Reflection on the Methods of Political Science on Both Sides of the Atlantic

For its Ninth Congress in Toulouse (5-7 September 2007), the French Political Science Association (AFSP) invited the American Political Science Association (APSA) to hold a joint “table-ronde”, comparing methods on both sides of the Atlantic. It took the form of three consecutive panels, devoted to qualitative and quantitative approaches, to the dimension of time and to contextual and inference problems. During three days, 18 papers were presented, over 60 participants attended, contrasting ways to validate theories and models were discussed at length, illustrated by concrete research examples. The objective here is less to sum up all that was said than to outline the main differences and convergences of our methodologies. (...

A semi-analytic model comparison - gas cooling and galaxy mergers

We use stripped-down versions of three semi-analytic galaxy formation models to study the influence of different assumptions about gas cooling and galaxy mergers. By running the three models on identical sets of merger trees extracted from high-resolution cosmological N-body simulations, we are able to perform both statistical analyses and halo-by-halo comparisons. Our study demonstrates that there is a good statistical agreement between the three models used here, when operating on the same merger trees, reflecting a general agreement in the underlying framework for semi-analytic models. We also show, however, that various assumptions that are commonly adopted to treat gas cooling and galaxy mergers can lead to significantly different results, at least in some regimes. In particular, we find that the different models adopted for gas cooling lead to similar results for mass scales comparable to that of our own Galaxy. Significant differences, however, arise at larger mass scales. These are largely (but not entirely) due to different treatments of the `rapid cooling' regime, and different assumptions about the hot gas distribution. At this mass regime, the predicted cooling rates can differ up to about one order of magnitude, with important implications on the relative weight that these models give to AGN feedback in order to counter-act excessive gas condensation in relatively massive haloes at low redshift. Different assumptions in the modelling of galaxy mergers can also result in significant differences in the timings of mergers, with important consequences for the formation and evolution of massive galaxies.Comment: 21 pages, 14 figures. Accepted for publication in MNRAS

Simulating Postbuckling Behaviour and Collapse of Stiffened CFRP Panels

Advanced composite materials are well known for their outstanding potential in weight-related stiffness and strength leading to an ever increasing share in aerospace structural components out of Carbon Fibre Reinforced Plastics (CFRP). In order to fully exploit the load-carrying capacity of such structures an accurate and reliable simulation is indispensable. Local buckling is not necessarily the load bearing limit for stiffened panels or shells; their full potential can be tapped only by utilizing the postbuckling region. That, however, requires fast tools which are capable of simulating the structural behaviour beyond bifurcation points including material degradation up to collapse. The most critical structural degradation mode is skin stringer separation; delamination, especially within the stringer, is a critical material degradation. A reliable prediction of collapse requires knowledge of degradation due to static as well as low cycle loading in the postbuckling region. Earlier projects have shown that it needs considerable experience in simulating the postbuckling behaviour. Though a great deal of knowledge about CFRP structural and material degradation is available its influence on collapse is not yet sufficiently investigated. It is the aim of the project COCOMAT (Improved MATerial exploitation at safe design of COmposite airframe structures by accurate simulation of COllapse) to develop means for and gain experience in fast and accurate simulation of the collapse load of stringer stiffened CFRP curved panels taking degradation and cyclic loading as well as geometric nonlinearity into account. COCOMAT is a Specific Targeted Research Project supported by the EU 6th Framework Programme; it started 2004 and runs for 4 years. Main deliverables are: • test results for buckling and collapse of undamaged and pre-damaged stiffened CFRP panels under static and cyclic loading, • improved material properties and degradation models, computational tools for design and certification of stiffened fibre composite panels which take postbuckling behaviour, degradation and collapse into account, • and finally design guidelines and industrial validation. The work will lead to an extended experimental data base, relevant degradation models and improved simulation tools for certification as well as for design. These results should allow setting up a future design scenario which exploits the existing reserves in primary fibre composite structures. The paper starts out from results provided by the forerunners of COCOMAT, describes the main objectives of the project, gives a general status of the progress reached so far and presents first results

Evolutionary Signatures In The Formation Of Low-Mass Protostars. II. Toward Reconciling Models And Observations

A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L(bol), T(bol), L(bol)/L(smm)) to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions; we include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T(bol) and L(bol)/L(smm)) can be considered reliable indicators of physical stage.NASA 1224608, 1288664, 1288658, RSA 1377304, NNX 07-AJ72GNSF AST0607793UT Austin University Continuing FellowshipAstronom

On the Origin of Exponential Galaxy Disks

We use a disk galaxy evolution model to investigate whether galaxies with exponential surface brightness profiles can be produced in a cosmologically motivated framework for disk galaxy formation. Our model follows the accretion, cooling, and ejection of baryonic mass, as a function of radius, inside growing dark matter haloes. The surface density profile of the disk is determined by detailed angular momentum conservation, starting from the distribution of specific angular momentum as found in cosmological simulations. Exponential and quasi-exponential disks can be produced by our model through a combination of supernova driven galactic outflows (which preferentially remove low angular momentum material), intrinsic variation in the angular momentum distribution of the halo gas, and the inefficiency of star formation at large radii. We use observations from the SDSS NYU-VAGC to show that the median Sersic index of blue galaxies is a strong function of stellar mass. For blue galaxies, low mass galaxies have a median n=1.3, while high mass galaxies have a median n=4. Our model with energy driven outflows correctly reproduces this trend, whereas our models with momentum driven outflows and no outflows over predict the Sersic indices in low mass galaxies. We show that the observed fraction of "bulge-less" exponential galaxies is a strong function of stellar mass. For Milky-Way mass galaxies less than 0.1% of blue galaxies are bulge-less, whereas for M33 mass galaxies bulge-less and quasi-bulgeless galaxies are typical. These results suggest that the difficulty of hierarchical formation models to produce bulge-less Milky-Way mass galaxies is in fact not a problem. However, the problem of producing M33 like galaxies remains, and will provide a key test for hierarchical galaxy formation models. [Abridged]Comment: 23 pages, 13 figures, accepted to MNRAS, two new figure

On the Origin of the Galaxy Star-Formation-Rate Sequence: Evolution and Scatter

We use a semi-analytic model for disk galaxies to explore the origin of the time evolution and small scatter of the galaxy SFR sequence -- the tight correlation between star-formation rate (SFR) and stellar mass (M_star). The steep decline of SFR from z~2 to the present, at fixed M_star, is a consequence of the following: First, disk galaxies are in a steady state with the SFR following the net (i.e., inflow minus outflow) gas accretion rate. The evolution of the SFR sequence is determined by evolution in the cosmological specific accretion rates, \propto (1+z)^{2.25}, but is found to be independent of feedback. Although feedback determines the outflow rates, it shifts galaxies along the SFR sequence, leaving its zero point invariant. Second, the conversion of accretion rate to SFR is materialized through gas density, not gas mass. Although the model SFR is an increasing function of both gas mass fraction and gas density, only the gas densities are predicted to evolve significantly with redshift. Third, star formation is fueled by molecular gas. Since the molecular gas fraction increases monotonically with increasing gas density, the model predicts strong evolution in the molecular gas fractions, increasing by an order of magnitude from z=0 to z~2. On the other hand, the model predicts that the effective surface density of atomic gas is ~10 M_sun pc^{-2}, independent of redshift, stellar mass or feedback. Our model suggests that the scatter in the SFR sequence reflects variations in the gas accretion history, and thus is insensitive to stellar mass, redshift or feedback. The large scatter in halo spin contributes negligibly, because it scatters galaxies along the SFR sequence. An observational consequence of this is that the scatter in the SFR sequence is independent of the size (both stellar and gaseous) of galaxy disks.Comment: 24 pages, 19 figures, accepted to MNRAS, minor changes to previous versio

Effects of Rotation on Stochasticity of Gravitational Waves in Nonlinear Phase of Core-Collapse Supernovae

By performing three-dimensional (3D) simulations that demonstrate the neutrino-driven core-collapse supernovae aided by the standing accretion shock instability (SASI), we study how the spiral modes of the SASI can have impacts on the properties of the gravitational-wave (GW) emission. To see the effects of rotation in the non-linear postbounce phase, we give a uniform rotation on the flow advecting from the outer boundary of the iron core, whose specific angular momentum is assumed to agree with recent stellar evolution models. We compute fifteen 3D models in which the initial angular momentum as well as the input neutrino luminosities from the protoneutron star are changed in a systematic manner. By performing a ray-tracing analysis, we accurately estimate the GW amplitudes generated by anisotropic neutrino emission. Our results show that the gravitational waveforms from neutrinos in models that include rotation exhibit a common feature otherwise they vary much more stochastically in the absence of rotation. The breaking of the stochasticity stems from the excess of the neutrino emission parallel to the spin axis. This is because the compression of matter is more enhanced in the vicinity of the equatorial plane due to the growth of the spiral SASI modes, leading to the formation of spiral flows circulating around the spin axis with higher temperatures. We point out that a recently proposed future space interferometers like Fabry-Perot type DECIGO would permit detection of these signals for a Galactic supernova.Comment: 13 Figures, ApJ in pres

Sunyaev-Zel'dovich clusters in millennium gas simulations

Large surveys using the Sunyaev–Zel’dovich (SZ) effect to find clusters of galaxies are now starting to yield large numbers of systems out to high redshift, many of which are new dis- coveries. In order to provide theoretical interpretation for the release of the full SZ cluster samples over the next few years, we have exploited the large-volume Millennium gas cosmo- logical N-body hydrodynamics simulations to study the SZ cluster population at low and high redshift, for three models with varying gas physics. We confirm previous results using smaller samplesthattheintrinsic(spherical)Y500–M500relationhasverylittlescatter(σlog10Y ≃0.04), is insensitive to cluster gas physics and evolves to redshift 1 in accordance with self-similar expectations. Our preheating and feedback models predict scaling relations that are in excel- lent agreement with the recent analysis from combined Planck and XMM–Newton data by the Planck Collaboration. This agreement is largely preserved when r500 and M500 are derived using thehydrostaticmassproxy,YX,500,albeitwithsignificantlyreducedscatter(σlog10Y ≃0.02),a result that is due to the tight correlation between Y500 and YX,500. Interestingly, this assumption also hides any bias in the relation due to dynamical activity. We also assess the importance of projection effects from large-scale structure along the line of sight, by extracting cluster Y500 values from 50 simulated 5 × 5-deg2 sky maps. Once the (model-dependent) mean signal is subtracted from the maps we find that the integrated SZ signal is unbiased with respect to the underlying clusters, although the scatter in the (cylindrical) Y500–M500 relation increases in the preheating case, where a significant amount of energy was injected into the intergalactic medium at high redshift. Finally, we study the hot gas pressure profiles to investigate the origin of the SZ signal and find that the largest contribution comes from radii close to r500 in all cases. The profiles themselves are well described by generalized Navarro, Frenk & White profiles but there is significant cluster-to-cluster scatter. In conclusion, our results support the notion that Y500 is a robust mass proxy for use in cosmological analyses with clusters