Abundance determination

The next level of compositional interpretation is the quantitative estimation of the abundances of the gases present. If the constituent is believed to be uniformly mixed in the atmosphere, then an attempt can be made to determine the single parameter describing the amount of gas present, that is, the constant mole fraction or the column abundance above some level. If the gas is not uniformly mixed, but the spectral information is minimal, then a one-parameter description, such as the vertical mean mole fraction, may still be used. If a significant amount of spectral information is available, it may be possible to obtain a detailed description of the vertical distribution of the gas. Many different approaches have been devised to obtain abundance information from measured spectra. As discussed earlier in this chapter it is convenient to divide these into two groups direct comparison methods and inversion methods. 

In the direct comparison approach, an atmospheric model is assumed, and the radiative transfer equation is used to calculate a spectrum, which is then compared with the measurements along the lines discussed in Chapters 4 and 6. The parameters of the model are varied and calculations are repeated until the theoretical spectrum agrees with the measured spectrum to within prescribed limits. This approach requires an independent knowledge of the temperature as a function of pressure in the atmosphere, which may be obtained either from another portion of the spectmm or from auxiliary measurements. A knowledge of the absorption coefficients for the gases that are optically active in the portion of the spectrum being analyzed is also necessary. The coefficients can be obtained from a combination of theory and laboratory measurements, as discussed in Chapter 3. 

With this approach, the unknown vertical distribution of the gas to be studied is usually parameterized in some way so that only a small number of quantities need be considered. These parameters can then be varied in a systematic way 

It was first necessary to retrieve the upper stratospheric thermal structure. This was accomplished using measurements at 1305 and 1358 cm which are insensitive to CH4. The mole fraction of CH4 was assumed to be known. Spectra were then calculated for various values of CH4 and compared with the measured spectra as shown for three cases in Fig. 8.3.1. The theoretical curves in the figure are labeled according to the ratio. By minimizing the mean residual 

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If a Ce reaction component of known skeletal configuration has been separated by (say) GPC from a reaction mixture, it will consist of a mixture of 13C positional isomers which we may designate X, Y, Z. . ., with corresponding mole fractions x, y, z. . . . If a (n = 4, 5) is the ion abundance determined for this mixture, it follows that... 

Only recently have large numbers of open clusters become the subject of elemental abundance determinations other than those of the lightest elements (Li, C). A survey of the literature (not guaranteed to be complete), including some unpublished data kindly made available for this contribution, revealed [Fe/H] determinations for 45 open clusters. Of these, 33 have determinations of at least some of the a-elements, and these are collected in Table 1. There are very few clusters in common between studies, so it is not possible to investigate systematic differences between studies, nor to bring these measures to a common system. 

A Comparison of Methods for Photospheric Abundance Determinations in K-Type Stars... 

Fig. 1. Model Spectra re-binned to CRIRES Resolution To demonstrate the potential for precise isotopic abundance determination two representative sample absorption spectra, normalized to unity, are shown. They result from a radiative transfer calculation using a hydrostatic MARCS model atmosphere for 3400 K. MARCS stands for Model Atmosphere in a Radiative Convective Scheme the methodology is described in detail e.g. in [1] and references therein. The models are calculated with a spectral bin size corresponding to a Doppler velocity of 1 They are re-binned to the nominal CRIRES resolution (3 p), which even for the slowest rotators is sufficient to resolve absorption lines. The spectral range covers ss of the CRIRES detector-array and has been centered at the band-head of a 29 Si16 O overtone transition at 4029 nm. In both spectra the band-head is clearly visible between the forest of well-separated low- and high-j transitions of the common isotope. The lower spectrum is based on the telluric ratio of the isotopes 28Si/29Si/30Si (92.23 4.67 3.10) whereas the upper spectrum, offset by 0.4 in y-direction, has been calculated for a ratio of 96.00 2.00 2.00.

IR molecular spectra also allow for abundance determination of metals in a fundamentally different way as compared to optical spectroscopy. In that sense CRIRES can also provide an invaluable cross-calibration of optical abundances. 

Abstract. Coronal abundances have been a subject of debate in the last years due to the availability of high-quality X-ray spectra of many cool stars. Coronal abundance determinations have generally been compared to solar photospheric abundances from this a number of general properties have been inferred, such as the presence of a coronal metal depletion with an inverse First Ionization Potential dependence, with a functional form dependent on the activity level. We report a detailed analysis of the coronal abundance of 4 stars with various levels of activity and with accurately known photospheric abundances. The coronal abundance is determined using a line flux analysis and a full determination of the differential emission measure. We show that, when coronal abundances are compared with real photospheric values for the individual stars, the resulting pattern can be very different some active stars with apparent Metal Abundance Deficiency in the corona have coronal abundances that are actually consistent with their photospheric counterparts. 

Abstract. A review is presented on abundance determinations in stars of the Galactic bulge, both in the field and in globular clusters. Previous low-resolution spectroscopy results are revised. Recent high resolution and high S/N spectroscopy results based on Keck-Hires, Gemini-Phoenix and VLT-UVES data are presented. Finally, recent analyses of FLAMES data are discussed. 

The Galactic Bulge contains about 20 percent of the Galaxy s stellar mass. Theories of its formation include a primordial free-fall collapse, remnants of accretion episodes, or secular evolution of bar instabilities ([1]). Accurate stellar abundance determinations can help distinguish between these models. 

We measured the IR triplet of the sulphur, that is to say the following lines 9212.863, 9228.093 and 9237.538. The abundance determination leads to the confirmation that sulphur behaves like an a element, and we can confirm that the [S/Fe] ratio reaches a plateau when the metallicity decreases and becomes lower than —2. The value of this plateau is close to 0,4 dex. 

Table 1 lists our abundances of Fe, C, O and a-elements for the observed clusters. An overall excess of a-elements is shared by all the clusters up to solar metallicity, consistent with SNell being responsible for the gas enrichment. Our findings are also in good agreement both with all previous high resolution studies (see [2], [3], [6]) and with recent abundance determinations for field stars in the Galactic Bulge (see [10]). 

Fig. 4. A summary of the time evolution of primordial 4He abundance determinations (mass fraction Yp) from observations of metal-poor, extragalactic Hu regions (see the text for references). The solid horizontal line is the SBBN-predicted 4He abundance expected for the WMAP (and/or D) inferred baryon density. The two dashed lines show the la uncertainty in this prediction.

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. 

Gustafsson, B. 1980, in P.E. Nissen K. Kjar (eds.), Workshop on Methods of Abundance Determination for Stars, Geneva ESO. 

Pathogen risk assessment Soil inoculum bioassay Indicates potential disease abundance determined in nonstandard laboratory with specialized equipment host specific time-consuming Van Bruggen and Griinwald (1996)... 

Limiting ourselves to the observation of isotopic abundances determined by the 4n + 2 and 4n + 3 decay series, we can construct a concordia diagram (Wetherill, 1956) relating (206p /238 j 207pj /235 j ratios developing at... 

The interest of this method is to make accurate abundance determinations at distances (epochs) that would be inaccessible to conventional stellar and galactic astronomy. The only problem is that the absorbing systems belong to... 

Boynton, W. V, Taylor, G. J., Karunatillake, S., Reedy, R. C. and Keller, J. M. (2008) Elemental abundances determined via the Mars Odyssey GRS. In The Martian Surface Composition, Mineralogy, and Physical Properties, ed. Bell, J. F. Cambridge Cambridge University Press, pp. 105-124. 

Up to now, the sdB/sdOB stars, the classical sdOs and the extremely helium-rich luminous sdOs have been analyzed for the most important (and accessible) metal abundances. The analyses usually require extensive non-LTE line formation calculations to solve the statistical equilibrium in detailed model atoms simultaneously with the radiative transfer equations for all relevant frequencies. With the advent of computer codes based on modern powerful solution algorithms (Auer and Heasley, 1976 Werner and Husfeld, 1985) it has now become possible to test (and eventually remove) approximations necessary in older computations. This and the availability of improved atomic data make the non-LTE predictions more reliable, and obstacles in obtaining accurate abundance determinations come now mainly from the observational side where high-quality spectra are needed to identify and to measure weak... 

A number of WN stars also have circumstellar ring nebulae, for which abundance determinations are of obvious relevance. Nitrogen and helium enrichments have been demonstrated in several of them by Talent and Dufour (1979), Contini and Shaviv (1980), and Kwitter (1981, 1984). 

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