159 research outputs found
Improved angular momentum evolution model for solar-like stars
We present new models for the rotational evolution of solar-like stars
between 1 Myr and 10 Gyr with the aim to reproduce the distributions of
rotational periods observed for star forming regions and young open clusters
within this age range. The models include a new wind braking law based on
recent numerical simulations of magnetized stellar winds and specific dynamo
and mass-loss prescriptions are adopted to tie angular momentum loss to angular
velocity. The model additionally assume constant angular velocity during the
disk accretion phase and allow for decoupling between the radiative core and
the convective envelope as soon as the former develops. We have developed
rotational evolution models for slow, median and fast rotators with initial
periods of 10, 7, and 1.4d, respectively. The models reproduce reasonably well
the rotational behaviour of solar-type stars between 1 Myr and 4.5 Gyr,
including PMS to ZAMS spin up, prompt ZAMS spin down, and the early-MS
convergence of surface rotation rates. We find the model parameters accounting
for the slow and median rotators are very similar to each other, with a disk
lifetime of 5 Myr and a core-envelope coupling timescale of 28-30 Myr. In
contrast, fast rotators have both shorter disk lifetime (2.5 Myr) and
core-envelope coupling timescale (12 Myr). We emphasize that these results are
highly dependent on the adopted braking law. We also report a tentative
correlation between initial rotational period and disk lifetime, which suggests
that protostellar spin-down by massive disks in the embedded phase is at the
origin of the initial dispersion of rotation rates in young stars. We conclude
that this class of semi-empirical models successfully grasp the main trends of
the rotational behaviour of solar-type stars as they evolve and make specific
predictions that may serve as a guide for further development.Comment: 16 pages, 5 figures, 4 table, accepted for publication by A&A. New
version that include the linguistic correctio
Improved angular momentum evolution model for solar-like stars II. Exploring the mass dependence
We developed angular momentum evolution models for 0.5 and 0.8
stars. The parametric models include a new wind braking law based on recent
numerical simulations of magnetised stellar winds, specific dynamo and
mass-loss rate prescriptions, as well as core/envelope decoupling. We compare
model predictions to the distributions of rotational periods measured for low
mass stars belonging to star forming regions and young open clusters.
Furthermore, we explore the mass dependence of model parameters by comparing
these new models to the solar-mass models we developed earlier. Rotational
evolution models are computed for slow, median, and fast rotators at each
stellar mass. The models reproduce reasonably well the rotational behaviour of
low-mass stars between 1 Myr and 8-10 Gyr, including pre-main sequence to
zero-age main sequence spin up, prompt zero-age main sequence spin down, and
early-main sequence convergence of the surface rotation rates. Fast rotators
are found to have systematically shorter disk lifetimes than moderate and slow
rotators, thus enabling dramatic pre-main sequence spin up. They also have
shorter core-envelope coupling timescales, i.e., more uniform internal
rotation. As to the mass dependence, lower mass stars require significantly
longer core-envelope coupling timescale than solar-type ones, which results in
strong differential rotation developing in the stellar interior on the early
main sequence. Lower mass stars also require a weaker braking torque to account
for their longer spin down timescale on the early main sequence, while they
ultimately converge towards lower rotational velocities than solar-type stars
on the longer term due to their reduced moment of inertia. We also find
evidence that the mass-dependence of the wind braking efficiency may be related
to a change of the magnetic topology in lower mass stars.Comment: 17 pages, 11 figures, accepted for publication in A&
A circumbinary disc model for the variability of the eclipsing binary CoRoT 223992193
We calculate the flux received from a binary system obscured by a
circumbinary disc. The disc is modelled using two dimensional hydrodynamical
simulations, and the vertical structure is derived by assuming it is
isothermal. The gravitational torque from the binary creates a cavity in the
disc's inner parts. If the line of sight along which the system is observed has
a high inclination , it intersects the disc and some absorption is produced.
As the system is not axisymmetric, the resulting light curve displays
variability. We calculate the absorption and produce light curves for different
values of the dust disc aspect ratio and mass of dust in the cavity
. This model is applied to the high inclination ()
eclipsing binary CoRoT 223992193, which shows 5-10% residual photometric
variability after the eclipses and a spot model are subtracted. We find that
such variations for can be obtained for and
M. For higher , would
have to be close to this lower value and somewhat less than .
Our results show that such variability in a system where the stars are at least
90% visible at all phases can be obtained only if absorption is produced by
dust located inside the cavity. If absorption is dominated by the parts of the
disc located close to or beyond the edge of the cavity, the stars are
significantly obscured.Comment: 17 pages, 6 figures, accepted for publication in MNRA
Rotating models of young solar-type stars : Exploring braking laws and angular momentum transport processes
We study the predicted rotational evolution of solar-type stars from the
pre-main sequence to the solar age with 1D rotating evolutionary models
including physical ingredients. We computed rotating evolution models of
solar-type stars including an external stellar wind torque and internal
transport of angular momentum following the method of Maeder and Zahn with the
code STAREVOL. We explored different formalisms and prescriptions available
from the literature. We tested the predictions of the models against recent
rotational period data from extensive photometric surveys, lithium abundances
of solar-mass stars in young clusters, and the helioseismic rotation profile of
the Sun. We find a best-matching combination of prescriptions for both internal
transport and surface extraction of angular momentum. This combination provides
a very good fit to the observed evolution of rotational periods for solar-type
stars from early evolution to the age of the Sun. Additionally, we show that
fast rotators experience a stronger coupling between their radiative region and
the convective envelope. Regardless of the set of prescriptions, however, we
cannot simultaneously reproduce surface angular velocity and the internal
profile of the Sun or the evolution of lithium abundance. We confirm the idea
that additional transport mechanisms must occur in solar-type stars until they
reach the age of the Sun. Whether these processes are the same as those needed
to explain recent asteroseismic data in more advanced evolutionary phases is
still an open question.Comment: 16 pages, 16 figures, accepted for publication in A&
The magnetic obliquity of accreting T Tauri stars
Classical T Tauri stars (CTTS) accrete material from their discs through
their magnetospheres. The geometry of the accretion flow strongly depends on
the magnetic obliquity, i.e., the angle between the rotational and magnetic
axes. We aim at deriving the distribution of magnetic obliquities in a sample
of 10 CTTSs. For this, we monitored the radial velocity variations of the
HeI5876 line in these stars' spectra along their rotational cycle. HeI
is produced in the accretion shock, close to the magnetic pole. When the
magnetic and rotational axes are not aligned, the radial velocity of this line
is modulated by stellar rotation. The amplitude of modulation is related to the
star's projected rotational velocity, , and the latitude of the
hotspot. By deriving and HeI5876 radial velocity curves from
our spectra we thus obtain an estimate of the magnetic obliquities. We find an
average obliquity in our sample of 11.4 with an rms dispersion of
5.4. The magnetic axis thus seems nearly, but not exactly aligned
with the rotational axis in these accreting T Tauri stars, somewhat in
disagreement with studies of spectropolarimetry, which have found a significant
misalignment () for several CTTSs. This could simply be an
effect of low number statistics, or it may be due to a selection bias of our
sample. We discuss possible biases that our sample may be subject to. We also
find tentative evidence that the magnetic obliquity may vary according to the
stellar interior and that there may be a significant difference between fully
convective and partly radiative stars.Comment: 28 pages (including online material), 25 figure
From solar to stellar corona: the role of wind, rotation and magnetism
Observations of surface magnetic fields are now within reach for many stellar
types thanks to the development of Zeeman-Doppler Imaging. These observations
are extremely useful for constraining rotational evolution models of stars, as
well as for characterizing the generation of magnetic field. We recently
demonstrated that the impact of coronal magnetic field topology on the
rotational braking of a star can be parametrized with a scalar parameter: the
open magnetic flux. However, without running costly numerical simulations of
the stellar wind, reconstructing the coronal structure of the large scale
magnetic field is not trivial. An alternative -broadly used in solar physics-
is to extrapolate the surface magnetic field assuming a potential field in the
corona, to describe the opening of the field lines by the magnetized wind. This
technique relies on the definition of a so-called source surface radius, which
is often fixed to the canonical value of 2.5Rsun. However this value likely
varies from star to star. To resolve this issue, we use our extended set of
2.5D wind simulations published in 2015, to provide a criteria for the opening
of field lines as well as a simple tool to assess the source surface radius and
the open magnetic flux. This allows us to derive the magnetic torque applied to
the star by the wind from any spectropolarimetric observation. We conclude by
discussing some estimations of spin-down time scales made using our technique,
and compare them to observational requirements.Comment: Accepted for publication in the Astrophysical Journa
Evolution dynamique des amas stellaires jeunes
Comprendre le processus de formation stellaire est un objectif majeur en astronomie. Sur ce sujet les observations ne donnent que très peu d'information, et les modèles numériques sont donc naturellement privilégiés. De tels modèles s'attachent à suivre la dynamique du gaz, sous l'effet de processus physique variés, ce qui nécessite un temps de calcul très important et ne permet pas de modéliser l'évolution au delà de 0.2 Myr environ. Or les résultats observationnels sont essentiellement issus du champ galactique proche, des amas évolués, voire des regions jeunes ou associations d'étoiles, dont l'âge peut varier de 1 Myr à quelques Gyr. Par conséquent, il est nécessaire pour comparer les résultats des modèles aux observations de comprendre ce qu'il se passe durant cet intervalle de temps. La formation stellaire tend à produire des étoiles en groupes, à partir de l'effondrement gravitationnel d'un nuage moléculaire turbulent. A mesure que les étoiles se forment, le gaz est éjecté et l'évolution est dominée par les interactions gravitationnelles. Suivre l'évolution sous l'effet de ces interactions est couramment utilisé afin de contraindre les modèles et de mieux comprendre l'origine des populations stellaires observées. Les étoiles se forment en sous-groupes ou structures hiérarchisées, qui peuvent ensuite fusionner pour donner des amas stellaires proche des amas ouverts, ou au contraire finir en associations distinctes. Dans ma thèse, je me suis intéressé à l'évolution dynamique de petits groupes d'étoiles, jusqu'alors peu étudiés par rapport aux groupes à 1000 ou 10^4 étoiles. J'ai simulé l'évolution de groupes à N < 100, dans le but d'en étudier la dynamique d'un point de vue statistique, grâce notamment au grand nombre de simulations effectuées, et afin d'identifier les signatures observationnelles propres à une situation initiale donnée. A partir d'un grand nombre de configurations initiales (avec N=20, 50, 100, un rayon typique de 0.025 pc à 1 pc) et 500 simulations par configurations, j'ai étudié l'évolution dynamique de groupes composés d'étoiles de même masse ou comprenant un spectre de masse, et sans population de binaire initiale. L'évolution de tels groupes s'est révélée similaire à celle de groupes plus grands, mais avec une phase d'effondrement plus rapide et surtout moins prononcée. Je décris le comportement moyen menant à une lente expansion de l'amas, ainsi qu'une voie d'évolution très différente, apparaissant dans 17% des cas étudiés, où l'amas est complètement dispersé suite à l'éjection d'une binaire centrale serrée. J'ai également recherché dans quelle mesure les données en densité et en vitesse 3D pouvaient permettre d'identifier l'état dynamique initial d'un groupe. L'utilisation de ces seules données suffisait dans certain cas à déterminer la densité initiale, mais elles devraient être complétées par des données concernant la population de binaire. Ce travail pourra être mis en application pour étudier l'origine dynamique d'association ou de groupes stellaires connus. Enfin, j'ai effectué un grand nombre de simulations numériques dans le but de reproduire l'état observé de l'amas eta Chamaeleontis par pure évolution dynamique à partir de conditions initiales standards. Cette association présente des caractéristiques d'amas évolué, telle que son spectre de masse pauvre en objets de faible masse et l'absence de binaires larges. Je montre que ces propriétés ne peuvent pas être reproduites uniquement par la dynamique, et sont donc les traces d'un processus de formation non standard.Understanding the star formation process is a key issue in astronomy. Since direct observation provide only very limited information, this issue is investigated by models. Such models need to take into account complex physical processes while following the gas dynamics, so that simulations need a lot of time to run and do not follow the star formation process for longer than 0.2 Myr. The best known observational results concerns the field population, evolved open clusters or younger clusters or associations, which are between 1 Myr and a few Gyr old. Therefore in order to compare the results from models to known observations, we need to bridge the gap between the two. Star formation appears to produce groups of stars from the collapse of turbulent molecular clouds. As stars form, the gas is progressively ejected from the cluster, and the evolution is dominated by gravitational interactions. Following the dynamical evolution of a group of star using N-Body codes is a standard way used to constraint the models and understand the origin of the different populations. Star formation may produce sub-structure or small groups that merge to form bigger entities, or end up as loose association. In my thesis I focused on the dynamics of small groups, that have not been investigated as thoroughly as 1000 or 10^4 star groups. I performed N-Body simulations of small stellar groups, with N<100, in order to study their dynamics using a statistical approach, made possible by running a large number of simulations, and to find some observational signatures of given initial conditions. This approach enable to take full account of stochastic effects due to dynamical interactions. Using a large number of initial configurations (with N=20, 50, 100, a typical radius from 0.025 pc to 1 pc) and a sample of 500 simulations per configuration, I looked at equal mass groups as well as groups having a mass spectrum, without any binary initially. Such small groups show similar evolution to bigger groups, but with faster and less pronounced collapse phase. I described the average behaviour of slow expansion of the cluster, and an alternative evolution, occurring with 17% probability, that ended in the complete dissolution of the group due to ejection of a central binary. Searching for a way to identify the initial configuration from observational measure, I looked at the complementarity of density and 3D velocity and was able to show that it could be sufficient in some cases to determine the initial density. Further investigations are needed to take into account the information on the binary population and will be used to investigate the formation of known associations or young regions. Finally, I ran a large number of simulations, aiming at reproducing the observed state of the eta Chamaeleontis from standard initial conditions and pure dynamical evolution. This association properties are consistent with a dynamical evolved cluster, namely low-mass object poor and having only tight binaries. I showed that these properties cannot be reproduced with pure dynamical evolution from standard initial mass function and binary population, meaning that its particular features must have been pristine.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
Spin alignment of stars in old open clusters
Stellar clusters form by gravitational collapse of turbulent molecular
clouds, with up to several thousand stars per cluster. They are thought to be
the birthplace of most stars and therefore play an important role in our
understanding of star formation, a fundamental problem in astrophysics. The
initial conditions of the molecular cloud establish its dynamical history until
the stellar cluster is born. However, the evolution of the cloud's angular
momentum during cluster formation is not well understood. Current observations
have suggested that turbulence scrambles the angular momentum of the
cluster-forming cloud, preventing spin alignment amongst stars within a
cluster. Here we use asteroseismology to measure the inclination angles of spin
axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The
stars within each cluster show strong alignment. Three-dimensional
hydrodynamical simulations of proto-cluster formation show that at least 50 %
of the initial proto-cluster kinetic energy has to be rotational in order to
obtain strong stellar-spin alignment within a cluster. Our result indicates
that the global angular momentum of the cluster-forming clouds was efficiently
transferred to each star and that its imprint has survived after several
gigayears since the clusters formed.Comment: 14 pages, 3 figures, 1 table. Published in Nature Astronom
The 2008-2009 outburst of the young binary system Z CMa unraveled by interferometry with high spectral resolution
Z CMa is a young binary system consisting of an Herbig primary and a FU Ori
companion. Both components seem to be surrounded by active accretion disks and
a jet was associated to the Herbig B0. In Nov. 2008, K. Grankin discovered that
Z CMa was exhibiting an outburst with an amplitude larger than any photometric
variations recorded in the last 25 years. To study the innermost regions in
which the outburst occurs and understand its origin, we have observed both
binary components with AMBER/VLTI across the Br{\gamma} emission line in Dec.
2009 in medium and high spectral resolution modes. Our observations show that
the Herbig Be, responsible for the increase of luminosity, also produces a
strong Br{\gamma} emission, and they allow us to disentangle from various
origins by locating the emission at each velocities through the line.
Considering a model of a Keplerian disk alone fails at reproducing the
asymmetric spectro-astrometric measurements, suggesting a major contribution
from an outflow.Comment: To be published in the proceedings of the SPIE'2010 conference on
"Optical and Infrared Interferometry II
L’émergence en question
Jérôme Dokic, Frederic Nef, Bernard Walliser, directeurs d’étudesAlban Bouvier, professeur à l’Université Aix-Marseille-II/Provence L’émergence en question : les niveaux d’explication Ce séminaire transversal a traité du problème de l’émergence entendue comme apparition de propriétés macroscopiques à partir d’éléments microscopiques en interaction. Des séances spécialisées ont donné la parole à des spécialistes de disciplines diverses qui ont exposé comment l’émergence était traitée dans leur..
- …