3 Determination of fundamental cluster parameters

BA86a and BA87 analyzed the behaviour of metallic and Balmer-line equivalent widths as well as the continuum energy distribution in the spectral range 3700-9700 Å from integrated spectra of Galactic and Magellanic Cloud star clusters. Shortly afterwards, Bica (1988) created a library of template cluster spectra with known parameters grouped according to their evolutionary stages. The construction of spectral libraries of star clusters of all ages is a fundamental reference in order to study clusters in distant galaxies. Efforts in this direction have been made in different spectral ranges (e.g. Bica et al. 1990; Bonatto et al. 1995; Santos et al. 1995).

In this study, we have simultaneously estimated age and foreground interstellar reddening by comparing the continuum (and lines) of the observed cluster spectra with those of template cluster spectra. This procedure has been described by Santos & Bica (1993). A direct reddening-independent age estimate was first obtained from EWs of the Balmer lines in absorption in each spectrum by interpolating these values in the age calibration of BA86b (their Table II). Notice that the Balmer lines peak at t(age) $\approx$ 500 Myr, but spectra of intermediate-age clusters have strong metallic lines in the blue range like K CaII. This is not the case of the present sample which ensures that we are dealing with ages younger than about 500 Myr. We then selected an appropriate set of template spectra according to the age provided by the Balmer lines and varied reddening and template to get the best match of continuum and lines of the observed spectrum to that of the template that most resembles it. To perform reddening corrections, we used the normal reddening law $A_{\lambda}$= 0.65 A_v(1/$\lambda-$0.35) (Seaton 1979), the relation A_v= 3E(B-V), and the SPEED spectral analysis software (Schmidt 1988). We have adopted the above reddening law to keep internal consistency with the database of template spectra established using this law and the factor 3 for the ratio of total to selective absorption (e.g., Bica 1988; Santos et al. 1995). The resulting reddenings and ages for the cluster sample are shown in Table 5. We note that the uncertainty in the adopted reddening represents the lowest reddening variation necessary to distinguish the cluster spectrum from that of the corresponding template. As shown in Table 5, the uncertainties in the E(B-V) determinations range from 0.02 to 0.05 magnitudes.

 

 

Table 5: Reddening and age determinations

Cluster	E(B-V)	Age (Balmer)	Age (template match)	Adopted age
		(Myr)	(Myr)	(Myr)
Ruprecht144	0.32 $\pm$ 0.02	200	100	150 $\pm$ 50
Melotte105	0.31 $\pm$ 0.02	300	100	200 $\pm$ 100
BH132	0.60 $\pm$ 0.05	200	100	150 $\pm$ 50
Hogg15a	1.05 $\pm$ 0.05	30	3-6	5 $\pm$ 2
Pismis21	1.50 $\pm$ 0.03	110	50	80 $\pm$ 30
Lyngå11	0.12 $\pm$ 0.03	400	500	450 $\pm$ 50
BH217	0.80 $\pm$ 0.03	20	50	35 $\pm$ 15

^a We adopted the template method age, since for very young ages the Balmer lines lose sensitivity (BA86a).

The template groups from Bica (1988) and Santos et al. (1995) found to be particularly useful in the present study are YA.LMC (age $\approx$ 3-6 Myr), YB.LMC (6-9 Myr), Y2 (50 Myr), Y3A and Y3B (100 Myr) and Y4 (500 Myr). The group Y3A has an enhanced population of massive asymptotic giant branch stars with respect to Y3B (BA87). The template sequence depends essentially on age, the metallicity effects being negligible in the blue-visual ranges.

As discussed by BA87, CaII triplet and other absorption windows in the near-IR can provide metallicity estimates in young clusters. The metallicity values interpolated from Table A3 of BA87, however, should be taken as indicative and, for definitive results for each cluster, complementary information from deep CMDs and/or spectroscopy of individual member giants would be necessary. Notice that for lower surface brightness clusters (e.g., Lyngå11, Fig. 6), residuals of telluric line corrections may be important. Nevertheless, the CaII triplet region is little affected by such problems (BA87), which allows one to confidently measure them in such cases.