The extended structure of the remote cluster B514 in M31. Detection of extra-tidal stars

We present a study of the density profile of the remote M31 globular cluster B514, obtained from HST/ACS observations. Coupling the analysis of the distribution of the integrated light with star counts we can reliably follow the profile of the cluster out to r~35", corresponding to ~130pc. The profile is well fitted, out to ~15 core radii, by a King Model having C=1.65. With an estimated core radius r_c=0.38", this corresponds to a tidal radius of r_t~17" (~65pc). We find that both the light and the star counts profiles show a departure from the best fit King model for r>~8" - as a surface brightness excess at large radii, and the star counts profile shows a clear break in correspondence of the estimated tidal radius. Both features are interpreted as the signature of the presence of extratidal stars around the cluster. We also show that B514 has a half-light radius significantly larger than ordinary globular clusters of the same luminosity. In the M_V vs. log r_h plane, B514 lies in a region inhabited by peculiar clusters, like Omega Cen, G1, NGC2419 and others, as well as by the nuclei of dwarf elliptical galaxies.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy & Astrophysic

Correlations of Globular Cluster Properties: Their Interpretations and Uses

Correlations among the independently measured physical properties of globular clusters (GCs) can provide powerful tests for theoretical models and new insights into their dynamics, formation, and evolution. We review briefly some of the previous work, and present preliminary results from a comparative study of GC correlations in the Local Group galaxies. The results so far indicate that these diverse GC systems follow the same fundamental correlations, suggesting a commonality of formative and evolutionary processes which produce them.Comment: An invited review, to appear in "New Horizons in Globular Cluster Astronomy", eds. G. Piotto, G. Meylan, S.G. Djorgovski, and M. Riello, ASPCS, in press (2003). Latex file, 8 pages, 5 eps figures, style files include

Photometry of the Globular Cluster NGC 5466: Red Giants and Blue Stragglers

We present wide-field BVI photometry for about 11,500 stars in the low-metallicity cluster NGC 5466. We have detected the red giant branch bump for the first time, although it is at least 0.2 mag fainter than expected relative to the turnoff. The number of red giants (relative to main sequence turnoff stars) is in excellent agreement with stellar models from the Yonsei-Yale and Teramo groups, and slightly high compared to Victoria-Regina models. This adds to evidence that an abnormally large ratio of red giant to main-sequence stars is not correlated with cluster metallicity. We discuss theoretical predictions from different research groups and find that the inclusion or exclusion of helium diffusion and strong limit Coulomb interactions may be partly responsible. We also examine indicators of dynamical history: the mass function exponent and the blue straggler frequency. NGC 5466 has a very shallow mass function, consistent with large mass loss and recently-discovered tidal tails. The blue straggler sample is significantly more centrally concentrated than the HB or RGB stars. We see no evidence of an upturn in the blue straggler frequency at large distances from the center. Dynamical friction timescales indicate that the stragglers should be more concentrated if the cluster's present density structure has existed for most of its history. NGC 5466 also has an unusually low central density compared to clusters of similar luminosity. In spite of this, the specific frequency of blue stragglers that puts it right on the frequency -- cluster M_V relation observed for other clusters.Comment: 51 pages, 21 figures, 1 electronic table, accepted to Ap

Non-monotonic current-to-rate response function in a novel integrate-and-fire model neuron

A novel integrate-and-fire model neuron is proposed to account for a non-monotonic f-I response function, as experimentally observed. As opposed to classical forms of adaptation, the present integrate- and-fire model the spike-emission process incorporates a state - dependent inactivation that makes the probability of emitting a spike decreasing as a function of the mean depolarization level instead of the mean firing rate. \ua9 Springer-Verlag Berlin Heidelberg 2002

A New Version of Reimers' law of Mass Loss Based on a Physical Approach

We present a new semi-empirical relation for the mass loss of cool stellar winds, which so far has frequently been described by "Reimers' law". Originally, this relation was based solely on dimensional scaling arguments without any physical interpretation. In our approach, the wind is assumed to result from the spill-over of the extended chromosphere, possibly associated with the action of waves, especially Alfven waves, which are used as guidance in the derivation of the new formula. We obtain a relation akin to the original Reimers law, but which includes two new factors. They reflect how the chromospheric height depends on gravity and how the mechanical energy flux depends, mainly, on effective temperature. The new relation is tested and sensitively calibrated by modelling the blue end of the Horizontal Branch of globular clusters. The most significant difference from mass loss rates predicted by the Reimers relation is an increase by up to a factor of 3 for luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter

Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump

A Monte Carlo simulation exploring uncertainties in standard stellar evolution theory on the red giant branch of metal-poor globular clusters has been conducted. Confidence limits are derived on the absolute V-band magnitude of the bump in the red giant branch luminosity function (M_v,b) and the excess number of stars in thebump, R_b. The analysis takes into account uncertainties in the primordial helium abundance, abundance of alpha-capture elements, radiative and conductive opacities, nuclear reaction rates, neutrino energy losses, the treatments of diffusion and convection, the surface boundary conditions, and color transformations. The uncertainty in theoretical values for the red giant bump magnitude varies with metallicity between +0.13/-0.12 mag at [Fe/H] = -2.4 and +0.23/-0.21 mag at [Fe/H] = -1.0$. The dominant sources of uncertainty are the abundance of the alpha-capture elements, the mixing length, and the low-temperature opacities. The theoretical values of M_v,b are in good agreement with observations. The uncertainty in the theoretical value of R_b is +/-0.01 at all metallicities studied. The dominant sources of uncertainty are the abundance of the alpha-capture elements, the mixing length, and the high-temperature opacities. The median value of R_b varies from 0.44 at [Fe/H] = -2.4$ to 0.50 at [Fe/H] = -1.0. These theoretical values for R_b are in agreement with observations.Comment: 30 pages, 6 figures. To appear in Ap

The shape of the Red Giant Branch Bump as a diagnostic of partial mixing processes in low-mass stars

We suggest to use the shape of the Red Giant Branch (RGB) Bump in metal-rich globular clusters as a diagnostic of partial mixing processes between the base of the convective envelope and the H-burning shell. The Bump located along the differential luminosity function of cluster RGB stars is a key observable to constrain the H-profile inside these structures. In fact, standard evolutionary models that account for complete mixing in the convective unstable layers and radiative equilibrium in the innermost regions do predict that the first dredge-up lefts over a very sharp H-discontinuity at the bottom of the convective region. Interestingly enough we found that both atomic diffusion and a moderate convective overshooting at the base of the convective region marginally affects the shape of the RGB Bump in the differential Luminosity Function (LF). As a consequence, we performed several numerical experiments to estimate whether plausible assumptions concerning the smoothing of the H-discontinuity, due to the possible occurrence of extra-mixing below the convective boundary, affects the shape of the RGB Bump. We found that the difference between the shape of RGB Bump predicted by standard and by smoothed models can be detected if the H-discontinuity is smoothed over an envelope region whose thickness is equal or larger than 0.5 pressure scale heights. Finally, we briefly discuss the comparison between theoretical predictions and empirical data in metal-rich, reddening free Galactic Globular Clusters (GGCs) to constrain the sharpness of the H-profile inside RGB stars.Comment: 15 pages, 8 postscript figures, ApJ in pres

Polynuclear Complexes: Two Amino-Phenol Macrocycles Spaced by Several Linear Polyamines; Synthesis, Binding Properties, and Crystal Structure

The synthesis and characterization of the new polytopic ligands 1,14-bis(3,6,9-triaza-15-hydroxybicyclo[9.3.1]pentadeca-11,13,115-trien-6-yl)-3,6,9,12-tetraazatetradecane L1, 1,15-bis(3,6,9-triaza-15-hydroxybicyclo[9.3.1]pentadeca-11,13,115-trien-6-yl)-3,6,10,13-tetraazapentadecane L2, and 1,16-bis(3,6,9-triaza-15-hydroxybicyclo[9.3.1]pentadeca-11,13,115-trien-6-yl)-3,7,10,14-tetraazahexadecane L3, containing two equal amino-phenol macrocycles spaced by several linear tetraamines, are reported. The basicity and coordination behavior toward the Cu(II) ion were potentiometrically determined in aqueous solution at 298.1 K. All the ligands show similar acid-base properties behaving as octaprotic bases in the examined pH range (pH = 2-12). The acid protons of L1-L3 cannot be removed under the experimental conditions used; thus, the main deprotonated species obtainable in aqueous solution are the neutral ligands, having amphionic character as demonstrated by UV-vis experiments. These species are able to form mono-, di-, and trinuclear Cu(II) complexes having stoichiometry [CuL]2+, [Cu2L]4+, and [Cu3L]6+, respectively, that can lose one or two protons giving rise to [CuH-1L]+, [Cu2H-2L]2+, and [Cu3H-2L]4+. Depending on the used ligand to metal molar ratio, the mono-, di-, or trinuclear species prevail over the others in solution. Both di- and trinuclear complexes are able to add secondary ligands (such as OH-), and in some cases two Cu(II) can cooperate to stabilize themby coordinating the guest in a bridged conformation. The structure of the [Cu2L3]4+ cation was resolved by X-ray analysis of the {[Cu2L3](ClO4)4 3 3H2O}2 3 H2O crystalline complex. It shows that each Cu(II) is penta-coordinated by one phenolate oxygen, two amine functions, belonging to one macrocyclic unit, and two amine functions of the spacer; in this species the distance between the two Cu(II) is about 5.3 Å

The Age Dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump

Color-magnitude diagrams of globular clusters often exhibit a prominent horizontal branch (HB) and may also show features such as the red giant branch (RGB) bump and the asymptotic giant branch (AGB) bump. Stellar evolution theory predicts that the luminosities of these features will depend on the metallicity and age of the cluster. We calculate theoretical lines of 2 to 12 Gyr constant age RGB-bumps and AGB-bumps in the V(HB-Bump)--[Fe/H] diagram, which shows the brightness difference between the bump and the HB as a function of metallicity. In order to test the predictions, we identify giant branch bumps in new Hubble Space Telescope color-magnitude diagrams for 8 SMC clusters. First, we conclude that the SMC cluster bumps are RGB-bumps. The data for clusters younger than ~6 Gyr are in fair agreement the relative age dependent luminosities of the HB and RGB-bump. The V(HB-Bump)--[Fe/H] data for clusters older then ~6 Gyr demonstrate a less satisfactory agreement with our calculations. We conclude that ~6 Gyr is a lower bound to the age of clusters for which the Galactic globular cluster, age independent V(HB-Bump)--[Fe/H] calibration is valid. Application of the V(HB-bump)--[Fe/H] diagram to stellar population studies is discussed.Comment: Accepted for publication in the Astrophysical Journal, 30 pages, Latex aaspp4.sty, including 7 postscript figure