168 research outputs found
The radio lighthouse CU Virginis: the spindown of a single main sequence star
The fast rotating star CU Virginis is a magnetic chemically peculiar star
with an oblique dipolar magnetic field. The continuum radio emission has been
interpreted as gyrosyncrotron emission arising from a thin magnetospheric
layer. Previous radio observations at 1.4 GHz showed that a 100% circular
polarized and highly directive emission component overlaps to the continuum
emission two times per rotation, when the magnetic axis lies in the plane of
the sky. This sort of radio lighthouse has been proposed to be due to cyclotron
maser emission generated above the magnetic pole and propagating
perpendicularly to the magnetic axis. Observations carried out with the
Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this
discovery show that this radio emission is still present, meaning that the
phenomenon responsible for this process is steady on a timescale of years. The
emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On
the light of recent results on the physics of the magnetosphere of this star,
the possibility of plasma radiation is ruled out. The characteristics of this
radio lighthouse provides us a good marker of the rotation period, since the
peaks are visible at particular rotational phases. After one year, they show a
delay of about 15 minutes. This is interpreted as a new abrupt spinning down of
the star. Among several possibilities, a quick emptying of the equatorial
magnetic belt after reaching the maximum density can account for the magnitude
of the breaking. The study of the coherent emission in stars like CU Vir, as
well as in pre main sequence stars, can give important insight into the angular
momentum evolution in young stars. This is a promising field of investigation
that high sensitivity radio interferometers such as SKA can exploit.Comment: Accepted to MNRAS, 8 pages, 7 figures, updated versio
Making judgements about students making work : lecturersâ assessment practices in art and design.
This research study explores the assessment practices in two higher education art and design departments. The key aim of this research was to explore art and design studio assessment practices as lived by and experienced by art and design lecturers. This work draws on two bodies of pre existing research. Firstly this study adopted innovative methodological approaches that have been employed to good effect to explore assessment in text based subjects (think aloud) and moderation mark agreement (observation). Secondly the study builds on existing research into the assessment of creative practice. By applying thinking aloud methodologies into a creative practice assessment context the authors seek to illuminate the âin practiceâ rather than espoused assessment approaches adopted. The analysis suggests that lecturers in the study employed three macro conceptions of quality to support the judgement process. These were; the demonstration of significant learning over time, the demonstration of effective studentship and the presentation of meaningful art/design work
Gravity compensation in complex plasmas by application of a temperature gradient
Micron sized particles are suspended or even lifted up in a gas by
thermophoresis. This allows the study of many processes occurring in strongly
coupled complex plasmas at the kinetic level in a relatively stress-free
environment. First results are presented. The technique is also of interest for
technological applications.Comment: 4 pages, 4 figures, final version to be published in Phys. Rev. Let
Twisted Dust Acoustic Waves in Dusty Plasmas
We examine linear dust acoustic waves (DAWs) in a dusty plasma with strongly
correlated dust grains, and discuss possibility of a twisted DA vortex beam
carrying orbital angular momentum (OAM). For our purposes, we use the Boltzmann
distributed electron and ion density perturbations, the dust continuity and
generalized viscoelastic dust momentum equations, and Poisson's equation to
obtain a dispersion relation for the modified DAWs. The effects of the
polarization force, strong dust couplings, and dust charge fluctuations on the
DAW spectrum are examined. Furthermore, we demonstrate that the DAW can
propagate as a twisted vortex beam carrying OAM. A twisted DA vortex structure
can trap and transport dust particles in dusty plasmas.Comment: arXiv admin note: text overlap with arXiv:1205.594
Observations of radio pulses from CU Virginis
The magnetic chemically peculiar star CU Virginis is a unique astrophysical
laboratory for stellar magnetospheres and coherent emission processes. It is
the only known main sequence star to emit a radio pulse every rotation period.
Here we report on new observations of the CU Virginis pulse profile in the 13
and 20\,cm radio bands. The profile is known to be characterised by two peaks
of 100 circularly polarised emission that are thought to arise in an
electron-cyclotron maser mechanism. We find that the trailing peak is stable at
both 13 and 20\,cm, whereas the leading peak is intermittent at 13\,cm. Our
measured pulse arrival times confirm the discrepancy previously reported
between the putative stellar rotation rates measured with optical data and with
radio observations. We suggest that this period discrepancy might be caused by
an unknown companion or by instabilities in the emission region. Regular
long-term pulse timing and simultaneous multi-wavelength observations are
essential to clarify the behaviour of this emerging class of transient radio
source.Comment: Accepted by MNRAS Letters; 5 pages, 2 figures, 3 table
The red-sky enigma over Svalbard in December 2002
On 6 December 2002, during winter darkness, an extraordinary event occurred in the sky, as viewed from Longyearbyen (78° N, 15° E), Svalbard, Norway. At 07:30 UT the southeast sky was surprisingly lit up in a deep red colour. The light increased in intensity and spread out across the sky, and at 10:00 UT the illumination was observed to reach the zenith. The event died out at about 12:30 UT. Spectral measurements from the Auroral Station in Adventdalen confirm that the light was scattered sunlight. Even though the Sun was between 11.8 and 14.6deg below the horizon during the event, the measured intensities of scattered light on the southern horizon from the scanning photometers coincided with the rise and setting of the Sun. Calculations of actual heights, including refraction and atmospheric screening, indicate that the event most likely was scattered solar light from a target below the horizon. This is also confirmed by the OSIRIS instrument on board the Odin satellite. The deduced height profile indicates that the scattering target is located 18â23km up in the stratosphere at a latitude close to 73â75° N, southeast of Longyearbyen. The temperatures in this region were found to be low enough for Polar Stratospheric Clouds (PSC) to be formed. The target was also identified as PSC by the LIDAR systems at the Koldewey Station in Ny-Ă
lesund (79° N, 12° E). The event was most likely caused by solar illuminated type II Polar Stratospheric Clouds that scattered light towards Svalbard. Two types of scenarios are presented to explain how light is scattered.publishedVersio
The Effects of Alfven Waves and Radiation Pressure in Dust Winds of Late-type Stars
In the present study, we analyze the effects of a flux of Alfven waves acting
together with radiation pressure on grains as an acceleration mechanism of the
wind of late-type stars. In the wind model we simulate the presence of grains
through a strong damping of the waves, we used a non-isothermal profile for
temperature, coherent with grain formation theories. We examine the changes in
the velocity profile of the wind and we show that if the grains are created in
the region 1.1 < r/r_0 < 2.0 their presence will affect the mass loss and
terminal velocity. The model is applied to a K5 supergiant star and for
Betelgeuse (alpha Ori).Comment: 6 pages, 3 figures, accepted in ApJ (Sep, 2002
Time-dependent simulations of steady C-type shocks
Using a time-dependent multifluid, magnetohydrodynamic code, we calculated
the structure of steady perpendicular and oblique C-type shocks in dusty
plasmas. We included relevant processes to describe mass transfer between the
different fluids, radiative cooling by emission lines and grain charging and
studied the effect of single-sized and multiple sized grains on the shock
structure. Our models are the first of oblique fast-mode molecular shocks in
which such a rigorous treatment of the dust grain dynamics has been combined
with a self-consistent calculation of the thermal and ionisation structures
including appropriate microphysics. At low densities the grains do not play any
significant role in the shock dynamics. At high densities, the ionisation
fraction is sufficiently low that dust grains are important charge and current
carriers and, thus, determine the shock structure. We find that the magnetic
field in the shock front has a significant rotation out of the initial upstream
plane. This is most pronounced for single-sized grains and small angles of the
shock normal with the magnetic field. Our results are similar to previous
studies of steady C-type shocks showing that our method is efficient, rigorous
and robust. Unlike the method employed in the previous most detailed treatment
of dust in steady oblique fast-mode shocks, ours allows a reliable calculation
even when chemical or other conditions deviate from local statistical
equilibrium. We are also able to model transient phenomena.Comment: 9 pages, 4 figures, accepted for publication in MNRA
The Effect of Rotational Gravity Darkening on Magnetically Torqued Be Star Disks
In the magnetically torqued disk (MTD) model for hot star disks, as proposed
and formulated by \citet{cassi02}, stellar wind mass loss was taken to be
uniform over the stellar surface. Here account is taken of the fact that as
stellar spin rate S_o (=\sqrt {\Omega_o^2 R^3/GM}) is increased, and the
stellar equator is gravity darkened, the equatorial mass flux and terminal
speed are reduced, compared to the poles, for a given total \mdot. As a result,
the distribution of equatorial disk density, determined by the impact of north
and southbound flows, is shifted further out from the star. This results, for
high S_o (\gtrsim 0.5), in a fall in the disk mass and emission measure, and
hence in the observed emission line EW, scattering polarization, and IR
emission. Consequently, contrary to expectations, critical rotation S_o \to 1
is not the optimum for creation of hot star disks which, in terms of EM for
example, is found to occur in a broad peak around S_o\approx 0.5-0.6 depending
slightly on the wind velocity law.
The relationship of this analytic quasi-steady parametric MTD model to other
work on magnetically guided winds is discussed. In particular the failures of
the MTD model for Be-star disks alleged by \citet{owo03} are shown to revolve
largely around open observational tests, rather in the basic MTD physics, and
around their use of insufficiently strong fields.Comment: 25 pages with 11 figures, accepted by MNRA
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