Astron. Astrophys. 322, L13-L16 (1997)

6. First results

We have derived instrumental magnitudes on the y and b scales and, to some extent, on the v scale, in the uvby system. For these data, we made tentative zero point determinations. The resulting magnitudes and colour indices, y and (b-y), were used for the preliminary conclusions reported in this paper. Whilst solid enough for the conclusions drawn, the data are still on an instrumental system. System calibration will be made to high accuracy. This work is in progress, in collaboration with STScI colleagues.

The preliminary analysis of our recent HST uvby observations in the centre of the LMC Bar confirms their high quality. A comparison between our HST PC field and the corresponding field seen from the ground is given in Figures 1 and 2. The quality of our data is demonstrated in Figure 3.

Fig. 1. Our target field observed with the HST PC. The field is square with 35" side. A y picture is shown with a total exposure time of 53 min.

Fig. 2. Same field as in Figure 1 but observed from the ground with the ESO/MPA 2.2 m telescope at La Silla. A y picture is shown with 5 min exposure time and an image quality corresponding to FWHM = 0.9 arcsec.

Fig. 3. The standard deviation of averaged y magnitudes is displayed versus y, both expressed in magnitudes.

Figure 4 shows a colour magnitude diagram derived from our PC field. Various tests have shown that our distribution of stars is close to complete down to at least y = 22.5. Thus, down to this limit, number statistics can be used to derive the age distribution of stars in our target field. In this first step, effects of interstellar extinction, being modest (Ardeberg et al. 1985, Schwering and Israel 1991, Oestreicher et al. 1995), are neglected. For the young population of stars, we adopt a galactic luminosity function (LF) of the zero-age main sequence (ZAMS) based on galactic open clusters (Taff 1974). Using our number statistics and the LF, we estimate that the young stellar component contains around 30 % of the total amount of stars in the part of the Bar studied.

Fig. 4. A tentative colour-magnitude diagram for our LMC Bar field. y is displayed versus (b-y), both parameters on a zero point corrected instrumental system and expressed in magnitudes. Total exposure time in y is 53 min, in b 207 min.

As a check on our conclusion regarding age distribution, we employed an alternative way to estimate the fraction of young stars in our field. As before, we used the LF for the young stellar population, whilst we adopted isochrones to estimate the total contribution of the old population from the giant branch. The result is close to that obtained from number statistics and the young star LF.

From the morphology of the horizontal branch and the colour of the horizontal and giant branches, we estimate a metallicity of around [Me/H] = -0.5. A probably better estimate of the abundance of heavy elements will later be derived from the m₁ index. The reduction of the v-band data is still rather preliminary since all calibration observations have not been acquired. However, from a tentative m₁ -(b-y) diagram, we estimate [Me/H] = -0.7. There may be some indication of a real scatter in metallicity in the m₁ data, with the older population being more metal poor. This will be further investigated.

As estimated from isochrones by VandenBerg (1985), at least a considerable fraction of the young stellar component originated less than 500 million years ago. We tentatively ascribe this young population to the Bar. We will return to this topic with a statistical test.

From isochrones of Bergbusch and VandenBerg (1992), we estimate that the older population ranges in age between 2 and 9 Gyears. The very small number of stars in the rectangle in Fig 4, defined by 19.5 y 20.5, 0.3 (b-y) 0.5, as compared with the evolutionary tracks of VandenBerg (1985), suggests that star formation decreased considerably before the more recent burst occurred. Improved age estimates will be based on the entire WFPC2 field sample, again taking advantage of its usefulness for the brighter stars. At any rate, stars much older than 10 Gyears seem to be scarce.

Our preliminary luminosity function, down to y = 24, is given in Figure 5. Compared to a luminosity function derived from an observed field in the LMC halo close to the Bar (Ardeberg et al. 1985) and surface density measurements (de Vaucouleurs 1957), the new, tentative, luminosity function presents a rather similar slope and even agrees quite well in over-all shape.

Fig. 5. A tentative bright-end luminosity function for our LMC Bar field. The number of stars observed, expressed on a logarithmic scale, is given versus y, expressed in magnitudes. For the brightest stars, number statistics are weak.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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