On Low Mass AGB Stars

Abstract

ABSTRACT. Recent results on low mass AGB stars are presented. Observed amounts of AGB mass loss imply that thermal pulses will only be encountered for stars of initial mass less than about 4Μ Θ for Pop I and 3 MQ for Pop II. M C -L, M E -7j/, and M E -Tb relations are summarized. Carbon dredge-up has been found in low mass stars of both Pop I and Pop II; the mixing length parameter α is crucial to dredge-up, and its value must be normalized according to each author's opacities and mixing length treatment (e.g., via the Sun's T e and L). The "carbon star mystery" is nearing a solution, but a new "^-process mystery" has appeared: only in a narrow range of mass and metallicity have theoretical models been found that encounter the semiconvective 13 C s-process mechanism. IntroductionHistorically, low mass stars were among the first to be investigated in detail theoretically. However, much of the attention was soon shifted to intermediate and high mass stars, which do not encounter the helium core flash and therefore were considered to be easier to compute. Nevertheless, an understanding of low mass stars is crucial, since the majority of stars, including our Sun, belong to this class. Low mass stars, both in the red giant and the asymptotic giant branch (AGB) stage, are luminous and can contribute significantly to the red and infrared luminosity of galaxies and of old clusters. Low (and intermediate) mass stars on the AGB display rich spectra, showing an enrichment of carbon and s-process elements: they are the dominant source of these elements in the interstellar medium. Renewed attention was given to low mass stars due to the "carbon star mystery" (see, e.g., Iben 1981), in which theory could only provide massive carbon stars (M £ 5 MQ) of high luminosity (brighter than Mboi ~ -6), while observations found carbon stars only at lower luminosities (-3.5 > Mboi > -6)· This "carbon star mystery", and the new "s-process mystery", are discussed in Section 2.2.Low mass stars may be defined as those in the mass range Mburn < M < MueFj where urn is the lowest stellar mass for which hydrogen ignition can take place and Mu e F is the largest stellar mass for which helium ignition will take place in a degenerate core (leading to the helium core flash).

Nucleosynthesis of light elements inside thermally pulsing AGB stars

“…at least a factor of 10 or more) insufficient to explain the s−process that requires more than one neutron by seed 56 Fe. This so-called "s−process mystery" (Sackmann & Boothroyd 1991) is a common failure of all the recent stellar evolution models of TP-AGB stars. It is moreover reinforced by the fact that other lighter nuclides also capture part of the available neutrons (mainly, by increasing order of atomic mass, 12 C, 14 N, 17 O, 19 F, 24 Mg, 25 Mg, 26 Al g , 28 Si, 29 Si, 30 Si, 31 P, 32 S, 33 S, .…”