S-type star
The class was originally defined in 1922 by Paul Merrill for stars with unusual absorption lines and molecular bands now known to be due to s-process elements.
The intrinsic S-type stars are on the most luminous portion of the asymptotic giant branch, a stage of their lives lasting less than a million years.
Cool stars, particularly class M, show molecular bands, with titanium(II) oxide (TiO) especially strong.
A small proportion of these cool stars also show correspondingly strong bands of zirconium oxide (ZrO).
The existence of clearly detectable ZrO bands in visual spectra is the definition of an S-type star.
The absorption bands now recognised as due to ZrO are clearly listed as major features of the S-type spectra.
At that time, class M was not divided into numeric sub-classes, but into Ma, Mb, Mc, and Md.
It is a digit between 1 (although the smallest type actually listed is S1.5) and 9, intended to represent a temperature scale corresponding approximately to the sequence of M1 to M9.
This calculation generally yields a number which can be rounded down to give the abundance class digit, but this is modified for higher values:[1] In practice, spectral types for new stars would be assigned by referencing to the standard stars, since the intensity values are subjective and would be impossible to reproduce from spectra taken under different conditions.
[1] A number of drawbacks came to light as S stars were studied more closely and the mechanisms behind the spectra came to be understood.
[9] In 1979 Ake defined an abundance index based on the ZrO, TiO, and YO band intensities.
[3] The new abundance index is not calculated directly, but is assigned from the relative strengths of a number of spectral features.
Primarily the relative strength of ZrO and TiO bands forms a sequence from MS stars to abundance index 1 through 6.
Abundance indices 7 to 10 are the SC stars and ZrO is weak or absent so the relative strength of the sodium D lines and Cs bands is used.
[4] The derivation of the temperature class is also refined, to use line ratios in addition to the total ZrO and TiO strength.
The ratio of BaII and SrI lines is also useful at the same indices and for carbon-rich stars with abundance index 7 to 9.
[2] This table shows the spectral types of a number of well-known S stars as they were classified at various times.
The thermal pulses, created by flashes from the helium shell, cause strong convection within the upper layers of the star.
[14] The s-process elements include zirconium (Zr), yttrium (Y), lanthanum (La), technetium (Tc), barium (Ba), and strontium (Sr), which form the characteristic S class spectrum with ZrO, YO, and LaO bands, as well as Tc, Sr, and Ba lines.
[16] The Technetium isotope produced by neutron capture in the s-process is 99Tc and it has a half life of around 200,000 years in a stellar atmosphere.
Any of the isotope present when a star formed would have completely decayed by the time it became a giant, and any newly formed 99Tc dredged up in an AGB star would survive until the end of the AGB phase, making it difficult for a red giant to have other s-process elements in its atmosphere without technetium.
After a few hundred thousand years, the 99Tc will have decayed and a technetium-free star enriched with carbon and other s-process elements will remain.
When it evolves to temperatures cool enough for ZrO absorption bands to show in the spectrum, approximately M class, it will be classified as an S-type star.
They are less strongly concentrated in the galactic disc, indicating that they are from an older population of stars than the intrinsic group.
More massive stars reach equal levels of carbon and oxygen gradually during several small dredge-ups.
Typically the rates are around 1/10,000,000th the mass of the sun per year, although in extreme cases such as W Aquilae they can be more than ten times higher.
Infrared excesses indicate that there is dust around most intrinsic S stars, but the outflow has not been sufficient and longlasting enough to form a visible detached shell.
