arXiv subjects

T. Decressin

Publications and source records attributed to T. Decressin.

At least 19 recordsLinked to original sources

arXiv2017

The magnetic strip(s) in the advanced phases of stellar evolution - Theoretical convective turnover timescale and Rossby number for low- and intermediate-mass stars up to the AGB at various metallicities

Recent spectropolarimetric observations of otherwise ordinary G, K, and M giants revealed localized magnetic strips in the HRD coincident with the regions where the first dredge-up and core He-burning occur. We seek to understand the origin of magnetic fields in such late-type giant stars. In analogy with late-type dwarf stars, we focus primarily on parameters known to influence the generation of magnetic fields in the outer convective envelope. We compute the classical dynamo parameters along the evolutionary tracks of low- and intermediate-mass stars at various metallicities using stellar models that have been extensively tested by spectroscopic and asteroseismic observations. These include convective turnover timescales and convective Rossby numbers, computed from the PMS to the tip of the RGB or the early AGB. To investigate the effects of the very extended outer convective envelope, we compute these parameters both for the entire convective envelope and locally, that is, at different depths within the envelope. We also compute the turnover timescales and corresponding Rossby numbers for the convective cores of intermediate-mass stars on the main sequence. Our models show that the Rossby number of the convective envelope becomes lower than unity in the well-delimited locations of the Hertzsprung-Russell diagram where magnetic fields have indeed been detected. We show that alpha-omega dynamo processes might not be continuously operating, but that they are favored in the stellar convective envelope at two specific moments along the evolution tracks, that is, during the first dredge-up at the base of the RGB and during central helium burning in the helium-burning phase and early-AGB. This general behavior can explain the so-called magnetic strips recently discovered by dedicated spectropolarimetric surveys of evolved stars.

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arXiv2015

The extended Main Sequence Turn Off cluster NGC1856: rotational evolution in a coeval stellar ensemble

Multiple or extended turnoffs in young clusters in the Magellanic Clouds have recently received large attention. A number of studies have shown that they may be interpreted as the result of a significant age spread (several 10^8yr in clusters aged 1--2 Gyr), while others attribute them to a spread in stellar rotation. We focus on the cluster NGC 1856, showing a splitting in the upper part of the main sequence, well visible in the color m_{F336W}-m_{F555W}$, and a very wide turnoff region. Using population synthesis available from the Geneva stellar models, we show that the cluster data can be interpreted as superposition of two main populations having the same age (~350Myr), composed for 2/3 of very rapidly rotating stars, defining the upper turnoff region and the redder main sequence, and for 1/3 of slowly/non-rotating stars. Since rapid rotation is a common property of the B-A type stars, the main question raised by this model concerns the origin of the slowly/non-rotating component. Binary synchronization is a possible process behind the slowly/non-rotating population; in this case, many slowly/non-rotating stars should still be part of binary systems with orbital periods in the range from 4 to 500 days. Such periods imply that Roche lobe overflow occurs, during the evolution of the primary off the main sequence, so most primaries may not be able to ignite core helium burning, consistently why the lack of a red clump progeny of the slowly rotating population.

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arXiv2014

The Magnetic Fields at the Surface of Active Single G-K Giants

We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets and use the least-squares deconvolution (LSD) method. We also measure the classical S-index activity indicator, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times. We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars.The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, eta Psc) are detected with magnetic field strength at the sub-gauss level.

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arXiv2013

Magnetic field structure in single late-type giants: Beta Ceti in 2010 - 2012

The data were obtained using two spectropolarimeters - Narval at the Bernard Lyot Telescope, Pic du Midi, France, and ESPaDOnS at CFHT, Hawaii. Thirty-eight circularly-polarized spectra have been collected in the period June 2010 - January 2012. The Least Square Deconvolution method was applied for extracting high signal-to-noise ratio line profiles, from which we measure the surface-averaged longitudinal magnetic field Bl. Chromospheric activity indicators CaII K, H_alpha, CaII IR (854.2 nm) and radial velocity were simultaneously measured and their variability was analysed together with the behavior of Bl. The Zeeman Doppler Imaging (ZDI) inversion technique was employed for reconstruction of the large-scale magnetic field and two magnetic maps of Beta Ceti are presented for two periods (June 2010 - December 2010 and June 2011 - January 2012). Bl remains of positive polarity for the whole observational period. The behavior of the line activity indicators is in good agreement with the Bl variations. The two ZDI maps show a mainly axisymmetric and poloidal magnetic topology and a simple surface magnetic field configuration dominated by a dipole. Little evolution is observed between the two maps, in spite of a 1 yr interval between both subsets. We also use state-of-the-art stellar evolution models to constrain the evolutionary status of Beta Ceti. We derive a mass of 3.5 M_sun and propose that this star is already in the central-helium burning phase. Taking into account all our results and the evolutionary status of the star, we suggest that dynamo action alone may not be eficient enough to account for the high magnetic activity of Beta Ceti. As an alternate option, we propose that it may be an Ap star descendant presently undergoing central helium-burning and still exhibiting a remnant of the Ap star magnetic field.

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arXiv2013

Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars

We study the impact of internal gravity waves (IGW), meridional circulation, shear turbulence, and stellar contraction on the internal rotation profile and surface velocity evolution of solar metallicity low-mass pre-main sequence stars. We compute a grid of rotating stellar evolution models with masses between 0.6 and 2.0Msun taking these processes into account for the transport of angular momentum, as soon as the radiative core appears and assuming no more disk-locking from that moment on.IGW generation along the PMS is computed taking Reynolds-stress and buoyancy into account in the bulk of the stellar convective envelope and convective core (when present). Redistribution of angular momentum within the radiative layers accounts for damping of prograde and retrograde IGW by thermal diffusivity and viscosity in corotation resonance. Over the whole mass range considered, IGW are found to be efficiently generated by the convective envelope and to slow down the stellar core early on the PMS. In stars more massive than ~ 1.6Msun, IGW produced by the convective core also contribute to angular momentum redistribution close to the ZAMS. Overall, IGW are found to significantly change the internal rotation profile of PMS low-mass stars.

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arXiv2013

The role of massive stars in the turbulent infancy of Galactic globular clusters: Feedback on the intracluster medium, and detailed timeline

A major paradigm shift has recently revolutionized our picture of globular clusters (GC) that were long thought to be simple systems of coeval stars born out of homogeneous material. Indeed, detailed abundance studies of GC long-lived low-mass stars performed with 8-10m class telescopes, together with high-precision photometry of Galactic GCs obtained with HST,have brought compelling clues on the presence of multiple stellar populations in individual GCs. These stellar subgroups can be recognized thanks to their different chemical properties (more precisely by abundance differences in light elements from carbon to aluminium; see Bragaglia, this volume) and by the appearance of multimodal sequences in the colour-magnitude diagrams (see Piotto, this volume). This has a severe impact on our understanding of the early evolution of GCs, and in particular of the possible role that massive stars played in shaping the intra-cluster medium (ICM) and in inducing secondary star formation. Here we summarize the detailed timeline we have recently proposed for the first 40 Myrs in the lifetime of a typical GC following the general ideas of our so-called "Fast Rotating Massive stars scenario" (FRMS, Decressin et al. 2007b) and taking into account the dynamics of interstellar bubbles produced by stellar winds and supernovae. More details can be found in Krause et al. (2012, 2013).

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arXiv2012

How did globular clusters lose their gas?

We summarize the results presented in Krause et al. (2012) on the impact of supernova-driven shells and dark-remnant accretion on gas expulsion in globular cluster infancy.

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arXiv2012

Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities

The availability of asteroseismic constraints for a large sample of stars from the missions CoRoT and Kepler paves the way for various statistical studies of the seismic properties of stellar populations. In this paper, we evaluate the impact of rotation-induced mixing and thermohaline instability on the global asteroseismic parameters at different stages of the stellar evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic Giant Branch to distinguish stellar populations. We present a grid of stellar evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and 0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid we provide the usual stellar parameters (luminosity, effective temperature, lifetimes, ...), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relations, the frequency corresponding to the maximum oscillation power ν_{max}, the maximal amplitude A_{max}, the asymptotic period spacing of g-modes, and different acoustic radii. We discuss the signature of rotation-induced mixing on the global asteroseismic quantities, that can be detected observationally. Thermohaline mixing whose effects can be identified by spectroscopic studies cannot be caracterized with the global seismic parameters studied here. But it is not excluded that individual mode frequencies or other well chosen asteroseismic quantities might help constraining this mixing.

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arXiv2011

Thermohaline instability and rotation-induced mixing II- Yields of 3He for low- and intermediate-mass stars

Context. The 3He content of Galactic HII regions is very close to that of the Sun and the solar system, and only slightly higher than the primordial 3He abundance as predicted by the standard Big Bang nucleosynthesis. However, the classical theory of stellar evolution predicts a high production of 3He by low-mass stars, implying a strong increase of 3He with time in the Galaxy. This is the well-known "3He problem". Aims. We study the effects of thermohaline and rotation-induced mixings on the production and destruction of 3He over the lifetime of low- and intermediate-mass stars at various metallicities. Methods. We compute stellar evolutionary models in the mass range 1 to 6M\odot for four metallicities, taking into account thermohaline instability and rotation-induced mixing. For the thermohaline diffusivity we use the prescription based on the linear stability analysis, which reproduces Red Giant Branch (RGB) abundance patterns at all metallicities. Rotation-induced mixing is treated taking into account meridional circulation and shear turbulence. We discuss the effects of these processes on internal and surface abundances of 3He and on the net yields. Results. Over the whole mass and metallicity range investigated, rotation-induced mixing lowers the 3He production, as well as the upper mass limit at which stars destroy 3He. For low-mass stars, thermohaline mixing occuring beyond the RGB bump is the dominant process in strongly reducing the net 3He yield compared to standard computations. Yet these stars remain net 3He producers. Conclusions. Overall, the net 3He yields are strongly reduced compared to the standard framework predictions.

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arXiv2011

Tracing the evolution of NGC6397 through the chemical composition of its stellar populations

With the aim to constrain multiple populations in the metal-poor globular cluster NGC6397, we analyse and discuss the chemical compositions of a large number of elements in 21 red giant branch stars. High-resolution spectra were obtained with the FLAMES/UVES spectrograph on VLT. We have determined non-LTE abundances of Na and LTE abundances for the remaining 21 elements, including O, Mg, Al, alpha, iron-peak, and neutron-capture elements, many of which have not previously been analysed for this cluster. We have also considered the influence of possible He enrichment in the analysis of stellar spectra. We find that the Na abundances of evolved, as well as unevolved, stars show a distinct bimodality, which suggests the presence of two stellar populations; one primordial stellar generation with composition similar to field stars, and a second generation that is enriched in material processed through hydrogen-burning (enriched in Na and Al and depleted in O and Mg). The cluster is dominated (75%) by the second generation. The red giant branch show a similar bimodal distribution in the Stroemgren colour index c_y=c_1-(b-y), implying a large difference also in N abundance. The two populations have the same composition of all analysed elements heavier than Al, within the measurement uncertainty of the analysis, with the possible exception of [Y/Fe]. Using two stars with close to identical stellar parameters, one from each generation, we estimate the difference in He content, Delta Y=0.01+-0.06, given the assumption that the mass fraction of iron is the same for the stars. Finally, we show that winds from fast rotating massive stars of the first generation can be held responsible for the abundance patterns observed in NGC6397 second generation long-lived stars and estimate that the initial mass of the cluster were at least ten times higher than its present-day value.

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arXiv2010

Evolution of two stellar populations in globular clusters II. Effects of primordial gas expulsion

We investigate the early evolution of two distinct populations of low-mass stars in globular clusters under the influence of primordial gas expulsion driven by supernovae to study if this process can increase the fraction of second generation stars at the level required by observations. We analyse N-body models that take into account the effect of primordial gas expulsion. We divide the stars into two populations which mimic the chemical and dynamical properties of stars in globular clusters so that second generation stars start with a more centrally concentrated distribution. The main effect of gas expulsion is to eject preferentially first generation stars while second generation stars remain bound to the cluster. In the most favourable cases second generation stars can account for 60% of the bound stars we see today. We also find that at the end of the gas expulsion phase, the radial distribution of the two populations is still different, so that long-term evolution will further increase the fraction of second generation stars. The large fraction of chemically anomalous stars is readily explainable as a second generation of stars formed out of the slow winds of rapidly rotating massive stars if globular clusters suffer explosive residual gas expulsion for a star formation efficiency of about 0.33.

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arXiv2009

The chemical composition of TS 01, the most oxygen-deficient planetary nebula. AGB nucleosynthesis in a metal-poor binary star

The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A), with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h), is an exceptional object located in the Galactic halo. We have secured observational data in a complete wavelength range in order to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling taking into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Both stellar components contribute to the ionization: the ``cool'' one provides the bulk of hydrogen ionization, and the ``hot'' one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. The nebular abundances of C, N, O, and Ne are found to be respectively, 1/3.5, 1/4.2, 1/70, and 1/11 of the Solar value, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of Solar. Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an extreme oxygen deficiency in a star whose progenitor has an initial mass of about 1 msun requires an additional mixing process, which can be induced by stellar rotation and/or by the presence of the close companion. We have computed a stellar model with initial mass of 1 msun, appropriate metallicity, and initial rotation of 100 kms, and find that rotation greatly improves the agreement between the predicted and observed abundances.

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arXiv2009

CNO enrichment by rotating AGB stars in globular clusters

AGB stars have long been held responsible for the important star-to-star variations in light elements observed in Galactic globular clusters. We analyse the main impacts of a first generation of rotating intermediate-mass stars on the chemical properties of second-generation globular cluster stars. The rotating models were computed without magnetic fields and without the effects of internal gravity waves. They account for the transports by meridional currents and turbulence. We computed the evolution of both standard and rotating stellar models with initial masses between 2.5 and 8 Msun within the metallicity range covered by Galactic globular clusters. During central He-burning, rotational mixing transports fresh CO-rich material from the core towards the hydrogen-burning shell, leading to the production of primary 14N. In stars more massive than M > 4 Msun, the convective envelope reaches this reservoir during the second dredge-up episode, resulting in a large increase in the total C+N+O content at the stellar surface and in the stellar wind. The corresponding pollution depends on the initial metallicity. At low- and intermediate-metallicity, it is at odds with the constancy of C+N+O observed among globular cluster low-mass stars. With the given input physics, our models suggest that massive rotating AGB stars have not shaped the abundance patterns observed in low- and intermediate-metallicity globular clusters. Our non-rotating models, on the other hands, do not predict surface C+N+O enhancements, hence are in a better position as sources of the chemical anomalies in globular clusters showing the constancy of the C+N+O. However at the moment, there is no reason to think that intermediate mass stars were not rotating.

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arXiv2008

Dynamical mixing of two stellar populations in globular clusters

Stars in globular clusters (GCs) exhibit a peculiar chemical pattern with strong abundance variations in light elements along with a constant abundance in heavy elements. These abundance anomalies can be explained by a primordial pollution due to a first generation of fast rotating massive stars which released slow winds into the ISM from which a second generation of chemically anomalous stars can be formed. In particular the observed ratio of anomalous and standard stars in clusters can be used to constrain the dynamical evolution of GCs as around 95% of the standard stars need to be lost by the clusters. We show that both residual gas expulsion during the cluster formation and long term evolution are needed to achieve this ratio.

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arXiv2008

The ages of Galactic globular clusters in the context of self-enrichment

A significant fraction of stars in globular clusters (about 70%-85%) exhibit peculiar chemical patterns with strong abundance variations in light elements along with constant abundances in heavy elements. These abundance anomalies can be created in the H-burning core of a first generation of fast rotating massive stars and the corresponding elements are convoyed to the stellar surface thanks to rotational induced mixing. If the rotation of the stars is fast enough this matter is ejected at low velocity through a mechanical wind at the equator. It then pollutes the ISM from which a second generation of chemically anomalous stars can be formed. The proportion of anomalous to normal star observed today depends on at least two quantities : (1) the number of polluter stars; (2) the dynamical history of the cluster which may lose during its lifetime first and second generation stars in different proportions. Here we estimate these proportions based on dynamical models for globular clusters. When internal dynamical evolution and dissolution due to tidal forces are accounted for, starting from an initial fraction of anomalous stars of 10% produces a present day fraction of about 25%, still too small with respect to the observed 70-85%. In case gas expulsion by supernovae is accounted for, much higher fraction is expected to be produced. In this paper we also address the question of the evolution of the second generation stars that are He-rich, and deduce consequences for the age determination of globular clusters.

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arXiv2008

Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars

(Abridged) We present a detailed analysis of the main physical processes responsible for the transport of angular momentum and chemical species in the radiative regions of rotating stars. We focus on cases where meridional circulation and shear-induced turbulence only are included in the simulations. Our analysis is based on a 2-D representation of the secular hydrodynamics, which is treated using expansions in spherical harmonics. We present a full reconstruction of the meridional circulation and of the associated fluctuations of temperature and mean molecular weight along with diagnosis for the transport of angular momentum, heat and chemicals. In the present paper these tools are used to validate the analysis of two main sequence stellar models of 1.5 and 20 Msun for which the hydrodynamics has been previously extensively studied in the literature. We obtain a clear visualization and a precise estimation of the different terms entering the angular momentum and heat transport equations in radiative zones. This enables us to corroborate the main results obtained over the past decade by Zahn, Maeder, and collaborators concerning the secular hydrodynamics of such objects. We focus on the meridional circulation driven by angular momentum losses and structural readjustements. We confirm quantitatively for the first time through detailed computations and separation of the various components that the advection of entropy by this circulation is very well balanced by the barotropic effects and the thermal relaxation during most of the main sequence evolution. This enables us to derive simplifications for the thermal relaxation on this phase. The meridional currents in turn advect heat and generate temperature fluctuations that induce differential rotation through thermal wind thus closing the transport loop.

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arXiv2008

The evolution of two stellar populations in globular clusters I. The dynamical mixing timescale

We investigate the long-term dynamical evolution of two distinct stellar populations of low-mass stars in globular clusters in order to study whether the energy equipartition process can explain the high number of stars harbouring abundance anomalies seen in globular clusters. We analyse N-body models by artificially dividing the low-mass stars (m<0.9 Msun) into two populations: a small number of stars (second generation) consistent with an invariant IMF and with low specific energies initially concentrated towards the cluster-centre mimic stars with abundance anomalies. These stars form from the slow winds of fast-rotating massive stars. The main part of low-mass (first generation) stars has the pristine composition of the cluster. We study in detail how the two populations evolve under the influence of two-body elaxation and the tidal forces due to the host galaxy.Stars with low specific energy initially concentrated toward the cluster centre need about two relaxation times to achieve a complete homogenisation throughout the cluster. For realistic globular clusters, the number ratio between the two populations increases only by a factor 2.5 due to the preferential evaporation of the population of outlying first generation stars. We also find that the loss of information on the stellar orbital angular momentum occurs on the same timescale as spatial homogenisation.

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arXiv2007

Origin of the abundance patterns in Galactic globular clusters: constraints on dynamical and chemical properties of globular clusters

Aims. We analyse the effects of a first generation of fast rotating massive stars on the dynamical and chemical properties of globular clusters. Methods. We use stellar models of fast rotating massive stars, losing mass through a slow mechanical equatorial winds to produce material rich in H-burning products. We propose that stars with high Na and low O abundances (hereafter anomalous stars) are formed from matter made of slow winds of individual massive stars and of interstellar matter. The proportion of slow wind and of interstellar material is fixed in order to reproduce the observed Li-Na anticorrelation in NGC 6752. Results. In the case that globular clusters, during their lifetime, did not lose any stars, we found that to reproduce the observed ratio of normal to anomalous stars, a flat initial mass function (IMF) is needed, with typically a slope x=0.55 (a Salpeter's IMF has x=1.35). In the case that globular clusters suffer from an evaporation of normal stars, the IMF slope can be steeper: to have x=1.35, about 96% of the normal stars would be lost. We make predictions for the distribution of stars as a function of their [O/Na] and obtain quite reasonable agreement with that one observed for NGC 6752. Predictions for the number fraction of stars with different values of helium, of the 12C/13C and 16O/17O ratios are discussed, as well as the expected relations between values of [O/Na] and those of helium, of [C/N], of 12C/13C and of 16O/17O. Future observations might test these predictions. We also provide predictions for the present day mass of the clusters expressed in units of mass of the gas used to form stars, and for the way the present day mass is distributed between the first and second generation of stars and the stellar remnants.

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