Local Ignition in Carbon/Oxygen White Dwarfs -- I: One-zone Ignition and Spherical Shock Ignition of Detonations

The details of ignition of Type Ia supernovae remain fuzzy, despite the importance of this input for any large-scale model of the final explosion. Here, we begin a process of understanding the ignition of these hotspots by examining the burning of one zone of material, and then investigate the ignition of a detonation due to rapid heating at single point. We numerically measure the ignition delay time for onset of burning in mixtures of degenerate material and provide fitting formula for conditions of relevance in the Type Ia problem. Using the neon abundance as a proxy for the white dwarf metallicity, we then find that ignition times can decrease by ~20% with addition of even 5% of neon by mass. When temperature fluctuations that successfully kindle a region are very rare, such a reduction in ignition time can increase the probability of ignition by orders of magnitude. If the neon comes largely at the expense of carbon, a similar increase in the ignition time can occur. We then consider the ignition of a detonation by an explosive energy input in one localized zone, eg a Sedov blast wave leading to a shock-ignited detonation. Building on previous work on curved detonations, we find that surprisingly large inputs of energy are required to successfully launch a detonation, leading to required matchheads of ~4500 detonation thicknesses - tens of centimeters to hundreds of meters - which is orders of magnitude larger than naive considerations might suggest. This is a very difficult constraint to meet for some pictures of a deflagration-to-detonation transition, such as a Zel'dovich gradient mechanism ignition in the distributed burning regime.Comment: 29 pages; accepted to ApJ. Comments welcome at http://www.cita.utoronto.ca/~ljdursi/thisweek/ . Updated version addressing referee comment

Sur l'origine de l'augmentation apparente des inondations en région méditerranéenne

En septembre 2002, les régions méditerranéennes françaises et notamment le département du Gard ont été affectées par des précipitations d'une extrême intensité. On estime que 80% de ce département a été inondé, on dénombre 23 victimes et les dégâts ont été évalués à 1.2 milliards d'euros. Cette catastrophe hydrologique soulève à nouveau les problèmes de la fréquence de ces événements et de l'augmentation des forts cumuls de pluie ces dernières années. L'objet de cet article est d'apporter quelques éléments de réponse, notamment à travers l'analyse régionale des pluies extrêmes journalières ayant affecté la région Languedoc-Roussillon de 1958 à 2002.La fréquence régionale des pluies extrêmes est estimée en prenant en compte la superficie couverte par ces événements en fonction des hauteurs pluviométriques. A l'échelle régionale la période de retour de l'événement varie entre 80 ans pour la superficie touchée par au moins 200 mm à 140 ans pour celle couverte par 300 mm.La stationnarité des fréquences des pluies extrêmes est analysée à partir des chroniques du nombre annuel d'événements pluvieux dépassant 200 mm, 250 mm et 300 mm en 24h maximum, entre 1958 et 2002 sur la région. Les tests de stationnarité ne révèlent pas de tendance significative à l'augmentation de ces fréquences. Les données historiques aboutissent aux mêmes conclusions. L'augmentation réelle des inondations est en fait principalement liée à l'augmentation de la vulnérabilité des bassins.In September 2002, the Gard department in the South of France was affected by heavy precipitation that covered a broad geographical area. It was estimated that 80% of the department was flooded; there were 23 victims and the damage was evaluated to be 1.2 billion euros. This hydrological catastrophe raised questions about a possible increase in the frequency of these events during recent years, since several other severe flooding events have been observed in the region over the last 15 years. The aim of this article is to explore these questions through a regional analysis of the extreme daily rainfall that affected the Languedoc-Roussillon region between 1958 and 2002. The daily rain data were used because they are the most available type of information over the observation period. Usually, the rainfall hazard description is based on statistical analysis of the maximum rainfall depth observed at a given rain gauge. However, because the spatial variability of rainfall in the Mediterranean region, such results are only representative of local rainfall conditions. Moreover, this type of analysis does not take into account the spatial coverage of the precipitation, which is another factor influencing the resulting floods. Thus, the regional frequency of extreme rainfall was estimated by taking into account the area covered according to a given rainfall depth. For each rainfall event, a rain field was built using a kriging interpolation (NEPPEL et al., 1997). The isohyet area defined a rainfall threshold from 10 to 300 mm with a step of 10 mm calculated for each rainfall event. For each rainfall depth from 10 to 300 mm with a step of 10 mm, the probability distribution of the isohyet area was estimated. The regional rainfall hazards were described with the Depth-Area-Frequency curves (DAF) for 24-h periods. It was shown that at on regional scale, the return period of the last event varied between 80 years for the surface affected by at least 200 mm and 140 years for the surface covered by 300 mm. Compared with other major events that have occurred in the region, it appears that the September 2002 event one was characterized by :1. the spatial extension of the heavy rainfall, for example more than 1800 km² were affected by at least 400 mm in less than 24 h;2. the spatial localisation of the heaviest rainfall depths, which were measured over the highest relief (1000 m to 1500 m) as usual in the 'cévenols' meteorological situation, but rather in the plain where the altitude lies between 200 m and 300 m.The stationnarity analysis of the extreme rainfall frequency was based on the annual number of events exceeding 200 mm, 250 mm and 300 mm over a 24 h maximum duration, between 1958 and 2002. The hypothesis of random events against the hypothesis of a trend or a sudden break in the mean was examined through several statistical tests. The procedures used were the rank correlation test, PETTITT's test, BUISHAND's test, HUBERT's segmentation procedure, a linear regression procedure, and the turning points procedure. Detailed descriptions of these tests can be found in KENDALL and STUART (1977), LUBES-NIEL et al. (1998) and WMO (2000). Except for the rank correlation test, all the procedures led to the conclusion that the three series are randomly distributed at the level of significance 1%, 5% and 10% respectively. Thus no significant increase in extreme rainfall frequency seems to appear. Although the study period was short, 45 years, compared with climatological variability, LUBES-NIEL et al. (1998) show that the procedures used were adapted in detecting trends in 50-yr time series. In considering historical rainfall data before 1958 in the same region, at least two extreme rainfall events could be compared with the event on 8-9 September 2002: in October 1940, 840 mm of rainfall were measured during 24 h in the Pyrénées-Orientales district and in September 1900, 940 mm were observed over 24 h in Valleraugue, upstream in the Herault catchment. Furthermore, if the evolution of the rain gauge network density is taken into account, one can argue that such an event could have occurred more frequently. Indeed, the number of rain gauges has varied from 162 gauges in 1900 to 330 today. It has been shown that the number of observed rainfall events varied according to the area of the events and the network density (NEPPEL et al., 1998b). For example, an event of 150 km2 (corresponding to the area covered by more than 600 mm in September 2002) had a probability of 70% to be observed by the network between 1958 and 1993. If one considers the period 1920-1939, this probability decreases to 30%.In addition, the basin vulnerability has increased. The regional population has grown from 1,460,000 inhabitants in 1949 to 2,300,000 in 2000. At the same time, urbanization has expanded widely. Moreover, this new population came from other districts, and they are not familiar with the Mediterranean rainfall regime and the resulting flash floods. Buildings have often been constructed near rivers, which are attractive building sites, and sometimes even in the river's main channel, increasing the flooding risk and the flood damages. Thus, rather than climate change, for which the effect on extreme rainfalls cannot be proved, the development of basin urbanisation and vulnerability could explain the apparent increase in floods. As the regional population is expected to reach more than 3,000,000 by 2030, it is necessary to take into account the flood risk in future urban planning

Differential Effects of Attention-, Compassion-, and Socio-Cognitively Based Mental Practices on Self-Reports of Mindfulness and Compassion

Research on the effects of mindfulness- and compassion-based interventions is flourishing along with self-report scales to assess facets of these broad concepts. However, debates remain as to which mental practices are most appropriate to develop the attentional, cognitive, and socio-affective facets of mindfulness and compassion. One crucial question is whether present-moment, attention-focused mindfulness practices are sufficient to induce a cascade of changes across the different proposed facets of mindfulness, including nonjudgmental acceptance, as well as compassion or whether explicit socio-affective training is required. Here, we address these questions in the context of a 9-month longitudinal study (the ReSource Project) by examining the differential effects of three different 3-month mental training modules on subscales of mindfulness and compassion questionnaires. The “Presence” module, which aimed at cultivating present-moment-focused attention and body awareness, led to increases in the observing, nonreacting, and presence subscales, but not to increases in acceptance or nonjudging. These latter facets benefitted from specific cultivation through the socio-cognitive “Perspective” module and socio-affective, compassion-based “Affect” module, respectively. These modules also led to further increases in scores on the subscales affected by the Presence module. Moreover, scores on the compassion scales were uniquely influenced by the Affect module. Thus, whereas a present-moment attention-focused training, as implemented in many mindfulness-based programs, was indeed able to increase attentional facets of mindfulness, only socio-cognitive and compassion-based practices led to broad changes in ethical-motivational qualities like a nonjudgmental attitude, compassion, and self-compassion

An empirical examination of the factor structure of compassion

Compassion has long been regarded as a core part of our humanity by contemplative traditions, and in recent years, it has received growing research interest. Following a recent review of existing conceptualisations, compassion has been defined as consisting of the following five elements: 1) recognising suffering, 2) understanding the universality of suffering in human experience, 3) feeling moved by the person suffering and emotionally connecting with their distress, 4) tolerating uncomfortable feelings aroused (e.g., fear, distress) so that we remain open to and accepting of the person suffering, and 5) acting or being motivated to act to alleviate suffering. As a prerequisite to developing a high quality compassion measure and furthering research in this field, the current study empirically investigated the factor structure of the five-element definition using a combination of existing and newly generated self-report items. This study consisted of three stages: a systematic consultation with experts to review items from existing self-report measures of compassion and generate additional items (Stage 1), exploratory factor analysis of items gathered from Stage 1 to identify the underlying structure of compassion (Stage 2), and confirmatory factor analysis to validate the identified factor structure (Stage 3). Findings showed preliminary empirical support for a five-factor structure of compassion consistent with the five-element definition. However, findings indicated that the ‘tolerating’ factor may be problematic and not a core aspect of compassion. This possibility requires further empirical testing. Limitations with items from included measures lead us to recommend against using these items collectively to assess compassion. Instead, we call for the development of a new self-report measure of compassion, using the five-element definition to guide item generation. We recommend including newly generated ‘tolerating’ items in the initial item pool, to determine whether or not factor-level issues are resolved once item-level issues are addressed

The lived experiences of experienced Vipassana Mahasi meditators: an interpretative phenomenological analysis.

Research into the effects and mechanisms of mindfulness training draws predominantly on quantitative research. There is a lack of understanding about the subjective experiences of experienced mindfulness meditators, which may provide additional insights into the effects, processes and context of mindfulness training. This qualitative study explored the lived experiences of a novel group of experienced mindfulness meditators who practise Vipassana Mahasi (VM) meditation. The study aimed to understand how experienced VM practitioners make sense of the effects of practice and what processes they ascribe to it. Participants attended semistructured interviews, and their responses were analysed using interpretative phenomenological analysis. Results yielded overarching themes including (a) improvements in hedonic and eudaimonic well-being; (b) insights into self, others and perception of reality; (c) attaining equanimity; and (d) physical and interpersonal difficulties. Participants perceived VM as a ‘cleansing’ process whereby maladaptive responses were eliminated through mindfulness, other supportive mental qualities, decentering and nonattachment. The findings revealed a complex and dynamic set of interdependent outcomes and processes, which are reinforced by Buddhist teachings and ethical practices. This study highlights the need for additional interdisciplinary research into topics such as insight generation and supportive mental qualities cultivated during VM, novel states of well-being informed by Buddhist constructs and interpersonal difficulties related to long-term practice. Findings also suggest that incorporating Buddhist teachings and ethics into mindfulness-based interventions may enhance practitioner understanding and implementation of meditation techniques.N/

Time perception and the experience of agency in meditation and hypnosis

Mindfulness meditation and hypnosis are related in opposing ways to awareness of intentions. The cold control theory of hypnosis proposes that hypnotic responding involves the experience of involuntariness while performing an actually intentional action. Hypnosis therefore relies upon inaccurate metacognition about intentional actions and experiences. Mindfulness meditation centrally involves awareness of intentions and is associated with improved metacognitive access to intentions. Therefore, mindfulness meditators and highly hypnotizable people may lie at opposite ends of a spectrum with regard to metacognitive access to intention‐related information. Here we review the theoretical background and evidence for differences in the metacognition of intentions in these groups, as revealed by chronometric measures of the awareness of voluntary action: the timing of an intention to move (Libet's “W” judgments) and the compressed perception of time between an intentional action and its outcome (“intentional binding”). We review these measures and critically evaluate their proposed connection to the experience of volition and sense of agency

Timescales of Multineuronal Activity Patterns Reflect Temporal Structure of Visual Stimuli

The investigation of distributed coding across multiple neurons in the cortex remains to this date a challenge. Our current understanding of collective encoding of information and the relevant timescales is still limited. Most results are restricted to disparate timescales, focused on either very fast, e.g., spike-synchrony, or slow timescales, e.g., firing rate. Here, we investigated systematically multineuronal activity patterns evolving on different timescales, spanning the whole range from spike-synchrony to mean firing rate. Using multi-electrode recordings from cat visual cortex, we show that cortical responses can be described as trajectories in a high-dimensional pattern space. Patterns evolve on a continuum of coexisting timescales that strongly relate to the temporal properties of stimuli. Timescales consistent with the time constants of neuronal membranes and fast synaptic transmission (5–20 ms) play a particularly salient role in encoding a large amount of stimulus-related information. Thus, to faithfully encode the properties of visual stimuli the brain engages multiple neurons into activity patterns evolving on multiple timescales

Molecular Insights into Reprogramming-Initiation Events Mediated by the OSKM Gene Regulatory Network

Somatic cells can be reprogrammed to induced pluripotent stem cells by over-expression of OCT4, SOX2, KLF4 and c-MYC (OSKM). With the aim of unveiling the early mechanisms underlying the induction of pluripotency, we have analyzed transcriptional profiles at 24, 48 and 72 hours post-transduction of OSKM into human foreskin fibroblasts. Experiments confirmed that upon viral transduction, the immediate response is innate immunity, which induces free radical generation, oxidative DNA damage, p53 activation, senescence, and apoptosis, ultimately leading to a reduction in the reprogramming efficiency. Conversely, nucleofection of OSKM plasmids does not elicit the same cellular stress, suggesting viral response as an early reprogramming roadblock. Additional initiation events include the activation of surface markers associated with pluripotency and the suppression of epithelial-to-mesenchymal transition. Furthermore, reconstruction of an OSKM interaction network highlights intermediate path nodes as candidates for improvement intervention. Overall, the results suggest three strategies to improve reprogramming efficiency employing: 1) anti-inflammatory modulation of innate immune response, 2) pre-selection of cells expressing pluripotency-associated surface antigens, 3) activation of specific interaction paths that amplify the pluripotency signal

CuBIC: cumulant based inference of higher-order correlations in massively parallel spike trains

Recent developments in electrophysiological and optical recording techniques enable the simultaneous observation of large numbers of neurons. A meaningful interpretation of the resulting multivariate data, however, presents a serious challenge. In particular, the estimation of higher-order correlations that characterize the cooperative dynamics of groups of neurons is impeded by the combinatorial explosion of the parameter space. The resulting requirements with respect to sample size and recording time has rendered the detection of coordinated neuronal groups exceedingly difficult. Here we describe a novel approach to infer higher-order correlations in massively parallel spike trains that is less susceptible to these problems. Based on the superimposed activity of all recorded neurons, the cumulant-based inference of higher-order correlations (CuBIC) presented here exploits the fact that the absence of higher-order correlations imposes also strong constraints on correlations of lower order. Thus, estimates of only few lower-order cumulants suffice to infer higher-order correlations in the population. As a consequence, CuBIC is much better compatible with the constraints of in vivo recordings than previous approaches, which is shown by a systematic analysis of its parameter dependence