Other
Scientific paper
Dec 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...311..753i&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 311, Dec. 15, 1986, p. 753-761. Research supported by the John Simon Guggen
Other
84
Binary Stars, Dwarf Stars, Mass Distribution, Star Distribution, Stellar Evolution, Stellar Mass, Helium, Stellar Luminosity, Stellar Models
Scientific paper
We construct a theoretical distribution function giving birthrate versus mass for degenerate dwarfs produced in close binaries as well as one giving the birthrate versus mass for dwarfs produced by single stars and wide binaries. The overall birthrate distribution function has two major peaks one which has a sharp maximum at 0.55 Msun, near the minimum white dwarf mass which a single star or component of a wide binary can produce, and another which has a maximum at˜ 0.2 Msun. This second peak is made up primarily of helium degenerate dwarfs of mean mass 0.25 Msun produced in low-mass close binaries. Our birthrate distribution function can be made consistent with number-mass distributions defined by observed white dwarfs if we assume that the majority of low-mass helium dwarfs merge on a time scale short compared with the age of our Galaxy. This assumption is consistent with rough estimates of the degree of orbital shrinkage caused by a magnetic stellar wind and/or by common envelope action during the formation of each degenerate dwarf and with the rate of orbital shrinkage due to gravitational wave radiation after the formation of each degenerate pair. Using parameterized algorithms, we estimate that the minimum mass of a merged product is ˜0.3 Msun and that the maximum mass depends on the initial mass ratio, being typically 0.65 Msun. Some of the hydrogen caught between two merging dwarfs may mix outward by turbulent mixing, and gravitational diffusion may lead to a nearly pure hydrogen surface layer. The lightest merged products may therefore appear initially as DA white dwarfs, 40 Eri B being a possible example. The most massive merged products may ignite helium and become sdB stars. Since these stars require a long wait between the formation of the degenerate pair precursors and merging, the scale height defined by sdB products will be significantly larger than that defined by young disk stars, consistent with observations. The thin layer of surface hydrogen may be abstracted by a modest radiative wind, and all merged products may eventually become non-DA white dwarfs. The total number of non-DA white dwarfs formed in this way relative to the total number of white dwarfs may be ˜10/100.
Iben Icko Jr.
Tutukov Alexander V.
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