11,402 research outputs found
Reduction of the COSMOS Southern Sky galaxy survey data to the RC3 standard system
After having cross-identified a subsample of LEDA galaxies in the COSMOS
database, we defined the best relations to convert COSMOS parameters
(coordinates, position angle, diameter, axis ratio and apparent magnitude) into
RC3 system used in the LEDA database. Tiny secondary effects can be tested:
distance to plate cenetrs effect and air-mass effect. The converted COSMOS
parameters are used to add missing parameters on LEDA galaxies.
Key words: galaxies - catalogue - photometryComment: 5 pages, postcript including figures, to appear in MNRAS, reprint
requests: [email protected]
Wind Channeling, Magnetospheres, And Spindown Of Magnetic Massive Stars
A subpopulation (~10%) of hot, luminous, massive stars have been revealed through spectropolarimetry to harbor strong (hundreds to tens of thousand Gauss), steady, large-scale (often significantly dipolar) magnetic fields. This review focuses on the role of such fields in channeling and trapping the radiatively driven wind of massive stars, including both in the strongly perturbed outflow from open field regions, and the wind-fed “magnetospheres” that develop from closed magnetic loops. For B-type stars with weak winds and moderately fast rotation, one finds “centrifugal magnetospheres”, in which rotational support allows magnetically trapped wind to accumulate to a large density, with quite distinctive observational signatures, e.g. in Balmer line emission. In contrast, more luminous O-type stars have generally been spun down by magnetic braking from angular momentum loss in their much stronger winds. The lack of centrifugal support means their closed loops form a “dynamical magnetosphere”, with trapped material falling back to the star on a dynamical timescale; nonetheless, the much stronger wind feeding leads to a circumstellar density that is still high enough to give substantial Balmer emission. Overall, this review describes MHD simulations and semi-analytic dynamical methods for modeling the magnetospheres, the magnetically channeled wind outflows, and the associated spin-down of these magnetic massive stars
An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical emission from slowly rotating magnetic massive stars
Slowly rotating magnetic massive stars develop "dynamical magnetospheres"
(DM's), characterized by trapping of stellar wind outflow in closed magnetic
loops, shock heating from collision of the upflow from opposite loop
footpoints, and subsequent gravitational infall of radiatively cooled material.
In 2D and 3D magnetohydrodynamic (MHD) simulations the interplay among these
three components is spatially complex and temporally variable, making it
difficult to derive observational signatures and discern their overall scaling
trends.Within a simplified, steady-state analysis based on overall conservation
principles, we present here an "analytic dynamical magnetosphere" (ADM) model
that provides explicit formulae for density, temperature and flow speed in each
of these three components -- wind outflow, hot post-shock gas, and cooled
inflow -- as a function of colatitude and radius within the closed (presumed
dipole) field lines of the magnetosphere. We compare these scalings with
time-averaged results from MHD simulations, and provide initial examples of
application of this ADM model for deriving two key observational diagnostics,
namely hydrogen H-alpha emission line profiles from the cooled infall, and
X-ray emission from the hot post-shock gas. We conclude with a discussion of
key issues and advantages in applying this ADM formalism toward derivation of a
broader set of observational diagnostics and scaling trends for massive stars
with such dynamical magnetospheres.Comment: 15 pages, 11 figures, accepted for MNRA
A MiMeS analysis of the magnetic field and circumstellar environment of the weak-wind O9 sub-giant star HD 57682
I will review our recent analysis of the magnetic properties of the O9IV star
HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the
Canada-France-Hawaii telescope within the context of the Magnetism in Massive
Stars (MiMeS) Large Program. I discuss our most recent determination of the
rotational period from longitudinal magnetic field measurements and Halpha
variability - the latter obtained from over a decade's worth of professional
and amateur spectroscopic observations. Lastly, I will report on our
investigation of the magnetic field geometry and the effects of the field on
the circumstellar environment.Comment: 2 pages, 2 figures, IAUS272 - Active OB Stars: Structure, Evolution,
Mass Loss and Critical Limit
Structural behavior of uranium dioxide under pressure by LSDA+U calculations
The structural behavior of UO2 under high pressure up to 300GPa has been
studied by first-principles calculations with LSDA+U approximation. The results
show that a pressure-induced structural transition to the cotunnite-type
(orthorhombic Pnma) phase occurs at 38GPa. It agrees well with the
experimentally observed ~42 GPa. An isostructural transition following that is
also predicted to take place from 80 to 130GPa, which has not yet been observed
in experiments. Further high compression beyond 226GPa will result in a
metallic and paramagnetic transition. It corresponds to a volume of 90A^3 per
cell, in good agreement with a previous theoretical analysis in the reduction
of volume required to delocalize 5f states.Comment: 10 pages, 8 figure
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