Star, formation

Planetary systems are now generally believed to be by-products of the process of star formation. Star formation, therefore, is the natural starting point for discussions of planet formation. Almost all stars are born as members of stellar clusters that, in turn, are bom in molecular clouds. Formation of isolated stars seems to be possible according to observations, but this is a rare process. Whether the Sun and its associated planetary system formed in isolation or as member of a cluster is not known some indications hint to formation in a cluster (see Hester Desch 2005 Gounelle Meibom 2008 and Chapter 9, this volume). 

For more concentrated initial distributions, the collapse time is even shorter. 

During this initial collapse phase the rotational degrees of freedom of H2 are not excited (A /Xc = 512 K for the J = 2—0 transition). If the temperature approaches 100K the ratio y = cp/cv of specific heats changes from y = 5/3 (rotation of H2 not excited), to y =7/5 (rotation excited). The critical y for a self-gravitating adiabatic gaseous sphere to be stable is y =4/3. The pressure in the central core then supports the matter against further collapse and the inflow steepens to a shock at the border of the opaque core. The first core has formed. 

The continued addition of matter increases the density and temperature of the core until H2 begins to dissociate. The dissociation consumes heat, which holds temperature approximately constant, i.e. the heat capacity becomes very high and y - 1. The stability condition y 4/3 becomes violated and a new collapse of the core ensues. The core collapses until all H2 is dissociated and the H finally becomes ionized. The temperature then increases again with further contraction and the second core is formed that approaches stellar density. The second collapse phase is short and lasts for a solar-type star of the order of 103 years. By this event a protostellar embryo is born, which continues to grow in mass by collecting the remaining material from its environment. 

Note Results of Wuchterl Tscharnuter (2003) for three different final masses and for spherically symmetrical collapse. 

It is a long journey from the formation of the Universe at 13.7 Ga ago to the formation of the Earth at about 4.57 Ga. This journey is the subject of the next sections of this chapter, and in them we shall consider the relationship between the Earth and its host - the solar system. In so doing we shall discuss the processes which have led to star formation, to the formation of the chemical elements, the condensation of the solar system, and ultimately to a model for planetary accretion and hence the Earth. 

In this section we consider the processes which led from the Big Bang to the formation of stars and galaxies. 

As cosmologists began to accumulate measurements of the Cosmic Background radiation at the edge of the Universe they were impressed by the uniformity of the results. 

This unevenness in the distribution of matter and energy has been described as the lumpiness, or more correctly, the anisotropy of the early Universe. Elsewhere it is stated that the Universe is homogeneous or isotropic. This is not a contradiction, rather a difference of scale. On a large scale the Universe is homogeneous and everywhere has a temperature of 2.73 K at its edge. On a fine scale however, microvariations in temperature have been measured. 

formed due to gravitational forces working against the expansion of the Universe. 

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The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

The formation of stars in the interiors of dense interstellar clouds affects the chemistry of the immediate environment in a variety of ways depending on many factors such as the stage in the evolution of star formation, the mass of the star or protostar, and the density and temperature of the surrounding material. In general, the dynamics of the material in the vicinity of a newly forming star are complex and show many manifestations. Table 3 contains a list of some of the better studied such manifestations, which tend to have distinctive chemistries. These are discussed individually below. 

R = the average rate of star formation in our galaxy fs = the fraction of stars that are suitable suns for planetary systems fp = the fraction of suitable suns with planetary systems ne = the mean number of planets that are capable of supporting life fi = the fraction of such planets on which life actually originates fi = the fraction of such planets on which some form of intelligence arises fc = the fraction of such intelligent species that develop the ability and desire to communicate with other civilizations L = the mean lifetime (in years) of a communicative civilization... 

The thick disk shows evidence of extensive star formation... 

A true age-gap between the end of star formation in the thick disk and the onset of star formation in the thin disk... 

Finally Age-metallicity relation in the thick disk - this is a very tentative statement and, to our knowledge, there is only one study that claims the possibility of such an age-metallicity relation ([5]). This would suggest an extended period of star formation in the thick disk... 

In an early phase with enhanced star formation ([7])... 

Predictions no abundance gradients the thick disk thin disk stars will all have [Fe/H] larger than found in the thick disk period of star formation in thick disk was short < 1 billion years thin disk always younger than thick disk Observations no vertical gradients metallicity distributions for the disks overlap star formation in thick disk includes SNela and AGB star formation in thick disk probably > 1 billion years... 

Fig. 3. Summary of current observational knowledge about the thin and the thick disks in the Milky Way. The two trends for [O/Fe] vs [Fe/H] are depicted in blue, dashed line (thin disk) and red (thick disk). The most debated issues are marked in blue (i.e. thick disk all the way to [Fe/H]=0, hiatus in star formation, SF, and AMR in thick disk) and topics of some debate in purple ( knee and <5age between various sub-populations, such as Sage between the youngest thin disk and the oldest thick disk). The one issue all agree upon, the a-enhancement, is indicated in green (light grey)...

Predictions infalling satellite probably has to be large thin disk younger than thick disk and an age gap between the disks likely that the abundance trends differ gradients will be preserved (if they exist in pre-existing thin disk) star formation in original thin disk could be as long as needed (e.g. to create the knee in the a-element trends)... 

Observations no vertical gradients abundance trends differ star formation in thick disk includes SNela and AGB... 

The resulting [Ca/Fe] versus [Fe/H] plot is shown in Fig. 1, where except for a few outliers that will have to be manually inspected, a clear trend appears [Ca/Fe] slowly rises with [Fe/H] until it reaches a maximum and then declines again for the most metal-rich stars (RGB-a according to [4]). This nicely confirms a previous finding by [8] and [9]. If the metal-rich stars have evolved within the cluster in a process of self-enrichment, the only way to lower their a-enhancement would be SNe type la intervention. No simple explanation is provided for the rise of [Ca/Fe] at low [Fe/H], although a series of star formation bursts should be the likely cause. 

The growing scenario for the Bulge is pointing toward an early formation and a rapid chemical enrichment, as for the Galactic Halo but possibly at higher star formation rate. 

The originally proposed hypothesis that there might have been more than an episode of star formation within M 67 and that, accordingly, Li-rich/poor cluster stars might represent the young/old population ([10]) can be excluded, since we now know that Li at old ages is not necessarily low (see Fig. 1). The scatter in M 67 indeed reinforces the conclusion that at least one further parameter besides age and mass drives Li depletion, the possible additional parameters being the presence of planets, chemical composition ([10], [14]) and rotation and/or rotational history ([9]). 

Scl is a close companion of the Milky Way, at a distance of 72 5 kpc [7], with a low total (dynamical) mass, (1.4 0.6) x 107Mq [8], and modest luminosity, My = —10.7 0.5, and central surface brightness, Soy = 23.5 0.5 mag/arcsec2 [9] with no HI gas [10]. CMD analysis, including the oldest Main Sequence turnoffs, has determined that this galaxy is predominantly old and that the entire star formation history can have lasted only a few Gyr [11]. 

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

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. 

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