156 research outputs found
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
Gravity modes in rapidly rotating stars. Limits of perturbative methods
CoRoT and Kepler missions are now providing high-quality asteroseismic data
for a large number of stars. Among intermediate-mass and massive stars, fast
rotators are common objects. Taking the rotation effects into account is needed
to correctly understand, identify, and interpret the observed oscillation
frequencies of these stars. A classical approach is to consider the rotation as
a perturbation. In this paper, we focus on gravity modes, such as those
occurring in gamma Doradus, slowly pulsating B (SPB), or Be stars. We aim to
define the suitability of perturbative methods. With the two-dimensional
oscillation program (TOP), we performed complete computations of gravity modes
-including the Coriolis force, the centrifugal distortion, and compressible
effects- in 2-D distorted polytropic models of stars. We started with the modes
l=1, n=1-14, and l=2-3, n=1-5,16-20 of a nonrotating star, and followed these
modes by increasing the rotation rate up to 70% of the break-up rotation rate.
We then derived perturbative coefficients and determined the domains of
validity of the perturbative methods. Second-order perturbative methods are
suited to computing low-order, low-degree mode frequencies up to rotation
speeds ~100 km/s for typical gamma Dor stars or ~150 km/s for B stars. The
domains of validity can be extended by a few tens of km/s thanks to the
third-order terms. For higher order modes, the domains of validity are
noticeably reduced. Moreover, perturbative methods are inefficient for modes
with frequencies lower than the Coriolis frequency 2Omega. We interpret this
failure as a consequence of a modification in the shape of the resonant cavity
that is not taken into account in the perturbative approach.Comment: 8 pages, 6 figures, Astronomy & Astrophysics (in press
Long-term magnetic field stability of Vega
We present new spectropolarimetric observations of the normal A-type star
Vega, obtained during the summer of 2010 with NARVAL at T\'elescope Bernard
Lyot (Pic du Midi Observatory). This new time-series is constituted of 615
spectra collected over 6 different nights. We use the
Least-Square-Deconvolution technique to compute, from each spectrum, a mean
line profile with a signal-to-noise ratio close to 20,000. After averaging all
615 polarized observations, we detect a circularly polarized Zeeman signature
consistent in shape and amplitude with the signatures previously reported from
our observations of 2008 and 2009. The surface magnetic geometry of the star,
reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the
maps obtained in 2008 and 2009, showing that most recognizable features of the
photospheric field of Vega are only weakly distorted by large-scale surface
flows (differential rotation or meridional circulation).Comment: Proceedings of the conference "Stellar polarimetry: from birth to
death", 2011 Jun 27-30, Madiso
Strong magnetic fields detected in the cores of 11 red giant stars using gravity-mode period spacings
Despite their importance in stellar evolution, little is known about magnetic
fields in the interior of stars. The recent seismic detection of magnetic
fields in the core of several red giant stars has given measurements of their
strength and information on their topology. We revisit the puzzling case of
hydrogen-shell burning giants that show deviations from the expected regular
period spacing of gravity modes. These stars also tend to have a too low
measured period spacing compared to their counterparts. We here show that these
two features are well accounted for by strong magnetic fields in the cores of
these stars. For 11 Kepler red giants showing these anomalies, we place lower
limits on the core field strengths ranging from 40 to 610 kG. For one star, the
measured field exceeds the critical field above which gravity waves no longer
propagate in the core. We find that this star shows mixed mode suppression at
low frequency, which further suggests that this phenomenon might be related to
strong core magnetic fields.Comment: 10 pages, 7 figures, accepted as an Letter in A&
Discovery of starspots on Vega - First spectroscopic detection of surface structures on a normal A-type star
The theoretically studied impact of rapid rotation on stellar evolution needs
to be confronted with the results of high resolution spectroscopy-velocimetry
observations. A weak surface magnetic field had recently been detected in the
A0 prototype star Vega, potentially leading to a (yet undetected) structured
surface. The goal of this article is to present a thorough analysis of the line
profile variations and associated estimators in the early-type standard star
Vega (A0) in order reveal potential activity tracers, exoplanet companions and
stellar oscillations. Vega was monitored in high-resolution spectroscopy with
the velocimeter Sophie/OHP. A total of 2588 high S/N spectra was obtained
during 5 nights (August 2012) at R = 75000 and covering the visible domain. For
each reduced spectrum, Least Square Deconvolved (LSD) equivalent photospheric
profiles were calculated with a Teff = 9500 and logg = 4.0 spectral line mask.
Several methods were applied to study the dynamic behavior of the profile
variations (evolution of radial velocity, bisectors, vspan, 2D profiles,
amongst others). We present the discovery of a starspotted stellar surface in
an A-type standard star with faint spot amplitudes Delta F/Fc ~5 10^{-4}. A
rotational modulation of spectral lines with a period of rotation P = 0.68 d
has clearly been exhibited, confirming the results of previous
spectropolarimetric studies. Either a very thin convective layer can be
responsible for magnetic field generation at small amplitudes, or a new
mechanism has to be invoked in order to explain the existence of activity
tracing starspots. This first strong evidence that standard A-type stars can
show surface structures opens a new field of research and asks the question
about a potential link with the recently discovered weak magnetic field
discoveries in this category of stars.Comment: accepted for publication by Astronomy & Astrophysics (23rd of March
2015
Wave chaos in rapidly rotating stars
Effects of rapid stellar rotation on acoustic oscillation modes are poorly
understood. We study the dynamics of acoustic rays in rotating polytropic stars
and show using quantum chaos concepts that the eigenfrequency spectrum is a
superposition of regular frequency patterns and an irregular frequency subset
respectively associated with near-integrable and chaotic phase space regions.
This opens new perspectives for rapidly rotating star seismology and also
provides a new and potentially observable manifestation of wave chaos in a
large scale natural system.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev.
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