Stellar population

A stellar population is a group of stars that resemble each other in spatial distribution, chemical composition (metallicity) and age. In astrophysics all elements heavier than iron are called metals. Therefore, metallicity denotes the content of elements heavier than helium in a star. When we say e.g. a nebula is rich in oxygen or nitrogen it does not mean that it is rich in these elements from the point of view of chemistry we know on Earth. 

The metallicity of an astronomical object may provide a clue to its age. During the Big Bang practically no metals were produced. Therefore older stars have a 

Population III stars exist only in theory. It is assumed that they consisted only of the primordial chemical elements H and He. They were extremely massive and therefore they evolved rapidly and ended in a supernova explosion. Rollinde et al., 2009 [279] investigated the influence of population III stars on the chemical evolution of the universe. Population III stars with masses between 30 and 40 M could explain a sharp decrease of the number of low-mass stars at very low iron abundance, which is not reproduced in models with only PopII/I stars. 

The oldest observed stars are population II stars. They have very low metallic-ities. Stars of population I are the youngest stars with a relatively high metallicity. The metallicity of the Sun is approximately 1.6% by mass. 

The distribution of different stellar populations throughout the Galaxy is illustrated in Fig. 8.10. 

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Age Stellar ages can be derived using isochrones. However, such data can not be used to select stars for studies of the thick disk as this must imply a prior knowledge of the age of the stellar population in this disk. This is knowledge that we seek, but do not have. 

There are a number of important observational facts that we now have established for the stellar population of solar neighbourhood stars that have kinemat-... 

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. 

With these accurate kinematical data and the future determination of the global metallicity for each star, we plan to study the possible mixing of stellar populations. 

Abstract. We present preliminary results of an extensive low and high-resolution ESO-VLT spectroscopic survey of Subgiant stars in the stellar system uj Centauri. Basing on infrared Ca II triplet lines we derived metallicities and radial velocities for more than 110 stars belonging to different stellar populations of the system. The most metal rich component, the SGB-a, appears to have metallicity [Fe/H] -0.5. Moreover, SGB-a stars have been found to stray from the dynamical behaviour of the bulk population. Such evidence adds new puzzling questions on the formation and the chemical enrichment history of this stellar system. 

How will we identify the extra astrophysics required to reconcile the properties of CDM dark haloes with those of luminous galaxies We can start by developing knowledge of the evolutionary history of at least one place in at least one galaxy. We would be unlucky if that place were far from the norm alternatively, any theory that predicts such a history to be very unusual might be suspect -the galaxian Copernican principle. Kinematics and current spatial location are of course critical parameters, so that traditional stellar populations analyses are... 

That is, the straightforward interpretation of abundance data for Galactic field stars in terms of stellar populations is feasible only because the Galaxy apparently acquired its gas early, or at a rate which was well-matched to the star formation rate across the whole volume now sampled by local halo stars, and kept this gas well-mixed and because the stellar IMF is (close to) invariant over time and metallicity. Neither deduction was obvious, nor is the underlying physics understood. However, these two deductions apply so well they have become assumed authors use any violation to rule out some possible Galaxy merger histories, as in the Venn et al. analysis from which Figure 1 is taken. 

We now briefly consider in turn the abundance constraints on the major Galactic stellar populations, highlighting the poorly known aspects. 

Fig. 2. The distribution of specific angular momentum in the dominant Galactic stellar populations. This indicates the similarity in this fundamental parameter, and presumably in origin, between bulge and halo (the two curves near the top left of the figure), and the quite distinct bahaviour of the Galactic thin and thick disks (the two curves through the centre of the figure). This figure is from Wyse Gilmore 1992.

Figure 1 above) have shown there is a very small scatter in element ratios at any [Fe/H] value, particularly within a stellar population, yet there is an extremely large scatter in the age-[Fe/H] relation at every age (Nordstrom et al. 2004). Linking the local and global remains a challenge. 

Fig. 1. Variation in [Fe/H] versus Galactic rotational velocity for stars assigned to Galactic stellar populations based purely on their kinematics thin disk (red), thick disk (green), halo (cyan), plunging orbits (blue), extreme retrograde orbits (black).

Fig. 2. Variation in [a/Fe] in Galactic stars (color coded by stellar population as in Fig. 1) and dSph stars (black squares).

Dwarf Spheroidal galaxies are the smallest and faintest galaxies known. They are typically dominated by old stellar populations (e.g. Sculptor and Sextans), but some of them (e.g. Fornax) exhibit more recent star formation episodes (2-8 Gyr ago). Analysis of the horizontal branch morphology shows that Red HB stars are more centrally concentrated than Blue HB stars which could be interpreted either as an age or a metallicity gradient or both ([1]). Only spectroscopic observations can unambiguously separate metallicity gradients and make a link with the kinematics. 

The accretion history of a parent galaxy is constructed using a semi-analytical code. The full phase-space evolution during each accretion event is then followed separately with numerical simulations [2]. Star-formation and chemical evolution models are implemented within each satellite. The star formation prescription matches the number and luminosity of present-day galaxies in the Local Group, whereas the chemical evolution model takes into account the metal enrichment of successive stellar populations as well as feedback processes. Below we present results of a sample of four such simulated galaxy halos, denoted as Halos HI, H2, H3 and H4. 

Fig. 5. Quantitative tests of numerical galaxy models are now becoming feasible. It is interesting to note how little apparent substructure is evident in the best conserved quantity, angular momentum, and how complex is the evolutionary history of a stellar population , such as the thick disk, in this model. This figure is from Abadi et al. 2003.

The problem of interpreting integrated spectra has been studied by many investigators starting with Whipple (1935). One approach, known as stellar population... 

In the last few years, there has been great progress in the more a-priori method of evolutionary population synthesis , pioneered by Tinsley (1968), using the same stellar libraries as in the empirical fitting, and calculating the spectral evolution of single stellar populations (SSP) as a function of age and metallicity. One problem... 

Planetary nebulae (PN) display basic abundance patterns characteristic of the age and location of their parent stellar populations, on which are superimposed the effects of evolution through giant, AGB and post-AGB stages of their own central... 

Multicolour photometric systems that have been used for various sorts of stellar and stellar-population classification are listed with their spectral reponse curves in the website... 

M. Salaris and S. Cassisi, Evolution of Stars and Stellar Populations, John Wiley, Chichester, 2005. 

Figure 3.38 shows a schematic cross-sectional view of the Galaxy, indicating the main stellar population groups (disk, bulge, halo and solar cylinder) that will figure in subsequent discussions and Table 7.9 gives some relevant statistics. 

Equation (8.6) refers to abundances in the gas and young stars. For the average abundance of a stellar population (with Z0 = 0) we have instead... 

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