Low mass stars

Three sources have been proposed to produce fluorine in the Galaxy. The first was suggested by Forestini et al. (1992) and refers to production in low-mass stars during the AGB phase while two others are related to massive stars production in Wolf-Rayet stars (Meynet Arnould 2000) and in type II Supernovae, via the neutrino-induced nucleosynthesis (Woosley et al. 1990). 

Fig. 1. Evolutionary tracks (labelled in Mq) and isochrones (in Myr) for low-mass stars taken from two models [8,31]. The epochs of photospheric Li depletion (and hence Li-burning in the core of a fully convective star or at the convection zone base otherwise) and the development of a radiative core are indicated. The numbers to the right of the tracks indicate the fraction of photospheric Li remaining at the point where the radiative core develops and at the end of Li burning.

C. Charbonnel Rotation and Internal Gravity Waves in Low-Mass Stars . In This volume. 

We have also measured the lithium abundances in the samples of unmixed and mixed stars [6]. When low mass stars, such as those in our sample, evolve through the red giant branch, the degree of dilution of the lithium increases as the convective zone penetrates deeper and thus we expect a decline of the lithium abundance. In the mixed stars the lithium has never been detected, the upper limit of the lithium abundance is log N(Li) < 0.0, on the contrary in all the unmixed stars but one, the lithium line is visible and log N(Li) is > 0.20. In these stars as expected, the lithium abundance decreases when the gravity decreases (Fig. 3-b). 

How much the ages of young PMS object depend on the starting stellar evolution model Baraffe et al. (2002) show that the first million year(s) are very uncertain for low mass stars (the statement refers to masses O.IMq or smaller), as they... 

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

Extra-Mixing During the Red Giant Branch Evolution of Low-Mass Stars... 

While this theory has been shown to be very successful in the case of massive stars (Maeder and Meynet, this volume) and Population 1 low-mass stars (Charbonnel, this volume), full and self-consistent application in the case of globular... 

It is evident from Fig. 1 that consistency with the observed abundance of 3He in the Galaxy is achieved only if the fraction of low-mass stars (M < 2.5 Mq) undergoing extra-mixing is larger than 90%, assuming the 3He yields of... 

Boothroyd Sackmann (1999). Thus, to solve the 3He problem in terms of extra-mixing in low-mass stars, the vast majority of them (90%-100%) must be affected by this phenomenon (Galli et al. 1997). The same conclusion has been reached independently by Charbonnel do Nascimento (1998) on the basis of the statistics of carbon isotopic ratios in a sample of red-giant stars with accurate Hipparcos parallaxes. 

The most direct, model independent, way to test the validity of the mixing solution is to measure the 3He abundance in the ejecta of low-mass stars, i.e. in planetary nebulae (PNe). The search for 3He in the ejecta of PNe via the 8.667 GHz spin-flip transition of 3He+, painstakingly carried out by Rood and coworkers at the Green Bank radiotelescope since 1992 (see summary of results in Balser et al. 1997), has produced so far one solid detection (NGC 3242, see Rood, Bania, Wilson 1992 confirmed with the Effelsberg radiotelescope by... 

White dwarf The compact remnant of a low-mass star that resists further collapse by the internal degenerate electron gas. 

Figure 5.15 gives an overview of stellar evolution in the HR diagram. Both intermediate- and low-mass stars end their lives as white dwarfs after having expelled a substantial amount of mass in winds and planetary nebulae, the basic reason being the formation of a degenerate CO core that is not massive enough... 

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