15 research outputs found
SPB stars in the open SMC cluster NGC 371
Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which
depends critically on the metallicity. It has therefore been suggested that
beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower
metallicities than the solar neighborhood. To test this prediction we have
observed the open SMC cluster NGC 371 for 12 nights in order to search for beta
Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in
the upper part of the main sequence, many of which are probably SPB stars. This
result indicates that pulsation is still driven by the kappa mechanism even in
low metallicity environments. All the identified variables have periods longer
than the fundamental radial period which means that they cannot be beta Cep
stars. Within an amplitude detection limit of 5 mmag no stars in the top of the
HR-diagram show variability with periods shorter than the fundamental radial
period. So if beta Cep stars are present in the cluster they oscillate with
amplitudes below 5 mmag, which is significantly lower than the mean amplitude
of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is
more common in the upper part of the main sequence than in the lower. We have
also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the
cluster field.Comment: 8 pages, 11 figures. Accepted for publication in MNRA
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Configuration control, fluctuations, and transport in low-collisionality plasmas in the ATF Torsatron
In low-collisionality plasmas confined in tokamaks and stellarators, instabilities driven by particles trapped in inhomogeneities of the magnetic fields could be important in increasing plasma transport coefficients. In the Advanced Toroidal Facility (ATF), an {ell} = 2, M = 12 field-period stellarator device with major radius R = 2.1 m, average plasma minor radius a = 0.27 m, central and edge rotational transforms {chi}{sub 0} {approx} 0.3, {chi}{sub a} {approx} 1, the effects of electron trapping in the helical stellarator field are expected to be important in plasmas with {bar n}{sub e} {approx} 5 {times} 10{sup 12} cm{sup {minus}3}, T{sub e0} {approx} 1 keV. Such plasmas have already been sustained for long-pulses (20 s) using 150--400 kW of 53.2-GHz ECH power at B = 0.95 T. Transport analysis shows that for {rho} = r/a {le} 1/3, the electron anomalous transport is {le}10 times the neoclassical value, while at {rho} = 2/3 it is 10--100 times neoclassical; this is compatible with expectations for transport enhancement due to dissipative trapped-electron modes. 4 refs., 3 figs
Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006-2007
We present radio-to-optical data taken by the WEBT, supplemented by VLBA and
RXTE observations, of 3C 279. Our goal is to use this extensive database to
draw inferences regarding the physics of the relativistic jet. We assemble
multifrequency light curves with data from 30 ground-based observatories and
the space-based instruments, along with linear polarization vs. time in the
optical R band. In addition, we present a sequence of 22 images (with
polarization vectors) at 43 GHz at resolution 0.15 milliarcsec, obtained with
the VLBA. We analyse the light curves and polarization, as well as the spectral
energy distributions at different epochs, corresponding to different brightness
states. The IR-optical-UV continuum spectrum of the variable component
corresponds to a power law with a constant slope of -1.6, while in the 2.4-10
keV X-ray band it varies in slope from -1.1 to -1.6. The steepest X-ray
spectrum occurs at a flux minimum. During a decline in flux from maximum in
late 2006, the optical and 43 GHz core polarization vectors rotate by ~300
degrees. The continuum spectrum agrees with steady injection of relativistic
electrons with a power-law energy distribution of slope -3.2 that is steepened
to -4.2 at high energies by radiative losses. The X-ray emission at flux
minimum comes most likely from a new component that starts in an upstream
section of the jet where inverse Compton scattering of seed photons from
outside the jet is important. The rotation of the polarization vector implies
that the jet contains a helical magnetic field that extends ~20 pc past the 43
GHz core.Comment: 12 pages, aa.cls style; accepted for publication in A&
How well do state-of-the-art Atmosphere-Ocean general circulation models reproduce atmospheric teleconnection patterns?
This article evaluates the ability of state-of-the-art climate models to reproduce the low-frequency variability of the mid-tropospheric winter flow of the Northern Hemisphere in terms of atmospheric teleconnection patterns. Therefore, multi-model simulations for present-day conditions, performed for the 4th assessment report of the Intergovernmental Panel on Climate Change, have been analysed and compared with re-analysis data sets. The spatial patterns of atmospheric teleconnections are reproduced reasonably by most of the models. The comparison of coupled with atmosphere-only runs confirmed that a better representation of the forcing by sea surface temperatures has the potential to slightly improve the representation of only wave train-like patterns. Due to internally generated climate variability, the models are not able to reproduce the observed temporal behaviour. Insights into the dynamical reasons for the limited skill of climate models in reproducing teleconnections have been obtained by studying the relation between major teleconnections and zonal wind variability patterns. About half of the models are able to reproduce the observed relationship. For these cases, the quality of simulated teleconnection patterns is largely determined by the quality of zonal wind variability patterns. Therefore, improvements of simulated eddy-mean flow interaction have the potential to improve the atmospheric teleconnections
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The effects of chromium and titanium gettering on the operation of the Advanced Toroidal Facility
Plasmas in the Advanced Toroidal Facility (ATF), an {ell} = 2 torsatron with 12 field periods, are produced by 200--400 kW of electron cyclotron heating (ECH) and up to 1.5 MW of neutral-beam injection (NBI). The characteristics of the plasmas are sensitive to the type of wall conditioning employed. A progression of techniques, beginning with glow discharge cleaning and baking and evolving to gettering with chromium and titanium, has significantly reduced the low-Z impurity content, lowered the fraction of radiated power, and permitted improved control over the electron density. As a result, plasma parameters and machine performance have been enhanced significantly. The maximum values achieved for storage energy, line-averaged density, and confinement times are 28 kJ, 1.2 {times} 10{sup 20} m{sup {minus}3}, and 25 ms. These parameters are comparable to those obtained in the ISX-B tokamak. In addition, with titanium gettering, quasisteady operation of 200 ms of neutral beam injection has been obtained without the collapses that characterized earlier periods of operation. 4 refs., 4 figs., 1 tab
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Bootstrap Current Studies in the Advanced Toroidal Facility
The toroidal current observed during electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron is identified as bootstrap current. The observed current ranges between +3 kA and {minus}1 kA with negligible Ohmic heating (OH) and ECH-driven currents. The observed currents agree well with predictions of neoclassical theory in magnitude (to within 30%) and parametric dependence (including the reversal of the current), as determined by systematic scans of quadrupole (shaping) and dipole (vacuum axis shift) moments of the vertical field. These results show that the current flow in ATF is well described by the neoclassical theory of bootstrap current despite the presence of anomalies in particle and heat flows. In addition, the results demonstrate the ability to control the toroidal current with the vertical field for currentless operation in stellarators. 5 refs., 5 figs