Spectral class G to K giants, whose spectra indicate an overabundance of s-process elements
Barium stars arespectral classG toK stars whosespectra indicate an overabundance ofs-process elements by the presence of singly ionizedbarium, Ba II, atλ 455.4 nm. Barium stars also show enhanced spectral features ofcarbon, the bands of the molecules CH, CN andC2. The class was originally recognized and defined byWilliam P. Bidelman andPhilip Keenan.[1] Initially, after their discovery, they were thought to be red giants, but the same chemical signature has been observed in main-sequence stars[2][3] as well.
Barium stars are believed to be the result ofmass transfer in abinary star system. The mass transfer occurred when the now-observed giant star was on themain sequence. Its companion, the donor star, was acarbon star on theasymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior. These nuclear fusion products were mixed byconvection to its surface. Some of that matter "polluted" the surface layers of the main-sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf. These systems are being observed at an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf.[11][12] Depending on the initial properties of the binary system, the polluted star can be found at different evolutionary stages.[13]
During its evolution, the barium star will at times be larger and cooler than the limits of the spectral types G or K. When this happens, ordinarily such a star is spectral typeM, but its s-process excesses may cause it to show its altered composition as another spectral peculiarity. While the star's surface temperature is in the M-type regime, the star may show molecular features of the s-process elementzirconium, zirconium oxide (ZrO) bands. When this happens, the star will appear as an "extrinsic"S star.
Historically, barium stars posed a puzzle, because in standardstellar evolution theory G and K giants are not far enough along in their evolution to have synthesized carbon and s-process elements and mix them to their surfaces. The discovery of the stars' binary nature resolved the puzzle, putting the source of their spectral peculiarities into a companion star which should have produced such material. The mass transfer episode is believed to be quite brief on an astronomical timescale.
TheCH stars arePopulation II stars with similar evolutionary state, spectral peculiarities, and orbital statistics, and are believed to be the older, metal-poor analogs of the barium stars.[14]
^McClure, R. D.; Fletcher, J. M.; Nemec, J. M. (1980), "The binary nature of the barium stars",Astrophysical Journal Letters,238: L35,Bibcode:1980ApJ...238L..35M,doi:10.1086/183252
^McClure, R. D.; Woodsworth, A. W. (1990), "The binary nature of the barium and CH stars. III – Orbital parameters",Astrophysical Journal,352: 709,Bibcode:1990ApJ...352..709M,doi:10.1086/168573
^Jorissen, A.; Mayor, M. (1988), "Radial velocity monitoring of a sample of barium and S stars using CORAVEL – Towards an evolutionary link between barium and S stars?",Astronomy and Astrophysics,198: 187,Bibcode:1988A&A...198..187J
^McClure, R. D. (1985), "The carbon and related stars",Journal of the Royal Astronomical Society of Canada,79: 277,Bibcode:1985JRASC..79..277M
^Boffin, H. M. J.; Jorissen, A. (1988), "Can a barium star be produced by wind accretion in a detached binary?",Astronomy and Astrophysics,205: 155,Bibcode:1988A&A...205..155B
