30 research outputs found
Infrared Morphology of Regions of Ionized Hydrogen
A search for infrared ring nebulae associated with regions of ionized
hydrogen has been carried out. The New GPS Very Large Array survey at 20 cm
forms the basis of the search, together with observations obtained with the
Spitzer Space Telescope at 8 and 24 m and the Herschel Space Telescope at
70 m. Objects having ring-like morphologies at 8 m and displaying
extended emission at 20 cm were selected visually. Emission at 24 m having
the form of an inner ring or central peak is also observed in the selected
objects. A catalog of 99 ring nebulae whose shapes at 8 and 70 m are well
approximated by ellipses has been compiled. The catalog contains 32 objects
whose shapes are close to circular (eccentricities of the fitted ellipses at 8
m no greater than 0.6, angular radius exceeding 20). These objects are
promising for comparisons with the results of one-dimensional hydrodynamical
simulations of expanding regions of ionized hydrogen.Comment: Astronomy Reports, Volume 61, Issue 12, pp.1015-1030 (ARep Homepage
Photometric observations of the supernova 2009nr
We present the results of our UBVRI CCD photometry for the second brightest
supernova of 2009, SN 2009nr, discovered during a sky survey with the
telescopes of the MASTER robotic network. Its light and color curves and
bolometric light curves have been constructed. The light-curve parameters and
the maximum luminosity have been determined. SN 2009nr is shown to be similar
in light-curve shape and maximum luminosity to SN 1991T, which is the prototype
of the class of supernovae Ia with an enhanced luminosity. SN 2009nr exploded
far from the center of the spiral galaxy UGC 8255 and most likely belongs to
its old halo population. We hypothesize that this explosion is a consequence of
the merger of white dwarfs
Self-Trapped Exciton Defects in a Charge Density Wave: Electronic Excitations of BaBiO3
In the previous paper, it was shown that holes doped into BaBiO3 self-trap as
small polarons and bipolarons. These point defects are energetically favorable
partly because they undo locally the strain in the charge-density-wave (Peierls
insulator) ground state. In this paper the neutral excitations of the same
model are discussed. The lowest electronic excitation is predicted to be a
self-trapped exciton, consisting of an electron and a hole located on adjacent
Bi atoms. This excitation has been seen experimentally (but not identified as
such) via the Urbach tail in optical absorption, and the multi-phonon spectrum
of the ``breathing mode'' seen in Raman scattering. These two phenomena occur
because of the Franck-Condon effect associated with oxygen displacement in the
excited state.Comment: 5 pages with 7 embedded figures. See also cond-mat/0108089 on
polarons and bipolarons in BaBiO3 contains background informatio
Multi-phonon Resonant Raman Scattering Predicted in LaMnO3 from the Franck-Condon Process via Self-Trapped Excitons
Resonant behavior of the Raman process is predicted when the laser frequency
is close to the orbital excitation energy of LaMnO3 at 2 eV. The incident
photon creates a vibrationally excited self-trapped ``orbiton'' state from the
orbitally-ordered Jahn-Teller (JT) ground state. Trapping occurs by local
oxygen rearrangement. Then the Franck-Condon mechanism activates multiphonon
Raman scattering. The amplitude of the -phonon process is first order in the
electron-phonon coupling . The resonance occurs {\it via} a dipole forbidden
to transition. We previously suggested that this transition (also seen
in optical reflectivity) becomes allowed because of asymmetric oxygen
fluctuations. Here we calculate the magnitude of the corresponding matrix
element using local spin-density functional theory. This calculation agrees to
better than a factor of two with our previous value extracted from experiment.
This allows us to calculate the absolute value of the Raman tensor for
multiphonon scattering. Observation of this effect would be a direct
confirmation of the importance of the JT electron-phonon term and the presence
of self-trapped orbital excitons, or ``orbitons''.Comment: 8 pages and 3 embedded figures. The earlier short version is now
replaced by a more complete paper with a slightly different title. This
version includes a caculation by density-functional theory of the dipole
matrix element for exciting the self-trapped orbital exciton which activates
the multiphonon Raman signal