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Search Results: 1 - 10 of 208649 matches for " Harriet L. Dinerstein "
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Zinc abundances of planetary nebulae
Christina L. Smith,Albert A. Zijlstra,Harriet L. Dinerstein
Physics , 2014, DOI: 10.1093/mnras/stu696
Abstract: Zinc is a useful surrogate element for measuring Fe/H as, unlike iron, it is not depleted in the gas phase media. Zn/H and O/Zn ratios have been derived using the [Zn IV] emission line at 3.625um for a sample of nine Galactic planetary nebulae, seven of which are based upon new observations using the VLT. Based on photoionization models, O/O++ is the most reliable ionisation correction factor for zinc that can readily be determined from optical emission lines, with an estimated accuracy of 10% or better for all targets in our sample. The majority of the sample is found to be sub-solar in [Zn/H]. [O/Zn] in half of the sample is found to be consistent with Solar within uncertainties, whereas the remaining half are enhanced in [O/Zn]. [Zn/H] and [O/Zn] as functions of Galactocentric distance have been investigated and there is little evidence to support a trend in either case.
ISO SWS Observations of H II Regions in NGC 6822 and I ZW 36: Sulfur Abundances and Temperature Fluctuations
Joshua G. Nollenberg,Evan D. Skillman,Donald R. Garnett,Harriet L. Dinerstein
Physics , 2002, DOI: 10.1086/344299
Abstract: We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine ionic sulfur abundances in these H II regions. There is relatively good agreement between our observations and predictions of S^+3 abundances based on photoionization calculations, although there is an offset in the sense that the models overpredict the S^+3 abundances. We emphasize a need for more observations of this type in order to place nebular sulfur abundance determinations on firmer ground. The S/O ratios derived using the ISO observations in combination with optical data are consistent with values of S/O, derived from optical measurements of other metal-poor galaxies. We present a new formalism for the simultaneous determination of the temperature, temperature fluctuations, and abundances in a nebula, given a mix of optical and infrared observed line ratios. The uncertainties in our ISO measurements and the lack of observations of [S III] lambda 9532 or lambda 9069 do not allow an accurate determination of the amplitude of temperature fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we illustrate the diagnostic power and limitations of our new method.
Observations of [S IV] 10.5 micron and [Ne II] 12.8 micron in Two Halo Planetary Nebulae: Implications for Chemical Self-Enrichment
Harriet L. Dinerstein,Matthew J. Richter,John H. Lacy,K. Sellgren
Physics , 2002, DOI: 10.1086/345572
Abstract: We have detected the [S IV] 10.5 micron and [Ne II] 12.8 micron fine-structure lines in the halo population planetary nebula (PN) DdDm 1, and set upper limits on their intensities in the halo PN H 4-1. We also present new measurements of optical lines from various ions of S, Ne, O, and H for DdDm 1, based on a high-dispersion spectrum covering the spectral range 3800 A to 1 micron. These nebulae have similar O/H abundances, (O/H) = 1e-4, but S/H and Ne/H are about half an order of magnitude lower in H 4-1 than in DdDm 1; thus H 4-1 appears to belong to a more metal-poor population. This supports previous suggestions that PNe arising from metal-poor progenitor stars can have elevated oxygen abundances due to internal nucleosynthesis and convective dredge-up. It is generally accepted that high abundances of carbon in many PNe results from self-enrichment. To the extent that oxygen can also be affected, the use of nebular O/H values to infer the overall metallicity of a parent stellar population (for example, in external galaxies) may be suspect, particularly for low metallicities.
Infrared Emission of Normal Galaxies from 2.5 to 12 Microns: ISO Spectra, Near-Infrared Continuum and Mid-Infrared Emission Features
Nanyao Lu,George Helou,Michael W. Werner,Harriet L. Dinerstein,Daniel A. Dale,Nancy A. Silbermann,Sangeeta Malhotra,Charles A. Beichman,Thomas H. Jarrett
Physics , 2003, DOI: 10.1086/374043
Abstract: We present ISO-PHOT spectra of the regions 2.5-4.9um and 5.8-11.6um for a sample of 45 disk galaxies from the U.S. ISO Key Project on Normal Galaxies. The spectra can be decomposed into three spectral components: (1) continuum emission from stellar photospheres, which dominates the near-infrared (2.5- 4.9um; NIR) spectral region; (2) a weak NIR excess continuum, which has a color temperature of ~ 1000K, carries a luminosity of a few percent of the total far-infrared luminosity L(FIR), and most likely arises from the ISM; and (3) the well-known broad emission features at 6.2, 7.7, 8.6 and 11.3 um, which are generally attributed to aromatic carbon particles. These aromatic features in emission (AFEs) dominate the mid-infrared (5.8-11.6 um; MIR) part of the spectrum, and resemble the so-called Type-A spectra observed in many non-stellar sources and the diffuse ISM in our own Galaxy. The relative strengths of the AFEs vary by 15-25% among the galaxies. However, little correlation is seen between these variations and either IRAS 60um-to-100um flux density ratio R(60/100) or the FIR-to-blue luminosity ratio L(FIR)/L(B), suggesting that the observed variations are not a direct consequence of the radiation field differences among the galaxies. We demonstrate that the NIR excess continuum and AFE emission are correlated, suggesting that they are produced by similar mechanisms and similar (or the same) material. On the other hand, as the current star-formation activity increases, the overall strengths of the AFEs and the NIR excess continuum drop significantly with respect to that of the far-infrared emission from large dust grains. This is likely a consequence of the preferential destruction in intense radiation fields of the small carriers responsible for the NIR/AFE emission.
Food Commodity Prices Volatility: The Role of Biofuels  [PDF]
Christopher L. Gilbert, Harriet K. Mugera
Natural Resources (NR) , 2014, DOI: 10.4236/nr.2014.55019
Abstract:

Food commodity prices have recently increased sharply and become more volatile, highlighting greater uncertainty in markets and threatening global food security. High fuel prices combined with legislative mandates have increased biofuel production raising the average cost of food on the global market and particularly in developing countries and established a link between crude oil and agricultural prices. We investigate the role of biofuels in explaining increased volatility in food commodities. Multivariate GARCH models and volatility decompositions are estimated on grains and crude oil daily prices over a twelve-year sample from 2000-2011. We find increases in correlations and co-movements between grains and crude oils prices after 2006 and particularly in 2008 when crude oil prices were high. Increased volatility in grains during the 2008-09 spike was largely due to shocks transmitted from crude oil to grains especially corn, wheat and soybean prices.

An Infrared Survey of Neutron-Capture Elements in Planetary Nebulae
N. C. Sterling,H. L. Dinerstein
Revista mexicana de astronomía y astrofísica , 2005,
Abstract:
UV Observations of Neutron Capture Elements in Planetary Nebulae
N. C. Sterling,H. L. Dinerstein
Revista mexicana de astronomía y astrofísica , 2003,
Abstract: Se presentan resultados de una búsqueda entre los datos existentes del Far Ultraviolet Spectroscopic Explorer(FUSE) y del HST, de especies formandas por captura de neutrones en nebulosas planetarias (NP), que se pueden enriquecer por captura lenta de neutrones (proceso-s) en las estrellas progenitoras. La medición de tales enriquecimientos arroja luz sobre el proceso-s en estrellas AGB y el enriquecimiento de elementos pesados en el medio interestelar. Derivamos abundancias de Ge (Z=32) relativas a S ó Fe de observaciones de Ge III λ1088.46 para cinco NP; cuatro de estas tienen abundancias de Ge un factor >3-10 por arriba de la solar, dependiendo de las suposiciones sobre la pérdida en el polvo. En contraste, encontramos abundancia aproximadamente solar para el Ge en IC 4776, y tambien en el medio interestelar hacia Abell 36 segun se deriva de Ge II λ1237.06. Otro elemento formado por captura de neutrones, Ga (Z = 31), probablemente se ha detectado en SwSt 1 por medio de Ga III λ1495.05, con una intensidad que indica una gran abundancia de Ga. La evidencia mas fuerte de enriquecimiento de Ge se observa en NP con estrellas centrales Wolf-Rayet (WR) deficientes en H y ricas en C. Aunque el camino evolutivo que produce las estrellas centrales WR no se conoce bien, es probable que estos objetos han experimentado mezcla profunda y dragado de material procesado nuclearmente.
The o II recombination line abundance problem in planetary nebulae
D. R. Garnett,H. L. Dinerstein
Revista mexicana de astronomía y astrofísica , 2001,
Abstract: Presentamos observaciones de l neas de recombinaci on de O II en diez nebu- losas planetarias brillantes, y mediciones de O II y [O III] con resoluci on espacial en NGC 6720. Esto es con el n de estudiar la discrepancia entre las abundancias calculadas con l neas de recombinaci on de O II y aquellas derivadas de l neas de excitaci on colisional de [O III]. Observamos una gran variaci on en la discrepan- cia, desde tener resultados consistentes hasta una diferencia de un factor de seis. La magnitud de esta discrepancia esta anti-correlacionada con el brillo super cial de la nebulosa (aquellas que son compactas y m as brillantes tienen las menores variaciones). Los niveles de O II poblados a trav es de recombinaci on di-electr onica dan abundancias mayores que otros niveles. Nuestra observaci on en rendija larga muestra que la emisi on en O II tiene su m aximo al interior de la c ascara brillante donde [O III] y H son m as brillantes. Con base en las correlaciones observadas, proponemos que la intensa emisi on en l neas de recombinaci on en nebulosas plan- etarias proviene de recombinaci on di-electr onica aumentada en el interior de la nebulosa, la cual posiblemente est e impulsada por un viento estelar caliente.
Spatially-Resolved O II Recombination Line Observations of the Ring Nebula, NGC 6720
D. R. Garnett,H. L. Dinerstein
Physics , 2001, DOI: 10.1086/322452
Abstract: We present spatially-resolved spectral of O II permitted lines and [O III] forbidden lines in the Ring Nebula NGC 6720. We find significant differences in the spatial distribution of the O II and [O III] lines. The [O III] emission follows the H-beta emission measure; however, the O II emission peaks closer to the central star. This suggests that radiative recombination may not be the primary mechanism for producing the O II lines. O+2 abundances derived from O II lines are 5-10 times larger than those derived from [O III] in the region within 20" of the central star, but agree to within 0.2-0.3 dex outside this region. The [O III] electron temperature rises smoothly from about 10,000 K in the outer shell to about 12,000 K in the center; we see no evidence for a temperature jump that would be associated with a shock. If temperature fluctuations are responsible for the discrepancy in O+2 abundances, the average temperature would have to be approximately 6,500 K in the He$^{+2}$ zone and about 9,000 K in the outer shell in order to force the [O III]-derived abundance to equal that derived from O II. We therefore argue that temperature fluctuations can not explain the abundance discrepancy. The O II emission does not peak at the locations of dusty knots, creating difficulties for models which explain the O II - [O III] discrepancy by density fluctuations. We examine the possibility high-temperature dielectronic recombination in a central hot bubble enhances the O II line strengths in the central nebula. However, comparison of recombination rates with collisional excitation rates shows that the increase in recombination emission due to dielectronic recombination at T ~ 10^5 K is not sufficient to overcome the increase in [O III] emission. (Abridged)
The O II Recombination Line Abundance Problem in Planetary Nebulae
D. R. Garnett,H. L. Dinerstein
Physics , 2001,
Abstract: We present new observations of O II recombination lines in ten bright planetary nebulae, along with spatially-resolved measurements of O II and [O III] in the Ring nebula NGC 6720, to study the discrepancy between abundances derived from O II recombination lines and those derived from collisionally-excited [O III]. We see a large range in the difference between O II- and [O III] derived abundances, from no difference up to a factor six difference. The size of this discrepancy is anti-correlated with nebular surface brightness; compact, high-surface-brightness nebulae have the smallest discrepancies. O II levels that are populated mainly by dielectronic recombination give larger abundances than other levels. Finally, our long-slit observation of the Ring nebula shows that the O II emission peaks interior to the bright shell where [O III] and H-beta are strongest. Based on the observed correlations, we propose that the strong recombination line emission in planetary nebulae is a result of enhanced dielectronic recombination in hot gas in the nebular interior, perhaps driven by a hot stellar wind.
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