Observational Data for Galactic Globular Clusters Brian A. Skiff Lowell Observatory 1400 West Mars Hill Road Flagstaff AZ 86001-4499 Internet: bas@lowell.edu Abstract An up-to-date tabular summary of total magnitudes, brightest stars, and journal references is given for all 142 confirmed Galactic globular clusters. The text discusses how the information can be used to plan and analyze visual observations. Introduction What you can observe depends a lot on what you know about. It is easy, for instance, to overlook a bright non-NGC galaxy in the same field as an NGC object merely because it is not plotted on the atlas you're using. To help improve this situation, over the years I have collected relevant data about all types of objects from the professional literature for use in analyzing my visual observations. Much of this material was condensed without attribution into Chris Luginbuhl's and my {\it Observing Handbook and Catalogue of Deep-sky Objects}. But a lot of data has come out since 1984, when work for the book was finished. Many of the most useful articles were mentioned in the `Scanning the Literature' column that appeared in the back pages of each issue of late, lamented {\it Deep Sky} magazine. I continue to get inquiries from amateurs about specific objects, and it is usually possible to supply a citation to a relevant journal article dealing with practically any fairly bright object. This task has become much easier in the last several years thanks to the availability of on-line databases, such as the NASA Extragalactic Database (NED) and SIMBAD, maintained by the Centre de Donne\'es astronomiques in Strasbourg, France. One file I've built up that has come in particularly handy is a list of `observables' for globular clusters. The list of globulars is taken from the compilation by Djorgovski & Meylan (ref. 1), and lists all confirmed clusters deemed to belong to the Galaxy and classified as globulars. Comments about objects omitted and undetermined cases are dicussed below. Included in the file are total V magnitudes according to the 1992 Peterson compilation (ref. 2). In addition, I went through the literature and found essentially every photometric study ever published of a globular cluster (hundreds of papers!). From these publications, I determined the V magnitude and B-V colour of the brightest stars in each cluster, and the magnitude of the `horizontal branch'. For each cluster I chose the best paper (sometimes more than one) dealing with the brighter stars. That is, I have listed in the bibliography the paper with the most useful finder chart for the field of each cluster, the one with magnitudes for bright field stars around the cluster---the best one for checking one's visual and photographic observations. This is not always the highest-quality or most recent study, just the one best for amateur purposes. This material is collected in Table 1. Colour-Magnitude Diagrams The purpose in making photometric measurements of globular cluster stars is to create a `colour-magnitude diagram'. This is merely the observational version of the well-known Herzsprung-Russell diagram. In the original HR diagram, the luminosity of a star (or group of stars) is plotted against the temperature. We cannot observe these two things directly, but we can measure the apparent brightness of a star and its `colour', that is, the difference in its brightness at two wavelengths. A hot star is brighter toward the blue end of the spectrum than toward the red end, and conversely for cool stars. In the commonly used UBV system, the temperature of a star can be estimated by the magnitude difference in the B (blue) and in the V (visual) band. The B-V ("B minus V") colour for a fairly hot star like Vega is close to zero in this system and is about 1.5 for ordinary cool giants such as Aldebaran. Some cool supergiant stars and carbon stars have much larger B-Vs, and appear quite red visually. The hottest ordinary stars, such as the stars in the Belt of Orion, have B-V about -0.2. Interstellar dust causes stars to appear redder than they would than otherwise, and so more distant stars are often `reddened' by significant amounts. Well, what happens when B and V magnitudes are measured for stars in a globular cluster? The figure shows an excellent example from a paper by Gonzalo Alcaino and William Liller (ref. 3). Notice first that the stars do not scatter everywhere in the plot of V versus B-V, but instead clump into narrowly-defined strands, called `sequences' or `branches'. The very brightest stars in any globular cluster are stars at the tip of the red-giant branch toward the upper right corner of the plot. These are giants and bright-giants of spectral class K and M. They have evolved from stars somewhat more massive than the Sun. Following the red-giant branch down, the more nearly vertical string of stars is called the subgiant branch, i.e., stars intermediate in brightness between main-sequence stars (like the Sun) and the brighter, cooler giants. Midway down this string, stars split off to the left (blueward) side of the diagram. These stars are called `horizontal- branch' stars, and are even more evolved than the red giants, i.e., stars that are heading toward extinction. In the middle of the horizontal branch is a band of variables, the RR Lyraes, indicated by small x's in this plot. These are A- and F-type stars (somewhat hotter than the Sun) that pulsate with amplitudes of around a magnitude with periods of about half a day. At the base of the red-giant/subgiant strip, the stars take a sharp turn redward as they get fainter. This is point where the most massive stars on the main-sequence are starting to evolve into giants. The stars at the turnoff in globular clusters are close to the Sun's temperature and mass. From here on down as far as anyone has been able to observe in any cluster, there are only unevolved main-sequence stars, becoming more and more numerous toward the faint end. The faintest stars measured in globulars so far are about absolute V magnitude 12, or eight magnitudes fainter than the Sun, corresponding to stars of only 0.2 of a solar mass. The most luminous stars at the tip of the red-giant branch, on the other hand, have absolute V magnitudes near -2.5, roughly half a million times brighter. Interpreting colour-magnitude diagrams for clusters in terms of a star's evolutionary history remains a central problem for astrophysics even 75 years after the first diagrams were constructed and their significance shown. For further reading on this subject, see for instance refs. 4 & 5. But these details need not concern us for the immediate purpose visual observation. What Are The Data Good For? In visual observation of a globular cluster, obviously if you are going to see any resolution at all, your telescope will have to show stars at least as faint as the brightest stars in the cluster. Thus it is of interest to know what the magnitudes of the stars in the cluster are in order to predict whether a cluster will be resolved, or perhaps to get an estimate of how faint you can see based on whether a cluster is resolved or not. So the table includes the magnitudes and colours for the very brightest stars in the cluster [V(tip) and B-V(tip)]. I note the B-V colour because the eye will see red stars fainter than their V magnitudes would indicate, since the peak dark-adapted visual response is shifted toward the blue from the standard V passband. The shift amounts to roughly: m(vis) = V + 0.2(B-V). For example, a star with B-V = 1.5 will appear 0.3 magnitudes fainter visually than a star of B-V = 0.0 that has an identical V magnitude. As you can see by looking down the list, stars at the tip of the giant branch in most globulars have B-V between 1.5 and 2.0. So although these stars will appear slightly fainter than blue stars of the same V magnitude, comparisons among different globulars can be made without taking this effect into account. In moderate-to-large apertures, most globulars are at least partially resolved, showing some dozens of stars. In the brightest objects, one often sees seemingly thousands of stars, and in a few such as 47 Tucanae and omega Centauri, it really is thousands. The cluster is `well resolved'. The point at which this happens is when your telescope has a limiting magnitude at or below the level of the horizontal branch. The reason is simply that the number of stars in any given magnitude interval takes a sudden leap at the magnitude of the horizontal branch. What about the Shapley-Sawyer concentration classes? These Roman numeral classes are often considered indicative of how easy a cluster is to resolve. The classes are actually correlated with central surface brightness, not the magnitudes of the stars, and my experience indicates they have little bearing on cluster resolution. In general, the highly-concentrated clusters are the most distant ones, and so are difficult to resolve simply because the stars are faint, not because they are close together. I have seen the brightest handful of stars in M15 and M2 using a 70mm refractor at 75x and 110x. They are both quite strongly concentrated objects, yet their brightest stars, between mag. 12.5 and 13.0, are just visible in this small aperture from a true-dark site. It may be that clusters with the same V(tip) but having different concentration classes will show a difference in resolution. Perhaps a reader can provide the necessary observations. Cluster Rankings Along with the main list, the supplementary lists (Tables 2,3,4) show how the clusters rank in terms of the three parameters: total magnitude, magnitude of the brightest stars, and by magnitude of the horizontal branch. The results are occasionally surprising compared to conventional wisdom. The first list shows the twenty-five clusters with total V of 7.0 or brighter. The two southern clusters omega Centauri and 47 Tucanae are roughly a magnitude-and-a-half brighter than the nearest competition. M5 and M13 rank seventh and eighth; they are nice clusters, but by comparison omega Cen and 47 Tuc will knock your socks off! A common debate among southern observers is which of the two is better. Some prefer the sheer richness of omega Cen, and others the star-density and remarkable (indeed unique) structure of 47 Tuc. Essentially all of the non-Messier clusters in the list are far-southern objects that certainly would have been catalogued by him had he worked at southern latitudes. NGC 3201 and NGC 6541 are accesible, however, to those at mid-northern latitudes. The other two lists give the top ten clusters in terms of brightest stars and brightest horizontal branches---the most easily resolved clusters. Note that neither omega Centauri nor 47 Tucanae appears at the top of these lists. Both are luminous objects with large numbers of stars, as anyone who has observed them can confirm, and their combined light sends them to the top of the total magnitude list. But because of their distance, the brightest stars are not as bright as nearer clusters. Instead the winner is NGC 6397, a far-southern object that is known as the closest or second-closest globular (it and Messier 4 are both about two kiloparsecs distant). This object and the next few in the ranking are partially resolved in ordinary handheld binoculars. For northern observers, the most easily resolved clusters are M22, M4, and M55, all of which are nevertheless south of -20 Dec. The brightest stars in M13 are nearly two magnitudes fainter than those in NGC 6397. On the horizontal-branch list, NGC 6397 also rates as #1, again with stars two magnitudes brighter than those in M13, which falls off the list altogether. Perhaps the most immediate lesson to be drawn from this is that to see the galactic globular system well at all, you need to head to the southern hemisphere! The Database The objects contained in Table 1 derive directly from Djorgovski & Meylan's list (ref. 1), which was presented at a 1992 conference. They included 143 objects, but one of them, known as the "Reticulum cluster", has since been shown to be an outlying member of the Large Magellanic Cloud. The cluster reported by Djorgovski as "Djorgovski 3" is identical with NGC 6540, which was previously considered to be an open cluster. This is the first NGC/IC object to be identified as a globular since the 1930s. Since they are relatively easy to pick up visually in moderate apertures, I have also included the five globular clusters in the Fornax system. The Clouds appear to contain only another eight or ten true globulars, and along with the "Fornax five", appear to be the only globulars outside the three major spirals in the Local Group (M31, M33, and Milky Way). Yes, I know Uranometria and the RNGC indicate scores of globulars in the LMC, but those clusters are not similar to galactic globulars. (Let's talk about this another time.) According to preliminary studies, Palomar 1 exhibits no horizontal- branch stars, and thus is likely to be an old open cluster. I've left it in the list for now since the work is not formally published. It is also worth noting that E 3, AM-2, and AM-4 are evidently clusters of intermediate age, i.e. between the ages of the oldest ordinary open clusters (6-8 billion years) and those of globulars (13-15 billion years). Although not uncommon in the Magellanic Clouds, these appear to be essentially absent from the Milky Way. One can suspect that, over the course of the dynamical evolution of the Local Group of galaxies, the Milky Way has glommed onto a couple of stray clusters from other galaxies. Indeed this may help explain the bi-modal character of the Galactic globular cluster system. If you are one of those cluster-fetishists familiar with obscure objects on older lists of globulars, you will notice a number of objects missing and others that are new. Most of the nebulous objects reported as possible globulars by A. Terzan and his collaborators turn out to be galaxies or planetary nebulae upon spectroscopic investigation. Still, for several of these no definitive conclusion has yet been reached. Djorgovski & Meylan indicate that the objects TJ 5, TJ 23, and TBJ 3 could be either distant, obscured globulars, chance groupings of stars, or background galaxies with Milky Way stars superposed. Other "suspect" objects are: ESO 166-SC11 = UKS 2, probably an open cluster; ESO 93-SC?08, probably an open cluster; ESO 224-SC08 = vdB-H 176, a sparse globular or an open cluster. More data are required for all these six objects. Among objects confirmed to be "not globular" are the Terzan objects TBJ 1 = TJ 17, TBJ 2 = TJ 16, TJ 15, and others, all of which are galaxies. Grindlay 1 and Kodaira 1 are both nonexistent, there being no cluster at the published coordinates of either. I maintain this list on a computer and keep it up to date as new journal articles are published. Contact me if you would like a copy of the latest version. Ordinary mail and electronic mail are the best ways to get my attention. If you have an e-mail connection (or know someone who does), it is easy for me to simply send you the current file. References 1. Djorgovski, S. G., and Meylan, G. 1993, "The Galactic Globular Cluster System: A List of the Known Globular Clusters and Their Positions"; in {\it Structure and Dynamics of Globular Clusters}, A.S.P. Conference Series vol. 50, S. G. Djorgovski and G. Meylan, eds., pg. 325. 2. Peterson C. J. 1993, "Integrated Photometric Properties of Globular Clusters"; in {\it Structure and Dynamics of Globular Clusters}, A.S.P. Conference Series vol. 50, S. G. Djorgovski and G. Meylan, eds., pg. 337. 3. Alcaino, G., and Liller, W. 1984, "BVRI Main-Sequence Photometry of the Globular Cluster M4", {\it Astrophys. J. Suppl.}, 56, 13. 4. Bok B. J., and Bok, P. F. {\it The Milky Way}, Harvard Univeristy Press, Cambridge MA, 1981. 5. Payne-Gaposchkin, C. {\it Stars and Clusters}, Harvard University Press, Cambridge MA, 1979. Table 2. Globular clusters brighter than V = 7.0 Name V N5139=omega Cen 3.9 N 104=47 Tuc 4.0 N6656=M22 5.2 N6397 5.3 N6752 5.3 N6121=M4 5.4 N5904=M5 5.7 N6205=M13 5.8 N6218=M12 6.1 N2808 6.2 N6809=M55 6.3 N6541 6.3 N5272=M3 6.3 N7078=M15 6.3 N6266=M62 6.4 N6341=M92 6.5 N6254=M10 6.6 N7089=M2 6.6 N 362 6.8 N6723 6.8 N6388 6.8 N6273=M19 6.8 N7099=M30 6.9 N3201 6.9 N6626=M28 6.9 Table 3. Ranking by brightest stars Name V N6397 10.0 N6752 10.5 N6656=M22 10.7 N6121=M4 10.8 N6809=M55 11.2 N5139=omega Cen 11.5 N3201 11.7 N 104=47 Tuc 11.7 N6205=M13 11.9 N6254=M10 12.0 Table 4. Ranking by horizontal-branch magnitude Name V N6397 12.9 N6121=M4 13.4 N6752 13.8 N 104=47 Tuc 14.1 N6656=M22 14.2 N6809=M55 14.4 N6838=M71 14.4 N5139=omega Cen 14.5 N6254=M10 14.7 N3201 14.8 Globular Cluster Diameters, Integrated Magnitudes, Brightest Stars, and Horizontal Branch Levels version: 2 May 1999 notes: ESO 452-SC11, IC 1257 added, AM-2 deleted (is old oc), Terzan 10 added Name RA (2000) Dec Diam(') Vt V(tip) B-V(tip) V(HB) mu22 mu25 NGC 104 = 47 Tuc 0 24 05 -72 04.9 24 50 4.0 11.7 1.7 14.1 NGC 288 0 52 48 -26 35.4 5.5 13 8.1: 12.6 1.8 15.3 NGC 362 1 03 14 -70 50.9 6.1 14 6.8 12.7 1.7 15.4 Fornax-1 2 37 02 -34 11.0 0.9 15.6 18.3 1.3 21.3 Fornax-2 2 38 44 -34 48.6 13.5 19.0 1.3 21.3 NGC 1049 = Fornax-3 2 39 48 -34 15.4 12.6 18.4 1.2 21.3 Fornax-4 2 40 07 -34 32.3 13.6 18.6 1.2 21.3 Fornax-5 2 42 21 -34 06.2 1.7 13.4 18.6 1.3 21.3 NGC 1261 3 12 15 -55 13.0 3.2 6.8 8.3 13.5 1.7 16.8 Palomar 1 3 33 23 +79 34.8 2.8 13.6: 16.3 1.2: AM-1 = E 1 3 55 03 -49 36.9 0.5 15.8: 18.2 1.5 20.9 Eridanus 4 24 45 -21 11.2 14.7: 17.6 1.5 20.4 Palomar 2 4 46 06 +31 22.9 2.2 13.0: 18.8: 21.7 NGC 1851 5 14 06 -40 02.8 4.5 12 7.1 13.2 1.7 16.1 NGC 1904 = M79 5 24 11 -24 31.5 3.8 9.6 7.7 13.1 1.6 16.2 NGC 2298 6 48 59 -36 00.3 2.2 5: 9.3 13.4 1.3 16.2 NGC 2419 7 38 08 +38 52.9 1.6 4.6 10.3 17.3 1.4 20.2 Pyxis 9 07 57 -37 13.6 2: 15.2 2.8 18.7 NGC 2808 9 12 03 -64 51.8 6.5 14 6.2 13.8 1.9 16.1 E 3 9 20 59 -77 17.0 11.4: 17.0: 1.1: -- Palomar 3 10 05 31 +0 04.3 1.6 13.9: 18.0 1.2 20.5 NGC 3201 10 17 37 -46 24.7 7.8 20 6.9 11.7 1.7 14.8 Palomar 4 11 29 17 +28 58.4 1.3 14.2: 18.0 1.4 20.8 NGC 4147 12 10 06 +18 32.5 1.6 4.4 10.4 14.5 1.2 16.9 NGC 4372 12 25 45 -72 39.4 5: 7.2 12.2 2.0 15.6 Ruprecht 106 12 38 40 -51 09.0 2: 10.9: 14.8 1.6 17.8 NGC 4590 = M68 12 39 28 -26 44.6 4.3 11 7.3 12.6 1.3 15.6 NGC 4833 12 59 35 -70 52.5 8.4 12.4 1.7 15.5 NGC 5024 = M53 13 12 55 +18 10.2 5.1 13 7.7 13.8 1.6 16.9 NGC 5053 13 16 27 +17 41.9 10 9.0 13.8 1.5 16.9 NGC 5139=omega Cen 13 26 46 -47 28.6 29 55 3.9 11.5 1.7 14.5 NGC 5272 = M3 13 42 11 +28 22.7 7.4 18 6.3 12.7 1.7 15.7 NGC 5286 13 46 27 -51 22.4 5.1 11 7.4 13.5 1.8 16.5 AM-4 13 56 21 -27 09.7 15.9 20.5: 0.5 21.6 NGC 5466 14 05 27 +28 32.1 3.0 9.0 9.2 13.8 1.3 16.6 NGC 5634 14 29 37 -5 58.6 1.8 5.5 9.5 17.8 NGC 5694 14 39 37 -26 32.3 1.6 4.3 10.2 15.5: 1.3: 18.5: IC 4499 15 00 19 -82 12.8 2.2 8.0 10.1 14.6 1.6 17.7 NGC 5824 15 03 59 -33 04.1 2.5 7.4 9.1 15.5 1.6 18.5 Palomar 5 15 16 05 -0 06.7 3.2 11.8: 15.5 1.1 17.4 NGC 5897 15 17 25 -21 00.6 4.5 11 8.4 13.3 1.8 16.3 NGC 5904 = M5 15 18 34 +2 05.0 11 23 5.7 12.2 1.6 15.1 NGC 5927 15 28 01 -50 40.4 4.6 6: 8.0 14.5 2.2 16.6 NGC 5946 15 35 29 -50 39.6 3: 8.4 17.2 NGC 5986 15 46 04 -37 47.2 9.6 7.6 13.2 1.7 16.5 Palomar 14 16 11 00 +14 57.8 14.7: 17.6 1.3 20.0 NGC 6093 = M80 16 17 03 -22 58.5 4.5 10 7.3 12.5 1.8 15.6 NGC 6121 = M4 16 23 36 -26 31.5 15 36 5.4 10.8 1.8 13.4 NGC 6101 16 25 49 -72 12.1 3.0 5: 9.2 13.5 1.8 16.6 NGC 6144 16 27 14 -26 01.5 3.7 17 9.0 13.4: 1.7: 16.5: NGC 6139 16 27 40 -38 50.9 2.7 8.2 9.1 15.0 1.5: 17.9 Terzan 3 16 28 41 -35 20.6 12.0: 15.0 2.2 17.3 NGC 6171 = M107 16 32 32 -13 03.2 4.2 13 7.8 13.0 1.9 15.6 ESO 452-SC11 16 39 25 -28 23.9 15.3: 16.6 NGC 6205 = M13 16 41 41 +36 27.6 10 20 5.8 11.9 1.6 15.0 NGC 6229 16 46 59 +47 31.7 2.0 4.5 9.4 15.5 1.5 18.0 NGC 6218 = M12 16 47 15 -1 56.9 7.4 16 6.1 12.0 1.7 14.6 NGC 6235 16 53 25 -22 10.6 2.2 5: 8.9 14.0: 1.8 16.7 NGC 6254 = M10 16 57 09 -4 06.0 8.5 20 6.6 12.0 1.6 14.7 NGC 6256 16 59 33 -37 07.3 1.2 4.1 11.3: 15.3: 2.7: 18.2: Palomar 15 16 59 51 -0 32.5 14.2 17.1 1.8 19.9 NGC 6266 = M62 17 01 13 -30 06.8 7.2 15 6.4 13.2: 2.0: 16.3: NGC 6273 = M19 17 02 38 -26 16.1 7.0 17 6.8 14.0? 1.8: 17.0 NGC 6284 17 04 29 -24 45.9 3.1 8.9 16.6 NGC 6287 17 05 09 -22 42.5 2.4 9.3 14.5: 1.8: 17.1: NGC 6293 17 10 10 -26 34.9 4.1 8.3 14.3 1.4 16.5: NGC 6304 17 14 32 -29 27.7 4.3 8: 8.3 14.2: 2.1: 16.2 NGC 6316 17 16 37 -28 08.4 2.7 8.1 16.0: 17.8 NGC 6341 = M92 17 17 07 +43 08.2 7.0 14 6.5 12.1 1.3 15.1 NGC 6325 17 17 59 -23 46.0 1.6 4.1 10.2 14.7? 2.4: 17.3: NGC 6333 = M9 17 19 12 -18 31.0 4.5 12 7.8 13.5 1.7 16.2: NGC 6342 17 21 10 -19 35.2 2.2 9.5 15.0: 16.9 NGC 6356 17 23 35 -17 48.8 3.9 10 8.2 15.1 2.2 17.7 NGC 6355 17 23 59 -26 21.2 2.1 8.6 17.2 NGC 6352 17 25 29 -48 25.4 9: 7.8 13.4: 1.7 15.1 IC 1257 17 27 08 -7 05.6 5: 13.1 17.5: 1.8: 19.8 Terzan 2 17 27 33 -30 48.2 0.6 14.3: 19.8: NGC 6366 17 27 44 -5 04.6 3.6 13 9.5: 13.6 2.2 15.7 Terzan 4 17 30 39 -31 35.7 16.0: 21.6? Haute-Provence 1 17 31 05 -29 58.9 0.4: 1.2 12.5: 16.0 18.6 NGC 6362 17 31 55 -67 02.9 5.6 15: 8.1 12.5 1.7 15.5 Liller 1 17 33 25 -33 23.4 15.8: 20.5: 24.4: NGC 6380 = Ton 1 17 34 28 -39 04.2 1.8 11.5: 17.0 19.5 Terzan 1 17 35 47 -30 28.9 15.9: 20.6: Ton 2 = Pis 26 17 36 11 -38 33.2 12.2 18.2 NGC 6388 17 36 17 -44 44.1 5.2 6.8 14.8 2.1 17.2 NGC 6402 = M14 17 37 36 -3 14.8 5.5 11 7.6 14.0 2.3 17.2 NGC 6401 17 38 37 -23 54.6 2.4 7.4 17.3 NGC 6397 17 40 41 -53 40.4 12 31 5.3 10.0 1.5 12.9 Palomar 6 17 43 42 -26 13.4 0.6 11.6: 19.1 NGC 6426 17 44 54 +3 10.2 1.0 4.2 10.9 15.2 1.8 18.1 Djorgovski 1 17 47 28 -33 03.9 13.6: Terzan 5 17 48 05 -24 46.8 0.4 2.4 13.9: 20.5: 22.5: NGC 6440 17 48 53 -20 21.6 2.2 9.3 16.7: 2.3: 18.7 NGC 6441 17 50 13 -37 03.1 4.8 7.2 15.4: 2.3: 17.1 Terzan 6 17 50 46 -31 16.5 0.2 1.4 13.9: 20.5: 2.5: 22.3: NGC 6453 17 50 52 -34 36.0 3.8 10.2 17.7 UKS 1 17 54 27 -24 08.7 17.3: 22? 25.5 NGC 6496 17 59 03 -44 16.0 2.8 8.6 14.3 1.8 16.5 Terzan 9 18 01 39 -26 50.4 0.2 16.0: 20.3: ESO 456-SC38 18 01 49 -27 49.6 9.9: 15.5: 17.5: NGC 6517 18 01 51 -8 57.5 1.5 4.0 10.1 16.0? 2.2: 18.0: Terzan 10 18 02 57 -26 04.0 19.7: 22: NGC 6522 18 03 35 -30 02.1 4.7 9.9 14.1: 2.1 16.9 NGC 6535 18 03 51 -0 17.8 0.8 3.4 9.3 12.8 1.9 15.7 NGC 6528 18 04 50 -30 03.4 2.5 9.6 15.5: 2.0: 17.1 NGC 6539 18 04 50 -7 35.2 2.1 7.9 8.9 15.9 18.3: NGC 6540 18 06 09 -27 45.9 NGC 6544 18 07 21 -24 59.9 4.6 7.5: 12.8: 2.1: 15.0: NGC 6541 18 08 02 -43 42.9 15 6.3 12.1 1.8 15.2 NGC 6553 18 09 16 -25 54.5 4.6 8.3 15.3 2.4 16.9 NGC 6558 18 10 18 -31 45.8 8.6 16.7 IC 1276 = Pal 7 18 10 45 -7 12.8 1.3 8.0 10.3: 15.7 2.4 17.7 Terzan 11 18 12 15 -22 44.5 16.4: 18.5? 20.5: NGC 6569 18 13 39 -31 49.6 3.2 8.4 17.1 NGC 6584 18 18 38 -52 12.9 3.3: 7.9 13.5 1.7 16.5 NGC 6624 18 23 41 -30 21.7 4.4 7.6 13.8 2.0 16.1 NGC 6626 = M28 18 24 33 -24 52.2 6.9 6.9 12.0 1.5 15.7 NGC 6638 18 30 56 -25 29.8 2.6 7.3 9.2 15.0? 1.6: 15.9 NGC 6637 = M69 18 31 23 -32 20.9 4.9 7.7 13.7 1.8 16.0 NGC 6642 18 31 54 -23 28.5 2.9 8.9 16.3 NGC 6652 18 35 46 -32 59.4 2.7 6.0 8.5 13.3 1.9 15.9 NGC 6656 = M22 18 36 24 -23 54.2 16 5.2 10.7 2.0 14.2 Palomar 8 18 41 30 -19 49.6 2.6 10.9 15.4 17.3 NGC 6681 = M70 18 43 13 -32 17.5 4.0 7.8 14.0? 1.2: 15.6 NGC 6712 18 53 04 -8 42.4 4.9: 8.1 13.3 2.0 16.3 NGC 6715 = M54 18 55 03 -30 28.7 4.1 12 7.7 15.2 1.3: 18.2 NGC 6717 = Pal 9 18 55 06 -22 42.1 2.7 8.4 14.0? 1.3: 15.6 NGC 6723 18 59 33 -36 37.9 6.4 13 6.8 12.8 1.6 15.5 NGC 6749 19 05 15 +1 54.1 ~4 12.4: 16.5 19.7 NGC 6752 19 10 52 -59 58.9 12 29 5.3 10.5 1.9 13.8 NGC 6760 19 11 12 +1 01.8 3.1 9.6 9.0 15.6 17.5: NGC 6779 = M56 19 16 36 +30 11.1 4.4 8.4 13.0: 1.8: 16.2 Terzan 7 19 17 44 -34 39.5 12.0 15.0 1.6 17.9 Palomar 10 19 18 02 +18 34.3 ~4 13.2 18 ? 19.4 Arp 2 19 28 44 -30 21.2 2.3 13.0 15.5 1.4 18.2 NGC 6809 = M55 19 39 59 -30 57.7 9.4 19 6.3 11.2 1.5 14.4 Terzan 8 19 41 44 -34 00.0 12.4: 15.0 1.5 18.0 Palomar 11 19 45 14 -8 00.4 5.0 9.8? 17.3 NGC 6838 = M71 19 53 46 +18 46.7 8.4 12.1 1.9 14.4 NGC 6864 = M75 20 06 05 -21 55.3 3.2 6.8 8.6 14.6 1.8 17.5 NGC 6934 20 34 12 +7 24.3 3.0 7.1 8.9 13.8 1.7 17.1 NGC 6981 = M72 20 53 28 -12 32.2 2.9 6.6 9.2 14.2 1.4 16.9 NGC 7006 21 01 29 +16 11.3 1.4 3.6 10.6 15.6 1.7 18.8 NGC 7078 = M15 21 29 58 +12 10.0 7.7 18 6.3 12.6 1.4 15.8 NGC 7089 = M2 21 33 27 -0 49.4 7.3 16 6.6 13.1 1.6 16.1 NGC 7099 = M30 21 40 22 -23 10.8 5.3 12 6.9 12.1 1.5 15.1 Palomar 12 21 46 39 -21 15.1 2.9 11.7: 14.6 1.6 17.1 Palomar 13 23 06 44 +12 46.3 0.7 13.8: 17.0: 1.0: 17.7 NGC 7492 23 08 27 -15 36.7 0.3 4.2 11.2: 15.5: 1.3: 17.6 ========== Globular Cluster Bibliography and Notes version: 2 May 1999 Name References N 104=47 Tuc Hesser & Hartwick 1977 ApJ Suppl 33,361; Alcaino & Liller 1985 A&A 146,389 N 288 Alcaino & Liller 1980 AJ 85,1592 N 362 Harris 1982 ApJ Suppl 50,573 Fornax-1 Buonanno et al 1985 A&A 152,65 Fornax-2 Buonanno et al 1985 A&A 152,65 N1049=Fornax-3 Buonanno et al 1985 A&A 152,65 Fornax-4 Buonanno et al 1985 A&A 152,65 Fornax-5 Buonanno et al 1985 A&A 152,65; mu25 diam from Grillmair, 1998 Washington AAS meeting, HST obs N1261 Alcaino 1979 A&A Suppl 38,61 Pal 1 Rosenberg et al. 1998 AJ 115,648 (no HB: prob an old open cluster) AM-1=E 1 Madore and Freedman 1989 ApJ 340,812 Eridanus Da Costa 1985 ApJ 291,230. Ortolani & Gratton 1989 A&A Suppl 79,155 V(tip)=18.0/1.29 Pal 2 N1851 Stetson 1981 AJ 86,687, Walker 1992 PASP 104,1063 N1904=M79 Stetson & Harris 1977 AJ 82,954 N2298 Alcaino 1974 A&A Suppl 13,55 N2419 Racine & Harris 1975 ApJ 196,413 Pyxis da Costa 1995 PASP 107,937; Irwin+ 1995 ApJ 453,L21; Sarajedini+ 1996 AJ 112,2013. N2808 Harris 1975 ApJ Suppl 29,397; Alcaino & Liller 1986 AJ 91,87 E 3 Gratton & Ortolani 1987 A&A Suppl 67,373. intermediate-age cluster? Pal 3 Ortolani & Gratton 1989 A&A Suppl 79,155 N3201 Lee 1977 A&A Suppl 28,409 Pal 4 Burbidge & Sandage 1958 ApJ 127,527; Christian & Heasley 1986 ApJ 303,216 N4147 Sandage & Walker 1955 AJ 60,230 N4372 Hartwick & Hesser 1973 ApJ 186,1171; Alcaino & Liller 1986 AJ 91,87 Ru 106 Buonanno et al. 1993 AJ 105,184 N4590=M68 Harris 1975 ApJ Suppl 29,397 N4833 Menzies 1972 MNRAS 156,207 N5024=M53 Cuffey 1965 AJ 70,732 N5053 Sandage et al. 1977 AJ 82,389 N5139=omega Cen Cannon & Stobie 1973 MNRAS 162,207 N5272=M3 Sandage 1953 AJ 58,61; Johnson & Sandage 1956 ApJ 124,379; center coords from Laget et al. 1998 A&A 332,93 from HST data N5286 Harris et al. 1976 ApJ Suppl 31,13; Samus et al. 1995 A&AS 112,439 AM-4 Inman & Carney 1987 AJ 93,1166. V(HB) is for m-s turnoff; intermediate-age cluster? N5466 Buonanno et al. 1984 A&A Suppl 56,79; Nemec & Harris 1987 ApJ 316,172 N5634 N5694 Harris 1975 ApJ Suppl 29,397 I4499 Walker+ 1996 AJ 112,2026. N5824 Harris 1975 ApJ Suppl 29,397; Cannon et al. 1990 MNRAS 243,151 Pal 5 Sandage & Hartwick 1977 AJ 82,459 N5897 Sandage & Katem 1968 ApJ 153,569 N5904=M5 Arp 1962 ApJ 135,311 N5927 Menzies 1974 MNRAS 169,79 N5946 N5986 Harris et al. 1976 ApJ Suppl 31,13 Pal 14 Harris & van den Bergh 1984 AJ 89,1816 (cen sfc br fainter than muV = 25.0). position from Archinal = GSC 0957-0359. N6093=M80 Harris & Racine 1974 AJ 79,472 N6121=M4 Alcaino & Liller 1984 ApJ Suppl 56,19; Cudworth & Rees 1990 AJ 99,1491 N6101 Alcaino 1974 A&A Suppl 18,9 N6144 Alcaino 1980 A&A Suppl 39,315 N6139 Terzan 3 Barbuy et al. 1998 A&AS 132,333 N6171=M107 Sandage & Katem 1964 ApJ 139,1088; Dickens & Rolland 1972 MNRAS 160,37 ESO 452-SC11 Bica et al. 1999 A&AS 136,363 N6205=M13 Arp & Johnson 1955 ApJ 122,171 N6218=M12 Racine 1971 AJ 76,331 N6229 Carney et al 1991 AJ 101,1699 N6235 Liller 1980 AJ 85,673 N6254=M10 Harris et al. 1976 ApJ Suppl 31,13 N6256 Alcaino 1983 A&A Suppl 53,47 Pal 15 Harris & van den Bergh 1984 AJ 89,1816; Seitzer & Carney 1990 AJ 99,229 N6266=M62 Alcaino 1978 A&A Suppl 32,379 N6273=M19 Harris et al. 1976 ApJ Suppl 31,13 N6284 N6287 Stetson & West 1994 PASP 106,726 N6293 Janes & Heasley 1991 AJ 101,2097 N6304 Hesser & Hartwick 1976 ApJ 203,113 N6316 Armandroff 1988 AJ 96,588 N6341=M92 Sandage & Walker 1966 ApJ 143,313 N6325 Harris 1975 ApJ Suppl 29,397 N6333=M9 Janes & Heasley 1991 AJ 101,2097 N6342 Armandroff 1988 AJ 96,588 N6356 Sandage & Wallerstein 1960 ApJ 131,598 N6355 N6352 Fullton et al. 1995 AJ 110,652 IC 1257 Harris et al. 1997, AJ 113,688 Terzan 2 N6366 Pike 1976 MNRAS 177,257 N6362 Alcaino 1972 A&A 16,220 Terzan 4 Haute-Provence 1 Ortolani et al. 1997, MNRAS 284,692 Liller 1 Ortolani et al. 1996, A&A 306,134 N6380 = Ton 1 Ortolani et al. 1998, A&AS 127,471 Terzan 1 Ton 2 = Pis 26 N6388 Alcaino 1981 A&A Suppl 44,33 N6402=M14 Smith-Kogon et al. 1974 AJ 79,387 N6401 N6397 Alcaino & Liller 1980 AJ 85,680 Pal 6 N6426 Zinn & Barnes 1996, AJ 112,1054. Djorgovski 1 Terzan 5 Ortolani+ 1996 A&A 308,733 N6440 Martins et al. 1980 AJ 85,521; Ortolani+ 1994 A&ASuppl 108,653 N6441 Hesser & Hartwick 1976 ApJ 203,97 Terzan 6 Barbuy+ 1997 A&AS 122,483. N6453 UKS 1 Ortolani et al. 1997, A&AS 126,319 N6496 Armandroff 1988 AJ 96,588. Terzan 9 E456-SC38 Ortolani et al. 1997, A&AS 126,319 Terzan 10 Ortolani et al. 1997, A&AS 126,319 N6517 Harris 1975 ApJ Suppl 29,397 N6522 Arp 1965 ApJ 141,43. N6535 Liller 1980 AJ 85,1480; Anthony-Twarog & Twarog 1985 ApJ 291,595 N6528 van den Bergh & Younger 1979 AJ 84,1305 N6539 Armandroff 1988 AJ 96,588 N6540 N6544 Alcaino 1983 A&A Suppl 52,105 N6541 Alcaino 1979 A&A Suppl 35,233 N6553 Hartwick 1975 PASP 87,77. Guarnieri et al. (1998 A&A 331,70) show that ground-based photom has serious zero-point offset due to crowding. V(tip) and V(HB) derive from this newer analysis. N6558 I1276=Pal 7 Barbuy et al. 1998 A&AS 132,333 Terzan 11 Ortolani et al. 1998, A&AS 127,471; approximate diameter 1'.5 from estimate on V-band CCD frame. N6569 N6584 Alcaino & Liller 1985 A&A Suppl 62,317 N6624 Liller & Carney 1978 ApJ 224,383 N6626=M28 Alcaino 1981 A&A Suppl 44,191; Rees and Cudworth 1991 AJ 102,152. N6638 Alcaino & Liller 1983 AJ 88,1166 N6637=M69 Hartwick & Sandage 1968 ApJ 153,715; Harris 1977 PASP 89,482 N6642 N6652 Ortolani+ 1994 A&A 286,444 N6656=M22 Alcaino & Liller 1983 AJ 88,1330; Cudworth 1986 AJ 92,348 Pal 8 Armandroff 1988 AJ 96,588 N6681=M70 Harris 1975 ApJ Suppl 29,397 N6712 Sandage & Smith 1966 ApJ 144,886; Cudworth 1988 AJ 96,105. N6715=M54 Harris 1975 ApJ Suppl 29,397; Sarajedini+ 1995, AJ N6717=Pal 9 Goranskii 1979 Sov AJ 23,284 (English translation); Hazen-Liller 1984 AJ 89,1551. N6723 Menzies 1974 MNRAS 168,177 N6749 Rosino et al. 1997, MNRAS 289,745; Kaisler et al. 1997 PASP 109,920 N6752 Buonanno 1986 A&A Suppl 66,79 N6760 Armandroff 1988 AJ 96,588 Terzan 7 Buonanno+ 1994 ApJ 430,L121 and 1995 AJ 109,663 N6779=M56 Barbon 1965 Asiago Contr no.175. Pal 10 Kailser et al. 1997 PASP 109,920. Arp 2 Buonanno+ 1994 ApJ 430,L121 and 1995 AJ 109,650 N6809=M55 Lee 1977 A&A Suppl 29,1 Terzan 8 Ortolani & Gratton 1990 A&A Suppl 82,71; Montegriffo et al. 1998 MNRAS 294,315 Pal 11 N6838=M71 Arp & Hartwick 1971 ApJ 167,499 and Cudworth 1985 AJ 90,65 N6864=M75 Harris 1975 ApJ Suppl 29,397 N6934 Harris & Racine 1973 AJ 78,242 N6981=M72 Dickens 1972 MNRAS 157,281 N7006 Sandage & Wildey 1967 ApJ 150,469 N7078=M15 Sandage 1970 ApJ 162,841; Buonanno et al. 1983 A&A Suppl 51,83 N7089=M2 Harris 1975 ApJ Suppl 29,397; Cudworth & Rauscher 1987 AJ 93,856 N7099=M30 Alcaino 1978 A&A Suppl 33,185; Alcaino & Wamsteker 1982 A&A Suppl 50,141 Pal 12 Harris & Canterna 1980 ApJ 239,815; Stetson et al. 1989 AJ 97,1360 Pal 13 Ortolani 1985 AJ 90,473 N7492 Buonanno et al. 1987 A&A Suppl 67,327