Barium star

Barium stars are giant stars of spectral type G to K, whose spectra show an overabundance of s -process elements, primarily by the presence of singly ionized barium, Ba II, at a wavelength λ = 455.4 nm

Spectral

Barium stars also show stronger spectral lines of carbon, bands of the molecules CH, CN and C2. The Star class was first recognized by William Bidelman and Philip C. Keenan and defined.

Formation

Double stars

The measurement results of investigations of their radial velocities show that all barium stars are double stars. Studies in the spectral range of the ultraviolet of the International Ultraviolet Explorer also found white dwarfs in many barium - star systems.

Cataclysm

It is believed, therefore, that this class of stars formed as a result of mass transfer in a binary system. It was transferred to the present giant barium stellar mass of his partner, as the barium star was still in the development phase of its main sequence phase. His companion, the donor star, was at this time a carbon star on the asymptotic giant branch (AGB: Asymptotic Giant Branch) and produced the carbon and s- process elements in its interior. These fusion products reached then convection in the upper regions of the atmosphere of the companion star.

Part of this material was then transferred to the main sequence star, as the giant star at the end of its AGB- development, at the end it evolved into a white dwarf, grows through its Roche limit and as mass is transferred to his partner. We observe this double star systems at a time in which the donor star has long been a White Dwarf and the barium star has become a red giant.

The episode of mass transfer is considered in astronomical time scales to be relatively short. This hypothesis also barium stars also predicts on the main sequence. An example in the literature is 107 HR dar.

Development

During its development phase of the barium star can be significantly larger and cooler than the typical stars of spectral types G and K. If this happens to such a star evolves typically spectral type M, but its s- process element overabundances show his change in composition and thus its spectral particularity. While the photosphere is in the range of the type of the M-Class, shows the star molecular features of the s- process element Zr as well as zirconium oxide ( ZrO ) molecular bands. At this time the star as extrinsic S star appears ( strange latin disturbed ).

History

Historically gave the barium star initially a mystery, since stars of spectral types G and K have not yet developed long enough to have enriched carbon and other s -process elements in their photospheres. The discovery of the binary nature solved the riddle by the source of these elements and thus the cause of the spectral peculiarities of Bariumsterne was moved to the much more developed binary partner.

Examples

Provide prototypes of the barium star ζ Capricorni, HR 774 and HR 4474 dar.

The CH stars are Population II stars with similar level of development, spectral characteristics and orbital characteristics and are held for the older, metal- poor counterparts of the barium stars.

More

• Pekuliärer star
• Star construction