In this contribution, we will overview the NANTEN observations of molecular clouds faced to H II regions, supershells, and interacting galaxies, which demonstrate that star/molecular cloud formation is being triggered by young OB associations, supershells, and collisions between galaxies. The large volume filling factor of explosive events like supernovae, ultraviolet radiation fields and stellar winds of massive stars suggest that most of the interstellar medium has been agitated by such strong impacts and triggered star formation is a common event at all scales from small molecular clouds to large galaxy-galaxy mergers. The consequence is the increase of star formation efficiency in many cases, and that more massive stars or clusters of more member stars tend to be formed by triggering than in spontaneous star formation.

Several (sub)millimeter-wave studies of nearby star-forming regions have revealed self-gravitating prestellar condensations that seem to be the direct progenitors of individual stars and whose mass distribution resembles the IMF. In a number of cases, small internal and relative motions have been measured for these condensations, indicating they are much less turbulent than their parent cloud and do not have time to interact before evolving into protostars and pre-main sequence stars. These findings suggest that the IMF is at least partly determined by pre-collapse cloud fragmentation and that one of the keys to understanding the origin of stellar masses lies in the physical mechanisms responsible for the formation and decoupling of prestellar cores within molecular clouds.

Although the subject of this meeting is triggered star formation in a turbulent interstellar medium, it remains unsettled what role magnetic fields play in the star formation process. This paper briefly reviews star formation model predictions for the ratio of mass to magnetic flux, describes how Zeeman observations can test these predictions, describes new results – an extensive OH Zeeman survey of dark cloud cores with the Arecibo telescope, and discusses the implications. Conclusions are that the new data support and extend the conclusions based on the older observational results – that observational data on magnetic fields in molecular clouds are consistent with the strong magnetic field model of star formation. In addition, the observational data on magnetic field strengths in the interstellar medium strongly suggest that molecular clouds must form primarily by accumulation of matter along field lines. Finally, a future observational project is described that could definitively test the ambipolar diffusion model for the formation of cores and hence of stars.

Young massive star-forming regions are known to produce hot molecular gas cores (HMCs) with a rich chemistry. While this chemistry is interesting in itself, it also allows to investigate important physical parameters. I will present recent results obtained with high-angular-resolution interferometers disentangling the small-scale structure and complexity of various molecular gas components. Early attempts to develop a chemical evolutionary sequence are discussed. Furthermore, I will outline the difficulty to isolate the right molecular lines capable to unambiguously trace potential massive accretion disks.

We present new BIMA observations of the massive star-forming region IRAS 23033+5951 in Cepheus. 3 mm continuum observations reveal that the source decomposes into at least three dusty clumps, each of which has sufficient mass to form a massive star. The most massive clump has a mass of about 225 M and appears to house the massive protostar which drives the prominent CO outflow seen in the region. Our H13CN, 1-0, N2H+ 1-0, and H13CO+ 1-0 maps show that the three continuum sources are all embedded in an elongated structure whose long axis is perpendicular to the outflow. Both H13CO+ and H13CN peak at the geometric center of this structure, which lies between the two prominent continuum peaks. All three lines – H13CN, H13CO+, and N2H+ –show the same velocity gradient along the long axis of their integrated intensity maps. Although the approximately 90,000 AU length of the elongated structure prohibits a disk interpretation, the fact that the dynamical and gas masses of the structure differ by only a factor of a few suggests that the structure may be partially rotationally supported. We also detect a signature of infall toward the center of the structure, seen as an asymmetrically blue HCO+ line where its optically thin isotope, H13CO+, is symmetric and single-peaked.

We present a multi-wavelength (NIR to radio) and multi-scale (1 AU to 10 pc) study of the S255–S257 complex of young high-mass (proto)stars. The complex consists of two evolved HII regions and a molecular gas filament in which new generations of high mass stars form. Four distinct regions are identified within this dusty filament: a young NIR/optical source cluster, a massive protostar binary, a (sub)millimetre continuum and molecular clump in global collapse and a reservoir of cold gas. Interestingly, the massive binary protostellar system is detected through methanol maser and mid-IR emission at the interface between the NIR cluster and the cold gas filament. The collapsing clump is located to the north of the NIR cluster and hosts a young high-mass star associated with an outflow that is observed in mid-IR, methanol maser and radio emission. We interpret this anatomy as the possible result of triggered star formation, starting with the formation of two HII regions, followed by the compression of a molecular gas filament in which a first generation of high-mass stars forms (the NIR cluster), which then triggers the formation of high mass protostars in its near environment (the massive protostellar binary). The global collapse of the northern clump might be due to both the expansion of the HII regions that squashes the filament. In conclusion, we witness the formation of four generations of clusters of high-mass stars in S255–S257.

Young stellar objects (YSOs) inject large amounts of momentum and kinetic energy into their surroundings. Feedback from low mass YSOs is dominated by their outflows. However, as stellar mass increases, UV photo-heating and ionization play increasingly important roles. Massive stars produce powerful stellar winds and explode as supernovae within 3 – 40 Myr after birth. While low-mass protostellar feedback can drive turbulence in cloud cores and even disrupt the star forming environment, feedback from massive stars plays important roles in the generation of cloud structure and motions in the entire ISM.

The ubiquity and high density of outflows from young stars in clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we discuss new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their ability to drive turbulent flows in molecular clouds. Our results are surprising in that they show that fossil cavities, rather than bow shocks from active outflows, constitute the mechanism of re-energizing turbulence. We first present simulations which show that collisions between active jets are ineffective at converting directed momentum and energy in outflows into turbulence. This effect comes from the ability of radiative cooling to constrain the surface area through which colliding outflows entrain ambient gas. We next discuss observational results which demonstrate that fossil cavities from “extinct” outflows are abundant in molecular material surrounding clusters such as NGC 1333. These structures, rather than the bow shocks of active outflows, comprise the missing link between outflow energy input and re-energizing turbulence. In a separate theoretical/simulation study we confirm that the evolution of cavities from decaying outflow sources leads to structures which match the observations of fossil cavities. Finally we present new results of outflow propagation in a fully turbulent medium exploring the explicit mechanisms for the transfer of energy and momentum between the driving wind and the turbulent environment.

Winds and outflows powered by the release of gravitational potential energy of infalling matter have been invoked as the triggering agent for further star-formation on scales from dwarf galaxies to individual, low-mass stars. This brief review will touch upon the circumstances under which outflows can serve as star-formation triggers, from specific observations of present-day, nearby protostellar flows.

Using radial velocities of vibrationally excited H2 emission in OMC1 we present the structure functions and the scaling of the structure functions with their order at scales ranging from 70 AU to 30000 AU extending earlier related studies to scales lower by two orders of magnitude. The structure functions for OMC1 show clear deviations from power laws at 1500 AU. The scaling of the higher order structure functions with order deviates from predicted theoretical scalings. Observational results are compared with simulations of supersonic hydrodynamic turbulence. The unusual scaling is explained as a selection effect of preferentially observing the shocked part of the gas. The simulations are unable to reproduce the deviations from power laws of the structure functions.

Extremely red objects were identified in the early Spitzer Space Telescope observations of the bright-rimmed globule IC 1396A; they were classified as Class I protostars Class II T Tauri stars with disks based on their colors. New spectroscopic observations covering 5.5–38 μ confirm this identification. The Class I sources have extremely red continua, still rising at 38 μm, with a deep silicate absorption at 9–11 μm, weaker silicate absorption around 18 μm, and weak ice features including CO2 at 15.2 μm and H2O at 6 μm. The Class II sources have warm, luminous disks, with a silicate emission feature at 9–11 μm. Optical spectra with the Palomar Hale 200-inch telescope show the Class II sources to be actively accreting, classical T Tauri stars with bright Hα and other emission lines. The Class I sources are located within the molecular globule, while the Class II sources are more widely scattered. This suggests two phases of star formation occurred in the region, the first one leading to the Class II sources including LkHα 349a, c that are located in the center of the globule, and a very recent one (less than 100,000 yr ago) that is occurring within the globule. This second phase was likely triggered by the wind and radiation of the central O star of the IC 1396 H II region, with possible additional contributions from the outflows of LkHα 349a, c and some nearby B stars.

Evidence for triggered star formation is difficult to establish because energy feedback from massive stars tend to erase the interstellar conditions that led to the star formation. Young stellar objects (YSOs) mark sites of current star formation whose ambient conditions have not been significantly altered. Spitzer observations of the Large Magellanic Cloud (LMC) effectively reveal massive YSOs. The inventory of massive YSOs, in conjunction with surveys of interstellar medium, allows us to examine the conditions for star formation: spontaneous or triggered. We examine the relationship between star formation and gravitational instability on a global scale, and we present evidence of triggered star formation on local scales in the LMC.

We recently launched a comprehensive ground based (ESO/VLT/NTT) and space (HST & SST) study of the present and past star formation in the Small Magellanic Cloud (SMC), in clusters and in the field, with the goal of understanding how star and cluster formation occur and propagate in an environment of low metallicity, with a gas and dust content that is significantly lower than in the Milky Way. In this paper, we present some preliminary results of the “young cluster” program, where we acquired deep F555W (~V), and F814W (~I) HST/ACS images of the four young and massive SMC star clusters: NGC 346, NGC 602, NGC 299, and NGC 376.

Near infrared dust extinction mapping is opening a new window on molecular cloud research. Applying a straightforward technique to near infrared large scale data of nearby molecular complexes one can easily construct density maps with dynamic ranges in column density covering, 3σ~ 0.5 < AV< 50 mag or 1021<N<1023 cm−2. These maps are unique in capturing the low column density distribution of gas in molecular cloud complexes, where most of the mass resides, and at the same time allow the identification of dense cores (n~104cm−3) which are the precursors of stars. For example, the application of this technique to the nearby Pipe Nebula complex revealed the presence of 159 dense cores (the largest sample of such object in one single complex) whose mass spectrum presents the first robust evidence for a departure from a single power-law. The form of this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor of about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%±10%, and that the stellar IMF may already be fixed during or before the earliest stages of core evolution. We are now extending this technique to extra-galactic mapping of Giant molecular Clouds (GMCs), and although a much less straightforward task, preliminary results indicate that the GMC mass spectrum in M83 and Centaurus A is a power-law characterized by α~−2 unlike CO results which suggest α~−1.

Starburst galaxies are characterized by uncommonly high star formation efficiencies, but it remains unclear what physical conditions in the molecular gas produce this high efficiency. Invariably, high star formation efficiency is associated with high column densities of molecular material (e.g. the Kennicutt-Schmidt law), but what are the conditions in the molecular clouds in starburst galaxies? Direct observations of starburst are difficult or impossible with current instruments, so I present the properties of GMCs in the Local Group as a starting case and then extend the analysis of GMC properties to nearby systems with surface densities of gas intermediate between the Local Group and starbursts. Rather than being constant, molecular cloud properties follow a continuum with significant variation across the Local Group and the intermediate surface density systems. Concomitant with these variations in the macroscopic properties are significant changes in the internal pressure and densities of molecular clouds, which implies significant variability in the initial conditions of the star formation process.

We show how the expansion of classical Galactic Hii regions can trigger massive-star formation via the collect & collapse process. We give examples of this process at work. We suggest that it also works in a turbulent medium.

According to a triggered star formation scenario (e.g. Martin-Pintado & Cernicharo 1987) outflows powered by young stellar objects shape the molecular clouds, can dig cavities, and trigger new star formation. NGC 1333 is an active site of low- and intermediate star formation in Perseus and is a suggested site of self-regulated star formation (Norman & Silk 1980). Therefore it is a suitable target for a study of triggered star formation (e.g. Sandell & Knee 2001, SK1). On the other hand, continuum sub-mm observations of star forming regions can detect dust thermal emission of embedded sources (which drive outflows), and further detailed structures.

Within the framework of our wide-field mapping of star formation regions in the Perseus and Orion molecular clouds using SCUBA at 850 and 450 μm, we mapped NCG 1333 with an area of around 14′× 21′. The maps show more structure than the previous maps of the region observed in sub-mm. We have unveiled the known embedded SK 1 source (in the dust shell of the SSV 13 ridge) and detailed structure of the region, among some other young protostars.

In agreement with the SK 1 observations, our map of the region shows lumpy filaments and shells/cavities that seem to be created by outflows. The measured mass of SK 1 (~0.07 M) is much less than its virial mass (~0.2-1 M). Our observations support the idea of SK 1 as an event triggered by outflow-driven shells in NGC 1333 (induced by an increase in gas pressure and density due to radiation pressure from the stellar winds that have presumably created the dust shell). This kind of evidences provides a more thorough understanding of the star formation regulation processes.

It is generally believed that most stars are born in groups and clusters, rather than in the field. It has also been demonstrated that merging galaxies produce large numbers of young massive star clusters, sometimes called super star clusters. Hence, understanding what triggers the formation of these young massive clusters may provide important information about what triggers the formation of stars in general. In recent years it has become apparent that most clusters do not survive more than ≈10 Myr (i.e., “infant mortality”). Hence, it is just as important to understand the disruption of star clusters as it is to understand their formation if we want to understand the demographics of both star clusters and field stars. This talk will first discuss what triggers star cluster formation in merging galaxies (primarily in the Antennae galaxies), will then demonstrate that most of the faint objects detected in the Antennae are clusters rather than individual stars (which shows that the initial mass function was a power law rather than a Gaussian), and will then outline a general framework designed to empirically fit observations of both star clusters and field stars in a wide variety of galaxies from mergers to quiescent spirals.

Star clusters are observed to form in a highly compact state and with low star-formation efficiencies, and only 10 per cent of all clusters appear to survive to middle- and old-dynamical age. If the residual gas is expelled on a dynamical time the clusters disrupt. Massive clusters may then feed a hot kinematical stellar component into their host-galaxy's field population thereby thickening galactic disks, a process that theories of galaxy formation and evolution need to accommodate. If the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function may transform within a few dozen Myr to a mass function with a turnover near 105M, thereby possibly explaining this universal empirical feature. Discordant empirical evidence on the mass function of star clusters leads to the insight that the physical processes shaping early cluster evolution remain an issue of cutting-edge research.

We review the structural properties of giant extragalactic HII regions and HII galaxies based on two dimensional hydrodynamic calculations, and propose an evolutionary sequence that accounts for their observed detailed structure. The model assumes a massive and young stellar cluster surrounded by a large collection of clouds. These are thus exposed to the most important star-formation feedback mechanisms: photoionization and the cluster wind. The models show how the two feedback mechanisms compete with each other in the disruption of clouds and lead to two different hydrodynamic solutions: The storage of clouds into a long lasting ragged shell that inhibits the expansion of the thermalized wind, and the steady filtering of the shocked wind gas through channels carved within the cloud stratum that results into the creation of large-scale superbubbles. Both solutions are here claimed to be concurrently at work in giant HII regions and HII galaxies, causing their detailed inner structure.