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Post Info TOPIC: Wolf-Rayet star WR123
Blobrana


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RE: Wolf-Rayet star WR123
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Title: A 10-hour period revealed in optical spectra of the highly variable WN8 Wolf-Rayet star WR 123
Authors: A.-N. Chené (U. de Concepción, U. de Valparaíso, NRC/HIA), C. Foellmi, S.V. Marchenko (Science Systems and Applications, Inc.), N. St-Louis (U. de Montréal), A. F. J. Moffat (U. de Montréal), D. Ballereau (Observatoire de Paris-Meudon), J. Chauville (Observatoire de Paris-Meudon), J. Zorec (IAP), C. A. Poteet (Ritter Observatory)

Aims. What is the origin of the large-amplitude variability in Wolf-Rayet WN8 stars in general and WR123 in particular? A dedicated spectroscopic campaign targets the ten-hour period previously found in the high-precision photometric data obtained by the MOST satellite. Methods. In June-August 2003 we obtained a series of high signal-to-noise, mid-resolution spectra from several sites in the {\lambda}{\lambda} 4000 - 6940 A° domain. We also followed the star with occasional broadband (Johnson V) photometry. The acquired spectroscopy allowed a detailed study of spectral variability on timescales from ~ 5 minutes to months. Results. We find that all observed spectral lines of a given chemical element tend to show similar variations and that there is a good correlation between the lines of different elements, without any significant time delays, save the strong absorption components of the Hei lines, which tend to vary differently from the emission parts. We find a single sustained periodicity, P ~ 9.8 h, which is likely related to the relatively stable pulsations found in MOST photometry obtained one year later. In addition, seemingly stochastic, large-amplitude variations are also seen in all spectral lines on timescales of several hours to several days.

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Blobrana


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Posts: 131433
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Physical Properties of Wolf-Rayet Stars
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Title: Physical Properties of Wolf-Rayet Stars
Authors: Paul A Crowther

The striking broad emission line spectroscopic appearance of Wolf-Rayet stars have long defied analysis due to the extreme physical conditions of their line and continuum forming regions. Recently, model atmosphere studies have advanced sufficiently to enable the determination of stellar temperatures, luminosities, elemental abundances, ionising fluxes and wind properties. The observed distribution of nitrogen (WN) and carbon (WC) sequence WR stars in the Milky Way and nearby star forming galaxies is discussed, from which lower limits to progenitor masses are 25, 40, 75 solar masses for hydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars, respectively. WR stars in massive binaries permit studies of wind-wind interactions and dust formation in WC systems, plus current mass determinations, revealing typically 10-25 solar masses, although extending up to 80Msun for H-rich WN stars. Theoretical and observational evidence in favour of a metallicity dependence of WR winds is presented, with implications for evolutionary models, ionising fluxes, and the role of WR stars within the context of core-collapse supernovae and long-duration gamma ray bursts.

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Blobrana


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Posts: 131433
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RE: Wolf-Rayet star WR123
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WR123, is one of 200 "Wolf-Rayet" stars known in our galaxy.
Wolf-Rayet represent the last stages - lasting just a few hundred thousand years - in the lives of stars born with the mass of at least 25 Suns.
 The brightness of WN8 stars appears to vary chaotically.
Previous ground-based observations showed WR123 had varied on a number of different timescales - from once every 10 hours to once every few days.
Wolf-Rayet stars blast trillions of tonnes of material into space every second and are also thought to produce fleeting blasts of high-energy photons, called gamma-ray bursts, when they die as supernovae.
"They produce the heavy elements we need for life - like carbon and oxygen - and they distribute them by blowing up." - Michael Corcoran, astronomer with the Universities Space Research Association in Greenbelt, Maryland, US.
WR123 is a member of a mysterious subclass called WN8.
 17  members are known and all lie far from any stellar nursery. This suggests WN8 stars once had stellar companions that exploded as supernovae, pushing the  star into regions of empty space.



Picture of WR124, which is similar to WR123 and shows the stellar wind blasting outwards.



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Blobrana


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It has been found that a hydrogen content of less than 2% by mass for WR123 demonstrats that not all WNL stars contain substantial hydrogen.
Since H-burning lasts by far the longest, some 90% of stars that shine are actually consuming hydrogen in their cores at a prodigious rate.
Fewer than 10% of stars are in the next stage, that of He-burning, while only a miniscule fraction occurs in the subsequent, ever-faster evolving stages. WR123 represents the fleeting final stages of helium-burning, before the rapid death-spiral to supernova.
The gases ejected from stars like WR123 will enrich the interstellar medium, and contribute to future generations of stars. Understanding such stars is vital if we are to properly understand the evolution of the Milky Way and other galaxies.

"We may be seeing an example of one of the key stages in the stellar lifecycle that led to the Sun, Earth, and us, being here." - Ms. Lefevre.


Position(2000): RA 19 03 59.0 Dec -04 19 02


-- Edited by Blobrana at 08:47, 2005-05-19

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Blobrana


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The Wolf-Rayet star WR123 is even weirder than astronomers ever suspected.
The new findings, by Laure Lefevre and Anthony Moffat (Universite de Montreal), Sergey Marchenko (Western Kentucky University) using the Canadian MOST astronomical space telescope, are based on over five weeks of non-stop monitoring of the light variations of WR123. They observed WR123 every 30 seconds for 39 days non-stop during June/July 2004.
WR123 is a member of the relatively rare class of Wolf-Rayet stars, predominantly with spectral type O4, (and named after two French astronomers who discovered their telltale strong plasma winds using a simple spectroscope in the Paris suburbs in 1867).
Wolf-Rayet stars like WR 123, are characterized by a very strong wind of particles propelled outward from its surface. They have long been known to exhibit complex brightness variations associated with the turbulent high-speed winds they eject into space.
But the nearly continuous coverage possible with the MOST (Microvariability & Oscillations of Stars) satellite has revealed a clock in the chaos - a stable variation repeating every 10 hours.
"Finding a clock in a star like WR123 is like finding the Rosetta stone for astronomers studying massive stars. However, although WR123 may vary like clockwork, it must be a very strange mechanism indeed." - Ms. Lefevre, a PhD student at the Universite de Montreal.

The only theories to explain the 10-hour clock in WR123 would be:

(1) The rotation of the star itself,
(2) the orbit of another small star around WR123, or
(3) vibrations in the structure of WR123 that are transmitted to its dense enveloping wind.

All of these ideas are equally strange.
If WR123 is spinning at that rate, the surface would be moving so fast (about 2000 kilometres per *second*, or over 7 million kph) that the star should throw itself apart, unless that is the actual source of the wind!

If the star is in a tight binary system, it's so tight that its companion would be orbiting inside the star itself. If pulsations are the right answer, theoreticians will have to completely revise their current understanding of this class of massive stars.
The same period was hinted at in spectroscopic data obtained from an Earthbound observatory a year earlier, but the MOST results leave little doubt as to the bizarre timing of this stellar clock.
One hundred times fainter than what the unaided eye can see, WR123 is located about 19,000 light-years from Earth, in the direction of the constellation Aquila ("the Eagle").
WR123 and other similar Wolf-Rayets are believed to have had very violent births, ejected by a supernova explosion in a binary system, or by a gravitational slingshot from a dense star cluster.
"Either way, WR123 was probably kicked out from the nest rather abruptly." - Dr. Moffat, who helped develop these formation theories in the late 1970's.
Stars that start off their lives with ten or more times the Sun's mass are capable of "burning" hydrogen into helium, helium into carbon, and so on up to the final nuclear ash, iron, before the iron-rich core collapses on itself in less than a second and produces the greatest of all stellar explosions, a supernova.


-- Edited by Blobrana at 08:44, 2005-05-19

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