7,384 research outputs found
Recovering a lost baseline: missing kelp forests from a metropolitan coast
© 2008 AuthorThere is concern about historical and continuing loss of canopy-forming algae across the world’s temperate coastline. In South Australia, the sparse cover of canopy-forming algae on the Adelaide metropolitan coast has been of public concern with continuous years of anecdotal evidence culminating in 2 competing views. One view considers that current patterns existed before the onset of urbanisation, whereas the alternate view is that they developed after urbanisation. We tested hypotheses to distinguish between these 2 models, each centred on the reconstruction of historical covers of canopies on the metropolitan coast. Historically, the metropolitan sites were indistinguishable from contemporary populations of reference sites across 70 km (i.e. Gulf St. Vincent), and could also represent a random subset of exposed coastal sites across 2100 km of the greater biogeographic province. Thus there was nothing ‘special’ about the metropolitan sites historically, but today they stand out because they have sparser covers of canopies compared to equivalent locations and times in the gulf and the greater province. This is evidence of wholesale loss of canopy-forming algae (up to 70%) on parts of the Adelaide metropolitan coast since major urbanisation. These findings not only set a research agenda based on the magnitude of loss, but they also bring into question the logic that smaller metropolitan populations of humans create impacts that are trivial relative to that of larger metropolitan centres. Instead, we highlight a need to recognise the ecological context that makes some coastal systems more vulnerable or resistant to increasing human-domination of the world’s coastlines. We discuss challenges to this kind of research that receive little ecological discussion, particularly better leadership and administration, recognising that the systems we study out-live the life spans of individual research groups and operate on spatial scales that exceed the capacity of single research providers.Sean D. Connell, Bayden D. Russell, David J. Turner, Scoresby A. Shepherd, Timothy Kildea, David Miller, Laura Airoldi, Anthony Cheshir
Exploring Accretion and Disk-Jet Connections in the LLAGN M81*
We report on a year-long effort to monitor the central supermassive black
hole in M81 in the X-ray and radio bands. Using Chandra and the VLA, we
obtained quasi-simultaneous observations of M81* on seven occasions during
2006. The X-ray and radio luminosity of M81* are not strongly correlated on the
approximately 20-day sampling timescale of our observations, which is
commensurate with viscous timescales in the inner flow and orbital timecales in
a radially-truncated disk. This suggests that short-term variations in black
hole activity may not be rigidly governed by the "fundamental plane", but
rather adhere to the plane in a time-averaged sense. Fits to the X-ray spectra
of M81* with bremsstrahlung models give temperatures that are inconsistent with
the outer regions of very simple advection-dominated inflows. However, our
results are consistent with the X-ray emission originating in a transition
region where a truncated disk and advective flow may overlap. We discuss our
results in the context of models for black holes accreting at small fractions
of their Eddington limit, and the fundamental plane of black hole accretion.Comment: Accepted for publication in Ap
An overview of jets and outflows in stellar mass black holes
In this book chapter, we will briefly review the current empirical
understanding of the relation between accretion state and and outflows in
accreting stellar mass black holes. The focus will be on the empirical
connections between X-ray states and relativistic (`radio') jets, although we
are now also able to draw accretion disc winds into the picture in a systematic
way. We will furthermore consider the latest attempts to measure/order jet
power, and to compare it to other (potentially) measurable quantities, most
importantly black hole spin.Comment: Accepted for publication in Space Science Reviews. Also to appear in
the Space Sciences Series of ISSI - The Physics of Accretion on to Black
Holes (Springer Publisher
The influence of spin on jet power in neutron star X-ray binaries
We investigate the role of the compact object in the production of jets from
neutron star X-ray binaries. The goal is to quantify the effect of the neutron
star spin, if any, in powering the jet. We compile all the available measures
or estimates of the neutron star spin frequency in jet-detected neutron star
X-ray binaries. We use as an estimate of the ranking jet power for each source,
the normalisation of the power law which fits the X-ray/radio and
X-ray/infrared luminosity correlations L_(radio/IR) proportional to
L_(X)^(Gamma) (using infrared data for which there is evidence for jet
emission). We find a possible relation between spin frequency and jet power
(Spearman rank 97%), when fitting the X-ray/radio luminosity correlation using
a power law with slope 1.4; Gamma=1.4 is observed in 4U 1728-34 and is
predicted for a radiatively efficient disc and a total jet power proportional
to the mass accretion rate. If we use a slope of 0.6, as observed in Aql X-1,
no significant relation is found. An indication for a similar positive
correlation is also found for accreting millisecond X-ray pulsars (Spearman
rank 92%), if we fit the X-ray/infrared luminosity correlation using a power
law with slope 1.4. While our use of the normalisation of the luminosity
correlations as a measure of the ranking jet power is subject to large
uncertainties, no better proxy for the jet power is available. However, we urge
caution in over-interpreting the spin-jet power correlations, particularly
given the strong dependence of our result on the (highly uncertain) assumed
power law index of the luminosity correlations. We discuss the results in the
framework of current models for jet formation in black holes and young stellar
objects and speculate on possible different jet production mechanisms for
neutron stars depending on the accretion mode.Comment: Accepted for publication in MNRA
Galactic X-ray binary jets
With their relatively fast variability time-scales, Galactic X-ray binaries
provide an excellent laboratory to explore the physics of accretion and related
phenomena, most notably outflows, over different regimes. After comparing the
phenomenology of jets in black hole X-ray binary systems to that of neutron
stars, here I discuss the role of the jet at very low Eddington ratios, and
present preliminary results obtained by fitting the broadband spectral energy
distribution of a quiescent black hole binary with a `maximally jet-dominated'
model.Comment: Refereed version, accepted for publication in Astrophysics & Space
Scienc
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Osteoprotegerin reduces osteoclast resorption activity without affecting osteogenesis on nanoparticulate mineralized collagen scaffolds.
The instructive capabilities of extracellular matrix-inspired materials for osteoprogenitor differentiation have sparked interest in understanding modulation of other cell types within the bone regenerative microenvironment. We previously demonstrated that nanoparticulate mineralized collagen glycosaminoglycan (MC-GAG) scaffolds efficiently induced osteoprogenitor differentiation and bone healing. In this work, we combined adenovirus-mediated delivery of osteoprotegerin (AdOPG), an endogenous anti-osteoclastogenic decoy receptor, in primary human mesenchymal stem cells (hMSCs) with MC-GAG to understand the role of osteoclast inactivation in augmentation of bone regeneration. Simultaneous differentiation of osteoprogenitors on MC-GAG and osteoclast progenitors resulted in bidirectional positive regulation. AdOPG expression did not affect osteogenic differentiation alone. In the presence of both cell types, AdOPG-transduced hMSCs on MC-GAG diminished osteoclast-mediated resorption in direct contact; however, osteoclast-mediated augmentation of osteogenic differentiation was unaffected. Thus, the combination of OPG with MC-GAG may represent a method for uncoupling osteogenic and osteoclastogenic differentiation to augment bone regeneration
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