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RE: R-stars
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A carbon star is a late type giant star similar to a red giant (or occasionally to a red dwarf) whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly red appearance.

N class was later enhanced by an R class for less deeply red stars sharing the characteristic carbon bands of the spectrum. Later correlation of this R to N scheme with conventional spectra, showed that the R-N sequence approximately run in parallel with c:a G7 to M10 with regards to star temperature.
MK-type R0 R3 R5 R8 Na Nb
giant equiv. G7-G8 K1-K2 ~K2-K3 K5-M0 ~M2-M3 M3-M4
Teff 4300 3900 ~3700 3450 --- ---

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L

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Type R carbon stars
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Scientists determine for the first time the chemical composition of a type of red giant star with more carbon than oxygen in its atmosphere

Researchers of the University of Granada have conducted the most complete worldwide analysis of the chemical composition and evolutionary state of a spectral type R carbon star
The presence of carbon is essential for the possible development of life in the Universe, and therefore explaining its origin is of vital importance.
This research work has determined the chemical composition of a 23 type-R star sample (both hot and cold), using spectrums in the optics with high-spectral resolution, in order to obtain information about the origin of this type of stars. To this end, the scientists performed observations with a 2.2-metre in diameter telescope placed in Calar Alto (Almeria), and carried out a chemical analysis of elements such as carbon, oxygen, nitrogen, lithium and other heavy metals, such as technetium, strontium, barium or lanthanum.

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Posts: 131433
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R-stars
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Title: A Small Step on the Long Road to Understanding the R-Stars: CNO Cycling in Candidate R-Star Progenitors
Authors: George Angelou, John Lattanzio

Recent work has proposed that a merger event between a red-giant and a He white dwarf may be responsible for the production of R-stars (Izzard et al, 2007). We investigate the proposed evolution and nucleosynthesis of such a model. We simulate the hypothesized late ignition of the core flash by increasing the neutrino losses until the ignition occurs sufficiently far from the centre that the subsequent evolution produces dredge-up of carbon to the extent that the post-flash object is a carbon star. Detailed nucleosynthesis is performed within this approximation, and we show that the overall properties are broadly consistent with the observations. Details will depend on the dynamics of the merger event.

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