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Spectroscopic Atlas for Amateur Astronomers 105

This classification can be supplemented with the following indices [107], [2]:

Index Specification

C2 – Index Intensity of the molecular C2 Swan bands Scale 1 – 5
CH – Index Intensity of the molecular CH absorption. Scale 1 – 6.

MS – Index Intensity of the Merrill Sanford Bands (SiC2), Scale 1 – 5
J – Index
Intensity ratio of the C2 molecular absorption with the Isotopes 12C13C and
12C12C, Scale 1 – 5.

Elements In some cases, for strong lithium and sodium lines index values are speci-
fied

Not included in this system is the still not understood class of the dC carbon dwarf stars
located on the Main Sequence.

23.5 Function of the Subclasses in the Evolution of Carbon Stars

Current professional publications provide a rather diffuse picture about the functions of
these subclasses within the evolution of carbon stars. Many details, so e.g. about the
"dredge up" processes, are obviously far from being fully understood. Further the theoreti-
cal modelling of "carbon atmospheres” ‫ܥ‬⁄ܱ > 1 seems to be difficult.

Analog to the S–Class also carbon stars on the AGB are referred as "intrinsic". It’s assumed,
that the subclass C–N, as well as the late representatives of C–R, form the last develop-
ment stage of the AGB sequence. These subgroups show a behaviour like M-giants with
very similar spectra. Furthermore they are also so-called "Low Mass Objects" with ≤ 3 solar
masses, typically show a mass loss and all of them are Variables.

Early representatives of C–R Type shows in contrast, neither variability nor a significant
mass loss. Further they seem not to be AGB stars and show a behaviour like K-giants. One
of the discussed scenarios postulates a star on the Horizontal Branch (HB) with a helium
“burning” core. However it remains here a mystery how the formed carbon will be conveyed
to the surface. A further hypothesis is a former binary system whose masses fused into one
single star.

The position of the C–J class on the AGB is unclear. It’s argued that these "Low Mass Ob-
jects" could be descendants of the C–R class on the AGB.

The C–H class seems to be "extrinsic". Most representatives are on the Horizontal Branch
(HB) and components of close binary stars. This suggests a mass transfer scenario similar
to the extrinsic S–Class.

23.6 Merrill Sanford Bands (MS)

Already at the beginning of the 20th Century in spectra of certain carbon stars, in the range
of some λλ 4640 to 5200, intensive absorptions attracted attention, which for a long time
could not be interpreted. The most intense band heads are at λλ 4977, 4909, 4867, 4640
and λ 4581. These absorption bands are named after the two explorers, who first described
these in 1926. Not until 1956 Kleman could prove with laboratory spectra that these bands
are caused by SiC2 silicon carbide. For this purpose he heated silicon in a graphite tube up
to 2500 K.

P. J. Sarre et al. have also shown [109] that the Merrill Sanford Bands are generated in
cooler layers of the stellar atmosphere, far beyond the photosphere. Merrill Sanford Bands
are most common in the entire C–J and the early C–N classes.