Evidence for quasi-chemically homogeneous evolution of massive stars up to solar metallicity

Long soft gamma ray bursts (LGRBs) are usually associated with the death of the most massive stars. A large amount of core angular momentum in the phases preceding the explosion is required to form LGRBs. A very high initial rotational velocity can provide this angular momentum. Such a velocity strongly influences the way the star evolves: it is chemically homogeneously mixed and evolves directly towards the blue part of the HR diagram from the main sequence. We have shown that chemically homogeneous evolution (CHE) takes place in the SMC, at low metallicity. We want to see if there is a metallicity threshold above which such an evolution does not exist. We perform a spectroscopic analysis of H-rich early-type WN stars in the LMC and the Galaxy. We use the code CMFGEN to determine the fundamental properties and the surface composition of the target stars. We then place the stars in the HR diagram and determine their evolution. We show that both the LMC and Galactic WNh stars we selected cannot be explained by standard stellar evolution. They are located on the left of the main sequence but show surface abundances typical of CN equilibrium. In addition, they still contain a large amount of hydrogen. They are thus core-H burning objects. Their properties are consistent with CHE. We determine the metallicity of the Galactic stars from their position and Galactic metallicity gradients, and conclude that they have 0.6<Z<1.0. A moderate coupling between the core and the envelope is required to explain that stellar winds do not extract to much angular momentum to prevent a blueward evolution. In view of the findings that some long gamma ray bursts appear in solar environments, CHE may be a viable way to form them over a wide range of metallicities.Comment: 10 pages, 10 figures. Accepted in Astronomy and Astrophysic

Controlled Shock Shells and Intracluster Fusion Reactions in the Explosion of Large Clusters

The ion phase-space dynamics in the Coulomb explosion of very large ($\sim 10^6 - 10^7$ atoms) deuterium clusters can be tailored using two consecutive laser pulses with different intensities and an appropriate time delay. For suitable sets of laser parameters (intensities and delay), large-scale shock shells form during the explosion, thus highly increasing the probability of fusion reactions within the single exploding clusters. In order to analyze the ion dynamics and evaluate the intracluster reaction rate, a one-dimensional theory is used, which approximately accounts for the electron expulsion from the clusters. It is found that, for very large clusters (initial radius $\sim$ 100 nm), and optimal laser parameters, the intracluster fusion yield becomes comparable to the intercluster fusion yield. The validity of the results is confirmed with three-dimensional particle-in-cell simulations.Comment: 25 pages, 11 figures, to appear in Physical Review

Bootstrap tests for time varying cointegration

This article proposes wild and the independent and identically distibuted (i.i.d.) parametric bootstrap implementations of the time-varying cointegration test of Bierens and Martins (2010). The bootstrap statistics and the original likelihood ratio test share the same first-order asymptotic null distribution. Monte Carlo results suggest that the bootstrap approximation to the finite-sample distribution is very accurate, in particular for the wild bootstrap case. The tests are applied to study the purchasing power parity hypothesis for twelve Organisation for Economic Cooperation and Development (OECD) countries and we only find evidence of a constant long-term equilibrium for the U.S.-U.K. relationship.info:eu-repo/semantics/acceptedVersio

Introducing programming to basic schools students using robotics

The present work reports on the development of programming activities with students from the 1st and 2nd cycles of schools in the town of Braga in the northwestern Portuguese region of Minho. These activities of promotion of computer programming were applied in order to promote the programming and innovative practices on science and technology education. The explored interdisciplinary methodologies in STEM teaching-learning processes, stimulate critical thinking and creativity while promoting the benefits of learning in collaborative environments. The active involvement of the students in these robot programming, “high tech” and trendy, activities is easy to achieve if the proposed challenges are set at an adequate level of difficulty and appealing enough to the age group and level of cognitive development of the student. Whenever possible to the students is given the possibility of choosing or even defining the problem/subject they will be exploring by programming a robot, which is seen as a mechanical artificial being the students will be able to understand, interact with and use and control. The teacher/educator should be available to provide to the students a proper empowering environment and to provide all support requested by the students giving, as much as possible, not straight answers but yes clues and small hints and examples leading the students to reach, themselves, to a solution to the problem the students face or to an answer to the students’ question that satisfy their own critical judgment. Through the programming testing process, it is possible to verify and see the level of perception and proficiency of the students assessing what students have learned and accomplished, creating immediate feedback for students and adjusting or re-orienting the students’ focus on a particular task or reasoning process. If well succeeded these activities can develop among the students a sound appreaciation towards Science Technology and Engineering while establishing relevant knowledge, creativity critical reasoning abilities and a large number of other competencies that will be valuable for the future development of the students in their studies and academic life but also in their future careers. The improvement of the self-esteem of the students when they realize they can actually “do it” is also a major benefit of this type of activities. As well in what concerns the boost of the self-esteem and selft-appreaciation of their teachers and educators, that often fear to explore this type of innovative approaches

A new mechanism for anticipating price exuberance

It is very important for investors, market regulators, and policy makers to possess a trustworthy ex-ante tool capable of anticipating price exuberance events. This paper proposes a new statistical mechanism to predict speculative bubbles by inferring a significant probability of exuberance at least one step ahead of a bubble peak period. Contrary to other approaches, we combine asset pricing modeling and non-stationarity statistical analysis and use both in the context of adaptive learning to build a dynamic model specification. Monte Carlo simulations show that the ex-ante prediction is improved enormously by adding the estimated abnormal returns into the model. In some cases our mechanism predicts 100% of the last bubbles of the sample up to five periods before the peak. Furthermore, the mechanism is able to successfully anticipate the technological bubble observed in the 1990’s by estimating a probability greater than 90%, one month before the bubble peak. Thus, this new mechanism provides an advantage for investors interested in performing a very profitable “bubble surfing” strategy and for market regulators whose responsibility is to maintain market efficiency.info:eu-repo/semantics/publishedVersio

HST's view of the youngest massive stars in the Magellanic Clouds

Accurate physical parameters of newborn massive stars are essential ingredients to shed light on their formation, which is still an unsolved problem. The rare class of compact H II regions in the Magellanic Clouds (MCs), termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter) harbor the youngest massive stars of the OB association/molecular cloud complexes in the MCs accessible through high-resolution near-IR and optical techniques. We present a brief overview of the results obtained with HST mainly on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).Comment: 5 pages, to appear in the Proceedings of the 41st ESLAB Symposium "The Impact of HST on European Astronomy", 29 May to 1 June 2007, ESTEC, Noordwijk, Netherlands; eds. Guido De Marchi and Duccio Macchett