The distinction between star clusters and associations

In Galactic studies a distinction is made between (open) star clusters and associations. For barely resolved objects at a distance of several Mpc this distinction is not trivial to make. Here we provide an objective definition by comparing the age of the stars to the crossing time of nearby stellar agglomerates. We find that a satisfactory separation can be made where this ratio equals unity. Stellar agglomerates for which the age of the stars exceeds the crossing time are bound, and are referred to as star clusters. Alternatively, those for which the crossing time exceeds the stellar age are unbound and are referred to as associations. This definition is useful whenever reliable measurements for the mass, radius and age are available.Comment: 2 pages, 2 figures, accepted for MNRAS Letter

High Performance Direct Gravitational N-body Simulations on Graphics Processing Units -- II: An implementation in CUDA

We present the results of gravitational direct $N$-body simulations using the Graphics Processing Unit (GPU) on a commercial NVIDIA GeForce 8800GTX designed for gaming computers. The force evaluation of the $N$-body problem is implemented in ``Compute Unified Device Architecture'' (CUDA) using the GPU to speed-up the calculations. We tested the implementation on three different $N$-body codes: two direct $N$-body integration codes, using the 4th order predictor-corrector Hermite integrator with block time-steps, and one Barnes-Hut treecode, which uses a 2nd order leapfrog integration scheme. The integration of the equations of motions for all codes is performed on the host CPU. We find that for $N > 512$ particles the GPU outperforms the GRAPE-6Af, if some softening in the force calculation is accepted. Without softening and for very small integration time steps the GRAPE still outperforms the GPU. We conclude that modern GPUs offer an attractive alternative to GRAPE-6Af special purpose hardware. Using the same time-step criterion, the total energy of the $N$-body system was conserved better than to one in $10^6$ on the GPU, only about an order of magnitude worse than obtained with GRAPE-6Af. For N \apgt 10^5 the 8800GTX outperforms the host CPU by a factor of about 100 and runs at about the same speed as the GRAPE-6Af.Comment: Accepted for publication in New Astronom

Mutation supply and the repeatability of selection for antibiotic resistance

Whether evolution can be predicted is a key question in evolutionary biology. Here we set out to better understand the repeatability of evolution. We explored experimentally the effect of mutation supply and the strength of selective pressure on the repeatability of selection from standing genetic variation. Different sizes of mutant libraries of an antibiotic resistance gene, TEM-1 $\beta$-lactamase in Escherichia coli, were subjected to different antibiotic concentrations. We determined whether populations went extinct or survived, and sequenced the TEM gene of the surviving populations. The distribution of mutations per allele in our mutant libraries- generated by error-prone PCR- followed a Poisson distribution. Extinction patterns could be explained by a simple stochastic model that assumed the sampling of beneficial mutations was key for survival. In most surviving populations, alleles containing at least one known large-effect beneficial mutation were present. These genotype data also support a model which only invokes sampling effects to describe the occurrence of alleles containing large-effect driver mutations. Hence, evolution is largely predictable given cursory knowledge of mutational fitness effects, the mutation rate and population size. There were no clear trends in the repeatability of selected mutants when we considered all mutations present. However, when only known large-effect mutations were considered, the outcome of selection is less repeatable for large libraries, in contrast to expectations. Furthermore, we show experimentally that alleles carrying multiple mutations selected from large libraries confer higher resistance levels relative to alleles with only a known large-effect mutation, suggesting that the scarcity of high-resistance alleles carrying multiple mutations may contribute to the decrease in repeatability at large library sizes.Comment: 31pages, 9 figure

Young massive star clusters

Young massive clusters are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few young massive clusters that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. Young massive clusters are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in young massive clusters, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review we focus on star clusters younger than $\sim100$ Myr, more than a few current crossing times old, and more massive than $\sim10^4$ \Msun, irrespective of cluster size or environment. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.Comment: Only 88 pages. To be published in ARAA. Final version to be submitted on Friday 12 Februar

Mass segregation in young star clusters: can it be detected from the integrated photometric properties?

We consider the effect of mass segregation on the observable integrated properties of star clusters. The measurable properties depend on a combination of the dynamical age of the cluster and the physical age of the stars in the cluster. To investigate all possible combinations of these two quantities we propose an analytical model for the mass function of segregated star clusters that agrees with the results of N-body simulations, in which any combination can be specified. For a realistic degree of mass segregation and a fixed density profile we find with increasing age an increase in the measured core radii and a central surface brightness that decreases in all filters more rapidly than what is expected from stellar evolution alone. Within a Gyr the measured core radius increases by a factor of two and the central surface density in all filters of a segregated cluster will be overestimated by a similar factor when not taking into account mass segregation in the conversion from light to mass. We find that the $V-I$ colour of mass segregated clusters decreases with radius by about 0.1-0.2 mag, which could be observable. From recent observations of partially resolved extra-galactic clusters a decreasing half-light radius with increasing wavelength was observed, which was attributed to mass segregation. These observations can not be reproduced by our models. We find that the differences between measured radii in different filters are always smaller than 5%.Comment: 8 pages, 4 figures, accepted by MNRAS Main Journa

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

N-Body Simulations of Compact Young Clusters near the Galactic Center

We investigate the dynamical evolution of compact young star clusters (CYCs) near the Galactic center (GC) using Aarseth's Nbody6 codes. The relatively small number of stars in the cluster (5,000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee (1999) have made a survey of cluster lifetimes for various initial conditions, and have found that clusters with a mass <~ 2x10^4 Msun evaporate in ~10 Myr. These results were, however, to be confirmed by N-body simulations because some extreme cluster conditions, such as strong tidal forces and a large stellar mass range participating in the dynamical evolution, might violate assumptions made in F-P models. Here we find that, in most cases, the CYC lifetimes of previous F-P calculations are 5-30% shorter than those from the present N-body simulations. The comparison of projected number density profiles and stellar mass functions between N-body simulations and HST/NICMOS observations by Figer et al. (1999) suggests that the current tidal radius of the Arches cluster is ~1.0 pc, and the following parameters for the initial conditions of that cluster: total mass of 2x10^4 Msun and mass function slope for intermediate-to-massive stars of 1.75 (the Salpeter function has 2.35). We also find that the lower stellar mass limit, the presence of primordial binaries, the amount of initial mass segregation, and the choice of initial density profile (King or Plummer models) do not significantly affect the dynamical evolution of CYCs.Comment: 20 pages including 6 figures, To appear in ApJ, Dec 20 issu

A Viral Protein Mediates Superinfection Exclusion at the Whole-Organism Level but Is Not Required for Exclusion at the Cellular Level

Superinfection exclusion (SIE), the ability of an established virus infection to interfere with a secondary infection by the same or a closely related virus, has been described for different viruses, including important pathogens of humans, animals, and plants. Citrus tristeza virus (CTV), a positive-sense RNA virus, represents a valuable model system for studying SIE due to the existence of several phylogenetically distinct strains. Furthermore, CTV allows SIE to be examined at the whole-organism level. Previously, we demonstrated that SIE by CTV is a virus-controlled function that requires the viral protein p33. In this study, we show that p33 mediates SIE at the whole-organism level, while it is not required for exclusion at the cellular level. Primary infection of a host with a fluorescent protein-tagged CTV variant lacking p33 did not interfere with the establishment of a secondary infection by the same virus labeled with a different fluorescent protein. However, cellular coinfection by both viruses was rare. The obtained observations, along with estimates of the cellular multiplicity of infection (MOI) and MOI model selection, suggested that low levels of cellular coinfection appear to be best explained by exclusion at the cellular level. Based on these results, we propose that SIE by CTV is operated at two levels-the cellular and the whole-organism levels-by two distinct mechanisms that could function independently. This novel aspect of viral SIE highlights the intriguing complexity of this phenomenon, further understanding of which may open up new avenues to manage virus diseases.Peer reviewe

Estimation of the in vivo recombination rate for a plant RNA virus

[EN] Phylogenomic evidence suggested that recombination is an important evolutionary force for potyviruses, one of the larger families of plant RNA viruses. However, mixed-genotype potyvirus infections are marked by low levels of cellular coinfection, precluding template switching and recombination events between virus genotypes during genomic RNA replication. To reconcile these conflicting observations, we evaluated the in vivo recombination rate (r(g)) of Tobacco etch virus (TEV; genus Potyvirus, family Potyviridae) by coinfecting plants with pairs of genotypes marked with engineered restriction sites as neutral markers. The recombination rate was then estimated using two different approaches: (i) a classical approach that assumed recombination between marked genotypes can occur in the whole virus population, rendering an estimate of r(g)=7.762x10(-8) recombination events per nucleotide site per generation, and (ii) an alternative method that assumed recombination between marked genotypes can occur only in coinfected cells, rendering a much higher estimate of r(g)=3.427x10(-8) recombination events per nucleotide site per generation. This last estimate is similar to the TEV mutation rate, suggesting that recombination should be at least as important as point mutation in creating variability. Finally, we compared our mutation and recombination rate estimates to those reported for animal RNA viruses. Our analysis suggested that high recombination rates may be an unavoidable consequence of selection for fast replication at the cost of low fidelity.We thank Francisca de la Iglesia and Angels Prosper for excellent technical assistance, Jose A. Dare's for methodological advice, Jose M. Cuevas for critical reading of the manuscript, and other lab members for helpful discussions. This work was supported by the Spanish Secretaria de Estado de Investigacion, Desarrollo e Innovacion (grants BFU2009-06993 and BFU2012-30805). N. T. was supported,by a pre-doctoral fellowship from the former Spanish Ministerio de Ciencia e Innovacion.Tromas, N.; Zwart, MP.; Poulain, M.; Elena Fito, SF. (2014). Estimation of the in vivo recombination rate for a plant RNA virus. Journal of General Virology. 95:724-732. https://doi.org/10.1099/vir.0.060822-0S7247329

Onset of virus systemic infection in plants is determined by speed of cell-to-cell movement and number of primary infection foci

The cornerstone of today's plant virology consists of deciphering the molecular and mechanistic basis of host-pathogen interactions. Among these interactions, the onset of systemic infection is a fundamental variable in studying both within-and between-host infection dynamics, with implications in epidemiology. Here, we developed a mechanistic model using probabilistic and spatio-temporal concepts to explain dynamic signatures of virus systemic infection. The model dealt with the inherent characteristic of plant viruses to use two different and sequential stages for their within-host propagation: cell-to-cell movement from the initial infected cell and systemic spread by reaching the vascular system. We identified the speed of cell-to-cell movement and the number of primary infection foci in the inoculated leaf as the key factors governing this dynamic process. Our results allowed us to quantitatively understand the timing of the onset of systemic infection, describing this global process as a consequence of local spread of viral populations. Finally, we considered the significance of our predictions for the evolution of plant RNA viruses.This work was supported by the grant no. BFU2012-30805 from Spain Ministerio de Economia y Competitividad (MINECO) to S. F. E. G. R. was supported by an EMBO long-term fellowship co-funded by Marie Curie actions (ALTF-1177-2011) and an AXA post-doctoral fellowship, and M.P.Z. by a Juan de la Cierva post-doctoral contract (JCI-2011-10379) from MINECO.Rodrigo Tarrega, G.; Zwart, MP.; Elena Fito, SF. (2014). Onset of virus systemic infection in plants is determined by speed of cell-to-cell movement and number of primary infection foci. Interface. 11(98):1-8. https://doi.org/10.1098/rsif.2014.0555S181198Waigmann, E., Ueki, S., Trutnyeva, K., & Citovsky, V. (2004). The Ins and Outs of Nondestructive Cell-to-Cell and Systemic Movement of Plant Viruses. Critical Reviews in Plant Sciences, 23(3), 195-250. doi:10.1080/07352680490452807Waterhouse, P. M., Wang, M.-B., & Lough, T. (2001). Gene silencing as an adaptive defence against viruses. Nature, 411(6839), 834-842. doi:10.1038/35081168Dunoyer, P., Lecellier, C.-H., Parizotto, E. A., Himber, C., & Voinnet, O. (2004). RETRACTED: Probing the MicroRNA and Small Interfering RNA Pathways with Virus-Encoded Suppressors of RNA Silencing. The Plant Cell, 16(5), 1235-1250. doi:10.1105/tpc.020719Kermack, W. O., & McKendrick, A. G. (1927). A Contribution to the Mathematical Theory of Epidemics. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 115(772), 700-721. doi:10.1098/rspa.1927.0118Segarra, J., Jeger, M. J., & van den Bosch, F. (2001). Epidemic Dynamics and Patterns of Plant Diseases. Phytopathology, 91(10), 1001-1010. doi:10.1094/phyto.2001.91.10.1001Keeling, M. (2005). The implications of network structure for epidemic dynamics. Theoretical Population Biology, 67(1), 1-8. doi:10.1016/j.tpb.2004.08.002Dolja, V. V., McBride, H. J., & Carrington, J. C. (1992). Tagging of plant potyvirus replication and movement by insertion of beta-glucuronidase into the viral polyprotein. Proceedings of the National Academy of Sciences, 89(21), 10208-10212. doi:10.1073/pnas.89.21.10208Zwart, M. P., Daròs, J.-A., & Elena, S. F. (2011). One Is Enough: In Vivo Effective Population Size Is Dose-Dependent for a Plant RNA Virus. PLoS Pathogens, 7(7), e1002122. doi:10.1371/journal.ppat.1002122Bedoya, L. C., Martínez, F., Orzáez, D., & Daròs, J.-A. (2012). Visual Tracking of Plant Virus Infection and Movement Using a Reporter MYB Transcription Factor That Activates Anthocyanin Biosynthesis. Plant Physiology, 158(3), 1130-1138. doi:10.1104/pp.111.192922Lafforgue, G., Tromas, N., Elena, S. F., & Zwart, M. P. (2012). Dynamics of the Establishment of Systemic Potyvirus Infection: Independent yet Cumulative Action of Primary Infection Sites. Journal of Virology, 86(23), 12912-12922. doi:10.1128/jvi.02207-12Holmes, F. O. (1929). Local Lesions in Tobacco Mosaic. Botanical Gazette, 87(1), 39-55. doi:10.1086/333923BALD, J. G. (1937). THE USE OF NUMBERS OF INFECTIONS FOR COMPARING THE CONCENTRATION OF PLANT VIRUS SUSPENSIONS: DILUTION EXPERIMENTS WITH PURIFIED SUSPENSIONS. Annals of Applied Biology, 24(1), 33-55. doi:10.1111/j.1744-7348.1937.tb05019.xBaulcombe, D. (2004). RNA silencing in plants. Nature, 431(7006), 356-363. doi:10.1038/nature02874Kunkel, B. N., & Brooks, D. M. (2002). Cross talk between signaling pathways in pathogen defense. Current Opinion in Plant Biology, 5(4), 325-331. doi:10.1016/s1369-5266(02)00275-3Kørner, C. J., Klauser, D., Niehl, A., Domínguez-Ferreras, A., Chinchilla, D., Boller, T., … Hann, D. R. (2013). The Immunity Regulator BAK1 Contributes to Resistance Against Diverse RNA Viruses. Molecular Plant-Microbe Interactions, 26(11), 1271-1280. doi:10.1094/mpmi-06-13-0179-rRodrigo, G., Carrera, J., Jaramillo, A., & Elena, S. F. (2010). Optimal viral strategies for bypassing RNA silencing. Journal of The Royal Society Interface, 8(55), 257-268. doi:10.1098/rsif.2010.0264Kleczkowski, A. (1950). Interpreting Relationships between the Concentrations of Plant Viruses and Numbers of Local Lesions. Journal of General Microbiology, 4(1), 53-69. doi:10.1099/00221287-4-1-53Van der Plank, J. E. (1965). Dynamics of Epidemics of Plant Disease: Population bursts of fungi, bacteria, or viruses in field and forest make an interesting dynamical study. Science, 147(3654), 120-124. doi:10.1126/science.147.3654.120Zwart, M. P., Daròs, J.-A., & Elena, S. F. (2012). Effects of Potyvirus Effective Population Size in Inoculated Leaves on Viral Accumulation and the Onset of Symptoms. Journal of Virology, 86(18), 9737-9747. doi:10.1128/jvi.00909-12Carrington, J. C., Kasschau, K. D., Mahajan, S. K., & Schaad, M. C. (1996). Cell-to-Cell and Long-Distance Transport of Viruses in Plants. The Plant Cell, 1669-1681. doi:10.1105/tpc.8.10.1669Gibbs, A. (1976). Viruses and Plasmodesmata. Intercellular Communication in Plants: Studies on Plasmodesmata, 149-164. doi:10.1007/978-3-642-66294-2_8Hillung, J., Elena, S. F., & Cuevas, J. M. (2013). Intra-specific variability and biological relevance of P3N-PIPO protein length in potyviruses. BMC Evolutionary Biology, 13(1), 249. doi:10.1186/1471-2148-13-249Dengler, N., & Kang, J. (2001). Vascular patterning and leaf shape. Current Opinion in Plant Biology, 4(1), 50-56. doi:10.1016/s1369-5266(00)00135-7SAMUEL, G. (1934). The Movement of Tobacco Mosaic Virus Within the Plant. Annals of Applied Biology, 21(1), 90-111. doi:10.1111/j.1744-7348.1934.tb06891.xKawakami, S., Watanabe, Y., & Beachy, R. N. (2004). Tobacco mosaic virus infection spreads cell to cell as intact replication complexes. Proceedings of the National Academy of Sciences, 101(16), 6291-6296. doi:10.1073/pnas.0401221101Bedoya, L., Martínez, F., Rubio, L., & Daròs, J.-A. (2010). Simultaneous equimolar expression of multiple proteins in plants from a disarmed potyvirus vector. Journal of Biotechnology, 150(2), 268-275. doi:10.1016/j.jbiotec.2010.08.006Wei, T., Zhang, C., Hong, J., Xiong, R., Kasschau, K. D., Zhou, X., … Wang, A. (2010). Formation of Complexes at Plasmodesmata for Potyvirus Intercellular Movement Is Mediated by the Viral Protein P3N-PIPO. PLoS Pathogens, 6(6), e1000962. doi:10.1371/journal.ppat.1000962Bragard, C., Caciagli, P., Lemaire, O., Lopez-Moya, J. J., MacFarlane, S., Peters, D., … Torrance, L. (2013). Status and Prospects of Plant Virus Control Through Interference with Vector Transmission. Annual Review of Phytopathology, 51(1), 177-201. doi:10.1146/annurev-phyto-082712-102346Sacristan, S., Diaz, M., Fraile, A., & Garcia-Arenal, F. (2011). Contact Transmission of Tobacco Mosaic Virus: a Quantitative Analysis of Parameters Relevant for Virus Evolution. Journal of Virology, 85(10), 4974-4981. doi:10.1128/jvi.00057-11Sanchez-Navarro, J. A., Zwart, M. P., & Elena, S. F. (2013). Effects of the Number of Genome Segments on Primary and Systemic Infections with a Multipartite Plant RNA Virus. Journal of Virology, 87(19), 10805-10815. doi:10.1128/jvi.01402-1