24 research outputs found
On the High--Velocity Ejecta of the Type Ia Supernova 1994D
Synthetic spectra generated with the parameterized supernova
synthetic-spectrum code SYNOW are compared to spectra of the Type Ia SN 1994D
that were obtained before the time of maximum brightness. Evidence is found for
the presence of two-component Fe II and Ca II features, forming in high
velocity ( \kms) and lower velocity ( \kms) matter.
Possible interpretations of these spectral splits, and implications for using
early--time spectra of SNe Ia to probe the metallicity of the progenitor white
dwarf and the nature of the nuclear burning front in the outer layers of the
explosion, are discussed.Comment: 15 pages, 3 figures, 3 tables, Astrophysical Journal, in pres
Subaru and Keck Observations of the Peculiar Type Ia Supernova 2006gz at Late Phases
Recently, a few peculiar Type Ia supernovae (SNe) that show exceptionally
large peak luminosity have been discovered. Their luminosity requires more than
1 Msun of 56Ni ejected during the explosion, suggesting that they might have
originated from super-Chandrasekhar mass white dwarfs. However, the nature of
these objects is not yet well understood. In particular, no data have been
taken at late phases, about one year after the explosion. We report on Subaru
and Keck optical spectroscopic and photometric observations of the SN Ia
2006gz, which had been classified as being one of these "overluminous" SNe Ia.
The late-time behavior is distinctly different from that of normal SNe Ia,
reinforcing the argument that SN 2006gz belongs to a different subclass than
normal SNe Ia. However, the peculiar features found at late times are not
readily connected to a large amount of 56Ni; the SN is faint, and it lacks [Fe
II] and [Fe III] emission. If the bulk of the radioactive energy escapes the SN
ejecta as visual light, as is the case in normal SNe Ia, the mass of 56Ni does
not exceed ~ 0.3 Msun. We discuss several possibilities to remedy the problem.
With the limited observations, however, we are unable to conclusively identify
which process is responsible. An interesting possibility is that the bulk of
the emission might be shifted to longer wavelengths, unlike the case in other
SNe Ia, which might be related to dense C-rich regions as indicated by the
early-phase data. Alternatively, it might be the case that SN 2006gz, though
peculiar, was actually not substantially overluminous at early times.Comment: 8 pages, 6 figures, 4 tables. Accepted for publication in The
Astrophysical Journa
Phase-controlled, heterodyne laser-induced transient grating measurements of thermal transport properties in opaque material
The methodology for a heterodyned laser-induced transient thermal grating
technique for non-contact, non-destructive measurements of thermal transport in
opaque material is presented. Phase-controlled heterodyne detection allows us
to isolate pure phase or amplitude transient grating signal contributions by
varying the relative phase between reference and probe beams. The phase grating
signal includes components associated with both transient reflectivity and
surface displacement whereas the amplitude grating contribution is governed by
transient reflectivity alone. By analyzing the latter with the two-dimensional
thermal diffusion model, we extract the in-plane thermal diffusivity of the
sample. Measurements on a 5 {\mu}m thick single crystal PbTe film yielded
excellent agreement with the model over a range of grating periods from 1.6 to
2.8 {\mu}m. The measured thermal diffusivity of 1.3 \times 10-6 m2/s was found
to be slightly lower than the bulk value.Comment: 19 pages, 6 figure
Statistical mechanics of semiflexible ribbon polymers
The statistical mechanics of a ribbon polymer made up of two semiflexible
chains is studied using both analytical techniques and simulation. The system
is found to have a crossover transition at some finite temperature, from a type
of short range order to a fundamentally different sort of short range order. In
the high temperature regime, the 2-point correlation functions of the object
are identical to worm-like chains, while in the low temperature regime they are
different due to a twist structure. The crossover happens when the persistence
length of individual strands becomes comparable to the thickness of the ribbon.
In the low temperature regime, the ribbon is observed to have a novel
``kink-rod'' structure with a mutual exclusion of twist and bend in contrast to
smooth worm-like chain behaviour. This is due to its anisotropic rigidity and
corresponds to an {\it infinitely} strong twist-bend coupling. The
double-stranded polymer is also studied in a confined geometry. It is shown
that when the polymer is restricted in a particular direction to a size less
than the bare persistence length of the individual strands, it develops zigzag
conformations which are indicated by an oscillatory tangent-tangent correlation
function in the direction of confinement. Increasing the separation of the
confining plates leads to a crossover to the free behaviour, which takes place
at separations close to the bare persistence length. These results are expected
to be relevant for experiments which involve complexation of two or more stiff
or semiflexible polymers.Comment: 20 pages, 11 figures. PRE (in press
The early spectral evolution of SN 2004dt
Aims. We study the optical spectroscopic properties of Type Ia Supernova (SN
Ia) 2004dt, focusing our attention on the early epochs.
Methods. Observation triggered soon after the SN 2004dt discovery allowed us
to obtain a spectrophotometric coverage from day -10 to almost one year (~353
days) after the B band maximum. Observations carried out on an almost daily
basis allowed us a good sampling of the fast spectroscopic evolution of SN
2004dt in the early stages. To obtain this result, low-resolution, long-slit
spectroscopy was obtained using a number of facilities.
Results. This supernova, which in some absorption lines of its early spectra
showed the highest degree of polarization ever measured in any SN Ia, has a
complex velocity structure in the outer layers of its ejecta. Unburnt oxygen is
present, moving at velocities as high as ~16,700 km/s, with some
intermediate-mass elements (Mg, Si, Ca) moving equally fast. Modeling of the
spectra based on standard density profiles of the ejecta fails to reproduce the
observed features, whereas enhancing the density of outer layers significantly
improves the fit. Our analysis indicates the presence of clumps of
high-velocity, intermediate-mass elements in the outermost layers, which is
also suggested by the spectropolarimetric data.Comment: 13 pages, 15 figures, accepted for pubblication in Astronomy and
Astrophysic
Photoconductivity of PbTe:In films with variable microstructure
It is shown that the microstructure and features of formation of surface states in nanocrystalline and polycrystalline PbTe:In films most significantly affect the character of photoconductivity in the spectral range of 1–2.5 THz. We present the results of a study and comparative analysis of the character of conductivity of PbTe:In films in the temperature range from 4.2 to 300 K in a static mode and in variable electric fields with a frequency of up to 1 MHz with illumination with white light and under the effect of high-power terahertz laser pulses with a wavelength of up to 280 μm