11,813 research outputs found
A new magnetic white dwarf : PG2329+267
We have discovered that the white dwarf PG 2329+267 is magnetic, and assuming
a centered dipole structure, has a dipole magnetic field strength of
approximately 2.3MG. This makes it one of only approximately 4% of isolated
white dwarfs with a detectable magnetic field. Linear Zeeman splitting as well
as quadratic Zeeman shifts are evident in the hydrogen Balmer sequence and
circular spectropolarimetry reveals 10% circular polarisation in the two
displaced sigma components of Halpha. We suggest from comparison with spectra
of white dwarfs of known mass that PG 2329+267 is more massive than typical
isolated white dwarfs, in agreement with the hypothesis that magnetic white
dwarfs evolve from magnetic chemically peculiar Ap and Bp type main sequence
stars.Comment: 5 pages, with 2 encapsulated postscipts figures include
A Survey for Photometric Variability in Isolated Magnetic White Dwarfs—Measuring their Spin Periods
We present the initial findings of a photometric survey of isolated magnetic white dwarfs (MWDs) carried out with the 1.0m Jacobus Kapteyn Telescope. Of our sample of 30 MWDs, we have observed variability in 17 (57%) over our observed timescales (minutes to years), with a further 11 requiring more data, and two that are non-variable at the 1% level. In total we have discovered possible variability in 15 targets that has not been reported before in the literature, and we have measured the spin period of five objects in our sample to within a few percent. We find no correlation between spin period, mass or temperature, but there may be a weak negative correlation between period and field strength for the short-period targets. We have identified 14 MWDs with low field strengths and low temperatures, which are candidates for having star spots on their surfaces and should be followed up with polarimetry. We have also found that three low-field, high temperature MWDs are unexpectedly variable, with no obvious mechanism to cause this
Macroscopic and microscopic studies of electrical properties of very thin silicon dioxide subject to electrical stress
The electrical characteristics of various size tunnel switch diode devices, composed of Al/SiO2/n-Si/p+-Si layers, which operate with a range of parameters (such as current densities in excess of 104 A/cm2) that stress the oxide layer far beyond the levels used in typical thin oxide metal-oxide semiconductor research have been examined. It is found that the first time a large current and electric field are applied to the device, a "forming" process enhances transport through the oxide in the vicinity of the edges of the gate electrode, but the oxide still retains its integrity as a tunnel barrier. The device operation is relatively stable to stresses of greater than 107 C/cm2 areally averaged, time-integrated charge injection. Duplication and characterization of these modified oxide tunneling properties was attempted using scanning tunneling microscopy (STM) to stress and probe the oxide. Electrical stressing with the STM tip creates regions of reduced conductivity, possibly resulting from trapped charge in the oxide. Lateral variations in the conductivity of the unstressed oxide over regions roughly 20–50 nm across were also found
Searching for nova shells around cataclysmic variables
We present the results of a search for nova shells around 101 cataclysmic
variables (CVs), using Halpha images taken with the 4.2-m William Herschel
Telescope (WHT) and the 2.5-m Isaac Newton Telescope Photometric Halpha Survey
of the Northern Galactic Plane (IPHAS). Both telescopes are located on La
Palma. We concentrated our WHT search on nova-like variables, whilst our IPHAS
search covered all CVs in the IPHAS footprint. We found one shell out of the 24
nova-like variables we examined. The newly discovered shell is around V1315 Aql
and has a radius of approx.2.5 arcmin, indicative of a nova eruption
approximately 120 years ago. This result is consistent with the idea that the
high mass-transfer rate exhibited by nova-like variables is due to enhanced
irradiation of the secondary by the hot white dwarf following a recent nova
eruption. The implications of our observations for the lifetime of the
nova-like variable phase are discussed. We also examined 4 asynchronous polars,
but found no new shells around any of them, so we are unable to confirm that a
recent nova eruption is the cause of the asynchronicity in the white dwarf
spin. We find tentative evidence of a faint shell around the dwarf nova V1363
Cyg. In addition, we find evidence for a light echo around the nova V2275 Cyg,
which erupted in 2001, indicative of an earlier nova eruption approx.300 years
ago, making V2275 Cyg a possible recurrent nova.Comment: 14 pages, 50 figures, 3 Table
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