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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

Abstract

In 1867 the two astronomers C.J.E. Wolf and G. Rayet working in the observatory of Paris identified a new category of star s . The group of Wolf-Rayet stars represents a short stage of life in the evolution of massive stars (about 15 and more)
With high temperatures of about 25000 to 50000 K and huge luminosit
ies ( ), they represent an extreme type of star.
T heir extraordinary status is received by their spectrum with broad emission lines. This is a consequence of their heavy stellar winds that are marked by the Wolf-Rayet stars' special chemical composition which is characterized by the products of further nuclear fusions (N, C, O) and less Hydrogen.
By these processes these heavy stars enrich our universe with the variety of higher elements that are so important for the human existence.
 

 

Classification

Generally the WR stars are divided in two main groups, the WN and the WC stars.

WN Stars are characterized by the presence of He and N, which is a product of the H-Burning (CN-cycle).
       

  • A subdivision into WNL (late = colder) and WNE (early = hotter) is necessary, because of differences in the nuclear synthesis and development.
    • The WNL are the youngest Wolf-Rayet stars with an incomplete H-burning. They are the heaviest and according to their size they are the most luminous. As a consequence of this great luminosity they are the most variable ones. They have a proportional high concentration of N while the concentration of O is low.
    • As a consequence of development the WNE have now a complete H-burning. Due to that there is not enough H left to keep the H-Fusion alive. N is still predominating. They are hotter and brighter than their predecessors.

    • ->  A possibility to decide between the WN subtypes could be made on the basis of the ratio N III / N V. Higher temperature causes higher ionisation, so if N V dominates it must be an earlier type.

 

  • In the class of the WC-stars the proportion of Carbon increases from wpe18.jpg (914 Byte)to 40 percent. 0xygen predominates and both Hydrogen and Nitrogen have disappeared completely. Despite being the hottest WR-types they represent the lower bound in luminosity. As they have ejected the main part of their atmosphere, they have shrinked and have lost a huge amount of their mass. They are the oldest and thus highest developed WR-stars.
    A subdivision is made between WC 5 till WC9 by decreasing number of C V in favour of C IV.
  • Some sources are reporting of another very rare third type, called WN+WC . It is assumed as a transition type between WN and WC for a very short period of time.

 

classification.jpg (22914 Byte)
Source: Group 142

Facts

The Wolf-Rayet class consists of very massive stars in a particular state of their lives. As a consequence of their short lifetime as WR-star with only a few million years, they are very rare (~200 in our galaxy). Like most of the heavy stars, WR stars will end in a supernova, or even in a hypernova. So being a WR star is almost the last stay in a massive star’s life. (see development)
One characteristic is that there is almost no hydrogen left and consequently the WR stars have started their 2nd or 3rd nuclear fusion, burning helium or other higher elements. So they are creating elements like carbon, nitrogen, oxygen, or even up to iron. One can prove such elements by strong emission lines in their spectra.
Due to their high temperature they are strong UV-radiators.
They are surrounded by the material of their heavy stellar winds and sometimes by enormous nebulas! But why? Due to their high temperature and the huge radiation-pressure, they a are blowing out their envelope with speeds about 2000 km/s. By these winds they loose a huge amount of mass, ~one earth mass per year. This is the reason, why about 40% of the mass of the whole star is now located in the surrounding nebulas.
But the high temperature causes also other things, such as the big ionisation, which can be seen for example in carbon having lost three of its four outer electrons.
So they can nearly be classified as stars of the spectral class O, but in difference to other typical O-class stars they have less mass.

Factbook: Wolf-Rayet Stars
mass 5 to 60
temperature 25.000 K to 100.000 K
stellar winds and mass loss
(data could be deduced from the spectra)
Average maximum velocity: 800 to 3.000 km/s
Mass loss
variability The higher the luminosity the higher the variability

 

 

Image1 -HD 50896 – WN5 – 400 to 700 nm

Image2 –HD 50896 – WN5 – 120 to 500 nm

Image2 – HD 15763 – WC5 – 400 to 700 nm

Source: Encyclopaedia of Astronomy and Astrophysics

The main source of informations about the Wolf-Rayet stars are their spectra. By the exact examination of their profiles many physical and chemical information can be deduced.
As a consequence of the heavy stellar winds, only a small percentage of the continuum arrives us as you can see both in the visible [image1] and the UV [image2].
The spectrum is determined by emission lines of the elements existing in the winds. In the WN stars especially Nitrogen [images 1 and 2], whereas in the spectra of WC [image 3] stars Carbon predominates. In both stars you can find heavy lines of helium. By the knowledge of the Doppler-effect one can derive the velocities of the blowing winds from the line-profile.

Developement

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.

  • 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.

  • 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.

  • 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 --> Of --> WR

    Nowadays by new theoretical considerations of Meader the two following phases of evolution were created:

  • For stars greater than 50 :
    O --> OIf --> BSG --> LBV --> WN --> WC --> supernova.

  • For stars between 35 and 50 there is an optional sequence:
    O --> BSG --> YSG --> RSG --> YSG --> WN --> WC --> supernova.

The minimum mass of a star that can become a WR is due to theoretical models 25 .

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

 

    graphic    
  lrgHRDWolfRayet2.gif (86448 Byte) Hertzsprung-Russell diagram in the area of the Wolf-Rayet
stars.
Eta 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).

Comparison

Comparison between WR and other, similar early type stars  

  • LBV
    Wolf-Rayet stars are very similar to the LBVs ( L uminous B lue V ariables). Types like S Dor or P Cygni are counted to this group. Both WR and LBV are characterized by their emission lines as a consequence of their stellar winds through which both types lose approximately the same amount of mass.
    One can separate them by their different time scales of variablity. The LBV's need much more time, up to several decades.
    According to the great similarity it seems logical that these stars are assumed as one of the possible predecessors.
  • Of
    These are the hottest, most massive and luminous stars. They can weigh up to 120 solar masses. They are more extreme then WR stars.
  • Central Stars of Planetary Nebula
    They have almost equal spectra and similar physical and chemical facts as WC stars. Besides having a wider range of temperature and a higher presence of hydrogen there are no bigger differences. Even the models developed for WC stars fit to this type either.
    But they only pretend being the same. Astronomers have achieved the knowledge that the Central Stars undergo a development that is very different from the Wolf-Rayet stars. As s consequence of their development from red giant stars, they have only a small percentage of the Wolf-Rayet stars' masses.

Examples

NGC 2359

WR104

Crescent Nebula

 

Example 1: NGC 2359

Facts:

   

 Image

 

Factbook: NGC 2359

 

Image

   
  wpe28.jpg (10675 Byte)  
Rec:     07h 18m 36s
Dec:    -13° 12' 00''
Constellation:    Canis Major
Distance:    15.000 ly
Diameter:    30 ly
   
 
   

www.astrophoto.com

     

antwrp.gsfc.nasa.gov/apod/

   

Observing information

     Sky Chart    
   
         
NGC 2359, which is according to its shape also known as "Duck Nebula" or "Thor's Helmet", is located in the north-eastern part of CMa. It has a visible size of 9‘ and a real extension of 25‘. Its apparent magnitude is about 9 mag, while its interesting central star, a Wolf Rayet, is 11th magnitude. In our regions (latitude +48°) it could be observed in winter and one should have at least a 130mm telescope. Not too big magnifcations are recommended (only about 70 X). An HII filter will probably help to identify more details.

 

The chart was created by "Cartes du Ciel" (www.astrosurf.com/astropc/), a freeware program.

 

Physical information:

The nebula was discovered in 1785 by William Herschel. It is about 15.000 light years away from earth and its bubble has a diameter of 30 light years. The WR star has lost about 20 solar masses through the years due to its heavy stellar winds, which are interacting with the interstellar medium forming the bubble. Another interesting thing is that it is one of the hottest stars known .

 

Example 2: WR 104

 

     Quiz   Factbook: WR 104
  wr 104 spiral1.gif (31313 Byte)   What does this picture show?
Rec:    18h 02' 04"
Dec:    -23 37' 42"
vmag:    14.90
spectral type:    WC 9

 

 

  • Embryo
  • Spiral Galaxy
  • WR - OB binary system
    www2.keck.hawaii.edu/news/
wr104.html
  source: [2]

Observing information:

With a hobby telescope WR 104 can only be observed as a point.

Physical information:

WR 104: (A new type of stellar nebula - WR-OR binary system)

The picture above shows the top view on a binary system consisting of the Wolf-Rayet star WR 104 and his companion, an OB star. Though not being a double of the W-R Star, nevertheless it has strong stellar winds of its own. As the winds of the two stars crash, a shock front arises.

By the assumption and the consequences of this theory (a companion and a shock-front), a problem for which astronomers haven't had a solution for years, could be explained.

The problem is that there is much dust in the stellar winds. According to the physical attributes of a Wolf-Rayet star it shouldn't be there. In the direct neighbourhood of the star the intense radiation would make it impossible for the dust to exist. If far enough away from the star, it would be too cold for the dust to condense.

     Graphic    
  wr 104 keck graph.gif (43596 Byte)  The binary system and the hot dust spiral
 
 
        www2.keck.hawaii.edu/news/wr104.html

The solution is the above mentioned shock front (see picture above). It causes high density and moderate temperature. By these circumstances dust can be born.

The spiral shape is caused by the superimposition of two movements. On the one hand the collision front moves with the OB star on its orbit (220 days). On the other hand the matter is radically swept outwards as a consequence of the stellar winds.

 

Example 3: Crescent Nebula

Facts:

     Image

 

Factbook: NGC 6888
 

crescent1.jpg (41548 Byte)

Q: What does this picture show?
Rec:    20h 12' 48"
Dec:    +38° 20'
central star:    HD 192163
spectral type:    WN 6

 

A: NGC 6888, the so called Crescent Nebula. Further information
    in the text.
Telescope: 0.9-meter telescope on Kitt Peak with the NOAO Mosaic CCD camera
    http://www.noao.edu/image_gallery/images/d6/04494a.jpg source: [3]

 

Observing information:

     Sky Chart    
   
         
NGC 6888 is located south of Gamma Cygni (Sadr). Its popular name is "Crescent Nebula".
Despite being one of the brightest Wolf-Rayet-Nebulas, still a 12" telescope is necessary to recognize details. In the 18' x 13' wide field one can find a faint arc. As a consequence of O III emission, a filter can help. Due to its shape it was erroneous assumed to be a supernova remnant.

 

 

The chart was created by "Cartes du Ciel" (www.astrosurf.com/astropc/), a freeware program.

 

Physical information:

The central star of NGC 6888 is a WN 6 Wolf-Rayet with a temperature of about 55.000 K . Its catalogue number is HD 192163 or WR 136. The structure of the shell gives evidence for the theory that a W-R star could evolve from a red giant.

In his red giant phase HD 192163 ejects his outer layers as consequence of his increasing size. About 200.000 years later, a very short time for a star, the immense radiation of the exposed inner layers causes heavy stellar winds of gases ( c haracteristics of a Wolf-Rayet Star). They flush with great velocities into the surrounding space. As the speed of this stellar wind is much higher then the one ejected by the former Red-Giant star, a crash is the consequence. The result is a shell and two shockwaves. One moves outwards, the other inwards forming a very hot X -ray emitting bubble. These phenomena could be seen on the image below.

     Image  

 

 

crescent-chandra.jpg (73991 Byte)

Q: What does this picture show?
A: The red colored part shows the shell. Green in the image is the outwards moving shockwave. And finally red is the hot X-Ray emitting inner bubble.
 
Telescope: Chandra X-Ray telescope
        http://www.noao.edu/image_gallery/images/d6/04494a.jpg

Because of having such a heavy mass the Wolf-Rayet star will end up as supernova. Though having lived only 4.5 million years, it is expected to do so in the next 10.000 years. A really dramatic end.

The crescent nebula co uld be photographed by amateurs.

     Amateur-Image  

 

  ngc6888.jpg (81006 Byte) Q: What does this picture show?
A: It shows NGC 6888 photographed by an amateur

 
Telescope: Astro-Physic's 130 EDT f/8
        http://www.astropix.com/HTML/E_SUM_N/NGC6888.HTM

 

Exercises

We would propose a three step exercise to amateur astronomers:

  1. Watching WR stars just with naked eyes, binoculars or telescope

    If you assume 6m as a limit for naked eye observation, you only find three stars that are within this range. ( Gamma2 Velorum [1,74m]; HD 152408[5,29m]; Theta Mus [5,88m]). The main problem is that all three of them are not observable from our latitude (+48°).
    The use of binoculars under good conditions opens some objects to our view, but a wider range of objects is only pos
    s ible with larger telescopes.
    We tried our luck by observing WR 133 (HD 190918), a 6.7
    m WN-5 star with a O9I companion. ( RA : 20:05:57, Dec : + 35° 47‘ 18“).

     

     

    pracex2.jpg (10974 Byte)
      Our group (Johannes, Manuel, Jürgen) during the observation.
  2. Taking pictures of the nebulas around WR-Stars

    If you have good amateur equipment and good atmospherical
    ly conditions you can even photograph the nebulous shells around the WR stars. One good result we found you can see in section examples. The third picture of the " Crescent Nebula " was made by an amateur.

  3. Photograph ing their characteristic spectr a

    With pictures taken by a good spectrograph you can clearly identify the emission lines.

         Spectra by an amateur    
       
        WR 136 (WN 6; 7,65 mag) WR 135 (WC 8; 8,36 mag)    

    For both the last points we didn‘t have the opportunities.

And finally an exercise for bad weather conditions:

     Exercise    
  wr104_Apr98_title.jpg (84521 Byte) Question 1:
How long does it take for the light to travel
this 160 AU?
 
Question 2:
What diameter of mirror would be at least necessary to seperate
these details, if you had a classical telescope
without further instruments?
 
        www2.keck.hawaii.edu/news/wr104.html

 

 

Sources

books:
abc Lexikon Astronomie, Spektrum Verlag
Lexikon der Physik, Spektrum Verlag
Sterne und ihre Spektren, James B. Kaler, Spektrum Verlag
Encyclopedia of Astronomy and Astrophysics, Nature Publishing group [1]
Das grosse Lexikon der Astronomie, Fraktum Lexikon Institut
Voigt: Abriss der Astronomie, BI-Wissenschaftsverlag

magazines:
Sterne und Weltraum 2/03
Sterne und Weltraum 6/03

Internet:
http://www.peripatus.gen.nz/Astronomy/WolRaySta.html
http://members.tripod.com/debnken/wolfrayet.htm
http://www.astroman.fsnet.co.uk/wr.htm
http://www.drao-ofr.hia-iha.nrc-cnrc.gc.ca/~smd/wr-ostars.html
http://www.ast.cam.ac.uk/AAO/images/captions/aat016.htm
http://astro.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node82.html
http://antwrp.gsfc.nasa.gov/apod/ap990409.html
http://www.star.ucl.ac.uk/~pac/paper/
http://www.astroscu.unam.mx/Publicaciones/RevistaMexAstron/SAHADE/underhill/node3.html
http://www.noao.edu/image_gallery/html/im0834.html
http://www.atnf.csiro.au/people/bkoribal/publications/definitions2.html
http://www.space.com/scienceastronomy/chandra_explosion_031015.html
http://www.astropix.com/HTML/E_SUM_N/NGC6888.HTM
http://www.aai.ee/~annuk/tekst.html
http://www.astro.umd.edu/~jph/eskimo.html
http://www.aas.org/publications/baas/v26n2/aas184/abs/S6705.html
http://www2.keck.hawaii.edu/news/wr104.html

star catalogues:
The Bright Star Catalogue
SKY2000 - Master Star Catalog
Spectrophotometry of Wolf-Rayet Stars (Torres-Dodgen+ 1988)[2]
Revised New General Catalogue (Sulentic+, 1973)[3]