17,895 research outputs found
Astrometric Discovery of GJ 802b: In the Brown Dwarf Oasis?
The Stellar Planet Survey is an ongoing astrometric search for giant planets
and brown dwarfs around a sample of about 30 M-dwarfs. We have discovered
several low-mass companions by measuring the motion of our target stars
relative to their reference frames. The lowest mass discovery thus far is GJ
802b, a companion to the M5-dwarf GJ 802A. The orbital period is 3.14 +/- 0.03
y, the system mass is 0.214 +/- 0.045 Msolar, and the semi-major axis is 1.28
+/- 0.10 AU or 81 +/- 6 mas. Imaging observations indicate that GJ 802b is
likely to be a brown dwarf with the astrometrically determined mass 0.058 +/-
0.021 Msolar (one sigma limits). The remaining uncertainty in the orbit is the
eccentricity that is now loosely constrained. We discuss how the system age
limits the mass and the prospects to further narrow the mass range when e is
more precisely determined.Comment: 13 pages, 6 figures, accepted for publication in ApJ on May 9, 200
Kinetic Monte Carlo simulations inspired by epitaxial graphene growth
Graphene, a flat monolayer of carbon atoms packed tightly into a two
dimensional hexagonal lattice, has unusual electronic properties which have
many promising nanoelectronic applications. Recent Low Energy Electron
Microscopy (LEEM) experiments show that the step edge velocity of epitaxially
grown 2D graphene islands on Ru(0001) varies with the fifth power of the
supersaturation of carbon adatoms. This suggests that graphene islands grow by
the addition of clusters of five atoms rather than by the usual mechanism of
single adatom attachment.
We have carried out Kinetic Monte Carlo (KMC) simulations in order to further
investigate the general scenario of epitaxial growth by the attachment of
mobile clusters of atoms. We did not seek to directly replicate the Gr/Ru(0001)
system but instead considered a model involving mobile tetramers of atoms on a
square lattice. Our results show that the energy barrier for tetramer break up
and the number of tetramers that must collide in order to nucleate an immobile
island are the important parameters for determining whether, as in the
Gr/Ru(0001) system, the adatom density at the onset of island nucleation is an
increasing function of temperature. A relatively large energy barrier for
adatom attachment to islands is required in order for our model to produce an
equilibrium adatom density that is a large fraction of the nucleation density.
A large energy barrier for tetramer attachment to islands is also needed for
the island density to dramatically decrease with increasing temperature. We
show that islands grow with a velocity that varies with the fourth power of the
supersaturation of adatoms when tetramer attachment is the dominant process for
island growth
Polarisation-sensitive terahertz detection by multicontact photoconductive receivers
We have developed a terahertz radiation detector that measures both the
amplitude and polarization of the electric field as a function of time. The
device is a three-contact photoconductive receiver designed so that two
orthogonal electric-field components of an arbitrary polarized electromagnetic
wave may be detected simultaneously. The detector was fabricated on Fe+
ion-implanted InP. Polarization-sensitive detection is demonstrated with an
extinction ratio better than 100:1. This type of device will have immediate
application in studies of birefringent and optically active materials in the
far-infrared region of the spectrum.Comment: 3 pages, 3 figure
Validity, reliability, acceptability, and utility of the Social Inclusion Questionnaire User Experience (SInQUE): a clinical tool to facilitate social inclusion amongst people with severe mental health problems.
BACKGROUND: Individuals with severe mental health problems are at risk of social exclusion, which may complicate their recovery. Mental health and social care staff have, until now, had no valid or reliable way of assessing their clients' social inclusion. The Social Inclusion Questionnaire User Experience (SInQUE) was developed to address this. It assesses five domains: social integration; productivity; consumption; access to services; and political engagement, in the year prior to first psychiatric admission (T1) and the year prior to interview (T2) from which a total score at each time point can be calculated. AIMS: To establish the validity, reliability, and acceptability of the SInQUE in individuals with a broad range of psychiatric diagnoses receiving care from community mental health services and its utility for mental health staff. METHOD: Participants were 192 mental health service users with psychosis, personality disorder, or common mental disorder (e.g., depression, anxiety) who completed the SInQUE alongside other validated outcome measures. Test-retest reliability was assessed in a sub-sample of 30 participants and inter-rater reliability was assessed in 11 participants. SInQUE ratings of 28 participants were compared with those of a sibling with no experience of mental illness to account for shared socio-cultural factors. Acceptability and utility of the tool were assessed using completion rates and focus groups with staff. RESULTS: The SInQUE demonstrated acceptable convergent validity. The total score and the Social Integration domain score were strongly correlated with quality of life, both in the full sample and in the three diagnostic groups. Discriminant validity and test-retest reliability were established across all domains, although the test-retest reliability on scores for the Service Access and Political Engagement domains prior to first admission to hospital (T1) was lower than other domains. Inter-rater reliability was excellent for all domains at T1 and T2. CONCLUSIONS: The component of the SInQUE that assesses current social inclusion has good psychometric properties and can be recommended for use by mental health staff
Biology, Ecology and Range of the Bobcat, Lynx Rufus in New York and its Inferred Interactions with Potentially Reintroduced Lynx, Lynx canadensis canadensis in Adirondack Park
1. To determine and describe the predation ecology, movement patterns, territorial behavior and habitat of bobcats in New York. 2. To determine vital population characteristics and exploitation levels of bobcats in New York. 3. To recommend management alternatives for bobcats in New York on the basis of an information synthesis, range map and model of current bobcat exploitation levels in the region. 4. To make recommendations concerning the feasibility of lynx reintroduction in Adirondack Park based on the inferred level of lynx-bobcat competition and a survey of potential lynx range in the Park
Keplerian discs around post-AGB stars: a common phenomenon?
Aims: We aim at showing that the broad-band SED characteristics of our sample
of post-AGB stars are best interpreted, assuming the circumstellar dust is
stored in Keplerian rotating passive discs.
Methods: We present a homogeneous and systematic study of the Spectral Energy
Distributions (SEDs) of a sample of 51 post-AGB objects. The selection criteria
to define the whole sample were tuned to cover the broad-band characteristics
of known binary post-AGB stars. The whole sample includes 20 dusty RV Tauri
stars from the General Catalogue of Variable Stars (GCVS). We supplemented our
own Geneva optical photometry with literature data to cover a broad range of
fluxes from the UV to the far-IR.
Results: All the SEDs display very similar characteristics: a large IR excess
with a dust excess starting near the sublimation temperature, irrespective of
the effective temperature of the central star. Moreover, when available, the
long wavelength fluxes show a black-body slope indicative of the presence of a
component of large mm sized grains.
Conclusions: We argue that in all systems, gravitationally bound dusty discs
are present. The discs must be puffed-up to cover a large opening angle for the
central star and we argue that the discs have some similarity with the passive
discs detected around young stellar objects. We interpret the presence of a
disc to be a signature for binarity of the central object, but this will need
confirmation by long-term monitoring of the radial velocities. We argue that
dusty RV Tauri stars are those binaries which happen to be in the Population II
instability strip.Comment: 29 pages, 5 figures, accepted for publication in A&
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Terahertz time-domain spectroscopy and scanning probe potentiometry were used
to investigate charge trapping in polymer field-effect transistors fabricated
on a silicon gate. The hole density in the transistor channel was determined
from the reduction in the transmitted terahertz radiation under an applied gate
voltage. Prolonged device operation creates an exponential decay in the
differential terahertz transmission, compatible with an increase in the density
of trapped holes in the polymer channel. Taken in combination with scanning
probe potentionmetry measurements, these results indicate that device
degradation is largely a consequence of hole trapping, rather than of changes
to the mobility of free holes in the polymer.Comment: 4 pages, 3 figure
Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs
The carrier dynamics of photoexcited electrons in the vicinity of the surface
of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission
spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy
measurements, coupled with Monte Carlo simulations of THz emission, revealed
that the surface electric field of GaAs reverses after passivation. The
conductivity of photoexcited electrons was determined via optical-pump
THz-probe spectroscopy, and was found to double after passivation. These
experiments demonstrate that passivation significantly reduces the surface
state density and surface recombination velocity of GaAs. Finally, we have
demonstrated that passivation leads to an enhancement in the power radiated by
photoconductive switch THz emitters, thereby showing the important influence of
surface chemistry on the performance of ultrafast THz photonic devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Cost/benefit analysis for the Operational Applications of Satellite Snowcover Observations (OASSO)
The author has identified the following significant results. The total cost associated with satellite snow cover area measurement (SATSCAM) in the Colorado ASVT was 493k. The estimated total benefits to hydroeletric energy production is 38m, with the Lower Colorado region receiving the largest per acre benefit and the Pacific Northwest receiving the lowest
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