Mira star
The Mira stars are long-period (80 to 1000 days) pulsation-variable stars with large amplitudes and late spectra . They are named after their prototype Mira in the constellation whale ( lat . Cetus ).
definition
Mira stars are long-period red giants with emission lines and late spectra with the spectral classes Me, Se or Ce. The amplitude of the light change is between 2.5 and 11 mag . This corresponds to a change in brightness in the visual between a factor of 10 and 25,000, while the bolometric brightness only fluctuates by a factor of 2 to 3. They show a pronounced periodicity with periods between 80 and 1000 days. The amplitudes in the infrared are lower than in the visual and usually remain below 2.5 mag.
Since the brightness fluctuations are largely due to the change in the opacity of molecules such as titanium (II) oxide and the frequency of the molecules depends on the spectral type, stars with similar physical properties, depending on their chemical composition, become mira stars as well as the semi-regularly variable stars . Therefore, all variable red giants with periods of more than 50 days are included in the group of long-period stars, which includes both the Mira stars, the semi-regular and the irregularly variable stars .
Occurrence in star catalogs
The General Catalog of Variable Stars currently lists around 8000 stars with the abbreviation M , which means that around 15% of all stars in this catalog are classified as Mira stars.
properties
spectrum
Most of the Mira stars belong to the spectral class M with titanium oxide bands. Only a small part belongs to the carbon stars C (with absorption bands of the molecules CN and C 2 ) or the spectral class S with pronounced zirconium oxide bands. The division according to the spectral class is a result of the relative proportion of oxygen to carbon. If there is less oxygen than carbon, all the oxygen is bound in the carbon monoxide (CO) and the excess of carbon shows up in the CN and C 2 molecular bands of the C stars. If there is more oxygen than carbon in the atmosphere, all of the carbon is bound in the CO, which cannot be detected optically, and the remaining oxygen forms titanium (II) oxide. The S-stars have an approximately equal ratio of oxygen to carbon, so that other molecular bands appear in the spectrum (ZrO, LaO).
Regardless of the spectral class, the hydrogen lines and occasionally also the spectral lines of other elements are observed in emission in Mira stars . The emission is caused by shock waves traveling through the red giant's vast atmosphere.
The detection of lithium in the atmospheres of Mira and other AGB stars has long been a mystery. Lithium is destroyed by thermonuclear reactions at temperatures as low as 3,000,000 K below hydrogen burning . Since the star was still fully convective during this T-Tauri phase , all lithium should have been converted. The lithium content seems to increase with the pulsation period and thus with age. This is interpreted as a result of hot bottom burning . The convection zone extends into the bowl with hydrogen burning and transports freshly synthesized lithium to the surface.
Periods
The cycle length of the Mira stars is between 80 and up to 1000 days. The period length is inversely proportional to the surface temperature , i.e. it increases with decreasing temperature. The observed period changes are mostly of a purely statistical nature due to the changing shape of the light curve. These variations amount to up to 5 percent of the cycle length. Only a few Mira stars (e.g. R Aquilae , T Ursae Minoris , R Hydrae , BH Crucis and W Draconis ) show real period changes that are attributed to changes in radius after a helium flash . With such a thermal pulse, technetium and other heavy elements are produced by the s-process , but these could not be detected in all of the listed Mira stars. Alternative models describe the period changes in long-period variables as the result of a change in the oscillation mode or a chaotic interaction between the molecular opacity and the oscillation amplitude.
As a first approximation, the period of the light change only depends on the radius and the temperature of the star. Accordingly, a period-luminosity relationship can be derived as with the Cepheids. The following applies to the K band in the infrared: M ≈ 1.0 - 3.5 log P ( M : average absolute brightness; P : period duration in days).
Light curves
As a first approximation, the light curves of the Mira stars are sinusoidal. In contrast to the Cepheids , the light curves themselves are changeable and one cycle is always different from the previous one. In the rise to the maximum, depressions can occur in some Mira stars, which, like the Cepheids, are based on a 2: 1 resonance between the fundamental and the first harmonic . There is only a weak relationship between the shape of the mean light curve and stellar parameters.
In addition, some Mira stars show randomly distributed drops in brightness superimposed on the normal change of light. This has been linked to absorption by dust particles in the envelope of the red giants.
Cause of the light change
Like the Cepheids, Mira stars are pulsation-variable stars . Their pulsation mechanism is also based on the kappa mechanism , whereby the temporary energy storage, unlike the Cepheids, is not based on the ionization of helium , but that of hydrogen . Due to the structure of the atmosphere of red giants, there is no sharp transition layer like in the sun (keyword photosphere ), on which the density waves are reflected. The density waves therefore run 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 waves need between about a hundred and a few hundred days to pass through them. The visual fluctuations in brightness are amplified by three effects:
- Stefan-Boltzmann's law : The total amount of radiation increases with the fourth power of the temperature.
- Wien's law of displacement : At lower temperatures, a large part of the radiation is emitted in the infrared (invisible) and in the red (with scotopic vision, the eye is very insensitive there). The conversion factor candela / watt has very low values.
- When the temperature drops, molecules (e.g. titanium (II) oxide) condense in the outer atmosphere and absorb radiation of certain wavelengths.
Mira stars pulsate in the basic oscillation, which, however, can be superimposed by harmonics. While the pulsations inside the star are very regular according to theoretical models, the variability of the light curve is caused by convection currents and non-radial oscillations in the expanded atmosphere. Other AGB stars , the semi-regularly changing stars , pulsate in the first or second harmonic. A different period-luminosity relationship than given above applies to these .
The vibrations in the outer layers of the atmosphere of carbon stars can accelerate material which condenses into a cloud of soot at some distance from the star. This can lead to deep minima in some mira stars and the related semi-regular ones with a high carbon content due to the absorption of light by the dust particles. Interferometric observations support the assumption of an asymmetrical ejection of matter. The cause of the asymmetry is unknown.
Star wind
The shock waves transport matter into the outer atmosphere of the red giant. There, condensation takes place to form dust particles , which receive an additional impulse from the radiation pressure . This leads to a mass loss rate of up to 10 −8 to 10 −4 solar masses per year. The dust could be detected as silicate , silicon carbide and carbon dust in the infrared. The dust particles absorb radiation in the optical and near infrared range and re-emit it in the middle and far infrared. Mira stars are a significant source of heavy elements that are released into interstellar space for subsequent star generations.
development
Mira stars are stars of medium mass between approx. 0.8 to 3 solar masses on the asymptotic giant branch . They have a dense core of carbon topped with a helium-burning layer. On top of this there is a thin, hydrogen-rich layer in which hydrogen burning occurs only temporarily . They are the largest, coolest, and brightest red giants, between 3 and 10 billion years old. The Mira stage itself is quite short-lived, lasting a few hundred thousand years. As the predecessors of the Mira stars, red giants with less light change are viewed as semi-regular variables . The successors are the protoplanetary nebula or post-AGB stars . With these the pulsation has ended 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 , which are completely hidden in dust covers and show an even higher loss of mass due to stellar winds . The typical grain - radiation of OH / IR stars could in some Mira stars are detected.
Examples
See also
literature
- C. Hoffmeister, G. Richter, W. Wenzel: Variable stars . 3. Edition. JA Barth, Leipzig 1990, ISBN 3-335-00224-5 .
- JR Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
- K. Szatmáry, LL Kiss, Zs. Bebesi: The He-shell flash in action: T Ursae Minoris revisited . In: Astronomy and Astrophysics . tape 398 , 2003, p. 277-284 , doi : 10.1051 / 0004-6361: 20021646 .
- S. Uttenthaler, K. van Stiphout, K. Voet, H. van Winckel, S. van Eck et al: The evolutionary state of Miras with changing pulsation periods . In: Astronomy and Astrophysics . tape 531 , 2011, p. 88-98 , doi : 10.1051 / 0004-6361 / 201116463 , arxiv : 1105.2198 .
- Patricia A. Whitelock: Asymptotic Giant Branch Variables in the Galaxy and the Local Group. In: Variable Stars, the Galactic Halo and Galaxy Formation. Proceedings of an international conference held in Zvenigorod, Russia, 12-16 October 2009. Sternberg Astronomical Institute of Moscow University, Moscow 2010, arxiv : 1201.2997 .
Web links
Individual evidence
- ↑ Lee Anne Willson, Massimo Marengo: Miras . In: The Journal of the American Association of Variable Star Observers . tape 40 , no. 1 , 2012, p. 516 , arxiv : 1207.4094 .
- ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved February 6, 2019 .
- ↑ C. Barnbaum, RPS Stone, PC Keenan: A Moderate-Resolution Spectral Atlas of Carbon Stars: R, J, N, CH, and Barium Stars . In: Astrophysical Journal Supplement . tape 105 , 1996, pp. 419 , bibcode : 1996ApJS..105..419B .
- ↑ S. Uttenthaler, T. Lebzelter, M. Busso, S. Palmerini, B. Aringer, M. Schultheis: Lithium destruction and production observed in red giant stars . In: Memorie della Societa Astronomica Italiana Supplement . tape 22 , 2012, p. 56 , arxiv : 1206.2759v1 .
- ↑ S. Uttenthaler, K. van Stiphout, K. Voet, H. van Winckel, S. van Eck et al: The evolutionary state of Miras with changing pulsation periods . In: Astronomy and Astrophysics . tape 531 , 2011, p. 88-98 , doi : 10.1051 / 0004-6361 / 201116463 , arxiv : 1105.2198 .
- ^ T. Lebzelter: The shapes of light curves of Mira-type variables . In: Astronomical News . tape 332 , no. 2 , 2011, p. 140-146 , arxiv : 1010.2672 .
- ↑ C. Paladini, S. Sacuto, D. Klotz, K. Ohnaka, M. Wittkowski, W. Nowotny, A. Jorissen, J. Hron: Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI / MIDI . In: Astronomy & Astrophysics . tape 544 , 2012, p. 1–6 , doi : 10.1051 / 0004-6361 / 201219831 , arxiv : 1207.3910 .