Abstract

The formation of massive stars is currently an unsolved problem in astrophysics. Understanding the formation of massive stars is essential because they dominate the luminous, kinematic and chemical output of stars. Furthermore, their feedback is likely to play a dominant role in the evolution of molecular clouds and any subsequent star formation therein. Although significant progress has been made observationally and theoretically, we still do not have a consensus as to how massive stars form. There are two contending models to explain the formation of massive stars: core accretion and competitive accretion. They differ primarily in how and when the mass that ultimately makes up the massive star is gathered. In the core accretion model, the mass is gathered in a pre-stellar stage due to the overlying pressure of a stellar cluster or a massive pre-cluster cloud clump. In contrast, competitive accretion envisions that the mass is gathered during the star formation process itself, being funnelled to the centre of a stellar cluster by the gravitational potential of the stellar cluster. Although these differences may not appear overly significant, they involve significant differences in terms of the physical processes involved. Furthermore, the differences also have important implications in terms of the evolutionary phases of massive star formation and ultimately that of stellar clusters and star formation on larger scales. Here, we review the dominant models and discuss prospects for developing a better understanding of massive star formation in the future.

Introduction

During the last two decades, the focus of star formation research has shifted from understanding the collapse of a single dense core into a star to studying the formation of hundreds to thousands of stars in molecular clouds. In this chapter, we overview recent observational and theoretical progress towards understanding star formation on the scale of molecular clouds and complexes, i.e. the macrophysics of star formation (McKee & Ostriker 2007). We begin with an overview of recent surveys of young stellar objects (YSOs) in molecular clouds and embedded clusters, and we outline an emerging picture of cluster formation. We then discuss the role of turbulence to both support clouds and create dense, gravitationally unstable structures, with an emphasis on the role of magnetic fields (in the case of distributed stars), and feedback (in the case of clusters) to slow turbulent decay and mediate the rate and density of star formation. The discussion is followed by an overview of how gravity and turbulence may produce observed scaling laws for the properties of molecular clouds, stars and star clusters and how the observed, star formation rate (SFR) may result from self-regulated star formation. We end with some concluding remarks, including a number of questions to be addressed by future observations and simulations.

Observations of clustered and distributed populations in molecular clouds

Our knowledge of the distribution and kinematics of young stars, protostars and dense cores in molecular clouds is being rapidly improved by wide-field observations at X-ray, optical, infrared and (sub)millimeter wavelengths (Allen et al. 2007; Feigelson et al. 2007).

Abstract

Computational gas dynamics has become a prominent research field in both astrophysics and cosmology. In the first part of this chapter, we intend to briefly describe several of the numerical methods used in this field, discuss their range of application and present strategies for converting conditionally stable numerical methods into unconditionally stable solution procedures. The underlying aim of the conversion is to enhance the robustness and unification of numerical methods and subsequently enlarge their range of applications considerably. In the second part, Heitsch presents and discusses the implementation of a time-explicit magneto hydrodynamic (MHD) Boltzmann solver.

PART I

Numerical methods in AFD

Astrophysical fluid dynamics (AFD) deals with the properties of gaseous matter under a wide variety of circumstances. Most astrophysical fluid flows evolve over a large variety of different time and length scales, henceforth making their analytical treatment unfeasible.

On the contrary, numerical treatments by means of computer codes have witnessed an exponential growth during the last two decades due to the rapid development of hardware technology. Nowadays, the vast majority of numerical codes are capable of treating large and sophisticated multi-scale fluid problems with high resolutions and even in 3D.

The numerical methods employed in AFD can be classified into two categories (see Figure 5.1):

1. Microscopic-oriented methods: These are mostly based on N-body (NB), Monte Carlo (MC) and on the Smoothed Particle Hydrodynamics (SPH).

2. Grid-oriented methods: To this category belong the finite difference (FDM), finite volume (FVM) and finite element methods (FEM).

We conducted an optical survey (Keck Telescope, 3,700–7,000 Å) of 24 high-metallicity (Z) starburst galaxies to investigate whether high-Zenvironments favor the formation of Wolf–Rayet (WR) stars. We searched for the presence of the He II 4686 Å line produced by the massive WR stars. We detected this feature in six galaxies (25% of the sample). We also used a stellar-population-synthesis code to determine their ages. We find that (i) all galaxies hosting considerable numbers of WR stars are very young systems, with ages log(t) > 8, with t in years; (ii) not all young star-forming galaxies host WR stars, or at least that population cannot be detected in their integrated spectra; and (iii) most galaxies hosting WR populations are found in interacting systems. We for the first time detect WR populations in galaxies ESO 485-G003, NGC 6090, and NGC 2798.

Reports of high metallicities in galactic systems have always been controversial. I disuss whether observational claims both for nebulae and for stars are well-founded, and try to form a rational view of just how metal-rich some regions of galaxies do become. Metallicity is linked to the evolution of star formation in a galaxy through the yield, the mass of metals produced each time star formation locks up unit mass of interstellar material. The mechanisms by which real or apparent high yields might be achieved are examined – global and local gas flows, poor mixing, star formation and metallicity effects in stellar evolution. As perhaps expected, it turns out to be not so easy to ‘get rich’, quickly or otherwise – suggesting that sorting out the lingering uncertainties in the abundance analysis of H ii regions and stars remains a priority.

We review some of the models of chemical evolution of ellipticals and bulges of spirals. In particular, we focus on the star-formation histories of ellipticals and their influence on chemical properties such as [α/Fe] versus [Fe/H], galactic mass and visual magnitudes. By comparing models with observational properties, we can constrain the timescales for the formation of these galaxies. The observational properties of stellar populations suggest that the more-massive ellipticals formed on a shorter timescale than less-massive ones, in the sense that both the star-formation rate and the mass-assembly rate, strictly linked properties, were greater for the most-massive objects. Observational properties of true bulges seem to suggest that they are very similar to ellipticals and that they formed on a very short timescale: for the bulge of the Milky Way we suggest a timescale of 0.1 Gyr. This leads us to conclude that the bulge evolved in a quite independent way from the Galactic disk.

We present results for the chemical evolution of the Galactic bulge in the context of an inside-out formation model of the Galaxy. A supernovadriven wind was also included in analogy with elliptical galaxies. New observations of chemical-abundance ratios and the metallicity distribution have been employed in order to check the model results. We confirm previous findings that the bulge formed on a very short timescale with quite a high star-formation efficiency and an initial mass function more skewed towards high masses than the one suitable for the Solar neighbourhood. A certain amount of primary nitrogen from massive stars might be required in order to reproduce the nitrogen data at low and intermediate metallicities.

We have derived metallicity, masses, and ages for two samples of nearby giant stars, which have been observed with the aim of understanding their nature of the radial-velocity (RV) variability and to search for planetary companions. Our stars have reliable Hipparcos parallaxes, and for several we also have measured angular diameters; the parameters we retrieve from our inversion process are in very good agreement with the observed ones. Among our results, we find that the stars regarded as candidates to host planetary companions are not preferencially metal-rich, which is at odds with what is found for main-sequence stars. We also find that stars younger than ∼1 Gyr can be described by a single metallicity and that an age–metallicity relationship applies to our samples.

Galactic open clusters provide a key tool to address a variety of issues related to the formation and evolution of stars and the Galactic disk. In the last few years a metallicity higher than Solar has been derived/confirmed spectroscopically for a few clusters, the most famous example being the very old NGC 6791, for which a metallicity [Fe/H] ∼ 0.4 has recently been reported. In this paper current knowledge of these supersolarmetallicity clusters is reviewed and their properties and abundance patterns are compared with those of non-metal-rich clusters and other Galactic populations. Possible implications for their origin and for the metallicity gradient in the disk are briefly discussed. A summary of recent surveys for planets in metal-rich clusters is also provided, together with new results on Li abundances for the 3-Gyr-old metal-rich cluster NGC 6253.

The chemical evolution of the Galactic bulge is calculated by adopting a single-zone framework, with accretion of primordial gas on a free-fall timescale, assuming (i) a correspondingly rapid timescale for star formation and (ii) an initial mass function biased towards massive stars. We emphasise here the uncertainties associated with the underlying physics (specifically, stellar nucleosynthesis) and how those uncertainties are manifested in the predicted abundance-ratio patterns in the resulting present-day Galactic-bulge stellar populations.

Does the initial mass function (IMF) vary? Is it significantly different in metal-rich environments versus metal-poor ones? Theoretical work predicts this to be the case, but in order to provide robust empirical evidence for this, the researcher must understand all possible biases affecting the derivation of the stellar mass function. Apart from the very difficult observational challenges, this turns out to be highly non-trivial, relying on an exact understanding of how stars evolve, how stellar populations in galaxies are assembled dynamically and how individual star clusters and associations evolve. N-body modelling is therefore an unavoidable tool in this game: the case can be made that without complete dynamical modelling of star clusters and associations any statements about the variation of the IMF with physical conditions are most probably wrong. The calculations that do exist demonstrate time and again that the IMF is invariant: there exists no statistically meaningful evidence for a variation of the IMF on going from metal-poor to metal-rich populations. This means that currently existing star-formation theory fails to describe the stellar outcome. Indirect evidence, based on chemical-evolution calculations, however, indicates that the extreme starbursts that assembled bulges and elliptical galaxies may have had a top-heavy IMF.

Galactic chemical evolution witnesses the enrichment of the interstellar medium with elements heavier than H, He, and Li that originate from the Big Bang. These heavier elements can be traced via the surface compositions of low-mass stars of various ages, which have remained unaltered since their formation and therefore measure the composition in the interstellar medium at the time of their birth. Thus, the metallicity [Fe/H] is a measure of the enrichment with nucleosynthesis products and indirectly of the ongoing duration of galactic evolution. For very early times, when the interstellar medium was essentially pristine, this interpretation might be wrong and perhaps we see the ejecta of individual supernovae where the amount of H with which these ejecta mix is dependent on the energy of the explosion and the mass of the stellar progenitor. Certain effects are qualitatively well understood, i.e. the early ratios of alpha elements (O, Ne, Mg, Si, S, Ar, Ca, Ti) to Fe, which represent typical values from Type-II supernova explosions that originate from rapidly evolving massive stars. On the other hand, Type-Ia supernovae, which are responsible for the majority of Fe-group elements and are the products of binary evolution of lower-mass stars, later emit their ejecta and reduce the alpha/Fe ratio. In addition to being a measure of time, the metallicity [Fe/H] also enters stellar nucleosynthesis in two other ways. (i) Some nucleosynthesis processes are of secondary nature, e.g. the s-process, requiring initial Fe in stellar He-burning. (ii) Other processes are of primary nature, e.g. the production of Fe-group elements in both types of supernovae.

As well as being the realm of the first stars, the high-redshift regime is a window on some of the most metal-rich components in our Universe, the massive galaxies destined to become today's ellipticals and the black holes at their centres at a time of peak activity. While much has been learnt in recent years about these ‘get-rich-quick’ objects, progress is still hampered by the same limitations as apply to nearby metal-rich stars and H II regions: our methods for exploring the super solar-metallicity regime require considerable improvement before they can be considered to be reliable. I illustrate this conclusion with a few recent case studies of active galactic nuclei, star-forming galaxies and damped Lyman-alpha systems.

The subject of metal-rich stars has been controversial for over 40 years, and I review some of the major developments in the subject area during that period, emphasizing those papers that set the subject on its presentday course. Metals emerge in the Universe at very high redshift, and galaxies with roughly Solar metallicity are documented even at redshift 3. In the local Universe, disks and bulges are often metal-rich, but metal-rich stars can also be found in distant halo populations, likely ejected into those environments by merger events. The Galactic bulge has a mean abundance of slightly subsolar but contains stars as metal-rich as [Fe/H] ∼+0.5; these stars have a complicated enhancement of light elements.

We present abundance results for 53 bulge giant stars using highresolution spectra obtained with FLAMES/UVES at the ESO/VLT for various regions of the Bulge (−12 < b < −4). The trend of the four light elements O, Na, Mg and Al indicates a chemical enrichment of the bulge dominated by massive stars at all metallicities. For [Fe/H] > −0.5, [O/Fe], [Na/Fe], [Mg/Fe] and [Al/Fe] are enhanced relative to both the thin- and the thick-disc trend. This suggests that the bulge formed on a shorter timescale than did the Galactic discs.

Using Mg as a proxy for metallicity (instead of Fe) we further show the following (i) The [O/Mg] ratio for bulge stars follows and extends to higher metallicities the decreasing trend of [O/Mg] found in the galactic discs. (ii) The [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three distinct sequences in the thin disc, the thick disc, and the bulge. The bulge trend is well represented by the predicted metallicity- dependent yields of massive stars, whereas the galactic discs have Na/Mg ratios that are too high at low metallicities, indicating an additional source of Na from AGB stars. (iii) In contrast to the case with Na, there appears to be no systematic difference in the [Al/Mg] ratio between bulge and disc stars, and the theoretical yields for massive stars agree with the observed ratios, leaving no space for an AGB contribution to Al.

We discuss recent results based on our ongoing work on the study of the chemical abundances in the central part of spiral galaxies. A robust technique has been used to extrapolate the derived radial abundance gradients of oxygen to the center of the respective galaxies, taking into account the recent ff relation of Pilyugin (2005) and the new model-independent correction for electron-temperature structure within the H II regions as well as the contribution of a possible oxygen depletion by dust grains. In this way, a typical value for the expected maximum O/H abundance in spiral galaxies is derived. Implications of this result for the metallicity–luminosity relation and for the chemical abundances derived for high-redshift-galaxy samples are briefly discussed.