Simple Stellar Population Models as probed by the Large Magellanic Cloud Star Cluster ESO 121-SC03

The presence of blue straggler stars (BSs) in star clusters has proven a challenge to conventional simple stellar population (SSP) models. Conventional SSP models are based on the evolution theory of single stars. Meanwhile, the typical locations of BSs in the colour-magnitude diagram of a cluster are brighter and bluer than the main sequence turn-off point. Such loci cannot be predicted by single-star evolution theory. However, stars with such properties contribute significantly to the integrated light of the cluster. In this paper, we reconstruct the integrated properties of the Large Magellanic Cloud cluster ESO 121-SC03, based on a detailed exploration of the individual cluster stars, and with particular emphasis on the cluster's BSs. We find that the integrated light properties of ESO 121-SC03 are dramatically modified by its BS component. The integrated spectral energy distribution (ISED) flux level is significantly enhanced toward shorter wavelengths, and all broad-band colours become bluer. When fitting the fully integrated ISED of this cluster based on conventional SSP models, the best-fitting values of age and metallicity are significantly underestimated compared to the true cluster parameters. The age underestimate is $\sim40$ per cent if we only include the BSs within the cluster's half-light radius and $\sim60$ per cent if all BSs are included. The corresponding underestimates of the cluster's metallicity are $\sim30$ and $\sim60$ per cent, respectively. The populous star clusters in the Magellanic Clouds are ideal objects to explore the potential importance of BSs for the integrated light properties of more distant unresolved star clusters in a statistically robust manner, since they cover a large range in age and metallicity.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA

The Astrophysical Multipurpose Software Environment

We present the open source Astrophysical Multi-purpose Software Environment (AMUSE, www.amusecode.org), a component library for performing astrophysical simulations involving different physical domains and scales. It couples existing codes within a Python framework based on a communication layer using MPI. The interfaces are standardized for each domain and their implementation based on MPI guarantees that the whole framework is well-suited for distributed computation. It includes facilities for unit handling and data storage. Currently it includes codes for gravitational dynamics, stellar evolution, hydrodynamics and radiative transfer. Within each domain the interfaces to the codes are as similar as possible. We describe the design and implementation of AMUSE, as well as the main components and community codes currently supported and we discuss the code interactions facilitated by the framework. Additionally, we demonstrate how AMUSE can be used to resolve complex astrophysical problems by presenting example applications.Comment: 23 pages, 25 figures, accepted for A&

Photoinduced Halogen-Atom Transfer by N-Heterocyclic Carbene-Ligated Boryl Radicals for C(sp³)-C(sp³) Bond Formation

Herein, we present a comprehensive study on the use of N-heterocyclic carbene (NHC)-ligated boryl radicals to enable C(sp3)–C(sp3) bond formation under visible-light irradiation via Halogen-Atom Transfer (XAT). The methodology relies on the use of an acridinium dye to generate the boron-centered radicals from the corresponding NHC-ligated boranes via single-electron transfer (SET) and deprotonation. These boryl radicals subsequently engage with alkyl halides in an XAT step, delivering the desired nucleophilic alkyl radicals. The present XAT strategy is very mild and accommodates a broad scope of alkyl halides, including medicinally relevant compounds and biologically active molecules. The key role of NHC-ligated boryl radicals in the operative reaction mechanism has been elucidated through a combination of experimental, spectroscopic, and computational studies. This methodology stands as a significant advancement in the chemistry of NHC-ligated boryl radicals, which had long been restricted to radical reductions, enabling C–C bond formation under visible-light photoredox conditions

A Neutron Star with a Massive Progenitor in Westerlund 1

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2-455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of Pdot<2e-10 s/s, which implies that the spin-down luminosity is Edot<3e33 erg/s. The X-ray luminosity of the pulsar is L_X = 3(+10,-2)e33 (D/5 kpc)^2 erg/s, and the spectrum can be described by a kT = 0.61+/-0.02 keV blackbody with a radius of R_bb = 0.27+/-0.03 (D/5 kpc}) km. Deep infrared observations reveal no counterpart with K1 Msun. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2-455216 only 1.6' from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses M_i=35 Msun and >50 post-main-sequence stars that indicate the cluster is 4+/-1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i>40 Msun. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.Comment: 4 pages, 5 figures. Final version to match ApJL (added one figure since v2

A log-quadratic relation for predicting supermassive black hole masses from the host bulge Sersic index

We reinvestigate the correlation between black hole mass and bulge concentration. With an increased galaxy sample, updated estimates of galaxy distances, black hole masses, and Sersic indices `n' - a measure of concentration - we perform a least-squares regression analysis to obtain a relation suitable for the purpose of predicting black hole masses in other galaxies. In addition to the linear relation, log(M_bh) = 7.81(+/-0.08) + 2.69(+/-0.28)[log(n/3)] with epsilon_(intrin)=0.31 dex, we investigated the possibility of a higher order M_bh-n relation, finding the second order term in the best-fitting quadratic relation to be inconsistent with a value of zero at greater than the 99.99% confidence level. The optimal relation is given by log(M_bh) = 7.98(+/-0.09) + 3.70(+/-0.46)[log(n/3)] - 3.10(+/-0.84)[log(n/3)]^2, with epsilon_(intrin)=0.18 dex and a total absolute scatter of 0.31 dex. Extrapolating the quadratic relation, it predicts black holes with masses of ~10^3 M_sun in n=0.5 dwarf elliptical galaxies, compared to ~10^5 M_sun from the linear relation, and an upper bound on the largest black hole masses in the local universe, equal to 1.2^{+2.6}_{-0.4}x10^9 M_sun}. In addition, we show that the nuclear star clusters at the centers of low-luminosity elliptical galaxies follow an extrapolation of the same quadratic relation. Moreover, we speculate that the merger of two such nucleated galaxies, accompanied by the merger and runaway collision of their central star clusters, may result in the late-time formation of some supermassive black holes. Finally, we predict the existence of, and provide equations for, a relation between M_bh and the central surface brightness of the host bulge

An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters

We present a simple analytical description of the disruption of star clusters in a tidal field, which agrees excellently with detailed N-body simulations. The analytic expression can be used to predict the mass and age histograms of surviving clusters for any cluster initial mass function and any cluster formation history. The method is applied to open clusters in the solar neighbourhood, based on the new cluster sample of Kharchenko et al. From a comparison between the observed and predicted age distributions in the age range between 10 Myr to 3 Gyr we find the following results: (1) The disruption time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr. This is a factor 5 shorter than derived from N-body simulations of clusters in the tidal field of the galaxy. (2) The present starformation rate in bound clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7 10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher than before and after. (4) The present star formation rate in bound clusters is half as small as that derived from the study of embedded clusters. The difference suggests that half of the clusters in the solar neighbourhood become unbound within 10 Myr. (5) The most massive clusters within 600 pc had an initial mass of 3*10^4 M_sun. This is in agreement with the statistically expected value based on a cluster initial mass function with a slope of -2, even if the physical upper mass limit is as high as 10^6 M_sun.Comment: 14 pages, 15 figures, to appear in Astronomy & Astrophysic

Empirical evidence for definitions of episode, remission, recovery, relapse and recurrence in depression:a systematic review

AIMS: For the past quarter of a century, Frank et al.'s (1991) consensus-based definitions of major depressive disorder (MDD) episode, remission, recovery, relapse and recurrence have been the paramount driving forces for consistency in MDD research as well as in clinical practice. This study aims to review the evidence for the empirical validation of Frank et al.'s proposed concept definitions and to discuss evidence-based modifications. METHODS: A literature search of Web of Science and PubMed from 1/1/1991 to 08/30/2017 identified all publications which referenced Frank et al.'s request for definition validation. Publications with data relevant for validation were included and checked for referencing other studies providing such data. RESULTS: A total of 56 studies involving 39 315 subjects were included, mainly presenting data to validate the severity and duration thresholds for defining remission and recovery. Most studies indicated that the severity threshold for defining remission should decrease. Additionally, specific duration thresholds to separate remission from recovery did not add any predictive value to the notion that increased remission duration alleviates the risk of reoccurrence of depressive symptoms. Only limited data were available to validate the severity and duration criteria for defining a depressive episode. CONCLUSIONS: Remission can best be defined as a less symptomatic state than previously assumed (Hamilton Rating Scale for Depression, 17-item version (HAMD-17) ⩽4 instead of ⩽7), without applying a duration criterion. Duration thresholds to separate remission from recovery are not meaningful. The minimal duration of depressive symptoms to define a depressive episode should be longer than 2 weeks, although further studies are required to recommend an exact duration threshold. These results are relevant for researchers and clinicians aiming to use evidence-based depression outcomes

Disruption time scales of star clusters in different galaxies

The observed average lifetime of the population of star clusters in the Solar Neighbourhood, the Small Magellanic Cloud and in selected regions of M51 and M33 is compared with simple theoretical predictions and with the results of N-body simulations. The empirically derived lifetimes (or disruption times) of star clusters depend on their initial mass as t_dis ~ Mcl^0.60 in all four galaxies. N-body simulations have shown that the predicted disruption time of clusters in a tidal field scales as t_dis^pred ~ t_rh^0.75 t_cr^0.25, where t_rh is the initial half-mass relaxation time and t_cr is the crossing time for a cluster in equilibrium. We show that this can be approximated accurately by t_dis^pred ~ M_cl^0.62 for clusters in the mass range of about 10^3 to 10^6 M_sun, in excellent agreement with the observations. Observations of clusters in different extragalactic environments show that t_dis also depends on the ambient density in the galaxies where the clusters reside. Linear analysis predicts that the disruption time will depend on the ambient density of the cluster environment as t_dis ~ rho_amb^-0.5. This relation is consistent with N-body simulations.Comment: 7 pages, 4 figures. Accepted for publication in A&

The photometric evolution of dissolving star clusters I: First predictions

We calculated the broad-band photometric evolution of unresolved star clusters, including the preferential loss of low-mass stars due to mass segregation. The stellar mass function of a cluster evolves due to three effects: (a) the evolution of massive stars; (b) early tidal effects reduce the mass function independently of the stellar mass; (c) after mass segregation has completed, tidal effects preferentially remove the lowest-mass stars from the cluster. Results: (1) During the first ~40% of the lifetime of a cluster the cluster simply gets fainter due to the loss of stars by tidal effects. (2) Between ~40 and ~80% of its lifetime the cluster gets bluer due to the loss of low-mass stars. This will result in an underestimate of the age of clusters if standard cluster evolution models are used (0.15 -- 0.5 dex). (3) After ~80% of the total lifetime of a cluster it will rapidly get redder. This is because stars at the low-mass end of the main sequence, which are preferentially lost, are bluer than the AGB stars that dominate the light at long wavelengths, resulting in an age overestimate. (4) Clusters with mass segregation and the preferential loss of low-mass stars evolve along almost the same tracks in colour-colour diagrams as clusters without mass segregation. Therefore it will be difficult to distinguish this effect from that due to the cluster age for unresolved clusters, unless the total lifetime of the clusters can be estimated. (5) The changes in the colour evolution of unresolved clusters due to the preferential loss of low-mass stars will affect the determination of the SFHs. (6) The preferential loss of low-mass stars might explain the presence of old (~13 Gyr) clusters in NGC 4365 which are photometrically disguised as intermediate-age clusters (2 - 5 Gyr). [Abridged]Comment: accepted for publication in A&