9,853 research outputs found
The Las Campanas Infra-red Survey. V. Keck Spectroscopy of a large sample of Extremely Red Objects
(Abridged) We present deep Keck spectroscopy, using the DEIMOS and LRIS
spectrographs, of a large and representative sample of 67 ``Extremely Red
Objects'' (EROs) to H=20.5, with I-H>3.0, in three of the Las Campanas Infrared
Survey fields. Spectroscopic redshifts are determined for 44 sources, of which
only two are contaminating low mass stars. When allowance is made for
incompleteness, the spectroscopic redshift distribution closely matches that
predicted earlier on the basis of photometric data. Our spectra are of
sufficient quality that we can address the important question of the nature and
homogeneity of the z>0.8 ERO population. A dominant old stellar population is
inferred for 75% of our spectroscopic sample; a higher fraction than that seen
in smaller, less-complete samples with broader photometric selection criteria
(e.g. R-K). However, only 28% have spectra with no evidence of recent star
formation activity, such as would be expected for a strictly passively-evolving
population. More than ~30% of our absorption line spectra are of the `E+A' type
with prominent Balmer absorption consistent, on average, with mass growth of
5-15% in the past Gyr. We use our spectroscopic redshifts to improve earlier
estimates of the spatial clustering of this population as well as to understand
the significant field-to-field variation. Our spectroscopy enables us to
pinpoint a filamentary structure at z=1.22 in the Chandra Deep Field South.
Overall, our study suggests that the bulk of the ERO population is an
established population of clustered massive galaxies undergoing intermittent
activity consistent with continued growth over the redshift interval 0.8<z<1.6.Comment: 27 pages, including 14 figures and appendix of spectra (at low
resolution). Full resolution paper can be found at
http://www.ast.cam.ac.uk/~md . To appear in MNRA
Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model
The impact of convection on tropospheric O<sub>3</sub> and its precursors has been examined in a coupled chemistry-climate model. There are two ways that convection affects O<sub>3</sub>. First, convection affects O<sub>3</sub> by vertical mixing of O<sub>3</sub> itself. Convection lifts lower tropospheric air to regions where the O<sub>3</sub> lifetime is longer, whilst mass-balance subsidence mixes O<sub>3</sub>-rich upper tropospheric (UT) air downwards to regions where the O<sub>3</sub> lifetime is shorter. This tends to decrease UT O<sub>3</sub> and the overall tropospheric column of O<sub>3</sub>. Secondly, convection affects O<sub>3</sub> by vertical mixing of O<sub>3</sub> precursors. This affects O<sub>3</sub> chemical production and destruction. Convection transports isoprene and its degradation products to the UT where they interact with lightning NO<sub>x</sub> to produce PAN, at the expense of NO<sub>x</sub>. In our model, we find that convection reduces UT NO<sub>x</sub> through this mechanism; convective down-mixing also flattens our imposed profile of lightning emissions, further reducing UT NO<sub>x</sub>. Over tropical land, which has large lightning NO<sub>x</sub> emissions in the UT, we find convective lofting of NO<sub>x</sub> from surface sources appears relatively unimportant. Despite UT NO<sub>x</sub> decreases, UT O<sub>3</sub> production increases as a result of UT HO<sub>x</sub> increases driven by isoprene oxidation chemistry. However, UT O<sub>3</sub> tends to decrease, as the effect of convective overturning of O<sub>3</sub> itself dominates over changes in O<sub>3</sub> chemistry. Convective transport also reduces UT O<sub>3</sub> in the mid-latitudes resulting in a 13% decrease in the global tropospheric O<sub>3</sub> burden. These results contrast with an earlier study that uses a model of similar chemical complexity. Differences in convection schemes as well as chemistry schemes – in particular isoprene-driven changes are the most likely causes of such discrepancies. Further modelling studies are needed to constrain this uncertainty range
NHEJ protects mycobacteria in stationary phase against the harmful effects of desiccation
The physiological role of the non-homologous end-joining (NHEJ) pathway in the repair of DNA double-strand breaks (DSBs) was examined in Mycobacterium smegmatis using DNA repair mutants (DeltarecA, Deltaku, DeltaligD, Deltaku/ligD, DeltarecA/ku/ligD). Wild-type and mutant strains were exposed to a range of doses of ionizing radiation at specific points in their life-cycle. NHEJ-mutant strains (Deltaku, DeltaligD, Deltaku/ligD) were significantly more sensitive to ionizing radiation (IR) during stationary phase than wild-type M. smegmatis. However, there was little difference in IR sensitivity between NHEJ-mutant and wild-type strains in logarithmic phase. Similarly, NHEJ-mutant strains were more sensitive to prolonged desiccation than wild-type M. smegmatis. A DeltarecA mutant strain was more sensitive to desiccation and IR during both stationary and especially in logarithmic phase, compared to wild-type strain, but it was significantly less sensitive to IR than the DeltarecA/ku/ligD triple mutant during stationary phase. These data suggest that NHEJ and homologous recombination are the preferred DSB repair pathways employed by M. smegmatis during stationary and logarithmic phases, respectively
Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements
The production of the elements heavier than iron via slow neutron captures
(the s process) is a main feature of the contribution of asymptotic giant
branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos.
However, our understanding of the main neutron source, the 13C(alpha,n)16O
reaction, is still incomplete. It is commonly assumed that in AGB stars mixing
beyond convective borders drives the formation of 13C pockets. However, there
is no agreement on the nature of such mixing and free parameters are present.
By means of a parametric model we investigate the impact of different mixing
functions on the final s-process abundances in low-mass AGB models. Typically,
changing the shape of the mixing function or the mass extent of the region
affected by the mixing produce the same results. Variations in the relative
abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are
generally within +/-0.2 dex, similar to the observational error bars. We
conclude that other stellar uncertainties - the effect of rotation and of
overshoot into the C-O core - play a more important role than the details of
the mixing function. The exception is at low metallicity, where the Pb
abundance is significantly affected. In relation to the composition observed in
stardust SiC grains from AGB stars, the models are relatively close to the data
only when assuming the most extreme variation in the mixing profile.Comment: 17 pages, 8 figures, 6 tables, accepted for publications on Monthly
Notices of the Royal Astronomical Societ
From valuing design to designing value
This paper presents findings of an embedded action research project within a small to medium sized enterprise (SME). Through the implementation of design-led innovation processes, this research aims to identify the changes experienced in the participating company during a shift in the perspective of design from a product focus towards a strategic focus. Staff interviews and a reflective journal were used as methods to collect data from a range of design interventions that were facilitated throughout the engagement. A shift in perspective of design was evident through three cultural changes within the firm. First, the perceived outcome focus of design became increasingly long-term. Second, the value of design outcomes became less directed towards current projects, and more directed towards future possibilities. Finally, the perceived tangibility of design outcomes shifted from tangible to intangible. For example, design activities which produced customer insights, rather than product features, became seen as beneficial to the firm. These three components are proposed as cultural stepping stones which describe how a company transitions from an exclusively product-focused perspective and utilisation of design towards design as a company based process. Implications of this research provide considerations for designers who are attempting to facilitate a similar transformation within a business in the future
Arkansas Cotton Variety Test 2004
The primary aim of the Arkansas Cotton Variety Test is to provide unbiased data regarding the agronomic performance of cotton varieties and advanced breeding lines in the major cotton-growing areas of Arkansas. This information helps seed dealers establish marketing strategies and assists producers in choosing varieties to plant
Semiclassical theory of cavity-assisted atom cooling
We present a systematic semiclassical model for the simulation of the
dynamics of a single two-level atom strongly coupled to a driven high-finesse
optical cavity. From the Fokker-Planck equation of the combined atom-field
Wigner function we derive stochastic differential equations for the atomic
motion and the cavity field. The corresponding noise sources exhibit strong
correlations between the atomic momentum fluctuations and the noise in the
phase quadrature of the cavity field. The model provides an effective tool to
investigate localisation effects as well as cooling and trapping times. In
addition, we can continuously study the transition from a few photon quantum
field to the classical limit of a large coherent field amplitude.Comment: 10 pages, 8 figure
Assignment of the NV0 575 nm zero-phonon line in diamond to a 2E-2A2 transition
The time-averaged emission spectrum of single nitrogen-vacancy defects in
diamond gives zero-phonon lines of both the negative charge state at 637 nm
(1.945 eV) and the neutral charge state at 575 nm (2.156 eV). This occurs
through photo-conversion between the two charge states. Due to strain in the
diamond the zero-phonon lines are split and it is found that the splitting and
polarization of the two zero-phonon lines are the same. From this observation
and consideration of the electronic structure of the nitrogen-vacancy center it
is concluded that the excited state of the neutral center has A2 orbital
symmetry. The assignment of the 575 nm transition to a 2E - 2A2 transition has
not been established previously.Comment: 5 pages, 5 figure
On the suppression of the diffusion and the quantum nature of a cavity mode. Optical bistability; forces and friction in driven cavities
A new analytical method is presented here, offering a physical view of driven
cavities where the external field cannot be neglected. We introduce a new
dimensionless complex parameter, intrinsically linked to the cooperativity
parameter of optical bistability, and analogous to the scaled Rabbi frequency
for driven systems where the field is classical. Classes of steady states are
iteratively constructed and expressions for the diffusion and friction
coefficients at lowest order also derived. They have in most cases the same
mathematical form as their free-space analog. The method offers a semiclassical
explanation for two recent experiments of one atom trapping in a high Q cavity
where the excited state is significantly saturated. Our results refute both
claims of atom trapping by a quantized cavity mode, single or not. Finally, it
is argued that the parameter newly constructed, as well as the groundwork of
this method, are at least companions of the cooperativity parameter and its
mother theory. In particular, we lay the stress on the apparently more
fundamental role of our structure parameter.Comment: 24 pages, 7 figures. Submitted to J. Phys. B: At. Mol. Opt. Phy
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