A 'super' star cluster grown old: the most massive star cluster in the Local Group

We independently redetermine the reddening and age of the globular cluster (GC) 037−B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037−B327 has long been known to have a very large reddening value, which we confirm to be   E ( B − V ) = 1.360 ± 0.013  , in good agreement with the previous results. We redetermine its most likely age at  12.4 ± 3.2 Gyr  .
037−B327 is a prime example of an unusually bright early counterpart to the ubiquitous 'super' star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar initial mass function (IMF) cannot have been top-heavy. Using this constraint, and a variety of simple stellar population (SSP) models, we determine a photometric mass of     , somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, makes this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (one-dimensional) velocity dispersion of the order of  (72 ± 13) km s⁻¹  . As a surviving 'super' star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution.     

Proper motions in the field of the globular cluster NGC 6934

We measured relative proper motions with a typical accuracy of 1.0 milliarcsec/year (mas/a) for 2000 stars in a 1°4 × 1°4 field around the low-latitude globular cluster NGC 6934. Four plates taken with the Bonn double refractor, spanning an epoch difference of 62 years, were digitized completely. Within the tidal radius of the cluster, we find 106 stars with proper motion errors less than 5 mas/a. Membership probalilities are computed taking into account the individual proper motion errors and the radial distances to the cluster centre. We derive the mean relative proper motion of NGC 6934 using stars with high membership probabilities from radial velocities (Smith and Bell 1986) or from their location in the colour-magnitude diagram (Harris and Racine 1973). The relative proper motions of four Hipparcos stars in the field will be used to obtain the absolute proper motion of NGC 6934 once the extragalactically calibrated Hipparcos Output Catalogue is available.     

The globular cluster system of the young elliptical NGC 6702

We study the globular cluster (GC) system of the dust-lane elliptical galaxy NGC 6702, using B -, V - and I -band imaging observations carried out at the Keck telescope. This galaxy has a spectroscopic age of ≈2 Gyr suggesting recent star formation. We find strong evidence for a bimodal GC colour distribution, with the blue peak having a colour similar to that of the Galactic halo GCs. Assuming that the blue GCs are indeed old and metal-poor, we estimate an age of 2–5 Gyr and supersolar metallicity for the red GC subpopulation. Despite the large uncertainties, this is in reasonable agreement with the spectroscopic galaxy age. Additionally, we estimate a specific frequency of S _N =2.3±1.1 for NGC 6702. We predict that passive evolution of NGC 6702 will further increase its specific frequency to S _N ≈2.7 within 10 Gyr, in closer agreement to that of typical present-day ellipticals. We also discuss evidence that the merger/accretion event that took place a few Gyr ago involved a high gas fraction.     

Blueshifted galaxies in the virgo cluster

We examine a sample of 65 galaxies in the Virgo cluster with negative radial velocities relative to the Local Group. Some features of this sample are pointed out. All of these objects are positioned compactly within a virial zone of radius 6o in the cluster, but their centroid is displaced relative to the dynamic center of the cluster, M87, by 1o.1 to the northwest. The dwarf galaxies in this sample are clumped on a scale of ∼10' (50 kpc). The observed asymmetry in the distribution of the blueshifted galaxies may be caused by infall of a group of galaxies around M86 onto the main body of the cluster. We offer another attempt to explain this phenomenon, assuming a mutual tangential velocity of ∼300 km/s^-1 between the Local Group and the Virgo cluster owing to their being repelled from the local cosmological void.     

Revisiting the open cluster Lo 807

We have conducted an astrometric study of the open cluster Lo 807 using proper motion data from UCAC2 catalog. We applied the maximum likelihood method to determine cluster membership by using the proper motion data. The Padova isochrones lines with solar metallicity were fitted upon the Color Magnitude Diagram in near-infrared to derive physical parameters of the cluster. We do not detect significant variable reddening within the cluster. The present result is compared with our previous study.     

New stellar members of the Coma Berenices open star cluster

We present the results of a survey of the Coma Berenices open star cluster (Melotte 111), undertaken using proper motions from the USNO-B1.0 (United States Naval Observatory) and photometry from the Two-Micron All-Sky Survey (2MASS) Point Source catalogues. We have identified 60 new candidate members with masses in the range  1.007 < M < 0.269 M_⊙  . For each we have estimated a membership probability by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being members, more than doubling the number of known cluster members. The new luminosity function for the cluster peaks at bright magnitudes, but is rising at   K ≈ 12  , indicating that it is likely that lower mass members may exist. The mass function also supports this hypothesis.     

The axial zone of avoidance in the globular cluster system and the distance to the galactic center

We have checked the existence of a zone of avoidance oriented along the Galactic rotation axis in the globular cluster (GC) system of the Galaxy and performed a parametrization of this zone in the axisymmetric approximation. The possibility of the presence of such a structure in the shape of a double cone has previously been discussed in the literature. We show that an unambiguous conclusion about the existence of an axial zone of avoidance and its parameters cannot be reached based on the maximization of the formal cone of avoidance due to the discreteness of the GC system. The ambiguity allows the construction of the representation of voids in the GC system by a set of largest-radius meridional cylindrical voids to be overcome. As a result of our structural study of this set for northern and southern GCs independently, we have managed to identify ordered, vertically connected axial zones of avoidance with similar characteristics. Our mapping of the combined axial zone of avoidance in the separate and joint analyses of the northern and southern voids shows that this structure is traceable at |Z| ? 1 kpc, it is similar in shape to a double cone whose axis crosses the region of greatest GC number density, and the southern cavity of the zone has a less regular shape than the northern one. By modeling the distribution ofGalactocentric latitudes forGCs, we have determined the half-angle of the cone of avoidance α₀ = 15?. 0_{?4?. 1} ^{+2?. 1} and the distance to the Galactic center R ₀ = 7.3 ± 0.5 kpc (in the scale of the Harris (1996) catalog, the 2010 version) as the distance from the Sun to the point of intersection of the cone axis with the center–anticenter line. A correction to the calibration of the GC distance scale obtained in the same version of the Harris catalog from Galactic objects leads to an estimate of R ₀ = 7.2±0.5|_stat ±0.3|_calib kpc. The systematic error in R ₀ due to the observational incompleteness of GCs for this method is insignificant. The probability that the zone of avoidance at the characteristics found is random in nature is ≤2%. We have revealed evidence for the elongation of the zone of avoidance in the direction orthogonal to the center–anticenter axis, which, just as the north–south difference in this zone, may be attributable to the influence of the Magellanic Clouds. The detectability of similar zones of avoidance in the GC systems of external galaxies is discussed.     

Globular cluster systems in nearby dwarf galaxies – I. HST/ACS observations and dynamical properties of globular clusters at low environmental density

We investigate the old globular cluster (GC) population of 68 faint  ( M _V > −16 mag)  dwarf galaxies located in the halo regions of nearby (≲12 Mpc) loose galaxy groups and in the field environment based on archival Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS) images in F606W and F814W filters. The combined colour distribution of 175 GC candidates peaks at  ( V − I ) = 0.96 ± 0.07 mag  and the GC luminosity function turnover for the entire sample is found at   M _{V ,TO}=−7.6 ± 0.11 mag  , similar to the old metal-poor Large Magellanic Cloud (LMC) GC population. Our data reveal a tentative trend of   M _{V ,TO}  becoming fainter from late- to early-type galaxies. The luminosity and colour distributions of GCs in dIrrs show a lack of faint blue GCs (bGCs). Our analysis reveals that this might reflect a relatively younger GC system than typically found in luminous early-type galaxies. If verified by spectroscopy, this would suggest a later formation epoch of the first metal-poor star clusters in dwarf galaxies. We find several bright (massive) GCs which reside in the nuclear regions of their host galaxies. These nuclear clusters have similar luminosities and structural parameters as the peculiar Galactic clusters suspected of being the remnant nuclei of accreted dwarf galaxies, such as M54 and ωCen. Except for these nuclear clusters, the distribution of GCs in dIrrs in the half-light radius versus cluster mass plane is very similar to that of Galactic young halo clusters, which suggests comparable formation and dynamical evolution histories. A comparison with theoretical models of cluster disruption indicates that GCs in low-mass galaxies evolve dynamically as self-gravitating systems in a benign tidal environment.     

X-Ray emission from the pleiades cluster

Detection of a new X-ray source and its identification with the Pleiades cluster is reported here. The new X-ray source has an X-ray luminosity of ～10³² ergs s^?1 in the 0.15–3.0 keV energy band. The observed X-ray emission could be explained as integrated emission from a large number of stars in the cluster.     

Ages and masses of delta scuti stars in the praesepe cluster

Physical parameters and radial pulsation modes are determined for three Delta Scuti stars in the Praesepe cluster: HD 73345, HD 73712, and HD 73746. Their agest and evolutionary massesM _eare interpolated in the track systems of Iben (1967) and Paczyski (1970). By combining these age estimates with our previous results for nine other Delta Scuti-stars in the Praesepe cluster (Tsvetkov, 1989), the following weighted mean age estimates of this cluster are obtained: (14.2 ± 1.3) × 10⁸ yr (Iben) and (5.4 ± 0.4) × 10⁸ yr (Paczyski). Age and evolutionary mass estimates of the twelve cluster variables are also obtained in the modern track system of Maeder and Meynet (1988), in which the weighted mean cluster age is (15.3 ± 1.0) × 10⁸ yr.Pulsation massesM _Qcalculated from the fitting formulae of Faulkner (1977) and Fitch (1981) as well as massesM _g=gR ² / G were evaluated for the twelve cluster variables by Tsvetkov (1990) or in the present paper. In most cases there is a satisfactory agreement between our estimates of massesM _e,M _g, andM _Q.Due to a large disagreement between the long period and low luminosity of the star HD 73746, its radial pulsation mode and pulsation massM _Qcannot be evaluated.     

Membership in the open cluster NGC 2301

Astrometric cluster membership probabilities have been derived from proper motion of the open cluster NGC 2301. A comparison with previous investigations is presented.     

Further probing the nature of FSR 1767

With Two-Micron All-Sky Survey (2MASS) photometry and proper motions, Bonatto et al. suggested that FSR 1767 is a globular cluster (GC), while with J and K NTT/SOFI photometry Froebrich, Meusinger & Scholz concluded that it is not a star cluster. In this study, we combine previous and new evidence that are consistent with a GC. For instance, we show that the horizontal branch (HB) and red giant branch (RGB) stars, besides sharing a common proper motion, have radial density profiles that consistently follow the King's law independently. Reddening maps around FSR 1767 are built using the bulge RGB as reference and also Schlegel's extinction values to study local absorptions. Both approaches provide similar maps and show that FSR 1767 is not located in a dust window, which otherwise might have produced the stellar overdensity. Besides, neighbouring regions of similar reddening as FSR 1767 do not present the blue HB stars that are a conspicuous feature in the colour–magnitude diagram of FSR 1767. We report the presence of a compact group of stars located in the central parts of FSR 1767. It appears to be a detached post-collapse core, similar to those of other nearby low-luminosity GCs projected towards the bulge. We note that while the NTT/SOFI photometry of the star cluster FSR 1716 matches perfectly that from 2MASS, it shows a considerable offset for FSR 1767. We discuss the possible reasons why both photometries differ. We confirm our previous structural and photometric fundamental parameters for FSR 1767, which are consistent with a GC.     

Spatially resolved X-ray spectroscopy of the core of the Centaurus cluster

We present Chandra data from a 31.7-ks observation of the Centaurus cluster, using the ACIS-S detector. Images of the X-ray emission show a plume-like feature at the centre of the cluster, of extent 60 arcsec (20 kpc in projection). The feature has the same metallicity as gas at a similar radius, but is cooler. Using adaptive binning, we generate temperature, abundance and absorption maps of the cluster core. The radial abundance profile shows that the previously known, steep abundance gradient peaks with a metallicity of  1.3–1.8 Z_⊙  at a radius of about 45 arcsec (15 kpc), before falling back to 0.4 Z_⊙ at the centre of the cluster. A radial temperature profile shows that the temperature decreases inwards. We determine the spatial distributions of each of two temperature components, where applicable. The radiative cooling time of the cooler component within the inner 10 arcsec (3 kpc) is less than  2×10⁷ yr  . X-ray holes in the image coincident with the radio lobes are seen, as well as two outer sharp temperature drops, or cold fronts. The origin of the plume is unclear. The existence of the strong abundance gradient is a strong constraint on extensive convection or gas motion driven by a central radio source.     

An analysis of infrared emission spectra from the regions near the Galactic Centre

We present consistent modelling of line and continuum infrared (IR) spectra in the region close to the Galactic Centre. The models account for the coupled effect of shocks and photoionization from an external source. The results show that the shock velocities range between ∼65 and 80 km s⁻¹ and the pre-shock densities between 1 cm⁻³ in the interstellar medium (ISM) to 200 cm⁻³ in the filamentary structures. The pre-shock magnetic field increases from 5 × 10⁻⁶ G in the surrounding ISM to ∼8 × 10⁻⁵ G in the arched filaments. The stellar temperatures are ∼38 000 K in the Quintuplet cluster and ∼27 000 K in the Arches Cluster. The ionization parameter is relatively low (<0.01) with the highest values near the clusters, reaching a maximum >0.01 near the Arches Cluster. Depletion from the gaseous phase of Si is found throughout the whole observed region, indicating the presence of silicate dust. Grains including iron are concentrated throughout the arched filaments. The modelling of the continuum spectral energy distribution in the IR range indicates that a component of dust at temperatures of ∼100–200 K is present in the central region of the Galaxy. Radio emission appears to be thermal bremsstrahlung in the E2–W1 filaments crossing strip; however, a synchrotron component is not excluded. More data are necessary to resolve these questions.     

The mass content in the region of the cluster NGC 1750

Kinematical information has been used to solve the Jeans’ equation to provide a mass estimate for the open cluster NGC 1750. Using published data we have also applied the Michie–King model to estimate the mass in the cluster. The comparison of these masses gives the best estimation of mass in the region of this cluster as 1.01×10⁴ M_⊙, which indicates that the cluster may be a massive cluster. Checks of the density distribution and isotropy of the stellar velocity distribution are given.     

The product of the globular cluster collapse

In this paper we calculate the number of close binaries formed during the evolution process of a globular cluster core. The globular cluster core is assumed to contain a massive black hole at its center. We show that the central black hole can drive binaries formation in the core and the rate of binaries formation depends on the mass of the black hole at its center. When the massM of the black hole is between 10² M and 3×10³ M , there will be a few binaries formed. When the mass of the black hole is 4×10³ M M6×10³ M , the number of binary star formation will suddenly increase with a jump to the maximum value 58. When the mass of the black hole is 7×10³ M M9×10³ M , the number of binary star will immediately decrease. Whether cluster X-ray is produced mainly by the central black hole or by binaries in the core depends on the mass of the central black hole. Therefore, two cases arise: namely, black hole accretion domination and binaries radiation domination. We do think that we cannot exclude the possibility of the existence of a central black hole even when binary radiation characteristics have been observed in globular cluster X-ray sources.     

Research on the membership and spatial motion of the open cluster NGC6530

or stars in the region around the open cluster NGC6530, cluster memberships are calculated based on the maximum likelihood principle using proper motions obtained from photographic plates acquired with the 40 cm refraction telescope at Shanghai Astronomical Observatory. Analyses show that the membership determination is very effective with 250 stars having membership probabilities p ≥ 0.9. The spatial motion of the star cluster is also determined and a simple discussion is carried out.     

Photometry of the galactic cluster NGC 2169

The results of photoelectricU, B, V photometry of the galactic cluster NGC 2169 are presented. The colour excessE(B-V) is 0^m.18. The distance modulus to the cluster and its age are respectively estimated at 9^m.6 and 0.9×10⁷ yr. Star No. 10 is found to be variable.     

Detecting the effect of globular cluster impacts on the disc of the Milky Way

The crossing of the Galactic disc by a globular cluster (GC) could produce star formation due to gravitational focusing or compression of disc material. We report on simulations of the effect on disc material which reveal that the crossing can sometimes cause local gravitational focusing of disc material. We also present the salient points of a little-known paper by Levy, which shows that strong compression can result from the shock wave generated by GC disc crossing. The main thrust of our paper is a search for remnants of disc crossings by GCs. Using the gravitational potential of the Galaxy to locate the position of the most recent crossings of a subset of fifty-four GCs reveals that systematic errors and uncertainties in initial conditions limit the scope for unequivocal identification. From the subset of fifty-four, six possible search sites with the best constraints are retained for further scrutiny. Three of the six potentially promising search areas in the disc are from GCs NGC 3201, 6397 and 6838, for which we cannot rule out some observed star associations observed nearby as being remnants. The other three of the six areas are too large to provide meaningful identification of remnants. Also, a possible remnant (open cluster NGC 6231) is shown not to be due to GC impact, contrary to a previous report. In a more wide-ranging screening of 155 GCs, we identify which GCs are compatible with being responsible for the formation of any of the Galaxy's five most prominent star superclusters.     

The open cluster Tr 14

New photoelectric UBVRI data for stars in the young open cluster Tr 14 in the Great Carina Nebula (NGC 3372) are presented. The two-color diagram separates clearly the member from the nonmember stars. Thus, the membership of about 39 stars is suggested. Those located in the compact core of the cluster, have smaller reddening dispersion than those around it. No star appears above the Main Sequence at the faint end of the color-magnitude diagram. A distance modulus ofV ₀?M_v=12.20±0.2 is obtained, which gives an age of about 5×10⁶ yr. From only the photometry it is concluded to be slightly older than the nearby cluster Tr16.