Abundances nebulae

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed, or usually assumed to be identical to that of the sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the sun s interior by nuclear reactions. The composition of the sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy, and accordingly the relative abundances of the elements in the sun are referred to as "cosmic abundances." Although the cosmic abundance pattern is commonly seen in other stars there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as in the case with neutron stars. The... 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

Aqueous chemistry is one of the oldest forces of change in the solar system. It started less than 20 million years after the gases of the solar nebula began to coalesce into solid objects.2 Water is also the most abundant volatile molecule in comets. On the earth, the oceans alone contain about 1.4 x 1021 kilograms or 320,000,000 cubic miles of water. Another 0.8 x 1021 kilogram is held within the rocks of the earth s crust, existing in the form of water of hydration. The human... 

Abstract. We present recent advances in the determination of chemical abundances of galactic Planetary Nebulae and discuss implications resulting from the comparison with theoretical predictions. From the analysis of diagrams of abundances of N/O vs He/H, N/O vs N/H and N/O vs O/H we argue that very likely the often used solar photospheric abundance of oxygen of 8.9, in usual units, is overestimated by a factor of 2-3, as suggested by very recent work in the Sun. This would solve an astrophysical problem with the measured abundances in planetaries. 

We mention that recently (c.f. Liu et al., 2004) the dichotomy between chemical abundances of heavy elements derived using a) the classical intense forbidden lines, as in the traditional method or b) the much fainter recombination lines, appears solved with both methods correct a) probes the main body of the nebula, ionized and warm (10.000 K or so), while b) refers to a minor fraction in mass (few per cent) of the nebula (ionized but cold 1000-2000 K). 

Fig. 1. Interstellar 3He/H abundances as a function of source metallicity [2], The [3He/H] abundances by number derived for the H n region sample are given with respect to the solar ratio. Also shown is the abundance derived for the planetary nebula NGC3242 (triangle). We note that there is no trend in the 3He/H abundance with source metallicity...

Abundance Variations in the Galactic Disk Planetary Nebulae, Open Clusters and Field Stars... 

Fig. 1. Li EW (left panel) and abundances (right panel) of NGC 6530 stars compared to those of the coeval Orion Nebula cluster, using the same growth curves. Most of our targets show very strong Li lines increasing at lower temperatures. The results are not corrected for NLTE and veiling effects, as in Orion in [3]. Absolute values are overestimated but, relative values do not suffer from systematic errors.

Hu region abundances in gas-rich dwarfs. Richer, McCall, Stasinska (1998) compared dlrr H n region O abundances with O abundances of planetary nebulae (PNe) in dSphs. While the offset persisted, PNe have only been detected in the two most luminous dSphs and trace primarily intermediate-age populations as opposed to the present-day abundances in Hu regions. 

Radioastronomers first learned of 3He in 1955 at the fourth I.A.U. Symposium in Jodrell Bank, when the frequency of the hyperfine 3He+ line at 8.666 GHz (3.46 cm) was included by Charles Townes in a list of radio-frequency lines of interest to astronomy (Townes 1957). The line was (probably) detected for the first time only twenty years later, by Rood, Wilson Steigman (1979) in W51, opening the way to the determination of the 3He abundance in the interstellar gas of our Galaxy via direct (although technically challenging) radioastronomical observations. In the last two decades, a considerable collection of 3He+ abundance determinations has been assembled in Hi I regions and planetary nebulae. The relevance of these results will be discussed in Sect. 4 and 5 respectively. 

The most direct, model independent, way to test the validity of the mixing solution is to measure the 3He abundance in the ejecta of low-mass stars, i.e. in planetary nebulae (PNe). The search for 3He in the ejecta of PNe via the 8.667 GHz spin-flip transition of 3He+, painstakingly carried out by Rood and coworkers at the Green Bank radiotelescope since 1992 (see summary of results in Balser et al. 1997), has produced so far one solid detection (NGC 3242, see Rood, Bania, Wilson 1992 confirmed with the Effelsberg radiotelescope by... 

The Sun formed some 4.5 Gyr ago (Gyr is a Gigayear or 109 years) from its own gas cloud called the solar nebula, which consisted of mainly hydrogen but also all of the heavier elements that are observed in the spectrum of the Sun. Similarly, the elemental abundance on the Earth and all of the planets was defined by the composition of the solar nebula and so was ultimately responsible for the molecular inventory necessary for life. The solar system formed from a slowly rotating nebula that contracted around the proto-sun, forming the system of planets called the solar system. Astronomers have recently discovered solar systems around... 

The volatile-trapping mechanism has a further problem associated with the temperature. Very volatile molecules such as N2, CO and CH4 are not easily trapped in laboratory ice simulation experiments unless the ice temperature is 75 K, which is somewhat lower than the estimated Saturnian subnebula temperature. This has led to the suggestion that the primary source of nitrogen within the Titan surface ices was NH3, which became rapidly photolysed to produce H2 and N2 upon release from the ice. The surface gravity is insufficient to trap the H2 formed and this would be lost to space. However, the origin of methane on Titan is an interesting question. Methane is a minor component of comets, with a CH4/CO ratio of clCT1 compared with the present atmospheric ratio of > 102. The D/H ratio is also intermediate between that of comets and the solar nebula, so there must be an alternative source of methane that maintains the carbon isotope ratio and the D/H isotope ratio and explains the abundance on Titan. 

Fig. 3.40. Abundances in Galactic stars, H n regions and planetary nebulae, as a function of Galactocentric distance, with the Sun shown for comparison. After Hou, Prantzos and Boissier (2000). The curves show a model calculation by the authors nitrogen is underproduced in the model because only massive stars were considered. 

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