The determination of the pregalactic, or primordial, helium abundance by mass Yp is paramount for the study of cosmology, the physics of elementary particles, and the chemical evolution of galaxies (e.g. Boesgaard & Steigman 1985; Fields & Olive 1998; Izotov et al. 1999; Peimbert & Torres-Peimbert 1999; Olive & Skillman 2000 and references therein).
We will call Yp(nHc) those Yp values in the literature derived under the assumption of no contribution to the hydrogen Balmer lines due to collisional excitation.
The best determinations of Yp(nHc) in the literature are those of Izotov & Thuan (1998); Izotov et al. (1999); and Peimbert, Peimbert, & Ruiz (2000) that amount to 0.2443 ± 0.0015, 0.2452 ± 0.0015, and 0.2345 ± 0.0026 respectively. These determinations are based on 45, 2, and 1 extragalactic H II regions respectively and the differences between the first two and the last one amount to at least 3.
To study the source of this discrepancy Peimbert & Peimbert (2001) and Peimbert, Peimbert, & Luridiana (2001) decided to compute Yp(nHc) based on the data by Izotov & Thuan (1998) and Izotov et al. (1999). From two different subsamples of the best observed objects, comprising 12 and 5 objects, found that Yp(nHc) amounts to 0.2371 ± 0.0015 and 0.2360 ± 0.0025 respectively. These results are in good agreement with the value derived by Peimbert et al. (2000) and are significantly smaller than the values derived by Izotov & Thuan (1998) and Izotov et al. (1999).
The main source of the discrepancy between both groups of authors is due to the treatment of the temperature structure inside the nebulae; while Izotov & Thuan (1998) and Izotov et al. (1999) adopt T(O III) to derive the helium abundance, Peimbert & Peimbert (2001) and Peimbert et al. (2001) from the He I line intensities and adopting t2 > 0.00 determine T(He II) values 6-11% smaller than T(O III). In the self-consistent solutions the smaller T(He II) values imply higher densities; the higher the density the higher the collisional contribution to the He I line intensities and, consequently, the lower the helium abundances.
The baryon energy density, b, values derived by Peimbert & Peimbert (2001) and Peimbert et al. (2001) from the Yp(nHc) values are significantly smaller than the b value derived from the Dp determination by O'Meara et al. (2001). Before we conclude that a non-standard big bang nucleosynthesis model is needed to reconcile the differences it is necessary to analyze further two possible systematic effects: a) the ionization structure of the H II regions, and b) the collisional excitation of the hydrogen lines.
To determine very accurate He/H values of a given H II region we need to consider its ionization structure. The total He/H value is given by:
For objects of low degree of ionization it is necessary to consider the presence of He0 inside the H+ zone, while for objects of high degree of ionization it is necessary to consider the possible presence of H0 inside the He+ zone. For objects of low degree of ionization ICF(He) might be larger than 1.00, while for objects of high degree of ionization ICF(He) might be smaller than 1.00. The deviations from unity in the ICF(He) value occur in and near the ionization boundary of a given H II region, therefore those H II regions that are density bounded in all directions have an ICF(He) = 1.00. The ICF(He) problem has been discussed by many authors (e.g. Shields 1974; Stasinska 1983; Peña 1986; Vílchez & Pagel 1988; Pagel et al. 1992; Armour et al. 1999; Peimbert & Peimbert 2000; Viegas, Gruenwald, & Steigman 2000; Viegas & Gruenwald 2000; Ballantyne, Ferland, & Martin 2000; Sauer & Jedamzik 2001).
Based on models of metal poor H II regions Luridiana et al. (2001) find that the ICF(He) for some of the best observed objects is very close to 1.00 and consequently that the main difference between the b value derived from Yp(nHc) and Dp is not due to the ICF(He). Relaño et al. (2001) from the spectral types of the ionizing stars of NGC 346 find that about half of the ionizing photons escape the nebula favoring an ICF(He) = 1.00, this result is also supported by the fit of the lines of low degree of ionization by their photoionization model. From the work by Zurita, Rozas, & Beckman (2000) on the ionization of the diffuse interstellar medium in external galaxies it is expected that a large fraction of the ionizing photons escapes from the most luminous H II regions, which favors the assumption that the ICF(He) is very close to 1.00.
Davidson & Kinman (1985) were the first to estimate the collisional contribution to the Balmer lines and its effect on the determination of Yp; they made a crude estimate for I Zw 18 and concluded that the collisional contribution to I(H) may be roughly 2%. All the subsequent determinations of Yp in the literature have been derived under the assumption of no contribution to the hydrogen Balmer lines due to collisional excitation, I have referred to these determinations in this paper as Yp(nHc).
Notice that to a very good approximation the collisional excitation of the Balmer lines does not affect the maximum likelihood method determinations of Ne(He II), T(He II) (3889), and T(O III).
From a series of CLOUDY models it is found that the collisional contribution to I(H) for I Zw 18 and SBS 0335-052 is in the 2% to 6% range, for H 29 and NGC 2363 in the 1% to 2% range, and for NGC 346 in the 0.6% to 1.2% range. Our preliminary results indicate that the primordial helium abundance including hydrogen collisions, Yp(+Hc), is about 0.0050 larger than Yp(nHc). This problem together with the CLOUDY models for I Zw 18, SBS 0335-052, and H 29 will be discussed elsewhere Luridiana et al. (2001). The CLOUDY models for NGC 2363 and NGC 346 are those by Luridiana et al. (1999) and Relaño et al. (2001), respectively.
I should like to express my thanks to Jose Franco, the members of the Scientific Organizing Committee, and the members of the Local Organizing Committee for the idea of holding this symposium in honor of Silvia and me. I am also grateful to all the participants for a very stimulating and enjoyable meeting.