1982Apj. . .261. . .85R the Astrophysical Journal, 261:85-101, 1982 October 1 © 1982. the American Astronomical Society. All Ri
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.85R . .261. The Astrophysical Journal, 261:85-101, 1982 October 1 . © 1982. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1982ApJ. ANALYSIS OF INTEGRATED SPECTRA OF RED STAR CLUSTERS IN THE MAGELLANIC CLOUDS Douglas Rabin1 Department of Astronomy, California Institute of Technology Received 1981 August 3; accepted 1982 April 8 ABSTRACT SIT digital spectra of the integrated light of 16 “red” star clusters in the Magellanic Clouds exhibit a wide variation of hydrogen line strength from cluster to cluster. In a plot of Balmer line against Ca K line equivalent width, almost all of the clusters in the sample lie apart from an empirical sequence of Galactic globular clusters. At given metallic line strength, the Cloud clusters have stronger Balmer lines. This is interpreted as indicating that few among the populous clusters in the Magellanic Clouds are as old as globular clusters. According to models of the integrated light, Balmer line strength remains sensitive to stars at the turnoff, and therefore to age, for faint turnoffs that are difficult to reach in a color-magnitude study. In the case of NGC 2209, for which both methods may be applied, good agreement is found for an age 2.1 ± 0.6 x 109 yr and a metallicity [Fe/H] ~ — 1. In the case of Kron 3, it is predicted that its turnoff, when securely known, will be fainter than F = 22. When combined with synthetic spectra for a range of age and composition, the hydrogen-metals diagnostic diagram has the potential to yield quantitative, independent estimates of age and metallicity with a modest investment of observing time. Integrated spectra of star clusters are a practical tool for exploring the history of chemical enrichment in galaxies too distant for the application of techniques based on single stars. Subject headings: clusters: globular — galaxies: Magellanic Clouds — stars: abundances I. INTRODUCTION taminated to greater or lesser degree by the general field Spectroscopy provided the first clue that populous star of the Magellanic Clouds. Less tangible, but perhaps of clusters in the Magellanic Clouds fall into two distinct equal importance, have been the consequences of forcing the Cloud clusters into the classification scheme devised groups: Cannon classified the integrated spectra of eight 2 clusters as early A-type, earlier than any globular cluster for the spectra of globular clusters by Morgan (1956) in the Galaxy (Shapley 1930). Subsequently, emphasis and Kinman (1959). The influence was the more subtle shifted to other observational techniques. For example, in that “forcing” was apparently unnecessary: Andrews it was with the advent of photoelectric photometry that and Lloyd Evans (1971) could describe N419 as having the division between red and blue clusters became “a typical halo-type spectrum.” Ford (1970) also manifest (Gascoigne and Kron 1952). Recently, Searle, employed the Kinman classification criteria, and his Wilkinson, and Bagnuolo (1980; hereafter SWB) used interpretation of N419 and N1783 as intermediate age intermediate band photometry to construct the first clusters was apparently dictated as much by the color- convincing classification of the populous clusters, one magnitude diagrams as by evidence within the spectra. which embraces in a single sequence both the red and The simple fact is that the visual appearance of a the blue groups and which introduces subdivisions within photographic spectrum of N419 is quite similar to the the red group. Color-magnitude diagrams (CMDs) have spectrum of a moderately metal-poor globular cluster. been the other main avenue of advancement, from the Despite these difficulties, there are sound reasons for early work of Arp, Hodge, Tifft and others, through continuing to study integrated spectra. First, the turnoff Gascoigne’s (1966) influential survey of 17 clusters, to region of a red cluster remains difficult to reach current programs which are beginning to yield turnoff photographically (with the usual calibration problems at ages for a few red clusters (reviewed by Gascoigne faint magnitudes) and nearly inaccessible to photoelectric 1980). photometry; but the turnoff contributes importantly to In contrast, the analysis of spectroscopic data has the integrated spectrum, particularly for the many contributed comparatively little to our understanding of clusters that have a weak giant branch and a red the populous clusters. In part, this may be attributed to horizontal branch or giant clump. Second, with observational difficulties. Most of the clusters have low photometric data the issue of metallic line strength must surface brightness, and their integrated light is con- always be approached indirectly and with some trepidation, even for single stars, whereas the integrated 1 Visiting Astronomer at Cerro Tololo Inter-American Observatory, which is supported by the National Science Foundation under 2 In what follows, “globular cluster” refers exclusively to globular Contract AST 78-27879. clusters in the Galaxy. The prefix NGC is abbreviated to N. 85 © American Astronomical Society • Provided by the NASA Astrophysics Data System 86 RABIN Vol. 261 spectrum allows individual metallic features to be TABLE 1 compared among themselves and with hydrogen lines. Characteristics of Spectrograph and Detector Finally, reddening is a serious problem in most photometric studies of clusters at low latitudes in our Grating 600 1 mm - \ first order own Galaxy, as well as in external systems, whereas Slit width 5" for objects 10" for standard stars equivalent widths are unaffected by reddening. SIT pixel size 35 ¡mi square Calibrated by reference to direct determinations of age 3.3 Á along dispersion and metallicity and assisted by theoretical models of the 3" on sky integrated light, the analysis of integrated spectra has the Format 512 pixels along dispersion potential to become the most practical tool for exploring 128 pixels (384") along slit Usable spectral range AA3750-5170 the history of cluster formation and chemical enrichment Spectral resolution 8.5 Â in nearby galaxies. The present work is intended as a first step in realizing that potential. Specific impetus for a new investigation was provided over a range chosen at the telescope, considering both the by the availability of an integrating vidicon system at surface brightness of the cluster and the density of the Cerro Tololo. This two-dimensional detector offered background field; this range is noted in Table 2. All several advantages over the image tubes used in previous observations were made with the telescope in focus. work: (1) higher signal-to-noise ratio (because of the There was no moonlight. ability to sum the signal over the area of the cluster and to combine exposures); (2) higher fidelity to the b) Reduction Procedure true spectrum (because of the ability to subtract the mean The data were reduced with the software system spectrum of the sky and the surrounding field); and developed for SIT spectrographs on Mount Palomar. (3) superior accuracy in the measurement of equivalent A high-resolution television display was used to choose widths and central depths (because the detector is the limits of integration along the slit for object and for intrinsically linear). sky; to verify the adequacy of the compensation for S-distortion of the spectrum; and to monitor over the II. OBSERVATIONS whole frame small changes in dark level or alignment— changes which are difficult to detect in a selection of a) Program and Instrumentation cuts across the target but which, uncompensated, The availability of a modern color-magnitude diagram materially degrade the quality of the final result. (CMD) was the principal criterion in the selection of Photometric standard stars were chosen from the Oke clusters for the program. Thus, N2209, with good scanner standards (Oke 1964; Hayes 1970) and estimates of age and abundance (Gascoigne et al 1976 ; secondarily calibrated standards in the Clouds. The Gustafsson, Bell, and Hejlesen 1977), and Krön 3, wavelength response of the neutral density filter required with a problematic CMD (Walker 1970; Gascoigne to observe these bright stars was freshly calibrated. 1980), were included despite the long exposures required. Standard extinction coefficients for Cerro Tololo were N1987, although lacking a CMD, was considered adopted from Hayes and Latham (1975). Because the important because it is the only cluster in van den slit was scanned over each cluster, differential refraction Bergh and Hagen’s (1968) UBV study which cannot be does not affect the energy distributions. assigned unequivocally either to the red or to the blue Flat field corrections were found to be unnecessary, group. For the rest of the sample, red clusters were since the uncorrected target response was locally smooth chosen exclusively. Because of the rapid evolution of at the 3 %-4 % level, in agreement with the specification the main-sequence turnoff for clusters <109 yr (1 Gyr) given by Atwood et al (1979). To within the accuracy old, the integrated color of a blue cluster is already a with which equivalent widths are reported in Table 3, good indicator of its age; and as to metallicity, the flattened and unflattened spectra give the same result. A-type integrated spectrum offers little hold for an A slight flaw in the target was avoided in the placement analysis at low dispersion. of a cluster along the slit, but a small part of the Integrated spectra of 16 red clusters in the Magellanic spectrum near 4500 Â (marked in Fig. 1) was occasionally Clouds and seven globular clusters were obtained with affected. This region was ignored in fitting the pseudo- the SIT vidicon (Atwood et al. 1979) at the Cassegrain continuum described below, and there were no measured focus of the 1.5 m telescope on Cerro Tololo. Table 1 absorption features near it. summarizes the parameters of the spectrograph-detector The reduction procedure produces final energy distri- combination. Of 170 pixels (8'.5) exposed along the slit, butions which are smooth, reproducible, and in only the central 128 pixels (6'.4) were retained in the satisfactory agreement with independent data (van den reduction procedure.