3,583 research outputs found
The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum
We report the serendipitous discovery of a population of low mass, pre-main
sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system
gamma^{2} Vel and the Vela OB2 association. We argue that gamma^{2} Vel and the
low mass stars are truly associated, are approximately coeval and that both are
at distances between 360-490 pc, disagreeing at the 2 sigma level with the
recent Hipparcos parallax of gamma^{2} Vel, but consistent with older distance
estimates. Our results clearly have implications for the physical parameters of
the gamma^{2} Vel system, but also offer an exciting opportunity to investigate
the influence of high mass stars on the mass function and circumstellar disc
lifetimes of their lower mass PMS siblings.Comment: Monthly Notices of the Royal Astronomical Society, Letters - in pres
Ages of young stars
Determining the sequence of events in the formation of stars and planetary
systems and their time-scales is essential for understanding those processes,
yet establishing ages is fundamentally difficult because we lack direct
indicators. In this review we discuss the age challenge for young stars,
specifically those less than ~100 Myr old. Most age determination methods that
we discuss are primarily applicable to groups of stars but can be used to
estimate the age of individual objects. A reliable age scale is established
above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young
clusters, and consistency is shown between these ages and those from the upper
main sequence and the main sequence turn-off -- if modest core convection and
rotation is included in the models of higher-mass stars. Other available
methods for age estimation include the kinematics of young groups, placing
stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface
gravity measurement, rotation and activity, and lithium abundance. We review
each of these methods and present known strengths and weaknesses. Below ~20
Myr, both model-dependent and observational uncertainties grow, the situation
is confused by the possibility of age spreads, and no reliable absolute ages
yet exist. The lack of absolute age calibration below 20 Myr should be borne in
mind when considering the lifetimes of protostellar phases and circumstellar
material.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Non-classical photon streams using rephased amplified spontaneous emission
We present a fully quantum mechanical treatment of optically rephased photon
echoes. These echoes exhibit noise due to amplified spontaneous emission,
however this noise can be seen as a consequence of the entanglement between the
atoms and the output light. With a rephasing pulse one can get an "echo" of the
amplified spontaneous emission, leading to light with nonclassical correlations
at points separated in time, which is of interest in the context of building
wide bandwidth quantum repeaters. We also suggest a wideband version of DLCZ
protocol based on the same ideas.Comment: 5 pages, 4 figures. Added section
The lithium depletion boundary and the age of NGC 2547
We present the results of a photometric and spectroscopic survey of cool M
dwarf candidates in the young open cluster NGC 2547. Using the 2dF fiber
spectrograph, we have searched for the luminosity at which lithium remains
unburned in an attempt to constrain the cluster age. The lack of a population
of individual lithium-rich objects towards the faint end of our sample places a
very strong lower limit to the cluster age of 35 Myr. However, the detection of
lithium in the averaged spectra of our faintest targets suggests that the
lithium depletion boundary lies at 9.5 < M(I) < 10.0 and that the cluster age
is < 54 Myr. The age of NGC 2547 judged from fitting isochrones to low-mass
pre-main-sequence stars in colour-magnitude diagrams is 20-35 Myr using the
same evolutionary models. The sense and size of the discrepancy in age
determined by these two techniques is similar to that found in another young
cluster, IC 2391, and in the low-mass pre main-sequence binary system, GJ
871.1AB. We suggest that the inclusion of rotation or dynamo-generated magnetic
fields in the evolutionary models could reconcile the two age determinations,
but only at the expense of increasing the cluster ages beyond that currently
indicated by the lithium depletion. Alternatively, some mechanism is required
that increases the rate of lithium depletion in young, very low-mass fully
convective stars.Comment: 14 pages, 7 figures, 3 tables, accepted by MNRA
Photon echo quantum memories in inhomogeneously broadened two level atoms
Here we propose a solid-state quantum memory that does not require spectral
holeburning, instead using strong rephasing pulses like traditional photon echo
techniques. The memory uses external broadening fields to reduce the optical
depth and so switch off the collective atom-light interaction when desired. The
proposed memory should allow operation with reasonable efficiency in a much
broader range of material systems, for instance Er3+ doped crystals which have
a transition at 1.5 um. We present analytic theory supported by numerical
calculations and initial experiments.Comment: 7 pages, 8 figure
Criminal benefit, the confiscation order and the post-conviction confiscation regime
Governments and law enforcement agencies around the world seek to identify and confiscate the 'proceeds of crime' on the assertion that doing so will deter offending and symbolise to citizens and communities that 'crime does not pay'. In the UK such assertions have underpinned the enactment of legislation, the investment in law enforcement agents and the development of wide ranging new technologies to facilitate the identification of assets and their recovery. This paper critically considers two key concepts which fundamentally drive the post-conviction confiscation regime in the UK. First, 'criminal benefit' which is the amount that a defendant is adjudged to have made from 'criminal conduct'. Second, the 'available amount' which is the amount that the state hopes to recover from a defendant via the court ordered 'confiscation order'. In so doing, this paper explores the assumptions at the heart of the 2002 Proceeds of Crime Act and their application in practice, concentrating on the nature of the powers accorded to financial investigators and how these powers have been interpreted and applied. It is argued that far from representing the 'profit' generated from crime these values are constructs founded in the relationship between legislation, the discretional practice of police officers and financial investigators, organisational restrictions and constraints and informal negotiation and compromise between the defence and prosecution. This has implications for both conceptualising the nature of the post-conviction confiscation regime as well as for shaping what the state might expect to recover from defendants. © 2014 Springer Science+Business Media Dordrecht
Resonance line-profile calculations based on hydrodynamical models of cataclysmic variable winds
We present synthetic line profiles as predicted by the models of 2-D line-
driven disk winds due to Proga, Stone & Drew. We compare the model line
profiles with HST observations of the cataclysmic variable IX Vel. The model
wind consists of a slow outflow that is bounded on the polar side by a fast
stream. We find that these two components of the wind produce distinct spectral
features. The fast stream produces profiles which show features consistent with
observations. These include the appearance of the P-Cygni shape for a range of
inclinations, the location of the maximum depth of the absorption component at
velocities less than the terminal velocity, and the transition from absorption
to emission with increasing inclination. However the model profiles have too
little absorption or emission equivalent width. This quantitative difference
between our models and observations is not a surprise because the line-driven
wind models predict a mass loss rate that is lower than the rate required by
the observations. We note that the model profiles exhibit a double-humped
structure near the line center which is not echoed in observations. We identify
this structure with a non-negligible redshifted absorption which is formed in
the slow component of the wind where the rotational velocity dominates over
expansion velocity. We conclude that the next generation of disk wind models,
developed for application to CVs, needs to yield stronger wind driving out to
larger disk radii than do the present models.Comment: LaTeX, 19 pages, to appear in Ap
Experimental realization of light with time separated correlations by rephasing amplified spontaneous emission
Amplified spontaneous emission is a common noise source in active optical
systems, it is generally seen as being an incoherent process. Here we excite an
ensemble of rare earth ion dopants in a solid with a {\pi}-pulse, resulting in
amplified spontaneous emission. The application of a second {\pi}-pulse leads
to a coherent echo of the amplified spontaneous emission that is correlated in
both amplitude and phase. For small optical thicknesses, we see evidence that
the amplified spontaneous emission and its echo are entangled.Comment: 6 pages, 5 figures, the supplementary information pdf was uploaded
with latex source files. This version accepted for publication in PR
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