Carbon star | Wikipedia audio article


A carbon star is typically an asymptotic giant
branch star, a luminous red giant, 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 ruby red appearance. There are also some dwarf and supergiant carbon
stars, with the more common giant stars sometimes being called classical carbon stars to distinguish
them. In most stars (such as the Sun), the atmosphere
is richer in oxygen than carbon. Ordinary stars not exhibiting the characteristics
of carbon stars but cool enough to form carbon monoxide are therefore called oxygen-rich
stars. Carbon stars have quite distinctive spectral
characteristics, and they were first recognized by their spectra by Angelo Secchi in the 1860s,
a pioneering time in astronomical spectroscopy.==Spectra==By definition carbon stars have dominant spectral
Swan bands from the molecule C2. Many other carbon compounds may be present
at high levels, such as CH, CN (cyanogen), C3 and SiC2. Carbon is formed in the core and circulated
into its upper layers, dramatically changing the layers’ composition. In addition to carbon, S-process elements
such as barium, technetium, and zirconium are formed in the shell flashes and are “dredged
up” to the surface.When astronomers developed the spectral classification of the carbon
stars, they had considerable difficulty when trying to correlate the spectra to the stars’
effective temperatures. The trouble was with all the atmospheric carbon
hiding the absorption lines normally used as temperature indicators for the stars. Carbon stars also show a rich spectrum of
molecular lines at millimeter wavelengths and submillimeter wavelengths. In the carbon star CW Leonis more than 50
different circumstellar molecules have been detected. This star is often used to search for new
circumstellar molecules.===Secchi===
Carbon stars were discovered already in the 1860s when spectral classification pioneer
Angelo Secchi erected the Secchi class IV for the carbon stars, which in the late 1890s
were reclassified as N class stars.===Harvard===
Using this new Harvard classification, the 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.===Morgan–Keenan C system===
The later N classes correspond less well to the counterparting M types, because the Harvard
classification was only partially based on temperature, but also carbon abundance; so
it soon became clear that this kind of carbon star classification was incomplete. Instead a new dual number star class C was
erected so to deal with temperature and carbon abundance. Such a spectrum measured for Y Canum Venaticorum,
was determined to be C54, where 5 refers to temperature dependent features, and 4 to the
strength of the C2 Swan bands in the spectrum. (C54 is very often alternatively written C5,4). This Morgan–Keenan C system classification
replaced the older R-N classifications from 1960–1993.===The Revised Morgan–Keenan system===
The two-dimensional Morgan–Keenan C classification failed to fulfill the creators’ expectations: it failed to correlate to temperature measurements
based on infrared, originally being two-dimensional it was soon
enhanced by suffixes, CH, CN, j and other features making it impractical for en-masse
analyses of foreign galaxies’ carbon star populations,
and it gradually occurred that the old R and N stars actually were two distinct types of
carbon stars, having real astrophysical significance.A new revised Morgan–Keenan classification
was published in 1993 by Philip Keenan, defining the classes: C-N, C-R and C-H. Later the classes C-J and C-Hd were added. This constitutes the established classification
system used today.==Astrophysical mechanisms==
Carbon stars can be explained by more than one astrophysical mechanism. Classical carbon stars are distinguished from
non-classical ones on the grounds of mass, with classical carbon stars being the more
massive.In the classical carbon stars, those belonging to the modern spectral types C-R
and C-N, the abundance of carbon is thought to be a product of helium fusion, specifically
the triple-alpha process within a star, which giants reach near the end of their lives in
the asymptotic giant branch (AGB). These fusion products have been brought to
the stellar surface by episodes of convection (the so-called third dredge-up) after the
carbon and other products were made. Normally this kind of AGB carbon star fuses
hydrogen in a hydrogen burning shell, but in episodes separated by 104-105 years, the
star transforms to burning helium in a shell, while the hydrogen fusion temporarily ceases. In this phase, the star’s luminosity rises,
and material from the interior of the star (notably carbon) moves up. Since the luminosity rises, the star expands
so that the helium fusion ceases, and the hydrogen shell burning restarts. During these shell helium flashes, the mass
loss from the star is significant, and after many shell helium flashes, an AGB star is
transformed into a hot white dwarf and its atmosphere becomes material for a planetary
nebula. The non-classical kinds of carbon stars, belonging
to the types C-J and C-H, are believed to be binary stars, where one star is observed
to be a giant star (or occasionally a red dwarf) and the other a white dwarf. The star presently observed to be a giant
star accreted carbon-rich material when it was still a main-sequence star from its companion
(that is, the star that is now the white dwarf) when the latter was still a classical carbon
star. That phase of stellar evolution is relatively
brief, and most such stars ultimately end up as white dwarfs. These systems are now being observed a comparatively
long time after the mass transfer event, so the extra carbon observed in the present red
giant was not produced within that star. This scenario is also accepted as the origin
of the barium stars, which are also characterized as having strong spectral features of carbon
molecules and of barium (an s-process element). Sometimes the stars whose excess carbon came
from this mass transfer are called “extrinsic” carbon stars to distinguish them from the
“intrinsic” AGB stars which produce the carbon internally. Many of these extrinsic carbon stars are not
luminous or cool enough to have made their own carbon, which was a puzzle until their
binary nature was discovered. The enigmatic hydrogen deficient carbon stars
(HdC), belonging to the spectral class C-Hd, seems to have some relation to R Coronae Borealis
variables (RCB), but are not variable themselves and lack a certain infrared radiation typical
for RCB:s. Only five HdC:s are known, and none is known
to be binary, so the relation to the non-classical carbon stars is not known. Other less convincing theories, such as CNO
cycle unbalancing and core helium flash have also been proposed as mechanisms for carbon
enrichment in the atmospheres of smaller carbon stars.==Other characteristics==Most classical carbon stars are variable stars
of the long period variable types.===Observing carbon stars===
Due to the insensitivity of night vision to red and a slow adaption of the red sensitive
eye rods to the light of the stars, astronomers making magnitude estimates of red variable
stars, especially carbon stars, have to know how to deal with the Purkinje effect in order
not to underestimate the magnitude of the observed star.===Generation of interstellar dust===
Owing to its low surface gravity, as much as half (or more) of the total mass of a carbon
star may be lost by way of powerful stellar winds. The star’s remnants, carbon-rich “dust” similar
to graphite, therefore become part of the interstellar dust. This dust is believed to be a significant
factor in providing the raw materials for the creation of subsequent generations of
stars and their planetary systems. The material surrounding a carbon star may
blanket it to the extent that the dust absorbs all visible light.==Other classifications==
Other types of carbon stars include: CCS – Cool Carbon Star
CEMP – Carbon-Enhanced Metal-Poor CEMP-no – Carbon-Enhanced Metal-Poor star
with no enhancement of elements produced by the r-process or s-process nucleosynthesis
CEMP-r – Carbon-Enhanced Metal-Poor star with an enhancement of elements produced by
r-process nucleosynthesis CEMP-s – Carbon-Enhanced Metal-Poor star
with an enhancement of elements produced by s-process nucleosynthesis
CEMP-r/s – Carbon-Enhanced Metal-Poor star with an enhancement of elements produced by
both r-process and s-process nucleosynthesis CGCS – Cool Galactic Carbon Star==
See also==Barium star – Spectral class G to K giants,
whose spectra indicate an overabundance of s-process elements by the presence of singly
ionized barium S-type star – A cool giant with approximately
equal quantities of carbon and oxygen in its atmosphere
Technetium star – Star whose stellar spectrum contains absorption lines of technetium
Marc Aaronson, American astronomer and noted researcher of carbon starsSpecimens: R Leporis, Hind’s Crimson Star: an example
of a carbon star IRC +10216, CW Leonis: the most studied carbon
star, and also the brightest star in the sky at N-band
La Superba, Y Canum Venaticorum: one of the brighter carbon stars

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