338 research outputs found
Position and momentum observables on R and on R^3
We characterize all position and momentum observables on R and on R^3. We
study some of their operational properties and discuss their covariant joint
observables.Comment: 18 page
X-ray Emission from Magnetically Torqued Disks of Oe/Be Stars
We focus attention on the Oe/Be stars to test the concept that the disks of
these stars form by magnetic channeling of wind material toward the equator.
Calculations are made of the X-rays expected from the Magnetically Torqued Disk
(MTD) model for Be stars discussed by Cassinelli et al. (2002), by Maheswaran
(2003), and by Brown et al. (2004). The dominant parameters in the model are
the value of the velocity law, the rotation rate of the star, ,
and the ratio of the magnetic field energy density to the disk gravitational
energy density, .
The model predictions are compared with the observations obtained for
an O9.5 star Oph from \Berghofer et al. (1996) and for 7 Be stars from
Cohen et al. (1997). Extra considerations are also given here to the well
studied Oe star Oph for which we have observations of the
X-ray line profiles of the triad of He-like lines from the ion Mg XI.Comment: 28 pages with 6 figures. Accepted for publication in Ap
New Challenges For Wind Shock Models: The Chandra Spectrum Of The Hot Star Delta Orionis
The Chandra spectrum of delta Ori A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with N VII Lyalpha and lines from ions in the range Fe XVII-Fe XXI In contrast to the broad lines seen in zeta Pup and zeta Ori (850 +/- 40 and 1000 +/- 240 km s(-1) half-width at half-maximum [HWHM], respectively), these lines are broadened to only 430 +/- 60 km s(-1) HWHM. This is much lower than the measured wind terminal velocity of 2000 km s(-1). The forbidden, intercombination, and resonance (fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray-absorbing wind, and it is therefore surprising that the emission lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line pro le generation in hot stars, but none of them offers a fully satisfactory explanation for the observed line profiles
On the structure of covariant phase observables
We study the mathematical structure of covariant phase observables. Such an
observable can alternatively be expressed as a phase matrix, as a sequence of
unit vectors, as a sequence of phase states, or as an equivalent class of
covariant trace-preserving operations. Covariant generalized operator measures
are defined by structure matrices which form a W*-algebra with phase matrices
as its subset. The properties of the Radon-Nikodym derivatives of phase
probability measures are studied.Comment: 11 page
Highly Accelerated Diamagnetic Plasmoids: A New X-ray Production Mechanism for OB Stellar Winds
The observed X-ray source temperature distributions in OB stellar winds, as
determined from high energy resolution Chandra observations, show that the
highest temperatures occur near the star, and then steadily decrease outward
through the wind. To explain this unexpected behavior, we propose a shock model
concept that utilizes a well-known magnetic propulsion mechanism; the surface
ejection of "diamagnetic plasmoids" into a diverging external magnetic field.
This produces rapidly accelerating self-contained structures that plow through
an ambient wind and form bow shocks that generate a range in X-ray temperatures
determined by the plasmoid-wind relative velocities. The model free parameters
are the plasmoid initial Alfven speed, the initial plasma-beta of the external
medium, and the divergence rate of the external field. These are determined by
fitting the predicted bow shock temperatures with the observed OB supergiant
X-ray temperature distribution. We find that the initial external plasma-beta
has a range between 0 and 2, and the assumed radially-decreasing external
magnetic field strength that scales as r^{-S} has a value of S lying between 2
and 3. Most importantly, the initial plasmoid Alfven speed is found to be
well-constrained at a value of 0.6 times the terminal velocity, which appears
to represent an upper limit for all normal OB stars. This intriguing new limit
on OB magnetic properties, as derived from Chandra observations, emphasizes the
need for further studies of magnetic propulsion mechanisms in these stars.Comment: Accepted for publication in ApJ Letters, 4 pages, 4 figures (color),
1 tabl
The Correlation between X-Ray Line Ionization and Optical Spectral Types of the OB Stars
Marked correlations are reported between the ionization of the X-ray line
spectra of normal OB stars, as observed by the Chandra X-Ray Observatory, and
their optical spectral types. These correlations include the progressive
weakening of the higher ionization relative to the lower ionization X-ray lines
with advancing spectral type, and the similarly decreasing intensity ratios of
the H-like to He-like lines of the alpha ions. These relationships were not
predicted by models, nor have they been clearly evident in astrophysical
studies of a few objects; rather, they have emerged from morphological analysis
of an adequate (albeit still small) sample, from which known peculiar objects
such as magnetic stars and very rapid rotators have been isolated to reveal the
normal trends. This process is analogous to that which first demonstrated the
strong relationships between the UV wind profiles and the optical spectral
types of normal OB stars, which likely bear a physical as well as a historical
connection to the present X-ray results. Since the optical spectral types are
calibrated in terms of fundamental stellar parameters, it follows that the
winds and X-ray spectra are determined by the latter. These observations
provide strong guidance for further astrophysical modeling of these phenomena.Comment: 19 pages, 7 figures, 2 tables; ApJ accepte
X-ray line emission from a fragmented stellar wind
We discuss X-ray line formation in dense O star winds. A random distribution
of wind shocks is assumed to emit X-rays that are partially absorbed by cooler
wind gas. The cool gas resides in highly compressed fragments oriented
perpendicular to the radial flow direction. For fully opaque fragments, we find
that the blueshifted part of X-ray line profiles remains flat-topped even after
severe wind attenuation, whereas the red part shows a steep decline. These
box-type, blueshifted profiles resemble recent Chandra observations of the O3
star zeta Pup. For partially transparent fragments, the emission lines become
similar to those from a homogeneous wind.Comment: 8 pages, 7 figures, Latex, to be published in A&
Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds
We propose collisionless damping of fast MHD waves as an important mechanism
for the heating and acceleration of winds from rotating stars. Stellar rotation
causes magnetic field lines anchored at the surface to form a spiral pattern
and magneto-rotational winds can be driven. If the structure is a magnetically
dominated, fast MHD waves generated at the surface can propagate almost
radially outward and cross the field lines. The propagating waves undergo
collisionless damping owing to interactions with particles surfing on magnetic
mirrors that are formed by the waves themselves. The damping is especially
effective where the angle between the wave propagation and the field lines
becomes moderately large ( to ). The angle tends naturally
to increase into this range because the field in magneto-rotational winds
develops an increasingly large azimuthal component. The dissipation of the wave
energy produces heating and acceleration of the outflow. We show using
specified wind structures that this damping process can be important in both
solar-type stars and massive stars that have moderately large rotation rates.
This mechanism can play a role in coronae of young solar-type stars which are
rapidly rotating and show X-ray luminosities much larger than the sun. The
mechanism could also be important for producing the extended X-ray emitting
regions inferred to exist in massive stars of spectral type middle B and later.Comment: 12 pages, including 7 figures, accepted for publication in Ap
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