2,053 research outputs found
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
Phase-resolved HST/STIS spectroscopy of the exposed white dwarf in the high-field polar AR UMa
Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR
UMa confirms that the WD photospheric Ly alpha Zeeman features are formed in a
magnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+
components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes
``enabled'' in the presence of both strong magnetic and electric fields. Our
attempt in fitting the overall optical+UV low state spectrum with single
temperature magnetic WD models remains rather unsatisfactory, indicating either
a shortcoming in the present models or a new physical process acting in AR UMa.
As a result, our estimate of the WD temperature remains somewhat uncertain,
Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A and
present throughout the entire binary orbit, and a second bump near ~1650A,
which appears only near the inferior conjunction of the secondary star. These
are suggestive of low harmonic cyclotron emission produced by low-level
(M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is no
evidence in the power spectrum of light variations for accretion in gas blobs.
The observed Ly alpha emission line shows a strong phase dependence with
maximum flux and redshift near orbital phase phi~0.3, strongly indicating an
origin on the trailing hemisphere of the secondary star. An additional Ly alpha
absorption feature with similar phasing as the Ly alpha emission, but a
\~700km/s blueshift could tentatively be ascribed to absorption of WD emission
in a moderately fast wind. We derive a column density of neutral hydrogen of
NH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.Comment: 26 pages, 10 figures, AAS TeX 5.0, accepted for publication in the
Astrophysical Journa
Duration discrimination in the range of milliseconds and seconds in children with ADHD and their unaffected siblings
Background Detecting genetic factors involved in attention deficit hyperactivity disorder (ADHD) is complicated because of their small effect sizes and complex interactions. The endophenotype approach eases this by coming closer to the relevant genes. Different aspects of temporal information processing are known to be affected in ADHD. Thus, some of these aspects could represent candidate endophenotypes for ADHD. Method Fifty-four sib-pairs with at least one child with ADHD and 40 control children aged 6-18 years were recruited and asked to perform two duration discrimination tasks, one with a base duration of 50 ms on automatic timing and one with a base duration of 1000 ms on cognitively controlled timing. Results Whereas children with ADHD, but not their unaffected siblings, were impaired in discrimination of longer intervals, both groups were impaired in discriminating brief intervals. Furthermore, a significant within-family correlation was found for discrimination of brief intervals. Task performances of subjects of the control group correlated with individual levels of hyperactivity/impulsivity for discrimination of brief intervals, but not of longer intervals. Conclusions Cognitively controlled and also automatic processes of temporal information processing are impaired in children with ADHD. Discrimination of longer intervals appears as a typical âdisease marker' whereas discrimination of brief intervals shows up as a âvulnerability marker'. Discrimination of brief intervals was found to be familial and linked to levels of hyperactivity/impulsivity. Taken together, discrimination of brief intervals represents a candidate endophenotype of ADH
Surface Sediments of the Pearl River Estuary (South China Sea) - Spatial Distribution of Sedimentological/Geochemical Properties and Environmental Interpretation
The Pearl River Delta (South China) is one of the densest populated regions of the world. This study aims at the investigation and interpretation of the spatial distribution of grain size parameters and geochemical parameters obtained from surface sediment samples. These samples have been taken during cruises in 2003, 2004 and 2005. Investigations of the spatial correlations of the parameters obtained reveal an approximately north-south directed trend for the majority of the parameters. The trend was removed before applying Ordinary Kriging for interpolation. The maps obtained show non-uniform distribution patterns of the sedimentological and geochemical parameters. Here e.g. the concentrations of the As, Co, Cu, Hg and Ni decrease to the more marine influenced southeastern parts and show a higher concentration in the central part and at the western shoals of the estuary.The Pearl River Delta (South China) is one of the densest populated regions of the world. This study aims at the investigation and interpretation of the spatial distribution of grain size parameters and geochemical parameters obtained from surface sediment samples. These samples have been taken during cruises in 2003, 2004 and 2005. Investigations of the spatial correlations of the parameters obtained reveal an approximately north-south directed trend for the majority of the parameters. The trend was removed before applying Ordinary Kriging for interpolation. The maps obtained show non-uniform distribution patterns of the sedimentological and geochemical parameters. Here e.g. the concentrations of the As, Co, Cu, Hg and Ni decrease to the more marine influenced southeastern parts and show a higher concentration in the central part and at the western shoals of the estuary
New Integrable Structures in Large-N QCD
We study the anomalous dimensions of single trace operators composed of field
strengths in large-N QCD. The matrix of anomalous dimensions is
the Hamiltonian of a compact spin chain with two spin one representations at
each vertex corresponding to the selfdual and anti-selfdual components of
. Due to the special form of the interaction it is possible to
study separately renormalization of purely selfdual components. In this sector
the Hamiltonian is integrable and can be exactly solved by Bethe ansatz. Its
continuum limit is described by the level two SU(2) WZW model.Comment: 12 pages; V2: ref. added, V3: refs. added, explicit expression for
the spin ladder and other text improvement
A rotating cavity for high-field angle-dependent microwave spectroscopy of low-dimensional conductors and magnets
The cavity perturbation technique is an extremely powerful method for
measuring the electrodynamic response of a material in the millimeter- and
sub-millimeter spectral range (10 GHz to 1 THz), particularly in the case of
high-field/frequency magnetic resonance spectroscopy. However, the application
of such techniques within the limited space of a high-field magnet presents
significant technical challenges. We describe a 7.62 mm x 7.62 mm (diameter x
length) rotating cylindrical cavity which overcomes these problems.Comment: 11 pages including 8 figure
FUSE and HST/STIS far-ultraviolet observations of AM Herculis in an extended low state
We have obtained FUSE and HST/STIS time-resolved spectroscopy of the polar AM
Herculis during a deep low state. The spectra are entirely dominated by the
emission of the white dwarf. Both the far-ultraviolet (FUV) flux as well as the
spectral shape vary substantially over the orbital period, with maximum flux
occurring at the same phase as during the high state. The variations are due to
the presence of a hot spot on the white dwarf, which we model quantitatively.
The white dwarf parameters can be determined from a spectral fit to the faint
phase data, when the hot spot is self-eclipsed. Adopting the distance of
79+8-6pc determined by Thorstensen, we find an effective temperature of
19800+-700K and a mass of Mwd=0.78+0.12-0.17Msun. The hot spot has a lower
temperature than during the high state, ~34000-40000K, but covers a similar
area, ~10% of the white dwarf surface. Low state FUSE and STIS spectra taken
during four different epochs in 2002/3 show no variation of the FUV flux level
or spectral shape, implying that the white dwarf temperature and the hot spot
temperature, size, and location do not depend on the amount of time the system
has spent in the low state. Possible explanations are ongoing accretion at a
low level, or deep heating, both alternatives have some weaknesses that we
discuss. No photospheric metal absorption lines are detected in the FUSE and
STIS spectra, suggesting that the average metal abundances in the white dwarf
atmosphere are lower than 1e-3 times their solar values.Comment: ApJ in press, 12 pages, 11 figure
The Far-Ultraviolet Spectrum and Short Timescale Variability of AM Herculis from Observations with the Hopkins Ultraviolet Telescope
Using the Hopkins Ultraviolet Telescope (HUT), we have obtained 850-1850
angstrom spectra of the magnetic cataclysmic variable star AM Her in the high
state. These observations provide high time resolution spectra of AM Her in the
FUV and sample much of the orbital period of the system. The spectra are not
well-modelled in terms of simple white dwarf (WD) atmospheres, especially at
wavelengths shortward of Lyman alpha. The continuum flux changes by a factor of
2 near the Lyman limit as a function of orbital phase; the peak fluxes are
observed near magnetic phase 0.6 when the accreting pole of the WD is most
clearly visible. The spectrum of the hotspot can be modelled in terms of a 100
000 K WD atmosphere covering 2% of the WD surface. The high time resolution of
the HUT data allows an analysis of the short term variability and shows the UV
luminosity to change by as much as 50% on timescales as short as 10 s. This
rapid variability is shown to be inconsistent with the clumpy accretion model
proposed to account for the soft X-ray excess in polars. We see an increase in
narrow line emission during these flares when the heated face of the secondary
is in view. The He II narrow line flux is partially eclipsed at secondary
conjunction, implying that the inclination of the system is greater than 45
degrees. We also present results from models of the heated face of the
secondary. These models show that reprocessing on the face of the secondary
star of X-ray/EUV emission from the accretion region near the WD can account
for the intensities and kinematics of most of the narrow line components
observed.Comment: 19 pp., 12 fig., 3 tbl. To appear in The Astrophysical Journal. Also
available at http://greeley.pha.jhu.edu/papers/amherpp.ps.g
Keck-Nirspec Infrared OH Lines: Oxygen Abundances in Metal-Poor Stars Down to [Fe/H] = -2.9
Infrared OH lines at 1.5 - 1.7 um in the H band were obtained with the
NIRSPEC high-resolution spectrograph at the 10m Keck Telescope for a sample of
seven metal-poor stars. Detailed analyses have been carried out, based on
optical high-resolution data obtained with the FEROS spectrograph at ESO.
Stellar parameters were derived by adopting infrared flux method effective
temperatures, trigonometric and/or evolutionary gravities and metallicities
from FeII lines. We obtain that the sample stars with metallicities [Fe/H] <
-2.2 show a mean oxygen abundance [O/Fe] ~ 0.54, for a solar oxygen abundance
of epsilon(O) = 8.87, or [O/Fe] ~ 0.64 if epsilon(O) = 8.77 is assumed.Comment: To be published in ApJ 575 (August 10
A Brown Dwarf Companion for the Accreting Millisecond Pulsar SAX J1808.4-3658
BeppoSAX has revealed a population of faint neutron star X-ray transients in
the Galactic bulge. King conjectured that these neutron stars are accreting
from brown dwarfs (BD) with a time-averaged mass transfer rate \approx
10^{-11} M_sun/year. We show that the orbital parameters of the 401 Hz
accreting millisecond pulsar SAX J1808.4-3658 support this hypothesis. A
main-sequence mass donor requires a nearly face-on inclination and a higher
than observed. However, the range of allowed inclinations is
substantially relaxed and the predicted mass transfer rate is consistent with
that observed if a hot 0.05 M_sun BD is the donor. We then explain the BD
radius required (0.13 R_\sun) to fill the Roche lobe. All transiently accreting
neutron stars have a minimum luminosity in quiescence set by the time-averaged
mass transfer rate onto the neutron star. We show that the constant heating of
the BD by this emission can maintain the higher entropy implied by a 0.13
R_\sun radius. Our work bolsters the case that SAX J1808.4-3658 is a progenitor
to compact ms radio pulsar binaries (e.g. like those found in 47 Tuc). The very
low 's implies that the progenitors to these radio pulsars are
long-lived (Gyrs) transient systems, rather than short-lived (Myrs)
Eddington-limited accretors. Hence, the accreting progenitor population to
millisecond radio pulsars in 47 Tuc could still be present and found in
quiescence with Chandra.(abridged)Comment: To appear in Astrophysical Journa
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