arXiv subjects

Bruno V. Castilho

Publications and source records attributed to Bruno V. Castilho.

3 recordsLinked to original sources

arXiv2022

CUBES Phase A design overview -- The Cassegrain U-Band Efficient Spectrograph for the Very Large Telescope

We present the baseline conceptual design of the Cassegrain U-Band Efficient Spectrograph (CUBES) for the Very Large Telescope. CUBES will provide unprecedented sensitivity for spectroscopy on a 8 - 10 m class telescope in the ground ultraviolet (UV), spanning a bandwidth of > 100 nm that starts at 300 nm, the shortest wavelength accessible from the ground. The design has been optimized for end-to-end efficiency and provides a spectral resolving power of R > 20000, that will unlock a broad range of new topics across solar system, Galactic and extraglactic astronomy. The design also features a second, lower-resolution (R \sim 7000) mode and has the option of a fiberlink to the UVES instrument for simultaneous observations at longer wavelengths. Here we present the optical, mechanical and software design of the various subsystems of the instrument after the Phase A study of the project. We discuss the expected performances for the layout choices and highlight some of the performance trade-offs considered to best meet the instrument top-level requirements. We also introduce the model-based system engineering approach used to organize and manage the project activities and interfaces, in the context that it is increasingly necessary to integrate such tools in the development of complex astronomical projects.

astro-ph.IM↗

arXiv2002

Population Synthesis in the Blue I. Synthesis of the Integrated Spectrum of 47 Tucanae from its Color-Magnitude Diagram

We perform an empirical synthesis of the blue integrated spectrum of the metal-rich globular cluster 47 Tucanae, based directly on the color-magnitude diagram of the cluster coupled to a moderately high-resolution spectral library. Freed from any significant dependence on theory, we are able to perform a fundamental test of the adequacy of the spectral library and its associated stellar parameters. Excellent fits are achieved for almost all absorption-line indices studied, provided the computations are corrected for two limitations of the spectral library, namely, the lack of a representative set of metal-poor giants and the absence of CN-strong stars. The latter effect is corrected by means of spectrum synthesis from model photospheres, considering the abundance pattern of CN-strong and CN-normal stars. We also need to perform a slight correction of the metallicity of the cluster (by --0.05 dex in relation to the standard value [Fe/H]=-0.7) in order to match the metal-line index measurements in the cluster spectrum. After these relatively small adjustments, the overall spectral agreement is good. Good fits are achieved for Hbeta, Hgamma, Mgb, , Ca4227 and Fe4383, and only Hdelta_F is overpredicted. Thus, ages inferred from Hdelta_F are slightly older than the ages based on the other Balmer lines, by ~ 3 Gyrs. The success of this exercise suggests that previous failures to synthesize the spectrum of 47 Tuc must have arisen from inadequacies in the theoretical evolutionary isochrones and/or luminosity functions. Such a possibility is considered in a companion paper.

astro-ph↗

arXiv2002

Population Synthesis in the Blue II. The Spectroscopic Age of 47 Tucanae

We develop a new set of models for intermediate-metallicity single stellar populations in the blue/optical region and use those models to determine the spectroscopic age of 47 Tuc. The models are based on a moderately high-resolution (1.8 A FWHM) empirical spectral library, state-of-the-art theoretical isochrones from M. Salaris and the most recent set from the Padova group, and new semi-empirical calibrations between fundamental stellar parameters and observables. We highlight the importance of correctly modeling the luminosity function (LF) of the cluster at the level of the giant branch, in order to achieve a good reproduction of the integrated spectrum; agreement between the spectroscopic age and the age based on the cluster's color-magnitude diagram (CMD) is achieved only if the observed LF is used rather than the theoretical ones, which either do not include AGB stars (Salaris) or underpredict the total number counts of bright giants in the cluster by a factor of two(Padova). After all corrections are made, the CMD and the spectroscopic ages (from Hgamma and Hbeta) are in close agreement: 11--12 Gyrs for Salaris isochrones and about 13 Gyrs for Padova. The difference between the model ages is due to the inclusion of atomic diffusion in the Salaris models. Previously older spectroscopic ages were due to the underestimate of the number of red giants and/or the use of isochrones that neglected the effects of He-diffusion and alpha-enhancement. A similar underestimate in the bright giant LF, if it exists in current super-solar metallicity models, would cause spectroscopic ages of elliptical galaxies inferred from such models to be too high by approximately 30%.

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