14 research outputs found
Non-adiabatic oscillations of fast-rotating stars: the example of Rasalhague
Early-type stars generally tend to be fast rotators. In these stars, mode
identification is very challenging as the effects of rotation are not well
known. We consider here the example of Ophiuchi, for which dozens of
oscillation frequencies have been measured. We model the star using the
two-dimensional structure code ESTER, and we compute both adiabatic and
non-adiabatic oscillations using the TOP code. Both calculations yield very
complex spectra, and we used various diagnostic tools to try and identify the
observed pulsations. While we have not reached a satisfactory mode-to-mode
identification, this paper presents promising early results.Comment: 4 pages, 3 figures. SF2A 2017 proceeding
Gravito-inertial waves in a differentially rotating spherical shell
The gravito-inertial waves propagating over a shellular baroclinic flow
inside a rotating spherical shell are analysed using the Boussinesq
approximation. The wave properties are examined by computing paths of
characteristics in the non-dissipative limit, and by solving the full
dissipative eigenvalue problem using a high-resolution spectral method.
Gravito-inertial waves are found to obey a mixed-type second-order operator and
to be often focused around short-period attractors of characteristics or
trapped in a wedge formed by turning surfaces and boundaries. We also find
eigenmodes that show a weak dependence with respect to viscosity and heat
diffusion just like truly regular modes. Some axisymmetric modes are found
unstable and likely destabilized by baroclinic instabilities. Similarly, some
non-axisymmetric modes that meet a critical layer (or corotation resonance) can
turn unstable at sufficiently low diffusivities. In all cases, the instability
is driven by the differential rotation. For many modes of the spectrum, neat
power laws are found for the dependence of the damping rates with diffusion
coefficients, but the theoretical explanation for the exponent values remains
elusive in general. The eigenvalue spectrum turns out to be very rich and
complex, which lets us suppose an even richer and more complex spectrum for
rotating stars or planets that own a differential rotation driven by
baroclinicity.Comment: 33 pages, 14 figures, accepted for publication in Journal of Fluid
Mechanic
Detailed equilibrium and dynamical tides: impact on circularization and synchronization in open clusters
Binary stars evolve into chemically-peculiar objects and are a major driver
of the Galactic enrichment of heavy elements. During their evolution they
undergo interactions, including tides, that circularize orbits and synchronize
stellar spins, impacting both individual systems and stellar populations. Using
Zahn's tidal theory and MESA main-sequence model grids, we derive the governing
parameters and , and implement them in the new MINT library
of the stellar population code BINARY_C. Our MINT equilibrium tides are 2 to 5
times more efficient than the ubiquitous BSE prescriptions while the
radiative-tide efficiency drops sharply with increasing age. We also implement
precise initial distributions based on bias-corrected observations. We assess
the impact of tides and initial orbital-parameter distributions on
circularization and synchronization in eight open clusters, comparing synthetic
populations and observations through a bootstrapping method. We find that
changing the tidal prescription yields no statistically-significant improvement
as both calculations typically lie within 0.5. The initial
distribution, especially the primordial concentration of systems at
dominates the statistics even
when artificially increasing tidal strength. This confirms the inefficiency of
tides on the main sequence and shows that constraining tidal-efficiency
parameters using the distribution alone is difficult
or impossible. Orbital synchronization carries a more striking age-dependent
signature of tidal interactions. In M35 we find twice as many synchronized
rotators in our MINT calculation as with BSE. This measure of tidal efficiency
is verifiable with combined measurements of orbital parameters and stellar
spins.Comment: 24 pages, 29 figures includings appendices. Accepted for publication
in MNRA
Asteroseismology of massive stars with the TESS mission: the runaway Beta Cep pulsator PHL 346 = HN Aqr
We report an analysis of the first known Beta Cep pulsator observed by the
TESS mission, the runaway star PHL 346 = HN Aqr. The star, previously known as
a singly-periodic pulsator, has at least 34 oscillation modes excited, 12 of
those in the g-mode domain and 22 p modes. Analysis of archival data implies
that the amplitude and frequency of the dominant mode and the stellar radial
velocity were variable over time. A binary nature would be inconsistent with
the inferred ejection velocity from the Galactic disc of 420 km/s, which is too
large to be survivable by a runaway binary system. A kinematic analysis of the
star results in an age constraint (23 +- 1 Myr) that can be imposed on
asteroseismic modelling and that can be used to remove degeneracies in the
modelling process. Our attempts to match the excitation of the observed
frequency spectrum resulted in pulsation models that were too young. Hence,
asteroseismic studies of runaway pulsators can become vital not only in tracing
the evolutionary history of such objects, but to understand the interior
structure of massive stars in general. TESS is now opening up these stars for
detailed asteroseismic investigation.Comment: accepted for ApJ
Non-adiabatic oscillations of fast-rotating stars: the example of Rasalhague
Early-type stars generally tend to be fast rotators. In these stars, mode identification is very challenging as the effects of rotation are not well known. We consider here the example of α Ophiuchi, for which dozens of oscillation frequencies have been measured. We model the star using the two-dimensional structure code ESTER, and we compute both adiabatic and non-adiabatic oscillations using the TOP code. Both calculations yield very complex spectra, and we used various diagnostic tools to try and identify the observed pulsations. While we have not reached a satisfactory mode-to-mode identification, this paper presents promising early results
Gravito-inertial waves in a differentially rotating spherical shell
The gravito-inertial waves propagating over a shellular baroclinic flow inside a rotating spherical shell are analysed using the Boussinesq approximation. The wave properties are examined by computing paths of characteristics in the non-dissipative limit, and by solving the full dissipative eigenvalue problem using a high-resolution spectral method. Gravito-inertial waves are found to obey a mixed-type second-order operator and to be often focused around short-period attractors of characteristics or trapped in a wedge formed by turning surfaces and boundaries. We also find eigenmodes that show a weak dependence with respect to viscosity and heat diffusion just like truly regular modes. Some axisymmetric modes are found unstable and likely destabilized by baroclinic instabilities. Similarly, some non-axisymmetric modes that meet a critical layer (or corotation resonance) can turn unstable at sufficiently low diffusivities. In all cases, the instability is driven by the differential rotation. For many modes of the spectrum, neat power laws are found for the dependence of the damping rates with diffusion coefficients, but the theoretical explanation for the exponent values remains elusive in general. The eigenvalue spectrum turns out to be very rich and complex, which lets us suppose an even richer and more complex spectrum for rotating stars or planets that own a differential rotation driven by baroclinicity
Gravito-inertial modes in a differentially rotating spherical shell
While many intermediate- and high-mass main sequence stars are rapidly and differentially rotating, the effects of rotation on oscillation modes are poorly known. In this communication we present a first study of axisymmetric gravito-inertial modes in the radiative zone of a differentially rotating star. We consider a simplified model where the radiative zone of the star is a linearly stratified rotating fluid within a spherical shell, with differential rotation due to baroclinic effects. We solve the eigenvalue problem with high-resolution spectral computations and determine the propagation domain of the waves through the theory of characteristics. We explore the propagation properties of two kinds of modes: those that can propagate in the entire shell and those that are restricted to a sub-domain. Some of the modes that we find concentrate kinetic energy around short-period shear layers known as attractors. We describe various geometries for the propagation domains, conditioning the surface visibility of the corresponding modes
Gravito-inertial modes in a differentially rotating spherical shell
Oscillations have been detected in a variety of stars, including intermediate- and high-mass main sequence stars. While many of these stars are rapidly and differentially rotating, the effects of rotation on oscillation modes are poorly known. In this communication we present a first study on axisymmetric gravito-inertial modes in the radiative zone of a differentially rotating star. These modes probe the deep layers of the star around its convective core. We consider a simplified model where the radiative zone of a star is a linearly stratified rotating fluid within a spherical shell, with differential rotation due to baroclinic effects. We solve the eigenvalue problem with high-resolution spectral simulations and determine the propagation domain of the waves through the theory of characteristics. We explore the propagation properties of two kinds of modes: those that can propagate in the entire shell and those that are restricted to a subdomain. Some of the modes that we find concentrate kinetic energy around short-period shear layers known as attractors. We characterise these attractors by the dependence of their Lyapunov exponent with the ____BV frequency of the background and the oscillation frequency of the mode. Finally, we note that, as modes associated with short-period attractors form dissipative structures, they could play an important role for tidal interactions but should be dismissed in the interpretation of observed oscillation frequencies
[Ömer Seyfettin'in "İnci" dergisinde yayımlanmış kapak fotoğrafı ve vefatının duyurulması ]
Taha Toros Arşivi, Dosya No: 23-24-Ömer Seyfetti