The chemical compositions of 10 new sub-DLAs and strong Lyman-limit systems at z≲ 1.5     

Joseph D. Meiring  Varsha P. Kulkarni  James T. Lauroesch  Celine Péroux  Pushpa Khare  Donald G. York  Arlin P. S. Crotts 《Monthly notices of the Royal Astronomical Society》2008,384(3):1015-1033

We present chemical abundance measurements from medium-resolution observations of eight subdamped Lyα (sub-DLA) absorber and two strong Lyman-limit systems at   z ≲ 1.5  observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in   z _abs≲ 1.5  quasar absorption line systems focusing on sub-DLA systems. These observations increase the sample of Zn measurements in   z _abs≲ 1.5  sub-DLAs by ∼50 per cent. Lines of Mg  i , Mg  ii , Al  ii , Al  iii , Ca  ii , Mn  ii , Fe  ii and Zn  ii were detected and column densities were determined. Zn  ii , a relatively undepleted element and tracer of the gas-phase metallicity is detected in two of these systems, with  [Zn/H]=−0.05 ± 0.12  and  [Zn/H] > +0.86  . The latter one is however a weak system with   N _{H  I} < 18.8  , and therefore may need significant ionization corrections to the abundances. Fe  ii lines were detected in all systems, with an average Fe abundance of  〈[Fe/H]〉=−0.68  , higher than typical Fe abundances for DLA systems at these redshifts. This high mean [Fe/H] could be due to less depletion of Fe on to dust grains, or to higher abundances in these systems. We also discuss the relative abundances in these absorbers. The systems with high metallicity show high ratios of [Mn/Fe] and [Zn/Fe], as seen previously in another sub-DLA. These higher values of [Mn/Fe] could be a result of heavy depletion of Fe on to grains, unmixed gas, or an intrinsically non-solar abundance pattern. Based on cloudy modelling, we do not expect ionization effects to cause this phenomenon.  相似文献   

The impact of extra-tropical transitioning on storm surge and waves in catastrophe risk modelling: application to the Japanese coastline     

Nicolas Bruneau  Juergen Grieser  Thomas Loridan  Enrica Bellone  Shree Khare 《Natural Hazards》2017,85(2):649-667

Catastrophe risk models are used to assess and manage the economic and societal impacts of natural perils such as tropical cyclones. Large ensembles of event simulations are required to generate useful model output. For example, to estimate the risk due to wind-driven storm surge and waves in tropical cyclone risk models, computationally efficient parametric representations of the wind forcing are required to enable the generation of large ensembles. This paper presents new results on the impact of including explicit representations of extra-tropical transitioning in parametric wind models used to force storm surge and wave simulations in a catastrophe risk modelling context. Extra-tropical transitioning is particularly important in modelling risk on the Japanese coastline, as roughly 40 % of typhoons hitting the Japanese mainland are transitioning before landfall. Using both a historical and idealized track set, we compare maximum storm surge and wave footprints along the Japanese coastline for models that include, and do not include, explicit representations of extra-tropical transitioning. We find that the inclusion of extra-tropical transitioning leads to lower storm surge (10–20 %) and waves (5–15 %) on the southern Japanese coast, with significantly higher storm surge and waves along the northern coast (25–50 %). The results of this paper demonstrate that useful risk assessment of coastal flood risk in Japan must consider the extra-tropical transitioning process.  相似文献   

Organic dust synthesized in reducing environments by ultraviolet radiation or electric discharge     

B. N. Khare  C. Sagan 《Astrophysics and Space Science》1979,65(2):309-312

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Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter     

Sagan C  Khare BN  Thompson WR  McDonald GD  Wing MR  Bada JL  Vo-Dinh T  Arakawa ET 《The Astrophysical journal》1993,414(1):399-405

Polycyclic aromatic hydrocarbons (PAHs) are important components of the interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze. Titan tholin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionization mass spectrometry. A range of two- to four-ring PAHs, some with one to four alkylation sites are identified, with net abundance approximately 10(-4) g g-1 (grams per gram) of tholins produced. Synchronous fluorescence techniques confirm this detection. Titan tholins have proportionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chrysene, prominent in some discussions of interstellar grains, is found in Jupiter tholins. Solid state 13C NMR spectroscopy suggests approximately equal to 25% of the total C in both tholins is tied up in aromatic and/or aliphatic alkenes. IR spectra indicate an upper limit in both tholins of approximately equal to 6% by mass in benzenes, heterocyclics, and PAHs with more than four rings. Condensed PAHs may contribute at most approximately 10% to the observed detached limb haze layers on Titan. As with interstellar PAHs, the synthesis route of planetary PAHs is likely to be via acetylene addition reactions.  相似文献   

Red clouds in reducing atmospheres     

B.N. Khare  Carl Sagan 《Icarus》1973,20(3):311-321

A dark reddish-brown high-molecular-weight polymer is produced by long wavelength ultraviolet irradiation of abundant gases in reducing planetary atmospheres. The polymer i examined by paper chromatography, mass spectrometry, and infrared, visible, and ultraviolet spectroscopy. High carbon-number straight-chain alkanes with NH₂ and, probably, OH and CO groups are identified, along with the previously reported amino acids. There are chemical similarities between this polymer and organic compounds recovered from carbonaceous chondrites and precambrian sediments. The visible and near-ultraviolet transmission spectrum of the polymer shows its absorption optical depth to be redder than λ^?2 and perhaps similar in coloration to the clouds of Jupiter, Saturn, and Titan. The near-ultraviolet absorption coefficient is ～10³ cm^?1, and typical grain sizes ～30 μm. The nitrile content is small, and the polymer should be semitransparent in the 5 μm atmospheric window. Such polymers may be a common constituent of clouds in the outer solar system and on the early Earth.  相似文献   

Average extinction curves and relative abundances for quasi-stellar object absorption-line systems at 1 ≤z_abs < 2     

Donald G. York  Pushpa Khare  Daniel Vanden Berk  Varsha P. Kulkarni  Arlin P. S. Crotts  James T. Lauroesch  Gordon T. Richards  Donald P. Schneider  Daniel E. Welty  Yusra Alsayyad  Abhishek Kumar  Britt Lundgren  Natela Shanidze  Tristan Smith  Johnny Vanlandingham  Britt Baugher  Patrick B. Hall  Edward B. Jenkins  Brice Menard  Sandhya Rao  Jason Tumlinson  David Turnshek  Ching-Wa Yip  Jon Brinkmann 《Monthly notices of the Royal Astronomical Society》2006,367(3):945-978

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New abundance determinations in z < 1.5 QSO absorbers: seven sub-DLAs and one DLA     

Joseph D. Meiring  James T. Lauroesch  Varsha P. Kulkarni  Celine Péroux  Pushpa Khare  Donald G. York  Arlin P. S. Crotts 《Monthly notices of the Royal Astronomical Society》2007,376(2):557-572

We present chemical abundance measurements from high-resolution observations of seven subdamped Lyα (sub-DLA) absorbers and one DLA system at   z < 1.5  . Three of these objects have high metallicity, with near or supersolar Zn abundance. Grids of cloudy models for each system were constructed to look for possible ionization effects in these systems. For the systems in which we could constrain the ionization parameter, we find that the ionization corrections as predicted by the cloudy models are generally small and within the typical error bars (∼0.15 dex), in general agreement with previous studies. The Al  iii to Al  ii ratio for these and other absorbers from the literature are compared, and we find that while the sub-DLAs have a larger scatter in the Al  iii to Al  ii ratios than the DLAs, there appears to be little correlation between the ratio and   N _{H  i}   . The relationship between the metallicity and the velocity width of the profile for these systems is investigated. We show that the sub-DLAs that have been observed to date follow a similar trend as DLA absorbers, with the more metal rich systems exhibiting large velocity widths. We also find that the systems at the upper edge of this relationship with high metallicities and large velocity widths are more likely to be sub-DLAs than DLA absorbers, perhaps implying that the sub-DLA absorbers are more representative of massive galaxies.  相似文献   

Intergalactic UV background radiation field     

Snigdha Das  Pushpa Khare 《Journal of Astrophysics and Astronomy》1997,18(2-3):133-143

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Bianchi type-I cosmological models with time dependent q and Λ-term in general relativity     

Anirudh Pradhan  Rekha Jaiswal  Rajeev Kumar Khare 《Astrophysics and Space Science》2013,343(1):489-497

On getting motivation from increasing evidence for the need of a geometry that resembles Bianchi morphology to explain the observed anisotropy in the WMAP data, Einstein’s field equations with variable cosmological “constant” are considered in presence of perfect fluid for a homogeneous and anisotropic Bianchi type-I space-time. Einstein’s field equations are solved by considering a time dependent deceleration parameter which affords a late time acceleration in the universe. The cosmological constant Λ is found to be a decreasing function of time and it approaches a small positive value at the present epoch which is corroborated by consequences from recent supernovae Ia observations. From recently developed Statefinder pair, the behavior of different stages of the evolution of the universe has been studied. The physical significance of the cosmological models have also been discussed.  相似文献   

Indicative proxy evidences of the changing monsoonal precipitation during the last millennium over central west coast of India: A possible link to solar variability     

N. Khare 《Oceanology》2012,52(4):495-504

Proxy records of paleomonsoonal variations were obtained, utilizing foraminiferal tracers in two shallow water sediment cores collected from the inner shelf region off Karwar, central west coast of India. The preliminary results record the average signature of relatively better monsoonal precipitation at around 1400?C1410, 1575?C1590 and 1750 AD with intermittent dry phases from around (1290?C1355 AD), (1425?C1535 AD) and (1600?C1665 AD) due to general reduction in the intensity of the rainfall. A comparison with the solar variability (Wolf, Sporer and Maunder sun spot minima) during the last thousand years suggests a possible linkage between the rainfall pattern and solar variability. The presented results though preliminary in nature due to limited dating of the sediments put forth only a possibility of inter-correspondence between solar variability and rainfall intensity. To further augment the reasoning, a number of sediment cores with close spaced dating control need to be studied from geographically distinct regions.  相似文献