Analytic Solutions for the Evolution of Radiative Supernova Remnants

We present the general analytic solution for the evolution of radiative supernova remnants in a uniform interstellar medium, under thin-shell approximation. This approximation is shown to be very accurate approach to this task. For a given set of parameters, our solution closely matches the results of numerical models, showing a transient in which the deceleration parameter reaches a maximum value of 0.33, followed by a slow convergence to the asymptotic value 2/7. Oort (1951) and McKee and Ostriker (1977) analytic solutions are discussed, as special cases of the general solution we have found.Comment: 5 pages, 1 figure, Astronomy and Astrophysics, accepte

Social Connections and Incentives: Evidence from Personnel Data

We present evidence on the effect of social connections between workers and managers on productivity in the workplace. To evaluate whether the existence of social connections is beneficial to the firm's overall performance, we explore how the effects of social connections vary with the strength of managerial incentives and worker's ability. To do so, we combine panel data on individual worker's productivity from personnel records with a natural field experiment in which we engineered an exogenous change in managerial incentives, from fixed wages to bonuses based on the average productivity of the workers managed. We find that when managers are paid fixed wages, they favor workers to whom they are socially connected irrespective of the worker's ability, but when they are paid performance bonuses, they target their effort toward high ability workers irrespective of whether they are socially connected to them or not. Although social connections increase the performance of connected workers, we find that favoring connected workers is detrimental for the firm's overall performance

Cooperation in effective action

The ability to cooperate in collective action problems — such as those relating to the use of common property resources or the provision of local public goods — is a key determinant of economic performance. In this paper we discuss two aspects of collective action problems in developing countries. First, which institutions discourage opportunistic behavior and promote cooperation? Second, what are the characteristics of the individuals involved that determine the degree to which they cooperate? We first review the evidence from field studies, laboratory experiments, and cross community studies. We then present new results from an individual level panel data set of rural workers

The evolution of cooperative norms: evidence from a natural field experiment

We document the establishment and evolution of a cooperative norm among workers using evidence from a natural field experiment on a leading UK farm. Workers are paid according to a relative incentive scheme under which increasing individual effort raises a worker's own pay but imposes a negative externality on the pay of all co-workers, thus creating a rationale for cooperation. As a counterfactual, we analyze worker behavior when workers are paid piece rates and thus have no incentive to cooperate. We find that workers cooperate more as their exposure to the relative incentive scheme increases. We also find that individual and group exposure are substitutes, namely workers who work alongside colleagues with higher exposure cooperate more. Shocks to the workforce in the form of new worker arrivals disrupt cooperation in the short term but are then quickly integrated into the norm. Individual exposure, group exposure, and the arrival of new workers have no effect on productivity when workers and paid piece rates and there is no incentive to cooperate

Social Incentives in the Workplace

We present evidence on social incentives in the workplace, namely on whether workers' behaviour is affected by the presence of those they are socially tied to, even in settings where there are no externalities among workers due to either the production technology or the compensation scheme in place. To do so, we combine data on individual worker productivity from a firm's personnel records with information on each worker's social network of friends in the firm. We find that compared to when she has no social ties with her co-workers, a given worker's productivity is significantly higher when she works alongside friends who are more able than her, and significantly lower when she works with friends who are less able than her. As workers are paid piece rates based on individual productivity, social incentives can be quantified in monetary terms and are such that (i) workers who are more able than their friends are willing to exert less effort and forgo 10% of their earnings; (ii) workers who have at least one friend who is more able than themselves are willing to increase their effort and hence productivity by 10%. The distribution of worker ability is such that the net effect of social incentives on the firm's aggregate performance is positive. The results suggest that firms can exploit social incentives as an alternative to monetary incentives to motivate workers

Social capital in the workplace: Evidence on its formation and consequences

The existence of social ties between co-workers affect many aspects of firm and worker behavior, such as how workers respond to a given set of incentives, the optimal compensation structures for workers at different tiers of the firm hierarchy, and the optimal organizational design for the firm. This paper presents evidence on the social capital in one particular firm, as embodied in the friendship ties among its workers. We describe the structure of the friendship network as a whole and present evidence on the determinants of social ties. Finally, we review evidence from a field experiment we conducted in the firm to highlight one particular mechanism through which social capital significantly affects worker performance. (C) 2007 Elsevier B.V. All rights reserved

Radio polarization maps of shell-type SNRs II. Sedov models with evolution of turbulent magnetic field

Polarized radio emission has been mapped with great detail in several Galactic supernova remnants (SNRs), but has not yet been exploited to the extent it deserves. We have developed a method to model maps of the Stokes parameters for shell-like SNRs during their Sedov evolution phase. At first, 3-dimensional structure of a SNR has been computed, by modeling the distribution of the magnetohydrodynamic parameters and of the accelerated particles. The generation and dissipation of the turbulent component of magnetic field everywhere in SNR are also considered taking into account its interaction with accelerated particles. Then, in order to model the emission, we have used a generalization of the classical synchrotron theory, valid for the case in which the magnetic field has ordered and disordered components. Finally, 2-dimensional projected maps have been derived, for different orientations of SNR and of interstellar magnetic field with respect to the observer. An important effect to consider is the Faraday rotation of the polarization planes inside the SNR interior. In this paper we present details of the model, and describe general properties of the images.Comment: accepted in MNRA

A Dynamical Model for the Evolution of a Pulsar Wind Nebula inside a Non-Radiative Supernova Remnant

A pulsar wind nebula inside a supernova remnant provides a unique insight into the properties of the central neutron star, the relativistic wind powered by its loss of rotational energy, its progenitor supernova, and the surrounding environment. In this paper, we present a new semi-analytic model for the evolution of such a pulsar wind nebula which couples the dynamical and radiative evolution of the pulsar wind nebulae, traces the evolution of the pulsar wind nebulae throughout the lifetime of the supernova remnant produced by the progenitor explosion, and predicts both the dynamical and radiative properties of the pulsar wind nebula during this period. We also discuss the expected evolution for a particular set of these parameters, and show it reproduces many puzzling features of known young and old pulsar wind nebulae. The model also predicts spectral features during different phases of its evolution detectable with new radio and gamma-ray observing facilities. Finally, this model has implications for determining if pulsar wind nebulae can explain the recent measurements of the cosmic ray positron fraction by PAMELA and the cosmic ray lepton spectrum by ATIC and HESS.Comment: To be submitted to the Astrophysical Journal. Figures are included as GIF files, and a version containing the high-resolution figures is available http://cosmo.nyu.edu/~jg168/pwn/ms.pd