284 research outputs found
The dynamics of the radiative envelope of rapidly rotating stars. I. A spherical Boussinesq model
Context: The observations of rapidly rotating stars are increasingly detailed
and precise thanks to interferometry and asteroseismology; two-dimensional
models taking into account the hydrodynamics of these stars are very much
needed.
Aims: A model for studying the dynamics of baroclinic stellar envelope is
presented.
Methods: This models treats the stellar fluid at the Boussinesq approximation
and assumes that it is contained in a rigid spherical domain. The temperature
field along with the rotation of the system generate the baroclinic flow.
Results: We manage to give an analytical solution to the asymptotic problem
at small Ekman and Prandtl numbers. We show that, provided the Brunt-Vaisala
frequency profile is smooth enough, differential rotation of a stably
stratified envelope takes the form a fast rotating pole and a slow equator
while it is the opposite in a convective envelope. We also show that at low
Prandtl numbers and without -barriers, the jump in viscosity at the
core-envelope boundary generates a shear layer staying along the tangential
cylinder of the core. Its role in mixing processes is discussed.
Conclusions: Such a model provides an interesting tool for investigating the
fluid dynamics of rotating stars in particular for the study of the various
instabilities affecting baroclinic flows or, even more, of a dynamo effect.Comment: 17 pages, accepted in Astronomy and Astrophysic
Seismic signature of envelope penetrative convection: the CoRoT star HD 52265
Aims: We aim at characterizing the inward transition from convective to
radiative energy transport at the base of the convective envelope of the
solar-like oscillator HD 52265 recently observed by the CoRoT satellite.
Methods: We investigated the origin of one specific feature found in the HD
52265 frequency spectrum. We modelled the star to derive the internal structure
and the oscillation frequencies that best match the observations and used a
seismic indicator sensitive to the properties of the base of the envelope
convection zone. Results: The seismic indicators clearly reveal that to best
represent the observed properties of HD 52265, models must include penetrative
convection below the outer convective envelope. The penetrative distance is
estimated to be , which corresponds to an extent over a distance
representing 6.0 per cents of the total stellar radius, significantly larger
than what is found for the Sun. The inner boundary of the extra-mixing region
is found at where is the stellar radius.
Conclusions: These results contribute to the tachocline characterization in
stars other than the Sun.Comment: 4 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics Letter
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
Effects of Uniform and Differential Rotation on Stellar Pulsations
We have investigated the effects of uniform rotation and a specific model for
differential rotation on the pulsation frequencies of 10 \Msun\ stellar models.
Uniform rotation decreases the frequencies for all modes. Differential rotation
does not appear to have a significant effect on the frequencies, except for the
most extreme differentially rotating models. In all cases, the large and small
separations show the effects of rotation at lower velocities than do the
individual frequencies. Unfortunately, to a certain extent, differential
rotation mimics the effects o f more rapid rotation, and only the presence of
some specific observed frequencies with well identified modes will be able to
uniquely constrain the internal rotation of pulsating stars.Comment: 33 pages, 16 figures. Accepted for publication in Ap
The CoRoT primary target HD 52265: models and seismic tests
HD 52265 is the only known exoplanet-host star selected as a main target for
the seismology programme of the CoRoT satellite. As such, it will be observed
continuously during five months, which is of particular interest in the
framework of planetary systems studies. This star was misclassified as a giant
in the Bright Star Catalog, while it is more probably on the main-sequence or
at the beginning of the subgiant branch. We performed an extensive analysis of
this star, showing how asteroseismology may lead to a precise determination of
its external parameters and internal structure. We first reviewed the
observational constraints on the metallicity, the gravity and the effective
temperature derived from the spectroscopic observations of HD 52265. We also
derived its luminosity using the Hipparcos parallax. We computed the
evolutionary tracks for models of various metallicities which cross the
relevant observational error boxes in the gravity-effective temperature plane.
We selected eight different stellar models which satisfy the observational
constraints, computed their p-modes frequencies and analysed specific seismic
tests. The possible models for HD 52265, which satisfy the constraints derived
from the spectroscopic observations, are different in both their external and
internal parameters. They lie either on the main sequence or at the beginning
of the subgiant branch. The differences in the models lead to quite different
properties of their oscillation frequencies. We give evidences of an
interesting specific behaviour of these frequencies in case of helium-rich
cores: the ``small separations'' may become negative and give constraints on
the size of the core. We expect that the observations of this star by the CoRoT
satellite wi ll allow choosing between these possible models.Comment: 11 pages, 7 figures, to be published in Astronomy and Astrophysic
Merger of binary neutron stars with realistic equations of state in full general relativity
We present numerical results of three-dimensional simulations for the merger
of binary neutron stars (BNSs) in full general relativity. Hybrid equations of
state (EOSs) are adopted to mimic realistic nuclear EOSs. In this approach, we
divide the EOSs into two parts, i.e., the thermal part and the cold part. For
the cold part, we assign a fitting formula for realistic EOSs of cold nuclear
matter slightly modifying the formula developed by Haensel and Potekhin. We
adopt the SLy and FPS EOSs for which the maximum allowed ADM mass of cold and
spherical neutron stars (NSs) is ~ 2.04Mo and 1.80Mo, respectively. Simulations
are performed for BNSs of the total ADM mass in the range between 2.4Mo and
2.8Mo with the rest-mass ratio Q_M to be in the range 0.9 < Q_M < 1. It is
found that if the total ADM mass of the system is larger than a threshold
M_{thr}, a black hole (BH) is promptly formed in the merger irrespective of the
mass ratios. In the other case, the outcome is a hypermassive NS of a large
ellipticity, which results from the large adiabatic index of the realistic EOSs
adopted. The value of M_{thr} depends on the EOS: ~ 2.7Mo and ~ 2.5Mo for the
SLy and FPS EOSs, respectively. Gravitational waves are computed in terms of a
gauge-invariant wave extraction technique. In the formation of the hypermassive
NS, quasiperiodic gravitational waves of a large amplitude and of frequency
between 3 and 4 kHz are emitted. The estimated emission time scale is < 100 ms,
after which the hypermassive NS collapses to a BH. Because of the long emission
time, the effective amplitude may be large enough to be detected by advanced
laser interferometric gravitational wave detectors if the distance to the
source is smaller than ~ 100 Mpc.Comment: Typos corrected, 2 references and comments on them added, 26 pages,
54 Postscript figures, Phys.Rev.D in pres
On the structure of the Sun and alpha Centauri A and B in the light of seismic and non-seismic constraints
The small separation (delta nu 01, delta nu 02 and delta nu 13) between the
oscillations with low degree l is dependent primarily on the sound speed
profile within the stellar core, where nuclear evolution occurs. The detection
of such oscillations for a star offers a very good opportunity to determine the
stage of its nuclear evolution, and hence its age. In this context, we
investigate the Sun and alpha Cen A and B. For alpha Cen A and B, each of the
small separations delta nu 01, delta nu 02 and delta nu 13 gives a different
age. Therefore, in our fitting process, we also employ the second difference,
defined as nu n2 - 2 nu n1 + nu n0, which is 2 delta nu 01- delta nu 02. In
addition to this, we also use frequency ratio (nu n0/ nu n2). For the Sun,
these expressions areequivalent and give an age of about 4.9-5.0 Gyr. For alpha
Cen A and B, however, the small separation and the second difference give very
different ages. This conflict may be solved by the detection of oscillation
frequencies that can be measured much more precisely than the current
frequencies. When we fit the models to the observations, we find (i) Z 0=0.020,
t=3.50 Gyr and M B=1.006 Msun from the small separations delta nu 01, delta nu
02 and delta nu 13 of alpha Cen B; and (ii) a variety of solutions from the
non-seismic constraints and delta nu 02 of alpha Cen A and B, in which the
masses of alpha Cen A and B are slightly modified and the age of the system is
about 5.2-5.3 Gyr. For Z=0.025, the closest masses we find to the observed
masses are M B=0.922 Msun and M A=1.115 Msun.The differences between these
masses and the corresponding observed masses are about 0.01 Msun.Comment: 9 Pages and 9 Figure
Protodiscs around Hot Magnetic Rotator Stars
We develop equations and obtain solutions for the structure and evolution of
a protodisc region that is initially formed with no radial motion and
super-Keplerian rotation speed when wind material from a hot rotating star is
channelled towards its equatorial plane by a dipole-type magnetic field. Its
temperature is around K because of shock heating and the inflow of wind
material causes its equatorial density to increase with time. The centrifugal
force and thermal pressure increase relative to the magnetic force and material
escapes at its outer edge. The protodisc region of a uniformly rotating star
has almost uniform rotation and will shrink radially unless some instability
intervenes. In a star with angular velocity increasing along its surface
towards the equator, the angular velocity of the protodisc region decreases
radially outwards and magnetorotational instability (MRI) can occur within a
few hours or days. Viscosity resulting from MRI will readjust the angular
velocity distribution of the protodisc material and may assist in the formation
of a quasi-steady disc. Thus, the centrifugal breakout found in numerical
simulations for uniformly rotating stars does not imply that quasi-steady discs
with slow outflow cannot form around magnetic rotator stars with solar-type
differential rotation.Comment: Accepted for publication in MNRAS. 16 pages, 1 figure, 7 table
Regular Oscillation Sub-spectrum of Rapidly Rotating Stars
We present an asymptotic theory that describes regular frequency spacings of
pressure modes in rapidly rotating stars. We use an asymptotic method based on
an approximate solution of the pressure wave equation constructed from a stable
periodic solution of the ray limit. The approximate solution has a Gaussian
envelope around the stable ray, and its quantization yields the frequency
spectrum. We construct semi-analytical formulas for regular frequency spacings
and mode spatial distributions of a subclass of pressure modes in rapidly
rotating stars. The results of these formulas are in good agreement with
numerical data for oscillations in polytropic stellar models. The regular
frequency spacings depend explicitly on internal properties of the star, and
their computation for different rotation rates gives new insights on the
evolution of mode frequencies with rotation.Comment: 14 pages, 10 figure
Asteroseismological constraints on the pulsating planetary nebula nucleus (PG1159-type) RX J2117.1+3412
We present asteroseismological inferences on RX J2117.1+3412, the hottest
known pulsating PG1159 star. Our results are based on full PG1159 evolutionary
models recently presented by Miller Bertolami & Althaus (2006). We performed
extensive computations of adiabatic g-mode pulsation periods on PG1159
evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo. PG1159
stellar models are extracted from the complete evolution of progenitor stars
started from the ZAMS, through the thermally pulsing AGB and born-again phases
to the domain of the PG 1159 stars. We constrained the stellar mass of RX
J2117.1+3412 by comparing the observed period spacing with the asymptotic
period spacing and with the average of the computed period spacings. We also
employed the individual observed periods to find a representative seismological
model. We derive a stellar mass of 0.56-0.57 Mo from the period spacing data
alone. In addition, we found a best-fit model representative for RX
J2117.1+3412 with an effective temperature of 163,400 K, a stellar mass of
0.565 Mo, and a surface gravity log g= 6.61. The derived stellar luminosity and
radius are log(L/Lo)= 3.36 and log(R/Ro)= -1.23, respectively, and the He-rich
envelope thickness is Menv= 0.02 Mo. We derive a seismic distance of 452 pc and
a linear size of the planetary nebula of 1.72 pc. These inferences seem to
solve the discrepancy between the RX J2117.1+3412 evolutionary timescale and
the size of the nebula. All of the seismological tools we use concur to the
conclusion that RX J2117.1+3412 must have a stellar mass of 0.565 Mo much in
agreement with recent asteroseismology studies and in clear conflict with the
predictions of spectroscopy plus evolutionary tracks.Comment: 10 pages, 6 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics. Erratum available as a separate fil
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