The Astrophysical Journal, 520:L33±L36, 1999 July 20 q 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. AN INTERNAL SECOND-PARAMETER PROBLEM IN THE SCULPTOR DWARF SPHEROIDAL GALAXY S. R. Majewski,1 M. H. Siegel, Richard J. Patterson, and R. T. Rood Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818; [email protected], [email protected], [email protected], [email protected] Received 1999 March 1; accepted 1999 May 18; published 1999 June 22 ABSTRACT We present BV photometry of the Sculptor dwarf galaxy toV 5 22 . These data give evidence for a bimodality in Sculptor's metallicity distribution based on a discontinuity in the luminosities of horizontal-branch (HB) stars and by the presence of two distinct red giant branch (RGB) bumps. A consistent picture of the evolved stars in Sculptor is given by the presence of (1) a metal-poor population of[Fe/H] » 22.3 with an exclusively blue HB and that corresponds to the blueward side of the Sculptor RGB and the more luminous RGB bump, and (2) a less metal-poor population of[Fe/H] » 21.5 required to explain the less luminous red HB, the red side of the RGB, and a second, less luminous RGB bump. Best ®ts to the HB populations are obtained with enhanced oxygen abundances,[O/Fe] » 10.5 . Variations in the global HB and RGB morphology of Sculptor can be explained by differences in the radial distribution of the two different [Fe/H] populations. The presence of these two populations shows that the Sculptor dwarf galaxy has an internal second-parameter problem. Subject headings: galaxies: abundances Ð galaxies: dwarf Ð galaxies: individual (Sculptor) Ð galaxies: stellar content Ð Local Group Ð stars: horizontal-branch 1. INTRODUCTION moderate Sculptor HB morphology index of (B2R)/(B 1 V 1 R) 520.15 (see also the Grebel, Roberts, & van de Rydt 1994 The Sculptor dwarf spheroidal (dSph) galaxy was the ®rst CMD). Because their VI photometry of more than 6000 stars Galactic dSph to be identi®ed (Shapley 1938) and has a long withV ! 21.5 covered a much larger area than previous results, observational history. Its variables have been tabulated by a K95's signi®cantly increased BHB : RHB ratio suggests dif- number of studies (Baade & Hubble 1939; Thackeray 1950; ferences in the spatial distribution of BHB and RHB stars. This van Agt 1978; Goldsmith 1993, hereafter G93; Kaøuzny et might be due to abundance gradients in the dSph. However, al. 1995, hereafter K95), and the period distribution of since the most metal-rich population in Sculptor's RGB ap- RR Lyrae stars suggests a metallicity spread (G93; K95). While parently has[Fe/H] » 21.5 , which would normally give a uni- a range of abundances is generally accepted, impressions of form to blue HB, the variation in the RHB population must be the Sculptor horizontal-branch (HB) morphology outside of the due to spatial variation in the 2dP effect. instability strip have varied depending on photometric depth, In this Letter, we present BV photometry of the Sculptor ®eld size, and ®lter systems employed to construct the color- dSph galaxy. We ®nd the usual evidence for RGB stars rang- magnitude diagram (CMD). Kunkel & Demers' (1977, here- ing from metal poor ([Fe/H] » 21.5 ) to very metal poor 5 after KD77) Sculptor CMD (324 stars toV 20.6 ) yielded 43 ([Fe/H] » 22.3 ); however, on the basis of two distinct HBs red HB (RHB) out of 49 HB stars and a de®cit of stars with and two distinct RGB bumps, Sculptor's MDF may be better 2 ! 1 B V 0.3 as well as a red giant branch (RGB) well de- characterized as bimodal. This bimodality gives rise to one 52 scribed by a metal-poor population ([Fe/H] 1.9 ). Norris & population with a 2dP effect, and a second one with likely very Bessell (1978) reanalyzed the CMD in combination with two little HB 2dP effect. Differences in radial distributions for these 2 · spectra to argue for a Sculptor metallicity spread of 2.2 two populations can account for the variation in HB mor- · 2 [Fe/H] 1.5, and Smith & Dopita (1983) con®rmed an in- phology within Sculptor and among previous surveys of this homogeneous metallicity distribution function (MDF) via nar- galaxy. rowband photometry. Da Costa's (1984, hereafter D84) deep but small area photometry to the Sculptor main-sequence turn- 2. OBSERVATIONS AND REDUCTION off did not provide strong constraints on either the HB or RGB; however, it did show an abundance spread similar to previous We observed Sculptor on UT 1991 July 23 and August 1±2 results (con®rmed by Da Costa 1988) and a predominantly red with the Las Campanas 1 m Swope telescope using the thinned, HB (seven of 10 HB stars). The conclusion of these studies 10242 TEK2 CCD camera. Five overlapping, 109.4 wide point- was that Sculptor is a ªsecond-parameter (2dP) objectº that ings were arranged in a2 # 2 grid with a center frame over- shows a rather red global HB for its mean abundance (D84). lapping the other four to lock together the photometry. Each More recently, however, Schweitzer et al. (1995) produced ®eld was typically observed with one B and V exposure of a CMD with 1043 stars that reveals a prominent blue HB (BHB) 1800 and 900 s length, respectively. The data were reduced with more stars than KD77 and D84. In the ®rst wide-®eld, with the IRAF package CCDRED and photometered with the CCD survey of Sculptor, K95 reported the usual metallicity DAOPHOT II and ALLFRAME programs (Stetson 1987, spread based on the RGB (as did Schweitzer et al. 1995), but 1994). Detections were matched using DAOMASTER and then also substantiated the large BHB population and derived a more calibrated to observed Graham (1982) standard stars using our own code. This code compares calibrated magnitudes of stars 1 Visiting Research Associate, The Observatories of the Carnegie Institution in common on different CCD frames and determines minor of Washington, Pasadena, CA; David and Lucile Packard Foundation Fellow; frame-to-frame systematic errors (e.g., due to shuttering errors, Cottrell Scholar of the Research Corporation. transient transparency changes, errors in the photometric trans- L33 L34 SECOND PARAMETER IN SCULPTOR DWARF GALAXY Vol. 520 Fig. 2.ÐFits of model ZAHBs to the HB of Sculptor. (a) Oxygen-enhanced 5 1 Fig. 1.Ð(B2V, V) CMD for the Sculptor dwarf galaxy with overlaid ®- ([O/Fe] 0.5) models; (b) models with solar [O/Fe]. In each panel, the solid 0 5 2 5 ducials for M92 (dashed line; from Sandage 1970) and Pal 14 (solid line; from line shows the model with [Fe/H] 2.3, the dashed line shows [Fe/H] 2 5 2 Holland & Harris 1992). We adopt cluster distance moduli and reddenings 1.9, and the dotted line shows [Fe/H] 1.5. The ZAHBs are terminated from Harris (1996) and Holland & Harris (1992), respectively. The right panel at the red end at a mass of 0.85 M,, and a hash mark indicates a mass of highlights the RGB and red bump region. 0.80 M, (see text). formation equations). Because of photometric conditions, the 1.0 dex. This is consistent with reported [Fe/H] spreads from derived mean residuals for each frame (·0.1 mag on the basis ®tting isochrones to the Sculptor RGB. of ¸689 comparison stars) were used as offsets and applied The situation is, however, more complex because we are iteratively with new color determinations until convergence. comparing HB stars at different colors, and both the luminosity 5 Our resulting photometric precision is (jB, jV) (0.05, 0.05) of the theoretical zero-age HB (ZAHB) and bolometric cor- mag at the HB. rection vary with position along the HB. Moreover, the HB is strongly affected by the oxygen abundance. At a constant core 3. HORIZONTAL BRANCH mass (Mcore), increasing [O/Fe] increases LHB. However, in- creasing [O/Fe] also leads to a decrease in Mcore. All other things 2 2 Our (B V, V)0 (Fig. 1) and (B V, B)0 (not shown) CMDs being equal, a decrease in Mcore leads to a decrease in LHB. The show an HB that appears to be kinked over the RR Lyrae gap. net result is that the ZAHB variation with [O/Fe] can be rather All tests of the photometry pipeline have shown this ªkinkº to complex. In the Galaxy it is generally thought that for metal- be real, and a hint of this HB kink can be seen in KD77. licities appropriate for Sculptor, [O/Fe] is constant with a value Similarly kinked HBs have been noted previously in the CMDs in the range 10.3±0.5. There is no reason to assume that Sculp- of some ªbimodalº Galactic globular clusters (GGCs), e.g., tor has undergone the same chemical enrichment history as the NGC 6229 (Borissova et al. 1997), NGC 2808 (Ferraro et al. Galaxy, so we consider all 0.0 · [O/Fe] · 10.5 possible. Most 1990), and NGC 1851 (Walker 1992), to which the Sculptor recent HB models have an assumed [O/Fe], [Fe/H] relation. CMD bears some resemblance. Indeed, our derived B : V : R The only available models that allow us to explore the com- (blue : variable : red HB) ratio of (0.42 : 0.19 : 0.39) resembles position parameters independently are those of R. T. R. (un- 2 those of bimodal GGCs (see Borissova et al.