320,558 research outputs found
Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study
Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which
has been studied during the last years for its efficient photo-luminescence. In
contrast, the molecular geometry, providing the basis for the electronic and
optical properties, has been hardly investigated. In this paper, we apply a
density-functional-theory based molecular-dynamics approach to reveal the
molecular structure of PDPA in detail. We find that oligomers of this material
are limited in length, being stable only up to eight repeat units, while the
polymer is energetically unfavorable. These facts, which are in excellent
agreement with experimental findings, are explained through a detailed analysis
of the bond lengths. A consequence of the latter is the appearance of
pronounced torsion angles of the phenyl rings with respect to the plane of the
polyene backbone, ranging from up to . We point out
that such large torsion angles do not destroy the conjugation of the
electrons from the backbone to the side phenyl rings, as is evident from the
electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Accretion disc dynamics in extreme mass ratio compact binaries
An analysis is presented of a numerical investigation of the dynamics and
geometry of accretion discs in binary systems with mass ratios q < 0.1,
applicable to ultra-compact X-ray binaries, AM CVn stars and very short period
cataclysmic variables. The steady-state geometry of the disc in the binary
reference frame is found to be quite different from that expected at higher
mass ratios. For q ~ 0.1, the disc takes on the usual elliptical shape, with
the major axis aligned perpendicular to the line of centres of the two stars.
However, at smaller mass ratios the elliptical gaseous orbits in the outer
regions of the disc are rotated in the binary plane. The angle of rotation
increases with gas temperature, but is found to vary inversely with q. At q =
0.01, the major axis of these orbits is aligned almost parallel to the line of
centres of the two stars. These effects may be responsible for the similar disc
structure inferred from Doppler tomography of the AM CVn star GP Com
(Morales-Rueda et al. 2003), which has q = 0.02. The steady-state geometry at
low mass ratios is not predicted by an inviscid, restricted three-body model of
gaseous orbits; it is related to the effects of tidal-viscous truncation of the
disc near the Roche lobe boundary. Since the disc geometry can be inferred
observationally for some systems, it is proposed that this may offer a useful
diagnostic for the determination of mass ratios in ultra-compact binaries.Comment: 17 pages, 9 figures, 7 in colour. Accepted for publication in MNRAS.
Plain article formatting to get round arXiv problems with mn2e.st
Interplay between Physics and Geometry in Balmer filaments: the Case of SN 1006
The analysis of Balmer-dominated emission in supernova remnants is
potentially a very powerful way to derive information on the shock structure,
on the physical conditions of the ambient medium and on the cosmic-ray
acceleration efficiency. However, the outcome of models developed in
plane-parallel geometry is usually not easily comparable with the data, since
they often come from regions with rather a complex geometry. We present here a
general scheme to disentangle physical and geometrical effects in the data
interpretation, which is especially powerful when the transition zone of the
shock is spatially resolved and the spectral resolution is high enough to allow
a detailed investigation of spatial changes of the line profile. We then apply
this technique to re-analyze very high quality data of a region along the
northwestern limb of the remnant of SN~1006. We show how some observed
features, previously interpreted only in terms of spatial variations of
physical quantities, naturally arise from geometrical effects. With these
effects under control, we derive new constraints on physical quantities in the
analyzed region, like the ambient density (in the range 0.03-), the upstream neutral fraction (more likely in the range 0.01-0.1),
the level of face-on surface brightness variations (with factors up to ) and the typical scale lengths related to such variations (, corresponding to angular scales ).Comment: 22 pages, 24 figures, 3 tables, accepted on 2018 November 21 for
publication on MNRA
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