917 research outputs found
Effects of nonorthogonality in the time-dependent current through tunnel junctions
A theoretical technique which allows to include contributions from
non-orthogonality of the electron states in the leads connected to a tunneling
junction is derived. The theory is applied to a single barrier tunneling
structure and a simple expression for the time-dependent tunneling current is
derived showing explicit dependence of the overlap. The overlap proves to be
necessary for a better quantitative description of the tunneling current, and
our theory reproduces experimental results substantially better compared to
standard approaches.Comment: 4 pages, 1 table, 1 figur
Goodness-of-Fit Tests DIFF1 and DIFF2 for Locally-Normalized Supernova Spectra
Two quantitative tests DIFF1 and DIFF2 for measuring goodness-of-fit between
two locally-normalized supernova spectra are presented. Locally-normalized
spectra are obtained by dividing a spectrum by the same spectrum smoothed over
a wavelength interval relatively large compared to line features, but
relatively small compared to continuum features. DIFF1 essentially measures the
mean relative difference between the line patterns of locally-normalized
spectra and DIFF2 is DIFF1 minimized by a relative logarithmic wavelength shift
between the spectra: the shift is effectively an artificial relative Doppler
shift. Both DIFF1 and DIFF2 measure the physical similarity of line formation,
and thus of supernovae. DIFF1 puts more weight on overall physical similarity
of the supernovae than DIFF2 because the DIFF2 shift compensates somewhat for
some physical distinction in the supernovae. Both tests are useful in ordering
supernovae into empirical groupings for further analysis. We present some
examples of locally-normalized spectra for Type IIb supernova SN 1993J with
some analysis of these spectra. The UV parts of two of the SN 1993J spectra are
HST spectra that have not been published before. We also give an example of
fitted locally-normalized spectra and, as an example of the utility of DIFF1
and DIFF2, some preliminary statistical results for hydrogen-deficient
core-collapse (HDCC) supernova spectra. This paper makes use of and refers to
material to found at the first author's online supernova spectrum database
SUSPEND (SUpernovae Spectra PENDing further analysis: see
http://www.nhn.ou.edu/~jeffery/astro/sne/spectra/spectra.html)Comment: 6 coauthors, 53 pages, 6 Figures, accepted by the Astrophysical
Journal Supplement Series Version 2: Improved discussion from Version
Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves
We report on a simultaneous modelling of the expansion and radio light curves
of SN1993J. We have developed a simulation code capable of generating synthetic
expansion and radio light curves of supernovae by taking into consideration the
evolution of the expanding shock, magnetic fields, and relativistic electrons,
as well as the finite sensitivity of the interferometric arrays used in the
observations. Our software successfully fits all the available radio data of SN
1993J with an standard emission model for supernovae extended with some
physical considerations, as an evolution in the opacity of the ejecta material,
a radial drop of the magnetic fields inside the radiating region, and a
changing radial density profile of the circumstellar medium beyond day 3100
after explosion.Comment: 12 pages, 12 figures, accepted for publication in A&
Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data
We have performed a complete re-calibration and re-analysis of all the
available VLBI observations of supernova SN1993J, following an homogeneous and
well-defined methodology. Observations of SN1993J at 69 epochs, spanning 13
years, were performed by two teams, which used different strategies and
analysis tools. The results obtained by each group are similar, but their
conclusions on the supernova expansion and the shape and evolution of the
emitting region differ significantly. From our analysis of the combined set of
observations, we have obtained an expansion curve with unprecedented time
resolution and coverage. We find that the data from both teams are compatible
when analyzed with the same methodology. One expansion index () is enough to model the expansion observed at 1.7\,GHz, while two
expansion indices ( and ), separated
by a break time, days, are needed to model the data, at
frequencies higher than 1.7\,GHz, up to day 4000 after explosion. We thus
confirm the wavelength dependence of the size of the emitting region reported
by one of the groups. We also find that all sizes measured at epochs later than
day 4000 after explosion are systematically smaller than our model predictions.
We estimate the fractional shell width (, average of all epochs
and frequencies) and the level of opacity to the radio emission by the ejecta.
We find evidence of a spectral-index radial gradient in the supernova shell,
which is indicative of a frequency-dependent ejecta opacity. Finally, we study
the distribution and evolution of the azimuthal anisotropies (hot spots) found
around the radio shell during the expansion. These anisotropies have
intensities of % of the mean flux density of the shell, and appear to
systematically evolve during the expansion.Comment: 13 pages, 9 figures, accepted for publication in A&
SN Ib 1990I: Clumping and Dust in the Ejecta?
Photometry and spectra of the type Ib SN 1990I are presented and analysed,
covering about 400 days of evolution. The presence of optical helium lines is
shown. SN 1990I seems to show higher velocities compared to a sample of type Ib
events. The nebular emission lines display a high degree of asymmetry and the
presence of fine structures. Using the [O I] 6300,64A flux, we estimate a lower
limit on the oxygen mass to fall in the range (0.7-1.35) M_solar. The oxygen
mass requires a filling factor as small as 10^{-2} on day 254, indicating a
highly clumpy distribution of the oxygen material. A blueshift of the order 600
km/s is reported in the [O I] 6300,64A after day 254. The [Ca II] 7307.5A
emission profile appears blueshifted as well at late epochs . We recover the
quasi-bolometric "BVRI" light curve of SN 1990I. The constructed bolometric
light curve shows a change of slope at late phases, with an e-folding time of
60 +/-2 d in the [50:200] d time interval, considerably faster than the one of
{56}Co decay (i.e. 111.3 d), suggesting the gamma-rays escape with lower
deposition, owing to the low mass nature of the ejecta. After day 200, an
-folding time ~47 +/-2.8 d is measured. A simplified gamma-ray deposition
model is applied after adding a contribution of about 35% to the computed
pseudo-bolometric light curves to account for near-IR luminosities to estimate
the ejecta and {56}Ni masses (M({56}Ni)=0.11 M_solar and M_{ej}=3.7 M_solar).
The deficit in luminosity is estimated to be about 50% around day 308. The
observed spectral blueshift combined with the dramatic and sudden drop in the
pseudo-bolometric light curve and (B-V) colour is interpreted to be a
consequence of dust condensation in the ejecta of SN 1990I around day 250.Comment: 17 pages, 11 gigures and 5 tables. To appear in A & A (accepted
The tunnel magnetoresistance in chains of quantum dots weakly coupled to external leads
We analyze numerically the spin-dependent transport through coherent chains
of three coupled quantum dots weakly connected to external magnetic leads. In
particular, using the diagrammatic technique on the Keldysh contour, we
calculate the conductance, shot noise and tunnel magnetoresistance (TMR) in the
sequential and cotunneling regimes. We show that transport characteristics
greatly depend on the strength of the interdot Coulomb correlations, which
determines the spacial distribution of electron wave function in the chain.
When the correlations are relatively strong, depending on the transport regime,
we find both negative TMR as well as TMR enhanced above the Julliere value,
accompanied with negative differential conductance (NDC) and super-Poissonian
shot noise. This nontrivial behavior of tunnel magnetoresistance is associated
with selection rules that govern tunneling processes and various high-spin
states of the chain that are relevant for transport. For weak interdot
correlations, on the other hand, the TMR is always positive and not larger than
the Julliere TMR, although super-Poissonian shot noise and NDC can still be
observed
A Decade of SN1993J: Discovery of Wavelength Effects in the Expansion Rate
We have studied the growth of the shell-like radio structure of supernova
SN1993J in M81 from September 1993 through October 2003 with very-long-baseline
interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18cm. For
this purpose, we have developed a method to accurately determine the outer
radius (R) of any circularly symmetric compact radio structure like SN1993J.
The source structure of SN1993J remains circularly symmetric (with deviations
from circularity under 2%) over almost 4000 days. We characterize the
decelerated expansion of SN 1993J through approximately day 1500 after
explosion with an expansion parameter ().
However, from that day onwards the expansion is different when observed at 6
and 18cm. Indeed, at 18cm, the expansion can be well characterized by the same
as before day 1500, while at 6cm the expansion appears more decelerated,
and is characterized by another expansion parameter, .
Therefore, since about day 1500 on, the radio source size has been
progressively smaller at 6cm than at 18cm. These findings are in stark contrast
to previous reports by other authors on the details of the expansion. In our
interpretation the supernova expands with a single expansion parameter, , and the 6cm results beyond day 1500 are due to physical
effects, perhaps also coupled to instrumental limitations. Two physical effects
may be involved: (a) a changing opacity of the ejecta to the 6cm radiation, and
(b) a radial decrease of the magnetic field in the emitting region.
(Long abstract cut. Please, read full abstract in manuscript).Comment: 21 pages, 19 figures, accepted in A&
Fluid-Structural Coupling Effects on the Dynamics of Mistuned Bladed Disks
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77177/1/AIAA-23809-593.pd
An Intermediate Luminosity Transient in NGC300: The Eruption of a Dust-Enshrouded Massive Star
[abridged] We present multi-epoch high-resolution optical spectroscopy,
UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an
intermediate luminosity optical transient recently discovered in the nearby
galaxy NGC300. We find that the transient (NGC300 OT2008-1) has a peak absolute
magnitude of M_bol~-11.8 mag, intermediate between novae and supernovae, and
similar to the recent events M85 OT2006-1 and SN2008S. Our high-resolution
spectra, the first for this event, are dominated by intermediate velocity
(~200-1000 km/s) hydrogen Balmer lines and CaII emission and absorption lines
that point to a complex circumstellar environment, reminiscent of the yellow
hypergiant IRC+10420. In particular, we detect broad CaII H&K absorption with
an asymmetric red wing extending to ~1000 km/s, indicative of gas infall onto a
massive and relatively compact star (blue supergiant or Wolf-Rayet star); an
extended red supergiant progenitor is unlikely. The origin of the inflowing gas
may be a previous ejection from the progenitor or the wind of a massive binary
companion. The low luminosity, intermediate velocities, and overall similarity
to a known eruptive star indicate that the event did not result in a complete
disruption of the progenitor. We identify the progenitor in archival Spitzer
observations, with deep upper limits from Hubble data. The spectral energy
distribution points to a dust-enshrouded star with a luminosity of about 6x10^4
L_sun, indicative of a ~10-20 M_sun progenitor (or binary system). This
conclusion is in good agreement with our interpretation of the outburst and
circumstellar properties. The lack of significant extinction in the transient
spectrum indicates that the dust surrounding the progenitor was cleared by the
outburst.Comment: Submitted to ApJ; emulateapj style; 39 pages; 26 figure
Interaction-powered supernovae: Rise-time vs. peak-luminosity correlation and the shock-breakout velocity
Interaction of supernova (SN) ejecta with the optically thick circumstellar
medium (CSM) of a progenitor star can result in a bright, long-lived shock
breakout event. Candidates for such SNe include Type IIn and superluminous SNe.
If some of these SNe are powered by interaction, then there should be a
relation between their peak luminosity, bolometric light-curve rise time, and
shock-breakout velocity. Given that the shock velocity during shock breakout is
not measured, we expect a correlation, with a significant spread, between the
rise time and the peak luminosity of these SNe. Here, we present a sample of 15
SNe IIn for which we have good constraints on their rise time and peak
luminosity from observations obtained using the Palomar Transient Factory. We
report on a possible correlation between the R-band rise time and peak
luminosity of these SNe, with a false-alarm probability of 3%. Assuming that
these SNe are powered by interaction, combining these observables and theory
allows us to deduce lower limits on the shock-breakout velocity. The lower
limits on the shock velocity we find are consistent with what is expected for
SNe (i.e., ~10^4 km/s). This supports the suggestion that the early-time light
curves of SNe IIn are caused by shock breakout in a dense CSM. We note that
such a correlation can arise from other physical mechanisms. Performing such a
test on other classes of SNe (e.g., superluminous SNe) can be used to rule out
the interaction model for a class of events.Comment: Accepted to ApJ, 6 page
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