The metallicity distribution function of red giants in the Large Magellanic Cloud

We report new metallicity determinations for 39 red giants in a 220 arcmin( 2) region, 1.degrees 8 southwest of the bar of the Large Magellanic Cloud. These abundance measurements are based on spectroscopy of the Ca II infrare d triplet. We have carefully considered the effects of abundance ratios, th e physics of Ca II line formation, the variation of red clump magnitude, an d the contamination by foreground stars in our abundance analyses. The meta llicity distribution function (MDF) shows a strong peak at [Fe/H] = -0.57 /- 0.04; a tail to abundances at least as low as [Fe/H] approximate to -1.6 brings the average abundance down to [Fe/H] = -0.64 +/- 0.02. Half the red giants in our field fall within the range -0.83 less than or equal to [Fe/ H] less than or equal to -0.41. The MDF appears to be truncated at [Fe/H] a pproximate to -0.25; the exact value of the maximum abundance is subject to similar to 0.1 dex uncertainty in the calibration of the Ca II IR triplet for young, metal-rich stars. We find a striking contrast in the shape of th e MDF below [Fe/H] less than or equal to -1 between our inner disk field an d the distant outer field studied by Olszewski: red giants deficient by mor e than a factor of 10 in heavy elements relative to the Sun are extremely s carce in the inner disk of the LMC. Our held star sample does not reproduce the full MDF of the LMC star clusters but seems similar to that of the int ermediate-age (1-3 Gyr) clusters. Pie have also obtained abundance estimate s using Stromgren photometry for approximate to 10(3) red giants in the sam e field. Photometry is the only practical way to measure abundances for the large numbers of stars necessary to lift age-metallicity degeneracy from o ur high-precision color-magnitude diagrams. The Stromgren measurements, whi ch are sensitive to a combination of cyanogen and iron lines, correlate wel l with the Ca Ir measurements, but a metallicity-dependent offset is found. The offset may be due either to variations in the elemental abundance rati os due to galactic chemical evolution or to a metal-dependent mixing mechan ism in RGB stars. An empirical relation between photometric and spectroscop ic abundance estimates is derived. This will allow photometric abundance me asurements to be placed on a consistent metallicity scale with spectroscopi c metallicities, for very large numbers of stars.