Publications of the Astronomical Society of the Pacific 107: 937-944, 1995 October C J0907—372 (Pyxis): a New Distant Galactic

Publications of the Astronomical Society of the Pacific 107: 937-944, 1995 October

C J0907—372 (Pyxis): A New Distant Galactic Globular Cluster G. S. Da Costa Mount Stromlo and Siding Spring Observatories, The Australian National University, Private Bag, Weston P.O., ACT 2611, Australia Electronic mail: [email protected] Received 1995 May 10; revised 1995 July 7

ABSTRACT. The prime-focus camera of the Anglo-Australian Telescope has been used to image the object listed by Weinberger (PASP, 107, 58, 1995) as a possible distant star cluster or dwarf spheroidal galaxy. The CCD data reveal this object to be a distant globular cluster of low central concentration. In particular, the color-magnitude diagram for the cluster core shows a sparse giant branch, a prominent red horizontal branch at /? = 18.75 and a main-sequence tumoff near the limit of the data at 7^22.0. A population of possible blue stragglers is also evident. Based on comparisons with the giant branches of standard globular clusters, the reddening to this cluster, designated C J0907—372 (Pyxis), is estimated to be 0.25^E(B — ^)^0.40. The cluster is then approximately 35 kpc from the Sun and 37 kpc from the galactic center. With a core radius of —14 pc, an estimated absolute magnitude Mv^—5.1 and a dominant red horizontal branch, C J0907—372 (Pyxis) has very similar properties to the other low-luminosity globular clusters of the outer galactic halo.

1. INTRODUCTION For these reasons object No. 3 in the list of Weinberger (1995) is of considerable interest. Weinberger's list is a het- Over many decades studies of the globular-cluster system erogeneous compilation of six "interesting" objects discov- of our Galaxy have provided an increasingly detailed picture ered during the course of a long-term systematic search of of the formation of the galactic halo, with perhaps the most the optical sky surveys. Object No. 3 in this list lies in a recent information being the realization that accretion of relatively crowded field at/=2610, ¿7 = H-70 and is described globular clusters from the disruption of independently evolv- (Weinberger 1995) as a large number of very faint stars ap- ing satellite systems, at both early and late times, is a process proximately 1'8 in diameter, barely visible on the SERC J that cannot be ignored. In such studies it is frequently the Sky Survey film copies. Weinberger (1995) goes on to sug- clusters at large galactocentric distances that are particularly gest that this object could be either a distant faint star cluster important, since these clusters carry the greatest weight in or a dwarf spheroidal galaxy in the Local Group. any discussion involving radial gradients. For example, it Investigation of the true nature of this object is the pur- was the lack of any distinct abundance gradient among the pose of the present article. The acquisition and reduction of halo globular clusters, coupled with a radial gradient in the AAT prime focus CCD images are described in the next sec- so-called second parameter effect, in which clusters with red- tion while in Sec. 3 the characteristics of this object, which der horizontal branches than expected for their metallicities turns out to be a distant galactic globular cluster, are inves- are found more frequently at larger galactocentric distances, tigated. The results are summarized in Sec. 4. that led Searle and Zinn (1978) to formulate their "merging fragments" model for the formation of the galactic halo. The sample of distant galactic globular clusters however, is few in number. There are six known clusters with galactocentric 2. PHOTOMETRY distances beyond 60 kpc, none with 40^/^=^60 kpc, and only two with 30=^/^^40 kpc. Thus the discovery of addi- tional distant globular clusters is important since it can fur- 2.1 Observations ther constrain the properties of, and hence formation models for, the outer galactic halo. The observations were made with the prime focus CCD Similarly, among the nine dwarf spheroidal (dSph) com- camera of the Anglo-Australian Telescope on 1995 March panions to our Galaxy, the three dSph companions to M31 27. The camera incorporates a thinned Tek 1024X1024 CCD and the single isolated Local Group dSph Tucana, there is giving a field of view of 6'6 square at a scale of 0'.'39 pixel-1. both an underlying similarity, expressed in for example, the Single 300-s {R) and 900-s {B) exposures were made at the relations between mean abundance, length scale, absolute position listed by Weinberger (1995) for his object No. 3. magnitude, and surface brightness which these galaxies de- Conditions were photometric throughout the night but unfor- fine (e.g., Caldwell et al. 1992; Da Costa 1994), and an un- tunately the seeing was generally poor; on the R frame the derlying dissimilarity, particularly as regards star-formation FWHM of the images is 2"1 while it is T.l on the Β frame. history (e.g.. Da Costa 1992a). The discovery of further Lo- Exposures were also made of three standard-star fields cho- cal Group dSph systems could only help clarify the driving sen from the lists of Landolt (1992). Each of these fields mechanisms behind these similarities and differences. contains a number of standards covering a range of colors.

essence this process consists of defining the point-spread function (PSF) for each frame from the profiles of bright relatively uncrowded stars and then using this PSF with an initial list of image centers, magnitudes, and sky values, as input to the multiple star-fitting algorithm ALLSTAR. On both frames the PSF was taken to be spatially invariant and a gaussian was used for the PSF model function. The initial list of objects, their magnitudes (within an aperture equal to the PSF-fitting radius) and sky values came from the DAOFIND and PHOT algorithms. After the first ALLSTAR pass, DAOFIND was again run but this time on the frame from which the ALLSTAR fits had been subtracted in order to detect stars that had been missed in the first iteration. Aperture photometry was then performed on this list of new objects and the resulting file merged with the original photometry list to pro- duce a new input file to allstar. Inspection of the subtracted frames after the second allstar pass indicated that a third pass was not warranted. At this point the photometry consists of lists of magnitudes which measure for each star the amount of light above sky inside the fitting radius. Such magnitudes however, do Fig. 1—The /f-band CCD image of the field of C J0907—372 (Pyxis) ob- not contain all the stars' light and a "PSF magnitude to total tained at the prime focus of the AAT in 2'.'1 seeing. The 1018X1024 pixel magnitude" correction must be determined, via large- frame has North at the top and East at the left. The pixel scale is aperture measures on bright stars. To fix these corrections, 0'.'39 pixel-1 and the origin of the x,y coordinate system (cf. Table 1) is the upper left comer with χ increasing to the South and y increasing to the West. for each filter a frame was generated in which all but the brightest 20 or so stars with allstar magnitudes were subtracted from the original image. Aperture photometry, The raw data frames were overscan subtracted, trimmed, through a number of apertures of sufficiently large size to and flat-fielded using standard iraf1 ccdproc routines. For allow the definition of a "growth curve" and a "total mag- the R-band frames, a "dome flat," made from a number of nitude" (see, e.g., Da Costa 1992b), was then carried out for images of an illuminated section of the telescope windscreen, the least crowded subset of these bright stars. Care was taken was used to flatten the pictures, while for the Β band frames, to ensure that the wings of the numerous bright saturated a "twilight sky flat," made from a number of exposures of stars on the frames (which are not in the allstar lists and the dawn twilight sky, was employed. For both filters the are therefore not subtracted out) did not affect the large- final processed frames are flat to <1%. The .R-band image of aperture measures. The mean difference between the PSF the field of Weinberger (1995) object No. 3 is shown in Fig. magnitudes and the "total" magnitudes was then computed 1. The clustering of faint stellar images towards the center of for each frame. For both and R these corrections were well the frame, which led to the recognition of this object by Β Weinberger, is evident despite the significant contamination defined: for R the standard deviation of the (PSF-total) dif- by foreground stars. As it will turn out, most of these faint ferences was 0.011 mag for ten stars, for Β the corresponding stars are in fact in the vicinity of the main-sequence tumoff value was 0.006 for a sample of 13 stars. These corrections of a distant globular cluster. Following convention for single were then applied to the photometry lists. globular-cluster discoveries (e.g., C 0422-213 (Eridanus), In the next step the photometry lists were edited to re- Cesarsky et al. 1977) as distinct from lists of new globular move stars with significantly worse errors than the majority clusters such as the Palomar clusters (Abell 1955), the Ter- of stars at similar magnitudes. This was achieved by plotting zan (Terzan 1971) clusters, or the Arp-Madore (Madore and the magnitude errors and the CHI values, from the allstar Arp 1979, 1982) objects, this new globular cluster will be photometry lists, against magnitude for the Β and R frames. referred to as C J0907—372 (Pyxis), or the Pyxis globular Since the degree of crowding differs between the central cluster, Pyxis being the constellation in which it lies. parts of the frames and the outer regions, this editing process was done separately for the inner region [somewhat arbi- trarily chosen to be those stars within 270 pixels (105" or 2.2 Photometry — 1.3 rcore, see below) of the adopted center at χ=560, Photometry of the stellar images on the frames was car- j =590] and the outer region [stars more than 360 pix (140") ried out with the DAOPHOT π code (Stetson 1994 and refer- from the adopted center]. The number of stars removed in ences therein) as implemented within iraf Version 2.10.3. In this process was relatively small, —10% for the inner region and less for the outer region. The final step was then to compare the edited photometry lists for the two frames: stars ^raf is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, whose centers on the Β and R frames agree (after removing Inc. (AURA) under cooperative agreement with the National Science Foun- a small mean offset) to within a specified tolerance, in this dation. case approximately 15% of the FWHM on the Β frame, were

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System THE PYXIS GLOBULAR CLUSTER 939 retained. These matched and edited photometry lists then form the basis of the subsequent analysis. CNI 2.3 Standardization To determine the transformations from the instrumental magnitudes to the standard Β (Johnson), R (Cousins) system, O aperture photometry was performed on the standard-star < frames. In performing this task, since the seeing on these (/)er frames was often worse than for the Pyxis cluster data, care (0 was taken to measure the standard-star images through a σ number of apertures of sufficient size (up to 19" in diameter s 2 in the worst cases!) to again ensure that the growth curves en and total magnitudes were always well defined. The obser- —io ΙΟ vations of the standard-star fields did not cover a sufficient d range of airmass to permit a reliable determination of the extinction coefficients so mean values, determined from pre- vious runs with this instrument, were adopted. These coeffi- 1 o cients are kR=0.l5 mag airmass" and ^=0.30—0.015 (B -1 — R) mag airmass . Application of least-squares fitting to Log R (arcmin) the differences between the standard and extinction corrected (total) instrumental magnitudes then yields the following re- Fig. 2—The surface-density profile for the Pyxis globular cluster based on lations: the /?-band CCD frame. The logarithm of the number of cluster stars per sq. arcmin is plotted against the logarithm of the distance in arcmin from the /?=r+0.020(5—/^)+26.770 cluster center. Shown also is the surface-density profile of a single-mass King model with central concentration index c=0.65. The model fitted core Β0.070(5—/^)+26.521. radius is 1'38. The short dotted line represents the background density of Here Β and R are magnitudes on the standard system while b field objects on this frame. and r are the extinction corrected instrumental magnitudes in which the corresponding intensities are expressed in elec- bers of stars per square arcmin in successive 30 pixel (—12") trons per second above the sky background. The residuals width concentric annuli were determined and plotted against about these fitted relations are 0.010 mag for R (10 standards radial distance from the adopted cluster center at jc=560, with -0.19^5-5^1.71) and 0.009 mag for 5 (14 stan- y =590. This adopted center was determined by eye, the sig- dards with -0.29^5-5^1.75). Further, although the air- nificant level of field-star contamination and the resolved mass coverage is limited, there is no evidence for any corre- nature of the cluster making the application of more sophis- lation between the residuals from the fits and the airmass of ticated techniques inappropriate. This position is uncertain the standard-star observations. The Pyxis globular-cluster ob- by perhaps ±25 pixels (—10") but this uncertainty does not servations were made at lower aimasses (Ζ^1.01) than the materially affect the derived density profile. A check on the standard stars ((^)^1.21). adopted center is provided by the median χ and y coordi- Application of these transformation equations and the nates of the 17 candidate red-horizontal-branch stars (see adopted extinction coefficients then gives magnitudes and next section) that are found in the inner region photometry colors for the C J0907—372 (Pyxis) data on the standard dataset. These median χ and y values agreed with the system. It is worth noting that although formally the zero- adopted cluster center to within 30 pixels in both coordi- point uncertainties in the photometric transformations and nates. the uncertainties in the PSF magnitude to total aperture mag- The plot of stellar density versus radius reaches a constant nitude corrections are all less than 0.01 mag, because of the background value of 32.0 ±1.4 stars sq. arcmin-1 at radial poor seeing it is likely that the actual zeropoint uncertainty in distance beyond 3' from the cluster center. This background the C 10901-372 (Pyxis) photometry may be as large as stellar density was then subtracted from the raw densities and 0.03 mag. the logarithm of the resulting cluster star densities is shown as a function of the logarithm of the distance from the 3. RESULTS adopted cluster center in Fig. 2. The error bars on the figure represent the combination of the Poissonian uncertainty cal- 3.1 Surface-Density Profile culated from the number of stars in each annuli and the un- Although the image depicted in Fig. 1 clearly shows that certainty in the background density. Shown also on this fig- the Pyxis globular cluster is apparently of the order of 2 ure is the surface density profile of a single-mass King model arcmin in diameter, it is possible to use the quantitative in- (King 1966) with central concentration index c=log rtidaI/ formation available from the CCD frames to determine the rCore=0.65. This model profile is an adequate representation surface-density profile of the cluster. To carry out this task, of the cluster data and indicates a core radius for the Pyxis the ALLSTAR photometry of the 5 frame, which has the better globular cluster of approximately 83". Note that for this (and seeing and deeper limiting magnitude, was used. The num- other low central concentration King models), the fitted core

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 940 DA COSTA radius exceeds the radius where the surface density reaches half its central value. Anticipating the results of the next C J0908-373 (Pyxis) - Core Sample section in which the distance of C J0907—372 (Pyxis) is CD shown to be approximately 35 kpc, this core radius then corresponds to a linear distance of —14 pc. While comparatively large for a galactic globular cluster (see following paragraph), this value of the core radius im- OD mediately establishes that the Pyxis stellar system is indeed a globular cluster, and not a dwarf spheroidal galaxy. Dwarf spheroidal galaxies, which have core radii of order 200 pc or cu more, are significantly more diffuse than even the lowest- density globular clusters. This can be seen in, for example, CMo the lower right panel in Fig. 3 of Kormendy (1985), where it is evident that while some dSph galaxies have absolute magnitudes comparable to globular clusters, their core radii are an order of magnitude larger. The Pyxis system on the other CN(Ν hand, which has MB^-5 (see Sec. 3.3) and log [rcore(kpc)]—-1.85, falls among the low-luminosity, 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 low-density globular clusters in this diagram, which are well ' I I I I C J0908-373 (Pyxis) - Field Sample separated from the dE/dSph sequence. CD With its central concentration index c=0.65 and core radius pc, the Pyxis globular cluster is one of the most diffuse (i.e., low central concentration and large core 0<> r<3 radius) globular clusters known. For example, Trager et al. 00 (1993) and Trager et al. (1995) list only a few clusters (e.g., 0 0 0 o 2 8ft % Pal 5, 0=0.74; Pal 14, c=0.72, and NGC 6496, c=0.70) O O O00¾¾ o& O ς, O with central concentration indices as low as that found here 0o of coo ° for the Pyxis globular cluster. Similarly, Djorgovski (1993) O lists only five clusters (for data quality flags 1 or 2) that have CM Λ00 o8>cVo0A. core radii exceeding 10 pc. These are Pal 3 (r = 13 pc). Pal 00¾ core 0 0 , « o 4 (16 pc), NGC 5053 (11 pc). Pal 5 (19 pc) and Pal 14 (23 o 8 S pc). Three of these clusters (Pal 3, 4, and 14) are among the CM most distant from the galactic center and the other two are CN also at significant galactocentric distances (Rgc—17 kpc). In- deed as emphasized by van den Bergh (1994 and references therein), there is a general correlation between globular- 0.5 1 1.5 2 2.5 3 cluster size and galactocentric distance with the large-core, B-R low central concentration clusters lying predominantly at larger galactocentric distances. Thus the low central concen- Fig. 3—The upper panel shows the color-magnitude diagram for 296 stars tration and large core radius inferred here for the Pyxis within 1 core radius of the center of the Pyxis globular cluster. The lower globular cluster are consistent with its location at a relatively panel shows the color-magnitude diagram for 261 stars in the background large galactocentric distance, as derived below. Presumably field region. The background field region covers an area 2.4 times larger (see, e.g., van den Bergh 1994), the Pyxis globular cluster than the core field. has managed to survive despite its diffuse nature by having a low eccentricity orbit; it would not have remained intact if it 2.4 times larger than that of the core region. Comparing these were on an orbit that took it close to the galactic center. C-M diagrams, keeping the relative area scaling in mind, reveals the following. Most evident in the upper panel is the 3.2 Color-Magnitude Diagram large number of faint relatively blue stars at R~22.0. These stars are most simply interpreted as stars at or near the main- In the upper panel of Fig. 3, a color-magnitude (C-M) sequence tumoff in the Pyxis globular cluster. There are also diagram is plotted for the 296 stars in the matched, edited, approximately a dozen stars with B — R^ 1.0 and and standardized photometry lists that fall within the core 20.5^/^21.5 in the upper panel of Fig. 3 that have no region of the Pyxis globular cluster. The χ (increasing to the counterpart in the background field C-M diagram. These south) and y (increasing to the west) coordinates as well as stars are most probably blue straggler members of the Pyxis R, B — R photometry are given in Table 1 for the stars in this globular cluster. Also evident in the upper panel is the cluster panel that are brighter than R ^20.0. Similarly, the lower subgiant and giant branch rising from /?~21.5, B-R —1.6 panel of Fig. 3 shows the C-M diagram for the 261 stars in through /?~18.5, B — R~IJ and perhaps extending to the the background field region [i.e., stars with r^480 pixels two bright red stars at /?~17, B-R—2.0 and i?~16, (3'1)]. This background region covers an area approximately B — R—23. Spectroscopic observations of these two stars

Table 1 Photometry for Pyxis Globular-Cluster Core Region Stars with R =5¾ 20.0

Id R B-R Id R B-R

1813 550.44 442.63 16.033 2.339 755 647.78 489.77 18.728 1.731 2097 554.22 703.02 16.411 1.480 762 671.89 497.14 18.734 1.410 1655 523.02 720.80 16.468 1.278 915 396.05 701.03 18.736 1.322 846 695.60 605.09 16.613 1.433 1299 484.86 415.85 18.738 1.473 724 549.63 454.86 16.679 1.245 906 601.26 688.77 18.776 1.404 162 600.01 390.72 16.689 1.189 174 526.55 407.43 18.781 1.409 258 599.96 588.26 16.691 1.175 241 560.72 562.87 18.783 1.781 367 553.17 777.56 16.786 1.138 357 463.90 753.12 18.783 1.427 256 356.32 586.04 16.792 1.329 689 498.67 414.09 18.797 1.396 224 679.62 512.07 16.820 1.107 887 384.65 650.95 18.808 1.466 289 713.84 635.63 16.880 1.378 244 452.69 566.95 18.874 1.464 187 619.97 438.58 17.120 2.051 816 642.79 570.10 18.908 1.695 220 577.09 505.80 17.204 1.296 1454 687.34 712.71 18.939 1.292 178 585.18 412.71 17.234 1.098 292 656.57 639.51 19.028 2.324 232 648.73 534.69 17.401 1.311 831 627.71 587.45 19.055 1.283 1450 423.82 702.28 17.449 1.118 743 630.17 474.42 19.168 1.611 923 600.24 708.96 17.476 1.225 251 695.28 580.69 19.228 1.648 263 665.77 596.14 17.486 1.342 193 568.68 448.85 19.263 1.675 311 488.60 660.86 17.549 1.766 1464 675.37 753.97 19.298 1.419 838 635.50 599.56 17.647 1.329 217 708.76 499.65 19.345 1.684 233 755.42 534.93 17.772 1.850 1173 492.27 721.65 19.360 2.059 288 558.14 634.83 17.816 1.466 281 368.20 626.36 19.370 1.556 260 464.69 592.93 17.898 1.223 334 717.93 697.09 19.412 1.648 868 619.67 633.29 17.949 1.387 1426 684.88 637.43 19.414 1.659 1170 491.30 701.43 18.016 2.117 859 393.21 623.70 19.439 1.633 275 652.80 619.62 18.082 1.730 1569 498.94 720.55 19.469 1.614 889 585.71 656.23 18.182 1.707 804 715.76 554.71 19.507 1.639 286 461.20 632.15 18.237 1.665 806 635.40 559.93 19.546 1.805 242 501.02 565.08 18.302 1.476 1356 706.76 539.70 19.585 1.561 307 647.53 657.08 18.313 1.779 1382 679.94 572.41 19.588 1.621

264 535.83 603.83 18.334 1.849 719 638.87 444.00 19.596 1.280 1301 461.43 419.33 18.339 1.433 1360 566.84 548.00 19.597 1.845 1150 493.46 455.52 18.383 1.930 900 650.86 680.01 19.607 1.701 188 696.44 438.72 18.391 2.434 271 492.70 611.09 19.624 2.207 803 585.70 554.57 18.391 2.387 731 431.54 461.08 19.638 2.296 664 549.40 378.48 18.396 1.642 1432 674.68 662.41 19.654 1.575 1555 551.42 554.81 18.518 1.827 309 691.15 659.21 19.691 1.597 277 444.42 620.91 18.525 1.338 321 548.17 675.03 19.718 1.686 1169 484.47 699.15 18.561 1.100 1595 618.20 525.19 19.728 2.649 253 474.60 580.94 18.579 1.807 213 687.20 493.18 19.731 1.579

295 376.11 642.00 18.589 1.713 273 643.35 612.42 19.806 1.673 318 399.99 673.60 18.599 1.318 210 616.94 478.39 19.882 1.800 227 662.94 516.12 18.645 1.702 1357 635.03 540.40 19.894 1.600 279 680.95 626.26 18.647 1.450 721 500.63 448.56 19.947 1.976 1311 531.70 441.02 18.647 1.491 1424 373.73 631.66 19.957 1.372 1177 534.34 749.22 18.667 1.155 948 677.00 746.32 20.079 1.602 303 393.27 649.07 18.707 2.509 896 663.49 664.89 20.096 2.232 240 392.52 561.60 18.722 1.520 368 526.18 781.36 20.148 1.605

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 942 DA COSTA could prove fruitful. Finally, also apparent in Fig. 3, is a branches, the value of Vhb and the median {B — V) color of distinct clumping of stars at /? = 18.75±0.05 and 1.3 the horizontal branch, the latter read from the C-M diagrams ^B-R^l.55. These stars undoubtedly outline the red- of Lee (1977) and Harris (1982), respectively, were used horizontal branch (RHB) of the Pyxis globular cluster. Based together with a (B — V, B-R) relation for globular-cluster on the numbers of such stars in the field region and the giants established from the photometry of Green (1987), to relative areas, approximately a dozen of the candidate RHB fix Ruq , and thus MÄ(HB) for each cluster. The median stars are likely to be cluster members. There is however, no (B — R)0 color of the red-horizontal branch is also determined indication of any blue-horizontal-branch star component so in this process. For NGC 6397, which has no red-horizontal- that the horizontal-branch morphology index (B — R)/ branch stars, the value was assumed to apply for (B + V+R) for the Pyxis globular cluster is —1.0, as it is (B-y)0=0.20 which corresponds to {B — R)0=035. The for the majority of the outer galactic halo globular clusters corresponding /?HB then follows and use of the distance (e.g., Lee et al. 1994). modulus then yields MÄ(HB) for this cluster. The three sets Despite the fact that because of poor seeing, the photom- of [Μ/?, {B — R)0] data were then individually fit to the Pyxis etry presented here is,not very precise at the limit of the data photometry from the upper panel of Fig. 3 as follows. For 47 in the vicinity of R ^22.0, it is nevertheless evident that the Tue and NGC 362, the standard cluster red-horizontal-branch main-sequence turnoff in the Pyxis globular cluster is at least magnitude was first matched to that of the Pyxis data, and as faint as R =22.0. With R = 18.75 for the horizontal branch, then the standard cluster data were moved horizontally to this then indicates that Δ/?Το.ηβ exceeds 3.25 for the Pyxis best coincide with the Pyxis giant-branch stars. For NGC globular cluster. For comparison, the data of Green and Nor- 6397, the same procedure was followed except that the MR ds (1990) indicate Δ/?Χ0.ΗΒ=3.5 for the galactic globular of the NGC 6397 horizontal branch, which applies at a color cluster NGC 362, while Stryker et al. (1985) give approximately that of the blue edge of the instability strip, = Δ/?το-ηβ 3·1 — 0.2 for the SMC cluster NGC 121. This latter was matched to the Pyxis data under the assumption that it is cluster, as the ARto-ub value indicates, is younger than the 0.1 mag fainter than the Pyxis red-horizontal-branch stars galactic halo globular clusters (Stryker et al. 1985; Da Costa (see, e.g., Lee et al. 1987). The difference in color between 1993). Thus, while better data that reaches at least a magni- the Pyxis data and the fitted standard cluster giant branch tude fainter are required for a definitive answer, there is no then gives an estimate of the reddening while the difference reason to suspect from the current data that the Pyxis globu- between the R magnitude of the Pyxis horizontal-branch lar cluster is very different in age from the majority of halo stars (18.75) and the MÄ(HB) value of the standard cluster globular clusters. yields an estimate of the apparent R distance modulus. For 47 Tue, the inferred reddening for the Pyxis globular cluster is ( — )^0 , significantly lower than the minimum 3.3 Reddening, Distance Modulus, and Absolute Ε Β ν Λ value suggested from the Burstein and Heiles (1982) map. It Magnitude is therefore unlikely that the Pyxis globular cluster has an With a location at /=261!3 and Z? = +7?0, C J0907-372 abundance comparable to that of 47 Tue. The fits for the (Pyxis) does not lie on the reddening maps of Burstein and other two clusters are shown in the panels of Fig. 4. The Heiles (1982) which are limited to |¿?|^10o. However, the upper panel is the fit for NGC 362 which implies E(B — V) maps indicate that the reddening at the location of the Pyxis ^0.29 while the lower panel shows the NGC 6397 fit which globular cluster is at least is (ß —10=0.24. An improved es- implies E(B —1^)^035. The corresponding apparent moduli timate can be obtained however, by fitting standard globular- are (m — M)Ä = 18.48 and 18.50, respectively. There is little cluster giant branches to the data in the upper panel of Fig. 3. difference in the quality of these fits so that as a result there There is nevertheless a minor complication in that the pho- is no indication whether an abundance near that of NGC 362 tometry here is {R,B — R) rather than the more common ([Fe/H]= -1.28) or near that of NGC 6397 ([Fe/H]= -1.91), {V,B — V) or (/,y—/). This restriction limits the number of is more appropriate for the Pyxis globular cluster. Thus a standard clusters available for use as comparison objects to determination of the metal abundance of this cluster must NGC 104 (47 Tue) and NGC 6397, where {R,B-R) pho- await a spectroscopic study of its member stars. tometry of giant-branch stars is available from Green (1987) As a result of this process, the reddening of the and NGC 362, where {R,B-R) data are available from Pyxis globular cluster is limited to lie in the range 0.25 Green and Norris (1990). In the following the relations E(B ^£(5-V) ^0.40, while the apparent moduli imply — #) = 1.75 E(B-V) and AR=2A5 E{B-V), equivalent to — Π for the cluster, corresponding to a distance of Av=3.2 £(5-V), are assumed. approximately 35 kpc from the Sun. The galactocentric dis- For each of the three standard clusters the same basic tance is then —37 kpc and the cluster lies approximately 4.3 procedure was followed. First, the observed {R,B — R) giant kpc above the galactic plane. The Pyxis globular cluster is branch was corrected for reddening assuming E(B — V) thus comparable in galactocentric distance to the clusters Pal =0.04, 0.18, and 0.06, respectively, for the three clusters. 15 (/?gc~35 kpc) and NGC 7006 (flgc~36 kpc). With the Application of the distance moduli listed in Da Costa and exception of the six extreme outer halo globular clusters Armandroff (1990) then produces [Mr,(B — R)0] values. To (Pal 14, Eridanus, Pal 3, Pal 4, NGC 2419, and AM-1) which fit the Pyxis globular-cluster data though, a value of MÄ(HB) all lie at least 60 kpc from the galactic center, these three for the standard clusters at their adopted moduli is required. clusters are the most distant from the galactic center. For 47 Tue and NGC 362, which have red horizontal One final point can be addressed and that is the question

branches to the Pyxis globular cluster, the relative number of red-horizontal branch stars can be used as a scaling factor to CD estimate the total integrated magnitude. In the inner region of the Pyxis globular cluster (r^1.27 rcore) there are 17 stars with 18.6^^18.9 and 1.37^5-/^1.53. Based on the same limits and the relative areas, comparison with the back- OD ground region C-M diagram suggests that 13 or 14 of these 17 RHB stars are probably cluster members. In the Pal 4 C-M diagram of Christian and Heasley (1986) there are 27 er RHB stars; these same authors give Mv=-6.05 for this cluster. Scaling by the ratio of the RHB numbers and includ- o CNJ ing a contribution, based on the fitted c=0.65 King Model, for the light outside r= 1.27 rcore, the estimated absolute magnitude of the Pyxis globular cluster is My5.7. Using the RHB numbers in the C-M diagrams of Eridanus (Mv=—4.85, Da Costa 1985) and Pal 14 (My=-4.8, Da Costa et al. 1982) gives similar answers; —5.9 and —5.5, respectively. It then seems that Μγ^—5.Ί is a reasonable estimate for the absolute magnitude of C J0907—372 CD (Pyxis). The caution voiced by Da Costa (1985) however, should be kept in mind: when dealing with the absolute magnitudes of sparse globular clusters it is important to remem- ber that stochastic fluctuations in the number and luminosity 00 of the brighter giants can cause significant uncertainty in the Mv value; this effect can be as much as 0.3 or 0.4 mag for a My^—S.O cluster. With this value of M y 5.7, the Pyxis cd globular cluster is again very similar to the other outer halo globular clusters. For example, Djorgovski (1993) lists o (Ν Mv=—5.1, -4.7, and -6.0 for Pal 15, AM-1 and Pal 3, respectively.

4. SUMMARY 0.5 Ί 1.5 2 2.5 3 The object listed by Weinberger (1995) as a candidate B-R distant star cluster or a dwarf spheroidal galaxy has been shown via CCD imaging to be a moderately reddened globu- Fig. A—The upper panel shows the NGC 362 fiducial sequence from Green and Norris (1990) overlaid on the Pyxis globular-cluster core region data lar cluster at a distance of —35 kpc from the Sun and ~37 assuming E(B-R)=0.50 and (m-M)Ä = 18.48. In this case the NGC 362 kpc from the galactic center. The properties of this cluster, to red-horizontal branch coincides with the Pyxis red-horizontal branch in be known as C J0907—372 (Pyxis), are in general very simi- color as well as magnitude. The lower panel shows the fit of the lar to those of the majority of the globular clusters found in NGC 6397 fiducial giant branch from the photometry of Green (1987) to the Pyxis globular-cluster core region data using £(#-/?) =0.62 and the outskirts of the Galaxy. In particular, the Pyxis globular (m — M)Ä=18.50. The short horizontal line indicates where horizontal- cluster has a red-horizontal-branch morphology, a large core branch stars with {B-V)0^0.20 would fall. radius, a low central concentration, and a relatively faint absolute magnitude. A spectroscopic determination of the abundance of this globular cluster and a measurement of its radial of the absolute magnitude of the Pyxis globular cluster. For velocity will then allow Pyxis to be included in kinematic low-luminosity resolved globular clusters integrated absolute and abundance analyses of the galactic halo globular-cluster magnitudes are generally estimated by simply adding up the systems. For example, with its red-horizontal-branch mor- luminosities of the member stars down to a magnitude where phology, the Pyxis globular cluster almost certainly belongs incompleteness begins to set in and then including a correc- to the "younger halo" sample of globular clusters (Zinn tion, based on other better determined globular-cluster lumi- 1993; Da Costa and Armandroff 1995) for which at present nosity functions, to allow for the contributions of the fainter there are only approximately 20 members. stars. However, the large degree of field contamination makes this a difficult approach to take for the Pyxis globular The author is grateful to Dr. Magda Amaboldi and Pro- cluster. Instead, since globular clusters such as Pal 4 (Chris- fessor Ken Freeman for taking time from their scheduled tian and Heasley 1986), Eridanus (Da Costa 1985), and AAT observations to obtain the CCD frames on which this Pal 14 (Da Costa et al. 1982) have similar red-horizontal article is based.

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