TRIFFID observations of the cores of the three globular clusters M15, M92 and NGC 6712

We present the results of TRIFFID simultaneous V - and B -band observations of the cores of the globular clusters M15, M92 and NGC 6712. A variability search of their dense centres was made feasible through performing post-exposure image sharpening on the images, increasing the image resolution by a factor of ∼2. The isis implementation of the image subtraction technique developed by Alard & Lupton was then used to detect flux variations in our image sets. We have obtained periods for all observable variables (in our field of view) in NGC 6712 and we have found two new RR Lyrae variables (an RRab and an RRc). We have confirmed three variables in our field of view of the M92. For M15, we detect 48 variables in our field of view, 23 of which are new discoveries. We obtain periods and amplitudes for all variables and classify new ones based on the light-curve shape, the most significant period and the mean magnitude in the V band. Among the detected RR Lyrae we find 19 RRc, 12 RRab and two RRd types. In the subsequent analysis we find a marked increase in RRc over RRab variables in the core. In a refined procedure to search for fainter objects we find no dwarf novae in our field of view of M15. Simulations performed on the data set to quantify our sensitivity to such objects indicate that an upper limit of 10 dwarf novae (at 92 per cent probability) exist in our field of view. The implications this result has on globular clusters are discussed.     

UBRI photometry of globular clusters in the Leo group galaxy NGC 3379

We present wide-area UBRI photometry for globular clusters around the Leo group galaxy NGC 3379. Globular cluster candidates are selected from their B -band magnitudes and their  ( U − B ) _o   versus  ( B − I ) _o   colours. A colour–colour selection region was defined from photometry of the Milky Way and M31 globular cluster systems. We detect 133 globular cluster candidates, which supports previous claims of a low specific frequency for NGC 3379.
The Milky Way and M31 reveal blue and red subpopulations, with  ( U − B ) _o   and  ( B − I ) _o   colours indicating mean metallicities similar to those expected based on previous spectroscopic work. The stellar population models of Maraston and Brocato et al. are consistent with both subpopulations being old, and with metallicities of  [Fe/H]∼−1.5  and −0.6 for the blue and red subpopulations, respectively. The models of Worthey do not reproduce the  ( U − B ) _o   colours of the red (metal-rich) subpopulation for any modelled age.
For NGC 3379 we detect a blue subpopulation with similar colours, and presumably age/metallicity, to that of the Milky Way and M31 globular cluster systems. The red subpopulation is less well defined, perhaps due to increased photometric errors, but indicates a mean metallicity of [Fe/H]∼−0.6.     

Integrated infrared photometry of globular clusters

An integrating wide field infrared photometer recently built in the Astronomy Department at Manchester is described. This instrument contains a number of novel design features required for variable field aperture photometry and these are discussed in some detail.     

Photographic photometry of three RR Lyrae variables of M15

In 1979 we took plates of M15 with the new one metre telescope of Yunnan Observatory. The RR Lyrae variables Nos. 24, 28, and 43 were measured and reduced at Purple Mountain Observatory. Comparison stars were chosen that are clear of crowded regions and were further screened according to their residuals, as described in the paper. Our results on these three variables are compared with recent results from California /4/ and Mt. Wilson Observatory /5/. The light curves of Nos. 24 and 28 show abrupt changes. Our observations of No. 43 exclude its period being 0^d.395556, while fitting well the period 0^d.396013     

New age estimates of M31 globular clusters from multicolour photometry

The large majority of extragalactic star cluster studies performed to date essentially use multicolour photometry, combined with theoretical stellar synthesis models, to derive ages, masses, extinction estimates and metallicities. M31 offers a unique laboratory for studies of globular cluster (GC) systems. In this paper, we obtain new age estimates for 91 M31 GCs, based on improved photometric data, updated theoretical stellar synthesis models and sophisticated new fitting methods. In particular, we used photometric measurements from the Two Micron All Sky Survey (2MASS), which, in combination with optical photometry, can partially break the well-known age–metallicity degeneracy operating at ages in excess of a few Gyr. We show robustly that previous age determinations based on photometric data were affected significantly by this age–metallicity degeneracy. Except for one cluster, the ages of our other sample GCs are all older than 1 Gyr. Their age distribution shows populations of young- and intermediate-age GCs, peaking at ∼3 and 8 Gyr, respectively, as well as the 'usual' complement of well-known old GCs, i.e. GCs of similar age as the majority of the Galactic GCs. Our results also show that although there is significant scatter in metallicity at any age, there is a notable lack of young metal-poor and old metal-rich GCs, which might be indicative of an underlying age–metallicity relationship among the M31 GC population.     

Photoelectric catalogue of globular clusters in the Andromeda Nebula M31 and its companions NGC 147, NGC 185, and NGC 205

The photoelectric catalogue of the globular clusters (including the objects previously considered to be clusters) in the Andromeda Nebula M31 and its companions NGC 147, NGC185, and NGC 205 is presented. The catalogue based on all published data includes 338 objects withUBV, six objects withBV, and one object withV measurements.     

Ages and metallicities of globular clusters in NGC 4472

We have derived ages and metallicities from co-added spectra of 131 globular clusters associated with the giant elliptical galaxy NGC 4472. Based upon a calibration with Galactic globular clusters, we find that our sample of globular clusters in NGC 4472 span a metallicity range of approximately −1.6≤[Fe/H]≤0 dex. There is evidence of a radial metallicity gradient in the globular cluster system which is steeper than that seen in the underlying starlight. Determination of the absolute ages of the globular clusters is uncertain, but formally, the metal-poor population of globular clusters has an age of 14.5±4 Gyr and the metal-rich population is 13.8±6 Gyr old. Monte Carlo simulations indicate that the globular cluster populations present in these data are older than 6 Gyr at the 95 per cent confidence level. We find that within the uncertainties, the globular clusters are old and coeval, implying that the bimodality seen in the broadband colours primarily reflects metallicity and not age differences.     

CCD photometry of the Galactic globular cluster M13

In this paper, we present a B − V CCD colour–magnitude diagram (CMD) of the globular cluster M13 (NGC 6205), reaching down about 2 mag below the main sequence turn-off, based on a new, independent photometric calibration.
The most notable features of the CMD, including about 5500 stars, are (i) the long extension of the blue tail of the horizontal branch (HB), which can be traced down to V ∼19 and (moreover) shows the presence of two gaps; and (ii) the so-called bump on the red giant branch which is well-detected at V =14.75±0.05, using both the differential and integral luminosity functions.
No similar features have ever been detected in any previous ground-based photometry of this cluster.
A direct comparison between the CMD of M13 and M3, calibrated during the same run with the same apparatus, sets an upper limit of about 1.5 Gyr on their age difference, too small to explain the striking differences between their HB morphologies.     

Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds

We compare the conclusions reached using the coarse-graining technique employed by Henriksen for a one degree of freedom (per particle) collisionless system to those presented in a paper by Binney based on an exact one degree of freedom model. We find agreement in detail, but in addition we show that the isolated 1D system is self-similar and therefore unrelaxed. Fine graining of this system recovers much less prominent wave-like structure than in a spherically symmetric isotropic 3D system. The rate of central flattening is also reduced in the 1D system. We take this to be evidence that relaxation of collisionless systems proceeds ultimately by way of short wavelength Landau damping. N -body systems, both real and simulated, can be trapped in an incompletely relaxed state because of a break in the cascade of energy to small scales. This may be due to the rapid dissipation of the small-scale oscillations in an isolated system to the existence of conserved quantities such as angular momentum, or to the failure in simulations to resolve sub-Jeans length scales. Such a partially relaxed state appears to be the Navarro, Frenk and White (NFW) state and is to be expected especially in young systems. The NFW core is shown to be isolated. In non-isolated systems, continuing coarse-grained relaxation should be towards a density core in solid body rotation.     

Hubble Space Telescope photometry of globular clusters in the Sombrero galaxy

We explore the rich globular cluster (GC) system of the nearby Sa galaxy M104, the 'Sombrero' (NGC 4594), using archive Wide Field Planetary Camera 2 data. The GC colour distribution is found to be bimodal at the >99 per cent confidence level, with peaks at     and     . The inferred metallicities are very similar to those of GCs in our Galaxy and M31. However, the Sombrero reveals a much enhanced number of red (metal-rich) GCs compared to other well-studied spirals. Because the Sombrero is dominated by a huge bulge and only has a modest disc, we associate the two subpopulations with the halo and bulge components, respectively. Thus our analysis supports the view that the metal-rich GCs in spirals are associated with the bulge rather than with the disc. The Sombrero GCs have typical effective (half-light) radii of ∼2 pc with the red ones being ∼30 per cent smaller than the blue ones. We identify many similarities between the GC system of the Sombrero and those of both late-type spirals and early-type galaxies. Thus both the GC system and the Hubble type of the Sombrero galaxy appear to be intermediate in their nature.     

DDO integrated photometry of globular clusters and initial chemical evolution of the galaxy

Integrated photometry of 91 galactic globular clusters was carried out with theUBV and DDO systems. A method to determine reddening is presented. Metal abundances were obtained by means of a calibration ofC(42–45)₀ vs [Fe/H]. Thirteen other clusters whose metallicity values were made compatible with those observed here, were added to the sample totalizing a homogenous set of [Fe/H] estimates for 104 globular clusters. This sample is used to analyze the spatial distribution of metallicities. An estimate of the intrinsic metal abundance frequency distribution of the globular cluster system is provided. The chemical properties of the globular cluster system are compared with one-zone-chemical-enrichment models which take into account gas loss from star formation regions. It is concluded that these models cannot describe the complete chemical enrichment of the globular cluster system.     

RR Lyrae variables in the globular cluster M3 (NGC 5272) — I. BVI CCD photometry

New BVI CCD photometry is presented for 60 RR Lyrae variables in the globular cluster M3. Light curves have been constructed and ephemerides have been (re)-derived for all of them. Four stars (i.e. V29, V136, V155 and V209), although recognized as variables, had no previous period determinations. Also, the period derived for V129 is significantly different from the one published by Sawyer-Hogg. Light curve parameters, i.e. mean magnitudes, amplitudes and rise times, have been derived.   The discussion of these results in the framework of the stellar evolution and pulsation theories will be presented in a forthcoming paper.     

Color gradients in four king type globular clusters: NGC 2298, NGC 6402, NGC 6934, and NGC 7089

We present CCD surface photometry for four globular clusters: NGC 2298, NGC 6402, NGC 6934 and NGC 7089. Our photometry was aimed at the central 3 arcmin region in each cluster. We show that the surface brightness distributions of these clusters are well described by the isothermal King (1966) models. Our color analysis shows that NGC 7089 exhibits a remarkable color gradient in which the central region is bluer than the outer parts by the amplitude inB-V 0.1 mag/. Color gradients have also been detected in the central regions of NGC 6402 and NGC 6934, although the variation does not extend to outer part. No gradient has been detected in NGC 2298. Similar color gradients have been previsouly observed exclusively in highly concentrated systems classified as post core collapse clusters.