VLT/X-shooter spectroscopy of the GRB 120327A afterglow

We present a study of the environment of the Swift long gamma-ray burst GRB 120327A at z ~2.8 through optical spectroscopy of its afterglow. We analyzed medium-resolution, multi-epoch spectroscopic observations (~7000 - 12000, corresponding to ~ 15 - 23 km/s, S/N = 15- 30 and wavelength range 3000-25000AA) of the optical afterglow of GRB 120327A, taken with X-shooter at the VLT 2.13 and 27.65 hr after the GRB trigger. The first epoch spectrum shows that the ISM in the GRB host galaxy at z = 2.8145 is extremely rich in absorption features, with three components contributing to the line profiles. The hydrogen column density associated with GRB 120327A has log NH / cm^(-2) = 22.01 +/- 0.09, and the metallicity of the host galaxy is in the range [X/H] = -1.3 to -1.1. In addition to the ground state lines, we detect absorption features associated with excited states of CII, OI, SiII, FeII, and NiII, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The variability of the FeI\lambda2396 excited line between the two epochs proves that these features are excited by the GRB UV flux. Moreover, the distance of component I is found to be dI=200+100-60 pc, while component II is located closer to the GRB, at dII=100+40-30 pc. These values are among the lowest found in GRBs. Component III does not show excited transitions, so it should be located farther away from the GRB. The presence of H2 molecules is firmly established, with a molecular fraction in the range f=4 X 10^(-7) - 10^(-4). This particularly low value can be attributed to the small dust content. This represents the third positive detection of molecules in a GRB environment.Comment: 18 pages, 12 figures, accepted by A&

Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

Floristic and vegetation structure of a grassland plant community on shallow basalt in southern Brazil

Few studies have adequately described the floristic and structural features of natural grasslands associated with shallow basalt soils in southern Brazil. This study was carried out on natural grazing land used for livestock production in the municipality of Santana do Livramento, in the Campanha region of the state of Rio Grande do Sul, Brazil. The aim of the study was to describe the floristic and structural diversity of the area. The floristic list obtained comprises 229 plant taxa from 40 botanical families, with a predominance of the families Poaceae (62), Asteraceae (28), Fabaceae (16) and Cyperaceae (12). The estimated diversity and evenness in the community were 3.00 and 0.874, respectively. Bare soil and rock outcrops accounted for 19.3% of the area, resulting in limited forage availability. Multivariate analysis revealed two well-defined groups among the sampling units. One group showed a high degree of internal aggregation, associated with deep soils, and was characterized by the presence of tussocks, whereas the other was less aggregate and was characterized by prostrate species growing on shallow soil. Ordination analysis indicated a gradient of moisture and of soil depth in the study area, resulting in different vegetation patterns. These patterns were analogous to the vegetation physiognomies described for Uruguayan grasslands. Overall, the grassland community studied is similar to others found throughout southern Brazil, although it harbors more winter forage species. In addition, the rare grass Paspalum indecorum Mez is locally dominant in some patches, behaving similarly to P. notatum Fl., a widespread grass that dominates extensive grassland areas in southern Brazil

Variações espaço-temporais no estoque de sementes do solo na floresta amazônica

A dispersão eficiente, a longevidade e a capacidade das sementes de permanecer em estado latente a espera de condições adequadas de germinação no banco de sementes do solo da floresta garantem a presença de espécies arbóreas pioneiras nas áreas perturbadas. As variações estacionais e espaciais na densidade e na composição florística do banco de sementes em Florestas Tropicais Úmidas são assuntos ainda pouco compreendidos. Este trabalho verificou a existência de modificações espaço-temporais do banco de sementes presente em áreas de Floresta Tropical úmida localizadas próximas a Manaus, AM. Em cada uma das seis áreas estudadas, foram coletadas 40 amostras circulares de solo superficial (10 cm de diâmetro e 2 cm de profundidade) ao acaso. Essas amostras foram coletadas a cada dois meses, entre agosto/2004 e junho/2005,. As amostras de solo foram distribuídas em bandejas em casa de vegetação e a emergência das sementes presentes no solo foi acompanhada por 4 meses. Houve uma redução significativa (H: 14,09, p < 0,05) na densidade média de sementes no solo em junho (início da estação seca) em relação a fevereiro (meio da estação chuvosa). Houve também diferença significativa (H: 188,72, p < 0,05) na densidade média de sementes do solo presente nas diferentes áreas amostradas. Assim como para outras áreas de florestas tropicais, o banco de sementes permanente da floresta foi dominado por espécies pioneiras, principalmente da família Melastomataceae, enquanto as espécies típicas da Floresta Tropical madura foram raras no solo florestal

Procesamiento de imágenes, SIG y modelos ecológicos aplicados a la arqueología

Se emplea el enfoque de identificación de hábitat, con apoyo del procesamiento de imágenes y SIG, para diseñar un modelo espacial que permita predecir la ubicación de materiales arqueológicos en la Comarca Andina del Paralelo 42, Patagonia, Argentina. A partir de una imagen satelital Landsat TM se genera un mapa de cobertura de la tierra mediante clasificación supervisada. Sobre la base de un modelo general de respuestas humanas probables a diferentes ambientes (Binford, 2001) elaborado en función de datos ambientales y del análisis del comportamiento de cazadores-recolectores actuales, se reclasifica cada píxel asignando valores binarios con 1 para hábitat y 0 para no-hábitat. Con el mismo criterio se modelaron los posibles corredores utilizados por estos cazadores recolectores. El modelo espacial se valida con información acerca de sitios arqueológicos identificados en el campo

Selección de métricas de configuración espacial para la regionalización de un territorio antropizado

Los paisajes humanizados tienden a poseer elementos de formas más simples, bordes rectos y tamaños más homogéneos que los naturales y es posible diferenciarlos por su patrón espacial, el cual se cuantifica con índices de paisaje. El uso de estas métricas se ha popularizado, aunque existe bastante polémica con respecto a su significado ecológico, aplicabilidad, redundancia y sensibilidad. El descarte de unas u otras métricas con el argumento de la redundancia no es adecuado porque todas difieren en su sensibilidad a variables independientes. Se presenta un método que combina la tecnología SIG con el análisis multivariado para seleccionar las métricas según su capacidad de discriminación y generar un modelo predictivo para la clasificación de casos nuevos. El análisis discriminante redujo la cantidad de índices de 48 a 14. El modelo obtenido tiene un error medio de clasificación de 18%. Se discuten las aplicaciones y limitaciones de los índices de paisaje y del método discriminant

Procesamiento de imágenes, SIG y modelos ecológicos aplicados a la arqueología

Se emplea el enfoque de identificación de hábitat, con apoyo del procesamiento de imágenes y SIG, para diseñar un modelo espacial que permita predecir la ubicación de materiales arqueológicos en la Comarca Andina del Paralelo 42, Patagonia, Argentina. A partir de una imagen satelital Landsat TM se genera un mapa de cobertura de la tierra mediante clasificación supervisada. Sobre la base de un modelo general de respuestas humanas probables a diferentes ambientes (Binford, 2001) elaborado en función de datos ambientales y del análisis del comportamiento de cazadores-recolectores actuales, se reclasifica cada píxel asignando valores binarios con 1 para hábitat y 0 para no-hábitat. Con el mismo criterio se modelaron los posibles corredores utilizados por estos cazadores recolectores. El modelo espacial se valida con información acerca de sitios arqueológicos identificados en el campo