1,183 research outputs found

    Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle Observations of the Draco dwarf spheroidal

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    We introduce a Bayesian method for fitting faint, resolved stellar spectra in order to obtain simultaneous estimates of redshift and stellar-atmospheric parameters. We apply the method to thousands of spectra---covering 5160-5280 Angs. at resolution R~20,000---that we have acquired with the MMT/Hectochelle fibre spectrograph for red-giant and horizontal branch candidates along the line of sight to the Milky Way's dwarf spheroidal satellite in Draco. The observed stars subtend an area of ~4 deg^2, extending ~3 times beyond Draco's nominal `tidal' radius. For each spectrum we tabulate the first four moments---central value, variance, skewness and kurtosis---of posterior probability distribution functions representing estimates of the following physical parameters: line-of-sight velocity v_los, effective temperature (T_eff), surface gravity (logg) and metallicity ([Fe/H]). After rejecting low-quality measurements, we retain a new sample consisting of 2813 independent observations of 1565 unique stars, including 1879 observations for 631 stars with (as many as 13) repeat observations. Parameter estimates have median random errors of sigma_{v_los}=0.88 km/s, sigma_{T_eff}=162 K, sigma_logg=0.37 dex and sigma_[Fe/H]=0.20 dex. Our estimates of physical parameters distinguish ~470 likely Draco members from interlopers in the Galactic foreground.Comment: published in Monthly Notices of the Royal Astronomical Society, all data are publicly available at the following address: http://www.andrew.cmu.edu/user/mgwalker/hectochelle

    Massive black holes in stellar systems: 'quiescent' accretion and luminosity

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    Only a small fraction of local galaxies harbor an accreting black hole, classified as an active galactic nucleus (AGN). However, many stellar systems are plausibly expected to host black holes, from globular clusters to nuclear star clusters, to massive galaxies. The mere presence of stars in the vicinity of a black hole provides a source of fuel via mass loss of evolved stars. In this paper we assess the expected luminosities of black holes embedded in stellar systems of different sizes and properties, spanning a large range of masses. We model the distribution of stars and derive the amount of gas available to a central black hole through a geometrical model. We estimate the luminosity of the black holes under simple, but physically grounded, assumptions on the accretion flow. Finally we discuss the detectability of 'quiescent' black holes in the local Universe.Comment: ApJ in pres

    Semíramis: La influencia de la tragedia del horror en el teatro del siglo XVII

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    En el último siglo se ha producido un debate en torno a la importancia de los trágicos del fin del siglo XVI, productores de lo que se ha denominado tragedia del horror, en la concepción de la Comedia Nueva lopesca. Este trabajo trata de demostrar la influencia de este teatro no solo en Lope de Vega sino en todo el siglo XVII por medio de la comparación de dos obras que acogen un mismo tema: la reina Semíramis, símbolo de la tiranía y del peligro inherente a la belleza e inteligencia del ser humano. Estas obras son La gran Semíramis, escrita en torno a 1580 por Cristóbal de Virués y La hija del aire, obra en dos partes de Calderón de la Barca representada en 1653. Tomando a Lope de Vega como eje entre ambos autores, el presente trabajo estudiará el contexto cultural del final del siglo XVI y principios del XVII y, por medio del análisis de las dos obras, tratará de extraer los rasgos que perduraron en el teatro español hasta la época de Calderón. La tragedia del horror es un género que se origina en Valencia y que Lope de Vega debió de contemplar en su exilio de Madrid. El repaso histórico tratará de explicar la situación del teatro a finales del XVI y la manera en que este influyó al Fénix, impulsador de la Comedia Nueva y modelo vigente hasta Calderón. El análisis de las obras se hará atendiendo a los elementos que mejor muestran la evolución del teatro: al tema principal de cada una, al uso de las fuentes que cada dramaturgo hace, de forma que se constate la libertad con la que la fábula es contada, a la estructura de ambas obras y a la construcción del personaje principal, sobre el que ambas obras giran: Semíramis. De todo ello tratarán de extraerse aquellos rasgos propios del teatro finisecular que perduran en el teatro de Calderón, para así demostrar la importancia de esta práctica teatral tan habitualmente infravalorada

    Advances in the Hierarchical Emergent Behaviors (HEB) approach to autonomous vehicles

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    Widespread deployment of autonomous vehicles (AVs) presents formidable challenges in terms on handling scalability and complexity, particularly regarding vehicular reaction in the face of unforeseen corner cases. Hierarchical Emergent Behaviors (HEB) is a scalable architecture based on the concepts of emergent behaviors and hierarchical decomposition. It relies on a few simple but powerful rules to govern local vehicular interactions. Rather than requiring prescriptive programming of every possible scenario, HEB’s approach relies on global behaviors induced by the application of these local, well-understood rules. Our first two papers on HEB focused on a primal set of rules applied at the first hierarchical level. On the path to systematize a solid design methodology, this paper proposes additional rules for the second level, studies through simulations the resultant richer set of emergent behaviors, and discusses the communica-tion mechanisms between the different levels.Peer ReviewedPostprint (author's final draft

    An Extinction Map and Color Magnitude Diagram for the Globular Cluster NGC 3201

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    Differential EV−IE_{V-I} variations of up to ∼0.2\sim 0.2 mag on a scale of arcminutes across NGC 3201 are presented in the form of an extinction map. This map, created by calculating average EV−IE_{V-I} values for stars in small subregions of the field with respect to a fiducial region, greatly improves the appearance of the CMD of the cluster. We describe how we implemented this technique in detail with our data for NGC 3201. A comparison between our map and that of the same region extracted from the COBE/DIRBE reddening maps published by Schlegel, Finkbeiner, & Davis (1998) (SFD) displays larger-scale similarities between the two maps as well as smaller-scale features which show up in our map but not in the SFD map. Several methods of determining an EV−IE_{V-I} zeropoint to add to our differential extinction map are presented. Isochrone fitting proved to be the most successful one, but it produces an average EV−IE_{V-I} for the cluster which is smaller than previously published values by ∼1.5σ\sim 1.5\sigma. Finally, our results seem to support the statement by Arce & Goodman (1999) that the SFD maps overestimate the reddening in regions of high extinction.Comment: 19 pages, 12 figures, 1 table, accepted for publication in AJ (March 2001). Full resolution version may be obtained at http://www.astro.lsa.umich.edu/users/kaspar/html/ngc3201.pdf (PDF) and at http://www.astro.lsa.umich.edu/users/kaspar/html/ngc3201.ps.gz (PS
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