Deep high spectral resolution spectroscopy and chemical composition of ionized nebulae     

C. Esteban  J. García‐Rojas  A. Mesa‐Delgado  L. Toribio San Cipriano 《Astronomische Nachrichten》2014,335(1):73-78

High spectral resolution spectroscopy has proved to be very useful for the advancement of chemical abundances studies in photoionized nebulae, such as H II regions and planetary nebulae (PNe). Classical analyses make use of the intensity of bright collisionally excited lines (CELs), which have a strong dependence on the electron temperature and density. By using high resolution spectrophotometric data, our group has led the determination of chemical abundances of some heavy element ions, mainly O⁺⁺, O⁺, and C⁺⁺ from faint recombination lines (RLs), allowing us to deblend them from other nearby emission lines or sky features. The importance of these lines is that their emissivity depends weakly on the temperature and density structure of the gas. The unresolved issue in this field is that recombination lines of heavy element ions give abundances that are about 2–3 times higher than those derived from CELs – in H II regions – for the same ion, and can even be a factor of 70 times higher in some PNe. This uncertainty puts into doubt the validity of face values of metallicity that we use as representative not only for ionized nebulae in the Local Universe, but also for star‐forming dwarf and spiral galaxies at different redshifts. Additionally, high‐resolution data can allow us to detect and deblend faint lines of neutron capture element ions in PNe. This information would introduce further restrictions to evolution models of AGBs and would help to quantify the chemical enrichment in s‐elements produced by low and intermediate mass stars. The availability of an échelle spectrograph at the E‐ELT will be of paramount interest to: (a) extend the studies of heavyelement recombination lines to low metallicity objects, (b) to extend abundance determinations of s‐elements to planetary nebulae in the extragalactic domain and to bright Galactic and extragalactic H II regions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

A random solid‐porous model to simulate the retention curves of soils     

Jaime Horta  Eduardo Rojas  María L. Pérez‐Rea  Teresa López  Juan B. Zaragoza 《国际地质力学数值与分析法杂志》2013,37(8):932-944

This paper describes a random solid‐porous model capable of simulating the structure of porous materials. To this purpose, the grain and pore size distributions as well as the void ratio of the material are required. Solids and pores are distributed at random in the model's space according to a size strategy. Herein, the model is used to simulate the retention curves of soils. The Laplace equation is used to determine the size of the pores able to saturate or dry during a wetting or drying process, respectively. The continuous path principle is used to define those elements that effectively saturate or dry during these processes. With this procedure, it is possible to simulate the main retention curves as well as the scanning curves during wetting–drying cycles. Some experimental results reported in the international literature have been used to test the model. This model can be enhanced to study the mechanical behavior of unsaturated soils. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Sediment transport patterns determined from grain size parameters: Overview and state of the art   总被引：3，自引：0，他引：3  

J.P. Le Roux  E.M. Rojas 《Sedimentary Geology》2007,202(3):473-488

Grain size trends have been applied in many diverse sedimentary environments to determine sediment transport paths, generally coinciding with information from tracer studies, current measurements and the orientation of sedimentary structures. The different methods proposed to date are critically analysed and compared with reference to recent field studies. It is concluded that the two-dimensional methods produce comparable results and may in fact complement each other.In spite of the advances, several problems still exist, which include the sampling method and density, the choice of trend types, the relative weight of grain size parameters and the interpretation of results. These are discussed together with possible solutions.  相似文献   

Climate change and river floods in the European Union: Socio-economic consequences and the costs and benefits of adaptation     

Rodrigo Rojas  Luc Feyen  Paul Watkiss 《Global Environmental Change》2013,23(6):1737-1751

This study presents the first appraisal of the socio-economic impacts of river floods in the European Union in view of climate and socio-economic changes. The assessment is based on two trajectories: (a) no adaptation, where the current levels of protection are kept constant, and (b) adaptation, where the level of protection is increased to defend against future flooding events. As a basis for our analysis we use an ensemble-based pan-European flood hazard assessment for present and future conditions. Socio-economic impacts are estimated by combining flood inundation maps with information on assets exposure and vulnerability. Ensemble-based results indicate that current expected annual population affected of ca. 200,000 is projected to increase up to 360,000 due to the effects of socio-economic development and climate change. Under the no adaptation trajectory current expected annual damages of €5.5 billion/year are projected to reach €98 billion/year by the 2080s due to the combined effects of socio-economic and climate change. Under the adaptation trajectory the avoided damages (benefits) amount to €53 billion/year by the 2080s. An analysis of the potential costs of adaptation associated with the increase in protection suggests that adaptation could be highly cost-effective. There is, however, a wide range around these central numbers reflecting the variability in projected climate. Analysis at the country level shows high damages, and by association high costs of adaptation, in the United Kingdom, France, Italy, Romania, Hungary and Czech Republic. At the country level, there is an even wider range around these central values, thus, pointing to a need to consider climate uncertainty in formulating practical adaptation strategies.  相似文献   

Sensitivity analysis of prior model probabilities and the value of prior knowledge in the assessment of conceptual model uncertainty in groundwater modelling     

Rodrigo Rojas  Luc Feyen  Alain Dassargues 《水文研究》2009,23(8):1131-1146

A key point in the application of multi‐model Bayesian averaging techniques to assess the predictive uncertainty in groundwater modelling applications is the definition of prior model probabilities, which reflect the prior perception about the plausibility of alternative models. In this work the influence of prior knowledge and prior model probabilities on posterior model probabilities, multi‐model predictions, and conceptual model uncertainty estimations is analysed. The sensitivity to prior model probabilities is assessed using an extensive numerical analysis in which the prior probability space of a set of plausible conceptualizations is discretized to obtain a large ensemble of possible combinations of prior model probabilities. Additionally, the value of prior knowledge about alternative models in reducing conceptual model uncertainty is assessed by considering three example knowledge states, expressed as quantitative relations among the alternative models. A constrained maximum entropy approach is used to find the set of prior model probabilities that correspond to the different prior knowledge states. For illustrative purposes, a three‐dimensional hypothetical setup approximated by seven alternative conceptual models is employed. Results show that posterior model probabilities, leading moments of the predictive distributions and estimations of conceptual model uncertainty are very sensitive to prior model probabilities, indicating the relevance of selecting proper prior probabilities. Additionally, including proper prior knowledge improves the predictive performance of the multi‐model approach, expressed by reductions of the multi‐model prediction variances by up to 60% compared with a non‐informative case. However, the ratio between‐model to total variance does not substantially decrease. This suggests that the contribution of conceptual model uncertainty to the total variance cannot be further reduced based only on prior knowledge about the plausibility of alternative models. These results advocate including proper prior knowledge about alternative conceptualizations in combination with extra conditioning data to further reduce conceptual model uncertainty in groundwater modelling predictions. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

The 2009–2010 fade of Jupiter’s South Equatorial Belt: Vertical cloud structure models and zonal winds from visible imaging     

S. Pérez-Hoyos  J.F. Sanz-Requena  N. Barrado-Izagirre  J.F. Rojas  A. Sánchez-Lavega 《Icarus》2012,217(1):256-271

The South Equatorial Belt (SEB) of Jupiter is known to alternate its appearance at visible wavelengths from a classical belt-like band most of the time to a short-lived zone-like aspect which is called a “fade” of the belt, hereafter SEBF. The albedo change of the SEB is due to a change in the structure and properties of the clouds and upper hazes. Recent works based on infrared observations of the last SEBF have shown that the aerosol density below 1 bar increased in parallel with the reflectivity change. However, the nature of the change in the upper clouds and hazes that produces the visible reflectivity change and whether or not this reflectivity change is accompanied by a change in the winds at the upper cloud level remained unknown. In this paper we focus in the near ultraviolet to near infrared reflected sunlight (255–953 nm) to address these two issues. We characterize the vertical cloud structure above the ammonia condensation level from Hubble Space Telescope images, and the zonal wind velocities from long-term high-quality images coming from the International Outer Planet Watch database, both during the SEB and SEBF phases. We show that reflectivity changes do not happen simultaneously in this wavelength range, but they start earlier in the most deep-sensing filters and end in 2010 with just minor changes in those sensing the highest particle layers. Our models require a substantial increase of the optical thickness of the cloud deck at 1.0 ± 0.4 bar from τ_cloud = 6 ± 2 in July 2009 (SEB phase) to semiinfinite at visual wavelengths in 2010 (SEBF). Upper tropospheric particles (～240–1400 mbar) are also required to become substantially reflectant and their single scattering albedo in the blue changes from ?₀ = 0.905 ± 0.005 in November 2009 to ?₀ = 0.95 ± 0.01 in June 2010. No significant changes were found in the cloud top heights or in the particle density of the tropospheric haze. The disturbance travels from the levels below ～3 bar to a level about 400 ± 100 mbar. We derive an upward velocity of 0.15 ± 0.05 cm/s, in agreement with a diffusive process in Jupiter’s upper troposphere requiring a mean eddy coefficient K ～ 8 × 10⁵ cm² s^?1. On the other hand, cloud tracking on the IOPW imaging showed no significant changes in the zonal wind profile between the SEB and SEBF stages. As in other visually huge changes in Jupiter’s cloud morphology and structure, the wind profile remains robust, possibly indicating a deeply rooted dynamical regime.  相似文献   

Towards the main sequence: detailed analysis of weak line and post-T Tauri stars     

G. Rojas  J. Gregorio-Hetem  A. Hetem  Jr 《Monthly notices of the Royal Astronomical Society》2008,387(3):1335-1343

A detailed study was performed for a sample of low-mass pre-main-sequence (PMS) stars, previously identified as weak-line T Tauri stars, which are compared to members of the Tucanae and Horologium Associations. Aiming to verify if there is any pattern of abundances when comparing the young stars at different phases, we selected objects in the range from 1 to 100 Myr, which covers most of PMS evolution. High-resolution optical spectra were acquired at European Southern Observatory and Observatório do Pico dos Dias . The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of iron absorption lines. Chemical abundances were obtained via equivalent width calculations and spectral synthesis for 44 per cent of the sample, which shows metallicities within 0.5 dex solar. A classification was developed based on equivalent width of Li  i 6708 Å and Hα lines and spectral types of the studied stars. This classification allowed a separation of the sample into categories that correspond to different evolutive stages in the PMS. The position of these stars in the Hertzsprung–Russell diagram was also inspected in order to estimate their ages and masses. Among the studied objects, it was verified that our sample actually contains seven weak-line T Tauri stars, three are Classical T Tauri, 12 are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequence stars. An estimation of circumstellar luminosity was obtained using a disc model to reproduce the observed spectral energy distribution. Most of the stars show low levels of circumstellar emission, corresponding to less than 30 per cent of the total emission.  相似文献   

Sea level and paleotemperature records from a mid-Holocene reef on the North coast of Java, Indonesia     

Karem Azmy  Evan Edinger  Joyce Lundberg  Wilfredo Diegor 《International Journal of Earth Sciences》2010,99(1):231-244

Mid-Holocene age fossil-fringing reefs occur along the tectonically stable north coast of Java, Indonesia, presenting an opportunity for sea level and paleoclimate reconstruction. The fossil reef at Point Teluk Awur, near Jepara, Central Java, contains two directly superposed horizons of Porites lobata microatolls. Corals in the lower horizon, 80 cm above modern sea level, yielded Uranium series dates of 7090 ± 90 year BP, while corals in the upper horizon at 1.5 m grew at 6960 ± 60 year BP. These dates match the transgressive phase of regional sea-level curves, but suggest a mid-Holocene highstand somewhat older than that recorded on mid-Pacific islands. Paleotemperature was calibrated using Sr/Ca and δ¹⁸O values of a modern P. lobata coral and the locally measured sea surface temperature (SST), yielding SST–Sr/Ca and SST–δ¹⁸O calibration equations [T _Sr/Ca = 91.03–7.35(Sr/Ca) and T_{d¹⁸ \textO} T_{{\delta^{18} {\text{O}}}}  = −3.77 to −5.52(δ¹⁸O)]. The application of the local equations to Sr/Ca and δ¹⁸O measurements on these corals yielded a range of temperatures of 28.8 ± 1.7°C, comparable to that of the modern Java Sea (28.4 ± 0.7°C). A paleo-salinometer [Δδ¹⁸O = ∂δ¹⁸O/∂T ( T_{d¹⁸ \textO} T_{{\delta^{18} {\text{O}}}}  − T _Sr/Ca)], re-calculated using the local parameters, also suggests Java Sea mid-Holocene paleosalinity similar to modern values.  相似文献   

Downscaling extreme month-long anomalies in southern South America     

C. G. Menéndez  M. de Castro  J.-P. Boulanger  A. D’Onofrio  E. Sanchez  A. A. Sörensson  J. Blazquez  A. Elizalde  D. Jacob  H. Le Treut  Z. X. Li  M. N. Núñez  N. Pessacg  S. Pfeiffer  M. Rojas  A. Rolla  P. Samuelsson  S. A. Solman  C. Teichmann 《Climatic change》2010,98(3-4):379-403

We investigate the performance of one stretched-grid atmospheric global model, five different regional climate models and a statistical downscaling technique in simulating 3 months (January 1971, November 1986, July 1996) characterized by anomalous climate conditions in the southern La Plata Basin. Models were driven by reanalysis (ERA-40). The analysis has emphasized on the simulation of the precipitation over land and has provided a quantification of the biases of and scatter between the different regional simulations. Most but not all dynamical models underpredict precipitation amounts in south eastern South America during the three periods. Results suggest that models have regime dependence, performing better for some conditions than others. The models’ ensemble and the statistical technique succeed in reproducing the overall observed frequency of daily precipitation for all periods. But most models tend to underestimate the frequency of dry days and overestimate the amount of light rainfall days. The number of events with strong or heavy precipitation tends to be under simulated by the models.  相似文献   

Delimitation of ground failure zones due to land subsidence using gravity data and finite element modeling in the Querétaro valley, México     

Jesús Pacheco  Jorge Arzate  Eduardo Rojas  Moises Arroyo  Vsevolod Yutsis  Gil Ochoa   《Engineering Geology》2006,84(3-4):143-160

Sedimentary basins of arid and semiarid zones are often subject to regimes of intense ground-water withdrawal as it is normally the only source of water for development of communities. An associated phenomenon is land subsidence, which can develop to ground failures, and consequently, damage to infrastructure. Aquifer deformation can be analyzed using a stress–strain or a flux–force approach depending on the aquifer material (compact or loose) and on whether the water withdrawal forms a predominant flow direction toward a cone of depression. Geometry of the aquifer system also plays an important role as uneven thickness induces differential compaction and hence, tensional and shear stresses on the ground mass. In this work we present a stress–strain approach to analyze subsidence for an unconfined aquifer of varying thickness; this is done in two steps, namely when the aquifer is in equilibrium, and when it is totally depleted. Using this scheme in a region where ground failure is evident, a portion of the aquifer system of the Querétaro valley is analyzed. The geometry of the hydrologic basement is first modeled using gravity measurements properly correlated with wells and field data. Then a stress analysis is implemented using the finite element method in order to identify probable zones of ground weakness, which are calibrated with known ground failures. The results indicate that both, tensional and shear stress are present, which induce ground failure in the form of surface faults.  相似文献