Re: New Cepheid?

[Arne Henden <aah@nofs.navy.mil>]

Michael Koppelman wrote:
> Would you say that cepheid variable are more "important" than RR Lyrae 
> variables? It seem that RR's are very, very common. Are cepheids less 
> so? Is either one more reliable as a standard candle?
> 
They are "different".  Both are important.  Both need more study to help
pin down their respective P-L relations and improve distance determinations.
I'm sure Doug or Michael have a much longer list, but here is a quick
breakdown.

Cepheids
   - intrinsically brighter, so can be seen to greater
     distances
   - typically popI stars, highlighting different stellar
     populations than RR Lyrs
   - the P-L-C relation is reasonably reliable, giving ~10percent distance
     errors
   - a few are double-mode, nice for mass and metallicity determination
   - need to know which mode is primary since they have different luminosity

RR Lyrs
   - far more common, so if the object to be studied is close enough
     so that you can reach the Horizontal Branch, give better statistics
     especially when you want to study spatial structure
   - periods are short, so a single night on the telescope can give you
     periods.  The brighter cepheids are 10 days or more; hard to get
     such telescope time.
   - are PopII (metal poor) objects; not found in all area (such as open
     clusters)
   - often have the Blazhko effect that complicates solutions

Arne