New detections of H2O masers in planetary nebulae and post-AGB stars using the Robledo-70m antenna

Aims: We investigated the possible relationship between the evolutionary stage of post-AGB stars and planetary nebulae (PNe) and the presence of water masers in their envelopes. Methods: We have used NASA's 70-m antenna in Robledo de Chavela (Spain) to search for the water maser transition at 22235.08 MHz, towards a sample of 105 sources with IRAS colour characteristic of post-AGB stars and PNe at declination >-32 deg. 83% of the sources in the sample are post-AGB stars, 15% PNe or PN candidates, while only 2% seem to be HII regions. Results: We have detected five water masers, of which four are reported for the first time: two in PNe (IRAS 17443-2949 and IRAS 18061-2505), a ``water fountain'' in a post-AGB star (IRAS 16552-3050), and one in a source previously catalogued as a PN, but whose classification is uncertain (IRAS 17580-3111). Conclusions: The unexpected detections of water masers in two objects among the small subset of PNe led us to suggest that the PNe harbouring water masers are a special type of massive, rapidly evolving PNe.Comment: 15 pages, 7 figures. Accepted for publication in Astronomy and Astrophysic

An updated catalog of OH-maser-emitting planetary nebulae

Aims. We studied the characteristics of planetary nebulae (PNe) that show both OH maser and radio continuum emission (hereafter OHPNe). These have been proposed to be very young PNe, and therefore, they could be key objects for understanding the formation and evolution of PNe. Methods. We consulted the literature searching for interferometric observations of radio continuum and OH masers toward evolved stars, including the information from several surveys. We also processed radio continuum and OH maser observations toward PNe in the Very Large Array data archive. The high positional accuracy provided by interferometric observations allow us to confirm or reject the association between OH maser and radio continuum emission. Results. We found a total of six PNe that present both OH maser and radio continuum emissions, as confirmed with radio interferometric observations. These are bona fide OHPNe. The confirmed OHPNe present a bipolar morphology in resolved images of their ionized emission at different wavelengths, suggesting that the OH maser emission in PNe is related to nonspherical mass-loss phenomena. The OH maser spectra in PNe present a clear asymmetry, tending to show blueshifted emission with respect to the systemic velocity. Their infrared colors suggest that most of these objects are very young PNe. OHPNe do not form a homogeneous group, and seem to represent a variety of different evolutionary stages. We suggest that OH masers pumped in the AGB phase may disappear during the post-AGB phase, but reappear once the source becomes a PN and its radio continuum emission is amplified by the OH molecules. Therefore, OH maser emission could last significantly longer than the previously assumed 1000 yr after the end of the AGB phase. This maser lifetime may be longer in PNe with more massive central stars, which ionize a larger amount of gas in the envelope.Comment: 16 pages, 5 figures, 4 tables. Accepted for publication by Astronomy & Astrophysic

Quantum Phase Transitions detected by a local probe using Time Correlations and Violations of Leggett-Garg Inequalities

In the present paper we introduce a way of identifying quantum phase transitions of many-body systems by means of local time correlations and Leggett-Garg inequalities. This procedure allows to experimentally determine the quantum critical points not only of finite-order transitions but also those of infinite order, as the Kosterlitz-Thouless transition that is not always easy to detect with current methods. By means of simple analytical arguments for a general spin-$1 / 2$ Hamiltonian, and matrix product simulations of one-dimensional $X X Z$ and anisotropic $X Y$ models, we argue that finite-order quantum phase transitions can be determined by singularities of the time correlations or their derivatives at criticality. The same features are exhibited by corresponding Leggett-Garg functions, which noticeably indicate violation of the Leggett-Garg inequalities for early times and all the Hamiltonian parameters considered. In addition, we find that the infinite-order transition of the $X X Z$ model at the isotropic point can be revealed by the maximal violation of the Leggett-Garg inequalities. We thus show that quantum phase transitions can be identified by purely local measurements, and that many-body systems constitute important candidates to observe experimentally the violation of Leggett-Garg inequalities.Comment: Minor changes, 11 pages, 11 figures. Final version published in Phys. Rev.

Quantum Hysteresis in Coupled Light-Matter Systems

We investigate the non-equilibrium quantum dynamics of a canonical light-matter system, namely the Dicke model, when the light-matter interaction is ramped up and down through a cycle across the quantum phase transition. Our calculations reveal a rich set of dynamical behaviors determined by the cycle times, ranging from the slow, near adiabatic regime through to the fast, sudden quench regime. As the cycle time decreases, we uncover a crossover from an oscillatory exchange of quantum information between light and matter that approaches a reversible adiabatic process, to a dispersive regime that generates large values of light-matter entanglement. The phenomena uncovered in this work have implications in quantum control, quantum interferometry, as well as in quantum information theory.Comment: 9 pages and 4 figure

Neural Network Local Navigation of Mobile Robots in a Moving Obstacles Environment

IF AC Intelligent Components and Instruments for Control Applications, Budapest, Hungary, 1994This paper presents a local navigation method based on generalized predictive control. A modified cost function to avoid moving and static obstacles is presented. An Extended Kaiman Filter is proposed to predict the motions of the obstacles. A Neural Network implementation of this method is analysed. Simulation results are shown.Ministerio de Ciencia y Tecnología TAP93-0408Ministerio de Ciencia y Tecnología TAP93-058

Ideal codes over separable ring extensions

This paper investigates the application of the theoretical algebraic notion of a separable ring extension, in the realm of cyclic convolutional codes or, more generally, ideal codes. We work under very mild conditions, that cover all previously known as well as new non trivial examples. It is proved that ideal codes are direct summands as left ideals of the underlying non-commutative algebra, in analogy with cyclic block codes. This implies, in particular, that they are generated by an idempotent element. Hence, by using a suitable separability element, we design an efficient algorithm for computing one of such idempotents