2,720 research outputs found
A deep, wide-field search for substellar members in NGC 2264
We report the first results of our ongoing campaign to discover the first
brown dwarfs (BD) in NGC 2264, a young (3 Myr), populous star forming region
for which our optical studies have revealed a very high density of potential
candidates - 236 in 1 deg - from the substellar limit down to at least
20 M for zero reddening. Candidate BD were first selected
using wide field () band imaging with CFH12K, by reference to current
theoretical isochrones. Subsequently, 79 (33%) of the sample were found
to have near-infrared 2MASS photometry ( 0.3 mag. or better),
yielding dereddened magnitudes and allowing further investigation by comparison
with the location of NextGen and DUSTY isochrones in colour-colour and
colour-magnitude diagrams involving various combinations of ,, and
. We discuss the status and potential substellarity of a number of
relatively unreddened (A 5) likely low-mass members in our
sample, but in spite of the depth of our observations in , we are as yet
unable to unambiguously identify substellar candidates using only 2MASS data.
Nevertheless, there are excellent arguments for considering two faint (observed
18.4 and 21.2) objects as cluster candidates with masses
respectively at or rather below the hydrogen burning limit. More current
candidates could be proven to be cluster members with masses around 0.1
M {\it via} gravity-sensitive spectroscopy, and deeper near-infrared
imaging will surely reveal a hitherto unknown population of young brown dwarfs
in this region, accessible to the next generation of deep near-infrared
surveys.Comment: 10 pages, 12 figures, accepted by A&
Where is the warm H2 ? A search for H2 emission from disks around Herbig Ae/Be stars
Mid-IR emission lines of H2 are useful probes to determine the mass of warm
gas present in the surface layers of disks. Numerous observations of Herbig
Ae/Be stars (HAeBes) have been performed, but only 2 detections of mid-IR H2
toward HD97048 and AB Aur have been reported. We aim at tracing the warm gas in
the disks of 5 HAeBes with gas-rich environments and physical characteristics
close to those of AB Aur and HD97048, to discuss whether the detections toward
these 2 objects are suggestive of peculiar conditions for the gas. We search
for the H2 S(1) emission line at 17.035 \mu\m with VISIR, and complemented by
CH molecule observations with UVES. We gather the H2 measurements from the
literature to put the new results in context and search for a correlation with
some disk properties. None of the 5 VISIR targets shows evidence for H2
emission. From the 3sigma upper limits on the integrated line fluxes we
constrain the amount of optically thin warm gas to be less than 1.4 M_Jup in
the disk surface layers. There are now 20 HAeBes observed with VISIR and TEXES
instruments to search for warm H2, but only two detections (HD97048 and AB Aur)
were made so far. We find that the two stars with detected warm H2 show at the
same time high 30/13 \mu\m flux ratios and large PAH line fluxes at 8.6 and
11.3 \mu\m compared to the bulk of observed HAeBes and have emission CO lines
detected at 4.7 \mu\m. We detect the CH 4300.3A absorption line toward both
HD97048 and AB Aur with UVES. The CH to H2 abundance ratios that this would
imply if it were to arise from the same component as well as the radial
velocity of the CH lines both suggest that CH arises from a surrounding
envelope, while the detected H2 would reside in the disk. The two detections of
the S(1) line in the disks of HD97048 and AB Aur suggest either peculiar
physical conditions or a particular stage of evolution.Comment: accepted for publication in A&A : 10 pages, 6 figure
Intrinsic charge transport on the surface of organic semiconductors
The novel technique based on air-gap transistor stamps enabled realization of
the intrinsic (not dominated by static disorder) transport of the
electric-field-induced charge carriers on the surface of rubrene crystals over
a wide temperature range. The signatures of the intrinsic transport are the
anisotropy of the carrier mobility, mu, and the growth of mu with cooling. The
anisotropy of mu vanishes in the activation regime at lower temperatures, where
the charge transport becomes dominated by shallow traps. The deep traps,
deliberately introduced into the crystal by X-ray radiation, increase the
field-effect threshold without affecting the mobility. These traps filled above
the field-effect threshold do not scatter the mobile polaronic carriers.Comment: 10 pages, 4 figure
Assessing the UK policies for broadband adoption
Broadband technology has been introduced to the business community and the public as a rapid way of exploiting the Internet. The benefits of its use (fast reliable connections, and always on) have been widely realised and broadband diffusion is one of the items at the top of the agenda for technology related polices of governments worldwide. In this paper an examination of the impact of the UK governmentâs polices upon broadband adoption is undertaken. Based on institutional theory a consideration of the manipulation of supply push and demand pull forces in the diffusion of broadband is offered. Using primary and secondary data sources, an analysis of the specific institutional actions related to IT diffusion as pursued by the UK government in the case of broadband is provided. Bringing the time dimension into consideration it is revealed that the UK government has shifted its attention from supply push-only strategies to more interventional ones where the demand pull forces are also mobilised. It is believed that this research will assist in the extraction of the âsuccess factorsâ in government intervention that support the diffusion of technology with a view to render favourable results if applied to other national settings
Collapse of stamps for soft lithography due to interfacial adhesion
Collapse of elastomeric elements used for pattern transfer in soft lithography is studied through experimental measurements and theoretical modehng. The objective is to identify the driving force for such collapse. Two potential driving forces, the self-weight of the stamp and the interfacial adhesion, are investigated. An idealized configuration of periodic rectangular grooves and flat punches is considered. Experimental observations demonstrate that groove collapse occurs regardless of whether the gravitational force promotes or suppresses such collapse, indicating that self-weight is not the driving force. On the other hand, model predictions based on the postulation that interfacial adhesion is the driving force exhibit excellent agreement with the experimentally measured collapse behavior. The interfacial adhesion energy is also evaluated by matching an adhesion parameter in the model with the experimental data.open546
Effect of kinetic resonances on the stability of Resistive Wall Mode in Reversed Field Pinch
The kinetic effects, due to the mode resonance with thermal particle drift
motions in the reversed field pinch (RFP) plasmas, are numerically investigated
for the stability of the resistive wall mode, using a non-perturbative
MHD-kinetic hybrid formulation. The kinetic effects are generally found too
weak to substantially change the mode growth rate, or the stability margin,
re-enforcing the fact that the ideal MHD model is rather adequate for
describing the RWM physics in RFP experiments.Comment: Submitted to: Plasma Phys. Control. Fusio
Ketocarotenoid production in tomato triggers metabolic reprogramming and cellular adaptation: The quest for homeostasis
Plants are sessile and therefore have developed an extraordinary capacity to adapt to external signals. Here, the focus is on the plasticity of the plant cell to respond to new intracellular cues. Ketocarotenoids are high-value natural red pigments with potent antioxidant activity. In the present study, system-level analyses have revealed that the heterologous biosynthesis of ketocarotenoids in tomato initiated a series of cellular and metabolic mechanisms to cope with the formation of metabolites that are non-endogenous to the plant. The broad multilevel changes were linked to, among others, (i) the remodelling of the plastidial membrane, where the synthesis and storage of ketocarotenoids occurs; (ii) the recruiting of core metabolic pathways for the generation of metabolite precursors and energy; and (iii) redox control. The involvement of the metabolites as regulators of cellular processes shown here reinforces their pivotal role suggested in the remodelled âcentral dogmaâ concept. Furthermore, the role of metabolic reprogramming to ensure cellular homeostasis is propose
Non-stationary dynamo & magnetospheric accretion processes of the classical T Tauri star V2129 Oph
We report here the first results of a multi-wavelength campaign focussing on
magnetospheric accretion processes of the classical TTauri star (cTTS)
V2129Oph. In this paper, we present spectropolarimetric observations collected
in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT).
Circularly polarised Zeeman signatures are clearly detected, both in
photospheric absorption and accretion-powered emission lines, from time-series
of which we reconstruct new maps of the magnetic field, photospheric brightness
and accretion-powered emission at the surface of V2129Oph using our newest
tomographic imaging tool - to be compared with those derived from our old 2005
June data set, reanalyzed in the exact same way.
We find that in 2009 July, V2129Oph hosts octupolar & dipolar field
components of about 2.1 & 0.9kG respectively, both tilted by about 20deg with
respect to the rotation axis; we conclude that the large-scale magnetic
topology changed significantly since 2005 June (when the octupole and dipole
components were about 1.5 and 3 times weaker respectively), demonstrating that
the field of V2129Oph is generated by a non-stationary dynamo. We also show
that V2129Oph features a dark photospheric spot and a localised area of
accretion-powered emission, both close to the main surface magnetic region
(hosting fields of up to about 4kG in 2009 July). We finally obtain that the
surface shear of V2129Oph is about half as strong as solar.
From the fluxes of accretion-powered emission lines, we estimate that the
observed average logarithmic accretion rate (in Msun/yr) at the surface of
V2129Oph is -9.2+-0.3 at both epochs, peaking at -9.0 at magnetic maximum. It
implies in particular that the radius at which the magnetic field of V2129Oph
truncates the inner accretion disc is 0.93x and 0.50x the corotation radius in
2009 July and 2005 June respectively.Comment: MNRAS in press - 16 pages, 9 figure
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