The stars of Wolf-Rayet or Wolf-Rayet stars are stars massive, warm and evolved almost to the end of its cycle. They have an intense loss of material associated with strong stellar winds, they have a short duration, but they grow at a fairly high speed. On their surface these stars can easily reach 100. These stars emit a lot of radiation and sometimes combine to form a binary system.
Summary
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- 1 Discovery
- 2 Features
- 3 Properties
- 4 Variables
- 5 Evolution
- 6 Source
Discovery
They were discovered by the French astronomers Charles Wolf and Georges Rayet , who identified in the constellation of the Swan three peculiar stars with bright emission bands and yellow colors. Stars WR are identified by the initials WR and a number (for example WR 134).
Characteristics
Wolf Rayet stars are super-luminous, but very rare. Out of every 1000 stars, perhaps fewer than 30 are Wolf Rayet type. Just over 300 Wolf Rayet stars are known, none of them near the Solar System but almost all of them in the Galaxy. Wolf Rayet stars are so bright that they are easily detected in the other galaxies of the Local Cluster. Its great luminosity is due to the very high temperature of its surface, which can reach 90,000 k or more. No star in the Main Sequence can produce that much energy, which is why Wolf Rayet stars are special.
These types of stars have surface temperatures of more than 25,000 – 50,000 K, high luminosities, and are very blue, with their emission peak located in the ultraviolet. Their spectra show bright emission bands corresponding to ionized hydrogen or helium. The stellar surface also features broad emission lines for carbon, nitrogen, and oxygen. They constitute the spectral type W, which is divided into two types: WN (if nitrogen is abundant) and WC (if carbon is abundant). The brightest Wolf-Rayet stars are of the first type. They are often part of binary systems in which the other star is also a massive star of spectral types O and B, or in a few as a collapsed object such as a neutron star or a black hole is believed. The brightest star of this type is Gamma-2 Velorum, of apparent magnitude 1.9 and located in the constellation Vela. Wolf-Rayet galaxies are galaxies with a high number of WR-type stars.
The average mass of Wolf Rayet stars is about 10 solar masses, so their temperature appears to be excessive for such low mass. The spectrum of a Wolf Rayet -in fact- is anomalous: it has strong and widened emission lines that correspond to Helium, Carbon, Oxygen and ionized Nitrogen, but an almost total absence of absorption lines. The surprising thing about Wolf Rayet stars is their shortage of Hydrogen. All stars are supposed to transform Hydrogen into Helium to produce energy, but Wolf Rayet stars have little or no Hydrogen.
Current models of star formation and the evidence found show that stars are formed from molecular clouds – nebulae – abundant in Hydrogen. How is it possible that Wolf Rayet stars can be formed exclusively from Helium? This is impossible. The Wolf Rayet stars had to form in the same way, but some mechanism was responsible for removing the hydrogen in them.
Properties
Nebulosity or Bubbles
Stars like the Sun emit a continuous spectrum that appears interrupted by absorption lines caused by gases in their atmosphere. In Wolf Rayet stars there also appears to be a rapidly expanding atmosphere (3,000 km / sec) which is responsible for the emission lines observed. The excitation by the star’s radiation is so high that the gases do not produce absorption, but rather emit energy. The Wolf Rayet stars are detached so violently from these gas shells that the loss of mass must be very significant. The loss is massive, fast and constant.
The expanding envelope of gases detected in the spectrum is in some cases frankly visible as an intricate, luminous bubble surrounding the star Wolf Rayet. In some cases, it has been called a planetary nebula, although true planetary nebulae occur in lower mass stars such as the Sun.
The characteristics of the star Wolf Rayet suggest that it is a spectral type star Of that has lost its outer hydrogen envelope due to powerful stellar winds. The transformation from the Of to Wolf Rayet star is brief (100,000 years) and exhausting: in the process, the star loses more than 10 solar masses.
The Sun will also lose the outer layer of Hydrogen when it becomes a planetary nebula, but in its case the bare core is not a Wolf Rayet star but a white dwarf star. The main difference is that the white dwarf is off: it no longer produces nuclear fusion reactions, it is a dead star. But the Wolf Rayet star is the core of a type Of star that is still alive even when shelled.
About 50% of Wolf Rayet stars are in binary systems. Companion stars are usually massive spectral type O or B stars, with a luminosity lower than Wolf Rayet. Although in a binary system the Wolf Rayet is of lower mass than its companion, its advanced evolution implies that its original mass was higher, at least twice as large. The fact that the star Wolf Rayet is in an advanced stage of evolution means that its mass and temperature forced it to age earlier than its companion. The extreme closeness of the companion star can stimulate and accelerate the massive loss of the star Wolf Rayet.
Gamma Velorum (WC8 +07) is a Wolf Rayet star of spectral type O that resides in a binary system. In the case of single Wolf Rayet stars, the main cause of the massive detachment is due to the pressure exerted by radiation from the core towards the outer layers of the star.
Variables
WR HD 5980 Visual Curves
The Wolf and Rayet eruptive variables are massive objects (about 20 times more than the Sun) very luminous from Population I, with absolute magnitudes between -2.2 and -6.7 and surface temperatures between 25,000 and 50,000 degrees. They were discovered in 1867 by C. Wolf and G. Rayet while making spectroscopic observations of stars in the constellation Cygnus. They have very intense, dense and hot stellar winds with speeds between 1000 and 2500 km / s and extremely high rates of mass loss. The way in which this wind is produced, a product of radiation pressure, is unknown, but it undoubtedly plays an important role in the evolution of these objects. Thus, WRs lose mass in the form of wind at a rate of between 10-6 and 10-5 solar masses per year. As a comparison,
They show broad HeI, HeII emission lines as well as CII-CIV, OII-OV, or NIII-NV, making stars of this type easily identifiable by spectroscopy. They also present irregular brightness variations with an amplitude of up to 0.1 mag., Probably physical, caused by an unstable ejection of matter on their surface. From a spectroscopic point of view, not a photometric one, Wolf-Rayet stars are divided into 3 groups:
- WN: nitrogen and some carbon dominant.
- WC: carbon dominant, absence of nitrogen.
- WO: they are scarce, with the C / O <1 ratio.
About 10% of known WRs have an envelope, often called a nebular ring. These are spherical and hemispherical HII regions with radii of tens of parsecs, remnants of star formation. WR stars are also found in binary and multiple systems. Known periods for these systems range from a few hours to several years.
Evolution
Wolf-Rayet stars come from the most massive and brightest stars of all, the spectral type O stars. These stars have such powerful stellar winds that they cause them to lose mass very quickly, until the phenomenon mentioned above occurs. which accelerates the loss of mass even more, so that at the end of its life a star that could have started with 100 solar masses can have only 8 solar masses. A Wolf-Rayet star begins as a late WN spectral type (WN9). These stars are quite similar in luminosity and temperature to their progenitors.
As it loses mass, the star gets smaller and, although its temperature increases as it shows warmer inner layers while it moves to earlier spectral types (WN8, WN7, WN6, WN5 …), said increase in temperature does not it is enough to compensate for the decrease in brightness, so that the star is decreasing its luminosity (unlike what happens in small stars like the Sun, in which in their final stages of evolution they are brighter than in the initial ones). There comes a time when the star becomes a carbon-rich Wolf-Rayet (WC), which ends up exploding as a gamma-ray burst.
