Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results

Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast-rotating objects with apparently normal surface nitrogen abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing. Methods: We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity. Results: This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way.Comment: accepted for publication by A&

Sulfur Abundances in Orion B Stars

Sulfur abundances are derived for a sample of ten B MS star members of the Orion association. The analysis is based on LTE model atmospheres and non-LTE line formation theory by means of spectrum synthesis analysis of Sii and Siii lines. The abundance distribution obtained for the Orion targets is homogeneous within the errors in the analysis: A(S)=7.15±0.05. This abundance result is in agreement with the solar value and with results for the Orion nebula. The sulfur abundances for Orion combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of −0.037±0.012 dex Kpc−

On the age heterogeneity of the Pleiades, Hyades and Sirius moving groups

We investigate the nature of the classical low-velocity structures in the local velocity field, i.e. the Pleiades, Hyades and Sirius moving groups. After using a wavelet transform to locate them in velocity space, we study their relation with the open clusters kinematically associated with them. By directly comparing the location of moving group stars in parallax space to the isochrones of the embedded clusters, we check whether, within the observational errors on the parallax, all moving group stars could originate from the on-going evaporation of the associated cluster. We conclude that, in each moving group, the fraction of stars making up the velocity-space overdensity superimposed on the background is higher than the fraction of stars compatible with the isochrone of the associated cluster. These observations thus favour a dynamical (resonant) origin for the Pleiades, Hyades and Sirius moving groups.Comment: 8 pages, 8 figures, accepted for publication in A&

The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration based on Synthetic He I lines

We derive projected rotational velocities (vsini) for a sample of 156 Galactic OB star members of 35 clusters, HII regions, and associations. The HeI lines at $\lambda\lambda$4026, 4388, and 4471A were analyzed in order to define a calibration of the synthetic HeI full-widths at half maximum versus stellar vsini. A grid of synthetic spectra of HeI line profiles was calculated in non-LTE using an extensive helium model atom and updated atomic data. The vsini's for all stars were derived using the He I FWHM calibrations but also, for those target stars with relatively sharp lines, vsini values were obtained from best fit synthetic spectra of up to 40 lines of CII, NII, OII, AlIII, MgII, SiIII, and SIII. This calibration is a useful and efficient tool for estimating the projected rotational velocities of O9-B5 main-sequence stars. The distribution of vsini for an unbiased sample of early B stars in the unbound association Cep OB2 is consistent with the distribution reported elsewhere for other unbound associations.Comment: Accepted for publication in The Astronomical Journa

Estudo do Ca I como indicador de atividade em estrelas jovens

Bolsa de Iniciação científica CNPQO objetivo deste trabalho é verificar se a linha do Ca. I, λ 4226Ǻ pode ser utilizada como indicadora de atividade assim como acontece com algumas linhas de Ca II, que são linhas indicadoras clássicas. Para tal, analisamos o Huxo relativo desta linha de absorção nos espectros de 12 estrelas T-Tauri através da técnica de análise diferencial. A análise diferencial consiste em corrigir o espectro da estrela T-Tauri devido à rotação estelar projetada, v seni, e ao velamento contínuo e comparar o espectro resultante com o de uma. estrela padrão de tipo espectral semelhante, através da diferença e da razão destes espectros. A estrela padrão utilizada neste trabalho é a VA 404, pertencente ao aglomerado das Hyades, e que tem metalicidade e fotosfera, por suposição semelhantes às das estrelas T-Tauri. A análise qualitativa dos espectros de razão mostrou que a linha λ 4226Ǻ se evidencia com uma “pseudo-emissão”, indicando forte preenchimento seletivo. Este resultado nos leva à conclusão que a linha λ 4226Ǻ do Ca I é uma indicadora de atividade estelar na região do azul. O fluxo residual desta linha apresentou correlação com o acréscimo de matéria do disco à superfície da estrela (velamento). Este resultado vem reforçar a hipótese de que a magnetosfera de uma estrela T-Tauri desempenha um papel fundamental no acréscimo de matéria à superfície da estrela. Ao mesmo tempo, sugerimos que a correlação por nós encontrada é uma forma indireta de presenciar a formação de parte da linha do Ca I nestas regiões de acréscimo

Sulfur Abundances in the Orion Association B Stars

Sulfur abundances are derived for a sample of ten B main-sequence star members of the Orion association. The analysis is based on LTE plane-parallel model atmospheres and non-LTE line formation theory by means of a self-consistent spectrum synthesis analysis of lines from two ionization states of sulfur, SII and SIII. The observations are high-resolution spectra obtained with the ARCES spectrograph at the Apache Point Observatory. The abundance distribution obtained for the Orion targets is homogeneous within the expected errors in the analysis: A(S)=7.15+/-0.05. This average abundance result is in agreement with the recommended solar value (both from modelling of the photospheres in 1-D and 3-D, and meteorites) and indicates that little, if any, chemical evolution of sulfur has taken place in the last ~4.5 billion years. The sulfur abundances of the young stars in Orion are found to agree well with results for the Orion nebulae, and place strong constraints on the amount of sulfur depletion onto grains as being very modest or nonexistent. The sulfur abundances for Orion are consistent with other measurements at a similar galactocentric radius: combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of -0.037+/-0.012 dex/kpc.Comment: Accepted for publication in The Astronomical Journa

Detection methods of binary stars with low- and intermediate-mass components

This paper reviews methods which can be used to detect binaries involving low- and intermediate-mass stars, with special emphasis on evolved systems. Besides the traditional methods involving radial-velocity or photometric monitoring, the paper discusses as well less known methods involving astrometry or maser (non-)detection. An extensive list of internet resources (mostly catalogues/databases of orbits and individual measurements) for the study of binary stars is provided at the end of the paper.Comment: 53 pages, 30 figures. v2: Eq. 7 modified. A file with full-resolution figures is available at http://www.astro.ulb.ac.be/Html/ps.html#Binaries To appear in P. Pellegrini (ed.), XII Special Courses at the National Observatory of Rio de Janeiro, AIP Conference Proceedings. After it is published, it will be found at http://link.aip.org/link/?ap

Lectures on Dark Energy and Cosmic Acceleration

The discovery ten years ago that the expansion of the Universe is accelerating put in place the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. Yet the underlying cause of cosmic acceleration remains a mystery: it could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. In these lectures, I present the observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss a few of the theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that we hope will shed light on this enigma in the coming years.Comment: 36 pages, 11 figures, based on 5 lectures given at XII Ciclo de Cursos Especiais at Observatorio Nacional, Rio de Janeiro, Brazil, 1-5 Oct. 200

Boron Abundances in Early B Dwarfs of the Galactic Open Cluster NGC 3293

New boron abundances or upper limits have been determined for 8 early-B stars in the young Galactic open cluster NGC 3293, using ultraviolet spectra obtained by the Hubble Space Telescope Cosmic Origins Spectrograph. With previous observations, there are now 18 early-B stars in this cluster with boron measurements. Six of the newly observed stars have projected rotational velocities greater than 200 km/s, allowing new constraints on rotationally driven mixing in main-sequence stars. When comparing to synthetic model populations, we find that the majority of our sample stars agree well with the predicted trends of stronger boron depletion for larger rotation and for larger mass or luminosity. Based on those, a smaller than the canonical rotational mixing efficiency,(fc = 0.0165 vs the more standard value of 0.033), appears to be required. However, our five most slowly rotating stars are not well explained by rotational mixing, and we speculate that they originate from binary mergers.Comment: 18 pages,7 figures, Submitted to AAS Journal