RR Lyrae variable

RR Lyrae stars are pulsationsveränderliche star with a regular change of light and a period of 0.2 to 1.2 days. The brightness amplitudes are up to 2 mag and the spectral type A to F. They are also called heap variables due to their frequent occurrence in globular clusters.

History

Were discovered the RR Lyrae stars in 1895 by Solon Irving Bailey in the analysis of several recordings of globular clusters of the Boyden Station of the Harvard College Observatory in Arequipa, Peru. The variables found included in their light curves on strong resemblance to the Cepheids, but their periods were much shorter in comparison with the Cepheids with between 80 minutes to 20 hours. The first variable found in the galactic field of this type was probably Mu Leporis, but only the prototype RR Lyrae in the constellation Lyra was indistinguishable designated Pickering as the cluster variables.

Subgroups

RR Lyrae stars are divided into three subgroups based on their light curve:

• RRab: This sub-group is associated with a steep rise and a large amplitude, the majority of RR Lyrae stars discovered. Due to the greater probability of detection, the seemingly high incidence is a selection effect. The stars pulsate in the fundamental mode with a period between 0.3 and 0.9 days. They are also known as RR0 stars.
• RRc: The light change is sinusoidal and the amplitude does not exceed 0.6 mag. This star pulse usually in the first harmonic having a period of 0.2 to 0.5 days. An alternative name is RR1. A very small group of RRc stars pulsate probably use only the second harmonic and is called RR2 stars.
• RRd: In this subgroup of variable pulsates with two or more periods of comparable amplitude. When an oscillation at the fundamental frequency and the first harmonic would be called these stars as RR01. The proportion of RRd star in a star system or population is a few percent, with the value between 0.5 and 30 % may fluctuate. The ratio of P0 to P1 is 0.742 to 0.748, and the values ​​of the metallicity dependent. For RR Lyrae stars that pulsate in the fundamental and the second harmonic is the period ratio between 0.585 to 0.595.

Classification

RR Lyrae stars have about half a solar mass, a roughly five-fold solar diameter and the giant stars change their surface temperature over the period between 6000-7500 ° C. The cause of the variability is the kappa mechanism as for the Cepheids. It is evolved stars on the horizontal branch in the Hertzsprung -Russell diagram. They migrate from the Red Giant Branch Coming to the left and back, where they cross the instability strip. RR Lyrae stars are found in globular clusters, the galactic halo, the bulge of the Milky Way as well as recently in extragalactic systems. The proportion of heavy elements in its atmosphere is small and is from 0.00001 to 0.01 in the share of sun.

Period change

Since the RR Lyrae stars show a strictly periodic light changes, should lead by summing up over time to a shift in the timing of maximum brightness small changes. This makes it possible to measure the direction and speed when passing through the instability strip, the expected value of the model calculations of the period change should be at 0.01 days per million years. In contrast, the observations show a hard-to -interpret results. While the mean period change in line with expectations show only 40 % of all RRab star over the period of a century, a uniform period change. 15% may be as abrupt changes superimposed with a regular period change interpreted, while the other star point only irregular and abrupt changes in period. Most stars with irregular period changes also show a Blazhko effect., .

Blazhko effect

The periodic change of light may be a long-term modulation of the light curve between 10 and 500 days to be superimposed, wherein the amplitude of the fundamental wave can vary by up to 50 percent. In addition to the amplitude and the phase changes of the brightness is modulated. Approximately 40 to 50 percent of all RR Lyrae stars of type RRab and RRc show the Blazhko effect. There are several hypotheses have been developed to explain the Blazhko effect:

• A superimposed ( non- radial ) pulsation higher order
• Modification of the pulsation by a stellar magnetic field and rotation
• A 9:2 - resonance between the fundamental frequency and the harmonic 9
• A variable turbulent convection caused by a quasi- periodic variation of the stellar magnetic field
• A non-linear interaction between the fundamental and the first harmonic

These hypotheses are not supported by new observations of COROT and Kepler satellite missions, as major changes in the Blazhko period have already been observed from cycle to cycle. In addition to the RR Lyrae stars the Blazhko effect has also been observed in the Cepheids and Delta Scuti stars. Whether long periodic modulations is also based in the light curves of sdB stars and white dwarfs on the Blazhko effect the subject of current scientific discussions.

The case of V445 Lyrae: complex behavior

Recent observations indicate that this class of stars may show a considerably more complex, possibly chaotic behavior and the previous assumption, the RR Lyrae stars should be considered as radially pulsating variable is merely a period of simplification in order to understand the basic properties.

The satellite used for the Exoplanetensuche Kepler observed star fields intensively photometrically at high frequency, which also includes long-term observations of variable stars with.

The RR Lyrae star V445 Lyrae was doing the following properties, which were previously observed on RR Lyrae star CoRoT 105 288 363:

• Radial pulsations not only in the fundamental mode, but also with low amplitudes in the first and second harmonic
• At least one non-radial pulsation
• Other likely non-radial vibrations in the frequency band between the fundamental and first harmonic
• The Blazhko modulation is periodic variable with at least two cycles of lengths
• The radial oscillations show signs of period doubling, which means the transition from a stable oscillation in a chaotic state.

RR Lyrae stars as probes

The RR Lyrae stars are Pulsationsmassen of 0.7 solar masses and so they developed low-mass stars are at least 10 billion years old. Therefore, the RR Lyrae stars occur only in stellar populations of type II and are an easy indicator for determining the evolutionary history of a star system. In addition, it can be deduced with the help of these variable stars, both the metallicity and the distance within the local group.

The absolute brightness is between 0 and 1 M.5 M. It is dependent on the period of the mass and the mean surface temperature. These factors are combined into a period-luminosity relationship. Through this relationship, with RR Lyrae stars due to the dependence of the surface temperature, the absorbance within a star system are investigated, as this leads to a reddening of the starlight.

The large frequency and brightness of the RR Lyrae stars in the halo structures allows the Milky Way and in other galaxies to analyze the local group. In the halo of the Milky Way have been discovered, and it is likely to be the remains of the Milky Way cannibalized dwarf galaxies using the pulsating variable star numerous streams.

The metallicity, the proportion of the atmosphere with elements heavier than helium, can be derived from the light curve of RRab stars. It consists of both a relationship between the amplitude of the Sandage - metallicity, as well as a derived from a Fourier transformation parameters, the metallicity by Jurcsik and Kovacs, between the period and the metal content. It is therefore possible by means of the light curve to determine both the distance and the content of heavy elements and with little effort to analyze the historical development of the investigated star system.

The light curves of the RR Lyrae stars are also imitated by a rare group of stars that are not at the stage of core helium burning. RR Lyrae stars are evolved stars of low mass of less than one solar mass, who have already gone through the stage of the Red Riesens and have migrated after the ignition of helium burning in the core on the horizontal branch of the Hertzsprung- Russell diagram. OGLE- BLG - RRLYR - 02792 shows the light curve of an RR Lyrae star after both the shape and amplitude. Since the building is also an eclipsing binaries could with the help of radial velocity measurements, the mass can be calculated to be only 0.26 solar masses instead of about 0.7 solar masses for RR Lyrae stars. Also this Pulsationsveränderliche is two magnitudes fainter than the RR Lyrae stars. Probably these unusual RRLyr develop at two times the mass exchange in some binary systems. The determination of the distance to an RR Lyrae star is thus potentially inaccurate, since it could be an unusual RRLyr which is considerably fainter.

Preston's spectral index

The investigation of the spectra of RR Lyrae stars showed a lower metallicity, the frequency of elements heavier than lithium, as in the solar atmosphere. This lower frequency is quantified using Presto 's? S spectral index. It is defined as ten times the difference between the spectral lines derived from the hydrogen and the spectral lines derived from the calcium. RR Lyrae stars with? S <3 are located in the galactic plane and have periods less than 0.4 days. In contrast, the metal-poor RR Lyrae variables with? S are > 5 stars of galactic halos and their periods are greater than 0.4 days in general. In applying the period-luminosity relation with RR Lyrae stars, therefore, always the metallicity must be considered.

Oosterhoffsche dichotomy

1939 saw the Dutch astronomer P. Oosterhoff in the processing of RR Lyrae stars in globular clusters of the Milky Way, that there is no continuous distribution of periods. Either is the mean period of RRab stars, 0.55 days or 0.65 days. Even with RRc stars there is a corresponding division, which today are known as Oosterhoff groups I and II. The dichotomy is surprising, as there is no parameter for globular clusters, which occurs in two distinguishable characteristics such as age and chemical composition. For the dwarf galaxies of the Milky Way, in contrast to the globular clusters, a continuous distribution of the average period length of RR Lyrae stars was observed.

The explanation most commonly used for the Oosterhoffsche dichotomy assumes a hysteresis effect in the development of the RR Lyrae stars on the horizontal branch. Hysteresis hinders a change in the pulsation between the fundamental ( RRab ) and the harmonic ( RRc ). In the metal- poor group II RR Lyrae phase begins at higher temperatures and the development leads in the Hertzsprung -Russell diagram to the right. In contrast, the group I stars evolve from low temperatures to the left, and then later switch of the RRab - RRc in the phase.

In dwarf galaxies of the Local Group and its globular clusters the Oosterhoffsche Dichtonomie does not occur. The distribution of the periods of the RR Lyrae stars in these star systems is continuous. The sometimes Oosterhoffsche intermediate objects called variable stars represent in some globular clusters outside the Milky Way for the largest share of all RR Lyrae stars. This is not consistent with the assumption that the Milky Way has captured dwarf galaxies in the past and their globular clusters are now part of the Milky Way.