3,972 research outputs found
Determination of the lowest energy structure of Ag from first-principles calculations
The ground-state electronic and structural properties, and the electronic
excitations of the lowest energy isomers of the Ag cluster are calculated
using density functional theory (DFT) and time-dependent DFT (TDDFT) in real
time and real space scheme, respectively. The optical spectra provided by TDDFT
predict that the D dodecahedron isomer is the structural minimum of
Ag cluster. Indeed, it is borne out by the experimental findings.Comment: 4 pages, 2 figures. Accepted in Physical Review A as a brief repor
The white dwarf in dwarf nova SDSS J080434.20+510349.2: Entering the instability strip?
SDSS J080434.20+510349.2 is the WZ type binary that displayed rare outburst
in 2006 (Pavlenko et al., 2007). During the long-lasting tail of the late stage
of the outburst binary shown the two-humped or four-humped profile of the
orbital light modulation. The amplitude of orbital light curve decreased while
the mean brightness decreased, more over that occurred 10 times faster
during the fast outburst decline in respect to the late quiet state of slow
outburst fading. There were no white dwarf pulsations detected neither 1 - 1.5
months prior to the outburst nor in 1.5 - 2 months after the 2006 outburst in
this system. However the strong non-radial pulsations with period 12.6 minutes
and mean amplitude of 0.05^m were first detected in V band with 2.6-m Shajn
mirror telescope of the Crimean astrophysical observatory in ~ 8 months after
the outburst. The evolution of pulsations over two years in 2006 - 2008 is
considered. It is supposed that pulsations first appeared when the cooling
white dwarf (after the outburst) entered the instability strip although the
possibility of temporary lack of pulsations at some occasions also could not be
excluded.Comment: Submitted to Proceedings of 16th European White Dwarf Workshop
(EUROWD08
What Produced the Ultraluminous Supernova Remnant in NGC 6946?
The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known
SNR in X-rays, ~1000 times brighter than Cas A. To probe the nature of this
remnant and its progenitor, we have obtained high-dispersion optical echelle
spectra. The echelle spectra detect H-alpha, [N II], and [O III] lines, and
resolve these lines into a narrow (FWHM ~20--40 km/s) component from un-shocked
material and a broad (FWHM ~250 km/s) component from shocked material. Both
narrow and broad components have unusually high [N II]/H-alpha ratios, ~1.
Using the echelle observation, archival HST images, and archival ROSAT X-ray
observations, we conclude that the SNR was produced by a normal supernova,
whose progenitor was a massive star, either a WN star or a luminous blue
variable. The high luminosity of the remnant is caused by the supernova ejecta
expanding into a dense, nitrogen-rich circumstellar nebula created by the
progenitor.Comment: 20 pages, 5 figures. To be published in The Astronomical Journal,
March 200
- …