521 research outputs found
Stochastic excitation of non-radial modes I. High-angular-degree p modes
Turbulent motions in stellar convection zones generate acoustic energy, part
of which is then supplied to normal modes of the star. Their amplitudes result
from a balance between the efficiencies of excitation and damping processes in
the convection zones. We develop a formalism that provides the excitation rates
of non-radial global modes excited by turbulent convection. As a first
application, we estimate the impact of non-radial effects on excitation rates
and amplitudes of high-angular-degree modes which are observed on the Sun. A
model of stochastic excitation by turbulent convection has been developed to
compute the excitation rates, and it has been successfully applied to solar
radial modes (Samadi & Goupil 2001, Belkacem et al. 2006b). We generalize this
approach to the case of non-radial global modes. This enables us to estimate
the energy supplied to high-() acoustic modes. Qualitative arguments as
well as numerical calculations are used to illustrate the results. We find that
non-radial effects for modes are non-negligible:
- for high- modes (i.e. typically ) and for high values of ;
the power supplied to the oscillations depends on the mode inertia.
- for low- modes, independent of the value of , the excitation is
dominated by the non-diagonal components of the Reynolds stress term. We
carried out a numerical investigation of high- modes and we find that
the validity of the present formalism is limited to due to the
spatial separation of scale assumption. Thus, a model for very high-
-mode excitation rates calls for further theoretical developments, however
the formalism is valid for solar modes, which will be investigated in a
paper in preparation.Comment: 12 pages, accepted for publication in A&
Instability strips of main sequence B stars: a parametric study of iron enhancement
The discovery of beta Cephei stars in low metallicity environments, as well
as the difficulty to theoretically explain the excitation of the pulsation
modes observed in some beta Cephei and SPB stars, suggest that the iron opacity
``bump'' provided by standard models could be underestimated. We investigate,
by means of a parametric study, the effect of a local iron enhancement on the
location of the beta Cephei and SPB instability strips.Comment: 2 pages, to appear in the proceedings of "Vienna Workshop on the
Future of Asteroseismology", September 20-22, 200
Instability strips of SPB and beta Cephei stars: the effect of the updated OP opacities and of the metal mixture
The discovery of Cephei stars in low metallicity environments, as
well as the difficulty in theoretically explaining the excitation of the
pulsation modes observed in some Cephei and hybrid SPB- Cephei
pulsators, suggest that the ``iron opacity bump'' provided by stellar models
could be underestimated. We analyze the effect of uncertainties in the opacity
computations and in the solar metal mixture, on the excitation of pulsation
modes in B-type stars. We carry out a pulsational stability analysis for four
grids of main-sequence models with masses between 2.5 and 12
computed with OPAL and OP opacity tables and two different metal mixtures.
We find that in a typical Cephei model the OP opacity is 25% larger
than OPAL in the region where the driving of pulsation modes occurs.
Furthermore, the difference in the Fe mass fraction between the two metal
mixtures considered is of the order of 20%. The implication on the excitation
of pulsation modes is non-negligible: the blue border of the SPB instability
strip is displaced at higher effective temperatures, leading to a larger number
of models being hybrid SPB- Cephei pulsators. Moreover, higher overtone
p-modes are excited in Cephei models and unstable modes are found in a
larger number of models for lower metallicities, in particular Cephei
pulsations are also found in models with Z=0.01.Comment: Accepted for publication in MNRAS Letter
Theoretical power spectra of mixed modes in low mass red giant stars
CoRoT and Kepler observations of red giant stars revealed very rich spectra
of non-radial solar-like oscillations. Of particular interest was the detection
of mixed modes that exhibit significant amplitude, both in the core and at the
surface of the stars. It opens the possibility of probing the internal
structure from their inner-most layers up to their surface along their
evolution on the red giant branch as well as on the red-clump. Our objective is
primarily to provide physical insight into the physical mechanism responsible
for mixed-modes amplitudes and lifetimes. Subsequently, we aim at understanding
the evolution and structure of red giants spectra along with their evolution.
The study of energetic aspects of these oscillations is also of great
importance to predict the mode parameters in the power spectrum. Non-adiabatic
computations, including a time-dependent treatment of convection, are performed
and provide the lifetimes of radial and non-radial mixed modes. We then combine
these mode lifetimes and inertias with a stochastic excitation model that gives
us their heights in the power spectra. For stars representative of CoRoT and
Kepler observations, we show under which circumstances mixed modes have heights
comparable to radial ones. We stress the importance of the radiative damping in
the determination of the height of mixed modes. Finally, we derive an estimate
for the height ratio between a g-type and a p-type mode. This can thus be used
as a first estimate of the detectability of mixed-modes
Are the stars of a new class of variability detected in NGC~3766 fast rotating SPB stars?
A recent photometric survey in the NGC~3766 cluster led to the detection of
stars presenting an unexpected variability. They lie in a region of the
Hertzsprung-Russell (HR) diagram where no pulsation are theoretically expected,
in between the Scuti and slowly pulsating B (SPB) star instability
domains. Their variability periods, between 0.1--0.7~d, are outside the
expected domains of these well-known pulsators. The NCG~3766 cluster is known
to host fast rotating stars. Rotation can significantly affect the pulsation
properties of stars and alter their apparent luminosity through gravity
darkening. Therefore we inspect if the new variable stars could correspond to
fast rotating SPB stars. We carry out instability and visibility analysis of
SPB pulsation modes within the frame of the traditional approximation. The
effects of gravity darkening on typical SPB models are next studied. We find
that at the red border of the SPB instability strip, prograde sectoral (PS)
modes are preferentially excited, with periods shifted in the 0.2--0.5~d range
due to the Coriolis effect. These modes are best seen when the star is seen
equator-on. For such inclinations, low-mass SPB models can appear fainter due
to gravity darkening and as if they were located between the ~Scuti and
SPB instability strips.Comment: 6 pages, 2 figures, to appear in the proceedings of the IAU Symposium
307, New windows on massive stars: asteroseismology, interferometry, and
spectropolarimetr
Theoretical seismic properties of pre-main sequence gamma Doradus pulsators
Context. gamma Doradus (gamma Dor) are late A and F-type stars pulsating with
high order gravity modes (g-modes). The existence of different evolutionary
phases crossing the gamma Dor instability strip raises the question of the
existence of pre-main sequence (PMS) gamma Dor stars. Aims. We intend to study
the differences between the asteroseismic behaviour of PMS and main sequence
(MS) gamma Dor pulsators as it is predicted by the current theory of stellar
evolution and stability. Methods. We explore the adiabatic and non-adiabatic
properties of high order g-modes in a grid of PMS and MS models covering the
mass range 1.2 Msun < Mstar < 2.5 Msun. Results. We derive the theoretical
instability strip (IS) for the PMS gamma Dor pulsators. This IS covers the same
effective temperature range as the MS gamma Dor one. Nevertheless, the
frequency domain of unstable modes in PMS models with a fully radiative core is
larger than in MS models, even if they present the same number of unstable
modes. Moreover, the differences between MS and PMS internal structures are
reflected on the average values of the period spacing as well as on the
dependence of the period spacing on the radial order of the modes, opening the
window to the determination of the evolutionary phase of gamma Dor stars from
their pulsation spectra.Comment: 9 pages, 17 figures, accepted for publication in A&
Can an underestimation of opacity explain B-type pulsators in the SMC?
Slowly Pulsating B and Cephei are mechanism driven pulsating
B stars. That mechanism works since a peak in the opacity due to a
high number of atomic transitions from iron-group elements occurs in the area
of . Theoretical results predict very few SPBs and no
Cep to be encountered in low metallicity environments such as the Small
Magellanic Cloud. However recent variability surveys of B stars in the SMC
reported the detection of a significant number of SPB and Cep
candidates. Though the iron content plays a major role in the excitation of
Cep and SPB pulsations, the chemical mixture representative of the SMC
B stars such as recently derived does not leave room for a significant increase
of the iron abundance in these stars. Whilst abundance of iron-group elements
seems reliable, is the opacity in the iron-group elements bump underestimated?
We determine how the opacity profile in B-type stars should change to excite
SPB and Cep pulsations in early-type stars of the SMC.Comment: 5 pages, 7 figures, to appear under electronic form in : Proceedings
of the 4th HELAS International Conference: Seismological Challenges for
Stellar Structur
A new seismic analysis of Alpha Centauri
Models of alpha Cen A & B have been computed using the masses determined by
Pourbaix et al. (2002) and the data derived from the spectroscopic analysis of
Neuforge and Magain (1997). The seismological data obtained by Bouchy and
Carrier (2001, 2002) do help improve our knowledge of the evolutionary status
of the system. All the constraints are satisfied with a model which gives an
age of about 6 Gyr for the binary.Comment: to be published in Astronomy and Astrophysic
- …