Generalized limits to the number of light particle degrees of freedom from big bang nucleosynthesis

We compute the big bang nucleosynthesis limit on the number of light neutrino degrees of freedom in a model-independent likelihood analysis based on the abundances of He-4 and Li-7. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos for a range of He-4 abundances Y-p = 0.225-0.250. as well as a range in primordial Li-7 abundances from ( 1.6 to 4.1) x 10(-10). For (Li-7/H)(p) = 1.6 x 10(-10), as can be inferred from the Li-7 data from Population II halo stars, the upper limit to N-nu based on the current best estimate of the primordial He-4 abundance of Y-p = 0.238 is N-nu < 4.3 and varies from N-nu < 3.3 (at 95% C.L.) when Y-p = 0.225 to N-nu < 5.3 when Y-p = 0.250. If Li-7 is depleted in these stars the upper limit to N-nu is relaxed. Taking (Li-7/H)(p) = 4.1 x 10(-10), the limit varies from N-nu < 3.9 when Y-p = 0.225 to N-nu less than or similar to 6 when Y-p = 0.250. We also consider the consequences on the upper limit to N-nu if recent observations of deuterium in high-redshift quasar absorption-line systems are confirmed. (C) 1999 Elsevier Science B.V. All rights reserved.