Abstract
We present a second set of results from a wide-field photometric survey of the environs of Milky Way globular clusters. The clusters studied are NGC 1261, NGC 1851 and NGC 5824: all have ...data from the Dark Energy Camera on the Blanco 4 m telescope. NGC 5824 also has data from the Magellan Clay telescope with MegaCam. We confirm the existence of a large diffuse stellar envelope surrounding NGC 1851 of size at least 240 pc in radius. The radial density profile of the envelope follows a power-law decline with index γ = −1.5 ± 0.2 and the projected shape is slightly elliptical. For NGC 5824, there is no strong detection of a diffuse stellar envelope, but we find the cluster is remarkably extended and is similar in size (at least 230 pc in radius) to the envelope of NGC 1851. A stellar envelope is also revealed around NGC 1261. However, it is notably smaller in size with radius ∼105 pc. The radial density profile of the envelope is also much steeper with γ = −3.8 ± 0.2. We discuss the possible nature of the diffuse stellar envelopes, but are unable to draw definitive conclusions based on the current data. NGC 1851, and potentially NGC 5824, could be stripped dwarf galaxy nuclei, akin to the cases of ω Cen, M54 and M2. On the other hand, the different characteristics of the NGC 1261 envelope suggest that it may be the product of dynamical evolution of the cluster.
Electrostatic interactions between small molecules and their respective receptors are essential for molecular recognition and are also key contributors to the binding free energy. Assessing the ...electrostatic match of protein–ligand complexes therefore provides important insights into why ligands bind and what can be changed to improve binding. Ideally, the ligand and protein electrostatic potentials at the protein–ligand interaction interface should maximize their complementarity while minimizing desolvation penalties. In this work, we present a fast and efficient tool to calculate and visualize the electrostatic complementarity (EC) of protein–ligand complexes. We compiled benchmark sets demonstrating electrostatically driven structure-activity relationships (SAR) from literature data, including kinase, protein−protein interaction, and GPCR targets, and used these to demonstrate that the EC method can visualize, rationalize, and predict electrostatically driven ligand affinity changes and help to predict compound selectivity. The methodology presented here for the analysis of EC is a powerful and versatile tool for drug design.
We conducted a meta-analysis and empirical review of abusive supervision research in order to derive meta-analytic population estimates for the relationships between perceptions of abusive ...supervision and numerous demographic, justice, individual difference, leadership, and outcome variables. The use of psychometric correction enabled us to provide weighted mean correlations and population correlation estimates that accounted for attenuation due to measurement error and sampling error variance. Also, we conducted sensitivity analyses that removed the effects of large samples from analyses. Then, we conducted subgroup analyses using samples drawn from the United States to provide population correlation estimates that corrected for attenuation due to measurement error, sampling error variance, and indirect range restriction. Finally, we examined measurement artifacts resulting from various adaptations of Tepper’s abusive supervision measure. The results reveal that although the associations between perceptions of abusive supervision and outcome variables appear to be universally negative, the magnitude of the relationships between perceptions of abusive supervision and antecedent and outcome variables varies according to the design features of studies. Contributions to theory and practice, strengths and limitations, and directions for future research are discussed.
We have compiled a pseudo-snapshot data set of two-colour observations from the Hubble Space Telescope archive for a sample of 53 rich LMC clusters with ages of 106–1010 yr. We present surface ...brightness profiles for the entire sample, and derive structural parameters for each cluster, including core radii, and luminosity and mass estimates. Because we expect the results presented here to form the basis for several further projects, we describe in detail the data reduction and surface brightness profile construction processes, and compare our results with those of previous ground-based studies. The surface brightness profiles show a large amount of detail, including irregularities in the profiles of young clusters (such as bumps, dips and sharp shoulders), and evidence for both double clusters and post-core-collapse (PCC) clusters. In particular, we find power-law profiles in the inner regions of several candidate PCC clusters, with slopes of approximately −0.7, but showing considerable variation. We estimate that 20 ± 7 per cent of the old cluster population of the Large Magellanic Cloud (LMC) has entered PCC evolution, a similar fraction to that for the Galactic globular cluster system. In addition, we examine the profile of R136 in detail and show that it is probably not a PCC cluster. We also observe a trend in core radius with age that has been discovered and discussed in several previous publications by different authors. Our diagram has better resolution, however, and appears to show a bifurcation at several hundred Myr. We argue that this observed relationship reflects true physical evolution in LMC clusters, with some experiencing small-scale core expansion owing to mass loss, and others large-scale expansion owing to some unidentified characteristic or physical process.
This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar ...populations in the colour-magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ~45 per cent whereas it is ~25 per cent in NGC 339 and ~36 per cent in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way.
Abstract
We have recently shown that the ∼2 Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in N/Fe. These can be seen as ...a splitting or spread in the subgiant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Because of its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star formation epochs have occurred within NGC 1978. First, we use ultraviolet colour--magnitude diagrams (CMDs) to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of 1 ± 20 Myr between them. This is in tension with predictions from the asymptotic giant branch scenario for the origin of multiple populations. Second, we estimate the broadness of the main-sequence turn-off (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of ∼65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate-age clusters, while it fully supports predictions from the stellar rotation model.
In this paper we compare the properties of three subsystems of Galactic globular clusters, which are defined according to metallicity and horizontal branch morphology. We specifically focus on ...cluster luminosities, structures, surface brightnesses and ellipticities. It is shown that the so-called ‘young’ halo (YH) clusters, which are thought to have formed in external satellite galaxies, exhibit characteristics which are clearly distinct from those of the ‘old’ halo (OH) and bulge/disc (BD) clusters, the majority of which are believed to be Galactic natives. The properties of the YH objects are, in many respects, similar to those of clusters belonging to a number of present-day satellite dwarf galaxies. The OH and BD populations have apparently been strongly modified by destructive tidal forces and shocks in the inner Galaxy. By comparing the properties of the three cluster subsystems, we estimate that the present population of native Galactic clusters may only represent approximately two-thirds of the original population. Several clusters with low surface brightnesses are observed to be highly flattened. We briefly speculate on the possibility that this ellipticity reflects the intrinsic flattening of dark matter mini-haloes in which these optically dim clusters might be embedded. Finally, we examine the distribution of clusters on the size (logRh) versus luminosity (MV) plane. Three objects are seen to fall well above the sharp upper envelope of the main distribution of clusters on the size–luminosity plane: ω Centauri, M54 and NGC 2419. All three of these objects have previously, and independently, been suggested to be the stripped cores of former dwarf galaxies. This suspicion is strengthened by the additional observation that the massive cluster G1 in M31 plus a number of the most luminous clusters in NGC 5128 also fall in the same region of the logRh versus MV plane. All of the latter objects have previously been suggested as the stripped cores of now defunct dwarf galaxies.
ABSTRACT We report the discovery of SMSS J160540.18−144323.1, a new ultra metal-poor halo star discovered with the SkyMapper telescope. We measure $\left\rm {Fe}/\rm {H}\right= -6.2 \pm 0.2$ (1D ...LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, $\left\rm {C}/\rm {Fe}\right = 3.9 \pm 0.2$, while other abundances are compatible with an α-enhanced solar-like pattern with $\left\rm {Ca}/\rm {Fe}\right = 0.4 \pm 0.2$, $\left\rm {Mg}/\rm {Fe}\right = 0.6 \pm 0.2$, $\left\rm {Ti}/\rm {Fe}\right = 0.8 \pm 0.2$, and no significant s- or r-process enrichment, $\left\rm {Sr}/\rm {Fe}\right \lt 0.2$ and $\left\rm {Ba}/\rm {Fe}\right \lt 1.0$ (3σ limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about $10\, \rm M_\odot$ imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly $20\, \rm M_\odot$ are incompatible with the observed abundance pattern.
ABSTRACT
We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, ...we outline our photometric selection procedures and describe the low-resolution (R ≈ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity, and metallicity (Fe/H) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to −2.0, 41 per cent have Fe/H ≤ −2.75 and only approximately seven per cent have Fe/H > −2.0 dex. The most metal-poor candidate in the sample has Fe/H < −4.75 and is notably carbon rich. Except at the lowest metallicities (Fe/H < −4), the stars observed spectroscopically are dominated by a ‘carbon-normal’ population with C/Fe1D, LTE ≤ +1 dex. Consideration of the A(C)1D, LTE versus Fe/H1D, LTE diagram suggests that the current selection process is strongly biased against stars with A(C)1D, LTE > 7.3 (predominantly CEMP-s) while any bias against stars with A(C)1D, LTE < 7.3 and C/Fe1D,LTE > +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper v-band DR1.1 photometry. We find that the metallicity distribution function of the observed sample has a power-law slope of Δ(Log N)/ΔFe/H = 1.5 ± 0.1 dex per dex for −4.0 ≤ Fe/H ≤ −2.75, but appears to drop abruptly at Fe/H ≈ −4.2, in line with previous studies.
We present the results of a wide-field imaging survey of the periphery of the Milky Way globular cluster NGC 7089 (M2). Data were obtained with MegaCam on the Magellan Clay Telescope and the Dark ...Energy Camera on the Blanco Telescope. We find that M2 is embedded in a diffuse stellar envelope extending to a radial distance of at least ∼60 arcmin (∼210 pc) – five times the nominal tidal radius of the cluster. The envelope appears nearly circular in shape, has a radial density decline well described by a power law of index γ = −2.2 ± 0.2, and contains approximately 1.6 per cent of the luminosity of the entire system. While the origin of the envelope cannot be robustly identified using the presently available data, the fact that M2 also hosts stellar populations exhibiting a broad dispersion in the abundances of both iron and a variety of neutron capture elements suggests that this object might plausibly constitute the stripped nucleus of a dwarf galaxy that was long ago accreted and destroyed by the Milky Way.
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