THE ASTRONOMICAL JOURNAL VOLUME 90, NUMBER 11 NOVEMBER 1985 90.22210 THE URSA MINOR DWARF GALAXY: STILL AN OLD STELLAR SYSTEM Edward W. Olszewski^ Steward Observatory, University of Arizona, Tucson, Arizona 85721 1985AJ and Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, Victoria, British Columbia V8X 4M6, Canada Marc AARONSONa) Steward Observatory, University of Arizona, Tucson, Arizona 85721 Received 25 March 1985; revised 5 August 1985 ABSTRACT We have constructed a color-magnitude diagram of the Ursa Minor dwarf spheroidal to = 24.8 mag from charged-coupled device (CCD) observations with the Kitt Peak 4 m telescope. The main-sequence turnoff is easily visible. Fits to evolutionary isochrones and the globular M92 indicate that Ursa Minor has an age and metal abundance very similar to that of the latter cluster. No evidence for stars younger than about 16 billion years is seen, with the possible exception of approximately 20 stars believed to be blue stragglers. Ursa Minor is therefore an extreme-age galaxy, unlike superficially similar objects such as the Carina dwarf. Indeed, Ursa Minor may be the only outer-halo spheroidal whose stellar content lives up to the classical ideals of a Population II system. A distance modulus of (m — Af )0 = 19.0 mag is derived from a sliding fit to the M92 ridge lines. However, this modulus is uncertain by ~0.1 mag, for the horizontal branch in our color-magnitude diagram is poorly populated. The ratio of blue stragglers to anomalous Cepheids in Ursa Minor is estimated to be ~ 100, a number that may provide an impor- tant constraint on binary models for the origin of these stars. A surprising result of our study is the discovery of dumpiness in the distribution of stars. This finding may give more weight to the idea that dwarf spheroidal galaxies were previously dwarf irregular galaxies, although clearly, if so, Ursa Minor must have lost its gaseous content very soon after formation. I. INTRODUCTION to accumulate (Zinn 1978; Demers, Kunkel, and Hardy The dwarf spheroidal galaxies have in the last few years 1979; Kinman, Kraft, and Suntzeff 1981; Suntzeff et al. become intensely studied objects, and deservedly so, for they 1984; and Buonanno et al. 1985; but see Bell 1985). This provide an outstanding laboratory in which to investigate latter result is possibly not unexpected in systems such as evolutionary processes in the Milky Way’s outer halo. Excel- Fornax, where enrichment processes should have had suffi- lent reviews of these systems have been written by Hodge cient time to occur, but it perhaps does present difficulties in (1971) and Zinn (1980, 1985; see also Carney 1984). Since galaxies such as Draco and Ursa Minor, where there is little Zinn’s earlier review, a number of major developments have evidence as yet for an extended period of star formation. occurred. Some rather speculative ideas concerning the dwarf spher- To begin with, the presence of stars younger than those oidals have also been recently raised. First, Lin and Faber found in galactic globular clusters has now been firmly es- (1983) and Kormendy (1985) have argued that the spheroi- tablished for many of the spheroidals. This came about first dals are closer in kinship to dwarf irregulars rather than to through identification of luminous asymptotic-giant-branch dwarf ellipticals. Second, the possibility that the spheroidals (AGB) carbon stars (see Aaronson and Mould 1985), and may contain substantial amounts of dark matter has been more directly by deep color-magnitude diagrams (CMDs) suggested (see Aaronson 1983; Faber and Lin 1983; Lin and that have now been published for Carina (Mould and Aaron- Faber 1983). son 1983) and Sculptor (Da Costa 1984), which reach the Ursa Minor and Draco form a pair of galaxies whose con- level of the main-sequence turnoff. Such age effects seem to trasts may illuminate many of the remaining problems in account naturally for the prevalence of red horizontal understanding the stellar content of the dwarf galaxies. branches in the spheroidal systems. Furthermore, these sys- These systems are the two lowest-luminosity halo dwarfs tems not only appear to differ from each other in mean age, (Mv = — 8.8 mag and — 8.5 mag for Ursa Minor and Dra- but a considerable age range may be present within some of co, respectively, after Zinn 1985), and in this regard are most them. The most striking case in point is Fornax, whose stars comparable to galactic globulars. Both contain RR Lyrae appear to span an age from ~ 3 X109 to ~ 15 X 109 yr (Aar- variables and anomalous Cepheids, while neither are known onson and Mould 1985; and Buonanno etal. 1985). to possess red giant variables, which have occasionally Reasonably accurate metal abundances for all seven halo cropped up in several of the other spheroidals (Baade and spheroidals are now also available, and a mean metallicity- Swope 1961; van Agt 1973). Both have mean abundances absolute magnitude relation has been shown to exist (see similar to M92, but have some stars of very different metalli- Aaronson and Mould 1985; Buonanno etal. 1985, and refer- city, and both have stars that show enhanced CNO (e.g., ences therein). Furthermore, evidence that within a given Kinman eiû/. 1981; Stetson 1984; Suntzeff ei a/. 1984). Both spheroidal the stars exhibit a dispersion in [Fe/H] continues contain carbon stars (Aaronson, Liebert, and Stocke 1982; Aaronson, Olszewski, and Hodge 1983), but these are of the a)Visting Astronomer at Kitt Peak National Observatory, a division of low-luminosity, blue-color variety also found in co Cen and NO AO operated by AURA, Inc., under contract with the National Science several other galactic globulars. Foundation. On the other hand, Ursa Minor and Draco differ in two 2221 Astron. J. 90 (11), November 1985 0004-6256/85/112221-18$00.90 © 1985 Am. Astron. Soc. 2221 © American Astronomical Society • Provided by the NASA Astrophysics Data System 2222 E. W. OLSZEWSKI AND M. AARONSON: URSA MINOR 2222 90.22210 important respects. First, the characteristics of the Ursa Mi- III. DATA REDUCTIONS AND CALIBRATIONS nor RR Lyraes are very similar to those in OosterhofFType a) The Standard-Star Frames II clusters, while those in Draco cannot be readily placed in either of the Oosterhoff classes (e.g., Zinn 1980). This differ- Short exposures, ranging from 5 to 30 s, were taken of the 1985AJ ence may be related to Draco’s having a larger and slightly M92 CCD standard field (Davis 1984; Christian 1980; see more metal-rich abundance spread. Perhaps more signifi- also Christian et al. 1985; other photometry found in San- cant is the fact that Ursa Minor is the only spheroidal to have dage 1969) and the NGC 7790 videocamera field (Christian a blue horizontal branch. Draco, in contrast, has a red hori- 1980; Christian et al. 1985; photoelectric photometry also zontal branch like the other spheroidals, and given its mean available and used from Sandage 1958). All standard-star abundance exhibits perhaps the most extreme case known of observations were secured during photometric conditions. the famous “second-parameter” problem. In each case, two short and two longer exposures were made Although there are other possible ways to account for dif- for each observation of each object in each filter. M92 was ference in horizontal-branch type, it is very tempting to spec- observed at two different times during the night. The B —V ulate that Draco is a few billion years younger than Ursa colors of the observed standards cover a wide color base line Minor. The primary purpose of the present paper is to help from — 0.1 to 1.7. Twenty individual standard stars were test this hypothesis by providing a very deep color-magni- measured, 12 at two different airmasses. tude diagram for Ursa Minor. Two groups (Carney and Aperture photometry was then performed for each frame, Seitzer 1985; Stetson, VandenBerg, and McClure 1985) are yielding up to four observations of each standard star for currently supplying the companion data for Draco. Current each color at a given airmass. The instrumental total magni- detector technology readily enables the main sequence to be tudes were then determined by growing the radius until 1 reached in both systems, so that the crucial test can be made. pixel change in stellar magnitude was approximately equal The organization of the paper is as follows: A brief de- to the rms error in determining that quantity. This corre- scription of the observations is presented in Sec. II. The data sponded to a 7 pixel aperture, the last magnitude growth reduction, performed entirely with Peter Stetson’s dao from pixel 6 to pixel 7 being of order 0.005 mag (with the phot program, is fully described in Sec. III. Because this is scale — 0.6" per pixel). The total instrumental magnitudes one of the first papers to make extensive use of daophot, the and errors, and the standard-star magnitude with its asso- discussion in Sec. Ill has been made rather lengthy and de- ciated error, were then typed into two computer files. (Note tailed. The uninterested reader is invited to skip directly to that all instrumental magnitudes were derived using rou- Sec. IV, where the final color-mangitude diagram is present- tines in Stetson’s daophot program, and all transforma- ed, along with isochrone fits and the derivation of age and tions were made using daophot subsidiary programs, all distance modulus. A summary of our findings is given in Sec. also written by Stetson. The etymology of daophot is de- V. In Sec. VI we present and discuss the implications of a scribed in Stetson 1985.) A minor variation of the program startling and unexpected result pertaining to the presence of CCDSTD was then used to calculate the transformation to apparent stellar subclustering in the Ursa Minor field.