We summarize previous work on hydrodynamic ionization fronts and new work where magnetic fields are incorporated into their structures. We describe recent work on recombination front structures and outline refinements which need to be made to both them and ionization fronts.

Molecular outflows are found in most regions of massive star formation. However, evidence for highly collimated jets from massive protostars is still elusive. This paper briefly summarizes the observational status.

The formation of stars is a complex process which is poorly understood at present, although recently important progress has been made both on a theoretical and observational ground. It is clear that star formation must proceed through contraction of a large molecular clump into a dense optically thick proto-stellar core: the obvious consequence is that conservation of angular momentum must force the material to spin up and flatten. Thus, formation of disks around newly formed stars is a very sensible expectation. Indeed, the recent development of instruments like the Hubble Space Telescope (HST) and the millimeter interferometers has allowed detection of several disks around low-mass young stellar objects (YSOs), such as the Keplerian disk in GG Tau (Guilloteau et al. 1999) and that seen with the HST in HH 30 (Burrows et al. 1996). The situation is quite different for high-mass YSOs. In this case, the evidence for disks is scarce, although a priori one would expect these to be more massive than those in low-mass YSOs and hence easier to detect. Various effects may complicate this simple-minded picture: for instance, magnetic field is likely to play an important role coupling the inner part of the collapsing cloud to its outer layers, thus making angular momentum conservation difficult to apply to any single “portion” of the cloud; depending on the ratio between disk and stellar mass, the disk may be unstable and hence short-lived; the effects of the stellar wind and radiation have to be taken into account; finally, the mass and size of the disk depend on the accretion process, which is not well understood. All these caveats probably explain why disks around massive YSOs are difficult to detect.

In the past year several new observations with important implications for massive star formation (MSF) have been obtained. Among these were the detection of UC HII region precursor candidates at 350µm and the discovery of many hard X-ray point sources in the Orion and W3 MSF regions. These observations are summarized below.

Hunter et al. (1998, 2000) imaged 25 MSF regions at 350µm to search for candidate precursors of UC HII regions (i.e. luminous submm/FIR emission, maser sources, and no HII emission). Of the 28 sources detected, 10 appear to be UC HII precursor candidates.

Massive stars in our Galaxy are born predominantly within the dense cores of giant molecular clouds. They affect their environment very soon after a stellar core has formed through their large rate of ionizing photons and their strong stellar winds. Massive stars are formed in clusters with stellar densities n* ~ 104 stars/pc3 and sizes 0.2 — 0.4 pc, and seem to preferentially form near the central region of the cores. Disks have been found in several sources (see Cesaroni, this volume). Associated with massive stars also are bipolar molecular outflows, which have masses, mass loss rates, and energies that are factors of ~ 100 larger than those of low mass stars (see review of Churchwell 1999).

An understanding of the physical processes that dominate during the early stages of formation of massive stars and their influence back on the molecular gas from which they formed requires a detailed knowledge of the physical conditions of the environment prior to and after the formation of the star. Here we present an abstract of an extensive review on this subject by Garay & Lizano (1999). For compactness, most of the references have been omitted.

We summarize results of a study of combined ISO SWS-LWS grating spectra of compact HII regions in our Galaxy.

The utility of near-infrared, spectroscopic studies of central ionizing sources of UC HII regions is presented, in conjunction with a recently available, sophisticated atmospheric code, to constrain the physical conditions and environment of very massive stars at extremely early stages of evolution.

We calculate pre-Main Sequence evolutionary tracks with accretion rates growing with the actual stellar masses. We show that accretion rates growing at least as M1.5 are necessary to fit the constraints on the lifetimes and HR diagram. Most interestingly, such accretion rates growing with the stellar mass well correspond to those derived from observations of mass outflows (Churchwell 2000; Henning et al. 2000). These rates also lie in the permitted region of the dynamical models.

Giant HII (GHII) regions in our Galaxy are typically initially found by radio observations of their optically thin free-free continuum emission. Most of them are partially or totally obscured in the visible by the absorbing effect of intervening and/or local interstellar dust. We (Blum et al. 1999, 2000) have selected a list of the brightest GHII regions in our Galaxy (from Smith et al. 1978) and have begun a program of JHK imaging and K band spectroscopy to identify and classify the exciting stars. We have obtained near IR imaging of eight GHII regions (and data is available for four others). All of these, aside from W49, show the presence of a stellar cluster in the K band at the radio source position. The K,H — K diagrams are used to select the brightest stars. The J — Kvs. H — K diagrams distinguish those stars along the normal reddening line from those with K band excesses. The former group ought to have normal OB star spectra; the latter will have featureless continua in the K band due to emission from localized warm dust arising in a natal disc (Hanson et al. 1997). A disc geometry can also produce CO emission (or absorption) band features.

Globular clusters are ubiquitous in the local universe, and their younger and bluer siblings, “super star clusters” (SSCs), have been observed in a large number of starburst galaxies. Recently a handful of ultra-young SSCs have been found still embedded in their birth material (e.g. Kobulnicky & Johnson 1999 (here after KJ99); Turner et al. 2000; Beck et al. 2000; Tarchi et al. 2000). Because of their similarity (although on a vastly larger scale) to ultracompact HII regions in the Galaxy, KJ99 dub these embedded massive star clusters “ultradense HII regions” (UDHIIs). I will review the discovery of UDHIIs, their modeled properties, and their connection to the more familiar Galactic ultracompact HII regions.

A compact and bright radio and infrared source within the starburst center of the dwarf galaxy NGC 5253 appears to be an extremely large compact HII region. The high density (ne ~ few xlO4 cm-3) and size (∼1-2 pc) of the nebula require the ionization equivalent of at least 4000 O7 stars. We suggest that this optically obscured nebula is a forming super star cluster, or protoglobular cluster, with an age of a few hundred thousand years.

This Joint Discussion has been titled Massive Star Birth. Perhaps it is appropriate here to define what we mean by a massive star. The very word massive suggests we consider a minimum mass M below which one would speak of low (or intermediate) mass evolution, and above which is the realm of massive stars. It is natural to take this mass limit as that in which a (single) star will end its life as a supernova: 8M⊙. This corresponds to a (minimum) luminosity L of a few × 103L⊙, a (minimum) Teff of 20000 K, and a ZAMS spectral type of about B1.5V. Note that this mass division refers to the final evolution of a star, and might well have nothing to do with difference in physical processes between massive and low mass star birth. For example, the minimum Teff for a star to produce an UCHII region, a readily observable quantity, corresponds to a Teff closer to 30000 K and a mass of 15M⊙.

The recognition of the extensive population of bodies in the outer Solar System has been arguably the most significant discovery about the Solar System from groundbased observations during the twentieth century. Our understanding of the transneptunian population is still quite flawed in its details, although a plausible picture has been emerging.

Given the advances in our understanding of the dynamical structure in the Kuiper Belt since 1990, this paper re-examines nomenclature issues regarding dynamical structures in the belt.

We present our results from a wide area search for the brightest and rarest members of the Transneptunian and Centaur/Scattered-Disk asteroid populations. Our data encompasses 1483.8 square degrees of sky taken with the Spacewatch 0.9 meter telescope on Kitt Peak between September of 1995 and September of 1999. We found five new Transneptunian asteroids, three new Centaurs, and two new Scattered Disk Objects. These objects are among the larger members of the population and their relative brightness will make them ideal candidates for physical observations.

The purposes of classification and taxonomy are reviewed. Using examples from fields ranging from paleontology to planetology, I argue that non-exclusive classifications, which allow Pluto to be considered both a planet and a TNO, provide the most desirable approach to progress in our science.

We examine the question of planetary classification, making recommendations both for the criteria by which planethood should be evaluated, as well as for more detailed physical and dynamical subtype classification schemes.

A set of self-consistent simulations of the formation of Uranus and Neptune are performed to study the evolution of the native KBOs in the process. Our main goal is to have a deeper understanding of the impact of the formation of the outer planets on the present orbital structure of the trans-neptunian region. We aim to understand if resonance capture driven by the outward migration of Neptune can actually occur and its interplay with the invasion of massive planetesimals expelled from the Uranus-Neptune region as a byproduct of their formation. Also the putative present existence in the Oort reservoir of a population of objects originated in the Kuiper belt is analyzed.

Collisions have been a major process that shaped the Kuiper Belt that we see today. Collisional grinding likely played a significant role in removing mass from the trans-neptunian region and collisions are a mechanism for injecting fragments into resonances to start their journey to become short period comets. The Kuiper Belt preserves the accretional size distribution in bodies ≳ 100 km while the size distribution of smaller bodies is the result of collisional evolution. Observational confirmation of the transition size between these different regimes will constrain our understanding of the origin and evolution of the Kuiper Belt.