1,311 research outputs found
Where Are the Magnetic White Dwarfs with Detached, Nondegenerate Companions?
The Sloan Digital Sky Survey has already more than doubled the sample of white dwarfs with spectral classifications, the subset with detached M dwarf companions, and the subset of magnetic white dwarfs. In the course of assessing these new discoveries, we have noticed a curious, unexpected property of the total lists of magnetic white dwarfs and of white dwarf plus main-sequence binaries: there appears to be virtually zero overlap between the two samples! No confirmed magnetic white dwarf has yet been found in such a pairing with a main-sequence star. The same statement can be made for the samples of white dwarf–M dwarf pairs in wide, common proper motion systems. This contrasts with the situation for interacting binaries, in which an estimated 25% of the accreting systems have a magnetic white dwarf primary. Alternative explanations are discussed for the observed absence of magnetic white dwarf–main-sequence pairs, but the recent discoveries of very low accretion rate magnetic binaries pose difficulties for each. A plausible explanation may be that the presence of the companion and the likely large mass and small radius of the magnetic white dwarf (relative to nonmagnetic degenerate dwarfs) may provide a selection effect against the discovery of the latter in such binary systems. More careful analysis of the existing samples may yet uncover members of this class of binary, and the sample sizes will continue to grow. The question of whether the mass and field distributions of the magnetic primaries in interacting binaries are similar to those of the isolated magnetic white dwarfs (including those in wider binaries) must also be answered
Mid-infrared imaging of the massive young star AFGL 2591: Probing the circumstellar environment of an outflow source
Most, if not all, stars are now believed to produce energetic outflows during
their formation. Yet, almost 20 years after the discovery of bipolar outflows
from young stars, the origins of this violent phenomenon are not well
understood. One of the difficulties of probing the outflow process,
particularly in the case of massive embedded stars, is a deficit of high
spatial resolution observations. Here, we present sub-arcsecond-resolution
mid-infrared images of one massive young stellar object, AFGL 2591, and its
immediate surroundings. Our images, at 11.7, 12.5 and 18.0 microns, reveal a
knot of emission ~6'' SW of the star, which may be evidence for a recent
ejection event or an embedded companion star. This knot is roughly coincident
with a previously seen near-infrared reflection nebula and a radio source, and
lies within the known large-scale CO outflow. We also find a new faint NW
source which may be another embedded lower-luminosity star. The IRAS
mid-infrared spectrum of AFGL 2591 shows a large silicate absorption feature at
10 microns, implying that the primary source is surrounded by an optically
thick dusty envelope. We discuss the interrelationship of these phenomena and
suggest that mid-infrared imaging and spectroscopy provide powerful tools for
probing massive star birth.Comment: 14 pages, 3 PostScript figures, accepted for publication in The
Astrophysical Journal Letter
Dust temperature and density profiles of AGB and post-AGB stars from mid-infrared observations
First mid-infrared images of a sample of AGB and post-AGB carbon stars (V
Hya, IRC +10216, CIT 6 and Roberts 22) obtained at La Silla Observatory (ESO,
Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the
9.7m image, Roberts 22 shows an envelope slightly elongated in the
north-east direction while images of V Hya and IRC+10216 are roughly
spherically symmetric. Using inversion technique, the dust emissivity was
derived from the observed intensity profiles, allowing a determination of the
grain temperature and density distributions inside the envelope for these
stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216.
Our results are comparable to those obtained in previous studies if dust grains
have dimensions in the range 0.01 - 0.2 m. Color maps suggest the
presence of temperature inhomogeneities in the central regions of the dust
envelopes. In the case of V Hya, an eccentric hot point, which direction
coincides with the jet previously seen in [SII] emission, suggest that we are
observing a material ejected in a previous mass-loss event. Bipolar lobes are
clearly seen in the color maps of Roberts 22 and IRC +10216.Comment: accepted for publication in Astronomy and Astrophysic
Performance Limitations of Flat Histogram Methods and Optimality of Wang-Landau Sampling
We determine the optimal scaling of local-update flat-histogram methods with
system size by using a perfect flat-histogram scheme based on the exact density
of states of 2D Ising models.The typical tunneling time needed to sample the
entire bandwidth does not scale with the number of spins N as the minimal N^2
of an unbiased random walk in energy space. While the scaling is power law for
the ferromagnetic and fully frustrated Ising model, for the +/- J
nearest-neighbor spin glass the distribution of tunneling times is governed by
a fat-tailed Frechet extremal value distribution that obeys exponential
scaling. We find that the Wang-Landau algorithm shows the same scaling as the
perfect scheme and is thus optimal.Comment: 5 pages, 6 figure
Collective Charge Excitation in a Dimer Mott Insulating System
Charge dynamics in a dimer Mott insulating system, where a non-polar
dimer-Mott (DM) phase and a polar charge-ordered (CO) phase compete with each
other, are studied. In particular, collective charge excitations are analyzed
in the three different models where the internal-degree of freedom in a dimer
is taken into account. Collective charge excitation exists both in the
non-polar DM phase and the polar CO phase, and softens in the phase boundary.
This mode is observable by the optical conductivity spectra where the light
polarization is parallel to the electric polarization in the polar CO phase.
Connections between the present theory and the recent experimental results in
kappa-(BEDT-TTF)2Cu2(CN)3 are discussed.Comment: 5 pages, 4 figure
A tachyonic scalar field with mutually interacting components
We investigate the tachyonic cosmological potential in two
different cases of the quasi-exponential expansion of universe and discuss
various forms of interaction between the two components---matter and the
cosmological constant--- of the tachyonic scalar field, which leads to the
viable solutions of their respective energy densities. The distinction among
the interaction forms is shown to appear in the diagnostic. Further,
the role of the high- and low-redshift observations of the Hubble parameter is
discussed to determine the proportionality constants and hence the correct form
of matter--cosmological constant interaction.Comment: 14 page
Effect of dietary supplementation of cassia auriculata leaf powder on growth and immune responses of milkfish, Chanos chanos
A 30 days feeding trial was conducted to evaluate the effects of Cassia auriculata leaf powder on growth and
immunomological parameters of the juveniles of Milkfish, Chanos chanos and disease resistance against Vibrio
anguillarum. The juvenile fishes with an average weight of 20±3 g were stocked in four treatments each with
three replicates. Four practical diets were prepared with graded level of Cassia auriculata leaf powder (CAL) at the
rate of 0%, 0.5%, 1% and 1.5% and fed to four treatments respectively named as CAL0, CAL0.5, CAL1 and
CAL1.5. At the end of the experiment blood samples were collected for immunological parameters. After
sampling, the remaining fishes were challenged with 0.1 mL of virulent V. anguillarum suspension at a concentration
of 107
CFU mL-1 and mortality was observed for 7 days. Post challenge sampling was performed for the collection
of blood samples. The growth parameters such as Weight gain%, SGR%, PER were positively affected by Cassia
auriculata leaf powder incorporated diet. Higher weight gain %, SGR% and PER were noticed in CAL1.5 group
followed by CAL1, CAL0.5 and CAL0. The respiratory burst, lysozyme and phagocytic activities were increased
with increasing level of CAL in the diet which indicates better immune response of the fishes fed with CAL. The
higher respiratory burst (0.522 ± 0.03), lysozyme (70.07 ± 1.57) and phagocytic (52.91 ± 1.76) activitieswere
observed in CAL1.5, CAL1 and CAL1.5 groups respectively during pre-challenge and post challenge. There was
significant difference in the survival rate of control (CAL0) and CAL incorporated diet fed groups (P<0.05).
Maximum survival was witnessed in CAL1 followed by CAL1.5 and least survival was in CAL0 group. The results
indicate that diets supplemented with 1% CAL potentially enhance the immune system and effectively protects the
host against V. anguillarum infection and thereby improve the survivability of the fish against this dreadful
pathogen
A Mid-Infrared Imaging Survey of Proto-Planetary Nebula Candidates
We present the data from a mid-infrared imaging survey of 66 proto-planetary
nebula candidates using two mid-IR cameras (MIRAC2 and Berkcam) at the NASA
Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal
of this survey is to determine the size, flux, and morphology of the mid-IR
emission regions, which sample the inner regions of the circumstellar dust
shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with
narrow-band filters () at wavelengths of
notable dust features. With our typical angular resolution of 1\arcsec, we
resolve 17 sources, find 48 objects unresolved, and do not detect 1 source. For
several sources, we checked optical and infrared associations and positions of
the sources. In table format, we list the size and flux measurements for all
the detected objects and show figures of all the resolved sources. Images for
all the detected objects are available on line in FITS format from the
Astronomy Digital Image Library at the National Center for Supercomputing
Application. The proto-planetary nebula candidate sample includes, in addition
to the predominant proto-planetary nebulae, extreme asymptotic giant branch
stars, young planetary nebulae, a supergiant, and a luminous blue variable. We
find that dust shells which are cooler ( K) and brighter in the
infrared are more easily resolved. Eleven of the seventeen resolved sources are
extended and fall into one of two types of mid-IR morphological classes:
core/elliptical or toroidal. Core/elliptical structures show unresolved cores
with lower surface brightness elliptical nebulae. Toroidal structures show
limb-brightened peaks suggesting equatorial density enhancements. We argue that
core/ellipticals have denser dust shells than toroidals.Comment: 32 pages, 5 tables, 2 e/ps figures (fig3 is available through ADIL
[see text]), to be published in ApJS May 1999 issu
Pedunculopontine Nucleus Deep Brain Stimulation for Parkinsonian Disorders: A Case Series
BACKGROUND: Deep brain stimulation (DBS) of the pedunculopontine nucleus (PPN) has been investigated for the treatment of levodopa-refractory gait dysfunction in parkinsonian disorders, with equivocal results so far. OBJECTIVES: To summarize the clinical outcomes of PPN-DBS-treated patients at our centre and elicit any patterns that may guide future research. MATERIALS AND METHODS: Pre- and post-operative objective overall motor and gait subsection scores as well as patient-reported outcomes were recorded for 6 PPN-DBS-treated patients, 3 with Parkinson’s disease (PD), and 3 with progressive supranuclear palsy (PSP). Electrodes were implanted unilaterally in the first 3 patients and bilaterally in the latter 3, using an MRI-guided MRI-verified technique. Stimulation was initiated at 20–30 Hz and optimized in an iterative manner. RESULTS: Unilaterally treated patients did not demonstrate significant improvements in gait questionnaires, UPDRS-III or PSPRS scores or their respective gait subsections. This contrasted with at least an initial response in bilaterally treated patients. Diurnal cycling of stimulation in a PD patient with habituation to the initial benefit reproduced substantial improvements in freezing of gait (FOG) 3 years post-operatively. Among the PSP patients, 1 with a parkinsonian subtype had a sustained improvement in FOG while another with Richardson syndrome (PSP-RS) did not benefit. CONCLUSIONS: PPN-DBS remains an investigational treatment for levodopa-refractory FOG. This series corroborates some previously reported findings: bilateral stimulation may be more effective than unilateral stimulation; the response in PSP patients may depend on the disease subtype; and diurnal cycling of stimulation to overcome habituation merits further investigation
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