6,892 research outputs found
BFKL Physics in Jet Production at e+e- colliders
Virtual photon scattering in collisions can result in events with
the electron-positron pair at large rapidity separation with hadronic activity
in between. The BFKL equation resums large logarithms that dominate the cross
section for this process. We report here on a Monte Carlo method for solving
the BFKL equation that allows kinematic constraints to be taken into account
and show results for collisions.Comment: 3 pages, Latex, talk presented at the 2000 Meeting of the Division of
Particles and Fields of the APS, Columbus, OH, August 9--12, 200
An Optical Spectroscopic Study of T Tauri Stars. I. Photospheric Properties
Measurements of masses and ages of young stars from their location in the HR
diagram are limited by not only the typical observational uncertainties that
apply to field stars, but also by large systematic uncertainties related to
circumstellar phenomena. In this paper, we analyze flux calibrated optical
spectra to measure accurate spectral types and extinctions of 283 nearby T
Tauri stars. The primary advances in this paper are (1) the incorporation of a
simplistic accretion continuum in optical spectral type and extinction
measurements calculated over the full optical wavelength range and (2) the
uniform analysis of a large sample of stars. Comparisons between the
non-accreting TTS photospheric templates and stellar photosphere models are
used to derive conversions from spectral type to temperature. Differences
between spectral types can be subtle and difficult to discern, especially when
accounting for accretion and extinction. The spectral types measured here are
mostly consistent with spectral types measured over the past decade. However,
our new spectral types are 1-2 subclasses later than literature spectral types
for the original members of the TWA and are discrepant with literature values
for some well known Taurus CTTSs. Our extinction measurements are consistent
with other optical extinction measurements but are typically 1 mag lower than
nIR measurements, likely the result of methodological differences and the
presence of nIR excesses in most CTTSs. As an illustration of the impact of
accretion, SpT, and extinction uncertainties on the HR diagrams of young
clusters, we find that the resulting luminosity spread of stars in the TWA is
15-30%. The luminosity spread in the TWA and previously measured for binary
stars in Taurus suggests that for a majority of stars, protostellar accretion
rates are not large enough to significantly alter the subsequent evolution.Comment: Accepted by ApJ. 30 pages plus 12 pages of Tables and Reference
Empirical Isochrones for Low Mass Stars in Nearby Young Associations
Absolute ages of young stars are important for many issues in pre-main
sequence stellar and circumstellar evolution but are long recognized as
difficult to derive and calibrate. In this paper, we use literature spectral
types and photometry to construct empirical isochrones in HR diagrams for
low-mass stars and brown dwarfs in the eta Cha, epsilon Cha, and TW Hya
Associations and the beta Pic and Tuc-Hor Moving Groups. A successful theory of
pre-main sequence evolution should match the shapes of the stellar loci for
these groups of young stars. However, when comparing the combined empirical
isochrones to isochrones predicted from evolutionary models, discrepancies lead
to a spectral type (mass) dependence in stellar age estimates. Improved
prescriptions for convection and boundary conditions in the latest models of
pre-main sequence models lead to a significantly improved correspondence
between empirical and model isochrones, with small offsets at low temperatures
that may be explained by observational uncertainties or by model limitations.
Independent of model predictions, linear fits to combined stellar loci of these
regions provide a simple empirical method to order clusters by luminosity with
a reduced dependence on spectral type. Age estimates calculated from various
sets of modern models that reproduce Li depletion boundary ages of the beta Pic
Moving Group also imply a ~4 Myr age for the low mass members of the Upper Sco
OB Association, which is younger than the 11 Myr age that has been recently
estimated for intermediate mass members.Comment: Accepted by ApJ, 18 page
Progress Towards a Generator for BFKL Physics
In certain regions of phase space in jet production, large logarithms can
arise which are resummed by the BFKL equation. Linear colliders can potentially
be excellent places to study BFKL effects in jet production. We discuss an
approach to BFKL calculations which incorporates kinematic effects explicitly
and can be implemented in an event generator.Comment: 4 pages including 2 figures, Latex, uses epsfig, aipproc.sty; talk
presented at Linear Collider Workshop 2000, Fermilab, Batavia, IL, Oct.
24--28, 200
Good places for ageing in place : development of objective built environment measures for investigating links with older people's wellbeing
Background: There is renewed interest in the role of the built environment in public health. Relatively little
research to date investigates its impact on healthy ageing. Ageing in place has been adopted as a key strategy for
coping with the challenges of longevity. What is needed is a better understanding of how individual characteristics
of older peopleâs residential environments (from front door to wider neighbourhood) contribute to their wellbeing,
in order to provide the basis for evidence-based housing/urban design and development of interventions. This
research aimed to develop a tool to objectively measure a large range of built environment characteristics, as the
basis for a preliminary study of potential relationships with a number of âplace-relatedâ functional, emotional and
social wellbeing constructs.
Methods: Through a review of urban design literature, design documents, and existing measures, a new tool, the
NeDeCC (Neighbourhood Design Characteristics Checklist) was developed. It was piloted, refined, and its reliability
validated through inter-rater tests. A range of place-related wellbeing constructs were identified and measured
through interviews with 200 older people living in a wide variety of rural-urban environments and different types
of housing in England. The NeDeCC was used to measure the residential environment of each participant, and
significant bivariate relationships with wellbeing variables were identified.
Results: The NeDeCC was found to have convincing face and construct validity and good inter-rater and test/
retest reliability, though it would benefit from use of digital data sources such as Google Earth to eliminate the
need for on-site survey. The significant relationships found in the study suggest that there may be characteristics
of residential environments of potential relevance for older peopleâs lives that have been overlooked in research to
date, and that it may be worthwhile to question some of the assumptions about where and how older people
want to live (e.g. villages seem to be positive). They also point to the importance of considering non-linear
relationships.
Conclusions: The NeDeCC provides the basis for generation of evidence-based design guidance if it is used in
prospective controlled studies or ânatural experimentsâ in the future. Ultimately, this will facilitate the creation of
better places for ageing in place
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