Pulsation-variable star

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A pulsation- variable star is a variable star whose brightness fluctuates more or less regularly due to an internal excitation mechanism. In almost all pulsation-variable stars , this is the kappa mechanism .

history

As early as 1879, August Ritter proposed that stars can vibrate radially.

Before the change of light of variable stars with eclipsing effects in a binary star system or as a rotation light change was explained. However, Harlow Shapley showed in 1914 that the strictly periodic light change of δ Cephei cannot be explained with a cover light change ; the radial velocities observed in a binary star system would mean that the stars would have to orbit one another.

Types of vibration

Pulsating variables can vibrate:

The vibrations can occur:

The change in radius ranges from 0 for non-radial vibrations to 10 percent for Cepheids ; with red giants there is a continuous transition from the atmosphere to the interstellar medium , so that a change in radius cannot be specified.

Excitation mechanisms

The dominant excitation mechanism is the kappa mechanism . It is based on a non-linear opacity , the transparency in the star's atmosphere for the energy produced inside. If energy is partially absorbed in a layer , the outer layers try to return to equilibrium by contracting. The accumulation of energy in the absorption layer leads to an expansion, and thus the energy is released. The excess of energy in the upper atmospheric layers leads to an expansion of the atmosphere, while in the absorption layer the opacity rises again and the cycle begins again.

The epsilon mechanism, on the other hand, is based on a variable rate of energy production in nuclear fusion . This mechanism has often been suggested but not yet confirmed by observations.

The sun , sun-like stars and some red giants are convection excited stochastic oscillations: convective 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.

Vibrations of a star can also be excited by companions in a binary star system . Here it is tidal forces that are periodically transferred to the star through an eccentric orbit . Heartbeat stars are an example . The vibrations are stimulated in the perihelion . The internal friction dampens the vibrations and the amplitude decreases until the next perihelion.

meaning

The meanings of the pulsation variables in astrophysics are:

Nonlinear Effects

In the case of red giants , the vibrations are not reflected on a star's surface, but continue through the outer atmosphere. Due to the decreasing density , this leads to the creation of shock waves , which accelerate parts of the outer atmosphere beyond the escape speed. The result is a loss of mass that can reach up to 10 −4 solar masses per year for OH / IR stars .  

Subgroups

  • Alpha Cygni stars are non-radial pulsating supergiants with a spectral type from Bep to Aep of luminosity class Ia. The irregularly seeming changes in brightness are the result of a superposition of several periods that are close together. 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 between 1 and 130 days and amplitudes of up to 2 mag in the visual . The spectral type fluctuates in the course of the light change between F and K, with the spectral type becoming at a minimum later with the length of the period. The importance of the Cepheids lies in the period-luminosity relationship , which is why these variables are used as standard candles for measuring distances inside and outside the Milky Way . There are four subgroups among the Cepheids:
    • The classical Cepheids are young massive stars that have evolved away from the main sequence and cross the instability strip several times. They belong to the disk population and occur in open star clusters .
    • The bimodal Cepheids of the type CEP (B) vibrate with at least 2 periods.
    • The DCEPS subtype shows a low amplitude and symmetrical light curves . These Cepheids probably pulsate in the first harmonic .
    • The unusual Cepheids ( English anomalous Cepheids ) with the prototype BL Boo . These Cepheids have periods of less than a day like RR Lyrae stars with a luminosity more typical of Cepheids and 2 magnitudes higher than that of RR Lyrae stars.
    • Ultra Long Period Cepheids . Yellow giants with a period of 80 to 150 days whose light changes are similar to those of classic Cepheids.
    • Type II Cepheids are Cepheids made up of ancient stars that belong to the spherical population. They typically have a mass of less than one solar mass, while in the classical Cepheids it is more than 3 solar masses. The shape of the light curve differs between the two Cepheid species and so does their period-luminosity relationship .
      • The BL Herculis stars have a short period of a maximum of 8 days
      • The W Virginis stars have a period of 10 days to 20 days
      • RV Tauri stars : bright giants and supergiants with spectral types F to K and alternating deep and shallow minima. The periods are between approximately 30 and 150 days, with a long-period light change of the order of 1000 days being superimposed. The amplitude can reach up to 3 mag.
  • Delta Scuti stars : Short-period variables in the vicinity of the main sequence with periods of 0.02 to 0.3 days and mostly low amplitudes, which in exceptional cases can reach up to 0.8 mag. Spectral type A to F.
  • SX Phoenix stars : Like Delta Scuti stars, only belonging to Population II.
  • Gamma Doradus stars : A homogeneous group of stars with the spectral type F0 to F2 and a position close to or on the main sequence . The period ranges from 0.4 to 3 days and the amplitude reaches up to 0.1 mag.
  • Slow pulsating B star (SPB): blue variables of the spectral type B with periods around one day.
  • The long-period variables include:
    • Mira stars : Giant stars with a late spectral type (M, C or S) with emission lines. The light curves are variable and the periods range from 80 to 1000 days. The amplitude in the visual ranges from 2.5 to 8 mag.
    • Semi-regular (SR) and irregular (L): giants and supergiants of the mid to late spectral type. Quasi-periods in the range from 30 to a few thousand days occur.
  • Fast pulsating B sub-dwarfs with periods on the order of minutes at low amplitude
  • The PV Telescopii stars are helium- and carbon-rich variables with the spectral type Bp. The amplitudes do not exceed 0.1 mag for periods between 0.1 and a day.
  • RR Lyrae stars : Stars with a regular light change of up to 2 mag with a period between 0.2 and 1 day. The spectral type is A to F.
  • ZZ-Ceti stars : White dwarfs with very short periods of less than 20 minutes and low amplitudes
  • Newly discovered groups of pulsation- variable stars that have not yet been named in the General Catalog of Variable Stars :
    • Sun-like pulsators : The oscillation is not maintained by the κ-mechanism , but by convection currents .
    • A group of young stars in open star 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 , the stars lie between the SPB and Delta Scuti stars. The pulsations could be excited by the rotation of the stars.
    • RR-Lyrae-like stars show a light curve like RR-Lyrae stars, but they have a significantly lower luminosity and mass. These stars only develop through mass exchange in close binary star systems .
    • Low-mass white dwarfs with a thick atmosphere formed in a binary star system . These white dwarfs arise in binary star systems when a star wants to evolve into a red giant, but during its radius expansion matter loses most of its atmosphere to a companion.
    • Heartbeat Stars : Double star systems in which the pulsations are stimulated by the companion.
    • Hybrid pulsators : are only recently discovered, radially vibrating pulsators that vibrate both in the low-order / low amplitude p-mode and g-mode as well as in the high-order / high amplitude g-mode. You can therefore belong to several subgroups at the same time. Example: Kepler-11145123 (KIC 11145123) Most hybrid pulsators belong to the γ Doradus – δ Scuti class.

Comparison of selected classes

group typical spectral class typical mass ( M ☉ ) typical period Amplitude of the
apparent brightness (mag)
classic Cepheids F6 to K2 4 to 10 2 to 45 days 0.5 to 1.5
Type II Cepheids F to M 0.4 to 2 <1 day to about 150 days 0.5 to 4.5
Delta Scuti stars A0 to F8 1.5 to 2.5 0.02 to 0.3 days 0.02 to 0.8
RR Lyrae Stars A to F approx. 0.5 0.2 to 1.2 days 0.5 to 2
Alpha Cygni stars B to A 10 to 40 10 to 100 days 0.01 to 0.15
Beta Cephei Stars B0.5 to B2 8 to 18 3 to 7 hours 0.01 to 0.2
Gamma Doradus stars F0 to F2 1.5 to 2 0.4 to 3 days <0.1
Mira stars Me, Se, Ce 0.8 to 3 80 to 1000 days 2.5 to 11

Others

Despite the similarity of names, pulsars are among the rotating variables. The name comes from the pulsed radio signal that was received when the neutron stars were discovered.

See also

literature

Individual evidence

  1. ↑ a b Timothy R. Bedding et al .: Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars . In: Nature . tape 471 , no. 4 , 2011, p. 608–611 , doi : 10.1038 / nature09935 (English).
  2. ↑ Susan E. Thompson, Mark Everett, Fergal Mullally, Thomas Barclay, Steve B. Howell, Martin Still, Jason Rowe, Jessie L. Christiansen, Donald W. Kurtz, Kelly Hambleton, Joseph D. Twicken, Khadeejah A. Ibrahim, Bruce D. Clarke: A Class of Eccentric Binaries with Dynamic Tidal Distortions Discovered with Kepler . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.6115 (English).
  3. ↑ N. Mowlavi, F. Barblan, S. Saesen, L. Eyer: Stellar variability in open clusters. I. A new class of variable stars in NGC 3766 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.5266v2 (English).
  4. ↑ R. Smolec et al: Pulsation models for the 0.26M_sun star mimicking RR Lyrae pulsator. Model survey for the new class of variable stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.6030v2 (English).
  5. ↑ Pierre FL Maxted et al: Multi-periodic pulsations of a stripped red giant star in an eclipsing binary . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.1654v1 (English).
  6. ↑ Gerald Handler: Observational Asteroseismology . Vienna 2007, 1.3.3 Conclusions and outlook. , S. 14 (English, users.camk.edu.pl [PDF] Habilitation thesis at the Faculty of Geosciences, Geography and Astronomy of the University of Vienna): “[…] Several interesting individual objects were discovered recently ('hybrid' pulsators showing both low-order p and g modes as well as high-order g modes). [...] ”
  7. ↑ C. Aerts, J. Christensen-Dalsgaard, DW Kurtz: Asteroseismology (=  Astronomy and Astrophysics Library ). Springer Science + Business Media, Dordrecht et al. 2010, ISBN 978-1-4020-5803-5 , A. Summery of the Different Classes of Stellar Pulsators , p. 679 (English, limited preview in Google book search): “[…] Moreover, there is overlap between various classes where so-called hybrid pulsators, whose oscillations are excited into two different layers and / or by two different mechanisms, occure. [...] ”
  8. ↑ Laurent Gizon et al .: Shape of a slowly rotating star measured by asteroseismology . In: Science Advances . tape 2 , no. 11 , November 16, 2016, doi : 10.1126 / sciadv.1601777 (English, advances.sciencemag.org - article: e1601777).
  9. ↑ Laurent Gizon et al .: Shape of a slowly rotating star measured by asteroseismology . In: Science Advances . November 2016 (English, advances.sciencemag.org ): “Most known hybrid pulsators, including KIC 11145123, belong to the γ Doradus – δ Scuti class (13). Oscillations in these stars are likely to be excited by the opacity (p and mixed modes) and the convective-blocking (g modes) mechanisms. "