2 Spectroscopic data

2.1 Cluster sample and observations

Concentrated open clusters with small angular diameter (D) are certainly those most suitable to carry out integrated spectroscopic observations. This is so because the cluster as well as the surrounding background sky regions must extend along the whole slit. Besides, the angular diameter requirement results from the fact that the cluster integrated spectrum must reflect the synthesis of its stellar populations. Old or intermediate-age clusters with many red giants, for example, should reflect the presence of these stars in their integrated spectra. Similarly, integrated spectra of young clusters should present an enhanced blue continuum with comparatively strong Balmer lines.

In this study we have selected 7 compact ( $D~\leq4\hbox{$^\prime$ }$), relatively populous Galactic open clusters to allow good star sampling in the integrated spectra. The choice of these clusters is due not only to their compact nature and relatively high surface brightness, but also to the scarcity or absence of previous studies about them. Galactic coordinates and angular diameters taken from Lyngå  (1987) are listed in Table 1 for the cluster sample.

 

 

Table 1: Cluster sample

Cluster	$l(^\circ$)	$b(^\circ$)	D($^\prime$)
Ruprecht144	20.70	-1.25	4$^\prime$
Melotte105	292.89	-2.45	4$^\prime$
BH132	300.27	-1.33	3$^\prime$
Hogg15	302.04	-0.24	2$^\prime$
Pismis21	320.35	-1.80	2$^\prime$
Lyngå11	338.18	0.47	4$^\prime$
BH217	346.77	-1.51	4$^\prime$

The observations were carried out with the 2.15 m telescope at the Complejo Astronómico El Leoncito (CASLEO, San Juan, Argentina) in two runs in May 1995 and July 1998, respectively. In both runs, we employed a CCD camera containing a Tektronics chip of $1024 \times 1024$ pixels attached to a REOSC spectrograph, the size of each pixel being $24 \times 24~\mu$m. The slit was oriented in the east-west direction and the observations were made by scanning the slit across the objects in the north-south direction in order to get a proper sampling of cluster stars. The long slit, corresponding to 4.7$^\prime$ on the sky, allowed us to sample regions of background sky. A grating of 300 line/mm in two different set-ups, namely "blue nights'' and "red nights'', was used. During the blue nights, the spectral coverage was $\approx$ 3500-7000 Å, with an average dispersion in the observed region of $\approx$ 140 Å/mm (3.46 Å/pix). The slit width was 4 $.\!\!^{\prime\prime}$2 resulting in a resolution of $\approx$ 14 Å, as measured by the mean FWHM of the comparison lines. During the red nights, we obtained spectra over the range 5800-9200 Å, with a similar dispersion ($\approx$ 3.36 Å/pix) and a final resolution of $\approx$ 17 Å (same slit width). In order to eliminate the second-order contamination, an OG 550 filter was employed. Series of 15 minute exposures were taken for the objects. The standard stars LTT4364, EG274, and LTT7379 (Stone & Balwin 1983) and HD160233 (Gutiérrez-Moreno et al. 1988) were observed for flux calibrations. The hot standard HD160233 was also used for the telluric absorption corrections in the near-infrared. Bias, dome and twilight sky and tungsten lamp flat fields were taken and employed in the reductions. The logbook of observations is provided in Table 2, by Cols.: (1) cluster designation, (2) date of observation, (3) spectral range as explained above, (4) number of exposures and duration in seconds, (5) full width half maximum (FWHM) in arcseconds, determined by the seeing during observation, and (6) total area of the cluster scanned in our observations.

 

 

Table 2: Journal of observations

Cluster	Date	Spectral	Exposure	FWHM	AREA
	(UT)	range	Time (secs)	( $^{\prime\prime}$)	($^\prime$$\times$ $^\prime$)
Ruprecht144	7/20/98	blue	$6\times 900$	1.5	$3\times 4$
	7/22/98	red	$2\times 900$	2.9
	7/23/98	red	$2\times 900$	1.6
Melotte105	5/25/95	blue	$3\times 900$	2.0	$3.5\times 4$
	5/27/95	red	$3\times 900$	2.1
	5/29/95	red	$2\times 900$	2.2
BH132	7/18/98	blue	$4\times 900$	1.6	$2.5\times 2.5$
	7/20/98	blue	$2\times 900$	1.5
	7/24/98	red	$4\times 900$	1.5
Hogg15	5/24/95	blue	$4\times 900$	2.2	$1\times 1.5$
	5/23/95	red	$4\times 900$	2.0
	5/29/95	red	$2\times 900$	2.0
Pismis21	7/18/98	blue	$4\times 900$	1.9	$1\times 2$
	7/21/98	blue	$2\times 900$	1.2
	7/22/98	red	$4\times 900$	1.3
Lyngå11	7/19/98	blue	$4\times 900$	1.2	$3\times 4$
	7/22/98	red	$4\times 900$	1.3
BH217	7/19/98	blue	$6\times 900$	1.2	$3\times 3.5$
	7/24/98	red	$4\times 900$	1.5

Reductions were carried out with the IRAF package following standard procedures at the Observatorio Astronómico of the National University of Córdoba. We subtracted the bias and used flat-field frames - previously combined - to correct the frames for high and low spatial frequency variations. Background sky subtractions were then performed using pixel rows from the same frame after cleaning cosmic rays from the background sky regions. We controlled that no significant background sky residuals were present on the resulting spectra. The cluster spectra were extracted and wavelength-calibrated by fitting observed He-Ar comparison lamp spectra with template spectra. The rms errors involved in these calibrations are tipically 0.70 Å (0.20 pixel) for the blue nights and 0.35 Å (0.10 pixel) for the red ones. Extinction corrections and flux calibrations were then applied. Finally, we eliminated the telluric absorption in the near-IR following the precepts outlined in Bica & Alloin (1987).

Figure 1: Observed integrated spectrum of Ruprecht144 (bottom), the spectrum corrected for the derived reddening E(B-V) = 0.32 and the Y3B template which best matches the spectrum. An alternative match using the Y3A template (top) is also shown. Spectra are in relative $F_{\lambda }$ units normalized at $\approx$ 6000 Å. Constants have been added to the spectra, except for the bottom one

Figure 2: Comparison of observed (bottom) and reddening-corrected spectra of Melotte105 with the Y3B and Y2 templates. Units as in Fig. 1

Figure 3: Comparison of the observed (bottom) and reddening-corrected spectra of BH132 with the Y3B and Y4 templates. Units as in Fig. 1

Figure 4: Comparison of the observed (bottom) and reddening-corrected spectra of Hogg15 with the YA.LMC and YB.LMC templates. Units as in Fig. 1

Figure 5: Comparison of the observed (bottom) and reddening-corrected spectra of Pismis21 with the Y2 template which best matches the spectrum. Units as in Fig. 1

Figure 6: Comparison of the observed (bottom) and reddening-corrected spectra of Lyngå11 with the Y4 and Y3B templates. Units as in Fig. 1

Figure 7: Comparison of the observed (bottom) and reddening-corrected spectra of BH217 with the Y2 template. Units as in Fig. 1

In Figs. 1-7 (bottom) we present the calibrated integrated spectra of the observed clusters normalized to $F_{\lambda }$ = 1 at $\lambda$ $\approx$ 6000 Å. Indeed, the different slopes of the continuum energy distributions are the result of combined reddening and age effects. Although a certain amount of field star contamination may be expected mainly from stars located close to the cluster centre, only bright field stars could affect the spectra significantly but this is not the case in any of the observed clusters.

2.2 Equivalent width measurements

Equivalent widths (EWs) in some selected spectral windows defined by Bica & Alloin (1986a, 1986b, 1987, hereafter BA86a, BA86b, and BA87, respectively) were measured for the cluster sample using the spectral analysis program SPEED (Schmidt 1988). The EWs of the Balmer lines were measured taking into account both the spectral windows and flux points as defined by BA86a, BA86b and BA87. Two alternative fits using high and low continuum tracings were made in order to take into account the spectral noise in the measurements of the Balmer-line EWs. The resulting uncertainties in the EWs are found to be smaller than those associated with the intrinsic dispersions in Table 2 of BA86b. Equivalent widths for TiO features between (8200-8500) Å and CaII triplet lines were also measured in six of the seven clusters. The results of the measurements are presented in Tables 3 and 4, where the EWs are given in Angström units (Å).

 

 

Table 3: Measurements of equivalent widths of four Balmer lines

Cluster	H$_\delta$	H$_\gamma$	H$_\beta$	H$_\alpha$
Windows	(4082-4124) Å	(4318-4364) Å	(4846-4884) Å	(6540-6586) Å
Ruprecht144	9.0 $\pm$ 0.1	10.2 $\pm$ 0.4	8.4 $\pm$ 0.2	6.0 $\pm$ 0.2
Melotte105	11.4 $\pm$ 0.2	9.5 $\pm$ 0.3	10.2 $\pm$ 0.4	7.1 $\pm$ 0.4
BH132	15.6 $\pm$ 0.4	9.4 $\pm$ 0.1	7.2 $\pm$ 0.2	-
Hogg15	7.1 $\pm$ 0.8	4.0 $\pm$ 0.7	6.6 $\pm$ 0.7	1.2 $\pm$ 0.1
Pismis21	11.2 $\pm$ 0.2	7.5 $\pm$ 0.1	7.4 $\pm$ 0.1	-
Lyngå11	11.6 $\pm$ 0.1	9.0 $\pm$ 0.1	10.3 $\pm$ 0.1	-
BH217	9.2 $\pm$ 0.2	6.2 $\pm$ 0.1	7.2 $\pm$ 0.1	3.2 $\pm$ 0.1

 

 

Table 4: Measurements of equivalent widths of TiO and CaII triplet features

Cluster	TiO	CaII	CaII	CaII
Window	(8234-8476) Å	(8476-8520) Å	(8520-8564) Å	(8640-8700) Å
Ruprecht144	-	4 $\pm$ 1	5 $\pm$ 2	5 $\pm$ 1
Melotte105	-	4 $\pm$ 1	5 $\pm$ 1	5.3 $\pm$ 0.8
BH132	11 $\pm$ 2	4 $\pm$ 2	4 $\pm$ 1	5 $\pm$ 1
Hogg15	13 $\pm$ 2	3.2 $\pm$ 0.7	-	4.6 $\pm$ 0.7
Pismis21	13 $\pm$ 1	4 $\pm$ 2	5 $\pm$ 1	5 $\pm$ 1
Lyngå11	15 $\pm$ 2	-	5 $\pm$ 1	5 $\pm$ 1