3.2. Helium-3

The post-BBN evolution of ³He is considerably more complex and model dependent than that of D. Interstellar ³He incorporated into stars is burned to ⁴He (and beyond) in the hotter interiors, but preserved in the cooler, outer layers. Furthermore, while hydrogen burning in cooler, low-mass stars is a net producer of ³He (Iben 1967; Rood 1972; Dearborn, Schramm, & Steigman 1986; Vassiliadis & Wood 1993; Dearborn, Steigman, & Tosi 1996) it is unclear how much of this newly synthesized ³He is returned to the interstellar medium and how much of it is consumed in post-main sequence evolution (e.g., Sackmann & Boothroyd 1999a, b). Indeed, it is clear that when the data (Geiss & Gloeckler 1998; Rood et al. 1998; Bania, Rood, & Balser 2002) are compared to a large variety of chemical evolution models (Rood, Steigman, & Tinsley 1976; Dearborn et al. 1996; Galli et al. 1997; Palla et al. 2000; Chiappini, Renda, & Matteucci 2002), agreement is only possible for a very delicate balance between net production and net destruction of ³He. For a recent review of the current status of ³He evolution, see Romano et al. (2003). Given this state of affairs it is not possible to utilize ³He as a baryometer, but it may perhaps be used to provide a consistency check. To this end, the abundance inferred by Bania et al. (2002) from an HII region in the outer Galaxy, where post-BBN evolution might have been minimal, is adopted here: y₃ 10⁵(³He/H) = 1.1 ± 0.2.