Subgroups of RR-Lyrae

RRab (RR0)

RRc (RR1)

RRd (RR2)


P = 0,5 - 0,9 d
A = 0,2 - 1 mag

basic oscillation
light change: rapid increase, slow decrease

 

P = 0,2 - 0,5 d

A = <0,6 mag

oscillation in 1st harmonic, very few in 2nd harmonic

P = 

A = 

oscillation in two or more harmonics with similar amplitudes,

P0/P1~0,742-0,748   P0/P2~0,585-5,95


pulsing in RRab & RRc modes
pulsing in RRab & RRc modes

 

 

 

There is a clear difference in periods between RRab and RRc atars, amplitudes from RRab cover a slightly wider range than RRc stars.

Oosterhoff Dichotomy

Oosterhoff found, that in globular clusters, there are no continually distribution of the periods, either the average periode of RR-stars is 0,55d or 0,65d, also with RRc stars there is a similar dichotomy, today it is called Oosterhoff groups I and II;

contrary to globular cluster, this dichotomy was not found in dwarf galaxies, there the periods are continually spread,

most common root causes is explained with hysteresis effect on the horizontal branch in HR-diagram;

Peston Spectral Index

under abundance of metallicity compared to sun, quantification by Preston spectral index deltaS = 10(spectral typ from H-lines minus spec. lines from Ca-lines);

RR-stars with deltaS<3 you will find in plane of milky way with periods of <0,4 days,

RR-stars with poorer metallicity with deltaS>5 you will find in galactic halo with periods >0,4 days,

because of that, using luminosity-period-relation it is important to take care of metallicity