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

The Pre-Main-Sequence Eclipsing Binary ASAS J052821+0338.5

In this paper we present the first results of a detailed spectroscopic and photometric analysis of the V = 11.7m eclipsing binary ASAS J052821+0338.5. With the FIES spectrograph at the Nordic Optical Telescope we have obtained a series of high-resolution spectra (R=47000) covering the entire orbit of the system. In addition we obtained simultaneous broadband photometry from three small aperture telescopes. From these spectroscopic and photometric data we have derived the system's orbital parameters and determined the fundamental stellar parameters of the two components. Our results indicate that ASAS J052821+0338.5 is a K1/K3 pre-main-sequence eclipsing binary, with component masses of 1.38 M_sun and 1.33 M_sun and a period of 3.87 days, located at a distance of 280 +/- 30 pc. The kinematics, physical location and the evolutionary status of the two stars suggest that ASAS J052821+0338.5 is a member of the approximately 11 Myr old Orion OB1a subassociation. The systems also exhibits smooth 0.15m out-of-eclipse variations that are similar to those found in RS CVn binaries. Furthermore the parameters we derived are consistent with the 10-13 Myr isochrones of the popular Baraffe stellar evolutionary models.Comment: 9 pages, 8 figures, to appear in A&

A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies

We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with eleven telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges that we identify with even and odd values of the angular degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of about 1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is significantly shorter than the 2-4 days seen in the Sun.Comment: accepted for publication in Ap

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&

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

Spatially resolved instability measurements with a heavy ion beam probe

A heavy ion beam probe was used to make simultaneous measurements of the amplitude and phase of both density and space potential fluctuations in an energetic arc plasma. Detailed comparison with theoretical predictions identifies a 70 KHz coherent oscillation as a Kelvin-Helmholtz instability localized to a region of strong fluid shear. (auth

Unconventional heating experiments in EBT

Reported here are the results of two nonstandard heating experiments. In one, the fundamental ECH resonance at either 10.6 or 18 GHz was placed at the usual ring position in an attempt to generate rings solely with fundamental heating; this is the fundamental resonance configuration (FR). In a second experiment, an attempt was made to heat the ring and core separately by forming the ring as above with 10.6 GHz and then heating the core with the fundamental and second harmonic of 28 GHz; this is the separately coupled ring and toroidal core heating configuration (SCRATCH). In both cases, the hot electron annuli were formed and stabilized the core plasma as in the SR experiments. In FR, the rings form at the second harmonic and no significant potential structures were found. In SCRATCH, either a potential well or a potential hill could be formed. In both heating regimes, significant ion heating above that found in standard heating experiments was found