Superhumps of CC Cancri Revisited

We observed the 2001 November superoutburst of CC Cnc. This observation makes the first detailed coverage of a superoutburst of this object. The best-determined mean superhump period is 0.075518 +/- 0.000018 d, which is 2.7% longer than the reported orbital period. This fractional superhump excess is a quite typical value for a normal SU UMa-type dwarf nova, excluding the previously raised possibility that CC Cnc may have an anomalously large fractional superhump excess. During the superoutburst plateau, the object showed a decrease of the superhump period at dot(P)/P = -10.2(1.3) x 10^(-5), which is one of the largest negative period derivative known in all SU UMa-type dwarf novae.Comment: 5 pages, 5 figures, to appear in Publ. Astron. Soc. Japa

Photometric Observations of an SU UMa-type Dwarf Nova VW Coronae Borealis during Outbursts

We report the photometric observations of an SU UMa-type dwarf nova VW CrB during two superoutbursts in 2001 and 2003 and a normal outburst in 2003. Superhumps with a period of 0.07287(1) d were observed during the 2003 superoutburst. The change rate of the superhump period was positive. During the normal outburst, there are some hint of modulation up to a 0.2-mag amplitude. However, any periodicity was not found. The recurrence cycles of the normal outburst and the superoutburst, and the distance were estimated to be > ~50 d, 270-500 d, and 690(+230, -170) pc, respectively. These recurrence cycles are usual values for an SU UMa-type dwarf nova having this superhump period. The superhump period of VW CrB was the longest among those of the SU UMa stars with positive derivatives of the superhump period. The coverage of our observations was, however, not enough, and the variation of the Psh change rate of VW CrB is still unknown. A superhump regrowth and a brightening were seen near the end of the plateau phase. Measuring the deviation of the start timings of the brightening and the superhump regrowth (>2 days in VW CrB) will be a key to reveal the mechanism of these phenomena.Comment: 7 pages, 8 figures, to appear in PAS

Equation of motion for dislocations with inertial effects

An approximate equation of motion is proposed for screw and edge dislocations, which accounts for retardation and for relativistic effects in the subsonic range. Good quantitative agreement is found, in accelerated or in decelerated regimes, with numerical results of a more fundamental nature.Comment: 6 pages, 4 figures, LaTe

Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, in order to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. We show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure

Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals

The femtosecond optical pump-probe technique was used to study dynamics of photoexcited electrons and coherent optical phonons in transition metals Zn and Cd as a function of temperature and excitation level. The optical response in time domain is well fitted by linear combination of a damped harmonic oscillation because of excitation of coherent $E_{2g}$ phonon and a subpicosecond transient response due to electron-phonon thermalization. The electron-phonon thermalization time monotonically increases with temperature, consistent with the thermomodulation scenario, where at high temperatures the system can be well explained by the two-temperature model, while below $\approx$ 50 K the nonthermal electron model needs to be applied. As the lattice temperature increases, the damping of the coherent $E_{2g}$ phonon increases, while the amplitudes of both fast electronic response and the coherent $E_{2g}$ phonon decrease. The temperature dependence of the damping of the $E_{2g}$ phonon indicates that population decay of the coherent optical phonon due to anharmonic phonon-phonon coupling dominates the decay process. We present a model that accounts for the observed temperature dependence of the amplitude assuming the photoinduced absorption mechanism, where the signal amplitude is proportional to the photoinduced change in the quasiparticle density. The result that the amplitude of the $E_{2g}$ phonon follows the temperature dependence of the amplitude of the fast electronic transient indicates that under the resonant condition both electronic and phononic responses are proportional to the change in the dielectric function.Comment: 10 pages, 9 figures, to appear in Physical Review

An atlas of line profile studies for SU UMa type cataclysmic variables

We present H-alpha line-profile analyses for the seven SU UMa type dwarf novae AK Cnc, WX Cet, AQ Eri, VW Hyi, RZ Leo, TU Men, and HS Vir. All data sets are treated in the same manner, applying a sequence of techniques for each system. The basic ingredients of this sequence are the diagnostic diagram to determine the zero point of the orbital phase, and Doppler tomography to visualise the emission distribution. We furthermore introduce a new qualitative way of to evaluate the Doppler fit, by comparing the line profile of the reconstructed with the original spectrum in the form of the V/R plot. We present the results of the analysis in the compact form of an atlas, allowing a direct comparison of the emission distribution in our targets. Although most of the data sets were not taken with the intention of a line-profile analysis, we obtain significant results and are able to indicate the type of the additional emission in these systems. Our objects should have in principle very similar physical properties, i.e. they cover only a small range in orbital periods, mass ratios, and mass-transfer rates. Nevertheless, we find a large variety of phenomena both with respect to the individual systems and also within individual data sets of the same object. This includes `canonical' additional emission components from the secondary star and the bright spot, but also emission from the leading side of the accretion disc.Comment: 20 pages, 25 figures, accepted for publication in A&A, figures have been diminished in size and qualit