|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
7.2. Aluminum and Sodium
In the Galactic disk, the [Al/Fe] ratio increases with decreasing [Fe/H], reaching ~ +0.3 dex at [Fe/H] = -1 (Edvardsson et al 1993, Tomkin et al 1985), but this is only 0.2 dex above the mean [Al/Fe] ratio for solar metallicity stars. In the Edvardsson et al (1993) sample, these stars exhibit a 0.2-dex increase in [Na/Fe] from [Fe/H]=0 to -1; however, Tomkin et al (1985) found [Na/Fe] ~ 0.0 for their sample. Thus, from a phenomenological point of view, Al and perhaps Na could be classified as mild elements, even though their nuclei have odd numbers of protons, which is consistent with a significant component of Al and Na synthesis from SN II.
The [Al/Fe] ratio in halo stars spans a range of approximately 2-dex (see Figure 7). The most extensive recent study of Al is that of Shetrone (1996a), who showed that the globular cluster giants and the field halo giants have very different mean [Al/Fe] ratios, with little overlap. Shetrone (1996a) showed a plot of [Al/Fe] versus spectroscopic luminosity, indicating that the Al difference was not due to luminosity. However, luminosity is difficult to measure for field stars, so a more reliable estimate of the position of a star on the red giant branch for this comparison is the temperature; in this case, there is only a small region of overlap between field and globular cluster stars, near 4300 K. At 4300 K and above, for [Fe/H] ~ -1, no field giant has [Al/Fe] larger than +0.3, whereas many globular cluster giants with the same metallicity, luminosity, and temperature have [Al/Fe] larger than this value, up to a maximum value near +1 dex.
Figure 7. The trend of [Al/Fe] with metallicity for field stars, indicated by crosses (from Shetrone 1996a, Gratton & Sneden 1988), and globular clusters from Shetrone (1996a); open boxes for M71, open circles for M13, filled circles for M5 and open triangles for M92; filled triangles for M22, M4, and 47 Tuc from Brown et al (1992), and open stars for NGC 2298 from McWilliam et al (1992). Note that the lower bound of the globular cluster values is consistent with the field star trend.
In Figure 7, the lowest [Al/Fe] ratios found in globular cluster giants appear similar to the field halo giants. This, at least, is consistent with the idea of self-pollution of Al from proton burning and deep mixing in evolved red giants (e.g. Denisenkov & Denisenkova 1990). In metal-poor stars, the declining [Al/Fe] ratio, or [Al/Mg], with decreasing metallicity is well known (e.g. Aller & Greenstein 1960, Arpigny & Magain 1980, Spite & Spite 1980) and has long been interpreted as consistent with the metallicity-dependent Al yields from explosive carbon burning, predicted by Arnett (1971). The Arnett predictions indicate that [Na/Fe] ratios should also decline with decreasing metallicity, but this is not observed; in fact, [Na/Fe] ~ 0.0 from [Fe/H] = -1 to -4 (e.g. McWilliam et al 1995b). Pilachowski et al (1996) analyzed a sample of 60 halo subgiants, giants, and horizontal branch stars in the interval -3 [Fe/H] -1; they found a small [Na/Fe] deficiency of -0.17 dex in the mean and that bright field halo giants do not show the excess of sodium found in their globular cluster counterparts.
Globular cluster giants show large dispersions in Na and Al abundances. The Na and Al abundances are correlated, and they are correlated with N enhancements and O depletions. The abundance patterns have been interpreted either as evidence of internal nucleosynthesis and mixing operating in individual stars or, alternatively, as characteristic of a dispersion in the composition of the material out of which the stars formed (see Kraft 1994, Shetrone 1996a, b, Kraft et al 1997, and references therein for details). Presently, the source of the Al and Na dispersion remains contested. Thus it is difficult to use Na and Al as probes of Galactic chemical evolution until the effects of individual stars can be quantified.
Studies of globular cluster main-sequence stars suggest that there may be a problem with the notion that globular clusters formed from chemically homogeneous material. Briley et al (1991) observed 10 main-sequence stars in 47 Tuc and found the same frequency of CN bimodality as present in the more evolved giant sequences; later, Briley et al (1996) found correlated CN and Na enhancements in main-sequence stars of the globular cluster 47 Tuc. These abundances, if found in giant branch stars, would have been attributed to dredge-up of internal nucleosynthesis products, i.e. self-pollution; however, this is not possible for main-sequence stars. Therefore, it is most likely that either Briley's CN and Na results are due to mass transfer from evolved companions, or the stars in 47 Tuc were formed from gas that was inhomogeneous in C, N, and Na. In this regard, it is interesting that the high frequency of CN-strong stars (near 50%) far exceeds the frequency in the field (at 5%). Spiesman (1992) found a nitrogen-rich metal-poor dwarf with Na and Al enhanced by +0.5 dex; citing the difficulty in producing Na and Al enhancements in main-sequence stars, Spiesman concluded that in this star the N, Na, and Al abundance anomalies are primordial. Suntzeff (1989) found an anticorrelation between CN and CH for main-sequence stars in NGC 6752. Pilachowski & Armandroff (1996) used a fiber spectrograph to acquire spectra of the [O I] region, at 6300 Å, for 40 stars at the base of the giant branch in M13. These stars are not expected to show oxygen depletions in the standard theory (e.g. Kraft 1994). The combined spectrum was of high S/N (~ 300), yet the undetected [O I] line indicated a limiting oxygen abundance below the solar [O/Fe] ratio; such deficiences have been interpreted in the past as due to red giant evolution.
Comprehending Al in globular cluster giants will have to await careful abundance analysis of larger samples of globular cluster main-sequence and red giant branch stars; only then will it be possible to know the scale of the Al production on the giant branch. This is an area in which the new large telescopes can make a significant impact. If the Al and Na abundance anomalies are due to primordial inhomogeneities in the cluster composition, there would be significant implications for ideas of globular cluster formation.
The sample of Galactic bulge giants studied by McWilliam & Rich (1994) also show marked Al enhancements, at approximately +0.7 dex, even at solar metallicity. This observation is consistent with self-pollution by evolved giants because the bulge stars observed were fairly luminous; however, this does not constitute proof of the self-pollution picture.