Automatic Generation of Road Geometries to Create Challenging Scenarios for Automated Vehicles Based on the Sensor Setup

For the offline safety assessment of automated vehicles, the most challenging and critical scenarios must be identified efficiently. Therefore, we present a new approach to define challenging scenarios based on a sensor setup model of the ego-vehicle. First, a static optimal approaching path of a road user to the ego-vehicle is calculated using an A* algorithm. We consider a poor perception of the road user by the automated vehicle as optimal, because we want to define scenarios that are as critical as possible. The path is then transferred to a dynamic scenario, where the trajectory of the road user and the road layout are determined. The result is an optimal road geometry, so that the ego-vehicle can perceive an approaching object as poorly as possible. The focus of our work is on the highway as the Operational Design Domain (ODD).Comment: Accepted at the 2020 IEEE Intelligent Vehicles Symposium (IV), October 20-23, 202

The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808. Such stars are unexplained by canonical HB theory. In order to explore the origin of these subluminous stars, we evolved a set of low-mass stars from the main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous EHB stars, as well as the gap within the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash on the white-dwarf cooling curve. Under these conditions the flash convection will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium- burning interior. This phenomenon is analogous to the "born-again" scenario for producing hydrogen-deficient stars during a very late helium-shell flash. "Flash mixing" greatly enhances the envelope helium and carbon abundances and, as a result, leads to an abrupt increase in the HB effective temperature. We argue that the EHB gap in NGC 2808 is caused by this theoretically predicted dichotomy in the HB morphology. Using new helium- and carbon-rich stellar atmospheres, we show that the flash-mixed stars have the same reduced UV flux as the subluminous EHB stars. Moreover, we demonstrate that models without flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the observations. Flash mixing may also provide a new evolutionary channel for producing the high gravity, He-rich sdO and sdB stars.Comment: 8 pages, 5 figures, to appear in "Omega Centauri: a Unique Window into Astrophysics" (Cambridge, August, 2001), ASP Conf. Ser., edited by F. van Leeuwen, G. Piotto, and J. Hughe

The Discovery of Pulsating Hot Subdwarfs in NGC 2808

We present the results of a Hubble Space Telescope program to search for pulsating hot subdwarfs in the core of NGC 2808. These observations were motivated by the recent discovery of such stars in the outskirts of omega Cen. Both NGC 2808 and omega Cen are massive globular clusters exhibiting complex stellar populations and large numbers of extreme horizontal branch stars. Our far-UV photometric monitoring of over 100 hot evolved stars has revealed six pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of 2.0 to 6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these stars lie immediately below the canonical horizontal branch, a region populated by the subluminous "blue-hook" stars. For three of these six pulsators, we also have low-resolution far-UV spectroscopy that is sufficient to broadly constrain their atmospheric abundances and effective temperatures. Curiously, and in contrast to the omega Cen pulsators, the NGC 2808 pulsators do not exhibit the spectroscopic or photometric uniformity one might expect from a well-defined instability strip, although they all fall within a narrow band (0.2 mag) of far-UV luminosity.Comment: 5 pages, 1 table, 2 color and 2 grayscale figures. Accepted for publication in The Astrophysical Journal Letter

Impact of light scattering for efficiency enhancement in organic solar cells

Further efficiency enhancements in organic solar cells require a deeper understanding of the charge generation and transport in the cell as well as the employment of advanced light trapping mechanisms. Both electronic and optical device models for organic solar cells have been developed already in the past. This paper, however, for the first time presents a simulation tool that combines a state of the art driftdiffusion electrical model with a complex optical model able to simulate wave propagation in thin film optics but also ray-based light propagation in incoherent layers and scalar scattering. The combination of the light-scattering algorithm and this driftdiffusion model leads to a coupled opto-electronic cell model which represents a powerful design tool for cell characterization and optimization. This tool is then used to evaluate the gain of efficiency introduced by a light scattering interface made of a rough TCO in a bulk heterojunction (BHJ) solar cell. The results were found to be in good qualitative agreement with previously published experimental results

Subglobal Climate Agreements and Energy-Intensive Activities: Is there a Carbon Haven for Copper?

Subglobal climate policies induce changes in international competitiveness and favor a relocation of carbon-emitting activities. We argue that many energy-intensive activities are also capital-intensive, so that carbon policies could affect rents rather than abatement or location. Taking copper as an example, we formulate a plant-level spatial equilibrium model of the industry, and we estimate a set of elasticities to calibrate the behavioral parameters of the model. Given 2007 market conditions, Monte Carlo simulations suggest that a $50/tCO2 tax in industrialized countries induces emissions reductions of less than one percent in the copper industry, with a mean emission leakage rate of 25%. Our results conform with empirical findings on the pollution haven effect but challenge projections from computable general equilibrium models.Massachusetts Institute of Technology. Center for Energy and Environmental Policy Researc

The Blue Hook Populations of Massive Globular Clusters

We present new HST ultraviolet color-magnitude diagrams of 5 massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to investigate the "blue hook" phenomenon previously observed in UV images of the globular clusters omega Cen and NGC 2808. Blue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. By coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. Specifically, we compare our photometry to canonical models at standard cluster abundances, canonical models with enhanced helium (consistent with cluster self-enrichment at early times), and flash-mixed models formed via a late helium-core flash on the white dwarf cooling curve. We find that flash-mixed models are required to explain the faint luminosity of the blue hook stars, although neither the canonical models nor the flash-mixed models can explain the range of color observed in such stars, especially those in the most metal-rich clusters. Aside from the variation in the color range, no clear trends emerge in the morphology of the blue hook population with respect to metallicity.Comment: Accepted for publication in The Astrophysical Journal. Latex, 14 pages, 1 B&W and 6 color figure

Vulnerability of uneven-aged forests to storm damage

Uneven-aged forests are assumed to have a high stability against storm damage but have rarely been analysed for vulnerability to storm damage due to a lack of a sufficient empirical database. Here we model storm damage in uneven-aged forest to analyse major factors that may determine the sensitivity of this type of forests to storms based on a broad database. Data are derived of public forests in the canton Neuchâtel in West Switzerland that are dominated by silver fir and Norway spruce and managed since the beginning of the 20th century following a single-tree selection system. A unique dataset of periodical (every 5-10 years) full inventories measuring the diameter of every single tree including salvage cuttings was available for the investigation. The time series reached back until 1920 and covered an area of 16 000 ha divided into 3000 divisions. The effect of a major winter storm (‘Lothar') in December 1999 on these forests was investigated using a subset of 648 divisions. The influence of the vertical stand structure on the vulnerability of storm damage was studied using logistic regression models. To facilitate the analyses, an index of closeness to a J-shaped distribution (LikeJ) based on the number of trees in different diameter classes was developed. Besides structural indices, variables representing stand characteristics, soil-related and topography-related variables were included. The results of our study show that the overall damage level of the investigated forests was rather low. The variables that entered the model for the uneven-aged stands were different to those that are normally significant for even-aged stands. While variables like stand structure, the timing of the harvesting and topographic variables entered a multivariate statistical model as significant predictors, standard predictors for storm damage in even-aged stands such as stand density, thinning intensity or species composition were not significant. We hypothesize that the uneven-aged structure of the investigated forests may be one reason for the low damage level we observed but emphasize the need for more detailed research to support this conclusio

On The Age Estimation of LBDS 53W091

The recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators has suggested that this red galaxy at z=1.55 is at least 3.5 Gyr old. This imposes an important constraint on cosmology, suggesting that this galaxy formed at z > 6.5, assuming recent estimates of cosmological parameters. We have performed chi^2 tests to the continuum of this galaxy using its UV spectrum and photometric data (RJHK). We have used the updated Yi models that are based on the Yale tracks. We find it extremely difficult to reproduce such large age estimates, under the assumption of the most probable input parameters. Using the same configuration as in Spinrad et al. (solar abundance models), our analysis suggests an age of approximately 1.4 -- 1.8 Gyr. The discrepancy between Spinrad et al.'s age estimate (based on the 1997 Jimenez models) and ours originates from the large difference in the model integrated spectrum: the Jimenez models are much bluer than the Yi models and the Bruzual \& Charlot (BC) models. Preliminary tests favor the Yi and BC models. The updated age estimate of LBDS 53W091 would suggest that this galaxy formed approximately at z=2-3.Comment: LaTeX, 18 eps files Accepted for publication in ApJ (Feb 10, 2000, vol 530), uses emulateapj.st