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Corinne Charbonnel

Publications and source records attributed to Corinne Charbonnel.

At least 73 records · Page 4Linked to original sources
arXiv2007

Inhibition of thermohaline mixing by a magnetic field in Ap star descendants: Implications for the Galactic evolution of 3He

To reconcile the measurements of 3He/H in Galactic HII regions with high values of 3He in a couple of planetary nebulae, we propose that thermohaline mixing is inhibited by a fossil magnetic field in red giant stars that are descendants of Ap stars. We examine the effect of a magnetic field on the salt-finger instability, using a local analysis. We obtain a threshold for the magnetic field of 10^4 - 10^5 Gauss, above which it inhibits thermohaline mixing in red giant stars located at or above the bump. Fields of that order are expected in the descendants of the Ap stars, taking into account the contraction of their core. We conclude that in a large fraction of the descendants of Ap stars thermohaline mixing does not occur. As a consequence these objects must produce 3He as predicted by the standard theory of stellar evolution and as observed in the planetary nebulae NGC3242 and J320. The relative number of such stars with respect to non-magnetic objects that undergo thermohaline mixing is consistent with the statistical constraint coming from observations of the carbon isotopic ratio in red giant stars. It also satisfies the Galactic requirements for the evolution of the 3He abundance.

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arXiv2007

Angular momentum transport by internal gravity waves III - Wave excitation by core convection and the Coriolis effect

This is the third in a series of papers that deal with angular momentum transport by internal gravity waves. We concentrate on the waves excited by core convection in a 3Msun, Pop I main sequence star. Here, we want to examine the role of the Coriolis acceleration in the equations of motion that describe the behavior of waves and to evaluate its impact on angular momentum transport. We use the so-called traditional approximation of geophysics, which allows variable separation in radial and horizontal components. In the presence of rotation, the horizontal structure is described by Hough functions instead of spherical harmonics. The Coriolis acceleration has two main effects on waves. It transforms pure gravity waves into gravito-inertial waves that have a larger amplitude closer to the equator, and it introduces new waves whose restoring force is mainly the conservation of vorticity. Taking the Coriolis acceleration into account changes the subtle balance between prograde and retrograde waves in non-rotating stars. It also introduces new types of waves that are either purely prograde or retrograde. We show in this paper where the local deposition of angular momentum by such waves is important.

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arXiv2007

Light nuclei in galactic globular clusters : constraints on the self-enrichment scenario from nucleosynthesis

Hydrogen-burning is the root cause of the star-to-star abundance variations of light nuclei in Galactic globular clusters (GC). In the present work we constrain the physical conditions that gave rise to the observed abundance patterns of Li, C, N, O, Na, Mg, Al, as well as Mg isotopes in the typical case of NGC6752. We perform nucleosynthesis calculations at constant temperature, adopting realistic initial abundances for the proto-cluster gas. We use a detailed nuclear reaction network and state-of-the-art nuclear reaction rates. Although simplistic, our analysis provides original results and new constraints on the self-enrichment scenario for GCs. Our parametrized calculations allow us to determine a narrow range of temperature where the observed extreme abundances of all light elements and isotopes in NGC6752 are nicely reproduced simultaneously. This agreement is obtained after mixing of the H-processed material with 30 % of unprocessed gas. The observed C-N, O-Na, Mg-Al, Li-Na and F-Na anticorrelations, as well as the behaviour of the Mg isotopes can be recovered by assuming mixing with even larger dilution factors. Li production by the stars that build up the other abundance anomalies is not mandatory in the case of NGC 6752. Observations of O, Na, Mg and Al constrain the temperature range for H-burning; such temperatures are encountered in the two main candidate ``polluters'' proposed for GCs, namely massive AGBs and the most massive main-sequence stars. (Abridged).

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arXiv2007

Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants

Numerous spectroscopic observations provide compelling evidence for a non-canonical mixing process that modifies the surface abundances of Li, C and N of low-mass red giants when they reach the bump in the luminosity function. Eggleton et al. (2006) have proposed that a molecular weight inversion created by the 3He(3He,2p)4He reaction may be at the origin of this mixing, and relate it to the Rayleigh-Taylor instability. In the present work we argue that one is actually dealing with a double diffusive instability referred to as thermohaline convection and we discuss its influence on the red giant branch. We compute stellar models of various initial metallicities using the prescription by Ulrich (1972) (extended to the case of a non-perfect gas) for the turbulent diffusivity produced by the thermohaline instability in stellar radiation zones. Thermohaline mixing simultaneously accounts for the observed behaviour of the carbon isotopic ratio and of the abundances of Li, C and N on the upper part of the red giant branch. It reduces significantly the 3He production with respect to canonical evolution models as required by measurements of 3He/H in galactic HII regions. Thermohaline mixing is a fundamental physical process that must be included in stellar evolution modeling.

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arXiv2006

Abundance Anomalies in Galactic Globular Clusters - Looking for the Stellar Culprits

Galactic globular cluster stars exhibit abundance patterns which are not shared by their field counterparts. It is clear from recent spectroscopic observations of GC turnoff stars that these abundance anomalies were already present in the gas from which the observed stars formed. This provides undisputed support to the so-called self-enrichment scenario according to which a large fraction of GC low-mass stars have formed from material processed through hydrogen-burning at high temperatures and then lost by more massive and faster evolving stars (and perhaps mixed with some original gas). Within this framework we present a new method to derive the Initial Mass Function of the polluter stars.

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arXiv2006

On the self-enrichment scenario of galactic globular clusters: Constraints on the IMF

Galactic globular cluster (GC) stars exhibit abundance patterns which are not shared by their field counterparts, In the framework of the widely accepted "self-enrichment" scenario for GCs, we present a new method to derive the Initial Mass Function (IMF) of the polluter stars, by using the observed O/Na abundance distribution. We focus on NGC 2808, a GC for which the largest sample of O and Na abundance determinations is presently available. We consider two classes of possible "culprits" : massive Asymptotic Giant Branch (AGB) stars (4-9 Msun) and winds of massive stars (WMS) in the mass range 10-100 Msun. We obtain upper limits for the slope of the IMF (assumed to be given by a power-law) of the stars initially more massive than the present turnoff mass. We also derive lower limits for the amount of stellar residues. We find that the polluter IMF had to be much flatter than presently observed IMFs in stellar clusters, in agreement with the results of two other methods for GC IMF determination. Additionaly, we find that the present mass of the GC should be totally dominated by stellar remnants if the polluters were AGB stars, but not so in the case of WMS. We critically analyse the advantages and shortcomings of each potential polluter class, and we find the WMS scenario more attractive.

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arXiv2005

Influence of gravity waves on the internal rotation and Li abundance of solar-type stars

The Sun's rotation profile and lithium content have been difficult to understand in the context of conventional models of stellar evolution. Classical hydrodynamical models predict that the solar interior must rotate highly differentially, in disagreement with observations. It has recently been shown that internal waves produced by convection in solar-type stars produce an asymmetric, shear layer oscillation, similar to Earth's quasi-biennial oscillation, that leads to efficient angular momentum redistribution from the core to the envelope. We presents results of a model that successfully reproduces both the rotation profile and the surface abundance of lithium in solar-type stars of various ages.

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arXiv2004

'Sculptor'-ing the Galaxy? The Chemical Compositions of Red Giants in the Sculptor Dwarf Spheroidal Galaxy

We have used high-resolution, high signal-to-noise spectra obtained with the VLT and UVES to determine abundances of 17 elements in 4 red giants in the Sculptor dwarf spheroidal galaxy. Our [Fe/H] values range from --2.10 to --0.97, confirming previous findings of a large metallicity spread. We have combined our data with similar data for five Sculptor giants studied recently to form one of the largest samples of high resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range. These properties allow us to establish trends of [X/Fe] with [Fe/H] for many elements, X. The trends are significantly different from the trends seen in galactic halo and globular cluster stars. We compare our Sculptor sample to their most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between [X/Fe] as seen in Sculptor compared with Galactic field stars indicates that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Sculptor. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element-rich star. A very high percentage of such heavy element stars are now known in dwarf spheroidals compared to the halo, further mitigating against the formation of the halo from such objects.

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arXiv2004

Angular Momentum Transport by Internal Gravity Waves II- Pop II stars from the Li plateau to the horizontal branch

This paper is the second in a series where we examine the generation and filtering of internal gravity waves in stars and the consequences of wave induced transport of angular momentum at various stages of the stellar evolution. Here we concentrate on Pop II dwarf stars and we focus in particular on the differential properties of internal gravity waves as a function of the stellar mass. We show that, for the range of masses corresponding to the lithium plateau, gravity waves are fully efficient and should thus lead to a quasi-solid rotation state, similar to that of the Sun. In the slightly more massive progenitors of currently observed horizontal branch star however, internal wave generation is not efficient on the main sequence, and large internal differential rotation can thus be maintained. This leads to a natural explanation of the large rotational velocities measured on the horizontal branch in some globular clusters.

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arXiv2003

Angular Momentum Transport by Internal Gravity Waves. I - Pop I Main Sequence Stars

We examine the generation of gravity waves by the surface convection zone of low-mass main sequence stars with solar metallicity. It is found that the total momentum luminosity in waves rises with stellar mass, up to the quasi-disappearance of the convection zone around 6500K (corresponding to a mass of about 1.4 Msun for solar metallicity) where the luminosity drastically drops. We calculate the net momentum extraction associated with these waves and explain how the calculated mass dependence helps resolve the enigma of the Li dip in terms of rotational mixing, forming a coherent picture of mixing in all main sequence stars.

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arXiv2003

Rotating models for evolved low-mass stars

Low mass stars (< 2-2.5 M_sun) exhibit, at all the stages of their evolution, signatures of processes that require challenging modeling beyond the standard stellar theory. In this paper we focus on their peculiarities while they climb the red giant branch (RGB). We first compare the classical predictions for abundance variations due to the first dredge-up with observational data in various environments. We show how clear spectroscopic diagnostics probe the nucleosynthesis and the internal mixing mechanisms that drive RGB stars. Coherent data reveal in particular the existence of a non-standard mixing process that changes their surface abundances at the so-called RGB bump. By reviewing the models presented so far to explain the various abundance anomalies, we show that the occurrence of this extra-mixing process is certainly related to rotation. Finally we discuss the so-called Li-flash which is expected to occur at the very beginning of the extra-mixing episode.

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arXiv2002

Rotational mixing in low-mass stars : I Effect of the mu-gradients in main sequence and subgiant Pop I stars

We present a first set of results concerning stellar evolution of rotating low-mass stars. Our models include fully consistent transport of angular momentum and chemicals due to the combined action of rotation induced mixing (according to Maeder & Zahn 1998) and element segregation. The analysis of the effects of local variations of molecular weight due to the meridional circulation on the transport of angular momentum and chemicals are under the scope of this study. We apply this mechanism to low mass main sequence and subgiant stars of population I. We show that the so-called $μ$-currents are of major importance in setting the shape of the rotation profile, specially near the core. Furthermore, as shown by Talon & Charbonnel (1998) and Charbonnel & Talon (1999) using models without $μ$-currents, we confirm that rotation-induced mixing in stars braked via magnetic torquing can explain the blue side of the Li dip, as well as the low Li abundances observed in subgiants even when $μ$-currents are taken into account. We emphasize that $μ$ variations are not to be neglected when treating rotation-induced mixing, and that they could be of great importance for latter evolutionary stages.

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arXiv2001

On ionization effects and abundance ratios in damped Lyman alpha systems

The similarity between observed velocity structures of Al III and singly ionized species in damped Lyman-alpha systems (DLAs) suggests the presence of ionized gas in the regions where most metal absorption lines are formed. A simplified model consisting of Region 1) a plane-parallel ionization bounded region illuminated by an internal radiation field, and Region 2) a neutral region with a negligible metal content is considered. We calculate photoionization equilibrium models for region 1, and constrain the ionization parameter by the observed N(Al III)/N(Si II) column density ratio. Under these conditions we find that ionization effects are important. If these effects are taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in H II regions of local low-metallicity blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. No depletion of heavy elements on dust grains needs to be invoked, although our models do not exclude the presence of little depletion. Although highly simplified and relying on the strong assumption of a significantly lower metal content in region 2, our model appears to be supported by recent data on a local DLA and it is not in contradiction with the current knowledge on high redshift DLAs. If correct, it offers a clear simplification in the understanding of heavy element abundance ratios in DLAs and their comparison with the local Universe.

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arXiv2000

On ionisation effects and abundance ratios in damped Lyman-alpha systems

The similarity between observed velocity structures of Al III and singly ionised species in damped Lyman-alpha systems (DLAs) suggests the presence of ionised gas in the regions where most metal absorption lines are formed. To explore the possible implications of ionisation effects we construct a simplified two-region model for DLAs consisting of an ionisation bounded region with an internal radiation field and a neutral region with a lower metal content. Within this framework we find that ionisation effects are important. If taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in metal-poor H II regions in blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. From our models no dust depletion of heavy elements needs to be invoked; little depletion is however not excluded.

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arXiv2000

Sinks of Light Elements in Stars - Part II

The fragile light elements lithium, beryllium, and boron are easily destroyed in stellar interiors, and are thus superb probes of physical processes occuring in the outer stellar layers. The light elements are also excellent tracers of the chemical evolution of the Galaxy, and can test big bang nucleosynthesis (BBN). These inter-related topics are reviewed with an emphasis on stellar physics. In Part I (presented by CPD), an overview is given of the physical processes which can modify the surface abundances of the light elements, with emphasis on Population I dwarfs - convection; gravitational settling, thermal diffusion, and radiative levitation; slow mixing induced by gravity waves or rotation. We will discuss the increasingly large body of data which begin to enable us to discern the relative importance of these mechanisms in Population I main sequence stars. In Part II (presented by MHP), discussion is extended to the issue of whether or not the halo Li plateau is depleted, and includes the following topics: Li dispersion in field and globular cluster stars, Li production vs. destruction in Li-rich halo stars, and constraints from 6Li. Also discussed are trends with metal abundance and Teff and implications for chemical evolution and BBN. In Part III (presented by CC), evidence is reviewed that suggests that in situ mixing occurs in evolved low mass Population I and Population II stars. Theoretical mechanisms that can create such mixing are discussed, as well as their implications in stellar yields.

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arXiv2000

Sinks of Light Elements in Stars - Part III

The fragile light elements lithium, beryllium, and boron are easily destroyed in stellar interiors, and are thus superb probes of physical processes occuring in the outer stellar layers. The light elements are also excellent tracers of the chemical evolution of the Galaxy, and can test big bang nucleosynthesis (BBN). These inter-related topics are reviewed with an emphasis on stellar physics. In Part I (presented by CPD), an overview is given of the physical processes which can modify the surface abundances of the light elements, with emphasis on Population I dwarfs - convection; gravitational settling, thermal diffusion, and radiative levitation; slow mixing induced by gravity waves or rotation. We will discuss the increasingly large body of data which begin to enable us to discern the relative importance of these mechanisms in Population I main sequence stars. In Part II (presented by MHP), discussion is extended to the issue of whether or not the halo Li plateau is depleted, and includes the following topics: Li dispersion in field and globular cluster stars, Li production vs. destruction in Li-rich halo stars, and constraints from 6Li. Also discussed are trends with metal abundance and Teff and implications for chemical evolution and BBN. In Part III (presented by CC), evidence is reviewed that suggests that in situ mixing occurs in evolved low mass Population I and Population II stars. Theoretical mechanisms that can create such mixing are discussed, as well as their implications in stellar yields.

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arXiv2000

The Nature of the Lithium Rich Giants. Mixing episodes on the RGB and early-AGB

We present a critical analysis of the nature of the so-called Li-rich RGB stars. For a majority of the stars, we have used Hipparcos parallaxes to determine masses and evolutionary states by comparing their position on the Hertzsprung-Russell diagram with theoretical evolutionary tracks. Among the twenty Li-rich giants whose location on the HR diagram we were able to determine precisely, eight should be re-classified as Li-normal. For the remaining stars, the high Li abundance must be a result of fresh synthesis of this fragile element. We identify two distinct episodes of Li production which occur in advanced evolutionary phases depending upon the mass of the star. We suggest that the Li-rich stars are formed mainly as a precursor to the deeper mixing process which produces anomalously low carbon isotopic ratios. Low-mass RGB stars, which later undergo the helium flash, produce Li at the phase referred to as the bump in the luminosity function. In intermediate-mass stars, the Li-rich phase occurs on the early-AGB. The compiled data provide constraints on the time scales for extra mixing and some insight on processes suggested in the literature. However, our results do not suggest any specific trigger mechanism. Since the Li-rich phases are extremely short, enrichment of the Li content of the ISM as a result of these episodes is negligible.

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arXiv2000

Evolution and surface abundances of red giants experiencing deep mixing

We have calculated the evolution of low metallicity red giant stars under the assumption of deep mixing between the convective envelope and the hydrogen burning shell. We find that the extent of the observed abundance anomalies, and in particular the universal O-Na anticorrelation, can be totally explained by mixing which does not lead to significant helium enrichment of the envelope. On the other hand, models with extremely deep mixing and strong helium enrichment predict anomalies of sodium and oxygen, which are much larger than the observed ones. This latter result depends solely on the nucleosynthesis inside the hydrogen burning shell, but not on the details of the mixing descriptions. These, however, influence the evolution of surface abundances with brightness, which we compare with the limited observational material available. Our models allow, nevertheless, to infer details on the depth and speed of the mixing process in several clusters. Models with strong helium enrichment evolve to high luminosities and show an increased mass loss. However, under peculiar assumptions, red giants reach very high luminosities even without extreme helium mixing. Due to the consequently increased mass loss, such models could be candidates for blue horizontal branch stars, and, at the same time, would be consistent with the observed abundance anomalies.

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