1,399 research outputs found
The interior rotation of a sample of gamma Doradus stars from ensemble modelling of their gravity mode period spacings
CONTEXT. Gamma Doradus stars (hereafter gamma Dor stars) are known to exhibit
gravity- and/or gravito-intertial modes that probe the inner stellar region
near the convective core boundary. The non-equidistant spacing of the pulsation
periods is an observational signature of the stars' evolution and current
internal structure and is heavily influenced by rotation.
AIMS. We aim to constrain the near-core rotation rates for a sample of gamma
Dor stars, for which we have detected period spacing patterns.
METHODS. We combined the asymptotic period spacing with the traditional
approximation of stellar pulsation to fit the observed period spacing patterns
using chi-squared optimisation. The method was applied to the observed period
spacing patterns of a sample of stars and used for ensemble modelling.
RESULTS. For the majority of stars with an observed period spacing pattern we
successfully determined the rotation rates and the asymptotic period spacing
values, though the uncertainty margins on the latter were typically large. This
also resulted directly in the identification of the modes corresponding with
the detected pulsation frequencies, which for most stars were prograde dipole
gravity and gravito-inertial modes. The majority of the observed retrograde
modes were found to be Rossby modes. We further discuss the limitations of the
method due to the neglect of the centrifugal force and the incomplete treatment
of the Coriolis force.
CONCLUSION. Despite its current limitations, the proposed methodology was
successful to derive the rotation rates and to identify the modes from the
observed period spacing patterns. It forms the first step towards detailed
seismic modelling based on observed period spacing patterns of moderately to
rapidly rotating gamma Dor stars.Comment: 12 pages, 15 figures, 5 tables. Accepted for publication in Astronomy
& Astrophysic
Least-squares deconvolution based analysis of stellar spectra
In recent years, astronomical photometry has been revolutionised by space
missions such as MOST, CoRoT and Kepler. However, despite this progress,
high-quality spectroscopy is still required as well. Unfortunately,
high-resolution spectra can only be obtained using ground-based telescopes, and
since many interesting targets are rather faint, the spectra often have a
relatively low S/N. Consequently, we have developed an algorithm based on the
least-squares deconvolution profile, which allows to reconstruct an observed
spectrum, but with a higher S/N. We have successfully tested the method using
both synthetic and observed data, and in combination with several common
spectroscopic applications, such as e.g. the determination of atmospheric
parameter values, and frequency analysis and mode identification of stellar
pulsations.Comment: Proceedingspaper, 8 pages, 4 figures, appears in "Setting a New
Standard in the Analysis of Binary Stars", Eds K. Pavlovski, A. Tkachenko,
and G. Torres, EAS Publications Serie
The interior angular momentum of core hydrogen burning stars from gravity-mode oscillations
A major uncertainty in the theory of stellar evolution is the angular
momentum distribution inside stars and its change during stellar life. We
compose a sample of 67 stars in the core-hydrogen burning phase with a
value from high-resolution spectroscopy, as well as an asteroseismic
estimate of the near-core rotation rate derived from gravity-mode oscillations
detected in space photometry. This assembly includes 8 B-type stars and 59
AF-type stars, covering a mass range from 1.4 to 5\,M, i.e., it
concerns intermediate-mass stars born with a well-developed convective core.
The sample covers projected surface rotation velocities km\,s and core rotation rates up to Hz, which
corresponds to 50\% of the critical rotation frequency. We find deviations from
rigid rotation to be moderate in the single stars of this sample. We place the
near-core rotation rates in an evolutionary context and find that the core
rotation must drop drastically before or during the short phase between the end
of the core-hydrogen burning and the onset of core-helium burning. We compute
the spin parameter, which is the ratio of twice the rotation rate to the mode
frequency (also known as the inverse Rossby number), for 1682 gravity modes and
find the majority (95\%) to occur in the sub-inertial regime. The ten stars
with Rossby modes have spin parameters between 14 and 30, while the
gravito-inertial modes cover the range from 1 to 15.Comment: Manuscript of 5 pages, including 2 figures, accepted for publication
in The Astrophysical Journal Letter
Robust propagation direction of stresses in a minimal granular packing
By employing the adaptive network simulation method, we demonstrate that the
ensemble-averaged stress caused by a local force for packings of frictionless
rigid beads is concentrated along rays whose slope is consistent with unity:
forces propagate along lines at 45 degrees to the horizontal or vertical. This
slope is shown to be independent of polydispersity or the degree to which the
system is sheared. Further confirmation of this result comes from fitting the
components of the stress tensor to the null stress constitutive equation. The
magnitude of the response is also shown to fall off with the -1/2 power of
distance. We argue that our findings are a natural consequence of a system that
preserves its volume under small perturbations.Comment: 8 pages, 6 figures. Some extra clarification and minor improvements.
To appear in EPJ-
Tkachenko modes as sources of quasiperiodic pulsar spin variations
We study the long wavelength shear modes (Tkachenko waves) of triangular
lattices of singly quantized vortices in neutron star interiors taking into
account the mutual friction between the superfluid and the normal fluid and the
shear viscosity of the normal fluid. The set of Tkachenko modes that propagate
in the plane orthogonal to the spin vector are weakly damped if the coupling
between the superfluid and normal fluid is small. In strong coupling, their
oscillation frequencies are lower and are undamped for small and moderate shear
viscosities. The periods of these modes are consistent with the observed
~100-1000 day variations in spin of PSR 1828-11.Comment: 7 pages, 3 figures, uses RevTex, v2: added discussion/references,
matches published versio
Giant Vortex Lattice Deformations in Rapidly Rotating Bose-Einstein Condensates
We have performed numerical simulations of giant vortex structures in rapidly
rotating Bose-Einstein condensates within the Gross-Pitaevskii formalism. We
reproduce the qualitative features, such as oscillation of the giant vortex
core area, formation of toroidal density hole, and the precession of giant
vortices, observed in the recent experiment [Engels \emph{et.al.}, Phys. Rev.
Lett. {\bf 90}, 170405 (2003)]. We provide a mechanism which quantitatively
explains the observed core oscillation phenomenon. We demonstrate the clear
distinction between the mechanism of atom removal and a repulsive pinning
potential in creating giant vortices. In addition, we have been able to
simulate the transverse Tkachenko vortex lattice vibrations.Comment: 5 pages, 6 figures; revised description of core oscillation, new
subfigur
Transients influencing rocket engine ignition and popping Interim report
Engine design and operating parameters studied for effects on rocket engine ignition and poppin
Coulomb Charging Effects in an Open Quantum Dot
Low-temperature transport properties of a lateral quantum dot formed by
overlaying finger gates in a clean one-dimensional channel are investigated.
Continuous and periodic oscillations superimposed upon ballistic conductance
steps are observed, when the conductance G of the dot changes within a wide
range 0<G<6e^2/h. Calculations of the electrostatics confirm that the measured
periodic conductance oscillations correspond to successive change of the total
charge of the dot by . By modelling the transport it is shown that the
progression of the Coulomb oscillations into the region G>2e^2/h may be due to
suppression of inter-1D-subband scattering. Fully transmitted subbands
contribute to coherent background of conductance, while sequential tunneling
via weakly transmitted subbands leads to Coulomb charging of the dot.Comment: 12 pages, RevTeX, 15 eps figures included, submitted to Phys. Rev.
New Cataclysmic Variable 1RXS J073346.0+261933 in Gemini
In course of the search for the optical identifications associated with ROSAT
X-ray sources we have found a highly variable object with the very unusual
long-term behavior, color indices and high X-ray-to-optical flux ratio. We
report the archival photometric light curve from the Catalina Sky Survey,
optical spectroscopy from RTT150 and time-resolved photometry from
Astrotel-Caucasus telescope. The object appears to be the magnetic cataclysmic
variable (polar) with orbital period of P=3.20 hr.Comment: 8 pages, 5 figures. Submitted to Astronomy Letter
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