On the formation of hot DQ white dwarfs

We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white dwarfs. These calculations consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Our calculations provide strong support to the diffusive/convective-mixing picture for the formation of hot DQs. We find that the hot DQ stage is a short-lived stage and that the range of effective temperatures where hot DQ stars are found can be accounted for by different masses of residual helium and/or different initial stellar masses. In the frame of this scenario, a correlation between the effective temperature and the surface carbon abundance in DQs should be expected, with the largest carbon abundances expected in the hottest DQs. From our calculations, we suggest that most of the hot DQs could be the cooler descendants of some PG1159 stars characterized by He-rich envelopes markedly smaller than those predicted by the standard theory of stellar evolution. At least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an evolutionary link between this star and the massive PG1159 star H1504+65 is plausible.Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal Letter

L’efficacité des programmes de prévention de la toxicomanie axés sur les familles

Cette recension des écrits examine l'efficacité de programmes de prévention des toxicomanies auprès des familles. De nombreux travaux soulignent le rôle déterminant des familles dans la transmission des habitudes de consommation. De plus, selon plusieurs intervenants, la famille constitue un milieu propice pour l'instauration de tels programmes de prévention. Cet article propose une analyse critique de ces programmes. Sont présentés les facteurs de risque liés à l'abus de substances chez les jeunes, puis les programmes visant la famille dont la stratégie est l'information, ceux qui font appel à des stratégies multiples et ceux qui visent les parents les plus à risque. Une critique méthodologique des études évaluatives est présentée. Les auteurs concluent par des recommandations.This literature review examines the efficiency of drug addiction prevention programs centered on families. Many studies underline the determining role of families in the transmission of consumption habits. Moreover, according to many intervenors, the family constitutes a favourable milieu to institute these prevention programs. This article proposes a critical analysis of these programs. The risk factors related to substance abuse with youth are first presented followed by information strategy programs aimed at families, multiple-strategy programs as well as programs aimed at parents who are considered at high risk. A methodological review of evaluative studies is presented. The authors conclude with recommendations.Este censo de escritos examinarâ la eficacia de los programas de prevenciôn con las familias, de la toxicomania. Numerosos trabajos subrayan el papel déterminante de las familias en la transmisiôn de hâbitos de consumo. Ademâs, segun varios trabajadores, la familias constituye un medio propicio para la instauration de taies programas de prevenciôn. Este artïculo propone un anâlisis crîtico de esos programas. En principio se presentan los factores de reisgo ligados al abuso de sustancias en los jôvenes y después los programas hacia las familias cuya estrategia es la information, aquellos que Uaman a estrategias multiples y aquellos dirigidos hacia los padres a riesgo. Una critica metodolôgica de los estudios es presentada. Los autores concluyen con recomendaciones

L’aménagement des berges, ravins et monts urbains dans la conurbation du Haut-Saguenay : vers un espace plus fonctionnel

La croissance généralisée des densités urbaines dans le Québec méridional, l'inadéquation observée entre les qualités réelles des sites urbanisés et la nature des tissus urbains, la diminution progressive de l'espace vital à l'intérieur des périmètres urbanisés, l'utilisation anarchique des corridors interurbains et la dépendance accrue des schémas d'aménagement vis-à-vis de la circulation automobile constituent quelques-uns des principaux problèmes retenus dans l'analyse spatiale des processus d'urbanisation au Québec.Cet article présente le cas de la « conurbation du Haut-Saguenay », la troisième plus importante région urbanisée du Québec après Québec et Montréal. Les auteurs essaient de mesurer le taux de dégradation de ce site urbain et de déterminer la nature des correctifs qui seront nécessaires à la création d'un milieu de vie plus humain.Some of the main elements which have to be analysed when transforming cities spatially or creating new urban bodies are: the expansion of urban densities in the Southern Québec regions, the discrepancies between the qualities of urbanized areas and the nature of urban organization and structure, the slow reduction of vital space within cities, the anarchical uses of interurban corridors and the growing influence of the automobile on urban planning.In this article, which is concerned with the case of the « conurbation du Haut-Saguenay », one of the most important regions of Québec, the authors attempt to measure the state of deterioration of this urban territory and also to determine what transformations will be necessary in order to create a more humanized environment

Pulsation in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology

We present some of the results of a survey aimed at exploring the asteroseismological potential of the newly-discovered carbon-atmosphere white dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere white dwarfs may drive low-order gravity modes. We demonstrate that our theoretical results are consistent with the recent exciting discovery of luminosity variations in SDSS J1426+5752 and some null results obtained by a team of scientists at McDonald Observatory. We also present follow-up photometric observations carried out by ourselves at the Mount Bigelow 1.6-m telescope using the new Mont4K camera. The results of follow-up spectroscopic observations at the MMT are also briefly reported, including the surprising discovery that SDSS J1426+5752 is not only a pulsating star but that it is also a magnetic white dwarf with a surface field near 1.2 MG. The discovery of $g$-mode pulsations in SDSS J1426+5752 is quite significant in itself as it opens a fourth asteroseismological "window", after the GW Vir, V777 Her, and ZZ Ceti families, through which one may study white dwarfs.Comment: 7 pages, 4 figures, to appear in Journal of Physics Conference Proceedings for the 16th European White Dwarf Worksho

SDSS J142625.71+575218.3: A Prototype for A New Class of Variable White Dwarf

We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. On the basis of our theoretical calculations, the star SDSS J142625.71 + 575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s ( first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.NSF AST-0507639, AST-0602288, AST-0607480, AST-0307321Astronom

Effect of Hydrodynamic Force on Microcantilever Vibrations: Applications to Liquid-Phase Chemical Sensing

At the microscale, cantilever vibrations depend not only on the microstructure’s properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantilever’s vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, and improve sensitivity in sensing applications, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include in-plane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. However, such systems require sampling (no in-situ measurements) and a relatively large sample volume (a few milliliters). Moreover, the frequency range is limited to low frequencies (less than 200Hz). To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range

Regional assessment of the multi-decadal changes in braided riverscapes following large floods (Example of 12 reaches in South East of France)

Abstract. The district of the France Rhône basin is characterised by several braided reaches, preserved from the widespread disappearing occurred in the 20th century. Even if the overall pattern is evolving through a general river narrowing, some reaches have shown to be still active and have widened. The preliminary results suggest that differences in width pattern could be related to several factors, such as high magnitude and low frequency floods, and geographical position in the catchment which influences bedload delivery conditions and vegetation recruitment related to climate. From an initial set of 53 braided reaches, we selected 12 sites, distributed into four main hydro-geographical regions. Reaches were selected to be representative of the overall study area. We analysed the braiding width pattern and the vegetation pattern dynamic among five observation periods dating from the 1950s to the 2000s. We hypothesised that a comparative analysis of a detailed temporal trajectory (i.e. five dates) of a set of rivers within several hydro-geographical contexts would allow us to better distinguish the relative role of floods (in terms of magnitude and duration) and other controlling factors acting at the regional scale. We showed that active channel width is controlled mainly by Q10 flood and secondarily by bedload availability whereas island pattern is in large part independent of flood series characters. Moreover a clear regional differentiation, constant over time, in terms of riverscape response is observed, mainly opposing south-western and south-eastern reaches. This opposition depends on several concurring factors, i.e. the flood characters, the river activity, the human influence and the climate. Finally, these findings allowed us to highlight those sectors in which the braided pattern could disappear, and those sectors in which the braided pattern is still active, because critical processes responsible of channel dynamic are still present

Influence of Fluid-Structure Interaction on Microcantilever Vibrations: Applications to Rheological Fluid Measurement and Chemical Detection

At the microscale, cantilever vibrations depend not only on the microstructure’s properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantilever’s vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include inplane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range

On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65

We explore different evolutionary scenarios to explain the helium deficiency observed in H1504+65, the most massive known PG1159 star. We concentrate mainly on the possibility that this star could be the result of mass loss shortly after the born-again and during the subsequent evolution through the [WCL] stage. This possibility is sustained by recent observational evidence of extensive mass-loss events in Sakurai's object and is in line with the recent finding that such mass losses give rise to PG1159 models with thin helium-rich envelopes and large rates of period change, as demanded by the pulsating star PG1159-035. We compute the post born again evolution of massive sequences by taking into account different mass-loss rate histories. Our results show that stationary winds during the post-born-again evolution fail to remove completely the helium-rich envelope so as to explain the helium deficiency observed in H1504+65. Stationary winds during the Sakurai and [WCL] stages only remove at most half of the envelope surviving the violent hydrogen burning during the born-again phase. In view of our results, the recently suggested evolutionary connection born-again stars --> H1504+65 --> white dwarfs with carbon-rich atmospheres is difficult to sustain unless the whole helium-rich envelope could be ejected by non-stationary mass-loss episodes during the Sakurai stage.Comment: 5 pages, 2 figures. To be published in Astronomy & Astrophysic

Microscopic Nuclear Level Densities from Fe to Ge by the Shell Model Monte Carlo Method

We calculate microscopically total and parity-projected level densities for $\beta$-stable even-even nuclei between Fe and Ge, using the shell model Monte Carlo methods in the complete $(pf+0g_{9/2})$-shell. A single-particle level density parameter $a$ and backshift parameter $\Delta$ are extracted by fitting the calculated densities to a backshifted Bethe formula, and their systematics are studied across the region. Shell effects are observed in $\Delta$ for nuclei with Z=28 or N=28 and in the behavior of $A/a$ as a function of the number of neutrons. We find a significant parity-dependence of the level densities for nuclei with A \alt 60, which diminishes as $A$ increases.Comment: to be published in Phys. Lett. B; includes 5 eps figure