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Wolf-Rayet Stars |
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| Manuel Wolff ; Johannes
Zabl ; Jürgen Leschhorn Leonard Storz |
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Group 142 |
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Wolf-Rayet stars are good and nice, but you might ask how they have been formed and what their predecessors are.
Actually there are still a lot of questions concerning the past of WR-stars, but there are several hypothesis of their developement.
Generally the predecessors must be heavy stars like blue supergiants (BSG), red
supergiants (RSG), luminous blue variables (LBV) or WN/Of stars. The problem is, that the
evolution of heavy stars is not clearly understood yet.
So it’s important to look at similarities between these star types and WR-stars to
deduce the genesis of WR-stars.
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First there exist several WN/Of stars, which share the spectral characteristics of Of, an extreme type of O star off the main sequence, and WNL stars. So there is a link suggested between these two types of stars in their evolutionary phase.
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Second luminous blue variables (LBV) and WRs share an important characteristic: the variability. LBV’s have got a very irregular variability which lasts from hours to centuries. When there is an outburst the visual brightness and the bolometric magnitude increases. Then, like it is the case of WR-stars, extreme masses are ejected.
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But between these two similar objects (Of and LBV) there seems to be another link, that was recognized at a LMC (Large Magellan Cloud) Of/WN star, R127, which is now a WN11. This typical Of/WN star showed a characteristic LBV like outburst.
Because of this observation LBV’s are supposed to be a key phase in the live of massive stars. It’s assumed that a star ejects its hydrogen outer layers during the LBV phase, because there are not as many red supergiants as have been estimated without this theory. The mass loss mechanism of LBV’s is not yet understood but might be related to the Eddington limit which “provides a lower limit on the mass of a star, of a given luminosity, based on the assumption that the radiative force arising form the electron scattering apacity cannot exceed gravity.“ [1]
Due to all these links it’s believed, that O stars are the predecessors of WR-stars.
Following assumption
was made by the astronomer Conti in 1976:
O à Of à WR
Nowadays by new theoretical considerations of Meader the two following phases of evolution were created:
For stars greater than 50 M ¤ :
O à OIf à BSG à LBV à WN à WC à supernova.For stars between 35 and 50 M ¤ there is an optional sequence:
O à BSG à YSG à RSG à YSG à WN à WC à supernova.
The minimum mass of a star that can become a WR is due to theoretical models 25 M ¤ .
Interesting is, that they all end up as a
supernova because they have exceeded in mass the border that is necessary to finish the
life in this spectacular way.
But nevertheless the precise path, which seems to depend on mass, rotation rate and the
fact of having a binary (the majority of the WRs has a companion), of a star is yet
uncertain. There are still some inconsistencies in the whole theory of developement.
It is believed that on their way as WR stars they first become WNLs. In their
further live they become a WNE by getting hotter and ending their H-burning in the outer
shells. Then there is a short transition phase WN+WC. Afterwards their temperature
increases whereas their size decreases. They get a WC star as core fusion elements become
more complex ( classification).
In the end they explode in a supernova.
Hertzsprung-Russell Diagram
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Hertzsprung-Russell diagram in the area of
the Wolf-Rayet stars. |
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| E ta Carinae is a LBV star | ||||
As you can see in the diagram the WC stars are the less luminous descendants of the WN stars.
ZAMS is a derivation for "zero-age main sequence". This shows the place where a star starts its life on the main sequence. (H-ZAMS as a hydrogen burning star and He-ZAMS as a helium burning star).