The Exciting Lives of Giant Molecular Clouds

We present a detailed study of the evolution of GMCs in a galactic disc simulation. We follow individual GMCs (defined in our simulations by a total column density criterion), including their level of star formation, from their formation to dispersal. We find the evolution of GMCs is highly complex. GMCs often form from a combination of smaller clouds and ambient ISM, and similarly disperse by splitting into a number of smaller clouds and ambient ISM. However some clouds emerge as the result of the disruption of a more massive GMC, rather than from the assembly of smaller clouds. Likewise in some cases, clouds accrete onto more massive clouds rather than disperse. Because of the difficulty of determining a precursor or successor of a given GMC, determining GMC histories and lifetimes is highly non-trivial. Using a definition relating to the continuous evolution of a cloud, we obtain lifetimes typically of 4-25 Myr for >10^5 M$_{\odot}$ GMCs, over which time the star formation efficiency is about 1 %. We also relate the lifetime of GMCs to their crossing time. We find that the crossing time is a reasonable measure of the actual lifetime of the cloud, although there is considerable scatter. The scatter is found to be unavoidable because of the complex and varied shapes and dynamics of the clouds. We study cloud dispersal in detail and find both stellar feedback and shear contribute to cloud disruption. We also demonstrate that GMCs do not behave as ridge clouds, rather massive spiral arm GMCs evolve into smaller clouds in inter-arm spurs.Comment: 15 pages, 16 figures, accepted for publication in MNRA

Giant Molecular clouds: what are they made from, and how do they get there?

We analyse the results of four simulations of isolated galaxies: two with a rigid spiral potential of fixed pattern speed, but with different degrees of star-formation induced feedback, one with an axisymmetric galactic potential and one with a `live' self-gravitating stellar component. Since we use a Lagrangian method we are able to select gas that lies within giant molecular clouds (GMCs) at a particular timeframe, and to then study the properties of this gas at earlier and later times. We find that gas which forms GMCs is not typical of the interstellar medium at least 50 Myr before the clouds form and reaches mean densities within an order of magnitude of mean cloud densities by around 10 Myr before. The gas in GMCs takes at least 50 Myr to return to typical ISM gas after dispersal by stellar feedback, and in some cases the gas is never fully recycled. We also present a study of the two-dimensional, vertically-averaged velocity fields within the ISM. We show that the velocity fields corresponding to the shortest timescales (that is, those timescales closest to the immediate formation and dissipation of the clouds) can be readily understood in terms of the various cloud formation and dissipation mechanisms. Properties of the flow patterns can be used to distinguish the processes which drive converging flows (e.g.\ spiral shocks, supernovae) and thus molecular cloud formation, and we note that such properties may be detectable with future observations of nearby galaxies.Comment: 13 pages, 8 figures, accepted for publication in MNRA

Diagrammatic perturbation theory and the pseudogap

We study a model of quasiparticles on a two-dimensional square lattice coupled to Gaussian distributed dynamical fields. The model describes quasiparticles coupled to spin or charge fluctuations and is solved by a Monte Carlo sampling of the molecular field distributions. The non-perturbative solution is compared to various approximations based on diagrammatic perturbation theory. When the molecular field correlations are sufficiently weak, the diagrammatic calculations capture the qualitative aspects of the quasiparticle spectrum. For a range of model parameters near the magnetic boundary, we find that the quasiparticle spectrum is qualitatively different from that of a Fermi liquid in that it shows a double peak structure, and that the diagrammatic approximations we consider fail to reproduce, even qualitatively, the results of the Monte Carlo calculations. This suggests that the pseudogap induced by a coupling to antiferromagnetic fluctuations and the spin-splitting of the quasiparticle peak induced by a coupling to ferromagnetic spin-fluctuations lie beyond diagrammatic perturbation theory

Mecidea longula Stål (Heteroptera: Pentatomidae: Pentatominae: Mecideini) is established in south Florida

A Caribbean species of Mecidea Dallas, M. longula Stål, apparently established in south Florida, is reported from the United States for the first time. Specimens were first collected in February 2008 in a light trap operated in Miami-Dade County, Florida. Collections in that trap have continued through the present. Searches near the trap location resulted in several specimens being taken from smutgrass, Sporobolus indicus (L.), an exotic grass now established throughout much of the southeastern United States. The three North American species of Mecidea are keyed and illustrated. In addition to the Florida locality, M. longula is reported for the first time from the British Virgin Islands, St. Kitts, St. Martin, and the Turks and Caicos Islands

Age distributions of star clusters in spiral and barred galaxies as a test for theories of spiral structure

We consider models of gas flow in spiral galaxies in which the spiral structure has been excited by various possible mechanisms: a global steady density wave, self-gravity of the stellar disc and an external tidal interaction, as well as the case of a galaxy with a central rotating bar. In each model we estimate in a simple manner the likely current positions of star clusters of a variety of ages, ranging from ~ 2 Myr to around 130 Myr, depending on the model. We find that the spatial distribution of cluster of different ages varies markedly depending on the model, and propose that observations of the locations of age-dated stellar clusters is a possible discriminant between excitation mechanisms for spiral structure in an individual galaxy.Comment: 10 pages, 4 figures, accepted for publication in MNRA

LOCV calculations for polarized liquid 3He^3{He} with the spin-dependent correlation

We have used the lowest order constrained variational (LOCV) method to calculate some ground state properties of polarized liquid $^{3}He$ at zero temperature with the spin-dependent correlation function employing the Lennard-Jones and Aziz pair potentials. We have seen that the total energy of polarized liquid $^{3}He$ increases by increasing polarization. For all polarizations, it is shown that the total energy in the spin-dependent case is lower than the spin-independent case. We have seen that the difference between the energies of spin-dependent and spin-independent cases decreases by increasing polarization. We have shown that the main contribution of the potential energy comes from the spin-triplet state.Comment: 14 pages, 5 figures. Int. J. Mod. Phys. B (2008) in pres

Why are most molecular clouds not gravitationally bound?

The most recent observational evidence seems to indicate that giant molecular clouds are predominantly gravitationally unbound objects. In this paper we show that this is a natural consequence of a scenario in which cloud-cloud collisions and stellar feedback regulate the internal velocity dispersion of the gas, and so prevent global gravitational forces from becoming dominant. Thus, while the molecular gas is for the most part gravitationally unbound, local regions within the denser parts of the gas (within the clouds) do become bound and are able to form stars. We find that the observations, in terms of distributions of virial parameters and cloud structures, can be well modelled provided that the star formation efficiency in these bound regions is of order 5 - 10 percent. We also find that in this picture the constituent gas of individual molecular clouds changes over relatively short time scales, typically a few Myr.Comment: 9 pages, 8 figures, accepted for publication in MNRA

A technique for automatic real time scoring of several simultaneous sleep electroencephalograms

Automatic real-time scoring of simultaneous sleep electroencephalogram

Vacuum Energy: Myths and Reality

We discuss the main myths related to the vacuum energy and cosmological constant, such as: ``unbearable lightness of space-time''; the dominating contribution of zero point energy of quantum fields to the vacuum energy; non-zero vacuum energy of the false vacuum; dependence of the vacuum energy on the overall shift of energy; the absolute value of energy only has significance for gravity; the vacuum energy depends on the vacuum content; cosmological constant changes after the phase transition; zero-point energy of the vacuum between the plates in Casimir effect must gravitate, that is why the zero-point energy in the vacuum outside the plates must also gravitate; etc. All these and some other conjectures appear to be wrong when one considers the thermodynamics of the ground state of the quantum many-body system, which mimics macroscopic thermodynamics of quantum vacuum. In particular, in spite of the ultraviolet divergence of the zero-point energy, the natural value of the vacuum energy is comparable with the observed dark energy. That is why the vacuum energy is the plausible candidate for the dark energy.Comment: 24 pages, 2 figures, submitted to the special issue of Int. J. Mod. Phys. devoted to dark energy and dark matter, IJMP styl