Abundance analysis

Ultimately, however, one seeks abundances from high-resolution spectroscopy, and a full elemental abundance analysis. Data on a broad range of elemental abundances in open clusters have been limited until recently. Fortunately, this situation is beginning to change. 

NGC2506, for which new and better spectra have now been acquired), we will be able to correctly interpret the run of abundances with radius only when all measures will come from a precise and homogeneous spectroscopic analysis. Spectra of several OCs have already been collected using SARG, FEROS, UVES and FLAMES, but this part of the program is in a less advanced status, since abundance analysis for these metal rich stars is very time consuming. 

We are therefore developing a set of routines for an automatic or semiautomatic abundance analysis of stellar spectra based on equivalent widths (EW). The first product is DAOSPEC, a code developed by P. B. Stetson for automatic EW measurement (http //cadcwww.hia.nrc. ca/stetson/daospec/). The preliminary abundance analysis presented here is the first step of an iterative and automatic procedure under development at the Bologna Observatory. 

A weak but useful carbon line [Cl] 8727.13 A disappears in halo dwarfs with metallicities below —1. To measure carbon abundance in halo stars one can use four Cl high excitation lines near 9100 A and the CH band at 4300 A. The Cl lines at 9100 A together with the OI triplet at 7771 A have been used by Tomkin et al. (1992) and Akerman et al. (2004) to study the behaviour of C/O versus metallicity. However, Cl and OI lines employed in these papers are sensitive to a non-LTE effects and one has to bare in mind that this sensitivity is different for C and O. The CH band at 3145 A used by Israelian et al. (1999) is almost saturated in disk stars and several blends makes the abundance analysis less accurate. To ensure a homogeneous analysis of the C/O and N/O ratio from NH,CH and OH lines in the near-UV, we used the same model atmospheres and tools as in our previous studies. The oxygen abundances were compiled from Israelian et al. (1998, 2001) and Boesgaard et al. (1999). 

Fig. la shows the abundance ratio [Ba/Fe] for this sample as a function of [C/Fe]. Thirty stars (77% of the sample) have [Ba/Fe] > +0.7, while the others have [Ba/Fe] < 0.0. There is a clear gap in the Ba abundances between the two groups, suggesting at least two different origins of the carbon excesses. Ba-enhanced stars The Ba-enhanced stars exhibit a correlation between the Ba and C abundance ratios (Fig. la). This fact suggests that carbon was enriched in the same site as Ba. The Ba excesses in these objects presumably originated from the s-process, rather than the r-process, because (1) nine stars in this group for which detailed abundance analysis is available clearly show abundance patterns associated with the s-process [2], and (2) there is no evidence of an r-process excess in the other 21 objects. Hence, the carbon enrichment in these objects most likely arises from Asymptotic Giant Branch (AGB) stars, which are also the source of the s-process elements. 

Accurate abundance analysis of stellar absorption lines is an elaborate physical and numerical exercise involving the following steps (see Fig. 3.3) ... 

Fig. 3.3. Cartoon indicating steps in abundance analysis using model atmospheres. After Gustafsson (1980).

Damped Ly-a absorbers are well suited to abundance analysis because the large column density ensures that O and N are essentially all in their neutral state while sulphur and metals are essentially all singly ionized, with column densities leading to readily measurable spectral features. Also, the degree of depletion from the gas phase by the formation of dust is lower than in the local ISM. Some typical metallicities (based on zinc which is not seriously depleted even in the local... 

This new edition includes results from recent space missions, including WMAP and FUSE, new material on abundances from stellar populations, nebular analysis and meteoric isotopic anomalies, and abundance analysis of X-ray gas, and several extra problems at the end of chapters. 

In principle, mass spectrometry is not suitable to differentiate enantiomers. However, mass spectrometry is able to distinguish between diastereomers and has been applied to stereochemical problems in different areas of chemistry. In the field of chiral cluster chemistry, mass spectrometry, sometimes in combination with chiral chromatography, has been extensively applied to studies of proton- and metal-bound clusters, self-recognition processes, cyclodextrin and crown ethers inclusion complexes, carbohydrate complexes, and others. Several excellent reviews on this topic are nowadays available. A survey of the most relevant examples will be given in this section. Most of the studies was based on ion abundance analysis, often coupled with MIKE and CID ion fragmentation on MS " and FT-ICR mass spectrometric instruments, using Cl, MALDI, FAB, and ESI, and atmospheric pressure ionization (API) methods. 

The relative intensity of the M + 1 and M + 2 peaks can be used to deduce the empirical formula of the compound. Software packages can be used to perform this operation, based on the relative abundance analysis. This is a commonly used method for low-resolution mass spectra, but it has limitations. 

Comparatively low spectral resolution % 0.6 j) and difficulties with the intensity calibration did not warrant any detailed abundance analysis of our spectral material. Instead, the stars with similar spectral types were divided into four groups according to the strength of the lithium line. The groups may be described as follows (see Fig. 1) 1 - the lithium line is... 

The next feature that deserves attention in Fig. 2, is bimodal frequency distribution of lithium line strengths for MO - M4 giants. Here we get some hint at the existence of a gap in the distribution of the lithium abundances of the evolved stars. The results of a more precise abundance analysis of 25 M giants by Luck and... 

For a historical survey of stellar abundance analysis, see J.B. Hearnshaw, The Analysis of Starlight One Hundred and Fifty Years of Astronomical Spectroscopy (Cambridge,... 

Uranium with isotopic abundances different from that of natural uranium is the primary signature for HEU production activities. In any separation technology some enriched uranium will inevitably be released to the environment. Environmental samples taken at or near an enrichment facility can contain some of the enriched material altering the uranium isotopic abundance. Analysis of samples of vegetation, water and soil for uranium isotopic content using a sensitive analytical technique, such as thermal ionization mass spectrometry is recommended as the primary technique for the detection of HEU production. 

Lack of an abundance estimate for a trace element like boron has no effect on the accuracy of the abundance analysis for other elements but merely restricts astrophysical interpretations involving B. On the other hand, helium is an abundant elements with effects on the atmospheric structure and through this on the derived abundances of other elements. Although rarely stated explicitly, abundance analyses of cool stars are dependent on an assumption about the He/H ratio the assumption enters both into the model atmosphere and synthetic spectrum calculations. For normal stars, ignorance about the He/H ratio is mitigated by the fact that the He/H ratio is surely constrained within tight limits (Y = 0.24 to 0.26, see above). 

To black box users, the phrase curve of growth may denote an outdated method of stellar abundance analysis. I prefer to regard a curve of growth (CoG) as an inherent property of a set of photospheric absorption lines and would stress that recognition of basic features of the CoG may ease design of an abundance analysis. 

Reviews on atomic data for abundance analysis have been given by Mendoza (1983), Butler (1993), Storey (1997), Nahar (2002). On-line atomic data bases are available from different sites. For example http //plasma-gate.weizmann.ac.il/DBfAPP.html provides links to many sites of interest, including the site of CLOUDY. The XSTAR atomic data base, constructed by Bautista Kallman (2001) and used in the photoionization code XSTAR can be found at http //heasarc.gsfc.nasa.gov/docs/software/xstar/xstar.html. 

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