This article presents a review of all high-resolution ground-based mid-infrared observations of high mass protostellar objects (HMPOs) to date. An introduction to the characteristics of HMPOs and their relationship to the hot molecular core phase of massive star formation is outlined. I explain why observations in the mid-infrared of these types of sources are important, and in particular why high-resolution observations are needed. A summary is given of the earliest mid-infrared detections of HMPOs, followed by a detailed account of the most recent observations.

(Sub-)mm interferometry is the most favorable technique to investigate the earliest stages of massive star formation. I will outline general applications in that field and discuss results of different sub-topics (hot core chemistry and massive molecular outflows). Furthermore, recent data obtained with the Submillimeter Array will be shown to present the unique capabilities of this new instrument. Finally, I will give a short outlook on the main physical topics of massive star formation to be tackled with (sub-)mm interferometry within the next decade.

Protostellar core formation is probably much more dynamic, and magnetic fields are probably much less important, than has been previously assumed in the standard model of low-mass star formation. This revised picture has important consequences: it is easier to understand the observed rapidity of star formation in molecular clouds; cores are more likely to have structures favoring high infall rates at early times, helping to explain the differences between Class 0 and Class I protostars; and core structure and asymmetry will strongly favor post-collapse fragmentation into binary and multiple stellar systems.

A major gap in our understanding of star formation concerns the origins of binary stars although there is growing evidence that most stars form in binary and multiple systems. While some theoretical predictions of fragmentation models are indirectly supported by statistical studies of evolved binary stars at shorter wavelengths, direct observations of the formation phase became only possible with the advance of large millimeter interferometers. Molecular line spectroscopy and dust continuum observations at high angular resolution can address some of the key questions in binary star formation theories. Observing methods and results of recent studies of binary protostellar systems are reviewed in this paper.

Although much progress has been made in studying the multiplicity properties of pre-main-sequence T Tauri stars over the past decade, younger and more embedded protostellar systems are just now being surveyed for multiplicity. I examine several recent infrared imaging and spectroscopic studies which reveal the multiplicity properties of embedded protostars for the first time. The overall binary / multiplicity fraction of protostars is statistically indistinguishable from that of pre-main-sequence stars when measured over similar separation distances. Some individual components of protostellar binaries appear to be coeval from their spectral energy distributions or spectra. However, there are some systems which appear to consist of both a protostar and a pre-main-sequence star, and the coevality of these systems is very uncertain. There is much yet to be learned about the multiplicity properties of protostars, and future high resolution near-IR imaging surveys will likely make much headway.

New high-resolution adaptive optics systems provide an unprecedentedly detailed view of nearby star forming regions. In particular, young nearby T Tauri stars can be probed at much smaller physical scales (a few AU) than possible just a decade ago (several tens of AU). Of major importance is closing the sensitivity gap between imaging and spectral surveys for stellar companions. This allows for 1) calibration of pre-main-sequence evolutionary tracks by obtaining accurate dynamical masses, 2) resolving confusion problems arising by placing unresolved systems in colour-magnitude diagrams, and 3) well defined and determined multiplicity fractions of young stellar systems, important for discriminating star formation scenarios. This article briefly reviews the current status of high resolution imaging of T Tauri multiple systems, and what we can expect to learn from them in the near future.

I review recent measurements of the initial mass function of stars and brown dwarfs in star-forming regions and open clusters and summarize the implications of these data for theories of star formation.

In the last decade, our interest moved significantly to the youngest stellar phase, “protostars”. Evidence for infalls and outflows are implying violent star-forming activities close vicinity of the protostar itself. However, due to the very high extinction in optical and near-infrared band or even in soft X-ray band, we had no tool to access the star itself. In such situation, Chandra X-ray Observatory, with the high transparency in the hard X-ray band and the superior spatial resolution, are beginning to reveal X-ray emitting phenomena deep inside the protostellar cores. In this paper, the high energy activities in Class I and 0 sources detected with Chandra is reviewed.

We review recent results from hydrodynamical calculations of the formation of young stellar clusters. The calculations present a highly dynamical picture of star formation where the mass function of stars originates from competitive accretion between protostars and dynamical ejections which halt accretion. Large star clusters form hierarchically; a molecular cloud forms many small clusters that later merge into one large cluster, erasing the initial substructure. Using calculations that resolve fragmentation down to the opacity limit, we examine the implications of dynamical star formation for the IMF, the formation of brown dwarfs and close binaries, and the sizes of protoplanetary discs. Finally, we discuss the dependence of the results on the initial conditions.

Embedded clusters produce the greatest numbers of stars within our Galaxy but the process by which clusters themselves form is not well understood. The structures and kinematics of the very youngest embedded clusters, i.e., “protoclusters” with ages <106 yr, should provide needed insight. To gauge their utility in this regard, we examine recent observations of a nearby protocluster in Ophiuchus. Near-infrared images of the Oph A core reveal numerous young objects but the small sky coverages or low sensitivities to highly-embedded objects attained so far limit their usefulness in revealing cluster structure. Submillimeter continuum images do reveal the most embedded objects but their relatively low resolutions preclude effective comparisons with near-infrared images. Interferometric millimeter observations of N2H+ 1-0 show a strong spatial coincidence with the submillimeter continuum objects, and can be used to trace radial velocities of this population at least. The Oph A filament shows small radial velocity dispersions, e.g., ∼0.2 km s-1, at odds with predictions from some theories of cluster formation. The upcoming C2D and COMPLETE surveys of nearby star-forming molecular clouds will produce the sensitive, wide-field data needed to understand better the cluster formation process.

The newly upgraded Australia Telescope Compact Array (ATCA) at millimetre wavelengths is the first millimetre interferometer to be built in the Southern Hemisphere. The full array will be operational in 2004-2005 and will provide arcsec angular resolution at 3 mm and 12 mm. This will be a unique instrument to study at high angular resolution the interstellar chemistry and more generally the star formation process, especially in the bulk of the galactic plane and in the Magellanic Clouds. The upgraded ATCA will also be an excellent tool to detect dust emission from nearby protoplanetary disks. In this paper I will present the first results from the upgraded ATCA at 3 mm and 12 mm. The result review will cover the topics of massive star formation and hot molecular cores dust emission from star-forming regions and detection of protoplanetary disks.

The Submillimeter Array (SMA), a collaborative project of the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics of Taiwan (ASIAA), is approaching the end of the construction phase. In this paper, I review its current capability and describe the scientific programs carried out in recent months. I will also present some preliminary results obtained with the SMA in the area of star formation.

VLTI - the infrared interferometer integrated into the VLT complex on Paranal - is approaching operation as part of the scientific instrumentation of the VLTs, with the first call for proposals out for the observing period starting in spring 2004. Two first generation scientific instruments will become available: AMBER, working in the near-infrared and combining up to three telescope beams is scheduled to see first fringes and have commisioning early in 2004. MIDI, working in the 10 μm band and combining two telescope beams, already is working in its basic observing mode. This contribution concentrates on a brief description of the latter instrument and the preliminary first scientific results obtained with it.

The Keck Interferometer utilizes the two 10-meter Keck telescopes as a direct detection interferometer in the infrared. The 85-meter baseline produces a fringe spacing of 5 milliarcseconds at 2 microns, or 0.7 AU for sources in the Taurus star forming region. The first observations with the Keck Interferometer of T Tauri and Herbig Ae/Be sources are presented and the implications for circumstellar disk models are discussed.

We present results from our high-resolution, high-contrast imaging campaign targeting the circumstellar environments of young, nearby stars of different masses. The observations have been conducted using the ALFA/CA 3.5m and NACO UT4/VLT adaptive optics systems. In order to enhance the contrast we applied the methods PSF-subtraction and polarimetric differential imaging (PDI). The observations of young stars yielded the identification of numerous new companion candidates, the most interesting one being ∼ 0.5″ from FU Ori. We also obtained high-resolution near-infrared imaging of the circumstellar envelope of SU Aur and AB Aur. Our PDI of the TW Hya circumstellar disk traced back the disk emission as close as 0.1″ ≃ 6 AU from the star, the closest yet. Our results demonstrate the potential of the adaptive optics systems in achieving high-resolution and high-contrast imaging and thus in the study of circumstellar disks, envelopes and companions.

A cold near-infrared stellar coronagraph combined with adaptive optics (CIAO) is introduced. As an open-use instrument on the Subaru 8.2-m telescope, it has been used for several star formation studies with high spatial resolutions (from natural seeing of about 0.6 arcsec down to 0.07 arcsec with adaptive optics). A brief explanation is described of the instrument as well as its current main project of systematic surveys of disks and young companions around T Tauri stars and Herbig Ae/Be stars. In particular, observations of HL Tau are presented in some details. Our images of HL Tau show several new circumstellar features including the presence of a red H - K color region of ∼150 AU, probably corresponding to the small circumstellar disk. The observations of a high density stellar cluster of MWC 137 are also reported. It appears to be a cluster of very low-mass stars around Herbig Be star or a cluster of B stars around a super giant.

Over the past ten years, the concept of adaptive optics has evolved from early experimental stages to a standard observing tool now available at almost all major optical and near-infrared telescope facilities. Adaptive optics will also be essential in exploiting the full potential of the large optical/infrared interferometers currently under construction. Both observations with high-angular resolution and at high contrast, and with a high point source sensitivity are facilitated by adaptive optics. Among the areas which benefit most from the use of adaptive optics are studies of the circumstellar environment (envelopes, disks, outflows), substellar companions and multiple systems, and dense young stellar populations. This contribution highlights some of the recent advances in star formation studies facilitated by adaptive optics, and gives a brief tutorial on optimized observing and data reduction strategies.

Observational techniques with high angular and high spectral resolution have become available in recent years and have had a major impact on the study of Herbig-Haro outflows from very young stars. We discuss new results obtained using WFPC2, ACS and STIS onboard the Hubble Space Telescope, as well as spectroscopic data obtained at the Keck telescope and with the Integral Field Unit at the Gemini telescope. We also discuss centimeter radio continuum maps from the Very Large Array and their importance in testing scenarios for the formation of giant Herbig-Haro flows.

We present the results of a high angular resolution (< 5″) survey of protostars with outflows, at different evolutionary stages, using the Owens Valley Radio Observatory Millimeter Array. This survey aims to understand the evolution of the interaction between protostellar outflows and the infalling circumstellar envelopes. Our data enable us to probe the structure and kinematics of the molecular outflow and the circumstellar envelope, as well as the outflow-envelope interaction at scales of less than 0.02 pc, for every source in our sample. Our results indicate that outflows have a considerable impact on the dense circumstellar envelope. Also, we show that the nature of the outflow-envelope interaction depends on the protostar's evolutionary stage.

We have constructed the foundations to a series of diagnostics methods to probe the jet phenomena in young stars as observed at various optical forbidden lines and radio wavelengths. We calculate and model in a self-consistent manner the physical and radiative processes, which arise within an inner disk-wind driven magnetocentrifugally from the circumstellar accretion disk of a sun-like star. Comparing with real data taken at high angular resolution, our approach will provide the basis of systematic diagnostics for jets and their related young stellar objects, to attest the emission mechanisms of such phenomena. Such approach can help bring first-principle theoretical predictions to confront actual multi-wavelength observations, and will bridge the link between complex numerical simulations and observational data. Analysis methods discussed here are immediately applicable to new high-resolution data obtained with HST, Adaptic Optics, and radio interferometry.