163 research outputs found
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
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.Comment: 34 pages, 22 Figures, accepted for publication in Astronomy &
Astrophysic
Advances in secular magnetohydrodynamics of stellar interiors dedicated to asteroseismic spatial missions
With the first light of COROT, the preparation of KEPLER and the future
helioseismology spatial projects such as GOLF-NG, a coherent picture of the
evolution of rotating stars from their birth to their death is needed. We
describe here the modelling of the macroscopic transport of angular momentum
and matter in stellar interiors that we have undertaken to reach this goal.
First, we recall in detail the dynamical processes that are driving these
mechanisms in rotating stars and the theoretical advances we have achieved.
Then, we present our new results of numerical simulations which allow us to
follow in 2D the secular hydrodynamics of rotating stars, assuming that
anisotropic turbulence enforces a shellular rotation law. Finally, we show how
this work is leading to a dynamical vision of the Hertzsprung-Russel diagram
with the support of asteroseismology and helioseismology, seismic observables
giving constraints on the modelling of the internal transport and mixing
processes. In conclusion, we present the different processes that should be
studied in the next future to improve our description of stellar radiation
zones.Comment: 14 pages, 3 figures, Proceeding of the Joint HELAS and CoRoT/ESTA
Workshop (20-23 November 2006, CAUP, Porto - Portugal
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.Comment: 16 pages, 18 figures. Accepted for publication in A&
Constraints on mass loss and self-enrichment scenarios for the globular clusters of the Fornax dSph
Recently, high-dispersion spectroscopy has demonstrated conclusively that
four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy
are very metal-poor with [Fe/H]<-2. The remaining cluster, Fornax 4, has
[Fe/H]=-1.4. This is in stark contrast to the field star metallicity
distribution which shows a broad peak around [Fe/H]=-1 with only a few percent
of the stars having [Fe/H]<-2. If we only consider stars and clusters with
[Fe/H]<-2 we thus find an extremely high GC specific frequency, SN=400,
implying by far the highest ratio of GCs to field stars known anywhere. We
estimate that about 1/5-1/4 of all stars in the Fornax dSph with [Fe/H]<-2
belong to the four most metal-poor GCs. These GCs could, therefore, at most
have been a factor of 4-5 more massive initially. Yet, the Fornax GCs appear to
share the same anomalous chemical abundance patterns known from Milky Way GCs,
commonly attributed to the presence of multiple stellar generations within the
clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax
dSph is difficult to reconcile with scenarios for self-enrichment and early
evolution of GCs in which a large fraction (90%-95%) of the first-generation
stars have been lost. It also suggests that the GCs may not have formed as part
of a larger population of now disrupted clusters with an initial power-law mass
distribution. The Fornax dSph may be a rosetta stone for constraining theories
of the formation, self-enrichment and early dynamical evolution of star
clusters.Comment: 4 pages, 2 figures, accepted for A&A Letter
CP and related phenomena in the context of Stellar Evolution
We review the interaction in intermediate and high mass stars between their
evolution and magnetic and chemical properties. We describe the theory of
Ap-star `fossil' fields, before touching on the expected secular diffusive
processes which give rise to evolution of the field. We then present recent
results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that
magnetic fields of the kind seen on the main-sequence already exist during the
pre-main sequence phase, in agreement with fossil field theory, and that the
origin of the slow rotation of Ap/Bp stars also lies early in the pre-main
sequence evolution; we also present results confirming a lack of stars with
fields below a few hundred gauss. We then seek which macroscopic motions
compete with atomic diffusion in determining the surface abundances of AmFm
stars. While turbulent transport and mass loss, in competition with atomic
diffusion, are both able to explain observed surface abundances, the interior
abundance distribution is different enough to potentially lead to a test using
asterosismology. Finally we review progress on the turbulence-driving and
mixing processes in stellar radiative zones.Comment: Proceedings of IAU GA in Rio, JD4 on Ap stars; 10 pages, 7 figure
An empirical calibration of Lick indices using Milky Way Globular Clusters
To provide an empirical calibration relation in order to convert Lick indices
into abundances for the integrated light of old, simple stellar populations for
a large range in the observed [Fe/H] and [alpha/Fe]. This calibration
supersedes the previously adopted ones because it is be based on the real
abundance pattern of the stars instead of the commonly adopted metallicity
scale derived from the colours. We carried out a long-slit spectroscopic study
of 23 Galactic globular cluster for which detailed chemical abundances in stars
have been recently measured. The line-strength indices, as coded by the Lick
system and by Serven et al. (2005), were measured in low-resolution integrated
spectra of the GC light. The results were compared to average abundances and
abundance ratios in stars taken from the compilation by Pritzl et al. (2005) as
well as to synthetic models. Fe-related indices grow linearly as a function of
[Fe/H] for [Fe/H]>-2. Mg-related indices respond in a similar way to [Mg/H]
variations, however Mgb turns out to be a less reliable metallicity indicator
for [Z/H]<-1.5 . Despite the known Mg overabundance with respect to Fe in GC
stars, it proved impossible to infer a mean [Mg/Fe] for integrated spectra that
correlates with the resolved stars properties, because the sensitivity of the
indices to [Mg/Fe] is smaller at lower metallicities. We present empirical
calibrations for Ca, TiO, Ba and Eu indices as well as the measurements of
H_alpha and NaD.Comment: 16 pages, 15 figures, to appear on A&
Mass-loss and expansion of ultra compact dwarf galaxies through gas expulsion and stellar evolution for top-heavy stellar initial mass functions
(abridged) The dynamical V-band mass-to-light ratios of ultra compact dwarf
galaxies (UCDs) are higher than predicted by simple stellar population models
with the canonical stellar initial mass function (IMF). One way to explain this
finding is a top-heavy IMF, so that the unseen mass is provided by additional
remnants of high-mass stars. A possible explanation for why the IMF in UCDs
could be top-heavy while this is not the case in less massive stellar systems
is that encounters between proto-stars and stars become probable in forming
massive systems. However, the required number of additional stellar remnants
proves to be rather high, which raises the question of how their progenitors
would affect the early evolution of a UCD. We have therefore calculated the
first 200 Myr of the evolution of the UCDs, using the particle-mesh code
Superbox. It is assumed that the stellar populations of UCDs were created in an
initial starburst, which implies heavy mass loss during the following
approximately 40 Myr due to primordial gas expulsion and supernova explosions.
We find at the end of the simulations for various initial conditions and
(tabulated) mass-loss histories objects that roughly resemble UCDs. Thus, the
existence of UCDs does not contradict the notion that their stellar populations
formed rapidly and with a top-heavy IMF. We find tentative evidence that the
UCDs may have had densities as high as 10^8 M_sun/pc^3 at birth.Comment: 19 pages, 16 figures. Figure 4 has been modified in this version; it
now shows the quantities that were actually used in the paper. This
modification therefore does not imply any further changes to the paper, but
there are a few other, very minor changes (typos corrected, formulations
changed)
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