Helium in Three H II Galaxies and the Helium Abundance

Abstract

Extended abstractThe mass-fraction Y of helium in the interstellar medium is between 0.22 and 0.30 wherever it has been measured and it is believed to be the sum of two components: YP from Big Bang nucleosynthesis (BBNS) at about 100 s after the Big Bang (ABB) and a temperature near 0.1 MeV, and ΔY due to processing in stars. Precise measurements of Yp, along with balances of trace elements D, 3He, 7Li also resulting from BBNS, provide important tests of BBNS theory and of parameters of cosmology and particle physics, notably the contribution ΩBO of baryons to the mean density of matter in the universe (in units of the closure density), the number Nv of light neutrino flavours (or families of quarks and leptons) and the half-life т½ of the neutron (Shaver et al. 1983; Yang et al. 1984; Boesgaard and Steigman 1985). Figure 1 shows the predicted abundances from Standard BBNS theory (SBBN) as a function of η = μB/nλ the ratio of baryons to photons (unchanged since e± annihilation a few seconds ABB), which is proportional (through the known temperature of the microwave background) to ΩBOh20 where h0 is the Hubble constant in units of 100 km s−1 Mpc−1. SBBN theory (which assumes a homogeneous Friedmann universe and small lepton numbers), when combined with reasonable ideas on Galactic chemical evolution that predict a primordial (D + 3He)/H ratio below 10−4, imply that η ≥ 3 × 10−10 (shown by the tall vertical line in Fig. 1), which in turn implies YP≥0.210 if Nv = 3 and т½≥10.4 minutes. But this limit can be somewhat relaxed if т½ is smaller (current measurements permit values down to 9.0 minutes, e.g. Last et al. 1988) and/or if the quark-hadron phase transition around 200 MeV is first-order and leads to significant density fluctuations (Kurki-Sunonio et al. 1989; Reeves 1989).

The Pisa pre-main sequence tracks and isochrones

“…For the cosmological value of Y p , we used both the recent WMAP estimate Y p = 0.2485 (Cyburt et al 2004;Steigman 2006;Peimbert et al 2007a,b) and an older estimate of Y p = 0.230 (Lequeux et al 1979;Pagel & Simonson 1989;Olive et al 1991), usually referred to as canonical Y p in several stellar isochrone databases (e.g., VandenBerg et al 2000;Girardi et al 2000;Yi et al 2001;Girardi et al 2002;Cariulo et al 2004). The estimated value of helium to metal enrichment is affected by several uncertainties, thus we decided to choose both the typical value ΔY/ΔZ = 2 and a higher value ΔY/ΔZ = 5, which seems to be the upper extreme (Pagel & Portinari 1998;Jimenez et al 2003;Flynn 2004;Gennaro et al 2010 The models are available for three different values of the mixing length parameter α, namely 1.68 (solar calibrated), 1.2, and 1.9.…”

Testing pre-main-sequence models: the power of a Bayesian approach

“…For Y P we adopted both the recent Wilkinson Microwave Anisotropy Probe (WMAP) estimation Y P = 0.2485 (see e.g. Cyburt, Fields & Olive 2004; Steigman 2006) and a lower value Y P = 0.230 (Lequeux et al 1979; Pagel & Simonson 1989; Olive, Steigman & Walker 1991). In the first case, we used both Δ Y /Δ Z = 2 as commonly adopted in the literature (see e.g.…”