2,363 research outputs found
On the Nature of the Peculiar Hot Star in the Young LMC Cluster NGC1818
The blue star reported in the field of the young LMC cluster NGC1818 by Elson
et al. (1998) has the wrong luminosity and radius to be a "luminous white
dwarf" member of the cluster. In addition, unless the effective temperature
quoted by the authors is a drastic underestimate, the luminosity is much too
low for it to be a cluster member in the post-AGB phase. Other possibilities,
including that of binary evolution, are briefly discussed. However, the
implication that the massive main sequence turnoff stars in this cluster can
produce white dwarfs (instead of neutron stars) from single-star evolution
needs to be reconsidered.Comment: 5 pages, no figures, Ap J Letters in pres
PET and P300 Relationships in Early Alzheimer\u27s Disease
The P300 (P3) wave of the auditory brain event-related potential was investigated in patients with probable Alzheimer\u27s disease to determine whether P300 latency discriminated these patients from controls and whether prolonged P300 latency correlated with rates of brain glucose metabolism as measured by Positron Emission Tomography. P300 latency was prolonged by more than 1.5 standard deviations from age expectancy in 14 of 18 patients, but none of 17 controls. In these subjects P300 latency was shown to be inversely correlated with relative metabolic rates of parietal and, to a lesser extent, temporal and frontal association areas, but not with subcortical areas
The True Incidence of Magnetism among Field White Dwarfs
We study the incidence of magnetism in white dwarfs from three large and
well-observed samples of hot, cool, and nearby white dwarfs in order to test
whether the fraction of magnetic degenerates is biased, and whether it varies
with effective temperature, cooling age, or distance. The magnetic fraction is
considerably higher for the cool sample of Bergeron, Ruiz, and Leggett, and the
Holberg, Oswalt, and Sion sample of local white dwarfs that it is for the
generally-hotter white dwarfs of the Palomar Green Survey. We show that the
mean mass of magnetic white dwarfs in this survey is 0.93 solar masses or more,
so there may be a strong bias against their selection in the magnitude-limited
Palomar Green Survey. We argue that this bias is not as important in the
samples of cool and nearby white dwarfs. However, this bias may not account for
all of the difference in the magnetic fractions of these samples.
It is not clear that the magnetic white dwarfs in the cool and local samples
are drawn from the same population as the hotter PG stars. In particular, two
or threee of the cool sample are low-mass white dwarfs in unresolved binary
systems. Moreover, there is a suggestion from the local sample that the
fractional incidence may increase with decreasing temperature, luminosity,
and/or cooling age. Overall, the true incidence of magnetism at the level of 2
megagauss or greater is at least 10%, and could be higher. Limited studies
capable of detecting lower field strengths down to 10 kilogauss suggest by
implication that the total fraction may be substantially higher than 10%.Comment: 16 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
Cognitive Sparing during the Administration of Whole Brain Radiotherapy and Prophylactic Cranial Irradiation: Current Concepts and Approaches
Whole brain radiotherapy (WBRT) for the palliation of metastases, or as prophylaxis to prevent intracranial metastases, can be associated with subacute and late decline in memory and other cognitive functions. Moreover, these changes are often increased in both frequency and severity when cranial irradiation is combined with the use of systemic or intrathecal chemotherapy. Approaches to preventing or reducing this toxicity include the use of stereotactic radiosurgery (SRS) instead of WBRT; dose reduction for PCI; exclusion of the limbic circuit, hippocampal formation, and/or neural stem cell regions of the brain during radiotherapy; avoidance of intrathecal and/or systemic chemotherapy during radiotherapy; the use of high-dose, systemic chemotherapy in lieu of WBRT. This review discusses these concepts in detail as well as providing both neuroanatomic and radiobiologic background relevant to these issues
Towards Better Integrators for Dissipative Particle Dynamics Simulations
Coarse-grained models that preserve hydrodynamics provide a natural approach
to study collective properties of soft-matter systems. Here, we demonstrate
that commonly used integration schemes in dissipative particle dynamics give
rise to pronounced artifacts in physical quantities such as the compressibility
and the diffusion coefficient. We assess the quality of these integration
schemes, including variants based on a recently suggested self-consistent
approach, and examine their relative performance. Implications of
integrator-induced effects are discussed.Comment: 4 pages, 3 figures, 2 tables, accepted for publication in Phys. Rev.
E (Rapid Communication), tentative publication issue: 01 Dec 200
A Spatially Resolved `Inside-out' Outburst of IP Pegasi
We present a comprehensive photometric dataset taken over the entire outburst
of the eclipsing dwarf nova IP Peg in September/October 1997. Analysis of the
lightcurves taken over the long rise to the peak-of-outburst shows conclusively
that the outburst started near the centre of the disc and moved outwards. This
is the first dataset that spatially resolves such an outburst. The dataset is
consistent with the idea that long rise times are indicative of such
`inside-out' outbursts. We show how the thickness and the radius of the disc,
along with the mass transfer rate change over the whole outburst. In addition,
we show evidence of the secondary and the irradiation thereof. We discuss the
possibility of spiral shocks in the disc; however we find no conclusive
evidence of their existence in this dataset.Comment: 8 pages, 8 figures, to be appear in MNRA
X-ray properties of the white dwarf pulsar eRASSU J191213.9-441044
We report X-ray observations of the newly discovered pulsating white dwarf
eRASSU J191213.9-441044 with Spectrum Roentgen Gamma and eROSITA (SRG/eROSITA)
and XMM-Newton. The new source was discovered during the first eROSITA all-sky
survey at a flux level of fX (0.2 - 2.3 keV) = 3.3 e-13 erg cm-2 s-1 and found
to be spatially coincident with a G = 17.1 stellar Gaia-source at a distance of
237 pc. The flux dropped to about fX = 1 e-13 erg cm-2 s-1 during the three
following eROSITA all-sky surveys and remained at this lower level during
dedicated XMM-Newton observations performed in September 2022. With XMM-Newton,
pulsations with a period of 319 s were found at X-ray and ultraviolet
wavelengths occurring simultaneously in time, thus confirming the nature of
eRASSU J191213.9-441044 as the second white-dwarf pulsar. The X-ray and
UV-pulses correspond to broad optical pulses. Narrow optical pulses that
occurred occasionally during simultaneous XMM-Newton/ULTRACAM observations have
no X-ray counterpart. The orbital variability of the X-ray signal with a
roughly sinusoidal shape was observed with a pulsed fraction of ~28% and
maximum emission at orbital phase ~0.25. The ultraviolet light curve peaks at
around binary phase 0.45. The X-ray spectrum can be described with the sum of a
power law spectrum and a thermal component with a mean X-ray luminosity of
Lx(0.2-10 keV) = 1.4 e30 erg s-1. The spectral and variability properties could
indicate some residual accretion, in contrast to the case of the prototypical
object AR Sco.Comment: 6 pages, 7 figures, A&A Letter accepte
Singing in South Africa : monitoring the occurrence of humpback whale (Megaptera novaeangliae) song near the Western Cape
Humpback whales (Megaptera novaeangliae) are highly vocal, producing a wide repertoire of sounds often organised into song. Song is prolific at breeding sites but also documented along migration routes and at feeding sites, including along the west coast of South Africa (28°–34°S). Here we examine the occurrence of humpback whale song within False Bay, South Africa, using intermittent recording periods from moored hydrophones spanning September 2016 to January 2018. Recordings from four locations were scrutinised for humpback whale vocalisations using long-term spectral averages (LTSAs). In total, 7205 h were examined, with song identified in 3% (211 h) of recording hours. Song was exclusively documented in September and October 2016 and was more prevalent at the most westerly sites. Diel patterns of song presence were modelled, showing the likelihood of detection was higher in the early morning and late evening (GAM: p < 0.05). On 15 occasions, two or more singers were detected with temporally overlapping song components. These results indicate prevalent, albeit seasonal, song production by humpback whales off the coast of South Africa and highlight the utility of passive acoustic monitoring to indicate their presence, behaviour, and potential population linkages in the region.The Association for the Study of Animal Behaviour; Claude Leon Foundation; National Research Foundation and University Stellenbosch.http://www.tandfonline.comtoc/tbio202021-01-23hj2020Mammal Research Institut
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