Carbon Stars

Carbon star

A carbon star is a late type giant star similar to the red giants (or occasionally red dwarf) star 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 to human observers.

The spectral characteristics of these stars are quite distinctive, and they were first recognized by their spectra by Angelo Secchi in the 1860s — a pioneering time in astronomical spectroscopy. In "normal" stars (such as the Sun), the atmosphere is richer in oxygen than carbon.

Astrophysical mechanisms

Carbon stars are explained by more than one astrophysical mechanism. McClure distinguishes between classical carbon stars, and other non-classical ones that are less massive.

In the classical carbon stars, 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 so called Asymptotic Giant Branch (AGB). These fusion products have been brought to the stellar surface by episodes of convection 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 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. We are now seeing these systems 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.

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.

Carbon star 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. Other elements formed through helium fusion and the s-process are also "dredged up" in this way, including lithium and barium.

When astronomers developed the spectral classification of the carbon stars, they got into considerable hardships 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.

Secchi

Carbon stars were discovered already in the 1860s when spectral classification pioneer Pater Angelo Secchi erected the Secchi class IV for the carbon stars, who in the late 1890s were reclassified as N class stars.

Harvard

Using this new Harvard classification, the N class was later enhanced by a 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
T_eff	4300	3900	~3700	3450	---	---

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).

MK-type	C0	C1	C2	C3	C4	C5	C6	C7
giant equiv.	G4-G6	G7-G8	G9-K0	K1-K2	K3-K4	K5-M0	M1-M2	M3-M4
T_eff	4500	4300	4100	3900	3650	3450	---	---

The Revised Morgan-Keenan system

This two-dimensional classification replaced the older R-N classifications during the 1960-1993, but the Morgan-Keenan C system 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 :

class	spectrum	population	M_V	theory	temperature range (K)	example(s)	# known
classical carbon stars
C-R:	the old Harvard class R reborn: are still visible at the blue end of the spectrum, strong isotopic bands, no enhanced Ba line	medium disc pop I	0	red giants?	5100-2800	S Camelopardalis	~25
C-N:	the old Harvard class N reborn: heavy diffuse blue absorption, sometimes invisible in blue, s-process elements enhanced over solar abundance, weak isotopic bands	thin disc pop I	-2.2	AGB	3100-2600	R Leporis	~90
non-classical carbon stars
C-J:	very strong isotopic bands of C₂ and CN	unknown	unknown	unknown	3900-2800	Y Canum Venaticorum	~20
C-H:	very strong CH absorption	halo pop II	-1.8	bright giants, mass transfer (all C-H:s are binary )	5000-4100	V Arietis, TT Canum Venaticorum	~20
C-Hd:	hydrogen lines and CH bands weak or absent	thin disc pop I	-3.5	unknown	?	HD 137613	~7

Other qualities

Most classical carbon stars are variable stars: miras, irregular or semiregular variable stars.

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, amateur astronomers making magnitude estimates of red variable stars, especially carbon stars, have to know how to deal with the Purkinje effect in order to not overstate the luminosity of the observed star.

Interstellar carbon sowers

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.