arXiv subjects

G. Meynet

Publications and source records attributed to G. Meynet.

108 records · Page 6Linked to original sources

arXiv2004

Nucleosynthesis of 26Al in rotating Wolf-Rayet stars

The 26Al radionuclide can be detected through its decay emission line at 1.809 MeV, as was first observed by Mahoney et al. (1982). Since then, COMPTEL on board of the CGRO satellite, performed a sky survey in this energy range, and provided maps of the 26Al distribution in the Galaxy. These results revealed that the main contributors to the synthesis of 26Al are most likely the massive stars, which contribute through their winds (Wolf-Rayet stars) and through their supernova explosion. Comparison between these observations (in particular observations in localized regions such as the Vela region and the Cygnus region) and the models available at that moment, showed however the need for improvements from both theoretical and observational points of view, in order to improve our understanding of the 26Al galactic distribution as well as that of its synthesis. With the launch of the INTEGRAL satellite in October 2002, the observational part will hopefully be improved, and the construction of better resolution maps at 1.809 MeV is one of the main aims of the mission. From a theoretical point of view, we need the most up-to-date predictions in order to be able to interpret the forthcoming data. In this paper, we address this latter part, and present new results for 26Al production by rotating Wolf-Rayet stars and their contribution to the total amount observed in the Galaxy.

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arXiv2004

Stellar evolution with rotation XII: Pre-supernova models

We describe the latest developments of the Geneva stellar evolution code in order to model the pre-supernova evolution of rotating massive stars. Rotating and non-rotating stellar models at solar metallicity with masses equal to 12, 15, 20, 25, 40 and 60 solar masses were computed from the ZAMS until the end of the core silicon burning phase. We took into account meridional circulation, secular shear instabilities, horizontal turbulence and dynamical shear instabilities. Most of the differences between the pre-supernova structures obtained from rotating and non-rotating stellar models have their origin in the effects of rotation during the core hydrogen and helium burning phases. The effects of rotation on pre-supernova models are significant between 15 and 30 solar masses. Indeed, rotation increases the core sizes (and the yields) by a factor ~ 1.5. Above 20 solar masses, rotation may change the colour of the supernova progenitors (blue instead of red supergiant) and the supernova type (Ib instead of II). Rotation affects the lower mass limits for radiative core carbon burning, for iron core collapse and for black hole formation. For Wolf-Rayet stars (M > 30 solar masses), the pre-supernova structures are mostly affected by the intensities of the stellar winds and less by rotational mixing. Finally, the core of our rotating WR stars contain enough angular momentum to produce GRBs.

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arXiv2004

Chemical Abundances in the Secondary Star in the Black Hole Binary A0620-00

Using a high resolution spectrum of the secondary star in the black hole binary A0620-00, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a chi^2 minimization procedure to explore a grid of 800 000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni and Al. The Fe abundance of the star is [Fe/H]=0.14 +- 0.20. Except for Ca, we found the other elements moderately over-abundant as compared with stars in the solar neighborhood of similar iron content. Taking into account the small orbital separation, the mass transfer rate and the mass of the convection zone of the secondary star, a comparison with element yields in supernova explosion models suggests a possible explosive event with a mass cut comparable to the current mass of the compact object. We have also analyzed the Li abundance, which is unusually high for a star of this spectral type and relatively low mass.

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arXiv2004

Analysis of alpha Centauri AB including seismic constraints

Detailed models of alpha Cen A and B based on new seismological data for alpha Cen B by Carrier & Bourban (2003) have been computed using the Geneva evolution code including atomic diffusion. Taking into account the numerous observational constraints now available for the alpha Cen system, we find a stellar model which is in good agreement with the astrometric, photometric, spectroscopic and asteroseismic data. The global parameters of the alpha Cen system are now firmly constrained to an age of t=6.52+-0.30 Gyr, an initial helium mass fraction Y_i=0.275+-0.010 and an initial metallicity (Z/X)_i=0.0434+-0.0020. Thanks to these numerous observational constraints, we confirm that the mixing-length parameter alpha of the B component is larger than the one of the A component, as already suggested by many authors (Noels et al. 1991, Fernandes & Neuforge 1995 and Guenther & Demarque 2000): alpha_B is about 8% larger than alpha_A (alpha_A=1.83+-0.10 and alpha_B=1.97+-0.10). Moreover, we show that asteroseismic measurements enable to determine the radii of both stars with a very high precision (errors smaller than 0.3%). The radii deduced from seismological data are compatible with the new interferometric results of Kervella et al. (2003) even if they are slightly larger than the interferometric radii (differences smaller than 1%).

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arXiv2003

26Al yields from rotating Wolf--Rayet star models

We present new $^{26}$Al stellar yields from rotating Wolf--Rayet stellar models which, at solar metallicity, well reproduce the observed properties of the Wolf-Rayet populations. These new yields are enhanced with respect to non--rotating models, even with respect to non--rotating models computed with enhanced mass loss rates. We briefly discuss some implications of the use of these new yields for estimating the global contribution of Wolf-Rayet stars to the quantity of $^{26}$Al now present in the Milky Way.

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arXiv2003

Stellar evolution with rotation X: Wolf-Rayet star populations at solar metallicity

We examine the properties of Wolf--Rayet (WR) stars predicted by models of rotating stars taking account of the new mass loss rates for O--type stars and WR stars (Vink et al. \cite{Vink00}, \cite{Vink01}; Nugis & Lamers \cite{NuLa00}) and of the wind anisotropies induced by rotation. We find that the rotation velocities $v$ of WR stars are modest, i.e. about 50 km s$^{-1}$, not very dependant on the initial $v$ and masses. For the most massive stars, the evolution of $v$ is very strongly influenced by the values of the mass loss rates; below $\sim$12 M$_\odot$ the evolution of rotation during the MS phase and later phases is dominated by the internal coupling. Massive stars with extreme rotation may skip the LBV phase. Models having a typical $v$ for the O--type stars have WR lifetimes on the average two times longer than for non--rotating models. The increase of the WR lifetimes is mainly due to that of the H--rich eWNL phase. Rotation allows a transition WN/WC phase to be present for initial masses lower than 60 M$_\odot$. The durations of the other WR subphases are less affected by rotation. The mass threshold for forming WR stars is lowered from 37 to 22 M$_\odot$ for typical rotation. The comparisons of the predicted number ratios WR/O, WN/WC and of the number of transition WN/WC stars show very good agreement with models with rotation, while this is not the case for models with the present--day mass loss rates and no rotation. As to the chemical abundances in WR stars, rotation brings only very small changes for WN stars, since they have equilibrium CNO values. However, WC stars with rotation have on average lower C/He and O/He ratios. The luminosity distribution of WC stars is also influenced by rotation.

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arXiv2002

Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region

Gamma-ray and microwave observations of the Cygnus region reveal an intense signal of 1.809 MeV line emission, attributed to radioactive decay of 26Al, that is closely correlated with 53 GHz free-free emission, originating from the ionised interstellar medium. We modelled both emissions using a multi-wavelength evolutionary synthesis code for massive star associations that we applied to the known massive star populations in Cygnus. For all OB associations and young open clusters in the field, we determined the population age, distance, and richness as well as the uncertainties in all these quantities from published photometric and spectroscopic data. We propagate the population uncertainties in model uncertainties by means of a Bayesian method. The young globular cluster Cyg OB2 turns out to be the dominant 26Al nucleosynthesis and ionisation source in Cygnus. Our model reproduces the ionising luminosity of the Cygnus region very well, yet it underestimates 26Al production by about a factor of 2. We attribute this underestimation to shortcomings of current nucleosynthesis models, and suggest the inclusion of stellar rotation as possible mechanism to enhance 26Al production. We also modelled 60Fe nucleosynthesis in the Cygnus region, yet the small number of recent supernova events suggests only little 60Fe production. Consequently, a detection of the 1.137 MeV and 1.332 MeV decay lines of 60Fe from Cygnus by the upcoming INTEGRAL observatory is not expected.

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arXiv2002

The blue to red supergiant ratio in young clusters at various metallicities

We present new determinations of the blue to red supergiant ratio ($B/R$) in young open clusters at various metallicities. For this purpose, we examine the HR diagrams of 45 clusters in the Galaxy and of 4 clusters in the Magellanic Clouds. The identification of supergiants is based on spectroscopic measurements (with photometric counts to check the results). The new counts confirm the increase of the $B/R$ ratio when the metallicity increases with the following normalized relation : $\frac{B/R}{(B/R)_{\odot}} \cong 0.05\cdot e^{3 \frac{Z}{Z_{\odot}}}$, where $Z_{\odot}=0.02$ and $(B/R)_{\odot}$ is the value of $B/R$ at $Z_{\odot}$ which depends on the definition of $B$ and $R$ and on the age interval considered (e.g. for spectroscopic counts including clusters with $\log age$ between 6.8 and 7.5, $(B/R)_{\odot}\cong3$ when $B$ includes O, B and A supergiants).

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arXiv2001

Stellar evolution with rotation VII: Low metallicity models and the blue to red supergiant ratio in the SMC

We calculate a grid of models with and without the effects of axial rotation for massive stars in the range of 9 to 60 M$_{\odot}$ and metallicity $Z$ = 0.004 appropriate for the SMC. Remarkably, the ratios $Ω/Ω_{\mathrm{crit}}$ of the angular velocity to the break-up angular velocity grow strongly during the evolution of high mass stars, contrary to the situation at $Z$ = 0.020. The reason is that at low $Z$, mass loss is smaller and the removal of angular momentum during evolution much weaker, also there is an efficient outward transport of angular momentum by meridional circulation. Thus, a much larger fraction of the stars at lower $Z$ reach break-up velocities and rotation may thus be a dominant effect at low $Z$. The models with rotation well account for the long standing problem of the large numbers of red supergiants observed in low $Z$ galaxies, while current models with mass loss were predicting no red supergiants. We discuss in detail the physical effects of rotation which favour a redwards evolution in the HR diagram. The models also predict large N enrichments during the evolution of high mass stars. The predicted relative N-enrichments are larger at $Z$ lower than solar and this is in very good agreement with the observations for A-type supergiants in the SMC.

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arXiv2001

The Cygnus X region: a nucleosynthesis laboratory for INTEGRAL

The detection of 1.809 MeV gamma-ray line emission from the Cygnus X complex by the COMPTEL telescope is one of the most convincing proves of massive star nucleosynthesis in our Galaxy. The Cygnus X complex is an extremely active nearby region, containing several star forming regions, OB associations and young open star clusters. It houses some of the most massive stars known in our Galaxy and concentrates a large number of Wolf-Rayet stars. Thermal radio continuum emission and intense [C II] line emission reveals widespread ionisation, and at least 60 individual H II regions have been identified. In order to understand the 1.809 MeV gamma-ray line emission from the Cygnus X complex, and to compare the observations to theoretical nucleosynthesis calculations, we modelled the multi-wavelength spectrum of the region by means of an evolutionary synthesis model. Our investigation leads us to the following conclusions: 1. Stellar wind ejection is the dominant mechanism for the observed 26Al enrichment in the Cygnus X region. 2. Cyg OB2 is by far the dominating massive star association in Cygnus X and 1.809 MeV line emission from 26Al produced in this association should be detectable by the spectrometer SPI on INTEGRAL. 3. There is only low supernova activity in the Cygnus X complex and the 60Fe lines should be below INTEGRAL's detection sensitivity. The detectability of an individual massive star cluster (Cyg OB2) by INTEGRAL provides a fantastic opportunity for future nucleosynthesis studies using gamma-ray line spectroscopy. We will explore the scientific potential of such observations, and provide estimates for gamma-ray line intensity distributions based on the massive star census of the Cygnus X region.

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arXiv2000

Gamma-ray line emission from OB associations and young open clusters: I. Evolutionary synthesis models

We have developed a new diagnostic tool for the study of gamma-ray emission lines from radioactive isotopes (such as 26Al and 60Fe in conjunction with other multi-wavelength observables of Galactic clusters, associations, and alike objects. Our evolutionary synthesis models are based on the code of Cervino & Mas-Hesse (1994), which has been updated to include recent stellar evolution tracks, new stellar atmospheres for OB and WR stars, and nucleosynthetic yields from massive stars during hydrostatic burning phases and explosive SN II and SN Ib events. The temporal evolution of 26Al and 60Fe production, the equivalent yield of 26Al per ionising O7 V star (yal), and other observables are predicted for a coeval population. We also employed our model to estimate the steady state nucleosynthesis of a population of solar metallicity. To allow for a fully quantitative analysis of existing and future multi-wavelength observations, we propose a Bayesian approach that allows the inclusion of IMF richness effects and observational uncertainties in the analysis. In particular, a Monte Carlo technique is adopted to estimate probability distributions for all observables of interest. We outline the procedure of exploiting these distributions by applying our model to a fictive massive star association. Applications to existing observations of the Cygnus and Vela regions will be discussed in companion papers.

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arXiv2000

Synthesis of 19F in Wolf-Rayet stars

Meynet and Arnould (1993) have suggested that Wolf-Rayet (WR) stars could significantly contaminate the Galaxy with 19F. In their scenario, 19F is synthesized at the beginning of the He-burning phase from the 14N left over by the previous CNO-burning core, and is ejected in the interstellar medium when the star enters its WC phase. Recourse to CNO seeds makes the 19F yields metallicity-dependent. These yields are calculated on grounds of detailed stellar evolutionary sequences for an extended range of initial masses (from 25 to 120 Msol) and metallicities (Z = 0.008, 0.02 and 0.04). The adopted mass loss rate prescription enables to account for the observed variations of WR populations in different environments. The 19F abundance in the WR winds of 60 Msol model stars is found to be about 10 to 70 times higher than its initial value, depending on the metallicity. This prediction is used in conjunction with a very simple model for the chemical evolution of the Galaxy to predict that WR stars could be significant (dominant?) contributors to the solar system fluorine content. We also briefly discuss the implications of our model on the possible detection of fluorine at high redshift.

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arXiv1999

Wolf-Rayet stars and radioisotope production

Radioisotopes are natural clocks which can be used to estimate the age of the solar system. They also influence the shape of supernova light curves. In addition, the diffuse emission at 1.8 MeV from the decay of 26Al may provide a measure of the present day nucleosynthetic activity in the Galaxy. Therefore, even if radionuclides represent only a tiny fraction of the cosmic matter, they carry a unique piece of information. A large number of radioisotopes are produced by massive stars at the time of their supernova explosion. A more or less substantial fraction of them are also synthesized during the previous hydrostatic burning phases. These nuclides are then ejected either at the time of the supernova event, or through stellar winds during their hydrostatic burning phases. This paper focusses of the non explosive ejection of radionuclides by non-rotating or rotating Wolf-Rayet stars.

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arXiv1999

Some selected comments on cosmic radioactivities

Radionuclides with half-lives ranging from some years to billions of years presumably synthesized outside of the solar system are now recorded in `live' or `fossil' form in various types of materials, like meteorites or the galactic cosmic rays. They bring specific astrophysical messages the deciphering of which is briefly reviewed here, with special emphasis on the contribution of Jerry Wasserburg and his collaborators to this exciting field of research. A brief mention is also made to $γ$-ray line astrophysics. It provides an interesting information on live short-lived radionuclides in the present interstellar medium, and thus complements in a very important way the study of extinct radionuclides in meteorites.

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arXiv1999

No Increase of the Red-Giant-Branch Tip Luminosity Toward the Center of M31

We present observations with the Hubble Space Telescope Wide Field Planetary Camera 2 of three fields centered on super-metal-rich globular clusters in the bulge of M31. Our (I,V-I) color-magnitude diagrams reach as faint as I ~ 26.5 mag and clearly reveal the magnitude of the first ascent red giant branch (RGB) tip. We find that the apparent I magnitude of the RGB tip does not become brighter near the center of M31 as concluded by previous investigators. Our observations and artificial star experiments presented in this study strongly support the idea that previous very bright stars were likely the result of spurious detections of blended stars due to crowding in lower resolution images. On the contrary, our observations indicate that, at a mean projected galactocentric distance of 1.1 kpc, the RGB tip is some 1.3 magnitudes fainter than it is at 7 kpc. An analysis of this difference in RGB tip magnitude suggests that the M31 bulge stellar population has a mean metallicity close to that of the Sun.

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arXiv1998

Grids of stellar models. VIII. From 0.4 to 1.0 Msun at Z=0.020 and Z=0.001, with the MHD equation of state

We present stellar evolutionary models covering the mass range from 0.4 to 1 Msun calculated for metallicities Z=0.020 and 0.001 with the MHD equation of state (Hummer & Mihalas, 1988; Mihalas et al. 1988; Däppen et al. 1988). A parallel calculation using the OPAL (Rogers et al. 1996) equation of state has been made to demonstrate the adequacy of the MHD equation of state in the range of 1.0 to 0.8 Msun (the lower end of the OPAL tables). Below, down to 0.4 Msun, we have justified the use of the MHD equation of state by theoretical arguments and the findings of Chabrier & Baraffe (1997). We use the radiative opacities by Iglesias & Rogers (1996), completed with the atomic and molecular opacities by Alexander & Fergusson (1994). We follow the evolution from the Hayashi fully convective configuration up to the red giant tip for the most massive stars, and up to an age of 20 Gyr for the less massive ones. We compare our solar-metallicity models with recent models computed by other groups and with observations. The present stellar models complete the set of grids computed with the same up-to-date input physics by the Geneva group [Z=0.020 and 0.001, Schaller et al. (1992), Bernasconi (1996), and Charbonnel et al. (1996); Z=0.008, Schaerer et al. (1992); Z=0.004, Charbonnel et al. (1993); Z=0.040, Schaerer et al. (1993); Z=0.10, Mowlavi et al. (1998); enhanced mass loss rate evolutionary tracks, Meynet et al. (1994)].

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arXiv1998

The pulsating yellow supergiant V810 Centauri

The F8Ia supergiant V810 Centauri is part of a long-term high-precision photometric monitoring program on long period variables started twenty years ago. Time series analysis of this unique set of 500 data points, spanning almost fifteen years in the homogeneous Geneva photometric system, is presented. Cluster membership, physical parameters and evolutionary status of the star are reinvestigated. Radial velocity data do not support the cluster membership to Stock 14}. Ultraviolet and optical spectrophotometry is combined with optical and infrared photometry to evaluate the physical parameters of the yellow supergiant (Teff = 5970 K, M_bol = -8.5, R = 420 R_sun) and of its B0III companion. From theoretical stellar evolutionary tracks, an initial mass of 25 M_sun is estimated for V810 Cen, which is actually at the end of its first redward evolution. V810 Cen is a multi-periodic small amplitude variable star, whose amplitudes are variable with time. The period of the main mode, 156 d, is in agreement with the Period--Luminosity--Colour relation for supergiants. This mode is most probably the fundamental radial one. According to the theoretical pulsation periods for the radial modes, calculated from a linear non-adiabatic analysis, the period of the observed second mode, 107 d, is much too long to correspond to the first radial overtone . Thus, this second mode could be a non-radial p-mode. Other transient periods are observed, in particular at 187 d. The l ength of this period suggests a non-radial g-mode. Then, the complex variability of V810 Cen could be due to a mixing of unstable radial and non-radial p- and g-modes.

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arXiv1998

On some properties of very metal-rich stars

An analysis of some properties of stellar models as a function of metallicity Z (and helium content Y) is presented, with special attention to those stars with metallicities higher than twice or three times solar. It is shown that the stellar properties as a function of Z are mainly determined by the effects of the opacities at sub-solar metallicities, and by the effects of the mean molecular weight and stellar mass loss at higher metallicities. As a result, very metal-rich stars (Z >~0.05) exhibit properties that deviate from what is expected from the known characteristics at lower metallicities. In particular, they are more luminous and hotter than those at Z <~ 0.05 due to the effect of the mean molecular weight. They have main sequence lifetimes much shorter (60 % shorter at Z=0.1 than at Z=0.02) than those at solar metallicity due to their lower initial hydrogen content. Finally, the high mass loss rates at high metallicities affect significantly the population synthesis of massive stars in very metal-rich regions. An analysis of expected Wolf-Rayet and supernovae populations in such conditions is briefly presented.

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