8,950 research outputs found
The Expansion of the X-ray Remnant of Tycho's Supernova (SN1572)
Two ROSAT high resolution images separated by nearly five years have been
used to determine the expansion of the X-ray remnant of Tycho's supernova
(SN1572). The current expansion rate averaged over the entire remnant is
% per yr, which, when combined with the known age of the
remnant, determines the mean expansion parameter , defined as , to be . There are significant radial and azimuthal
variations of the X-ray expansion rate. The radial expansion in particular
shows highly significant evidence for the more rapid expansion of the forward
blast wave as compared to the reverse-shocked ejecta, an effect that has not
been seen previously. The expansion parameter varies from at
the outermost edge of Tycho's supernova remnant (SNR) to a value of
on the inside edge of the bright rim of emission. These values
are consistent with the rates expected for a remnant with constant density
ejecta evolving into a uniform interstellar medium during the ejecta-dominated
phase of evolution. Based on the size, age, and X-ray expansion rates, I obtain
values for the explosion energy and ambient density of and . As is also the case
for Cas A and Kepler's SNR, the X-ray expansion rate of Tycho's SNR appears to
be significantly higher than the radio expansion rate. In the case of Tycho's
SNR, however, the difference between radio and X-ray expansion rates is clearly
associated with the motion of the forward shock.Comment: 4 pages, including 2 postscript figs, LaTeX, emulateapj. Accepted by
ApJ Letter
Near-Infrared Classification Spectroscopy: H-band Spectra of Fundamental MK Standards
We present a catalogue of H-band spectra for 85 stars of approximately solar
abundance observed at a resolving power of 3000 with the KPNO Mayall 4m FTS.
The atlas covers spectral types O7-M5 and luminosity classes I-V as defined on
the MK system. We identify both atomic and molecular indices and line-ratios
which are temperature and luminosity sensitive allowing spectral classification
to be carried out in the H-band. The line ratios permit spectral classification
in the presence of continuum excess emission, which is commonly found in
pre-main sequence and evolved stars. We demonstrate that with spectra of R =
1000 obtained at SNR > 50 it is possible to derive spectral types within +- 2
subclasses for late-type stars. These data are available electronically through
the Astronomical Data Center in addition to being served on the World-Wide-Web.Comment: To appear in the November 20, 1998 issue of ApJ (Volume 508, #1
Diagnostic Line Emission from EUV and X-ray Illuminated Disks and Shocks around Low Mass stars
Extreme ultraviolet (EUV, 13.6 eV < h\nu \lta 100 eV) and X-rays in the
0.1-2 keV band can heat the surfaces of disks around young, low mass stars to
thousands of degrees and ionize species with ionization potentials greater than
13.6 eV. Shocks generated by protostellar winds can also heat and ionize the
same species close to the star/disk system. These processes produce diagnostic
lines (e.g., [NeII] 12.8 m and [OI] 6300 \AA) that we model as functions
of key parameters such as EUV luminosity and spectral shape, X-ray luminosity
and spectral shape, and wind mass loss rate and shock speed. Comparing our
models with observations, we conclude that either internal shocks in the winds
or X-rays incident on the disk surfaces often produce the observed [NeII] line,
although there are cases where EUV may dominate. Shocks created by the oblique
interaction of winds with disks are unlikely [NeII] sources because these
shocks are too weak to ionize Ne. Even if [NeII] is mainly produced by X-rays
or internal wind shocks, the neon observations typically place upper limits of
\lta 10^{42} s on the EUV photon luminosity of these young low mass
stars. The observed [OI] 6300 \AA line has both a low velocity component (LVC)
and a high velocity component. The latter likely arises in internal wind
shocks. For the former we find that X-rays likely produce more [OI] luminosity
than either the EUV layer, the transition layer between the EUV and X-ray
layer, or the shear layer where the protostellar wind shocks and entrains disk
material in a radial flow across the surface of the disk. Our soft X-ray models
produce [OI] LVCs with luminosities up to L, but may not be
able to explain the most luminous LVCs.Comment: 51 pages, 10 figures, accepted to Ap
Frequency behavior of Raman coupling coefficient in glasses
Low-frequency Raman coupling coefficient of 11 different glasses is
evaluated. It is found that the coupling coefficient demonstrates a universal
linear frequency behavior near the boson peak maximum and a superlinear
behavior at very low frequencies. The last observation suggests vanishing of
the coupling coefficient when frequency tends to zero. The results are
discussed in terms of the vibration wavefunction that combines features of
localized and extended modes.Comment: 8 pages, 9 figure
Broadband Dielectric Spectroscopy on Glass-Forming Propylene Carbonate
Dielectric spectroscopy covering more than 18 decades of frequency has been
performed on propylene carbonate in its liquid and supercooled-liquid state.
Using quasi-optic submillimeter and far-infrared spectroscopy the dielectric
response was investigated up to frequencies well into the microscopic regime.
We discuss the alpha-process whose characteristic timescale is observed over 14
decades of frequency and the excess wing showing up at frequencies some three
decades above the peak frequency. Special attention is given to the
high-frequency response of the dielectric loss in the crossover regime between
alpha-peak and boson-peak. Similar to our previous results in other glass
forming materials we find evidence for additional processes in the crossover
regime. However, significant differences concerning the spectral form at high
frequencies are found. We compare our results to the susceptibilities obtained
from light scattering and to the predictions of various models of the glass
transition.Comment: 13 pages, 9 figures, submitted to Phys. Rev.
Non exponential relaxation in fully frustrated models
We study the dynamical properties of the fully frustrated Ising model. Due to
the absence of disorder the model, contrary to spin glass, does not exhibit any
Griffiths phase, which has been associated to non-exponential relaxation
dynamics. Nevertheless we find numerically that the model exhibits a stretched
exponential behavior below a temperature T_p corresponding to the percolation
transition of the Kasteleyn-Fortuin clusters. We have also found that the
critical behavior of this clusters for a fully frustrated q-state spin model at
the percolation threshold is strongly affected by frustration. In fact while in
absence of frustration the q=1 limit gives random percolation, in presence of
frustration the critical behavior is in the same universality class of the
ferromagnetic q=1/2-state Potts model.Comment: 7 pages, RevTeX, 11 figs, to appear on Physical Review
Stratigraphic sequence and ages of volcanic units in the Gruithuisen region of the Moon
X-Ray Emission Line Imaging and Spectroscopy of Tycho's Supernova Remnant
We present X-ray images of Tycho's supernova remnant in emission line
features of Mg, Si, S, Ar, Ca, and Fe, plus the continuum, using data obtained
by the imaging spectrometers onboard the ASCA X-ray satellite. All the images
show the shell-like morphology characteristic of previously obtained broad-band
X-ray images, but are clearly distinct from each other. We use image
reconstruction techniques to achieve a spatial resolution of ~0.8'. Line
intensity ratios are used to make inferences about the remnant's physical
state, on average for the entire remnant, and with angular position around the
rim. The average temperature (T) of the Si and S ejecta in the remnant is
(0.8-1.1) X 10^7 K and the average ionization age (nt) is (0.8-1.3) X 10^11
cm^-3 s. For a constant nt, the observed relative brightness variations of Si
and S line image profiles with azimuthalangle imply differences of roughly a
factor of 1.3-1.8 in the temperature. We compare the radial brightness profiles
of our images to simple geometrical models, and find that a spherical emitting
geometry is favored over a torus. A spherical geometry is further supported by
the absence of systematic Doppler shifts across the remnant. The radial fit
results also suggest that some radial mixing of the ejecta has occurred.
However, the azimuthally averaged Fe K image peaks at a markedly lower radius
than the other images. The average Fe K / Fe L line intensity ratio and the
position of the Fe K energy centroid support a T several times higher and an nt
approx. a factor of ten lower than for the other elements, and imply that the
Fe ejecta must have retained some of its stratification. Although many of the
features in the 4-6 keV X-ray continuum correspond to those in the radio, there
is no obvious correlation between the relative brightness in these bands.Comment: 17 pages plus 9 figures, LaTeX type, uses aas2pp4.sty. To appear in
the Astrophysical Journa
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