The importance of massive stars in astrophysics is self-evident to all of us. However the full scope of this role has only become apparent over the last few years, as we begin to understand the central role that massive star formation and evolution plays in phenomena ranging from the structure and evolution of the interstellar medium, galaxy formation and evolution, nuclear activity in galaxies, and even the reionization of the universe itself. This paper briefly reviews this broad relevance of massive star formation, and the wide range in massive star formation environments found in the local universe.

Observations of the nearest regions of massive star formation such as Orion are reviewed. Early-type stars in the local OB associations, as well as their superbubbles and supershells provide a fossil record of massive star birth in the Solar vicinity over about the last 40 Myr. This record shows that most massive stars are born from dense, high-pressure, hot cores which spawn transient clusters that dissipate into the field soon after formation. A large fraction (15 to 30%) of massive stars are high-velocity runaways moving at more than 20 km s$^{-1}$. High-mass stars have a larger companion fraction than their lower-mass siblings. The Orion star forming complex contains the nearest site of on-going massive star formation. Studies of the Orion Nebula and the dense molecular cloud core located immediately behind the HII region provide our sharpest view of massive star birth. This region has formed a hierarchy of clusters within clusters. The Trapezium, OMC-1S, and OMC-1 regions represent three closely spaced sub-clusters within the more extended Orion Nebula Cluster. The oldest of these sub-clusters, which consists of the Trapezium stars, has completely emerged from its natal core. The OMC-1S and OMC-1 regions, are still highly embedded and forming clusters of additional moderate and high mass stars. Over a dozen YSOs embedded in OMC-1S are driving jets and outflows, many of which are injecting energy and momentum into the Orion Nebula. Recent proper motion measurements indicate that the Becklin-Neugebauer object is a high-velocity star moving away from the OMC1 core with a velocity of 30 km s$^{-1}$, making it the youngest high-velocity star known. Source I may be moving in the opposite direction with a velocity of about 12 km s$^{-1}$. The projected separation between source I and BN was less than few hundred AU about 500 years ago. The spectacular bipolar molecular outflow and system of shock-excited H$_2$ fingers emerging from OMC-1 has a dynamical age of about 1100 years. It is possible that a dynamical interaction between three or more stars in OMC-1 led to the formation of this eruptive outflow.

We summarize the properties of Infrared Dark Clouds, massive, dense, and cool aggregations of interstellar gas and dust that are found throughout the Galaxy in projection against the strong mid-infrared background. We describe their distribution and give an overview of their physical properties and chemistry. These objects appear to be the progenitors of high-mass stars and star clusters, but seem to be largely devoid of star formation, which however is taking place in localized spots.

The presence of ices along the line of sight of high mass star forming regions is clearly evidenced by infrared observations. In dense molecular clouds icy grain mantles form after direct freeze out of gas phase species and after surface reactions of atoms and radicals on grains. Thus chemical composition of icy mantles differs from that of the gas phase. Due to the presence of the protostar and of cosmic radiation, icy mantles suffer from ion bombardment, UV photolysis and thermal annealing. Most of our knowledge on the physical and chemical properties of ices is based on the comparison between observations and laboratory experiments performed at low temperature (10-80 K). Experimental results show that after ion irradiation and UV photolysis the chemical composition and the structure of the sample is modified. Both more volatile and less volatile species are formed and if a C-bearing species is present in the original sample a refractory residue is left over after warm-up to room temperature. After thermal annealing, segregation, crystallization, and sublimation take place. Thus molecular species are released to the gas phase which could be enriched by species formed in the solid phase. Here I will discuss some recent laboratory experiments relevant to our knowledge of the physico-chemical properties of ices in star forming regions.

The Submillimeter Array located atop of Mauna Kea in Hawaii is a collaborative project of the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. The high angular resolution provided by the SMA is particularly suitable for studying massive star-forming cores which often exhibit strong (sub)millimeter continuum and spectral features but mostly locate within crowded regions at large distances. We report the latest SMA status and recent results in the area of massive star formation obtained with the array.

High-quality K-band spectra of point sources, deeply embedded in massive star-forming regions, have revealed a population of 20 young massive stars showing no photospheric absorption lines, but only emission lines. The K-band spectra exhibit one or more features commonly associated with massive Young Stellar Objects surrounded by circumstellar material: a very red color $(J-K) = 2$, CO bandhead emission, hydrogen emission lines (sometimes doubly peaked), and FeII and/or MgII emission lines. The CO emission comes from a relatively dense (${\sim}10^{10} \mathrm{cm}^{-3}$) and hot ($T\sim 2000$–5000 K) region, sufficiently shielded from the intense UV radiation field of the young massive star. Modeling of the CO-first overtone emission shows that the CO gas is located within 5 AU of the star. The hydrogen emission is produced in an ionized medium exposed to UV radiation. The best geometrical configuration is a dense and neutral circumstellar disk causing the CO bandhead emission, and an ionized upper layer where the hydrogen lines are produced. We argue that the circumstellar disk is likely a remnant of the accretion via a circumstellar disk.

The scope of this contribution is to review and discuss recent findings concerning the molecular environment of high-mass star forming regions. Special attention is devoted to “hot molecular cores” and their role in the formation of massive stars. After analysing the relationship between such cores and the surrounding molecular clumps, we discuss the results of interferometric observations of these objects and propose an evolutionary sequence proceeding from cold, pre-stellar cores to ultracompact HII regions.

This paper reviews the chemistry of star-forming regions, with an emphasis on the formation of high-mass stars. We first outline the basic molecular processes in dense clouds, their implementation in chemical models, and techniques to measure molecular abundances. Then, recent observational, theoretical and laboratory developments are reviewed on the subjects of hot molecular cores, cosmic-ray ionization, depletion and deuteration, and oxygen chemistry. The paper concludes with a summary of outstanding problems and future opportunities.

We present numerical investigations into the formation of massive stars from turbulent cores of density gradient $\rho \propto r^{-1.5}$. The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra $P \propto k^{-4}$ and $P \propto k^{-3.5}$, and apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density ($10^{-14}$gcm$^{-3}$). Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term.

Massive stars are known to be formed in clusters. To understand their birth process it is essential to know the physical conditions of the parental cloud which is thought to play a critical role in determining the formation mechanism. In this contribution I summarize recent results from observations of dust, molecular and ionized gas emission we have made toward several massive star forming regions in the southern hemisphere. These observations are providing key evidence concerning the initial conditions for the formation of cluster of massive stars, allowing to characterize the physical properties of massive and dense cores and permitting to identify them in different stages of their early evolution.

We found that in regions of high mass star formation the CS emission correlates well with the dust continuum emission and is therefore a good tracer of the total mass while the N$_2$H$^+$ distribution is frequently very different. This is opposite to their typical behavior in low-mass cores. The behavior of other high density tracers varies from source to source but most of them are closer to CS. Radial density profiles in massive cores are fitted by power laws with indices about −1.6, as derived from the dust continuum emission. The radial temperature dependence on intermediate scales is close to the theoretically expected one for a centrally heated optically thin cloud. The velocity dispersion either remains constant or decreases from the core center to the edge. Several cores including those without known embedded IR sources show signs of infall motions. They can represent the earliest phases of massive protostars. There are implicit arguments in favor of small-scale clumpiness in the cores.

Magnetic fields may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio $M/\Phi$ and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Examples of massive star formation regions for which detailed observations have been made of magnetic field strengths and morphologies include DR21OH, OMC1, and S106; observational results for these regions and relevant results for the diffuse ISM and masers will be reviewed. Results are that the strength of interstellar magnetic fields remains invariant at $B {\sim} 6\mu$G between 0.1 cm$^{-3} < n(H) < 10^3$ cm$^{-3}$, but increases as $B \propto \rho^{0.4-0.5}$ for $10^3$ cm$^{-3} < n(H_2) < 10^{8}$ cm$^{-3}$. Moreover, $M/\Phi$ is significantly subcritical (strong $B$ with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend critically on the $M/\Phi$ ratio in each clump.

The physical parameters of HII regions span orders of magnitude in scale. The classes most closely linked to star formation are the smallest, densest, and, presumably, youngest stages: compact, ultracompact, and hypercompact HII regions.

Although hypercompact HII regions have been known for over ten years, until recently only a very small number of these regions were known. Moreover, it is only in the past several years that these regions have come to be recognized as a distinct class of HII region and that attempts have been made to understand their place in the scheme of massive star formation.

Here we present a summary of the observational studies to date. We give special emphasis to radio continuum studies, which indicate density gradients within the ionized gas, and to radio spectral line studies, which show unusually broad recombination line profiles. Possible interpretations of these aspects of hypercompact HII regions are discussed, and their implications for the interpretation of hypercompact HII regions as an evolutionary stage in the high-mass star formation process.

I review recent developments in the field of radio observations of ultracompact H II (UCHII) regions that have appeared in the literature since the last major review (Churchwell 2002). The morphology of UCHII regions continues to be a topic of research and recent studies of individual regions of star formation suggest modifications with respect to the results obtained from galactic plane surveys. I also briefly touch upon the question of the density structure of UCHII regions. Finally, I will also discuss the evidence of time variability in UCHII regions, and present examples of objects that are expanding, varying in flux density, or moving in the sky.

Most stars, including all massive stars, form in clusters. Here, I present (sub-)millimeter observations of young stellar clusters in two well kown massive star forming regions. First, I discuss relatively low mass cluster formation across the Rosette molecular cloud and the influence of the OB association, NGC 2244 (the Rosette nebula) on their properties. Second, I present SMA observations of the Trapezium cluster in Orion and the detection of emission from protoplanetary disks (proplyds) around 4 low mass stars. The implications for Solar System scale planet formation around low mass stars in high mass star forming environments are discussed.

Recent years have seen accumulating evidence for infall and rotation around disk like structures toward young stars of spectral types of early B and possibly late O. Observations indicate that the disks or tori are massive in comparison with the mass of the central stars and geometrically thick. Both properties distinguish them from the disks around low mass stars, in particular T-Tauri type stars.

I review observational techniques used in studying disks around massive young stars and summarize properties of disks derived from observations. I will discuss implications of these findings for the formation of massive stars.

IR imaging, K–band spectroscopy and X-ray data of the stellar cluster in M 17 suggests three categories of massive young objects: I) “Naked” early-type O-stars in the cluster center that exhibit X-ray emission and normal optical/IR-spectra; they do not show signs for any circumstellar material. II) Late O-type stars, that show X-ray emission and H/He absorption lines in their optical/IR-spectra. There is evidence for extended circumstellar material in the form of elongated L–band nebulae, circumstellar MIR emission from regions of about 10.000 AU in diameter and filaments of He I $1.083\,\mu$m. III) Massive stars without X-ray emission, K–band spectra with emission lines of H and He and in some cases with CO band head emission. They are heavily enshrouded and also show elongated L–band nebulae, circumstellar MIR emission from regions of about 4.000 AU and ionized He envelopes.

Obviously, M 17 harbors several generations of massive stars: Only the oldest “Category I”, located in the cluster center, is free of circumstellar material. “Category II” and “III” comprise younger dust-enshrouded objects of a second generation embedded in the interface of the HII region and the adjacent molecular cloud. Due to the presence of flattened dusty disks and perpendicular gaseous outflows, accretion seems to be important for the formation of stars up to type O8.

A population of X-ray emitting sources with($L_{\rm bol} \sim 10^4\,$L$_\odot$) shows CO band head absorption; one source was found to have variable CO absorption as known from FU Orionis stars.

New data on the 20.000 AU accretion disk in M 17 reveals a collimated H$_2$$2.122\,\mu$m jet with three distinct emission knots that emerges from the disk center. The broad H$\alpha$ and Ca II emission lines from the bipolar nebula display blue-shifted absorption originating from accretion disk-driven outflows with velocities of $\pm 500$ km/s.

We started (sub-)millimeter continuum and line studies of entire molecular cloud complexes located at intermediate distances from the Sun (1–3 kpc). Such an unbiased approach allows to identify and characterize the earliest phases of high-mass stars overlooked by IRAS or MSX. Our complete MAMBO-2 surveys of the Cygnus X and NGC 7538 complexes reveal a large population of ${\sim} 0.1$ pc-size massive young stellar objects (MYSOs) harboring high-mass infrared-quiet protostars. The determination of the nature of all the new millimeter sources is still in progress but we have already collected evidence that the infrared-quiet (or class 0-like) protostellar phase might last as long as the better-known infrared-bright protostellar phase. Besides, our complete census of MYSOs fails to discover the high-mass analogues of pre-stellar dense cores. We propose that the observed lower-density pre-stellar clumps (${>} 1$ pc) rapidly concentrate and collapse as also found in the kinematical studies of other prominent clumps. Indeed, CS and HCO$^+$ mappings in W43 and Cygnus X suggest global supersonic contraction with inward velocities of several km s−1 on parsec scales. Our work and similar studies of entire star-forming complexes will thus definitively contribute to a better knowledge of the earliest phases of high-mass star formation.

Massive stars begin their lives in cold, dense cores which are much more massive than the stars which form in them. We summarise the results of a program to find the earliest examples of massive star formation, and to examine the evolutionary sequence of events that occurs as such a star begins to form and heat its surroundings. Methanol maser emission has proved to be a particularly potent tool to locate such cores, though there are also clearly many massive cores which do not exhibit such maser emission. Our program began with a survey for 6.6 GHz methanol maser emission, but expanded to include dust continuum surveys in the mm and sub–mm, a survey for hot molecular cores associated with ‘isolated’ masers through mm-line CH3CN emission, and follow-up probing of some cores through sub-arcsecond, diffraction limited observations in the mid–IR. This program is outlined below.

A large body of theoretical and computational work shows that jets - modelled as magnetized disk winds - exert an external torque on their underlying disks that can efficiently remove angular momentum and act as major drivers of disk accretion. These predictions have recently been confirmed in direct HST measurements of the jet rotation and angular momentum transport in low mass protostellar systems. We review the theory of disc winds and show that their physics is universal and scales to jets from both low and high mass star forming regions. This explains the observed properties of outflows in massive star forming regions, before the central massive star generates an ultracompact HII region. We also discuss the recent numerical studies on the formation of massive accretion disks and outflows through gravitational collapse, including our own work on 3D Adaptive Mesh simulations (using the FLASH code) of the hydromagnetic collapse of an initial rotating, and cooling Bonner-Ebert sphere. Magnetized collapse gives rise to outflows. Our own simulations show that both a jet-like disk wind on sub AU scales, and a larger scale molecular outflow occur (Banerjee & Pudritz 2005).