9,113 research outputs found
Hydrodynamic Thermonuclear Runaways in Superbursts
We calculate the thermal and dynamical evolution of the surface layers of an
accreting neutron star during the rise of a superburst. For the first few hours
following unstable 12C ignition, the nuclear energy release is transported by
convection. However, as the base temperature rises, the heating time becomes
shorter than the eddy turnover time and convection becomes inefficient. This
results in a hydrodynamic nuclear runaway, in which the heating time becomes
shorter than the local dynamical time. Such hydrodynamic burning can drive
shock waves into the surrounding layers and may be the trigger for the normal
X-ray burst found to immediately precede the onset of the superburst in both
cases where the Rossi X-Ray Timing Explorer was observing.Comment: 4 pages, 3 figures (emulateapj), accepted to ApJ Letter
Inequality and Mobility of Wealth in Sweden 1983/84 - 1992/93.
Using longitudinal data which include real estate wealth, financial assets as well as consumer durables, changes in the distribution of wealth in Sweden are related to major changes in asset prices and in incentives to hold various assets in the 1980s and the beginning of the 1990s. Our analysis of the mobility of wealth indicates that mobility is higher in Sweden than in the United States, while the analysis of who is gaining and who is loosing shows results similar to those of previous studies.
On the validity of the method of reduction of dimensionality: area of contact, average interfacial separation and contact stiffness
It has recently been suggested that many contact mechanics problems between
solids can be accurately studied by mapping the problem on an effective one
dimensional (1D) elastic foundation model. Using this 1D mapping we calculate
the contact area and the average interfacial separation between elastic solids
with nominally flat but randomly rough surfaces. We show, by comparison to
exact numerical results, that the 1D mapping method fails even qualitatively.
We also calculate the normal interfacial stiffness and compare it with the
result of an analytical study. We attribute the failure of the elastic
foundation model to the neglect of the long-range elastic coupling between the
asperity contact regions.Comment: 5 pages, 4 figures, 29 reference
Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected in the Optical Transmission Spectrum
We present the first ground-based detection of sodium absorption in the
transmission spectrum of an extrasolar planet. Absorption due to the atmosphere
of the extrasolar planet HD189733b is detected in both lines of the NaI
doublet. High spectral resolution observations were taken of eleven transits
with the High Resolution Spectrograph (HRS) on the 9.2 meter Hobby-Eberly
Telescope (HET). The NaI absorption in the transmission spectrum due to
HD189733b is (-67.2 +/- 20.7) x 10^-5 deeper in the ``narrow'' spectral band
that encompasses both lines relative to adjacent bands. The 1-sigma error
includes both random and systematic errors, and the detection is >3-sigma. This
amount of relative absorption in NaI for HD189733b is ~3x larger than detected
for HD209458b by Charbonneau et al. (2002), and indicates these two
hot-Jupiters may have significantly different atmospheric properties.Comment: 12 pages, 2 figures; Accepted for publication in ApJ Letter
Fast all-optical nuclear spin echo technique based on EIT
We demonstrate an all-optical Raman spin echo technique, using
Electromagnetically Induced Transparency (EIT) to create the different pulses
of the spin echo sequence: initialization, pi-rotation, and readout. The first
pulse of the sequence induces coherence directly from a mixed state, and the
technique is used to measure the nuclear spin coherence of an inhomogeneously
broadened ensemble of rare-earth ions (Pr). In contrast to previous
experiments it does not require any preparatory hole burning pulse sequences,
which greatly shortens the total duration of the sequence. The effect of the
different pulses is characterized by quantum state tomography and is compared
with simulations. We demonstrate two applications of the technique by using the
spin echo sequence to accurately compensate a magnetic field across our sample,
and to measure the coherence time at high temperatures up to 11 K, where
standard preparation techniques are difficult to implement. We explore the
potential of the technique and possible applications.Comment: 8 pages, 6 figure
Presence of pathogenicity island genes in Enterococcus faecalis isolated from pigs in Denmark
Localization in orchards using Extended Kalman Filter for sensor-fusion - A FroboMind component
Making an automated vehicle navigate in rows of orchards is a feature, relevant for automating the plant nursing and cultivation of the trees. To be able to navigate accurate and reliably, the vehicle must know its position relative to the trees in the orchards
Thermonuclear .Ia Supernovae from Helium Shell Detonations: Explosion Models and Observables
During the early evolution of an AM CVn system, helium is accreted onto the
surface of a white dwarf under conditions suitable for unstable thermonuclear
ignition. The turbulent motions induced by the convective burning phase in the
He envelope become strong enough to influence the propagation of burning fronts
and may result in the onset of a detonation. Such an outcome would yield
radioactive isotopes and a faint rapidly rising thermonuclear ".Ia" supernova.
In this paper, we present hydrodynamic explosion models and observable outcomes
of these He shell detonations for a range of initial core and envelope masses.
The peak UVOIR bolometric luminosities range by a factor of 10 (from 5e41 -
5e42 erg/s), and the R-band peak varies from M_R,peak = -15 to -18. The rise
times in all bands are very rapid (<10 d), but the decline rate is slower in
the red than the blue due to a secondary near-IR brightening. The
nucleosynthesis primarily yields heavy alpha-chain elements (40Ca through 56Ni)
and unburnt He. Thus, the spectra around peak light lack signs of intermediate
mass elements and are dominated by CaII and TiII features, with the caveat that
our radiative transfer code does not include the non-thermal effects necessary
to produce He features.Comment: Accepted for publication in The Astrophysical Journal; 9 pages, 9
figures; v2: Minor changes to correct typos and clarify conten
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