Astron. Astrophys. 364, 455-466 (2000)

1. Introduction

This paper describes an investigation of reddenings and metallicities in two fields in each of the Magellanic Clouds by means of Strömgren CCD photometry. For the first time, the reddening variations on very small scales ( pc, limited by the surface density of B stars) have been examined from Strömgren photometry. A detailed analysis of the metallicities of GK giant stars is performed, with special emphasis on the problems that arise from an incomplete knowledge of the individual reddenings of the stars. The metallicities derived for GK giants from Strömgren photometry are very sensitive to the assumed reddening, but because we have an estimate of the scatter in the reddening distributions from the early-type stars we are able to give quantitative estimates of the uncertainty on metallicities for individual GK giants.

The disagreement in the literature on the reddening for Magellanic Cloud stars has been remarkably large. Part of the controversy probably arises from an insufficient distinction between foreground reddening and reddening intrinsically in the Clouds. Grieve & Madore (1986) found that some stars in the SMC have reddenings as high as with an average of around , while the maximum for the LMC is and the average is . Grieve & Madore (1986) also noted that in the LMC the stars with high reddenings are predominantly concentrated in the neighbourhood of the 30 Doradus complex. In a review paper, Bessel (1991) concluded that while it has been claimed that the reddening is as low as or 0.02 for both Clouds, several investigations arrive at considerably higher reddenings, up to or even more. From studies of a (fairly small) section of the LMC, Harris et al. (1997) determined a mean total of 0.20 with a non-Gaussian tail to high values. Oestreicher et al. (1995) and Oestreicher & Schmidt-Kaler (1996) determined foreground reddenings and intrinsic reddenings for the LMC, and found that the foreground reddening generally varies from to across the face of the LMC, while reddenings up to were seen for some of the brightest A and B supergiants. However, the extreme reddenings were interpreted as being caused by circumstellar dust shells, an assumption which was supported by the fact that for the fainter stars only more modest reddenings, up to , were detected. Schlegel et al. (1998) report typical foreground reddenings of (LMC) and 0.037 (SMC), respectively. Using UBVI photometry, Zaritsky (1999) also found a dependence of reddening on spectral type, with stars hotter than K having visual extinctions up to several tenths of a magnitude larger than cooler stars. Recently, Romaniello et al. (2000) found a mean of 0.20 mag around SN 1987A with a scatter of 0.072 mag, claimed to be at least twice the measurement errors.

Many investigations have aimed at deriving metallicities for different types of objects (HII regions, planetary nebulae, single stars) in the Magellanic Clouds. This is not the place to give an extensive review of all of them; we refer to Westerlund (1990 , 1997). A general picture of a roughly exponential increase in metal abundance with time over the last 10 Gyr is often quoted for both galaxies. From studies of the surroundings of six LMC clusters, Dirsch et al. (2000) have determined an age-metallicity relation which shows a strong increase in the metallicity starting around 2-3 Gyrs ago. For very young objects mean [Fe/H] values are about -0.2 (LMC) and -0.5 (SMC), respectively, whereas "canonical" field star metallicities of around [Fe/H] = -0.3 for the LMC and [Fe/H] = -0.65 for the SMC are often used.

The Strömgren photometric system was originally designed for the study of BAF stars (Strömgren 1966). It has, however, turned out to be very useful also for investigations of other types of stars. Bond (1980) demonstrated that metallicities can be derived for red giants by means of the Strömgren and indices. In nearby galaxies, such as the Magellanic Clouds, the Strömgren system thereby provides us with a potentially very powerful tool for studying the metallicities of large numbers of red field giant stars.

Several spectroscopic studies of cool supergiants reach the canonical metallicities and a scatter of about dex (e.g. Spite & Spite 1987; Olszewski et al. 1991; Hill 1999), whereas attempts to derive metallicities for red field giants from Strömgren photometry (Hilker et al. 1995 (HRG95); Grebel & Richtler 1992 (GR92); Dirsch et al. 2000) have failed to reproduce them. GR92 studied the neighbourhood of the young cluster NGC 330 in the SMC and found [Fe/H] = -1.26 for the cluster itself, whereas the surrounding field stars had an average metallicity of [Fe/H] with a large spread. HRG95 found [Fe/H] = for NGC 330 and metallicities in the range [Fe/H] = -2.0 to -0.2 for the field stars, with a peak at [Fe/H] . For the red giants in the LMC cluster NGC 1866, HRG95 found a metallicity of [Fe/H] = , while the field stars also here showed a large variation in metallicity with a peak at [Fe/H] = -0.7. Dirsch et al. (2000) generally find metallicities lower than -0.3 for six relatively young LMC clusters and their surrounding fields.

In this paper we first derive reddenings for early-type stars in each of the four fields using new CCD Strömgren photometry. Metallicities are then derived for red giant stars in the same four fields, also using Strömgren photometry. The uncertainties on the derived metallicities are then discussed, with particular emphasis on the scatter resulting from an inaccurate knowledge of the reddenings of individual red giant stars.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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