Raoul Haschke, Eva K. Grebel, Sonia Duffau, Shoko Jin
We present the first metallicity distribution functions of the old field
populations in the Magellanic Clouds. Our metallicities are based on the
Fourier decomposition of Type ab RR Lyrae light curves from the Optical
Gravitational Lensing Experiment (OGLE-III). On the metallicity scale of Zinn &
West; we find a mean metallicity of [Fe/H] = -1.50 +/- 0.24 dex based on 16776
RR Lyrae stars in the Large Magellanic Cloud (LMC). For the Small Magellanic
Cloud (SMC) we obtain -1.70 +/- 0.27 dex based on 1831 RR Lyrae stars. These
uncertainties represent the intrinsic spread in the population rather than the
standard deviation of the mean. Our results are in good agreement with the few
existing spectroscopic metallicity determinations for LMC RR Lyrae stars from
the literature. For both the LMC and the SMC the metallicity spread exceeds 1
dex in [Fe/H]. The distribution of metallicities in both Clouds is very
uniform, and no significant metallicity gradient is detectable. We also do not
find any pronounced populations of extremely metal-poor RR Lyrae candidates
with metallicities well below -2 dex, although we need to caution that the
photometric method used may overestimate the metallicities of metal-deficient
stars. Moreover, because of stellar evolutionary effects one does not expect to
observe many RR Lyrae stars among very metal-poor horizontal branch stars. We
suggest that the Magellanic Clouds experienced fairly rapid and efficient early
enrichment involving pre-enriched gas as well as possibly gas infall, while
metal loss through outflows does not seem to have played a significant role.
Moreover we suggest that the differences in the metallicities of the old
population of LMC and SMC make an origin from a single, common progenitor
unlikely, unless the separation happened very early on.
View original:
http://arxiv.org/abs/1111.6586
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