Iron, abundance

Astronomical Observatory, were used to carry out the calculations of theoretical equivalent widths of lines, synthetic spectra and a set of plane parallel, line-blanketed, flux constant LTE model atmospheres. The effective temperatures of the stars were determined from photometry, the infrared flux method and corrected, if needed, in order to achieve the LTE excitation balance in the iron abundance results. The gravities were found by forcing Fe I and Fe II to yield the same iron abundances. The microturbulent velocities were determined by forcing Fe I line abundances to be independent of the equivalent width. For more details on the method of analysis and atomic data see Tautvaisiene et al. (2001). 

In addition to results from this study, Table 1 includes two of the relatively more metal-rich globular clusters associated with the Sgr dSph. There appears to be little in common between the two metallicity groups in their < a>-abundances relative to iron. Abundances reported so far for in situ Sgr dSph field stars of comparable metallicities [4] are in accord with those of its metal-rich clusters. 

The microturbulent velocity was set by the need of having an iron abundance independent of the equivalent width of the lines. 

We have determined the iron abundances for a sub-sample of stars 13 Galactic, 13 LMC and 12 SMC Cepheids. Our main result is summarised in the Fig.l, where we plot the V-band residuals 5(My) of our stars from the standard PL relation of [3] as a function of the iron abundance we have derived from the... 

Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. Our data are incompatible with both no dependence of th PL relation on iron abundance and with the linearly decreasing behaviour often found in the literature (e.g. [5], [8]). On the other hand, non-linear theoretical models of [2] provide a fairly good description of the data. For an in-depth discussion see [7]. 

Chemical abundances are inferred from the EW of the lines. Selected lines and atomic data are from our previous paper [5], from [4] and. Stellar parameters were first inferred from Geneva photometry and Hipparcos parallaxes. Then temperatures, microturbulence velocities, gravities and metallicities were iteratively changed in order to i) obey the excitation equilibrium of the Fe I lines ii) require that Fe I and Fe II abundances agree within 0.1-0.15 dex and iii) require that Fe I lines with different equivalent widths (EW) give the same iron abundance. 

Abstract. We present preliminary iron abundances and a element (Ca, Mg) abundance ratios for a sample of 22 Red Giant Branch (RGB) Stars in the Sagittarius galaxy (Sgr), selected near the RGB-Tip. The sample is representative of the Sgr dominant population. The mean iron abundance is [Fe/H]=-0.49. The a element abundance ratios are slightly subsolar, in agreement with the results recently presented by [2]. 

Fig. 1. Upper panel metallicity distribution for the 22 stars analyzed so far. Lower panel a element abundance ratio vs iron abundance for 20 stars of the sample. The [a/Fe] abundance ratio is the average of [Mg/Fe] and [Ca/Fe]. A typical errorbar is also plotted.

Practically all sophisticated stellar evolution models predict the existence of processes altering photospheric abundances on long timescales (see e.g. Pinson-neault, these proceedings). For example, Richard et al. [6] predict iron abundances in turnoff stars of NGC 6397 to be lower by 0.2 dex than in red giants. 

Fig. 3.41. Iron abundances in intermediate-age open Galactic clusters as a function of Galactocentric distance, with the Sun shown by the axes. The line corresponds to a gradient of —0.06 dex kpc-1. Adapted from Friel et al. (2002).

Use the solar Fe I curve of growth in Fig. 3.12 to deduce the solar iron abundance, using 9i0n = 0.9 and given that Fe I has an ionization potential of 7.87 eV and that the partition function of Fe II is 42. Compare the result with the one in Table 3.4. 

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. 8.39. Chemo-spectrophotometric evolution of the solar neighbourhood (left) and the whole Milky Way (right) as a function of time. Panels aA show the oxygen and iron abundances, bB the mass of stars and gas and the star formation rate, cC the extinction in B, V and K bands along a line of sight normal to the plane, dD the luminosity in solar units (taking extinction into account), eE the colour indices and fF the supernova rates. Note that panels aA are in linear units (see Fig. 8.16), while the others are all logarithmic. After Boissier and Prantzos (1999).

Fig. 11.2. Mean stellar iron abundances as a function of luminosity in dwarf spheroidals (filled circles), dwarf ellipticals (open circles), dSph/dlrr transition galaxies (filled diamonds) and dwarf irregulars (open diamonds). Baryonic luminosity in the right panel includes the additional luminosity that irregulars would have if their gas were converted into stars. After Grebel, Gallagher and Harbeck (2003).

Regardless of the details concerning self-enrichment and winds, the existence of isolated star formation bursts will also affect the iron-oxygen and iron-a relations, introducing scatter in Fe/O and possibly gaps in the iron abundance distribution function. When the interval between successive bursts exceeds the evolution time for SN la (maybe about 1 Gyr), iron will build up in the ISM resulting in an enhanced Fe/O ratio in the second burst so that one can end up with [Fe/O] > 0 (Gilmore Wyse 1991) see Fig. 8.7. 

Lithium, on the other hand, displays almost no scatter and its abundance up to 0.1 Zq is independent of the iron abundance. 

SNII events alone explain the observed solar abundance distribution between oxygen and chromium. This can be taken as a major theoretical achievement. Complementary sources of hydrogen, helium, lithium, beryllium, boron, carbon and nitrogen are required, and these have been identified. They are the Big Bang, cosmic rays and intermediate-mass stars. Around iron and a little beyond, we must invoke a contribution from type la supernovas (Pig. 8.5). These must be included to reproduce the evolution of iron abundances, a fact which suggests... 

The abundance of boron varies in very old stars in proportion to their iron abundance. The measurements are very difficult because the useful transitions are in the ultraviolet. The very old stars formed before most of the stellar nucleosynthesis had occurred. The varying nucleosynthesis histories of the matter in differing stars are... 

The abundance of boron seems to increase in value in very old stars in proportion to the increase in the iron abundance. See 10B for this account, which implicates supernova explosions in the production of boron. 

The isotopes of Fe are not expected to occur in all natural samples in their usual proportions. Wide variations are expected within presolar grains but the small iron abundance within them has hindered measurements. 

S4Zn is the/irst stable nucleus after 9Be to have an alpha-particle binding energy less than 4 MeV. This is a manifestation 0/the general decline in binding energies above the iron abundance peak. 

Sensitive observations enable comparative surveys of silicate emission features from disks around low-mass, intermediate-mass, and Sun-like stars. While no strong correlations have been found with disk properties, flatter disks and disks around the coolest stars more often show crystalline silicate features. Cool stars and very low-mass disks display prominent crystalline silicate emission peaks (Apai et al. 2005 Merfn et al. 2001 Pascucci et al 2009). Thus, whatever processes are responsible for the presence of crystals around Sun-like stars must be capable of very efficiently producing crystals around low-mass stars, too. Interferometric measurements suggest that the amorphous/crystalline dust mass fraction is higher in the inner disk than at medium separations (van Boekel et al. 2004 Ratzka et al. 2007). The surveys also show that amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks. In contrast, those with crystalline silicates are always dominated by Mg-rich grains (e.g. Malfait et al. 1998 Bouwman et al. 2008). 

Silicates with olivine composition (MgxFe(i x))2Si04 are common in chondrites, comets, IDPs, and in protoplanetary disks. The Mg-rich end-member of the olivine family is forsterite, also often termed as Foioo the Fe-rich end-member is fayalite (Foo). The interstellar medium contains a similar concentration of the FeO- and MgO-rich silicates (see Chapter 2). Correspondingly, amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks, in cometary dust, and in chondritic IDPs. In stark contrast, crystalline dust is almost always dominated by Mg-rich grains in protoplanetary disks (e.g. Malfait et al. 1998 Bouwman etal. 2008), comet tails (e.g. Crovisier el al. 1997 Wooden et al 2004 Harker et al. 2005 Lisse et al. 2006), in the most primitive and least processed chondritic matrices, and IDPs (for a review, see Wooden et al. 2007). 

Figure 7 Observed evolution of the calcium to iron abundance ratio with metallicity (A Hartmann and Gehren (1998) Zhao and Magain (1990) Gratton and Sneden (1991) Edvardsson et al., (1993)).

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