Luminous blue changer

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The gun star and the fog emitted in an LBV outbreak. False color image from the Hubble Space Telescope . Source: NASA

Luminous blue variable (short LBV; English luminous blue variable ), according to the star S Doradus also S-Doradus star or Hubble Sandage-variable called, refers to a short period blue star of 20 to 150  solar masses with pronounced variable luminosity .

Overview

The LBV, which are among the hypergiants , have the greatest mass that a hydrostatically stable star can have ( Humphreys-Davidson limit ) and radiate for a short time with a luminosity that can be millions of times that of the sun. You achieve a bolometric luminosity of −9 to −11. Due to the high surface temperature of approx. 30,000 to 50,000  Kelvin , they appear blue and belong to the  O spectral class .

LBVs pulse in several modes simultaneously and are surrounded by a gas cloud due to a strong stellar wind . The mass loss rate due to the stellar wind is between 10 −6 and 10 −3  solar masses per year. As a result, they lose a considerable part of their mass within the LBV stage. The strong loss of mass prevents the development of the red supergiant . The minimum temperature LBVs can reach during outbreaks is around 8,000 K.

After their phase as LBVs, which lasted only a few tens of thousands of years, they can develop into Wolf-Rayet stars and end in a supernova - or (previously hypothetical) hypernova explosion. If the star does not lose enough mass, it could theoretically lead to a pair instability supernova . This hypothesis is questioned, however, since no Wolf-Rayet star has yet been identified as a precursor of supernovae.

Furthermore, theoretical models also allow luminous blue variables with rather low masses between 20 and 25 solar masses to explode directly as type IIb nuclear collapse supernovae . A possible example of an LBV that exploded as a supernova is the SN 2008ax in NGC 4490 .

In summary, the possible development paths are:

M☉ Development path
20th O-Star  → Blue Super Giant  → Red Super Giant  → Blue Super Giant / Hyper Giant → LBV  → Supernova
25th O star  → O super giant → Red super giant  → O super giant → LBV  → Supernova

Due to the shortness of the LBV stage, which is estimated to be 25,000 years old, the bright blue variables belong to the rarest class of stars. Only six LBVs are known in the Milky Way and a few more in neighboring galaxies of the local group .

Star class members and candidates

definition

A massive blue star is counted among the Luminous Blue Variables if the following conditions are met:

  • a pronounced P-Cygni profile of the spectral lines of hydrogen and helium
  • combined spectral and photometric variability
  • strong stellar wind of at least 10 −6  solar masses per year at a wind speed of 100 to 500 km / s
  • Excessive abundance of nitrogen, carbon and oxygen in the atmosphere or in the surrounding fog .

If one of the conditions is not met, the star is only one of the candidates for a luminous blue variable, c LBV for short  .

variability

The variability of the LBV is divided into three time scales :

  • Fluctuations with low amplitude on the order of days to months
  • Small eruptions or S-Dor eruptions with amplitudes of 1 to 2  may be on the order of years to decades
  • Large eruptions with amplitudes greater than 2 mag and a duration of up to a few 100 years. In the Milky Way, these eruptions have so far only been detected at P Cygni and Eta Carinae, and it is not clear whether all LBV go through this stage.

The first two changes in brightness are the result of the formation of density fluctuations in the pseudo photosphere . The variable stellar wind is so dense that the radiation given off by the star is absorbed and re- emitted in the outflowing envelope . However, due to the distance from the star, the re- emitted stellar wind is cooler, and the emission occurs at different frequencies than the directly emitted original stellar wind. The bolometric brightness of the luminous blue variable changes at most by a factor of 2, while there are greater fluctuations in brightness in the visual band .

Alternatively, the semi-periodic micro-variations on the order of days to weeks are described as radial pulsations in the atmosphere of the blue supergiants. Simulations show that in the outer layers of former red supergiants , which develop in the blue area of ​​the HR diagram , radial oscillations can develop through the kappa mechanism or through strange modes , as in the case of the alpha-cygni stars . The pulsations are possibly also responsible to a certain extent for the mass losses.

The cause of the major eruptions has not been clearly identified. When calculating star models, it was noticed that during the LBV phases in the outer layers of the stars the time scale of free fall becomes larger than that of thermal diffusion . This means that during an eruption the instabilities that control the eruption (e.g. a luminosity greater than the Eddington limit ) can spread into the star and thus carry away large amounts of material. This model is known as the geyser model. The deviation from the point symmetry of the nebulae of LBV stars suggests that the rotation plays an essential role and that matter is ejected preferentially along the poles of the axis of rotation .

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists only 13 with the abbreviation SDOR with 5 additional candidates. This makes the class of S-Doradus stars one of the rare groups in this catalog with a share of just 0.02%.

Surrounding fog

The surroundings of η Carinae, the Carina Nebula , in infrared light. Source: NASA

Mists consisting of both gas and dust can be observed around most LBVs . Several envelopes are always detected, which were created in different epochs of mass loss before and during the LBV phase. The dust and gas are distributed quite differently. Both dust and complex molecules such as polycyclic aromatic hydrocarbons only condense at a greater distance from the blue giant, where they are no longer dissociated by intense ultraviolet radiation . Most of the nebulae are likely to come from the major eruptions, in which a stellar wind  blows with a mass loss of more than 10 −5 solar masses per year. In quiet phases the mass loss is around 10 −6.5  solar masses per year. In the early history of the universe, LBVs, along with supernovae, were likely to have played a significant role in the enrichment of interstellar matter with dust and heavy elements.

The nebulae around LBV have a diameter of 0.5 to 2  parsecs and expansion speeds of some 10 km / s. From this, a dynamic age of 3,000 to 40,000 years can be estimated. The nebulae are mostly axially symmetrical with a bipolar or elliptical shape. The nebulae usually lie in an empty bubble around the star. It is likely that a fast stellar wind initially swept away the circumstellar material around the star before the LBV phase, and the nebulae were created by the large eruptions.

The fog around the LBV play a role in the eruptions of these variables: the accelerated to several hundred kilometers per second matter meets the circumstellar matter of the mist and is in a shock wave slowed down, with a large part of the converted kinetic energy in the infrared optical, , ultraviolet and X-ray radiation is emitted. These large eruptions show all signs of a faint supernova of the IIn type in spectrographic examinations and are therefore also called Supernova Impostors (supernova pretenders).

The circumstellar material around a luminous blue variable leads to modulated luminosity fluctuations in the radio light curve when the LBV explodes as a supernova. During the supernova, the outer shell of the LBV is repelled and interacts with the circumstellar material that is already present. Every time the supernova ejecta hits the dense remains of the great S-Dor eruptions, the bremsstrahlung in the radio range should increase. This phenomenon has also been observed in some supernovae, in which the luminosity in the range of radio radiation has fluctuated by more than a factor of 10,000 in the years after the explosion.

Supernova Impostors

The term supernova impostors (in German about supernova rockers) describes large eruptions of LBVs whose luminosity of 10 49 to 10 50 ergs is in the order of magnitude of nuclear collapse supernovae . The light curve, the ejection of the outer layers of the atmosphere at speeds of a few 1000 km / s and the exceeding of the Eddington luminosity are actually characteristic of a supernova, such as the great eruption of Eta Carinae in the middle of the 19th century. An example of a Supernova Impostor is SN1961V.

However, strong infrared excesses have not been detected in all Impostors , as would have been expected as a result of absorption of the ejected material. A subsequent increase in the optical brightness when the dust was dissolved in the following decades was also not evident in all supernova gazers. Maybe some of the jugglers were real supernovae after all, and the star now detectable at the location of the supernova is only a background or foreground object in the distant galaxy.

Supernova impostors can be the forerunners of real supernova outbreaks, as with the SN 2009ip. This Luminous Blue Variable showed two large eruptions in 2009 and 2010 to show all signs of a true IIn Type IIn core collapse supernova in 2012, with an envelope expanding at up to 13,000 km / s. This close temporal relationship between the LBV eruptions and the supernova explosion suggests that the supernova was triggered by a pulsation- controlled pair instability or by instabilities in the final phases of nucleosynthesis , when heavier elements are burned in thermonuclear reactions at ever shorter intervals .
Alternatively, SN 2009ip could also be the result of a merger burst . According to this, the small eruptions were the result of a periastron passage of a 100 and 30 solar mass double star that merged in another passage in 2012.

In the case of the 2010mc supernova, on the other hand, a pulsation mechanism is considered to be the cause of a small outbreak with a luminosity of 10 49 erg four weeks before the type IIn supernova outbreak . A corresponding instability is also held responsible for the large outbreaks in LBVs, and in SN 2010mc, too, the first eruption could not be distinguished from those of luminous blue variables.
Alternatively, the pre-eruption of SN 2010mc could have been triggered by an interacting double star . After that, before the final attempt to produce energy in the iron core, which led to the supernova explosion, the star produced energy from the fusion of a number of other elements. In response to the oxygen burning , the precursor star has expanded and has transferred matter to its companion in a close binary star system by means of a Roche boundary flow . This accretion of one-tenth solar mass onto the companion star has released the luminosity observed as the precursor explosion of the supernova and resulted in a bipolar discharge similar to that seen in large eruptions of LBVs.

Web links

Commons : LBV  - collection of pictures, videos and audio files

Individual evidence

  1. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  2. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  3. ↑ John J. Eldridge et al .: The death of massive stars - II. Observational constraints on the progenitors of type Ibc supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.1975 .
  4. ^ Jose H. Groh, Georges Meynet, and Sylvia Ekström: Massive star evolution: Luminous Blue Variables as unexpected Supernova progenitors . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.1519 .
  5. ^ ND Richardson et al .: The H-band Emitting Region of the Luminous Blue Variable P Cygni: Spectrophotometry and Interferometry of the Wind . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.1560v1 .
  6. ↑ Jorick S. Vink: Eta Carinae and the Luminous Blue Variables . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0905.3338 .
  7. ↑ Pavel Abolmasov: Stochastic variability of Luminous Blue Variables . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1103.3523 .
  8. ^ Hideyuki Saio, Cyril Georgy, Georges Meynet: Strange mode instability for micro-variations in Luminous Blue Variables . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.4728v1 .
  9. ↑ Georges Meynet, Cyril Georgy, Raphael Hirschi, Andre Maeder, Phil Massey, Norbert Przybilla, M.-Fernanda Nieva: Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: the single massive star perspective . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1101.5873 .
  10. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 4, 2019 .
  11. ↑ C. Agliozzo, G. Umana, C. Trigilio, C. Buemi, P. Leto, A. Ingallinera, T. Franzen, A. Noriega-Crespo: Radio detection of nebulae around four LBV stars in the LMC . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.5803v1 .
  12. ↑ C. Vamvatira-Nakou et al .: Herschel imaging and spectroscopy of the nebula around the luminous blue variable star WRAY 15-751 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.0759v1 .
  13. ↑ Nathan Smith: A Model for the 19th Century Eruption of Eta Carinae: CSM Interaction Like a Scaled-Down Type I In Supernova . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.6155 .
  14. ↑ Takashi J. Moriya, Jose H. Groh, Georges Meynet: Episodic modulations in supernova radio light curves from luminous blue variable supernova progenitor models . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.0605v1 .
  15. ↑ Schuyler D. Van Dyk, Thomas Matheson: IT'S ALIVE! THE SUPERNOVA IMPOSTOR 1961V . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.0299v1 .
  16. ^ CS Kochanek, DM Szczygieł, Stanek concentration camp: Unmasking the Supernova Impostors . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.0281v1 .
  17. ^ Jon C. Mauerhan et al .: The Unprecedented Third Outburst of SN 2009ip: A Luminous Blue Variable Becomes a Supernova . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.6320 .
  18. ↑ Noam Soker, Amit Kashi: Explaining the supernova impostor sn 2009ip as mergerburst . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.5388 .
  19. ↑ EO Ofek et al .: An outburst from a massive star 40 days before a supernova explosion . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.2633v1 .
  20. ↑ Noam Soker: A BINARY SCENARIO FOR THE PRE-EXPLOSION OUTBURST OF THE SUPERNOVA 2010mc . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.5037v1 .