Most Rotational Variables Dominated by a Single Bright Feature are α2\alpha^2 CVn Stars

We previously reported a rare class of variable star light curves isolated from a sample of 4.7 million candidate variables from the ATLAS survey. Dubbed `UCBH' light curves, they have broad minima and narrow, symmetrical maxima, with typical periods of 1-10 days and amplitudes of 0.05--0.20 mag. They maintain constant amplitude, shape, and phase coherence over multiple years, but do not match any known class of pulsating variables. A localized bright spot near the equator of a rotating star will produce a UCBH-type light curve for most viewing geometries. Most stars that exhibit rotational variability caused primarily by a single bright feature should therefore appear as UCBH stars, although a rotating bright spot is not the only thing that could produce a UCBH-type lightcurve. We have spectroscopically investigated fourteen UCBH stars and found ten of them to be Ap/Bp stars: A-type or B-type stars with greatly enhanced photospheric abundances of specific heavy elements. Rotationally variable Ap/Bp stars are referred to as $\alpha^2$ CVn variables. Most ATLAS UCBH stars are therefore $\alpha^2$ CVn stars, although only a minority of $\alpha^2$ CVn stars in the literature have UCBH light curves. The fact that $\alpha^2$ CVn stars dominate the UCBH class suggests that lone bright spots with sufficient size and contrast develop more readily on Ap/Bp stars than on any other type. The $\alpha^2$ CVn UCBH stars may be characterized by a specific magnetic field topology, making them intriguing targets for future Zeeman-Doppler imaging.Comment: 18 pages, 8 figures, accepted to A

Pulsating Front Speed-up and Quenching of Reaction by Fast Advection

We consider reaction-diffusion equations with combustion-type non-linearities in two dimensions and study speed-up of their pulsating fronts by general periodic incompressible flows with a cellular structure. We show that the occurence of front speed-up in the sense $\lim_{A\to\infty} c_*(A)=\infty$, with $A$ the amplitude of the flow and $c_*(A)$ the (minimal) front speed, only depends on the geometry of the flow and not on the reaction function. In particular, front speed-up happens for KPP reactions if and only if it does for ignition reactions. We also show that the flows which achieve this speed-up are precisely those which, when scaled properly, are able to quench any ignition reaction.Comment: 16p

Mechanism of Ambipolar Field-Effect Carrier Injections in One-Dimensional Mott Insulators

To clarify the mechanism of recently reported, ambipolar carrier injections into quasi-one-dimensional Mott insulators on which field-effect transistors are fabricated, we employ the one-dimensional Hubbard model attached to a tight-binding model for source and drain electrodes. To take account of the formation of Schottky barriers, we add scalar and vector potentials, which satisfy the Poisson equation with boundary values depending on the drain voltage, the gate bias, and the work-function difference. The current-voltage characteristics are obtained by solving the time-dependent Schr\"odinger equation in the unrestricted Hartree-Fock approximation. Its validity is discussed with the help of the Lanczos method applied to small systems. We find generally ambipolar carrier injections in Mott insulators even if the work function of the crystal is quite different from that of the electrodes. They result from balancing the correlation effect with the barrier effect. For the gate-bias polarity with higher Schottky barriers, the correlation effect is weakened accordingly, owing to collective transport in the one-dimensional correlated electron systems.Comment: 21 pages, 10 figures, to appear in J. Phys. Soc. Jp

First On-Sky High Contrast Imaging with an Apodizing Phase Plate

We present the first astronomical observations obtained with an Apodizing Phase Plate (APP). The plate is designed to suppress the stellar diffraction pattern by 5 magnitudes from 2-9 lambda/D over a 180 degree region. Stellar images were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope, with adaptive wavefront correction made with a deformable secondary mirror designed for low thermal background observations. The measured PSF shows a halo intensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering off as r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent with residual errors predicted for servo lag in the AO system. We project a 5 sigma contrast limit, set by residual atmospheric fluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hour exposure. This can be realised if static and quasi-static aberrations are removed by differential imaging, and is close to the sensitivity level set by thermal background photon noise for target stars with M'>3. The advantage of using the phase plate is the removal of speckle noise caused by the residuals in the diffraction pattern that remain after PSF subtraction. The APP gives higher sensitivity over the range 2-5 lambda/D compared to direct imaging techniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte

Intrinsic Photoconductivity of Ultracold Fermions in Optical Lattices

We report on the experimental observation of an analog to a persistent alternating photocurrent in an ultracold gas of fermionic atoms in an optical lattice. The dynamics is induced and sustained by an external harmonic confinement. While particles in the excited band exhibit long-lived oscillations with a momentum dependent frequency a strikingly different behavior is observed for holes in the lowest band. An initial fast collapse is followed by subsequent periodic revivals. Both observations are fully explained by mapping the system onto a nonlinear pendulum.Comment: 5+7 pages, 4+4 figure