1,567 research outputs found
Evidence for a Single Naphthylphthalamic Acid Binding Site on the Zucchini Plasma Membrane
Photosynthesis Under a Red Sun: Predicting the absorption characteristics of an extraterrestrial light-harvesting antenna
Here we discuss the feasibility of photosynthesis on Earth-like rocky planets in close orbit around ultra-cool red dwarf stars. Stars of this type have very limited emission in the \textit{photosynthetically active} region of the spectrum (400−700 nm), suggesting that they may not be able to support oxygenic photosynthesis. However, photoautotrophs on Earth frequently exploit very dim environments with the aid of highly structured and extremely efficient antenna systems. Moreover, the anoxygenic photosynthetic bacteria, which do not need to oxidize water to source electrons, can exploit far red and near infrared light. Here we apply a simple model of a photosynthetic antenna to a range of model stellar spectra, ranging from ultra-cool (2300 K) to Sun-like (5800 K). We assume that a photosynthetic organism will evolve an antenna that maximizes the rate of energy input while also minimizing fluctuations. The latter is the 'noise cancelling' principle recently reported by Arp et al. 2020. Applied to the Solar spectrum this predicts optimal antenna configurations in agreement with the chlorophyll Soret absorption bands. Applied to cooler stars, the optimal antenna peaks become redder with decreasing stellar temperature, crossing to the typical wavelength ranges associated with anoxygenic photoautotrophs at ∼3300 K. Lastly, we compare the relative input power delivered by antennae of equivalent size around different stars and find that the predicted variation is within the same order of magnitude. We conclude that low-mass stars do not automatically present light-limiting conditions for photosynthesis but they may select for anoxygenic organisms
Protoplanetary disc truncation mechanisms in stellar clusters: Comparing external photoevaporation and tidal encounters
Most stars form and spend their early life in regions of enhanced stellar
density. Therefore the evolution of protoplanetary discs (PPDs) hosted by such
stars are subject to the influence of other members of the cluster. Physically,
PPDs might be truncated either by photoevaporation due to ultraviolet flux from
massive stars, or tidal truncation due to close stellar encounters. Here we aim
to compare the two effects in real cluster environments. In this vein we first
review the properties of well studied stellar clusters with a focus on stellar
number density, which largely dictates the degree of tidal truncation, and far
ultraviolet (FUV) flux, which is indicative of the rate of external
photoevaporation. We then review the theoretical PPD truncation radius due to
an arbitrary encounter, additionally taking into account the role of eccentric
encounters that play a role in hot clusters with a 1D velocity dispersion
km/s. Our treatment is then applied statistically to varying
local environments to establish a canonical threshold for the local stellar
density ( pc) for which encounters can play a significant
role in shaping the distribution of PPD radii over a timescale Myr. By
combining theoretical mass loss rates due to FUV flux with viscous spreading in
a PPD we establish a similar threshold for which a massive disc is completely
destroyed by external photoevaporation. Comparing these thresholds in local
clusters we find that if either mechanism has a significant impact on the PPD
population then photoevaporation is always the dominating influence.ERC Advanced Grant grant agreement 34113
Flavone acetic acid (FAA) with recombinant interleukin-2 (rIL-2) in advanced malignant melanoma. III: Cytokine studies.
Twelve patients undergoing IL-2 and flavone acetic acid (FAA) combination immunotherapy for advanced melanoma were studied throughout treatment for the induction of measurable levels of bioactive TNF, GM-CSF and IL-6 in their serum. This was to assess the extent of secondary cytokine induction in these patients and the possible role of such cytokines in both the toxic and therapeutic responses. The nature of the treatment schedule enabled these cytokines to be measured in response to FAA alone, FAA/IL-2 and FAA alone following IL-2/FAA activation of target cells. A small rise in the serum levels of these cytokines was seen on the initial course of FAA/IL-2 but this was minor compared to the marked elevation in levels 2-8 h following the initiation of the third course of FAA given with or without IL-2 and at a time point which coincided with maximum toxicity in those patients who experienced it. These results show that FAA alone can induce cytokine release from primed target cells. This may be associated with the therapeutic effect and/or toxicity of the agent
Sound Synthesis with Auditory Distortion Products
This article describes methods of sound synthesis based on auditory distortion products, often called combination tones. In 1856, Helmholtz was the first to identify sum and difference tones as products of auditory distortion. Today this phenomenon is well studied in the context of otoacoustic emissions, and the “distortion” is understood as a product of what is termed the cochlear amplifier. These tones have had a rich history in the music of improvisers and drone artists. Until now, the use of distortion tones in technological music has largely been rudimentary and dependent on very high amplitudes in order for the distortion products to be heard by audiences. Discussed here are synthesis methods to render these tones more easily audible and lend them the dynamic properties of traditional acoustic sound, thus making auditory distortion a practical domain for sound synthesis. An adaptation of single-sideband synthesis is particularly effective for capturing the dynamic properties of audio inputs in real time. Also presented is an analytic solution for matching up to four harmonics of a target spectrum. Most interestingly, the spatial imagery produced by these techniques is very distinctive, and over loudspeakers the normal assumptions of spatial hearing do not apply. Audio examples are provided that illustrate the discussion
Fluctuations in the Irreversible Decay of Turbulent Energy
A fluctuation law of the energy in freely-decaying, homogeneous and isotropic
turbulence is derived within standard closure hypotheses for 3D incompressible
flow. In particular, a fluctuation-dissipation relation is derived which
relates the strength of a stochastic backscatter term in the energy decay
equation to the mean of the energy dissipation rate. The theory is based on the
so-called ``effective action'' of the energy history and illustrates a
Rayleigh-Ritz method recently developed to evaluate the effective action
approximately within probability density-function (PDF) closures. These
effective actions generalize the Onsager-Machlup action of nonequilibrium
statistical mechanics to turbulent flow. They yield detailed, concrete
predictions for fluctuations, such as multi-time correlation functions of
arbitrary order, which cannot be obtained by direct PDF methods. They also
characterize the mean histories by a variational principle.Comment: 26 pages, Latex Version 2.09, plus seceq.sty, a stylefile for
sequential numbering of equations by section. This version includes new
discussion of the physical interpretation of the formal Rayleigh-Ritz
approximation. The title is also change
Turbulence Fluctuations and New Universal Realizability Conditions in Modelling
General turbulent mean statistics are shown to be characterized by a
variational principle. The variational functionals, or ``effective actions'',
have experimental consequences for turbulence fluctuations and are subject to
realizability conditions of positivity and convexity. An efficient
Rayleigh-Ritz algorithm is available to calculate approximate effective actions
within PDF closures. Examples are given for Navier-Stokes and for a 3-mode
system of Lorenz. The new realizability conditions succeed at detecting {\em a
priori} the poor predictions of PDF closures even when the classical 2nd-order
moment realizability conditions are satisfied.Comment: 4 pages, LaTeX (Version 2.09), 3 figures, Postscript, Submitted to
Phys. Rev. Let
Recommended from our members
Primality-testing Mersenne numbers
Report of a systematic review of Mersenne numbers 2^p-1 for p < 62982
Rendering an Account: An Open-State Archive in Postgraduate Supervision
The paper begins with a brief account of the transformation of research degree studies under the pressures of global capitalism and neo-liberal governmentality. A parallel transformation is occurring in the conduct of research through the use of information and communication technologies. Yet the potential of ICTs to shape practices of surveillance or to produce new student-supervisor relations and enhance the processes of developing the dissertation has received almost no critical attention. As doctoral supervisor and student, we then describe the features and uses of a web-based open state archive of the student's work-in-progress, developed by the student and accessible to his supervisor. Our intention was to encourage more open conversations between data and theorising, student and supervisor, and ultimately between the student and professional community. However, we recognise that relations of accountability, as these have developed within a contemporary "audit revolution" (Power, 1994, 1997) in universities, create particular "lines of visibility" (Munro, 1996). Thus while the open-state archive may help to redefine in less managerial terms notions of quality, transparency, flexibility and accountability, it might also make possible greater supervisory surveillance. How should we think about the panoptical potential of this archive? We argue that the diverse kinds of interactional patterns and pedagogical intervention it encourages help to create shifting subjectivities. Moreover, the archive itself is multiple, in bringing together an array of diverse materials that can be read in various ways, by following multiple paths. It therefore constitutes a collage, which we identify as a mode of cognition and of accounting distinct from but related to argument and narrative. As a more "open" text (Iser, 1978) it has an indeterminacy which may render it less open to abuse for the technologies of managerial accountability
Testing external photoevaporation in the -Orionis cluster with spectroscopy and disk mass measurements
The evolution of protoplanetary disks is regulated by an interplay of several
processes, either internal to the system or related to the environment. As most
of the stars and planets have formed in massive stellar clusters, studying the
effects of UV radiation on disk evolution is of paramount importance. Here we
test the impact of external photoevaporation on the evolution of disks in the
Orionis cluster by conducting the first combined large-scale UV to IR
spectroscopic and mm-continuum survey of this region. We study a sample of 50
targets located at increasing distances from the central, OB system
Ori. We combine new VLT/X-Shooter spectra with new and previously published
ALMA measurements of disk dust and gas fluxes and masses. We confirm the
previously found decrease of in the inner 0.5 pc of the
cluster. This is particularly evident when considering the disks around the
more massive stars ( 0.4 ), where those located in the inner
part ( 0.5 pc) have about an order of magnitude lower than
the more distant ones. About half of the sample is located in the region of the
vs expected by models of external
photoevaporation, namely showing shorter disk lifetimes. These are observed for
all targets with projected separation from Ori 0.5 pc, proving
that the presence of a massive stellar system affects disk evolution. External
photoevaporation is a viable mechanism to explain the observed shorter disk
lifetimes and lower in the inner 0.5 pc of the cluster.
Follow-up observations of the low stellar mass targets are crucial to confirm
the dependence of the external photoevaporation process with stellar host mass.
This work confirms that the effects of external photoevaporation are
significant down to impinging radiation as low as G.Comment: Accepted for publication on Astronomy & Astrophysics. 13 pages, 7
figures + appendix. Abstract abridged to meet arXiv requirement
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