45 research outputs found
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&
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
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&
Tidal dissipation in rotating low-mass stars and implications for the orbital evolution of close-in planets I. From the PMS to the RGB at solar metallicity
Star-planet interactions must be taken into account in stellar models to
understand the dynamical evolution of close-in planets. The dependence of the
tidal interactions on the structural and rotational evolution of the star is of
peculiar importance and should be correctly treated. We quantify how tidal
dissipation in the convective envelope of rotating low-mass stars evolves from
the pre-main sequence up to the red-giant branch depending on the initial
stellar mass. We investigate the consequences of this evolution on planetary
orbital evolution. We couple the tidal dissipation formalism described in
Mathis (2015) to the stellar evolution code STAREVOL and apply it to rotating
stars with masses between 0.3 and 1.4 M. In addition, we generalize the
work of Bolmont & Mathis (2016) by following the orbital evolution of close-in
planets using the new tidal dissipation predictions for advanced phases of
stellar evolution. On the PMS the evolution of tidal dissipation is controlled
by the evolution of the internal structure of the contracting star. On the MS
it is strongly driven by the variation of surface rotation that is impacted by
magnetized stellar winds braking. The main effect of taking into account the
rotational evolution of the stars is to lower the tidal dissipation strength by
about four orders of magnitude on the main-sequence, compared to a normalized
dissipation rate that only takes into account structural changes. The evolution
of the dissipation strongly depends on the evolution of the internal structure
and rotation of the star. From the pre-main sequence up to the tip of the
red-giant branch, it varies by several orders of magnitude, with strong
consequences for the orbital evolution of close-in massive planets. These
effects are the strongest during the pre-main sequence, implying that the
planets are mainly sensitive to the star's early history.Comment: 13 pages, 7 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
The complex interplay between tidal inertial waves and zonal flows in differentially rotating stellar and planetary convective regions:I. Free waves
Quantifying tidal interactions in close-in two-body systems is of prime
interest since they have a crucial impact on the architecture and on the
rotational history of the bodies. Various studies have shown that the
dissipation of tides in either body is very sensitive to its structure and to
its dynamics, like differential rotation which exists in the outer convective
enveloppe of solar-like stars and giant gaseous planets. In particular, tidal
waves may strongly interact with zonal flows at the so-called corotation
resonances, where the wave's Doppler-shifted frequency cancels out. We aim to
provide a deep physical understanding of the dynamics of tidal inertial waves
at corotation resonances, in the presence of differential rotation profiles
typical of low-mass stars and giant planets. By developping an inclined
shearing box, we investigate the propagation and the transmission of free
inertial waves at corotation, and more generally at critical levels, which are
singularities in the governing wave differential equation. Through the
construction of an invariant called the wave action flux, we identify different
regimes of wave transmission at critical levels, which are confirmed with a
one-dimensional three-layer numerical model. We find that inertial waves can be
either fully transmitted, strongly damped, or even amplified after crossing a
critical level. The occurrence of these regimes depends on the assumed profile
of differential rotation, on the nature as well as the latitude of the critical
level, and on wave parameters such as the inertial frequency and the
longitudinal and vertical wavenumbers. Waves can thus either deposit their
action flux to the fluid when damped at critical levels, or they can extract
action flux to the fluid when amplified at critical levels. Both situations
could lead to significant angular momentum exchange between the tidally
interacting bodies.Comment: 25 pages, 12 figures, 4 tables, accepted for publication in Astronomy
& Astrophysic
Front Pediatr
BACKGROUND: Nutritional status is a major prognostic factor for breathing and the survival of patients with cystic fibrosis (CF). Since 2012, the development of CFTR modulators has considerably transformed the outcome of this disease. Indeed, both lung function and body mass index are improved by CFTR modulators, such as Lumacaftor/Ivacaftor. However, few data exist regarding the outcome of nutritional intakes under Lumacaftor/Ivacaftor. METHODS: We conducted a prospective single-center study in children with CF treated with Lumacaftor/Ivacaftor to evaluate their nutritional intake before and after treatment. RESULTS: Thirty-four children were included in this study, with a median age of 12.4 years [11.9; 14.7]. There was no significant improvement in weight, height or BMI. Patients' total energy intake was not significantly changed with Lumacaftor/Ivacaftor, while carbohydrate intakes decreased significantly. We found that blood levels of vitamin E and Selenium were significantly increased under Lumacaftor/Ivacaftor, without a significant increase in supplementation. In patients with a BMI Z-score < 0 at treatment initiation, there was a significant improvement in weight and BMI Z-score, while TEI and carbohydrate intakes were significantly lower. CONCLUSION: We showed that treatment with Lumacaftor/Ivacaftor improved the nutritional status of patients without necessarily being associated with an increase in nutritional intake. Although these data need to be confirmed in larger cohorts, they support the hypothesis that weight gain under modulators is multifactorial, and may be related to a decrease in energy expenditure or an improvement in absorption