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Leiden, Netherlands

Roos N.,Leiden Observatory
American Journal of Physics | Year: 2014

Interest in the concept of entropic forces has risen considerably since Verlinde proposed in 2011 to interpret the force in Newton's second law and gravity as entropic forces. Brownian motion-the motion of a small particle (pollen) driven by random impulses from the surrounding molecules-may be the first example of a stochastic process in which such forces are expected to emerge. In this article, it is shown that at least two types of entropic force can be identified in three-dimensional Brownian motion. This analysis yields simple derivations of known results of Brownian motion, Hooke's law, and-applying an external (non-radial) force-Curie's law and the Langevin-Debye equation. © 2014 American Association of Physics Teachers. Source

Jolissaint L.,Leiden Observatory
Journal of the European Optical Society | Year: 2010

We present an analytical model of a single natural guide star astronomical adaptive optics system, in closed loop mode. The model is used to simulate the long exposure system point spread function, using the spatial frequency (or Fourier) approach, and complements an initial open loop model. Applications range from system design, science case analysis and AO data reduction. All the classical phase errors have been included: deformable mirror fitting error, wavefront sensor spatial aliasing, wavefront sensor noise, and the correlated anisoplanatic and servo-lag error. The model includes the deformable mirror spatial transfer function, and the actuator array geometry can be different from the wavefront sensor lenslet array geometry. We also include the dispersion between the sensing and the correction wavelengths. Illustrative examples are given at the end of the paper. Source

Chiar J.E.,Search for Extraterrestrial Intelligence Institute | Tielens A.G.G.M.,Leiden Observatory | Adamson A.J.,Gemini Observatory | Ricca A.,Search for Extraterrestrial Intelligence Institute
Astrophysical Journal | Year: 2013

Many materials have been considered for the carrier of the hydrocarbon absorption bands observed in the diffuse interstellar medium (ISM). In order to refine the model for ISM hydrocarbon grains, we analyze the observed aromatic (3.28, 6.2 μm) and aliphatic (3.4 μm) hydrocarbon absorption features in the diffuse ISM along the line of sight toward the Galactic center Quintuplet Cluster. Observationally, sp 2 bonds can be measured in astronomical spectra using the 6.2 μm CC aromatic stretch feature, whereas the 3.4 μm aliphatic feature can be used to quantify the fraction of sp 3 bonds. The fractional abundance of these components allows us to place the Galactic diffuse ISM hydrocarbons on a ternary phase diagram. We conclude that the Galactic hydrocarbon dust has, on average, a low H/C ratio and sp 3 content and is highly aromatic. We have placed the results of our analysis within the context of the evolution of carbon dust in the ISM. We argue that interstellar carbon dust consists of a large core of aromatic carbon surrounded by a thin mantle of hydrogenated amorphous carbon (a-C:H), a structure that is a natural consequence of the processing of stardust grains in the ISM. © 2013. The American Astronomical Society. All rights reserved. Source

Kauffmann G.,Max Planck Institute for Astrophysics | Li C.,Max Planck Institute Partner Group | Zhang W.,CAS National Astronomical Observatories | Weinmann S.,Leiden Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2013

The observed correlation between star formation in central galaxies and in their neighbours (a phenomenon dubbed 'galactic conformity') is in need of a convincing physical explanation. To gain further insight, we use a volume-limited sample of galaxies with redshifts less than 0.03 drawn from the Sloan Digital Sky Survey Data Release 7 to investigate the scale dependence of the effect and how it changes as a function of the mass of the central galaxy. Conformity extends over a central galaxy stellar mass range spanning two orders of magnitude. The scale dependence and the precise nature of the effect depend on the mass of the central. In central galaxies with masses less than 1010M⊙, conformity extends out to scales in excess of 4 Mpc, well beyond the virial radii of their dark matter haloes. For low-mass central galaxies, conformity with neighbours on very large scales is only seen when they have lowstar formation rate or gas content. In contrast, at high stellar masses, conformity with neighbours applies in the gas-rich regime and is clearly confined to scales comparable to the virial radius of the dark matter halo of the central galaxy. Our analysis of a mock catalogue from the Guo et al. semianalytic models shows that conformity-like effects arise because gas-poor satellite galaxies are sometimes misclassified as centrals. However, the effects in the models are much weaker than observed. Misclassification only influences the low-end tail of the SFR/M* distribution of neighbouring galaxies at large distances from the primary. The median and the upper percentiles of the SFR/M* distribution remain almost unchanged, which is in contradiction with the data. We speculate that the conformity between low-mass, gas-poor central galaxies and their distant neighbours may be a signature of 'pre-heating' of the intergalactic gas at an earlier epoch. The smaller scale conformity between high-mass, gas-rich central galaxies and their close neighbours may be a signature of ongoing gas accretion on to central galaxies in a minority of massive dark matter haloes. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Berg K.I.,Harvard - Smithsonian Center for Astrophysics | Bottinelli S.,Roche Holding AG | Jorgensen J.K.,Copenhagen University | Van Dishoeck E.F.,Leiden Observatory | Van Dishoeck E.F.,Max Planck Institute for Extraterrestrial Physics
Astrophysical Journal | Year: 2010

Gas-phase complex organic molecules have been detected toward a range of high- and low-mass star-forming regions at abundances which cannot be explained by any known gas-phase chemistry. Recent laboratory experiments show that UV irradiation of CH3OH-rich ices may be an important mechanism for producing complex molecules and releasing them into the gas phase. To test this ice formation scenario, we mapped the B1-b dust core and nearby protostar in CH3OH gas using the IRAM 30 m telescope to identify locations of efficient non-thermal ice desorption. We find three CH3OH abundance peaks tracing two outflows and a quiescent region on the side of the core facing the protostar. The CH3OH gas has a rotational temperature of 10K at all locations. The quiescent CH3OH abundance peak and one outflow position were searched for complex molecules. Narrow, 0.6-0.8kms-1 wide, HCOOCH3 and CH3CHO lines originating in cold gas are clearly detected, CH3OCH3 is tentatively detected, and C2H5OH and HOCH2CHO are undetected toward the quiescent core, while no complex molecular lines were found toward the outflow. The core abundances with respect to CH3OH are 2.3% and 1.1% for HCOOCH3 and CH3CHO, respectively, and the upper limits are 0.7%-1.1%, which is similar to most other low-mass sources. The observed complex molecule characteristics toward B1-b and the pre-dominance of HCO-bearing species suggests a cold ice (below 25 K, the sublimation temperature of CO) formation pathway followed by non-thermal desorption through, e.g., UV photons traveling through outflow cavities. The observed complex gas composition together with the lack of any evidence of warm gas-phase chemistry provides clear evidence of efficient complex molecule formation in cold interstellar ices. © 2010. The American Astronomical Society. All rights reserved. Source

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