Gaia in-orbit realignment. Overview and data analysis

The ESA Gaia spacecraft has two Shack-Hartmann wavefront sensors (WFS) on its focal plane. They are required to refocus the telescope in-orbit due to launch settings and gravity release. They require bright stars to provide good signal to noise patterns. The centroiding precision achievable poses a limit on the minimum stellar brightness required and, ultimately, on the observing time required to reconstruct the wavefront. Maximum likelihood algorithms have been developed at the Gaia SOC. They provide optimum performance according to the Cr\'amer-Rao lower bound. Detailed wavefront reconstruction procedures, dealing with partial telescope pupil sampling and partial microlens illumination have also been developed. In this work, a brief overview of the WFS and an in depth description of the centroiding and wavefront reconstruction algorithms is provided.Comment: 14 pages, 6 figures, 2 tables, proceedings of SPIE Astronomical Telescopes + Instrumentation 2012 Conference 8442 (1-6 July 2012

Kinematics of the circumstellar gas around UXOR stars

This thesis presents the results of a high spectral resolution study of the circumstellar (CS) gas around the intermediate mass, pre-main sequence UXOR stars BF Ori, SV Cep, UX Ori, WW Vul and XY Per. The results are based on a set of 38 echelle spectra covering the spectral range 3800-5900 Angstroms, monitoring the stars on time scales of months, days and hours. All spectra show a large number of Balmer and metallic lines with variable blueshifted and redshifted absorption features superimposed to the photospheric stellar spectra. Synthetic Kurucz models are used to estimate rotational velocities, effective temperatures and gravities of the stars. The best photospheric models are subtracted from each observed spectrum to determine the variable absorption features due to the circumstellar gas; those features are characterized, via multigaussian fitting, in terms of their velocity, v, dispersion velocity, Delta_v, and residual absorption, R_max. The absorption components detected in each spectrum can be grouped by their similar radial velocities and are interpreted as the signature of the dynamical evolution of gaseous clumps. Most of the events undergo accelerations/decelerations at a rate of tenths of ms-2. The typical timescale for the duration of the events is a few days. The dispersion velocity and the relative absorption strength of the features do not show drastic changes during the lifetime of the events, which suggests that they are gaseous blobs preserving their geometrical and physical identity. See full abstract in the documentComment: 130 pages, 40 figures, PhD dissertatio

Spectroscopic properties of stars with debris discs

The question of the origin and evolution of planetary systems is of fundamental importance for astrophysics. Dusty debris discs are signatures of planetary systems and, therefore, constitute valuable tools to provide new light in our understanding of how planetary systems form and evolve. We present the first results of a spectroscopic programme of a sample of stars with debris discs. High-resolution echelle spectra are used to determine metallicities and abundances. Properties of stars with debris discs, are compared with those of stars hosting planets, as well as 'normal' starsThis work was supported by the Spanish Ministerio de Ciencia e Innovación (MICINN), Plan Nacional de Astronomía y Astrofísica, under grant AYA2008-0172

Accretion rates and accretion tracers of Herbig Ae/Be stars

Astronomy & Astrophysics 535 (2011): A99 reproduced with permission from Astronomy & AstrophysicsContext. The scarcity of accretion rate estimates and accretion tracers available for Herbig Ae/Be (HAeBe) stars contrasts with the extensive studies for lower mass objects. Aims. This work aims to derive accretion rates from the UV Balmer excess for a sample of 38 HAeBe stars. We look for possible empirical correlations with the strength of the Hα, [Oi]6300, and Brγ emission lines. Methods. Shock modelling within the context of magnetospheric accretion (MA) was applied to each star. We obtained the accretion rates from the excess in the Balmer discontinuity, derived from mean values of multi-epoch Johnson’s UB photometry. The accretion rates were related to both mean Hα luminosities, Hα 10% widths, and [Oi]6300 luminosities from simultaneous spectra, and to Brγ luminosities from the literature. Results. The typical -median- mass accretion rate is 2 × 10−7 M_ yr−1 in our sample, 36% of the stars showing values ≤10−7 M_ yr−1, 35% between 10−7 and 10−6, and 29% > 10−6 M_ yr−1. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T∗ > 12 000 K). When accretion is related to the stellar masses and luminosities (1 ≤ M∗/M_ ≤ 6; 2 ≤ L∗/L_ ≤ 103), we derive ˙Macc ∝ M5 ∗ and Lacc ∝ L1.2 ∗ , with scatter. Empirical calibrations relating the accretion and the Hα, [Oi]6300, and Brγ luminosities are provided. The slopes in our expressions are slightly shallower than those for lower mass stars, but the difference is within the uncertainties, except for the [Oi]6300 line. The Hα 10% width is uncorrelated with ˙Macc, unlike for the lower mass regime. The mean Hα width shows higher values as the projected rotational velocities of HAe stars increase, which agrees with MA. The accretion rate variations in the sample are typically lower than 0.5 dex on timescales of days to months. Our data suggest that the changes in the Balmer excess are uncorrelated to the simultaneous changes of the line luminosities. Conclusions. The Balmer excesses and Hα line widths of HAe stars can be interpreted within the context of MA, which is not the case for several HBes. The steep trend relating Macc and M∗ can be explained from the mass-age distribution characterizing HAeBe stars. The line luminosities used for low-mass objects are also valid to estimate typical accretion rates for the intermediate-mass regime under similar empirical expressions. However, we suggest that several of these calibrations are driven by the stellar luminosityC. Eiroa, I. Mendigutía, and B. Montesinos are partially supported by grant AYA-2008 01727. I. Mendigutía is grateful for financial support from the Space Telescope Science Institute during a two-month sta

Proposed instrumentation for PILOT

PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed Australian/European optical/infrared telescope for Dome C on the Antarctic Plateau, with target first light in 2012. The proposed telescope is 2.4m diameter, with overall focal ratio f/10, and a 1 degree field-of-view. In median seeing conditions, it delivers 0.3" FWHM widefield image quality, from 0.7-2.5 microns. In the best quartile of conditions, it delivers diffraction-limited imaging down to 1 micron, or even less with lucky imaging. The areas where PILOT offers the greatest advantages over existing ground-based telescopes are (a) very high resolution optical imaging, (b) high resolution wide-field optical imaging, and (c) all wide-field thermal infrared imaging. The proposed first generation instrumentation consists of (a) a fast, lownoise camera for diffraction-limited optical lucky imaging; (b) a gigapixel optical camera for seeing-limited imaging over a 1 degree field; (c) a 4K × 4K near-infrared (1-5 micron) camera with both wide-field and diffraction-limited modes; and (d) a double-beamed mid-infrared (7-40 micron) imaging spectrograph

Characterization of Hepatitis B virus (HBV) genotypes in patients from Rondônia, Brazil

<p>Abstract</p> <p>Background</p> <p>Hepatitis B virus (HBV) can be classified into nine genotypes (A-I) defined by sequence divergence of more than 8% based on the complete genome. This study aims to identify the genotypic distribution of HBV in 40 HBsAg-positive patients from Rondônia, Brazil. A fragment of 1306 bp partially comprising surface and polymerase overlapping genes was amplified by PCR. Amplified DNA was purified and sequenced. Amplified DNA was purified and sequenced on an ABI PRISM^®377 Automatic Sequencer (Applied Biosystems, Foster City, CA, USA). The obtained sequences were aligned with reference sequences obtained from the GenBank using Clustal X software and then edited with Se-Al software. Phylogenetic analyses were conducted by the Markov Chain Monte Carlo (MCMC) approach using BEAST v.1.5.3.</p> <p>Results</p> <p>The subgenotypes distribution was A1 (37.1%), D3 (22.8%), F2a (20.0%), D4 (17.1%) and D2 (2.8%).</p> <p>Conclusions</p> <p>These results for the first HBV genotypic characterization in Rondônia state are consistent with other studies in Brazil, showing the presence of several HBV genotypes that reflects the mixed origin of the population, involving descendants from Native Americans, Europeans, and Africans.</p

IAA : Información y actualidad astronómica (20)

Sumario : ¿Cómo se forma un Sol?.-- El observatorio virtual.-DECONSTRUCCIÓN Y otros ENSAYOS Estrellas peculiares.-- ACTUALIDAD.-- ENTRE BASTIDORES.-- CIENCIA: PILARES E INCERTIDUMBRES.-- HISTORIAS DE ASTRONOMÍA. Pero, ¿quién inventó el telescopio?.-- ACTIVIDADES IAA.Esta revista se publica con la ayuda de la Accion Complementaria CCT003-05-00325 del Programa Nacional de Fomento de la Cultura Cientifica y Tecnologica.N