Definition

RR Lyrae variables exist in large numbers. They were once known as “cluster variables” because the first of them were found in globular clusters. There are currently more than 6000 in the General Catalog.

The typical star is RR Lyrae, the brightest of this group. It varies from visual magnitude 7.06 to one of 8.12 with a period of 13h 36m or o.56682326d. The light curve is highly asymmetric with the rise to the maximum taking 2h 35m and the fall taking about 11h. The luminosity variation accompanies a significant change in color: B-V changes from +0.14 at max to +0.45 at minimum. So the amplitude of 1.06 in visual magnitude corresponds to one of 1.37 in the B magnitude. The spectra show a large change from A8 to F7.

Description

There are four subtypes: RR, RRAB, RRC, and RR(B).

RR (such as RR Lyr) is characterized by an amplitude near to, or greater than, one magnitude, a period of about 0.5 day and by marked asymmetry. It is believed that the oscillations take place at the fundamental frequency. The shortest period 0.22d, the longest(modal value) is 0.55d. The maximum periods, although rare, exceed 1.3 d.

RRAB (such as RX Leo) is characterized by less amplitude than the first group (0.5-0.8 magnitude), the period is longer (-0.7 day), the maximum is more rounded and more symmetric. It is believed that the oscillations take place at the fundamental frequency.

RRC (such as RZ Cep) is characterized by an almost sinusoidal light curve with a rounded maximum, 0.5 amplitude, and 0.3 day period. It is believed that the oscillations take place at the first harmonic frequency. They are slightly bluer, and so hotter than the first two groups. The shortest period 0.20d, the longest (modal value) is 0.34d. The maximum periods are 0.55 d. Very few of this type compared to the two above.

There are four subtypes: RR, RRAB, RRC, and RR(B).

RR (such as RR Lyr) is characterized by an amplitude near to, or greater than, one magnitude, a period of about 0.5 day and by marked asymmetry. It is believed that the oscillations take place at the fundamental frequency. The shortest period 0.22d, the longest(modal value) is 0.55d. The maximum periods, although rare, exceed 1.3 d.

RRAB (such as RX Leo) is characterized by less amplitude than the first group (0.5-0.8 magnitude), the period is longer (-0.7 day), the maximum is more rounded and more symmetric. It is believed that the oscillations take place at the fundamental frequency.

RRC (such as RZ Cep) is characterized by an almost sinusoidal light curve with a rounded maximum, 0.5 amplitude, and 0.3 day period. It is believed that the oscillations take place at the first harmonic frequency. They are slightly bluer, and so hotter than the first two groups. The shortest period 0.20d, the longest (modal value) is 0.34d. The maximum periods are 0.55 d. Very few of this type compared to the two above.

Distribution

RR Lyrae stars are population II stars so there are large numbers in the galactic nucleus, in the halo, and in the globular clusters (although some have none). They are “old” less massive stars being on average 0.43-0.48 that of the Sun, their radii about 4-5 times larger than the Sun, resulting in low density. They form a quasi-spherical system, density decreasing slowly from the galactic plane (they are still found at 10kpc from it) and increasing sharply towards the center of the Galaxy. At 4kpc the density is four times greater than near the Sun (9.5kpc). Unlike the Cepheid’s, there is no correlation between period and galactic position.

Life
Morphology