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991.
Howell G.M. Edwards Caroline D. Moody Susana E. Jorge Villar Michael J. Russell 《Icarus》2005,175(2):372-381
Raman spectra of an extremophile cyanobacterial colony in hydromagnesite from Lake Salda in Turkey have revealed a biogeological modification which is manifest as aragonite in the stratum associated with the colony. The presence of key spectral biomarkers of organic protectant molecules such as β-carotene and scytonemin indicate that the survival strategy of the cyanobacteria is significantly one of UV-radiation protection. The terrestrial location of this extremophile is worthy of consideration further because of its possible putative link with the “White Rock” formations in Sabaea Terra and Juventae Chasma on Mars. 相似文献
992.
993.
Michael M. Reddy Micaela B. Reddy Kenneth L. Kipp Aaron Burman Paul Schuster Peter S. Rawlik Jr. 《水文研究》2008,22(11):1713-1724
Water quality is a key aspect of the Everglades Restoration Project, the largest water reclamation and ecosystem management project proposed in the United States. Movement of nutrients and contaminants to and from Everglades peat porewater could have important consequences for Everglades water quality and ecosystem restoration activities. In a study of Everglades porewater, we observed complex, seasonally variable peat porewater chloride concentration profiles at several locations. Analyses and interpretation of these changing peat porewater chloride concentration profiles identifies processes controlling conservative solute movement at the peat–surface water interface, that is, solutes whose transport is minimally affected by chemical and biological reactions. We examine, with an advection–diffusion model, how alternating wet and dry climatic conditions in the Florida Everglades mediate movement of chloride between peat porewater and marsh surface water. Changing surface water–chloride concentrations alter gradients at the interface between peat and overlying water and hence alter chloride flux across that interface. Surface water chloride concentrations at two frequently monitored sites vary with marsh water depth, and a transfer function was developed to describe daily marsh surface water chloride concentration as a function of marsh water depth. Model results demonstrate that porewater chloride concentrations are driven by changing surface water chloride concentrations, and a sensitivity analysis suggests that inclusion of advective transport in the model improves the agreement between the calculated and the observed chloride concentration profiles. Published in 2007 by John Wiley & Sons, Ltd. 相似文献
994.
995.
996.
Jerry Edelstein Stuart Bowyer Eric J. Korpela Michael Lampton JoaquÍn Trapero José F. Gómez Carmen Morales Veronica Orozco 《Astrophysics and Space Science》2001,276(1):177-185
The hot interstellar medium (ISM) has far-reaching effect upon thestructure of galaxies. Although ISM heating processes are fairly wellunderstood, after decades of study, the processes that cool the hotinterstellar medium remain obscure. The EURD spectrograph was designed tomeasure the diffuse cosmic background from 350 to 1100 Å in order tostudy the hot ISM and the mechanisms by which it sheds its energy. Wepresent the first analysis of EURD observations of the cosmic background.These EURD observations have proven to be far more sensitive than previouswork; compared to previous results, we have improved the limits to theintensity of 450 to 900 Å line emission from the ISM by one to twoorders of magnitude. Our limit to OVI 1032 Å / 1038 Å doublet of 7900ph s-1 cm-2 str-1 is the lowest yet reported. The EURDlimits to line emission are less intense than predicted by a varietytheoretical models of the local ISM. 相似文献
997.
Steven J. OSTRO R. Scott HUDSON Lance A. M. BENNER Michael C. NOLAN Jon D. GIORGINI Daniel J. SCHEERES Raymond F. JURGENS Randy ROSE 《Meteoritics & planetary science》2001,36(9):1225-1236
Abstract— Goldstone and Arecibo delay‐Doppler radar imaging of asteroid 1998 ML 14 shortly after its discovery reveals a 1 km diameter spheroid with prominent topography on one side and subdued topography on the other. The object's radar and optical properties are typical for S‐class near‐Earth asteroids. The gravitational slopes of a shape model derived from the images and assumed to have a uniform density are shallow, exceeding 30° over only 4% of the surface. If 1998 ML14's density distribution is uniform, then its orbital environment is similar to a planetary body with a spheroidal gravitational field and is relatively stable. Integration of a radar‐refined orbit reveals that the 1998 apparition was the asteroid's closest approach to Earth since at least 1100 and until 2283, when it approaches to within 2.4 lunar distances. Outside of that time interval, orbit uncertainties based on the present set of observations preclude reliable prediction. 相似文献
998.
Liliana Formiggini Noah Brosch Elchanan Almoznino Stuart Bowyer Michael Lampton 《Monthly notices of the Royal Astronomical Society》2002,332(2):441-455
We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars.
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples. 相似文献
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples. 相似文献
999.
1000.
To accomplish its mission, the spaceborne observatory SNAP (SuperNova Acceleration Probe) requires a pointing stability of
<0.03 arcseconds during exposures lasting up to 500 sec. A Monte Carlo simulation of the photoelectron statistics from the
guiding star investigates geometrical (such as the pixel size of the detector or the plate scale) and physical parameters
(such as the magnitude of the star). It is shown that simple centroiding calculations can lead to the desired accuracy with
guide stars as faint as magnitude 16. Availability of these stars is verified thanks to the HST Guide Star Catalog complemented
with a statistical model of the distribution of stars. Thus a through-the-lens sensor that uses stars as faint as magnitude
16 to provide the necessary guiding signals is feasible.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献