|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
2.7. The Old and Metal-Poor Field Stars
There are two ways to find the ancient population of normal stars. The first is to derive abundances and velocities of red giants in a well-defined field to identify the most metal-poor stars. Simple photometry is inadequate because of Galactic foreground contamination. The second is to look for old and metal-poor main-sequence turnoffs and subgiant branches in the color-magnitude diagrams of the field.
There are two studies of complete samples of field giants in the Magellanic Clouds: Suntzeff et al (1986) for the field surrounding NGC 121 in the SMC and Olszewski (1993) for the NGC 2257 field in the LMC. The two RGB abundance distributions are quite different.
The NGC 2257 field is the same field used by Jones et al (1994) to derive the proper motion of the LMC as a whole. Many, but not all, foreground stars are therefore removed from the sample because they have large proper motions. This field is 8 kpc north of the LMC bar and would normally therefore be called a "pure halo" field. Its surface brightness is very low, ~ 27 mag arcsec-2, according to the surface brightness model of Bothun & Thompson (1988). The present-day neutral hydrogen density is less than 1.6 × 1020 cm-2. Potential giants were picked from the CMD by superposing ridge lines of metal-poor and metal-rich Milky Way clusters, with care to ensure that no potentially metal-poor stars would be missed. Spectra were obtained for all such stars brighter than the red giant branch clump. Only 8 or 9 of the 36 resultant member giants are more metal poor than [Fe/H] ~ -1.3. The vast majority of the stars in this "halo" field have abundances centered on [Fe/H] = -0.5 (see Figure 3 in Olszewski 1993). This abundance distribution is not expected for a "Population II halo." It is not yet understood how such a field can be dominated by stars more metal rich than 47 Tuc. The velocity dispersion of the stars more metal poor than [Fe/H] = -1.3 is 29 km s-1 (23 km s-1 with one extreme-velocity star removed). The velocity dispersion for the more metal-rich stars is 16 km s-1. These velocity dispersions are not obviously different from those of the clusters or OLPVs. Very large areas on the sky will need to be observed to derive the velocity dispersion of subsamples of the metal-poor population.
The NGC 121 field in the SMC is much more normal for a distant "halo" field. Astrometry was again used to cull foreground stars. Spectra were then obtained for 13 stars. The spectroscopy and photometry were then used to derive abundances for 31 stars in the field. The distribution of metallicities peaks at [Fe/H] = -1.6 (Suntzeff et al 1986, Figure 9) and is very similar to the abundance histogram of Galactic halo RR Lyraes and of Galactic halo globular clusters. There is no component similar to the dominant metal-rich component in the NGC 2257 field. The velocity dispersion is 24 km s-1.
Old and metal-poor main-sequence turnoffs are an indication of the progenitors of the giants discussed above. Elson et al (1994) attempted to find this population with pre-Costar HST images of the young and crowded 30 Dor region. If their assumptions about the errors in the photometry are correct, they see a small excess of stars blueward of the dominant main-sequence population. They attribute these stars to a metal-poor population. We believe that this result is inconclusive. Elson et al (1994, Figure 6) show a color cut across the main sequence at 22 < V < 22.5 in two 30 Dor fields approximately 30 arcmin apart. The color of the dominant population does not change, while the color of the putative metal-poor population changes by 0.08 mag. Fields known to contain BHB stars or strong subgiant branch populations will show the old metal-poor main sequence most clearly. This population is easily hidden. The clusters and RR Lyrae results suggest that the ancient population is no more than 10% of the luminous mass of these galaxies. Simple Monte Carlo simulations of synthetic color-magnitude diagrams show that the ancient population will be difficult to find because the lower main sequences of younger populations are superposed on the old turnoff.
The horizontal branch and red clump (RC) can be populated by populations with a wide range of ages (see Section 3.2). Gardiner & Hatzidimitriou (see references in Section 3.2) have made extensive studies of the color-magnitude diagrams of large areas of the SMC field using scans of SRC sky survey plates. A similar study is not currently published for the LMC, but surveys such as the MACHO project have already collected the appropriate photometry. Gardiner & Hatzidimitriou (1992) have argued that the majority of clump stars come from intermediate-age populations, though the bulk of the field populations near NGC 121 and L113 are claimed to be older than 10 Gyr. The RC stars closest to the instability strip can come from a population older than 10-12 Gyr (Hatzidimitriou 1991). Color cuts through the RC region of the CMD show this slightly bluer, fainter red HB population. More recent ages and abundances for the calibrating clusters (Armandroff et al 1992, Armandroff & Da Costa 1991, Buonanno et al 1990) lead to the conclusion that this population of red HB stars near the instability strip may be as much as several billion years younger than the oldest globulars. This subset of older clump stars is approximately 7% of the total number of clump stars. About 7% of the stars older than 2 Gyr are thus older than 10-12 Gyr. More importantly, there is little evidence for blue horizontal branch stars. Gardiner & Hatzidimitriou (1992) estimate that the number of BHB stars is an order of magnitude smaller than the number of old RHB stars. Given the low metal abundance of the SMC clusters and field it is more likely that BHB stars do not exist because ancient SMC stars do not exist. NGC 121 and its RR Lyraes are demonstrably younger than the oldest globulars. Therefore the SMC clusters do not contradict this reasoning. Important observational tests are needed to derive the metal abundance of the SMC RR Lyraes, to see if there exists significant populations with abundances more metal poor than [Fe/H] = -1.6. It is also important to derive the abundance distribution of the clump giants.
If one isolates a set of old clusters or field stars, then with the next generation of telescopes we can expect to make detailed analyses of elemental and isotopic ratios. The ratio most commonly discussed is [O/Fe], because oxygen is made in Type II supernovae, whose progenitors are very short lived, and iron is made in Type I supernovae, whose progenitors are very long lived. [Mg/Fe], [Ca/Fe], and [Si/Fe] should change in the same way as [O/Fe] and are easier to measure. As Gilmore & Wyse (1991) summarized, the ancient objects described in this review should be overabundant in oxygen if the initial star-formation burst were short, but could have a Solar ratio of [O/Fe] if these objects were formed near the end of a lengthy episode of star formation. If the clusters are not self polluted, we may be able to tell if the ancient clusters were formed before or after the ancient field.