Oxygen-17, abundance

Oxygen abundances in clusters show solar [O/Fe] ratios overall, and fall within the envelope of the distribution with [Fe/H] displayed by the disk field stars... 

The current status of HF abundances from infrared spectroscopy in samples of red-giants from different Galactic stellar populations are summarized in Figure 1. The abundance results displayed in this figure are from Cunha et al. (2003), plus new results for stars at the lowest metallicities, as well as two Orion pre-main-sequence stars. The run of fluorine with metallicity is now probed between oxygen abundances from roughly 7.7 to 8.7. 

A few comments can be drawn from the abundance results presented in Figure 1. The abundances obtained for the red giants in the globular cluster iv Cen-tauri seem to indicate the existence of a sharp decline in the [F/O] ratios as the metallicity approaches the lowest observed oxygen abundance in this globular... 

Abstract. Oxygen abundances of a large number of metal-rich stars, with and without known planets, were derived from the forbidden line [OI] 6300 A, the OI 7771-5 A triplet and from near-UV OH lines. Non-LTE corrections were calculated and applied to the LTE abundance results derived from the OI 7771-5 A triplet. Spectral synthesis was performed for several OH lines. Results from different indicators are compared. We study abundance trends in planet host and comparison sample stars. We find for all the indicators that, on average, [O/Fe] clearly decreases with [Fe/H], with significantly negative slopes in all the linear fits. 

Fig. 1. Oxygen abundances as a function of the activity index, Rx, derived from X-ray data (left-hand panels) and the excitation temperature Texc (right-hand panels). The bottom panels show the difference between [O/Fe] yielded by the OI triplet at about 7774 A and the [OI] A6300 line. Filled circles RS CVn binaries ([2] and [3]), filled squares field subgiants [3], filled triangles Pleiades stars, open triangles Hyades stars, open circles, squares and hexagons disk dwarfs. The source of the literature data for the open cluster and Galactic disk stars can be found in [4].

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). 

The models differ particularly in what concerns the Oxygen abundance of the ejecta in the FST case Oxygen is destroyed due to full CNO burning at the base of the envelope, therefore an lsO poor yield is expected. On the other hand, in the MLT models oxygen is not heavily destroyed, and it is also carried to the surface of the star in the latest AGB phases due to a deep 3rd dredge-up therefore, in this case we expect an oxygen content of the ejecta which is close to the initial value. 

Fig. 3. The 3He abundances (by number relative to hydrogen), j/3 = 10B(3He/H), derived from Galactic H n regions [4], as a function of galactocentric distance (filled circles). Also shown for comparison is the solar system abundance (solar symbol). The open circles are the oxygen abundances for the same H n regions (and for the Sun).

Fig. 3.25. Trends of nebular line strengths in H n regions with oxygen abundance. This figure shows oxygen abundance in H n regions of the Milky Way and spiral and irregular galaxies (determined using measured electron temperatures) vs. log R23, after Pilyugin (2003) the p parameter is the line ratio [O iii]/([0 11] + [O hi]).

Fig. 3.44. Metallicities in gas-poor galaxies (open symbols) and oxygen abundances at a representative radius in gas-rich disk galaxies (filled symbols), as a function of galaxy luminosity in blue light. The dotted lines in each panel represent identical trends for [Fe/H] and [O/H] and the ordinate 0.0 represents solar composition. Adapted from Zaritsky, Kennicutt and Huchra (1994).

Fig. 4.7. 3He/H in simple Galactic H n regions, i.e. those thought to be reasonably well represented by homogeneous spherical models (Balser et al. 1999), and one planetary nebula, as a function of their oxygen abundance. 3He/H is plotted on a logarithmic scale relative to the proto-solar value of 1.5 x 10-5. After Bania, Rood and Balser (2002). Reprinted by permission from Macmillan Publishers Ltd. Courtesy Tom Bania.

Given that the absorption cross-section of 07+ at the photo-ionization threshold is 10-19 cm2 and that the oxygen abundance is 10-2 by mass, find the bound-free opacity (cm2 gm-1) due to oxygen at that frequency, and compare it to the electron scattering opacity. 

According to Eq. (8.6), the primary abundances should increase as the logarithm of the gas fraction, the proportionality coefficient giving another estimate of the yield. For example, from the data in Table 7.9, assuming solar oxygen abundance in the local ISM, the effective yield is between 0.5 and 0.7 Z . Other gas-rich systems in which one may try to test this relationship (which obviously does not apply,... 

Fig. 8.12. Relation between oxygen abundance of H II regions in irregular (open squares) and spiral galaxies (filled circles, taking abundances at 0.4 of the de Vaucouleurs isophotal radius R25) plotted against the gas fraction, after Pilyugin, Vilchez and Contini (2004). The heavy curve shows expectation from the Simple model with an oxygen yield of 0.0027 (or about 0.5 Z ) and the broken curves show the same with the yield 1.5 x higher or lower, whereas the dotted curve shows a yield 4 x lower. The effective yield, defined as Zo/(— ln/z), increases systematically with luminosity, and the gas fraction decreases.

Fig. 8.13. H ii region oxygen abundance against rotational velocity for irregular and spiral galaxies (at a radius of 0.4/ s), after Pilyugin, Vflchez and Contini (2004).

The oxygen abundance then decreases more or less linearly from about 2 Z0 near the centre to Z0 in the solar neighbourhood, reaching about 0.3 Z0 at 20 kpc. 

Figure 8.19 shows an estimate of the distribution function of oxygen abundances among field stars of the Galactic halo and Fig. 8.20 shows the iron abundance... 

Fig. 8.19. Distribution function of oxygen abundances among halo field stars taking [Fe/H] from Fig. 8.15 and assuming an Fe-O relation similar to those in Fig. 8.5, after Pagel (1992b). With kind permission from Kluwer Academic Publishers.

Fig. 8.26. Oxygen abundance distribution function for disk stars. Data deduced from observation are shown by the boxes (same as in Fig. 8.23, after Sommer-Larsen 1991a) and by the dotted line and curve at lower left (after Beers Sommer-Larsen 1995). The solid curve shows the distribution given by the model of Pagel and Tautvaisiene (1995).

Fig. 8.38. Evolution of oxygen abundance as a function of time and Galac-tocentric radius, after Samland, Hensler and Theis (1997). Courtesy Gerhard Hensler.

Results for the Simple model are shown in Fig. 9.5. Oxygen abundance simply increases in proportion to u, whereas that of 7Li starts from a finite base and tends to a limiting value p].,/(a l — 1) owing to its eventual destruction by astration. Deuterium (p = 0 in Eq. 9.25) suffers little destruction as long as the oxygen abundance (or metallicity) is less than about 0.1 of the true yield (assumed to be comparable to solar abundance), e.g. in high-redshift absorption-line clouds, but... 

Fig. 9.5. Schematic of decline of D and growth of 7Li and oxygen abundances in the Simple or homogeneous outflow model, assuming instantaneous recycling with a = 0.67 and yields p(O) = 0.8 Zq(O) p(7Li) = 0.7 Z0(7Li) Zo(7Li) =...

Fig. 9.6. Beryllium abundance as a function of oxygen abundance, according to models (curves) and observations (open circles) by Gilmore et al. (1992). (a in the key is actually the quantity called a in the text.) After Pagel (1994). With kind permission from Kluwer Academic Publishers.

R. von Steiger, S. P. Christon, G. Gloeckler, and F. M. Ipavich. Variable Carbon and Oxygen Abundances in the Solar Wind as Observed in Earth s Magnetosheath by AMPTE/CCE, Astrophys. J., Parti, 389(1992) 791-799. 

It is known that the oxygen abundance in the interstellar medium increases all the time this nucleus is produced by type 11 supernovas which, one after the other, also contribute their iron production to the Galaxy (Fig. 8.7). The pO mechanism is thus likely to grow in importance as the Galaxy evolves. In other words, clues to the Op mechanism should be sought in the early phases of galactic evolution, that is, in halo stars. The fact remains that the two mechanisms induce different evolution in beryllium and boron as a function of oxygen. 

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