5.1. Cosmological Motivation
The universally high abundance of helium in stars and nebulae, Y 0.24, is considered to be one of the fundamental pieces of evidence in favor of the "standard" hot big-bang cosmological model. Calculations of nucleosynthesis in the early universe show that the helium to hydrogen abundance ratio is a function of several fundamental cosmological and physical parameters: the baryon to photon number ratio, number of neutrino families, and neutron half-life (e.g. Yang et al. 1984). Metal-poor H II regions, where nucleosynthetic activity and enrichment by stars has been minimal, offer extremely attractive sites for attempting to determine the primordial helium fraction YP. This realization has led to many attempts to measure helium abundances as precisely as possible for extragalactic H II regions. There are many thorough reviews of these efforts and their results in the astronomical literature; these include the proceedings of the 1983 ESO conference on helium abundances (Shaver, Kunth, and Kjär 1983), and more recent reviews by Shields (1986), Boesgaard and Steigman (1985), and Kunth (1986). In the following discussion, I emphasize the uncertainties and problems in determining YP; the interested reader may consult the above references for further details.
A great deal of excitement was generated about ten years ago by the announcement of anomalously low helium values derived from several extragalactic H II regions, YP < 0.22, by French (1980), French and Miller (1981), Rayo, Peimbert, and Torres-Peimbert (1982). Such low values presented a conundrum for cosmological models. However, the disagreement was significant only if the observational errors were substantially smaller than 10%, prompting further studies to re-examine the question. Kunth and Sargent (1983) studied a sample of about a dozen metal-poor H II regions, and took the average value of measured Y in this sample as being representative of the primordial value, i.e. YP = 0.245, which was not in conflict with the standard model. Nevertheless, since helium can be presumably be added but not removed by stellar activity, the existence of any H II region with a helium fraction less than a cosmologically "allowed" value would seem to disprove the simple big-bang model. The profound consequences of establishing a truly low value of Y in H II regions motivated a number of authors to pursue the issue further.
5.2. Demands on the Observations
5.2.1. Corrections to the Line Intensities. In order to definitively establish whether there is a conflict with standard cosmologies, it is necessary to determine the He/H ratio with a precision and accuracy which is unprecedented for astronomical determinations of chemical abundances. Therefore, it becomes essential to consider and correct for every possible source of observational uncertainty, including many effects that can usually be ignored in other contexts. Extensive discussions of the uncertainties involved in determining He/H values in extragalactic H II regions have been given by Davidson and Kinman (1985) and Dinerstein and Shields (1986), in the context of detailed studies of particular objects. Some of the most interesting galaxies (I Zw 18, for example) are so faint that achieving the necessary signal-to-noise in the helium line intensity measurements is a challenge, at least for the present generation of large telescopes (of apertures of 3 to 5 m). Going beyond such standard considerations, several other issues have been raised in the course of pursuing the helium problem. For example, most of the measurements of He/H in the literature were made with IDS (image dissector-scanner) instruments, which have been found to display slight non-linearities in the relationship between counts and flux. This non-linearity, while unimportant under most circumstances, becomes very important in the case of helium. Several different values have been proposed for the magnitude of this effect (e.g. Rosa 1985; Peimbert and Torres-Peimbert 1987); perhaps different individual instruments do indeed have different non-linearities. The issue may become moot, with IDS systems being replaced by CCDs, although the burden of proving the linearity of instruments will remain if the results are to be believed to the percent level.
Another problem affects only He I 5876Å, generally the strongest helium line observed, and therefore given more weight than other measured He I lines. For objects with small positive redshifts (which includes most of the key extragalactic H II regions), 5876Å shifts to the vicinity of the Na I lines at 5889, 5895Å. While telluric emission in the Na I lines can be removed by sky subtraction, it is not so easy to compensate for absorption by Na I in our own Galaxy, an effect which can be at least as large as 10-15% (Davidson and Kinman 1985; Davidson, Kinman, and Friedman 1989). Higher spectral resolution can help somewhat with this problem. Yet another factor which influences the He I line intensities is collisional excitation out of metastable levels. This concern was raised recently by Ferland (1986), who claimed that it could be a large effect; a reassessment by Clegg (1987), using newer calculations of the relevant cross-sections, found the effect on the derived helium abundances to be minor.
In many cases, the entrance aperture for the nebular observations includes not only ionized gas, but also continuum from the ionizing stars (see Section 1.2.). In the spectra of hot stars, the hydrogen and helium lines will be in absorption. Since these observations are generally made with spectral resolutions too low to resolve the narrower emission lines from the underlying absorption features, the emission line intensities will be weakened accordingly. However, unlike the emission decrement, the absorption line decrement is fairly flat; thus, given three or more hydrogen lines, it is possible to solve simultaneously for both interstellar reddening and the strengths of the absorption lines (e.g. Rayo, Peimbert, and Torres-Peimbert 1982; McCall, Rybski, and Shields 1985). In general, the emission equivalent widths of the first few hydrogen recombination lines (H, , Dinerstein and Shields 1986).
The problem of correcting for underlying absorption H and He absorption features in the hot star photospheres is not the only problem introduced by the stellar continuum. As discussed in Section 1.4., it is becoming apparent that many extragalactic H II regions contain Wolf-Rayet stars. Such stars produce broad, complex emission features, one of which falls near He I 5876Å. Figure 8 shows this spectral region as well as the region near He II 4686Å, for the dwarf irregular NGC 4861 (Dinerstein and Shields 1986). It is apparent from the figure that, unless one knows the intrinsic shape of the underlying continuum (i.e. whether there is net emission or absorption from the stars), there will be a substantial uncertainty in the strength of the nebular He I 5876Å line. Unfortunately, at present one cannot do much better than to guess at the continuum shape, since the Wolf-Rayet features in extragalactic H II regions display a variety of shapes (D'Odorico, Rosa, and Wampler 1983, and references given above). The best prospect for measuring accurate He I 5876Å lines is to avoid using apertures which contain starlight.
Figure 8. Two segments of the spectrum of the dwarf irregular galaxy NGC 4861 containing Wolf-Rayet emission features are shown. The upper panel shows how these features interfere with the measurement of nebular He I 5876Å. (Figure from Dinerstein and Shields 1986).
5.2.2. The Correction for Neutral Helium. So far we have discussed only the uncertainties in determining the ionic ratio He+ / H+. As with derivations of metal abundances, it is necessary to take into consideration the relative fractions of each element in the observed ions, in order to obtain the elemental abundance ratio. Of the other ions of helium, He++ produces visible recombination lines, primarily 4686Å. This line has been seen in several extragalactic H II regions, but there is some difficulty in distinguishing whether it arises from the nebula or from Wolf-Rayet stars; a true nebular emission line of He II would require the presence of at least some UV radiation from extremely hot stars (e.g. Rayo, Peimbert, and Torres-Peimbert 1982; Dinerstein and Shields 1986). However, even if nebular in origin, He++ represents only a couple of percent of the total helium abundance.
Neutral helium is a potentially much larger component, and, furthermore, it cannot be observed directly. The neutral helium fraction is presumably minimized in nebulae with a high degree of ionization, which, as mentioned above, is usually the case for H II regions with low metal abundances. However, it is still necessary to understand how much neutral helium might be present. Some workers have employed empirical ionization correction formulae for this correction; others have used nebular ionization models. One interesting point is that for T* 40,000 K, the He+ Strömgren sphere actually extends further out than that for H+; therefore the ionization correction factor is less than 1, but only by a few percent at most: He/H 0.98 × [He+ / H+] (Stasinska 1980; Shields and Dinerstein 1986). On the other hand, it is very difficult to establish that a particular nebular ionization model is a unique solution for an observed set of line intensities. Particularly for distant objects, there is always the possibility that one is observing several spatially distinct regions which are not resolved by the spectrophotometric measurements. In this case, it is possible for there to be a large amount of neutral helium "hidden" inside separate low-ionization nebulae ionized by cooler stars. Such "composite" models have been examined, for example, by Dinerstein and Shields (1986) and Peña (1986), who find that the correction for neutral helium could easily be as great as 10% in this case.
5.2.3. The Correction for Stellar Synthesis. There remains the question of whether or not to attempt to correct for the presumed contribution of helium synthesized by stars. It has been suggested that this contribution should be related linearly to the amount of heavier elements synthesized by the same stars or at least the same population of stars. If such a relationship can be established and the value of Y / Z determined, then any measured helium abundance can be extrapolated backwards to obtain YP. The problem is that there is no general agreement on the value of this coefficient. Estimates for Y / Z have varied from 1.7 (Lequeux et al. 1979) to 5.7 (Pagel, Terlevich, and Melnick 1986). Others tend to take a value of around 3 (Rayo, Peimbert, and Torres-Peimbert 1982). Meanwhile, Kunth and Sargent (1983), among others, have argued that there is no such correlation between Y and Z, within the low-metallicity domain. There is also the question of how one determines "Z". Originally it was calculated from O/H, but more recently many authors have suggested calculating the stellar helium contribution from the abundances of N or C, the rationale being that the sites of helium synthesis are also sources N and/or C (e.g. Pagel 1985; Vigroux, Stasinska, and Comte 1987; Steigman, Gallagher, and Schramm 1989; Torres-Peimbert, Peimbert, and Fierro 1989).
5.3. Current Status and Future Prospects
The current situation is that various groups have estimated the value of the primordial helium abundance to fall in the range 0.23 YP 0.24 (Torres-Peimbert, Peimbert, and Fierro 1989; Pagel and Simonson 1989). This is uncomfortably close to the lower limit for the standard cosmological model, but not in actual direct conflict with it (although it does rule out the possible existence of unknown families of neutrinos). However, there is still essentially no decisive proof of the existence of actual, as opposed to extrapolated, helium abundances lower than Y = 0.24. There is also no substantial evidence for variations in the primordial abundance from place to place (see Dinerstein and Shields 1986). In view of the caveats discussed in the last section, it seems clear that it is not going to be easy to improve on the current situation. The Hubble Space Telescope will at least provide better opportunities to measure the nebular spectrum without contamination by stellar continuum. It may also help with the correction for neutral helium, because it may be possible to spatially resolve the nebular ionization structure. However, the question of the correction for stellar-synthesized helium will remain. Thus, unless an actual, present-day, helium abundance lower than permitted by the standard cosmology is found, there probably will continue to be controversy about any further inferences regarding the primordial value.