The effect of the dynamical state of clusters on gas expulsion and infant mortality

The star formation efficiency (SFE) of a star cluster is thought to be the critical factor in determining if the cluster can survive for a significant (>50 Myr) time. There is an often quoted critical SFE of ~30 per cent for a cluster to survive gas expulsion. I reiterate that the SFE is not the critical factor, rather it is the dynamical state of the stars (as measured by their virial ratio) immediately before gas expulsion that is the critical factor. If the stars in a star cluster are born in an even slightly cold dynamical state then the survivability of a cluster can be greatly increased.Comment: 6 pages, 2 figures. Review talk given at the meeting on "Young massive star clusters - Initial conditions and environments", E. Perez, R. de Grijs, R. M. Gonzalez Delgado, eds., Granada (Spain), September 2007, Springer: Dordrecht. Replacement to correct mistake in a referenc

Numerical evidence for global bifurcations leading to switching phenomena in long Josephson junctions

Fluxons in long Josephson junctions are physical manifestations of travelling waves that connect rest states of the model partial differential equation (p.d.e.), which is a perturbed sine-Gordon equation. In the reduced traavelling wave ordinary differential equation (o.d.e.), fluxons correspond to heteroclinic connections between fixed points. In the absence of surface impedence effects, fluxons persist in parameter regimes until the fixed points disappear, after which the system ‘switches’ to another configuration. It is known that the presence of surface impedence produces a singular perturbation of the model equation, together with a new phenomenon: the fluxons switch in parameter regimes before the fixed points are lost. Why this occurs is unknown, and is the focus of this paper. Two disjoint possibilities are: (1) instability: fluxons still exist, but they become unstable in the p.d.e. due to surface impedance effects; (2) nonexistence: the fluxons fail to exist, even though the fixed points remain. Here, we provide compelling numerical evidence for the second scenario, characterized by a global bifurcation in the travelling wave phase space: a breakdown of heteroclinicorbits, undetected at the local linearized level. Moreover, this global o.d.e. bifurcation occurs at parameter values consistent with the p.d.e. switching phenomenon

The uncertain climate footprint of wetlands under human pressure

Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and managed wetlands, and cover a wide range of climatic regions, ecosystem types and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e. several centuries), typically offset by CO2 uptake, though with large spatio-temporal variability. Using a space-for-time analogy across ecological and climatic gradients we represent the chronosequence from natural to managed conditions in order to quantify the "cost" of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse-response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new IPCC guidelines accounting for both sustained CH4 emissions and cumulative CO2 exchange.JRC.H.7-Climate Risk Managemen

The uncertain climate footprint of wetlands under human pressure

Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence fro