Measuring proper motions of isolated neutron stars with Chandra     

Christian Motch  Adriana M. Pires  Frank Haberl  Axel Schwope 《Astrophysics and Space Science》2007,308(1-4):217-224

The excellent spatial resolution of the Chandra observatory offers the unprecedented possibility to measure proper motions at X-ray wavelength with relatively high accuracy using as reference the background of extragalactic or remote galactic X-ray sources. We took advantage of this capability to constrain the proper motion of RX J0806.4-4123 and RX J0420.0-5022, two X-ray bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and lacking an optical counterpart. In this paper, we present results from a preliminary analysis from which we derive 2σ upper limits of 76 mas/yr and 138 mas/yr on the proper motions of RX J0806.4-4123 and RX J0420.0-5022 respectively. We use these values together with those of other ROSAT discovered INSs to constrain the origin, distance and evolutionary status of this particular group of objects. We find that the tangential velocities of radio quiet ROSAT neutron stars are probably consistent with those of ‘normal’ pulsars. Their distribution on the sky and, for those having accurate proper motion vectors, their possible birth places, all point to a local population, probably created in the part of the Gould Belt nearest to the earth.   相似文献   

Numerical modelling of Quaternary terrace staircase formation in the Ebro foreland basin,southern Pyrenees,NE Iberia          下载免费PDF全文

Kurt Martin Stange  Ronald T. Van Balen  Daniel Garcia‐Castellanos  Sierd Cloetingh 《Basin Research》2016,28(1):124-146

The southern foreland basin of the Pyrenees (Ebro basin) is an exorheic drainage basin since Late Miocene times. Remnants of an early exorheic Ebro drainage system are not preserved, but morphology provides evidence for the Pliocene–Quaternary drainage development. The incision history of the Ebro system is denoted by (i) extensive, low gradient pedimentation surfaces which are associated with the denudation of the southern Pyrenean piedmont around the Pliocene–Quaternary transition and (ii) deeply entrenched Quaternary river valleys. Presumably since the Middle Pleistocene fluvial incision intensified involving the formation of extensive terrace staircase in the Ebro basin. Terrace exposure dating in major Ebro tributary rivers indicates climate‐triggered terrace formation in response to glacial–interglacial climate and glacier fluctuations in the Pyrenean headwaters. The overall (semi)parallel longitudinal terrace profiles argue for progressive base level lowering for the whole Ebro drainage network. The landscape evolution model, TISC, is used to evaluate climatic, tectonic and base level scenarios for terrace staircase formation in the Ebro drainage system. Model simulations are compared with morpho‐climatic, tectonic and chronologic data. Results show that climatic fluctuations cause terrace formation, but the incision magnitudes and convergent terrace profiles predicted by this climate model scenario are not consistent with the (semi)parallel terraces in the Ebro basin. A model including previous (late Pliocene) uplift of the lower Ebro basin results in rapid base‐level lowering and erosion along the drainage network, small late stage incision magnitudes and terrace convergence, which are not in agreement with observations. Instead, continuous Quaternary uplift of both the Pyrenees and the Ebro foreland basin triggers (semi)parallel terrace staircase formation in southern Pyrenean tributary rivers in consistency with the observed longitudinal terrace profiles and Middle–Late Pleistocene incision magnitudes. Forward model simulations indicate that the present Ebro drainage system is actively incising, providing further evidence for uplift.  相似文献   

The Northwest Geysers EGS Demonstration Project,California: Pre-stimulation Modeling and Interpretation of the Stimulation     

Jonny Rutqvist  Patrick F. Dobson  Julio Garcia  Craig Hartline  Pierre Jeanne  Curtis M. Oldenburg  Donald W. Vasco  Mark Walters 《Mathematical Geosciences》2015,47(1):3-29

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The green–blue swing: plasticity of plankton food‐webs in response to coastal oceanographic dynamics          下载免费PDF全文

Domenico D'Alelio  Maria Grazia Mazzocchi  Marina Montresor  Diana Sarno  Adriana Zingone  Iole Di Capua  Gayantonia Franzè  Francesca Margiotta  Vincenzo Saggiomo  Maurizio Ribera d'Alcalà 《Marine Ecology》2015,36(4):1155-1170

The internal organization of plankton communities plays a key role in biogeochemical cycles and in the functioning of aquatic ecosystems. In this study, the structure of a marine plankton community (including both unicellular and multicellular organisms) was inferred by applying an ecological network approach to species abundances observed weekly at the long‐term ecological research station MareChiara (LTER‐MC) in the Gulf of Naples (Tyrrhenian Sea, Mediterranean Sea) in the summers of 2002–2009. Two distinct conditions, characterized by different combination of salinity and chlorophyll values, alternated at the site: one influenced by coastal waters, herein named ‘green’, and the other reflecting more offshore conditions, named ‘blue’. The green and blue ‘phases’ showed different keystone biological elements: namely, large diatoms and small‐sized flagellates, respectively. Several correlations amongst species belonging to different trophic groups were found in both phases (connectance ~0.30). In the green phase, several links between phytoplankton and mesozooplankton and within the latter were detected, suggesting matter flow from microbes up to carnivorous zooplankton. A microbial‐loop‐like sub‐web, including mixo‐ and heterotrophic dinoflagellates and ciliates, was present in the green phase, but it was relatively more important in the blue phase. The latter observation suggests a more intense cycling of matter at the microbial trophic level in the blue phase. These results show that different modes of ecological organization can emerge from relatively small changes in the composition of aquatic communities coping with environmental variability. This highlights a significant plasticity in the internal structure of plankton webs, which should be taken into account in predictions of the potential effects of climatic oscillations on aquatic ecosystems and biogeochemical cycles therein.  相似文献   

Geochemistry and Provenance of Springs in a Baja California Sur Mountain Catchment     

Jory C. Lerback  Brenda B. Bowen  C. E. Humphrey  Diego. P. Fernandez  Jeremiah A. Bernau  Shane J. Macfarlan  Eric Schniter  J. J. Garcia 《Ground water》2022,60(2):295-308

Fractured rock aquifers cover much of Earth's surface and are important mountain sites for groundwater recharge but are poorly understood. To investigate groundwater systematics of a fractured-dominated aquifer in Baja California Sur, Mexico, we examined the spatial patterns of aquifer recharge and connectivity using the geochemistry of springs. We evaluate a range of geochemical data within the context of two endmember hypotheses describing spatial recharge patterns and fracture connectivity. Hypothesis 1 is that the aquifer system is segmented, and springs are fed by local recharge. Hypothesis 2 is that the aquifer system is well connected, with dominant recharge occurring in the higher elevations. The study site is a small <15 km² catchment. Thirty-four distinct springs and two wells were identified in the study area, and 24 of these sites were sampled for geochemical analyses along an elevation gradient and canyon transect. These analyses included major ion composition, trace element and strontium isotopes, δ¹⁸O and δ²H isotopes, radiocarbon, and tritium. δ¹⁸O and δ²H isotopes suggest that the precipitation feeding the groundwater system has at least two distinct sources. Carbon isotopes showed a change along the canyon transect, suggesting that shorter flowpaths feed springs in the top of the transect, and longer flowpaths discharge near the bottom. Geochemical interpretations support a combination of the two proposed hypotheses. Understanding of the connectivity and provenance of these springs is significant as they are the primary source of water for the communities that inhabit this region and may be impacted by changes in recharge and use.  相似文献   

North and South Hemispheric Solar Activity for Cycles 21 – 23: Asymmetry and Conditional Volatility of Plage Region Areas     

E. Gonçalves  N. Mendes-Lopes  I. Dorotovič  J. M. Fernandes  A. Garcia 《Solar physics》2014,289(6):2283-2296

We study the dynamic evolution of the time series describing the plage regions areas observed daily at the Observatório Astronómico da Universidade de Coimbra, in each one of the solar hemispheres during solar cycles 21?–?23. The classical ARMA model has proven to be insufficient to describe the time variations seen in the data because of the strong conditional variability. We found that the data are well fitted by ARMA mixed with power-δ TGARCH error models. The power index δ is non-integer; this property has recently been introduced in the literature on time-series analyses and indicates the presence of strong volatility and long memory in the data series. We also detected dynamic asymmetry in the plage region areas observed in the two hemispheres when two different temporal models were obtained to fit them. The finding of a dynamic asymmetry is also supported by the dynamic evolution of the daily difference (north–south) time series, which is significantly different from white noise. This statistical modeling of time series, taking into account new and different characteristics of the solar activity, will be very useful in subsequent forecast developments.  相似文献   

Light absorption by phytoplankton,non-algal particles and dissolved organic matter at the Patagonia shelf-break in spring and summer     

Amábile Ferreira  Virginia M.T. Garcia  Carlos A.E. Garcia 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(12):2162-2174

Satellite image studies and recent in situ sampling have identified conspicuous phytoplankton blooms during spring and summer along the Patagonia shelf-break front. The magnitudes and spectral characteristics of light absorption by total particulate matter (phytoplankton and detritus) and colored dissolved organic matter (CDOM) have been determined by spectrophotometry in that region for spring 2006 and late summer 2007 seasons. In spring, phytoplankton absorption was the dominant optical component of light absorption (60–85%), and CDOM showed variable and important contributions in summer (10–90%). However, there was a lack of correlation between phytoplankton biomass (chlorophyll-a concentration or [chl a]) and the non-algal compartment in both periods. A statistically significant difference was found between the two periods with respect to the CDOM spectral shape parameter (S_cdom), with means of 0.015 (spring) and 0.012 nm^?1 (summer). Nonetheless, the mean S_cdm values, which describe the slope of detritus plus CDOM spectra, did not differ between the periods (average of 0.013 nm^?1). Phytoplankton absorption values in this work showed deviations from mean parameterizations in previous studies, with respect to [chl a], as well as between the two study periods. In spring, despite the microplankton dominance, high specific absorption values and large dispersion were found (a*_ph(440)=0.04±0.03 m² mg [chl a]^?1), which could be attributed to an important influence of photo-protector accessory pigments. In summer, deviations from general trends, with values of a*_ph(440) even higher (0.09±0.02 m² mg [chl a]^?1), were due to the dominance of small cell sizes and also to accessory pigments. These results highlight the difficulty in deriving robust relationships between chlorophyll concentration and phytoplankton absorption coefficients regardless of the season period. The validity of a size parameter (S_f) derived from the absorption spectra has been demonstrated and was shown to describe the size structure of phytoplankton populations, independently of pigment concentration, with mean values of 0.41 in spring and 0.72 in summer. Our results emphasize the need for specific parameterization for the study region and seasonal sampling approach in order to model the inherent optical properties from water reflectance signatures.  相似文献   

Which objective function to calibrate rainfall–runoff models for low-flow index simulations?     

Florine Garcia  Nathalie Folton  Ludovic Oudin 《水文科学杂志》2017,62(7):1149-1166

Much has been written on the subject of objective functions to calibrate rainfall–runoff models. Many studies focus on the best choice for low-flow simulations or different multi-objective purposes. Only a few studies, however, investigate objective functions to optimize the simulations of low-flow indices that are important for water management. Here, we test different objective functions, from single objective functions with different discharge transformations or using low-flow indices, to combinations of single objective functions, and we evaluate their robustness and sensitivity to the rainfall–runoff model. We find that the Kling and Gupta efficiency (KGE) applied to a transformation of discharge is inadequate to fulfil all assessment criteria, whereas the mean of the KGE applied to the discharge and the KGE applied to the inverse of the discharge is sufficient. The robustness depends on the climate variability rather than the objective function and the results are insensitive to the model.