Efficient ortho-para conversion of H2 on interstellar grain surfaces

Context: Fast surface conversion between ortho- and para-H2 has been observed in laboratory studies, and this mechanism has been proposed to play a role in the control of the ortho-para ratio in the interstellar medium. Observations of rotational lines of H2 in Photo-Dissociation Regions (PDRs) have indeed found significantly lower ortho-para ratios than expected at equilibrium. The mechanisms controlling the balance of the ortho-para ratio in the interstellar medium thus remain incompletely understood, while this ratio can affect the thermodynamical properties of the gas (equation of state, cooling function). Aims: We aim to build an accurate model of ortho-para conversion on dust surfaces based on the most recent experimental and theoretical results, and to validate it by comparison to observations of H2 rotational lines in PDRs. Methods: We propose a statistical model of ortho-para conversion on dust grains with fluctuating dust temperatures, based on a master equation approach. This computation is then coupled to full PDR models and compared to PDR observations. Results: We show that the observations of rotational H2 lines indicate a high conversion efficiency on dust grains, and that this high efficiency can be accounted for if taking dust temperature fluctuations into account with our statistical model of surface conversion. Simpler models neglecting the dust temperature fluctuations do not reach the high efficiency deduced from the observations. Moreover, this high efficiency induced by dust temperature fluctuations is quite insensitive to the values of microphysical parameters of the model. Conclusions: Ortho-para conversion on grains is thus an efficient mechanism in most astrophysical conditions that can play a significant role in controlling the ortho-para ratio.Comment: Accepted in Astronomy & Astrophysic

An Anatomy of the French Labour Market

[Excerpt] Over the last decades, many European countries have experienced high and persistent unemployment rates. The bulk of labour market research has tackled this issue by emphasizing the effect of employment protection legislation, hereafter EPL, on labour market performance. As a result, the importance of labour market flexibility has been widely acknowledged. This view can be summarized by the expressed desire of the E.U. council to give member States incentives to “review and, where appropriate, reform overly restrictive elements in employment legislation that affect labour market dynamics [...] and to undertake other appropriate measures to promote a better balance between work and private life and between flexibility and security”. It is however striking that most of the reforms undertaken have contrasted sharply with this latter recommendation by favouring reforms at the margin. Those reforms have fostered two-tier systems, as the increase in labour market flexibility has taken place mainly through a series of marginal reforms that liberalized the use of fixed-term and/or non-standard employment contracts. Two-tier systems have promoted the emergence of dual employment protection which can be broadly defined as the coexistence of both long-term contracts, which benefit from stringent protection, and short-term contracts with little or no protection. It is often argued that this combination creates labour market segmentation, traps workers in a recurring sequence of frequent unemployment spells, favours unequal repartition of risk between workers and enhances inequalities. In particular, two-tier systems create excess labour turnover as they increase the incentives to create temporary rather than permanent jobs, reduce job destruction for stable jobs, but increase churning for temporary jobs. For instance in countries with stringent legal constraints on the termination of permanent jobs, such as France or Spain, it turns out that about 70 per cent to 90 per cent of entries into employment are in temporary jobs with very short duration (on average less than one month and a half in France). If excess labour turnover and its consequences are a concern for the economy as a whole, the dramatic spread of temporary jobs is even more a concern for young/less experienced workers as they are more likely to be negatively affected by the adverse effects of dual employment protection. The French labour market is no exception and has faced similar trends during the 1990s. Given the pervasiveness of temporary jobs on the labour market and their consequences on the society and economic outcomes, it is urgent to understand how two-tier systems shape the functioning of the labour market. This is the very purpose of the present report. After having described in details the salient features of the French dual labour market and having discussed the legislation at the root of French dualism, we review the different mechanisms through which dualism affect labour markets: labour market dynamics, wage inequality, human capital accumulation, job satisfaction, social integration and health. We consider whenever possible both theoretical insights and empirical evaluations. We finally conclude this report by providing possible directions to reform the labour market

Surface chemistry in the Interstellar Medium II. H2\mathrm{H}_2 formation on dust with random temperature fluctuations

The $\mathrm{H}_2$ formation on grains is known to be sensitive to dust temperature, which is also known to fluctuate for small grain sizes due to photon absorption. We aim at exploring the consequences of simultaneous fluctuations of the dust temperature and the adsorbed H-atom population on the $\mathrm{H}_2$ formation rate under the full range of astrophysically relevant UV intensities and gas conditions. The master equation approach is generalized to coupled fluctuations in both the grain's temperature and its surface population and solved numerically. The resolution can be simplified in the case of the Eley-Rideal mechanism, allowing a fast computation. For the Langmuir-Hinshelwood mechanism, it remains computationally expensive, and accurate approximations are constructed. We find the Langmuir-Hinshelwood mechanism to become an efficient formation mechanism in unshielded photon dominated region (PDR) edge conditions when taking those fluctuations into account, despite hot average dust temperatures. It reaches an importance comparable to the Eley-Rideal mechanism. However, we show that a simpler rate equation treatment gives qualitatively correct observable results in full cloud simulations under most astrophysically relevant conditions. Typical differences are a factor of 2-3 on the intensities of the $\mathrm{H}_2$ $v=0$ lines. We also find that rare fluctuations in cloud cores are sufficient to significantly reduce the formation efficiency. Our detailed analysis confirms that the usual approximations used in numerical models are adequate when interpreting observations, but a more sophisticated statistical analysis is required if one is interested in the details of surface processes.Comment: 21 pages, 28 figures, accepted in A&

Bankruptcy Risk, Product Market Competition and Horizontal Mergers

Debt, Bankruptcy, Horizontal Merger, Competition Policy, Oligopoly.

Thermoacoustic instability - a dynamical system and time domain analysis

This study focuses on the Rijke tube problem, which includes features relevant to the modeling of thermoacoustic coupling in reactive flows: a compact acoustic source, an empirical model for the heat source, and nonlinearities. This thermo-acoustic system features a complex dynamical behavior. In order to synthesize accurate time-series, we tackle this problem from a numerical point-of-view, and start by proposing a dedicated solver designed for dealing with the underlying stiffness, in particular, the retarded time and the discontinuity at the location of the heat source. Stability analysis is performed on the limit of low-amplitude disturbances by means of the projection method proposed by Jarlebring (2008), which alleviates the linearization with respect to the retarded time. The results are then compared to the analytical solution of the undamped system, and to Galerkin projection methods commonly used in this setting. This analysis provides insight into the consequences of the various assumptions and simplifications that justify the use of Galerkin expansions based on the eigenmodes of the unheated resonator. We illustrate that due to the presence of a discontinuity in the spatial domain, the eigenmodes in the heated case, predicted by using Galerkin expansion, show spurious oscillations resulting from the Gibbs phenomenon. By comparing the modes of the linear to that of the nonlinear regime, we are able to illustrate the mean-flow modulation and frequency switching. Finally, time-series in the fully nonlinear regime, where a limit cycle is established, are analyzed and dominant modes are extracted. The analysis of the saturated limit cycles shows the presence of higher frequency modes, which are linearly stable but become significant through nonlinear growth of the signal. This bimodal effect is not captured when the coupling between different frequencies is not accounted for.Comment: Submitted to Journal of Fluid Mechanic

Online VNF Scaling in Datacenters

Network Function Virtualization (NFV) is a promising technology that promises to significantly reduce the operational costs of network services by deploying virtualized network functions (VNFs) to commodity servers in place of dedicated hardware middleboxes. The VNFs are typically running on virtual machine instances in a cloud infrastructure, where the virtualization technology enables dynamic provisioning of VNF instances, to process the fluctuating traffic that needs to go through the network functions in a network service. In this paper, we target dynamic provisioning of enterprise network services - expressed as one or multiple service chains - in cloud datacenters, and design efficient online algorithms without requiring any information on future traffic rates. The key is to decide the number of instances of each VNF type to provision at each time, taking into consideration the server resource capacities and traffic rates between adjacent VNFs in a service chain. In the case of a single service chain, we discover an elegant structure of the problem and design an efficient randomized algorithm achieving a e/(e-1) competitive ratio. For multiple concurrent service chains, an online heuristic algorithm is proposed, which is O(1)-competitive. We demonstrate the effectiveness of our algorithms using solid theoretical analysis and trace-driven simulations.Comment: 9 pages, 4 figure

HI-to-H2 Transitions in the Perseus Molecular Cloud

We use the Sternberg et al. (2014) theory for interstellar atomic to molecular (HI-to-H$_2$) conversion to analyze HI-to-H$_2$ transitions in five (low-mass) star-forming and dark regions in the Perseus molecular cloud, B1, B1E, B5, IC348, and NGC1333. The observed HI mass surface densities of 6.3 to 9.2 M$_{\odot}$ pc$^{-2}$ are consistent with HI-to-H$_2$ transitions dominated by HI-dust shielding in predominantly atomic envelopes. For each source, we constrain the dimensionless parameter $\alpha G$, and the ratio $I_{\rm UV}/n$, of the FUV intensity to hydrogen gas density. We find $\alpha G$ values from 5.0 to 26.1, implying characteristic atomic hydrogen densities 11.8 to 1.8 cm$^{-3}$, for $I_{\rm UV} \approx 1$ appropriate for Perseus. Our analysis implies that the dusty HI shielding layers are probably multiphased, with thermally unstable UNM gas in addition to cold CNM within the 21 cm kinematic radius.Comment: 5 pages, 2 Figures. Minor improvements suggested by the referee. Accepted for publication in the Astrophysical Journa

LA CONSTRUCTION DE LA COMPTABILITE CARBONE : HISTOIRE, USAGES ET PERSPECTIVES

National audienceAfin d'intégrer de nouvelles problématiques sociétales et environnementales dans l'activité des organisations et rendre compte de dimensions jusque-là invisibles, de nouvelles formes de comptabilité sont expérimentées, comme l'illustre l'exemple de la comptabilité carbone. Pourtant, la complexité technique de ce sujet tend à dissimuler les enjeux managériaux et économiques sous-jacents. Que fait une entreprise lorsqu'elle cherche à comptabiliser le carbone ? Pour répondre à cette question, nous adoptons une posture réflexive en déconstruisant la notion de comptabilité carbone, à savoir ses fondements, ses usages dans les entreprises ainsi que les apprentissages et obstacles identifiés dans l'élaboration de conventions de calcul. Cette recherche interroge notamment les effets structurants de la comptabilité et de façon plus inédite, du processus de construction de cette comptabilité pour tâcher de comprendre « ce que comptabiliser le carbone veut dire », à la fois en termes d'action collective que de responsabilité des entreprises. Abstract : To take into account environmental issues, new accounting projects are created such as carbon accounting. However, the technical complexity of this project tends to hide the issues at stake. In fact, what does it mean for a corporation to account for carbon ? In this study, we adopt a reflexive position to answer this question. To do that, we deconstruct the « carbon accounting » concept, both its foundations and uses. Hence we study the structuring effects of a specific form of accounting and of the his creation process in a previously unseen way

Modélisation du milieu interstellaire sur la Grille

International audienceModélisation du milieu interstellaire sur la Grill

Studying performation: the arrangement of speech, calculation and writing acts within dispositifs: Carbon accounting for strategizing in a large corporation

International audienceThis paper aims at proposing an analytical framework for performation process that is performation through speech, calculation and writing acts connected within a “dispositif”. This analytical framework is put into practice in the case study of a French large corporation which has built a low-carbon strategy based on carbon accounting tools. We have found that low-carbon strategy is performed through carbon accounting tools since speech, calculation and writing acts are iterated and connected within a broader “dispositif” The theoretical contribution is first to suggest an analytical framework of the process by which « felicitous conditions » of a performative are realized, that is to say, its performation. The second theoretical contribution consists in linking the communicative school of performativity with management research focusing on instruments and strategic “dispositifs”. Finally, the empirical contribution involves an in-depth analysis of the low-carbon strategy’s implementation based on a carbon accounting tool in a French large company