Evidence for Low-Dimensional Chaos in Semiregular Variable Stars

An analysis of the photometric observations of the light curves of the five large amplitude, irregularly pulsating stars R UMi, RS Cyg, V CVn, UX Dra and SX Her is presented. First, multi-periodicity is eliminated for these pulsations, i.e. they are not caused by the excitation of a small number of pulsation modes with constant amplitudes. Next, on the basis of energetics we also eliminate stochasticity as a cause, leaving low dimensional chaos as the only alternative. We then use a global flow reconstruction technique in an attempt to extract quantitative information from the light curves, and to uncover common physical features in this class of irregular variable stars that straddle the RV Tau to the Mira variables. Evidence is presented that the pulsational behavior of R UMi, RS Cyg, V CVn and UX Dra takes place in a 4-dimensional dynamical phase space, suggesting that two vibrational modes are involved in the pulsation. A linear stability analysis of the fixed points of the maps further indicates the existence of a two-mode resonance, similar to the one we had uncovered earlier in R Sct: The irregular pulsations are the result of a continual energy exchange between two strongly nonadiabatic modes, a lower frequency pulsation mode and an overtone that are in a close 2:1 resonance. The evidence is particularly convincing for R UMi, RS Cyg and V CVn, but much weaker for UX Dra. In contrast, the pulsations of SX Her appear to be more complex and may require a 6D space.Comment: 20 pages, 14 figures, accepted in ApJ - paper with clearer figures is available at http://www.phys.ufl.edu/~buchler/publications/u12.ps.gz (1Mb

Breakdown in hydrogen and deuterium gases in static and radio-frequency fields

We report the results of a combined experimental and modeling study of the electrical breakdown of hydrogen and deuterium in static (DC) and radio-frequency (RF) (13.56 MHz) electric fields. For the simulations of the breakdown events, simplified models are used and only electrons are traced by Monte Carlo simulation. The experimental DC Paschen curve of hydrogen is used for the determination of the effective secondary electron emission coefficient. A very good agreement between the experimental and the calculated RF breakdown characteristics for hydrogen is found. For deuterium, on the other hand, presently available cross section sets do not allow a reproduction of RF breakdown characteristics

A Cepheid is No More: Hubble's Variable 19 in M33

We report on the remarkable evolution in the light curve of a variable star discovered by Hubble (1926) in M33 and classified by him as a Cepheid. Early in the 20th century, the variable, designated as V19, exhibited a 54.7 day period, an intensity-weighted mean B magnitude of 19.59+/-0.23 mag, and a B amplitude of 1.1 mag. Its position in the P-L plane was consistent with the relation derived by Hubble from a total of 35 variables. Modern observations by the DIRECT project show a dramatic change in the properties of V19: its mean B magnitude has risen to 19.08 +/- 0.05 mag and its B amplitude has decreased to less than 0.1 mag. V19 does not appear to be a classical (Population I) Cepheid variable at present, and its nature remains a mystery. It is not clear how frequent such objects are nor how often they could be mistaken for classical Cepheids.Comment: Accepted for publication in the Astrophysical Journal Letters. Finding charts and photometry data can be downloaded from http://cfa-www.harvard.edu/~kstanek/DIRECT

A trapped single ion inside a Bose-Einstein condensate

Improved control of the motional and internal quantum states of ultracold neutral atoms and ions has opened intriguing possibilities for quantum simulation and quantum computation. Many-body effects have been explored with hundreds of thousands of quantum-degenerate neutral atoms and coherent light-matter interfaces have been built. Systems of single or a few trapped ions have been used to demonstrate universal quantum computing algorithms and to detect variations of fundamental constants in precision atomic clocks. Until now, atomic quantum gases and single trapped ions have been treated separately in experiments. Here we investigate whether they can be advantageously combined into one hybrid system, by exploring the immersion of a single trapped ion into a Bose-Einstein condensate of neutral atoms. We demonstrate independent control over the two components within the hybrid system, study the fundamental interaction processes and observe sympathetic cooling of the single ion by the condensate. Our experiment calls for further research into the possibility of using this technique for the continuous cooling of quantum computers. We also anticipate that it will lead to explorations of entanglement in hybrid quantum systems and to fundamental studies of the decoherence of a single, locally controlled impurity particle coupled to a quantum environment