the
binary frequency in the halo was the same as the Solar neighborhood
(Latham et al. 2002) So this was a dead end -- probably the low
binary fraction in globular clusters is mostly due to dynamical
effects rather than low metalicity.

Finally, Robert if you are interested in the mass of RR Lyrae stars,
there is a well established (both theoretically and observationally)
period-mass-luminosity relation for these pulsators. (I know you
don't believe in physics, but I though I would mention this anyway.)
--Wayne
  #3  
Old November 19th 14, 09:15 AM posted to sci.astro.research
Richard D. Saam
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Default Take 2: Rarity of RR Lyrae Binaries

On 11/18/14, 4:27 AM, wlandsman wrote:
there is a well established (both theoretically and observationally)
period-mass-luminosity relation for these pulsators.
--Wayne

ref:
http://www.aavso.org/vsx/
12,137 RR Lyrae c-type variable stars (RRc)
have average period 0.320 days
standard deviation 0.047 days
What is the theoretical source of such an observed consistent period
that you allude to?

Richard D Saam
  #4  
Old November 21st 14, 11:38 AM posted to sci.astro.research
wlandsman
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Posts: 43
Default Take 2: Rarity of RR Lyrae Binaries

On Wednesday, November 19, 2014 3:15:50 AM UTC-5, Richard D. Saam wrote:

What is the theoretical source of such an observed consistent period
that you allude to?


I wouldn't say that 0.320 +/- 0.047 days is a consistent period, but
here are a couple of factors that influence the observed distribution
of RR Lyrae periods:

1. There are instability strips in the HR (temperature - luminosity)
diagram where stars are unstable against pulsations. These are the
temperatures and densities where opacity increases with temperature
( http://en.wikipedia.org/wiki/Kappa_mechanism )

2. One must then determine if the evolutionary tracks of stars put
them in the instability strip for a significant amount of time.
Following the helium flash at the tip of the red giant branch, core
helium burning stars are located along the "horizontal branch" in the
HR diagram. Most RR Lyrae stars are located in the fairly narrow
intersection region of the horizontal branch with the instability
strip. The periods vary from the blue end to the red end of this
intersection region but the range.

[Mod. note: reformatted -- mjh]
 




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