Wolf-Rayet star
Wolf-Rayet stars (after the French astronomers Charles Wolf and Georges Rayet ), also abbreviated to WR stars in technical literature , are the exposed cores of formerly massive stars . They are none of the usual spectral assigned, but are in a separate type classified .
properties
The masses of the Wolf-Rayet stars measured so far are between 10 and 265 solar masses (M ☉ ), although originally a theoretical upper limit of around 150 M ☉ was expected. The surface temperature is between 30,000 and 120,000 K and is thus higher than that of almost all other stars.
WR stars repel large amounts of matter into their surroundings. These stellar winds are accelerated to up to 4000 km / s by the radiation from the star, which superimposes strong, very broad emission lines on the continuous spectrum . A WR star can lose up to 10 −4 M ☉ per year; The rate of mass loss can rise to ten times this episodically .
The stellar wind of carbon- rich Wolf-Rayet stars with a late spectral type WC condenses into dust particles . This happens at a greater distance, where the dust is no longer dissociated by the intense ultraviolet radiation . It is not a continuous process, but rather discrete clouds form around the Wolf-Rayet star. As a result, there are fluctuations in brightness due to the variable absorption by the carbon-rich dust .
Furthermore, WR stars arise in close binary star systems : If a massive star begins to move away from the main sequence and expands in the process, it can cross the Roche limit. The outer stellar atmosphere is no longer bound to the star and can flow away. The further development of the star leads to further expansion, the outer layers are lost. What remains is a WR star with a spectral signature that shows the thermonuclear reactions of burning hydrogen and / or burning helium in the former core. The best known example of a WR star in a binary star system is the Colliding-Wind Binary V444 Cygni.
Classification
Wolf-Rayet stars are divided into two main categories according to the predominant elements of their emission lines (order also applies to the development over time, see below):
- The WN type mainly shows emission lines of helium and multiply ionized nitrogen .
- The WC type mainly shows emission lines of oxygen and multi-ionized carbon .
- The WO type is an extension of the toilet type. In WO stars, the oxygen lines dominate; Stars of this type are very rare.
These elements come from the Wolf-Rayet star's nucleosynthesis and become visible when it blows off its hydrogen- rich atmosphere.
Occurrence in star catalogs
The General Catalog of Variable Stars currently lists about 40 variable stars with the abbreviation WR , which means that only about 0.1% of all stars in this catalog belong to the Wolf-Rayet class.
development
The typical evolution of a Wolf-Rayet star depends on the initial mass of the original star. It should be noted that mass loss already takes place during the development to the Wolf-Rayet star, so that the masses of the WR stars can be significantly lower than the initial masses.
| Initial mass in M☉ | Development path |
|---|---|
| > 75 | O-star → WN (hydrogen-rich) → LBV → WN (low-hydrogen) → WC → SN Ic |
| 40-75 | O star → LBV → WN (low hydrogen) → WC → SN Ic |
| 25 - 40 | O star → LBV or Red Super Giant → WN (low in hydrogen) → SN Ib |
Despite extensive surveys such as the Palomar Transient Factory , it has not yet been possible to identify the precursors of type Ib / c supernovae on recordings before the eruption. The precursors should be bright Wolf-Rayet stars with an absolute visual brightness that is around 150 times higher than that of the average Wolf-Rayet stars.
Simulation calculations of massive WR stars that develop in type Ib / c supernovae show an almost complete loss of their helium atmosphere shortly before the core collapse . The surface temperature rises to over 150,000 K and, in accordance with Wien's law of displacement , most of the radiation is emitted in the far ultraviolet. Therefore, shortly before their core collapse, Wolf-Rayet stars are quite faint with absolute visual magnitudes of −2 and thus about a factor of 100 fainter than most WR stars. According to computational simulations, the lifespan of massive Wolf-Rayet stars should be on the order of 500,000 years.
According to the Kollapsar model, rapidly rotating Wolf-Rayet stars could also be the forerunners of long gamma-ray bursts . Firstly, the connection between long gamma-ray bursts and supernovae of type Ib / c has now been verified by observations, and secondly, blue-shifted absorption lines with speeds of 3,000 to 4,000 km / s have been detected in the optical spectra of the long gamma-ray bursts . The properties of these blue-shifted absorption lines match an interaction of the supernova with circumstellar matter, which was created by the stellar wind of a Wolf-Rayet star.
Central stars with planetary nebula
Due to the morphological similarities of the spectrum (strong and broad emission lines) about 10% of the central stars of planetary nebulae are also referred to as Wolf-Rayet stars. These are stars with lower mass (about 0.6 M ☉ , initial masses less than 8 M ☉ ) with a hydrogen- poor atmosphere. In order to avoid confusion, the Engl. Abbreviation WR-CSPN (Wolf-Rayet - Central Star with Planetary Nebula) or [WC] (with square brackets), sometimes also [WR] , enforced.
The mass loss rates as a result of the strong stellar winds are around 10 −7 to 10 −5 M und per year and thus around ten to one hundred times higher than in normal, hydrogen-rich central stars.
The central stars of IC 4663 and Pb8 are [WN] stars, the atmosphere of which consists of 95% helium. [WN] stars could result from the merging of two white dwarfs , as this would explain the high proportion of neon and nitrogen in the atmosphere of the central star .
An alternative scenario is a diffusion induced nova . The helium burning in a Post-AGB star ignites again, and the strong convection that is triggered mixes material from the CNO core area into the atmosphere. It is assumed that [WR] -CSPN are formed from post- AGB stars by a helium flash in which the majority of the hydrogen in the star is mixed down and burned there. The atmosphere that remains consists essentially of helium, carbon and oxygen. The star is now developing into a hydrogen-poor white dwarf via a PG1159 star , which can be explained by a simple cooling sequence.
Observations of planetary nebulae have shown no systematic difference between those of ordinary (hydrogen-rich) and such hydrogen-poor (WR) central stars. This suggests that the evolution to the hydrogen-poor central star is due to chance.
Examples
- Type WN:
- Type toilet:
- γ Velorum in the constellation Sails of the Ship , visible to the naked eye, type WC
- Type WHERE:
- Unknown:
- NGC300 X – 1: binary system made up of Wolf-Rayet star and black hole
Web links
- What is a Wolf-Rayet star from the alpha Centauri TV series(approx. 15 minutes). First broadcast on Jan 3, 2007.
Individual evidence
- ↑ Jonathan Amos: Astronomers detect 'monster star'. on: BBC News. July 21, 2010.
- ↑ Bergmann, Schäfer: Stars and Space. P. 251.
- ↑ A. Unsöld, B. Baschek: The new cosmos. 7th edition. Springer, Berlin 2002, ISBN 3-662-45992-2 , p. 254.
- ↑ Alexandre David-Uraz, Anthony FJ Moffat, André-Nicolas Chené, Jason F. Rowe, Nicholas Lange, David B. Guenther, Rainer Kuschnig, Jaymie M. Matthews, Slavek M. Rucinski, Dimitar Sasselov, Werner W. Weiss: Using MOST to reveal the secrets of the mischievous Wolf-Rayet binary CV Ser . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.6032v1 .
- ↑ 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 .
- ↑ A. Unsöld, B. Baschek: The new cosmos. 7th edition. 2002, p. 189.
- ↑ Wolf-Rayet Stars on harvard.edu
- ↑ Wolf-Rayet at the spectroscopy group of the VdS (PDF; 173 kB)
- ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 9, 2019 .
- ↑ LAS McClelland, JJ Eldridge: Helium Stars: Towards an Understanding of Wolf-Rayet Evolution . In: Monthly Notices of the Royal Astronomical Society . tape 459 , no. 2 , 2017, p. 1505–1518 , doi : 10.1093 / mnras / stw618 , arxiv : 1602.06358 , bibcode : 2016MNRAS.459.1505M .
- ↑ S.-C. Yoon, G. Gräfener, JS Vink, A. Kozyreva, RG Izzard: On the nature and detectability of Type Ib / c supernova progenitors . In: Astronomy & Astrophysics . tape 544 , 2012, p. L11 , doi : 10.1051 / 0004-6361 / 201219790 .
- ↑ R. Margutti et al: A panchromatic view of the restless SN 2009IP reveals the explosive ejection of a massive star envelope . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.0038v1 .
- ↑ G. Gräfener, JS Vink, TJ Harries, N. Langer: phase Rotating Wolf-Rayet stars in a post RSG / LBV. An evolutionary channel towards long-duration GRBs? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.1153 .
- ↑ PDF at www.crya.unam.mx
- ↑ Brent Miszalski, Paul A. Crowther, Orsola De Marco, Joachim Köppen, Anthony FJ Moffat, Agnes Acker, Todd C. Hillwig: IC4663: the first unambiguous [WN] Wolf-Rayet central star of a planetary nebula . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.0562 .
- ↑ H. Todt et al.: Abell 48 - a rare WN-type central star of a planetary nebula . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.1944 .