3,500 research outputs found
"Supersolid" self-bound Bose condensates via laser-induced interatomic forces
We show that the dipole-dipole interatomic forces induced by a single
off-resonant running laser beam can lead to a self-bound pencil-shaped Bose
condensate, even if the laser beam is a plane-wave. For an appropriate laser
intensity the ground state has a quasi-one dimensional density modulation --- a
Bose "supersolid".Comment: 4 pages, 3 eps figure
Hybrid fuzzy and sliding-mode control for motorised tether spin-up when coupled with axial vibration
A hybrid fuzzy sliding mode controller is applied to the control of motorised tether spin-up coupled with an axial oscillation phenomenon. A six degree of freedom dynamic model of a motorised momentum exchange tether is used as a basis for interplanetary payload exchange. The tether comprises a symmetrical double payload configuration, with an outrigger counter inertia and massive central facility. It is shown that including axial elasticity permits an enhanced level of performance prediction accuracy and a useful departure from the usual rigid body representations, particularly for accurate payload positioning at strategic points. A special simulation program has been devised in MATLAB and MATHEMATICA for a given initial condition data case
Depletion of a Bose-Einstein condensate by laser-iduced dipole-dipole interactions
We study a gaseous Bose-Einstein condensate with laser-induced dipole-dipole
interactions using the Hartree-Fock-Bogoliubov theory within the Popov
approximation. The dipolar interactions introduce long-range atom-atom
correlations, which manifest themselves as increased depletion at momenta
similar to that of the laser wavelength, as well as a "roton" dip in the
excitation spectrum. Surprisingly, the roton dip and the corresponding peak in
the depletion are enhanced by raising the temperature above absolute zero.Comment: 10 pages, 6 figure
Mutual Coherence of Polarized Light in Disordered Media: Two-Frequency Method Extended
The paper addresses the two-point correlations of electromagnetic waves in
general random, bi-anisotropic media whose constitutive tensors are complex
Hermitian, positive- or negative-definite matrices. A simplified version of the
two-frequency Wigner distribution (2f-WD) for polarized waves is introduced and
the closed form Wigner-Moyal equation is derived from the Maxwell equations. In
the weak-disorder regime with an arbitrarily varying background the
two-frequency radiative transfer (2f-RT) equations for the associated coherence matrices are derived from the Wigner-Moyal equation by using the
multiple scale expansion. In birefringent media, the coherence matrix becomes a
scalar and the 2f-RT equations take the scalar form due to the absence of
depolarization. A paraxial approximation is developed for spatialy anisotropic
media. Examples of isotropic, chiral, uniaxial and gyrotropic media are
discussed
Massive Star Feedback on the IMF
We have obtained the first measures of the mass accretion rates on stars of
the Trapezium Cluster. They appear systematically lower than those of similar
stars in the Taurus-Auriga association. Together with premature disk
evaporation, dramatically revealed by the HST images of the Orion proplyds,
this result suggests that low mass stars in a rich cluster may be ``dwarfed''
by the influence of nearby OB stars. This feedback mechanism affects the IMF,
producing an excess of low mass stars and brown dwarfs. The observed frequency
of low mass objects in Orion vs. Taurus seems to confirm this scenario.Comment: 3 page
New rotation periods in the Pleiades: Interpreting activity indicators
We present results of photometric monitoring campaigns of G, K and M dwarfs
in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation
periods for 18 stars in this cluster. In this paper, we examine the validity of
using observables such as X-ray activity and amplitude of photometric
variations as indicators of angular momentum loss. We report the discovery of
cool, slow rotators with high amplitudes of variation. This contradicts
previous conclusions about the use of amplitudes as an alternate diagnostic of
the saturation of angular momentum loss. We show that the X-ray data can be
used as observational indicators of mass-dependent saturation in the angular
momentum loss proposed on theoretical grounds.Comment: 24 pages, LaTex (AASTeX); includes 8 postscript figures and 4 Latex
tables. To appear in ApJ, Feb. 1, 1998. Postscript version of preprint can be
obtained from http://casa.colorado.edu/~anitak/pubs.htm
Morphology and Composition of the Helix Nebula
We present new narrow-band filter imagery in H-alpha and [N II] 6584 along
with UV and optical spectrophotometry measurements from 1200 to 9600 Angstroms
of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large
diameter and low surface brightness. Detailed models of the observable ionized
nebula support the recent claim that the Helix is actually a flattened disk
whose thickness is roughly one-third its diameter with an inner region
containing hot, highly ionized gas which is generally invisible in narrow-band
images. The outer visible ring structure is of lower ionization and temperature
and is brighter because of a thickening in the disk. We also confirm a central
star effective temperature and luminosity of 120,000K and 100L_sun, and we
estimate a lower limit to the nebular mass to be 0.30M_sun. Abundance
measurements indicate the following values: He/H=0.12 (+/-0.017),
O/H=4.60x10^-4 (+/-0.18), C/O=0.87 (+/-0.12), N/O=0.54 (+/-0.14), Ne/O=0.33
(+/-0.04), S/O=3.22x10^-3 (+/-0.26), and Ar/O=6.74x10^-3 (+/-0.76). Our carbon
abundance measurements represent the first of their kind for the Helix Nebula.
The S/O ratio which we derive is anomalously low; such values are found in only
a few other planetary nebulae. The central star properties, the super-solar
values of He/H and N/O, and a solar level of C/O are consistent with a 6.5M_sun
progenitor which underwent three phases of dredge-up and hot bottom burning
before forming the planetary nebula.Comment: 50-page manuscript plus 11 postscript figures. This revised version
corrects a typo in earlier submission. Nothing else has changed. Accepted for
publication in the Astrophysical Journa
Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity
New optical and radio data on the BL Lacertae object AO 0235+16 have been
collected in the last four years by a wide international collaboration, which
confirm the intense activity of this source. The optical data also include the
results of the Whole Earth Blazar Telescope (WEBT) first-light campaign
organized in November 1997. The optical spectrum is observed to basically
steepen when the source gets fainter. We have investigated the existence of
typical variability time scales and of possible correlations between the
optical and radio emissions by means of visual inspection, Discrete Correlation
Function analysis, and Discrete Fourier Transform technique. The major radio
outbursts are found to repeat quasi-regularly with a periodicity of about 5.7
years; this period is also in agreement with the occurrence of some of the
major optical outbursts, but not all of them.Comment: to be published in A&
Unraveling the Helix Nebula: Its Structure and Knots
Through HST imaging of the inner part of the main-ring of the Helix Nebula
together with CTIO 4-m images of the fainter outer parts, we have an
unprecedented-quality view of the nearest bright planetary nebula. These images
have allowed determination that the main-ring of the nebula is composed of an
inner-disk of about 499\arcsec diameter (0.52 pc) surrounded by an outer-ring
(in reality a torus) of 742\arcsec diameter (0.77 pc) whose plane is highly
inclined to the plane of the disk. This outer-ring is surrounded by an
outermost-ring of 1500\arcsec (1.76 pc) diameter which is flattened on the side
colliding with the ambient interstellar medium. The inner-disk has an extended
distribution of low density gas along its rotational axis of symmetry and the
disk is optically thick to ionizing radiation, as is the outer-ring.
Published radial velocities of the knots provides support for the
two-component structure of the main-ring of the nebula and to the idea that the
knots found there are expanding along with the nebular material from which it
recently originated. There is a change in the morphology of the knots as a
function of the distance from the local ionization front. This supports a
scenario in which the knots are formed in or near the ionization front and are
then sculpted by the stellar radiation from the central star as the ionization
front advances beyond them.Comment: 30 pages, 20 figures, many figures have reduce fidelity for astroph
preprint. Note: URLs in preprint were change
Flight Programs and X-ray Optics Development at MSFC
The X-ray astronomy group at the Marshall Space Flight Center is developing electroformed nickel/cobalt x-ray optics for suborbital and orbital experiments. Suborbital instruments include the Focusing X-ray Solar Imager (FOXSI) and Micro-X sounding rocket experiments and the HERO balloon payload. Our current orbital program is the fabrication of a series of mirror modules for the Astronomical Roentgen Telescope (ART) to be launched on board the Russian-German Spectrum Roentgen Gamma Mission (SRG.) The details and status of these various programs are presented. A second component of our work is the development of fabrication techniques and optical metrology to improve the angular resolution of thin shell optics to the arcsecond-level. The status of these x-ray optics technology developments is also presented
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