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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
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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 |
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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
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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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