12,951 research outputs found
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Celestus montanus
Number of Pages: 3Integrative BiologyGeological Science
Surface trapping and leakage of low-frequency g-modes in rotating early-type stars -- I. Qualitative analysis
A qualitative study of the surface trapping of low-frequency non-radial
g-modes in rotating early-type stars is undertaken within the Cowling,
adiabatic and traditional approximations. It is demonstrated that, at
frequencies below a cut-off, waves cannot be fully trapped within the star, and
will leak through the surface. Expressions for the cut-off frequency are
derived in both the non-rotating and rotating cases; it is found from these
expressions that the cut-off frequency increases with the rotation rate for all
but prograde sectoral modes.
The results are of possible relevance to the 53 Per and SPB classes of
variable star, which exhibit pulsation frequencies of the same order of
magnitude as the cut-off frequencies found for the stellar model. It is
suggested that observations either of an upper limit on variability periods
(corresponding to the cut-off), or of line-profile variations due to leaking
modes, may permit asteroseismological studies of the outer layers of these
systems.Comment: 8 pages, 2 figures, to be published in MNRA
Excitation of g modes in Wolf-Rayet stars by a deep opacity bump
We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich
Wolf-Rayet stellar models characterized by differing hydrogen abundances. We
find that modes with intermediate radial orders are destabilized by a kappa
mechanism operating on an opacity bump at an envelope temperature log T ~ 6.25.
This `deep opacity bump' is due primarily to L-shell bound-free transitions of
iron. Periods of the unstable modes span ~ 11-21 hr in the model containing
some hydrogen, and ~ 3-12 hr in the hydrogen-depleted model. Based on the
latter finding, we suggest that self-excited g modes may be the source of the
9.8 hr-periodic variation of WR 123 recently reported by Lefevre et al. (2005).Comment: 5 pages, 3 figures, accepted by MNRAS letter
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Tlalocohyla loquax
Number of Pages: 8Integrative BiologyGeological Science
Phenomenological optical potential analysis of proton-carbon elastic scattering at 200 MeV
Differential cross sections for 200 MeV protons elastically scattered from C-12 were analyzed utilizing a local, complex, spin-dependent optical potential with a harmonic well radial dependence. Analyses were performed using the WKB and eikonal approximations. For the latter, first-order corrections to he phase shifts were incorporated to account for the spin-orbit contribution. Large disagreement between theory and experiment was observed when the usual Thomas form for the spin-orbit potential was utilized. Substantial improvement was obtained by allowing the parameters in the central and spin-orbit potential terms to vary independently
Centrifugal Breakout of Magnetically Confined Line-Driven Stellar Winds
We present 2D MHD simulations of the radiatively driven outflow from a
rotating hot star with a dipole magnetic field aligned with the star's rotation
axis. We focus primarily on a model with moderately rapid rotation (half the
critical value), and also a large magnetic confinement parameter, . The magnetic field
channels and torques the wind outflow into an equatorial, rigidly rotating disk
extending from near the Kepler corotation radius outwards. Even with
fine-tuning at lower magnetic confinement, none of the MHD models produce a
stable Keplerian disk. Instead, material below the Kepler radius falls back on
to the stellar surface, while the strong centrifugal force on material beyond
the corotation escape radius stretches the magnetic loops outwards, leading to
episodic breakout of mass when the field reconnects. The associated dissipation
of magnetic energy heats material to temperatures of nearly K, high
enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection}
represents a novel mechanism for driving magnetic reconnection, and seems a
very promising basis for modeling X-ray flares recently observed in rotating
magnetic Bp stars like Ori E.Comment: 5 pages, 3 figures, accepted by ApJ
Integration of ground and on-board system for terminal count
The development of an integrated ground and onboard system for Space Shuttle terminal count management is discussed. The criteria considered in designing this system are outlined. Examples of problems encountered in the process of maturing the design are presented
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