Pressure and density gradients in H II Regions     

José Franco  Stan E. Kurtz  José A. GarcÍa-Barreto  Guillermo GarcÍa-Segura  Eduardo de la Fuente  Peter Hofner  Alejandro Esquivel 《Astrophysics and Space Science》2001,277(1):71-74

Here we discuss the effects of large ambient pressures and decreasing density gradients on the observed properties of dusty UC hii regions. Dust absorption can effectively reduce the size of the photo ionized region, and density gradients can modify the spectral index of the emission. The effects of the density gradients seem to be present in both galactic and extragalactic HII regions.  相似文献   

Geochemical characteristics of particulate matter in the atmosphere surrounding a ceramic industrialized area   总被引：3，自引：0，他引：3  

E.?T.?Gómez  T.?Sanfeliu  M.?M.?JordánEmail author  J.?Rius  C.?de la?Fuente 《Environmental Geology》2004,45(4):536-543

Total suspended particulate samples (TSP) were collected and concentrations measured during seventeen months in the vicinity of a ceramic industrial area. A method of fractionating was applied to the samples in order to obtain two fractions corresponding to mineral particulate coming from dust emissions (Upper-F fraction) and to amorphous matter (carbon plus small amounts of S, Ca, Fe, etc) coming mainly from traffic and other combustion processes. Also for TSP samples several element concentrations were measured following two previous treatments: extraction of elements mainly associated with the soluble fraction of the samples (B, Fe, P, As, NO^2–, NH⁴⁺, Cl^–, F) and acid digestion for trace metals and elements mainly associated with the non-soluble fractions of the samples (Fe, As, Cd, Ni, Pb, Zn Ca). Seasonal differences and the influence of meteorological parameters (temperature, relative humidity, pressure and wind conditions) on the air pollution levels, particles as well as ions, were studied.Results show different seasonal and weekly evolution for mineral and amorphous carbonaceous particles because of the different origins in dust emissions or combustion processes respectively, and the different physical properties such as size grain. Of the ions analyzed Fe, Ca and Zn were clearly associated to mineral phases and consequently related to dust emissions, and NO^2–, NH4+, P, Cl^– were related to amorphous matter coming from combustion. Ni and Cd show lower levels than those reported as guideline values and the source is mainly related to the enrichment of these elements in clay materials. B and As content result in elevated concentrations, with the tendency to increase during cold months. The emission of these elements was associated with vaporization or volatilization during high temperature ceramic processes. The original gaseous state is influenced by temperature. In the winter the content for B and As is higher due to enhanced condensation of gas-phase boron onto particles, while in the summer the increase of air temperature results in elevated evaporation.  相似文献   

Extreme maximum temperature events and their relationships with large-scale modes: potential hazard on the Iberian Peninsula     

Merino  Andrés  Martín  M. L.  Fernández-González  S.  Sánchez  J. L.  Valero  F. 《Theoretical and Applied Climatology》2018,133(1-2):531-550

Theoretical and Applied Climatology - The aim of this paper is to analyze spatiotemporal distribution of maximum temperatures in the Iberian Peninsula (IP) by using various extreme maximum...  相似文献   

Lead Removal from Aqueous Solution by Natural and Pretreated Zeolites     

Ismael S. Ismael  Ahmed Melegy  Tomas Kratochvíl 《Geotechnical and Geological Engineering》2012,30(1):253-262

Hazardous metal cations enter water through the natural geochemical route or from the industrial wastes. Their separation and removal can be achieved by adsorptive accumulation of the cations on a suitable adsorbent. In the present work, toxic Pb(II) ions are removed from water by accumulating it on the surface of natural zeolite in three different forms; one untreated and two treated samples, one sample treated with 2 M HCI solution and other is treated with 3 M NaOH solution. Natural zeolite is mainly composed of clinoptilolite, and mordenite, with amount of non-zeolite phase (smectite and illite) and C and CT opal. The adsorption experiments are carried out using a batch process in environments of different pH, initial Pb(II) concentration, interaction time and amount of zeolites. Treated zeolite samples show high exchange capacity for Pb(II) compared to untreated sample, however, acid-treated sample shows an exceedingly good exchange capacity. Equilibrium data fitted well with the Langmuir isotherm model with maximum adsorption capacity of 115, 126, and 132 mg g⁻¹ of untreated natural zeolites, alkali-treated zeolites and acid-treated zeolites respectively. The rates of adsorption were found to confirm to pseudo-first order kinetic with good correlation and the overall rate of lead ions uptake.  相似文献   

Search for Circumstellar Disks Around Herbig Be Stars     

A. Fuente  A. Rodríguez-Franco  L. Testi  A. Natta  R. Bachiller  R. Neri 《Astrophysics and Space Science》2004,292(1-4):465-468

We have carried out a high-sensitivity search for circumstellar disks around Herbig Be stars in the continuum at 1.3 mm and 2.7 mm using the IRAM interferometer at the Plateau de Bure (PdBI). We report data on three Herbig Be stars MWC1080, MWC137 and R Mon. We have observed two of them, MWC137 and R Mon, in the continuum at 0.7 cm and 1.3 cm using the NRAO Very Large Array (VLA). This allows us to build the complete SED at mm and cm wavelengths and fit a simple disk model. We report the detection of circumstellar disks around MWC1080 with M _d = 0.003M _⊙ and R Mon with M _d = 0.01M _⊙. A significant upper limit to the mass of the circumstellar disk around MWC137 has been obtained. Our results show that the ratio M _d/M * is at least an order of magnitude lower in Herbig Be stars than in Herbig Ae and T Tauri stars.  相似文献   

Large retrograde Centaurs: visitors from the Oort cloud?     

C. de la Fuente Marcos  R. de la Fuente Marcos 《Astrophysics and Space Science》2014,352(2):409-419

Among all the asteroid dynamical groups, Centaurs have the highest fraction of objects moving in retrograde orbits. The distribution in absolute magnitude, H, of known retrograde Centaurs with semi-major axes in the range 6–34 AU exhibits a remarkable trend: 10 % have H<10 mag, the rest have H>12 mag. The largest objects, namely (342842) 2008 YB₃, 2011 MM₄ and 2013 LU₂₈, move in almost polar, very eccentric paths; their nodal points are currently located near perihelion and aphelion. In the group of retrograde Centaurs, they are obvious outliers both in terms of dynamics and size. Here, we show that these objects are also trapped in retrograde resonances that make them unstable. Asteroid 2013 LU₂₈, the largest, is a candidate transient co-orbital to Uranus and it may be a recent visitor from the trans-Neptunian region. Asteroids 342842 and 2011 MM₄ are temporarily submitted to various high-order retrograde resonances with the Jovian planets but 342842 may be ejected towards the trans-Neptunian region within the next few hundred kyr. Asteroid 2011 MM₄ is far more stable. Our analysis shows that the large retrograde Centaurs form an heterogeneous group that may include objects from various sources. Asteroid 2011 MM₄ could be a visitor from the Oort cloud but an origin in a relatively stable closer reservoir cannot be ruled out. Minor bodies like 2011 MM₄ may represent the remnants of the primordial planetesimals and signal the size threshold for catastrophic collisions in the early Solar System.  相似文献   

Multifractal evolution in interacting galaxies: from supergiant molecular clouds to stellar superclusters in the Antennae     

R. de la Fuente Marcos  C. de la Fuente Marcos 《Monthly notices of the Royal Astronomical Society》2006,372(1):279-285

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SETI in star clusters: a theoretical approach     

R. de la Fuente Marcos  C. de la Fuente Marcos 《Astrophysics and Space Science》2003,284(3):1087-1096

For several decades, the search for extraterrestrial intelligence (SETI) has proceeded using advanced astronomical techniques. Different strategies have been proposed for target selection for targeted searches with goals of improving the chances of successful detection of signals from technological civilizations that may inhabit planets around solar-type stars, and to minimize the chances of missing signals from unexpected sites. In this paper we demonstrate that these goals are best achieved by observing star clusters. We show that standard open clusters are not appropriate for SETI scans because their disruption time scale is shorter than the characteristic time scale for the development of a protective atmospheric layer on a habitable planet. However, the old open clusters, those older than some Gy are optimal candidates for SETI surveys as their ages are older than the likely time for intelligent civilizations to emerge and the probability of catastrophic orbital modification as a result of close encounters with other cluster stars is, in general, rather negligible. The final performance of the proposed survey can be significantly increased by using initially a radio telescope beam larger than the cluster apparent size so that the entire cluster can be observed simultaneously. Globular clusters are also good candidates from the statistical point of view but only if hypothetical civilizations located in these clusters have been able to develop astronomical engineering technologies or have been involved in (rather speculative) cosmic colonization. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Stability and equation of state of post-aragonite BaCO3     

Joshua P. Townsend  Yun-Yuan Chang  Xiaoting Lou  Miguel Merino  Scott J. Kirklin  Jeff W. Doak  Ahmed Issa  Chris Wolverton  Sergey N. Tkachev  Przemyslaw Dera  Steven D. Jacobsen 《Physics and Chemistry of Minerals》2013,40(5):447-453

At ambient conditions, witherite is the stable form of BaCO₃ and has the aragonite structure with space group Pmcn. Above ~10 GPa, BaCO₃ adopts a post-aragonite structure with space group Pmmn. High-pressure and high-temperature synchrotron X-ray diffraction experiments were used to study the stability and equation of state of post-aragonite BaCO₃, which remained stable to the highest experimental P–T conditions of 150 GPa and 2,000 K. We obtained a bulk modulus K ₀ = 88(2) GPa with $K'$  = 4.8(3) and V ₀ = 128.1(5) ų using a third-order Birch-Murnaghan fit to the 300 K experimental data. We also carried out density functional theory (DFT) calculations of enthalpy (H) of two structures of BaCO₃ relative to the enthalpy of the post-aragonite phase. In agreement with previous studies and the current experiments, the calculations show aragonite to post-aragonite phase transitions at ~8 GPa. We also tested a potential high-pressure post–post-aragonite structure (space group C222 ₁ ) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, ΔH between the C222 ₁ structure and post-aragonite (Pmmn) decreases with pressure, but the Pmmn structure remains energetically favorable to pressures greater than 200 GPa. We conclude that post–post-aragonite phase transformations of carbonates do not follow systematic trends observed for post-aragonite transitions governed solely by the ionic radii of their metal cations.  相似文献   

Manmade Vulnerability of the Cancun Beach System: The Case of Hurricane Wilma   总被引：1，自引：0，他引：1  

Rodolfo Silva Casarin  Gabriel Ruiz Martinez  Ismael Mariño‐Tapia  Gregorio Posada Vanegas  Edgar Mendoza Baldwin  Edgar Escalante Mancera 《洁净——土壤、空气、水》2012,40(9):911-919

Climate change and resultant coastal erosion and flooding have been the focus of many recent analyses. Often these studies overlook the effects of manmade modifications to the coastline which have reduced its resilience to storm events. In this investigation, we integrate previous reports, historical photo analysis, field work, and the application of numerical models to better understand the effects of Wilma, the most destructive hurricane to affect Cancun, Mexico. Huge waves (of significant height, >12 m), long mean wave periods (>12 s), devastating winds (>250 km/h), and powerful currents (>2 m/s) removed >7 million cubic meters of sand from the Cancun beach system, leaving 68% of the sub‐aerial beach as bedrock, and the rest considerably eroded. Numerical simulations show that the modifications to the barrier island imposed by tourist infrastructure have considerably increased the rigidity of the system, increasing the potential erosion of the beach under extreme conditions. If there were no structural barriers, a series of breaches could occur along the beach, allowing exchange of water and alleviating storm surge on other sections of the beach. If the effects caused by anthropogenic changes to Cancun are ignored, the analysis is inaccurate and misleading.  相似文献