Shock formation around planets orbiting M-dwarf stars

Bow shocks can be formed around planets due to their interaction with the coronal medium of the host stars. The net velocity of the particles impacting on the planet determines the orientation of the shock. At the Earth's orbit, the (mainly radial) solar wind is primarily responsible for the formation of a shock facing towards the Sun. However, for close-in planets that possess high Keplerian velocities and are frequently located at regions where the host star's wind is still accelerating, a shock may develop ahead of the planet. If the compressed material is able to absorb stellar radiation, then the signature of bow shocks may be observed during transits. Bow-shock models have been investigated in a series of papers (Vidotto et al. 2010, 2011,a,b; Llama et al. 2011) for known transiting systems. Once the signature of a bow-shock is observed, one can infer the magnetic field intensity of the transiting planet. Here, we investigate the potential to use this model to detect magnetic fields of (hypothetical) planets orbiting inside the habitable zone of M-dwarf stars. For these cases, we show, by means of radiative transfer simulations, that the detection of bow-shocks of planets surrounding M-dwarf stars may be more difficult than for the case of close-in giant planets orbiting solar-type stars.Comment: Published in Astronomische Nachrichten, Vol. 9-10/2011, page 1055-1061. 7 pages, 5 figure

Dating U.S. Business Cycles with Macro Factors

A probit model is used to show that latent common factors estimated by principal components from a large number of macroeconomic time series have important predictive power for NBER recession dates. A pseudo out-of-sample forecasting exercise shows that predicted recession probabilities consistently rise during subsequently declared NBER recession dates. The latent variable in the factor-augmented probit model is interpreted as an index of real business conditions which can be used to assess the strength of an expansion or the depth of a recession.business cycle; forecasting; factors; probit model; Bayesian methods

On the dependence of the spectral parameters on the observational conditions in homogeneous time dependent models of the TeV blazars

Most of current models of TeV blazars emission assume a Synchrotron Self-Compton mechanism where relativistic particles emit both synchrotron radiation and Inverse Compton photons. For sake of simplicity, these models usually consider only steady state emission. The spectral features are thus only related to the shape of the particle distribution, and do not depend on the timing of observations. In this letter, we study the effect of, firstly, the lag between the beginning of the injection of the fresh particles and the trigger of the observation, and secondly, of a finite injection duration. We illustrate these effects considering an analytical time-dependent model of the synchrotron emission by a monoenergetic distribution of leptons. We point out that the spectral shape can be in fact very dependent on observational conditions if the particle injection term is time-dependent, particularly taking into account the effect of the time averaging procedure on the final shape of the SED. Consequences on the acceleration process are also discussed.Comment: Letter to Editor, accepted for publication in A&

Migrants and mafia as global public goods

Global public goods, differently from what it might be thought, are quite common in the real world. This work suggests that both the governments' struggle against Mafia and the prevention of immigration can be regarded as global public goods. We assume a federation of jurisdictions with two tiers of Government: the central and the local. Regional utility directly represents the preferences of citizens, since the local governments aim at individualistic utility maximization; central government uses the redistribution of resources among the members of the federation to maximize the social welfare which is given, as usual, by the sum of regional utilities. The Central Government aims at welfare maximization. To get its goal it has to find out the efficient way to fund and provide public goods taking into account not only their particular characteristics but also the fact that, in many circumstances, their production faces increasing cost, which may depend both on the quantity of good produced and on the type (high or low cost) of the producer (which, in this framework, coincides with the jurisdiction). Thus the first issue addressed by the paper concerns the choice between central and local provision. Furthermore, as far as the informational structure is concerned, the centre lacks information concerning the type of each region. Thus, the central government's key informational problem concerns the regional costs and quantities with regard both to the public and the private good. Indeed we assume that the centre can observe the expenditure levels but neither the costs nor the outputs associated with those expenditure levels.global public good, asymmetric information, adverse selection, redistribution, Mafia

Nash behaviour and public good

In this note we analyse the provision of a pure public good with non constant production cost in the context of a federation of jurisdictions with two tiers of Government: the central and the local. The central government aims at welfare maximization but this objective is constrained to the use of lump sum transfer. Local governments aim at their own utility maximization and they behave according to the Nash rule. The production cost for the public good is affected by the jurisdiction's type (high or low) and by the quantity of the good that is produced. It is shown that a social welfare improvement might take place, in some circumstances, even without any central government intervention. On the other hand a first best is unreachable under the hypothesis of Nash behaviour and lump sum transfer among jurisdictions.public good, Nash equilibrium, lump sum transfer