Publications OFTHK Astronomical Socik-N-Οκτπκ Pacific 101: 570-572, June 1989

Publications OFTHK Astronomical SociK-n-οκτπκ Pacific 101: 570-572, June 1989

A SURVEY FOR RR LYRAE VARIABLES IN FIVE SMALL MAGELLANIC CLOUD CLUSTERS

ALISTAIR R. WALKER Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories* Casilla 603, La Serena, Chile Received 1989 March 15

ABSTRACT Results are presented of a search for RR Lyrae variables in the SMC clusters NGC 121, Lindsay 1, Kron 3, Kron 7, and Kron 44. The techniques used rediscovered the four RR Lyraes in NGC 121, but no variables were found in the other clusters. It is concluded that RR Lyraes do not occur in SMC clusters younger than — 11 Gyr. Key words: star clusters-Magellanic Clouds-variable stars: RR Lyrae stars

1. Introduction Lindsay 1 but did not find any variables; however, the The Small Magellanic Cloud (SMC) contains a number majority of his plates were exposed in times of rather poor of populous clusters in the age range 3-12 Gyr, unlike seeing. A survey of several MC clusters for RR Lyraes was either the Galaxy or the Large Magellanic Cloud (LMC). made, using the Yale 1-m telescope and an image-tube Olszewski (1986) and Olszewski, Schommer, and camera, by Graham and Nemec (1984). They found many Aaronson (1987, henceforth OSA) point out that it should variables in old LMC clusters but none in the two SMC be possible to determine the age of the youngest cluster clusters blinked, NGC 339 and NGC 416. RR Lyraes by a suitable survey for the variables in the Since these surveys, color-magnitude diagrams candidate clusters. Traditionally, RR Lyraes indicate the (CMDs) for several SMC clusters older than a few Gyr presence of a population perhaps 14 or more Gyr old, have appeared in the literature. Table 1 lists the SMC although the more metal-rich, galactic-field RR Lyraes clusters thought to be older than 3 Gyr, together with may be several Gyr younger (Strugnell, Reid, and Murray their ages as estimated from the CMDs. Where neces- 1986). Renzini (1984) notes that CNO abundance varia- sary, the ages have been adjusted to an SMC modulus of tions can alter the age at which RR Lyraes first occur, 18.8 mag (Feast and Walker 1987) and metallicity [Fe/H] since CNO-poor core He-burning stars lie in a bluer zone - -1.3 (Feast 1988). After NGC 121, the two oldest SMC of the zero-age horizontal branch. If RR Lyraes can occur clusters are thought to be Lindsay 1 and Kron 3. at ages significantly younger than the galactic globular OSA suggest, on the basis of integrated colors, that the clusters then this has obvious relevance to our ideas on clusters Kron 7 and Kron 44 might also be old. Both these the evolution of old, solar-mass stars and also on the mix of clusters are rather sparse and lie in regions of high field stars used to construct population models of galaxies. star density. The discovery of RR Lyraes in SMC clusters in addition Modern techniques (CCDs and multistar psf-fitting to NGC 121, for which four RR Lyraes are known, would photometry) allow measurement of stars to the very cen- also help to determine distances to the clusters and, ters of these not very centrally condensed clusters. None hence, the SMC and perhaps aid in unraveling the com- TABLE 1 plex SMC geometry. Three RR Lyraes in NGC 121 were found by Thackeray SMC clusters older than 3 Gyr (1951); this discovery was crucial in confirming the factor two change in the distance scale proposed by Baade Cluster Age (Gyr) Reference (1952). Another RR Lyrae was found in NGC 121 by Graham (1975) during his survey of RR Lyraes in the SMC NGC 121 12 ± 2 Stryker, da Costa and Mould (1985) Lindsay 1 10 ± 2 Olszewski, Schonmer and Aaronson (1987) field near the cluster. Gascoigne (1966) blinked plates of Kron 3 8 ± 2 Rich, da Costa and Mould (1984) Kron 7 Kron 44 *Cerro Tololo Inter-American Observatory, National Optical Astron- Lindsay 113 5 ± 1 Mould, da Costa and Crawford (1984) omy Observatories, operated by the Association of Universities for NGC 339 3 Olszewski, Schommer and Aaronson (1987) Research in Astronomy, Inc., under contract with the National Science NGC 416 3? Durand, Hardy and Melnick (1984) Foundation.

570 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System RR LYRAE VARIABLES 571 of the above-mentioned CMD studies measure all the 3. Results cluster stars to a level fainter than the horizontal branch— 3.1 NGC 121 the aim of the present program. This cluster is centrally very condensed and photome- 2. Observations try was only attempted further than 40 arc sec from the Three nights of time on the CTIO 4-m telescope plus core. No new candidates for variables were found on the PFCCD system were allocated to this project. The TI No. resulting CMDs. Three U-band frames, all taken in very 1 CCD was used together with filters matching Johnson good seeing, were blinked. The four known RR Lyrae UBV. For each of the candidate clusters short-exposure variables were rediscovered both by blinking and from U, B, and V frames were taken in order to identify possi- the photometry. Two new variables were suspected —30 ble RR Lyrae candidates on V, (C/ — ß ) CMD diagrams. In arc sec from the cluster center but were not confirmed by addition, U-band exposures were taken for each cluster at comparison with a CCD frame taken on 1989 November intervals of a few hours. These frames were "blinked". 14 using the 1.5-m telescope and TI No. 2 CCD in excel- Observational details are given in Table 2. Many Graham lent (0.8 arc sec) seeing. The crowding is such that it is (1982) standards were observed on each of the three unlikely that ground-based photometry of V ~ 19 mag nights in order to provide zero points and extinction stars this close to the cluster center will be possible. coefficients and to check the color equations. 3.2 Lindsay 1 The data frames were trimmed, debiased, and flat fielded in the usual way (e.g., Walker 1984). The flat fields The best seeing L7-band frame (FWHM = 1.2 arc sec) were produced by median filtering many exposures of the was chosen and a star list prepared using the "find" option "white spot" in the 4-m dome. This procedure flattened of DAOPHOT with the level set so that all stars with U < object frames to ±0.5% (V), ±0.8% (ß ), and ±1.0% ((7), 22 were included. The same stars were then measured on if the unthinned edge and corner of the CCD were the short exposure Β and V frames. Five V,{U — B) and avoided. This accuracy is entirely adequate for the pur- one V, (β — V) CMDs were then examined; none showed pose here. any stars to the blue of the giant clump. DAOPHOT (Stetson 1987) was used to find and mea- Individual U magnitudes were then examined and any sure stars on the CCD frames. Only in NGC 121 is the stars brighter than U = 21.5 with significant s.d. (> 0.1 star density too high to allow stars to be measured to the mag) for the five U frames were marked for closer very center of the cluster. This paper gives the results of scrutiny. There were 21 such stars out of the 679 total. the search for RR Lyrae variables alone; details of the They were all found to be due to poor measurement of photometry and CMDs will be presented elsewhere. blends or lay near CCD defects on some of the frames but not on others. TABLE 2 The five U frames were blinked in various combina- Observing Log tions. No variables were found. It is estimated that any variable with an amplitude of more than 0.1 mag would be UT Exposures Seeing easily apparent. RR Lyraes have amplitudes several times (1988) (s) (arc sec) this limit, so it is concluded that there are none in this cluster. Oct 8 0130 Kron 7 40V,60B,120U,600U 1.8 0400 Kron 3 60B,40V,120U,600U 1.8 0420 Lindsay 1 120U,600U,60B,40V 1.8 3.3 Kron 3 0440 Kron 44 40V,40B,120U,600U 1.8 0715 Kron 44 600U 2.0 The analysis was similar to that for Lindsay 1 above. 0740 Kron 7 600U 2.0 Five (7-band frames were blinked in various combina- 0755 Kron 3 600U 2.0 0806 Lindsay 1 600U 2.0 tions. No candidate variables were found on the CMDs or 0818 Kron 44 600U 2.0 Oct 9 0350 Kron 7 200U 1.4 when blinking the frames. 0356 Kron 3 200U 1.4 0400 Lindsay 1 200U 1.4 3.4 Kron 7 0406 Kron 44 200U 1.4 0415 NGC 121 30U,300U,5B,50B,2V,20V 1.4 The analysis was similar to that for Lindsay 1 above. No Oct 10 0003 Kron 7 300U 1.3 0010 Kron 3 300U 1.2 variables were found, either from examination of the 0017 Lindsay 1 300U 1.2 CMDs or from blinking five (7-band frames. The CMDs 0024 NGC 121 300U 1.2 0031 Kron 44 300U,20B,10V,50V,100B 1.2 show that the cluster is only 3-4 Gyr old. 0722 Lindsay 1 300U 1.2 0729 Kron 3 300U 1.2 3.5 Kron 44 0736 Kron 7 300U 1.2 0743 Kron 44 300U 1.2 This cluster lies in a region with very high field star 0751 NGC 121 300U 1.2 density, and photometry has not yet been completed. Five (7-band frames were blinked and no variables were found.

4. Conclusions Feast, M. W., and Walker, A. R. 1987, Ann. Rev., 25, 345. Gascoigne, S. C. B. 1966, M.N.R.A.S., 134, 59. Of the clusters surveyed, only NGC 121 contains RR Graham, J. A. 1975, Pub. A.S.P., 87, 641. Lyrae variables. This is evidence that RR Lyraes are not 1982, Pub. A.S.P., 94, 244. formed in SMC clusters younger than eleven Gyr. Unfor- Graham, J. Α., and Nemec, J. M. 1984, in IAU Symposium 108, Struc- ture and Evolution of the Magellanic Clouds, ed. S. van den Bergh tunately, the ages of old SMC clusters are not determined and K. S. de Boer (Dordrecht: Reidel), p. 37. better than ± 2 Gyr so this estimate may be incorrect by Mould, J. R., Da Costa, G. S., and Crawford, M. D. 1984, Ap. J., 280, 2-3 Gyr either way. Deeper CMDs for these old clusters 595. may be possible with the current generation of 4-m tele- Olszewski, E. W. 1986, in IAU Symposium 126, Globular Cluster Systems in Galaxies, ed. J. E. Grindlay and A. G. David Philip scopes in very good seeing and is certainly possible from (Dordrecht: Kluwer), p. 159. space with the H ST. Observations to provide better Olszewski, E. W., Schommer, R. Α., and Aaronson, M. 1987, A./., 93, abundance estimates for these clusters are under way (D. 565 (OSA). Geisler, private communication). When these measure- Renzini, Α. 1984, in /AÍ7 Symposium 108, Structure and Evolution of the Magellanic Clouds, ed. S. van den Bergh and K. S. de Boer ments are available the age limit of RR Lyraes will be able (Dordrecht: Reidel), p. 215. to be refined further. Rich, R. M., Da Costa, G. S., and Mould, J. R. 1984, Ap./., 286, 517. Stetson, P. B., 1987, Pub. A.S.P., 99, 191. REFERENCES Strugnell, P., Reid, N., and Murray, C. A. 1986, M.N.R.A.S., 220, 413. Baade, W. 1952, Trans. I AU, 8, 397. Stryker, L. L., Da Costa, G. S., and Mould, J. R. 1985, Ap./., 298, 544. Durand, D., Hardy, Ε., and Melnick, J. 1984, Ap. /., 283, 552. Thackeray, A. D. 1951, Observatory, 71, 219. Feast, M. W. 1988, preprint. Walker, A. R. 1984, M.N.R.A.S., 209, 83.