Stellar pulsations

A pulsationsveränderlicher star is a variable star whose brightness varies more or less regularly by an internal excitation mechanism. In almost all pulsating variables stars this is the kappa mechanism.

History

In 1879, A. Ritter has proposed that stars can oscillate radially. Before the light change of variable stars had been declared with cover effects in a binary star system or rotation change of light. H. Shapley showed in 1914 that the strictly periodic light variation of δ Cephei can not be explained with a light cover change. The observed radial velocities in a binary system would mean that the stars would orbit each other.

Modes of vibration

Pulsating variable can oscillate radially or non-radially ( at a 90 ° angle to the radius vector standing). For non- radial pulsators, the restoring force can be gravity ( g -mode ), the static lift ( L -mode ) or the pressure (p -mode ). The vibrations may occur in the fundamental and / or harmonics. The change in radius ranges from 0 for non-radial oscillations of up to 10 percent of Cepheids. In red giant, there is a continuous transition of the atmosphere to the interstellar medium, so that a change in radius can not be specified.

Excitation mechanisms

The dominant excitation mechanism is the kappa mechanism. It is based on a nonlinear opacity, transparency in the atmosphere of the star for the energy produced inside. If energy is absorbed in a layer partially, so try the outer layers to come back into balance by contract. The storage of energy in the absorption layer leads to an expansion and consequently the energy is released. The excess energy in the upper layers of the atmosphere results in an expansion of the atmosphere, while in the absorption layer, the opacity is increased again and the cycle begins anew.

The epsilon- mechanism is based, however, on a variable power generation in the fusion rate. This mechanism has often been suspected, but has not yet been confirmed by observations. In the sun and in red giants, however, oscillations of the star are excited stochastically by convection.

Stochastic suggestions are the cause of pulsations in solar-type stars and some red giant. Convection heat transport by the rise of material into colder layers of the atmosphere and transfer it also kinetic energy can excite vibrations in the outer layers of the atmosphere.

Oscillations of a star can be excited by companion in a binary star system. These are tidal forces that are transmitted periodically by an eccentric orbit on the star. One example is the heartbeat stars. The oscillations are excited at perihelion. By the internal friction, the vibrations are damped and the amplitude increases to the next from perihelion.

Importance

The meanings of the pulsating variables in astrophysics are:

• Cepheids and RR Lyrae stars are used using the period-luminosity relationship for distance measurement.
• The asteroseismology allows a model-independent analysis of the stellar structure.
• For double- periodic pulsating variables to calculate the stellar mass is possible. This can otherwise be done only in binary systems. There, however, may differ from that of a single star by previous mass transfer of the structure of the star.

Nonlinear effects

The vibrations are not reflected in the red giant at a star's surface and continue to run through the outer atmosphere. Due to the decreasing density, this leads to the formation of shock waves that accelerate parts of the outer atmosphere above the escape velocity addition. The result is a loss of mass, which can reach up to 10-4 solar masses per year for OH / IR stars.

Subgroups

• Alpha Cygni stars are not radially pulsating supergiants with a spectral type of Bep to Aep of luminosity class Ia. The irregular translucent brightness changes are the result of a superposition of several closely spaced periods. The cycle length is a few days to weeks.
• Beta Cephei stars ( Beta Canis Majoris stars): main sequence stars with the spectral type B0.5 to B2 and low amplitudes
• Cepheids are radially pulsating supergiants with periods of 1-130 days and amplitudes of up to 2 mag in the visual. The spectral type varies during the light variation between F and K, where the spectral minimum in later runs the length of the period. The importance of Cepheids is in the period-luminosity relationship, which is why these variables are used as standard candles to measure distances within and outside the Milky Way. For the Cepheids, there are four sub-groups: The classical Cepheids are massive young stars that have evolved away from the main sequence and the instability strip crossing several times. They belong to the disk population and occur in open clusters.
• The W Virginis stars are Cepheids consisting of old stars that belong to the spherical population. They have a mass of less than one solar mass, while it is in the classical Cepheids with more than 3 solar masses. The shape of the light curve differs between the two types of Cepheids as well as their period-luminosity relationship.
• The bimodal Cepheids of type CEP (B ) swing with at least 2 periods.
• The DECPS subtype shows a low amplitude and symmetric light curves. These Cepheids pulsate probably in the first harmonic.
• The unusual Cepheids (English anomalous Cepheids ) with the prototype BL Boo. These Cepheids have periods of less than one day as RR Lyrae stars with a luminosity that is more typical of Cepheids and two magnitudes higher than that of RR Lyrae stars.

• Mira stars: giant stars with late spectral type (M, C or S) with emission lines. The light curves are variable and the periods range 80-1000 days. The amplitude of the visual ranges from 2.5 to 8 mag.
• Semi- Regular (SR) and Irregular (L): giants and supergiants of mid-to late spectral type. Occur at quasi-periods in the range of 30 to several thousand days.

• Sun-like pulsators: The vibration is maintained not by the κ - mechanism, but by convection currents.
• A group of young stars in open clusters with periods between 0.1 and 0.7 days and amplitudes in the range of less than 0.005 mag. In the HR diagram are the stars between the SPB and the Delta Scuti stars. The pulsations may be excited by the rotation of the star.
• RR Lyrae - type stars show a light curve as RR Lyrae stars, but where they have a much lower luminosity and mass. These stars evolve only through mass exchange in close binary systems.
• In binary system formed low-mass white dwarfs with a thick atmosphere. These white dwarfs in binary systems arise when a star will evolve into a red giant, but most of its atmosphere is losing a companion in his radius expansion matter.

Others

Pulsars are, despite the name similarity to the rotating variables. The name comes from the pulsed radio signal that is received at the discovery of the neutron star.