Abundances in the Galaxy

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

Recent observations of the HF (1-0) R9 line at 2.3 /tm with the Phoenix spectrograph on the Gemini-South telescope has opened a new window that sheds light on understanding the chemical evolution of fluorine and the nuclear processes that produce this element. Until recently, only a small number of observations of fluorine were available and the trend of fluorine abundances with metallicity had yet to be probed in the Galaxy. 

We have used the infra-red triplet (921.286 nm, 922.809 nm and 923.754 nm) to determine the sulphur abundance in the 32 giants from ESO s Large Program Galaxy Formation, Early Nucleosynthesis, and First Stars . 

In the Galaxy, we know 93 (3 Cephei (Stankov Handler 2004) and about 100 SPB-type stars (De Cat et al. 2004). They fall within the instability strips predicted by the theory. The K-mechanism driving pulsations in (3 Cephei and SPB stars strongly depends on the abundance of the iron-group ions in the driving zone at temperatures around 2 x 105 K (Dziembowski Pamyatnykh 1993, Dziembowski et al. 1993). Theoretical models predict that pulsations of (3 Cephei and SPB-type vanish for Z = 0.01 and Z = 0.006, respectively (Pamyatnykh 1999). 

Abstract. We report on preliminary results of VLT/FLAMES observations of the old open clusters NGC 2506, Mel 66 and Cr 261, obtained as part of our Guaranteed Time on this instrument. We focus in particular on the very old cluster Cr 261, one of the oldest open clusters in the Galaxy. We compare the derived Li abundances with those of other old clusters, and we discuss briefly Li depletion on the main-sequence from the age of the Hyades to 8 Gyr. 

Detailed elemental abundances are now available for several individual stars in the Galaxy s dwarf satellites (Shetrone et a1. 2001, 2003 Geisler et al. 2005 also see the reviews in this proceedings). A comparison of these abundance ratios to those of stars in the Galaxy can be used to address several questions related to galaxy formation and evolution, as well as stellar nucleosynthesis. 

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. 

It is evident from Fig. 1 that consistency with the observed abundance of 3He in the Galaxy is achieved only if the fraction of low-mass stars (M < 2.5 Mq) undergoing extra-mixing is larger than 90%, assuming the 3He yields of... 

Adding to this general context the fact that the FLAMES facility at the VLT was offered to the community one year earlier, and that it was starting to produce an impressive wealth of abundance data of stars in the Galaxy and in our neighborhoods, the broad concept of the ESO-Arcetri workshop on Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites was built. 

The existence and distribution of the chemical elements and their isotopes is a consequence of nuclear processes that have taken place in the past in the Big Bang and subsequently in stars and in the interstellar medium (ISM) where they are still ongoing. These processes are studied theoretically, experimentally and obser-vationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmo-chemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium. 

Broadly speaking, the abundances in nearby galaxies show two main features ... 

Consider star formation in a spherical galaxy of unit mass, consisting initially of gas. Stars are formed from the gas, which simultaneously contracts through the stars as a result of energy dissipation by cloud collisions etc. The abundance in the gas (assumed uniform) is still z = ln(l/g) as in the Simple model (Chapter 8), where g is the mass of gas remaining and z is in units of the yield. 

The motivation is the same as that which lannched spectroscopy in such a spectacular way over the past thirty years, that is, to test the theory of nucleosynthesis and the chemical evolntion of galaxies. Cosmologists are delighted to point ont that today we may assess chemical abundances in the remote Universe by pure observation, whereas even ten years ago, such a feat remained ont of reach. 

In order to reconstruct relative abundances of these nuclei at source, we must first expurgate all the fragmentation debris. This is done with the help of a model to be described shortly. The Galaxy is not totally closed as regards cosmic ray movements. Three dangers await any particle launched at high speed in the Galaxy ... 

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. 

The logical foundation of the model is as follows apart from the lightest elements, the history of the other elements in the Galaxy is dominated by nucleosynthesis in many generations of stars. Each one has a different history to all the others, at least, on the face of things. However, species with a common origin, for example, those produced abundantly by such and such a type of star, are likely to evolve in parallel. The picture we... 

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