Mira variable

The Mira stars are long-period (80 to 1000 days ) variable stars with large amplitudes and late spectra. They are named after their prototype Mira in the constellation Cetus (Latin cetus ).

Definition

Mira stars are long-period red giant with emission lines and late spectra with the spectral classes Me, Se or Ce. The amplitude of the light variation is from 2.5 to 11 mag. This corresponds to a brightness change in vision between the factor 10 and 25000, while the bolometric brightness varies only by a factor of 2 to 3. They show a distinct periodicity with periods of 80-1000 days. The amplitudes in the infrared are lower than in the visible and remain mostly below 2.5 mag.

Since the brightness variations due to a large part on the change of the opacity of molecules such as the titanium oxide and the frequency of the molecules from the spectral depends be star with similar physical characteristics, depending upon their chemical composition, both the Mira stars and the semi- periodically variable stars attributed. Therefore, all variable red giant stars with periods longer than 50 days, the group of the long-period stars are attributed to what both the Mira stars, the semi-regular and the irregular variable stars includes.

Spectrum

Most Mira stars are of spectral type M with titanium bands. Only a small part of one of the carbon stars of spectral type C or S with distinct zirconia bands. The classification according to the spectral class is a result of the relative proportion of oxygen to carbon. Is less oxygen than carbon present, all the oxygen in carbon monoxide (CO) is bound and the excess of carbon is reflected in the carbon bonds of C- stars. If more oxygen than carbon present in the atmosphere as all of the carbon is bound in the non-detectable in the optical CO and the remaining oxygen forms titanium oxide. In the S- stars is probably a nearly equal ratio of oxygen to carbon.

Regardless of the spectral lines of hydrogen and occasionally the spectral lines of other elements are observed in emission at Mira stars. The emission is caused by shock waves traveling through the extended atmosphere of the red giant.

The detection of lithium in the atmospheres of Mira and other AGB stars has long been a mystery. Lithium is destroyed at temperatures of 3,000,000 K below the hydrogen burning by thermonuclear reactions. Since the star during this T Tauri phase was still fully convective, all the lithium should have been converted. The lithium content seems to increase with the pulsation period and thus age. This is interpreted as a result of hot bottom burning. The convection zone extends into the bowl of hydrogen burning and transported freshly synthesized lithium to the surface.

Light curves

The light curves of Mira stars are sinusoidal in a first approximation. In contrast to the Cepheid light curves are themselves variable and a cycle always differs from the preceding one. In increase to the maximum depressions can occur, probably due, as in the Cepheids in a 2:1 resonance between the fundamental and the first harmonic in some Mira stars. There is no relationship between the shape of the mean light curve and stellar parameters.

In addition, show some Mira stars randomly distributed brightness dips superimposed on the normal light change. This is associated with an absorption by dust particles in the shell of a red giant in conjunction.

Cause of light variation

How Cepheids are Mira stars Pulsationsveränderliche. Your Pulsationsmechanismus is also based on the kappa mechanism, wherein the temporary storage of energy is not based as opposed to the Cepheids on the ionization of helium, but that of the hydrogen. Due to the structure of the atmosphere of the red giant lacks a sharp transition layer as in the sun (keyword photosphere ) where the density waves are reflected. Therefore, the density waves propagate as shock waves through the stellar atmosphere at speeds of up to 10 km / s Due to the expansion of the stellar atmosphere, the shock wave between a hundred and several hundred days need to go through them. The visual brightness fluctuations are amplified by three effects:

• Stefan- Boltzmann law: the total amount of radiation increases with the fourth power of the temperature.
• Wien's displacement law: At lower temperatures, a large part of the radiation is in the infrared ( invisible) and in the red ( the scotopic vision is the eye there is very insensitive ) radiated. The conversion factor Candela / Watt assumes very low values.
• With decreasing temperature condense molecular bands (for example, titanium oxide) in the outer atmosphere and to absorb visible radiation re-emitted in the infrared region as an invisible radiation.

Mira stars pulsate in the fundamental mode, which can be superimposed harmonics, however. While the pulsations take place very regularly inside the star according to theoretical models, the variability of the light curve by Konvektionsströmmungen and non- radial oscillations in the extended atmosphere caused. Only a small group of AGB stars, where hydrogen-rich with a hot - bottom- burning material from the outer layers into the hydrogen- burning zone reaches, pulsate in the first harmonic. For this a different period-luminosity relation is given as above.

The vibration in the outer layers of the atmosphere of carbon stars can accelerate material condensed in some distance from the star to a cloud of carbon black. This can lead to deep minima at some Mirasternen and the related semi- regular with a high carbon content due to the absorption of light by the dust particles. Interferometric observations support the assumption of an asymmetric ejection of matter. The cause of the unbalance is not known.

Stellar wind

The shock waves transport of matter in the outer atmosphere of the red giant. There, a condensation into particles of dust takes place, received an additional impulse via the radiation pressure. This leads to a mass loss rate of up to 10-8 solar masses per year. The dust could be detected as Silcate, silicon carbide and carbon dust in the infrared. Mira stars are a major source of heavy elements, which are released into the interstellar space for subsequent generations of stars.

Development

Mira stars are stars of medium mass of stars between 0.6 up to about 3 solar masses on the Asymptotic Giant Branch. They have a dense core of carbon is a helium - burning layer over the. In turn, there is a thin hydrogen-rich layer in which only temporarily runs a hydrogen burning. It is the biggest, coolest and most luminous red giants with ages 3 to 10 billion years ago. The Mira- stage itself is fairly short-lived with a duration of a few hundred thousand years. As a predecessor of Mira stars red giants with lesser light changes are considered to be semi- regular variables. As the successor to the kernel are as proto planetary nebula or post- AGB stars. In these, the pulsation is completed and the star moves to the left in the Hertzsprung -Russell diagram to higher temperatures.

Closely related to the Mira stars are the OH / IR stars that are completely hidden in dust shells and show an even higher mass loss by stellar wind. The typical maser radiation from OH / IR stars could also be detected in some Mira stars.

Periods

The cycle length of the Mira stars is between 80 and 1000 days. The period length is inversely proportional to the surface temperature, that is It increases with decreasing temperature. The observed period changes are usually purely statistical in nature due to the variable shape of the light curve. These variations up to 5 percent of the cycle length. Few Mira stars (R Aql, T UMi, R Hya, BH Cru and W Dra ) show real period changes that are attributed to changes in radius for a helium flash. With so a thermal pulse lithium and technetium produced by the s- process, but could not be detected at the listed Mira stars. Alternative models describe the period changes in long -period variables as the result of a change in the vibration mode or a chaotic interaction between the molecular opacity and the oscillation amplitude.

The period of the light variation is only dependent on the radius and the temperature of the star in the first approximation. Accordingly, a period-luminosity relation can be derived as in the Cepheids. For the K-band in the infrared applies: M ≈ 1.0 to 3.5 log P ( M: mean absolute brightness; P: Period in days).

Period-luminosity relation

Mira stars are in the infrared, a period-luminosity relationship. They are most often observed in the K band, since there the absorbance is very small by the circumstellar matter.

A small group of AGB stars of higher mass in the state of Hot Bottom Burning deviates from this period-luminosity relationship. The advantage of the distance determination by a period-luminosity relationship with Mira stars over the Cepheids is:

• Mira stars are in the far infrared luminous than Cepheids and can therefore be observed over a greater distance
• Mira stars are also found in galaxy types such as dwarf galaxies, where no Cepheids occur
• Mira stars can be observed in the halos of spiral galaxies. In contrast to the spiral arms they are not densely populated and the problem of superposition of several stars can be avoided.