We present a multi-wavelength photometric study of similar to 15,000 resolved stars in the nearby spiral galaxy M83 (NGC 5236, D = 4.61 Mpc) based on Hubble Space Telescope Wide Field Camera 3 observations using four filters: F336W, F438W, F555W, and F814W. We select 50 regions (an average size of 260 pc by 280 pc) in the spiral arm and inter-arm areas of M83 and determine the age distribution of the luminous stellar populations in each region. This is accomplished by correcting for extinction toward each individual star by comparing its colors with predictions from stellar isochrones. We compare the resulting luminosity-weighted mean ages of the luminous stars in the 50 regions with those determined from several independent methods, including the number ratio of red-to-blue supergiants, morphological appearance of the regions, surface brightness fluctuations, and the ages of clusters in the regions. We find reasonably good agreement between these methods. We also find that young stars are much more likely to be found in concentrated aggregates along spiral arms, while older stars are more dispersed. These results are consistent with the scenario that star formation is associated with the spiral arms, and stars form primarily in star clusters and then disperse on short timescales to form the field population. The locations of Wolf-Rayet stars are found to correlate with the positions of many of the youngest regions, providing additional support for our ability to accurately estimate ages. We address the effects of spatial resolution on the measured colors, magnitudes, and age estimates. While individual stars can occasionally show measurable differences in the colors and magnitudes, the age estimates for entire regions are only slightly affected.

The Si stable isotope fractionation between metal and silicate has been investigated experimentally at 1800, 2000, and 2200 degrees C. We find that there is a significant silicon stable isotope fractionation at high temperature between metal and silicate in agreement with Shahar et al. (2009). Further we find that this fractionation is insensitive to the structure and composition of the silicate as the fractionation between silicate melt and olivine is insignificant within the error of the analyses. The temperature-dependent silicon isotope fractionation is Delta Si-30(silicate-metal) = 7.45 +/- 0.41 x 10(6)/T-2. We also demonstrate the viability of using laser ablation MC-ICPMS as a tool for measuring silicon isotope ratios in high pressure and temperature experiments. (C) 2011 Elsevier Ltd. All rights reserved.

Doppler weather radar imaging enabled the rapid recovery of the Sutter's Mill meteorite after a rare 4-kiloton of TNT-equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand's parameter = 2.8 +/- 0.3). Sutter's Mill is a regolith breccia composed of CM (Mighei)-type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z similar or equal to 1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 arcmin(2) in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z similar or equal to 1-3 are selected using dropout selection criteria similar to high-redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout-selected LBGs is accurate to within few percent; (2) the UV spectral slope beta is redder than at high redshift (z > 3), where LBGs are less dusty; (3) on average, LBGs at z similar or equal to 1-3 are massive, dustier, and more highly star forming, compared to LBGs at higher redshifts with similar luminosities (0.1L* less than or similar to L less than or similar to 2.5L*), though their median values are similar within 1 sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, finds physically similar galaxies; and (4) the stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of similar to 0.46, and star formation rates are proportional to their stellar masses with a logarithmic slope of similar to 0.90. These relations hold true-within luminosities probed in this study-for LBGs from z similar or equal to 1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z similar or equal to 2, but to avoid any selection biases, and for direct comparison with LBGs at z > 3, a true Lyman break selection at z similar or equal to 2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties and their evolution.

High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using non-traditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors. (C) 2014 Elsevier B.V. All rights reserved.

We describe a unique and novel isotope ratio mass spectrometer (IRMS), the Panorama, developed explicitly for high-mass-resolution analysis of isotopologue ratios of gas samples. The double-focussing instrument routinely operates at a mass resolving power of 40,000 with a maximum useful MRP of similar to 80,000. The instrument achieves this exceptional MRP for a multi-collector using a Matsuda ion optical design with an ESA radius of 1018 mm and a magnetic sector radius of 800 mm. Collectors comprise 9 Faraday cups and a single channel of ion counting each with continuously variable collector slits. First results demonstrate both accuracy and precision comparable to, and in some cases, surpassing, other gas-source multi-collector IRMS instruments for singly-substituted species. For example, accurate bulk D/H and C-13/C-12 for methane gas measured with CH4 as the analyte are measured simultaneously with internal precision of 0.02-0.04 parts per thousand (1 std error) and similar to 0.006 parts per thousand (1 se), respectively. Ion counting with continuous rebalancing of sample and standard gases permits high-precision measurements of rare, multiply-substituted isotopologues with relative abundances as small as similar to 0.1 ppm. In the case of methane, both (CH3D)-C-13/(CH4)-C-12 and (CH2D2)-C-12/(CH4)-C-12 ratios are measured with precision of similar to 0.1 parts per thousand and similar to 0.5 parts per thousand, respectively. Accuracy of the multiply-substituted species measurements is demonstrated using isotope ratio mixing experiments. The ability to measure both Delta(CH3D)-C-13 and Delta CH2D2 (parts per thousand variations relative to the stochastic reference frame) provides heretofore unmatched capabilities to identify kinetic reaction pathways, isotope fractionation during transport, mixing, as well as temperatures of formation for methane gas. The high-resolution instrument can be used for a wide variety of applications. For example, it easily resolves Ar-36(+) from (OO+)-O-18-O-18 for oxygen bond-ordering studies. It also easily resolves (NO+)-N-14-O-16 from (NN+)-N-15-N-15 for measurements of the doubly-substituted N-2 species. (C) 2016 Elsevier B.V. All rights reserved.

We report measurements of resolved (CH2D2)-C-12 and (CH3D)-C-13 at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve (CH2D2)-C-12 from (CH3D)-C-13 provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Delta (CH2D2)-C-12 and Delta (CH3D)-C-13 temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H-2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where Delta (CH2D2)-C-12 and Delta (CH3D)-C-13 values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes. Deficits in (CH2D2)-C-12 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in (CH3D)-C-13 that accompany the low (CH2D2)-C-12 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Delta (CH2D2)-C-12 values are a key tracer of microbial recycling. (C) 2017 Elsevier Ltd. All rights reserved.

Stable isotope compositions of methane (delta C-13 and delta D) and of short-chain alkanes are commonly used to trace the origin and fate of carbon in the continental crust. In continental sedimentary systems, methane is typically produced through thermogenic cracking of organic matter and/or through microbial methanogenesis. However, secondary processes such as mixing, migration or biodegradation can alter the original isotopic and composition of the gas, making the identification and the quantification of primary sources challenging. The recently resolved methane 'clumped' isotopologues Delta(CH3D)-C-13 and Delta(CH2D2)-C-12 are unique indicators of whether methane is at thermodynamic isotopic equilibrium or not, thereby providing insights into formation temperatures and/or into kinetic processes controlling methane generation processes, including microbial methanogenesis.

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controlled attendant isotope fractionations.