Sergio Simón-Díaz

Publications (59) View all

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  Available from: Sergio Simón-Díaz

  Article: CoRoT observations of O stars: diverse origins of variability

  R. Blomme, M. Briquet, P. Degroote, L. Mahy, C. Aerts, J. Cuypers, M. Godart, E. Gosset, M. Hareter, J. Montalban, [......], S. Simon-Diaz, K. Smolders, P. Ventura, M. Vuckovic, M. Auvergne, A. Baglin, F. Baudin, C. Catala, E. Michel, R. Samadi
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  ABSTRACT: Six O-type stars were observed continuously by the CoRoT satellite during a 34.3-day run. The unprecedented quality of the data allows us to detect even low-amplitude stellar pulsations in some of these stars (HD 46202 and the binaries HD 46149 and Plaskett's star). These cover both opacity-driven modes and solar-like stochastic oscillations, both of importance to the asteroseismological modelling of O stars. Additional effects can be seen in the CoRoT light curves, such as binarity and rotational modulation. Some of the hottest O-type stars (HD 46223, HD 46150 and HD 46966) are dominated by the presence of red-noise: we speculate that this is related to a sub-surface convection zone.
  09/2011;
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  Available from: Sergio Simón-Díaz

  Article: The VLT-FLAMES Tarantula Survey I: Introduction and observational overview

  C. J. Evans, W. D. Taylor, V. Henault-Brunet, H. Sana, A. de Koter, S. Simon-Diaz, G. Carraro, T. Bagnoli, N. Bastian, J. M. Bestenlehner, [......], N. Markova, F. Najarro, J. Puls, O. H. Ramirez, C. Sabin-Sanjulian, S. J. Smartt, V. E. Stroud, J. Th. van Loon, J. S. Vink, N. R. Walborn
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  ABSTRACT: The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our scientific motivations and give an overview of the survey targets, including optical and near-infrared photometry and comprehensive details of the data reduction. One of the principal objectives was to detect massive binary systems via variations in their radial velocities, thus shaping the multi-epoch observing strategy. Spectral classifications are given for the massive emission-line stars observed by the survey, including the discovery of a new Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To illustrate the diversity of objects encompassed by the survey, we investigate the spectral properties of sixteen targets identified by Gruendl & Chu from Spitzer photometry as candidate young stellar objects or stars with notable mid-infrared excesses. Detailed spectral classification and quantitative analysis of the O- and B-type stars in the VFTS sample, paying particular attention to the effects of rotational mixing and binarity, will be presented in a series of future articles to address fundamental questions in both stellar and cluster evolution.
  03/2011;
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  Available from: Sergio Simón-Díaz

  Article: Spectroscopic and physical parameters of Galactic O-type stars. I. Effects of rotation and spectral resolving power in the spectral classification of dwarfs and giants

  N. Markova, J. Puls, S. Scuderi, S. Simon-Diaz, A. Herrero
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  ABSTRACT: The modern-era spectral classification of O-stars relies on either the Walborn or the Conti-Mathys scheme. Since both of these approaches have been developed using low-quality photographic data, their application to high-quality digital data might not be straightforward and be hampered by problems and complications that have not yet been appreciated. Using high-resolution spectra obtained with the ESO/MPG 2.2\,m telescope in La Silla and following the premises of the Walborn and Conti classification schemes, we determined the spectral types and luminosity classes of 19 Galactic O-type stars and compared them to those attributed by Walborn and Mathys based on low-quality data. Our analysis reveals that the morphological spectral types assigned using high-resolution data are systematically later (by up to 1.5 subtypes) then those attributed by Walborn. By means of line-profile simulations, we show that part of this discrepancy is more likely caused by the combined effect of stellar rotation and high spectral resolution on the depth of helium lines used as spectral type indicators. In addition, we demonstrate that at least for narrow-lined stars the "rotational effect" does not disappear when the high-resolution spectra are degraded to the resolution of the Walborn standards. We also find evidence of a systematic difference between our high-resolution quantitative spectral types and those assigned by Mathys. Rotation and spectral resolution are important third parameters in the spectral classification of O-type stars. To obtain reliable spectral classes within the Walborn approach, the unknown and the standard spectra must be compared at the same resolution and \vsini. Owing to resolution effects, the Conti approach might also need to be updated.
  03/2011;
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  Available from: Sergio Simón-Díaz

  Article: The FLAMES Tarantula Survey

  N. Markova, C Evans, N. Bastian, Y. Beletsky, J. Bestenlehner, I. Brott, M. Cantiello, G. Carraro, J. Clark, P. Crowther, [......], F. Najarro, J. Puls, H. Sana, S. Simon-Diaz, S. Smartt, V. Stroud, W. Taylor, J. van Loon, J. Vink, N. Walborn
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  ABSTRACT: The Tarantula survey is an ESO Large Programme which has obtained multi-epochs spectroscopy of over 800 massive stars in the 30 Dor region in the Large Magelanic Cloud. Here we briefly describe the main drivers of the survey and the observational material derived.
  03/2011;
• Article: Asteroseismic observations of OB stars

  Peter De Cat, Katrien Uytterhoeven, Juan Gutiérrez-Soto, Pieter Degroote, Sergio Simón-Díaz
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  ABSTRACT: The region of the hot end of the main-sequence is hosting pulsating stars of different types and flavours. Pulsations are not only observed for Slowly pulsating B stars (mid to late B-type stars; high order g-modes) and β Cephei stars (early B-type stars; low order p/g-modes) but are also causing variability in Be stars and OB-supergiants. In this review we give an overview of the asteroseismic observations that are currently available for these types of stars. The first asteroseismic results were solely based on ground-based observations. Recently, the arrival of space-based data gathered by space missions like most, corot and kepler has led to important discoveries for massive stars, highlighting their excellent asteroseismic potential. We show that, despite the unprecedented precision of the space-based data, there is still a clear need for ground-based follow-up observations.
  Proceedings of the International Astronomical Union 06/2010; 6:433 - 444.