4.2. Abundance patterns in the solar vicinity and the solar abundance discrepancy
Stars and nebulae provide a different perspective of the solar vicinity chemical composition. The methods for abundance determinations differ (and might be in error in different ways) and the astrophysical significance of the abundances is not necessarily the same. One expects a priori the surface composition of the Sun to be identical with that of other objects in the solar vicinity. It turns out that the abundances from nearby H II regions (Orion being the best example) are significantly smaller than the solar abundances from the works of Anders & Grevesse (1989) or Grevesse & Sauval (1998). It is to be noted that, despite of this fact, the reference abundance is often chosen to be that of the Sun from Anders & Grevesse (1989). Table 3 summarizes the abundances in the Sun, in the local interstellar medium (ISM) and in local B stars from recent references.
|Gas phase local interstellar medium|
|a Anders & Grevesse (1989)|
|b Grevesse & Sauval (1998)|
|c Holweger (2001)|
|d Allende Prieto et al. (2001)|
|e Meyer et al. (1998) (O), Meyer et al. (1997) (N), Sofia et al. (1997) (C), Cardelli et al. (1996), Sembach & Savage (1996) (Si, S, Fe, Ni) Sofia & (Meyer 2001) (Mg)|
|f Cunha & Lambert 1994|
|g Compilation from Sofia & Meyer (2001)|
Peimbert et al. (2001) notes that a decade ago, the oxygen abundance in the Sun was 0.44 dex higher than in Orion but when using the value from Esteban et al. (1998) with t2 = 0.024 and the solar value of Grevesse & Sauval (1998), the difference is only 0.19 dex. When accounting for the fraction of oxygen that is contained in dust grains (which can be done assuming a standard chemical composition of the dust grains, and the constraints provided by the Mg, Si and Fe abundances), the oxygen abundance is multiplied by a factor 1.2 and the difference between the Solar value and Orion is only 0.11 dex.
The oxygen abundance in Orion obtained with t2 = 0 is actually similar to the one in the local interstellar medium (obtained from high resolution and high signal to noise absorption measurements, Meyer et al. 1998) and in local B stars (e.g. Cunha & Lambert 1994). Several possible explanations have been invoked. The ones listed by Meyer et al. 1998 are: i) an early enrichment of the Solar system by a local supernova (not really tenable if the abundances of all the elements in the local ISM are 2/3 solar); ii) a recent infall of metal poor gas in the local Milky Way; iii) an outward diffusion of the Sun from a smaller Galactocentric distance. A more recent discussion Sofia & Meyer (2001) definitely rejects the hypothesis of the local ISM standard being 2/3 of the Sun. Indeed, new determinations give much smaller values for O/H: 544 ppM (Holweger 2001), 490 ppM (Allende Prieto et al. 2001). The support for the 2/3 solar value is also invalidated from carbon (see their discussion). Note that Sofia & Meyer (2001) also argue that B stars have metal abundances that are too low to be considered valid representations of the ISM. According to Meyer et al. (1998), the local standard oxygen abundance should be 540 ppM (gas + dust).
In conclusion, the "solar abundance discrepancy" has gradually disappeared, mostly because modern derivations of the solar oxygen abundance give much lower values than earlier ones. This reinforces confidence in H II regions as probes of the ISM abundances and in the methods used to analyze them. This is good news, since H II regions are practically the only way to derive oxygen abundance in external galaxies, if one excepts the abundance analysis in giant stars of local galaxies which require very large telescopes. Giant H II regions can be used as abundance indicators up to large redshifts (see Pettini in the same volume).