We theoretically investigate the impact of feedback and its metallicity dependence in massive star formation from prestellar cores at all metallicity range. We include the feedback by MHD disk winds, radiation pressure, and photoevaporation solving the evolution of protostars and accretion flows self-consistently. Interestingly, we find that the feedback does not set the upper mass limit of stellar birth mass at any metallicity. At the solar metallicity, the MHD disk wind is the dominant feedback to set the star formation efficiencies (SFEs) from the prestellar cores similar to low-mass star formation. The SFE is found to be lower at lower surface density environment. The photoevaporation becomes significant at the low metallicity of Z < 10−2 Z⊙. Considering this efficient photoevaporation, we conclude that the IMF slope is steeper, i.e., massive stars are rarer at the extremely metal-poor environment of 10−5 − 10−3Z⊙. Our study raises a question on the common assumption of the universal IMF with a truncated at 100M⊙. Since the total feedback strength in the cluster/galaxy scale is sensitive to the number fraction of massive stars, the re-evaluations of IMF at various environments are necessary.

I review massive star formation in our Galaxy, focussing on initial conditions in Infrared Dark Clouds (IRDCs), including the search for massive pre-stellar cores (PSCs), and modeling of later stages of massive protostars, i.e., hot molecular cores (HMCs). I highlight how developments in astrochemistry, coupled with rapidly improving theoretical/computational and observational capabilities are helping to improve our understanding of the complex process of massive star formation.

I review theoretical models of star formation and how they apply across the stellar mass spectrum. Several distinct theories are under active study for massive star formation, especially Turbulent Core Accretion, Competitive Accretion and Protostellar Mergers, leading to distinct observational predictions. These include the types of initial conditions, the structure of infall envelopes, disks and outflows, and the relation of massive star formation to star cluster formation. Even for Core Accretion models, there are several major uncertainties related to the timescale of collapse, the relative importance of different processes for preventing fragmentation in massive cores, and the nature of disks and outflows. I end by discussing some recent observational results that are helping to improve our understanding of these processes.

The Kepler-discovered Systems with Tightly-packed Inner Planets (STIPs), typically with several planets of Earth to super-Earth masses on well-aligned, sub-AU orbits may host the most common type of planets, including habitable planets, in the Galaxy. They pose a great challenge for planet formation theories, which fall into two broad classes: (1) formation further out followed by inward migration; (2) formation in situ, in the very inner regions of the protoplanetary disk. We review the pros and cons of these classes, before focusing on a new theory of sequential in situ formation from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm-m size) “pebbles,” drifting inward via gas drag. Pebbles first collect at the pressure trap associated with the transition from a magnetorotational instability (MRI)-inactive (“dead zone”) region to an inner, MRI-active zone. A pebble ring builds up that begins to dominate the local mass surface density of the disk and spawns a planet. The planet continues to grow, most likely by pebble accretion, until it becomes massive enough to isolate itself from the accretion flow via gap opening. This reduces the local gas density near the planet, leading to enhanced ionization and a retreat of the dead zone inner boundary. The process repeats with a new pebble ring gathering at the new pressure maximum associated with this boundary. We discuss the theory's predictions for planetary masses, relative mass scalings with orbital radius, and minimum orbital separations, and their comparison with observed systems. Finally, we discuss open questions, including potential causes of diversity of planetary system architectures, i.e., STIPs versus Solar System analogs.

Inside-Out Planet Formation (IOPF; Chatterjee & Tan 2014, hereafter CT14) is a scenario for sequential in situ planet formation at the pressure traps of retreating dead zone inner boundaries (DZIBs) motivated to explain the many systems with tightly packed inner planets (STIPs) discovered by Kepler. The scenario involves build-up of a pebble-dominated protoplanetary ring, supplied by radial drift of pebbles from the outer disk. It may also involve further build-up of planetary masses to gap-opening scales via continued pebble accretion. Here we study radial drift & growth of pebbles delivered to the DZIB in fiducial IOPF disk models.

All stars are born in molecular clouds, and most in giant molecular clouds (GMCs), which thus set the star formation activity of galaxies. We first review their observed properties, including measures of mass surface density, Σ, and thus mass, M. We discuss cloud dynamics, concluding most GMCs are gravitationally bound. Star formation is highly clustered within GMCs, but overall is very inefficient. We compare properties of star-forming clumps with those of young stellar clusters (YSCs). The high central densities of YSCs may result via dynamical evolution of already-formed stars during and after star cluster formation. We discuss theoretical models of GMC evolution, especially addressing how turbulence is maintained, and emphasizing the importance of GMC collisions. We describe how feedback limits total star formation efficiency, ε, in clumps. A turbulent and clumpy medium allows higher ε, permitting formation of bound clusters even when escape speeds are less than the ionized gas sound speed.

Asian vultures have undergone dramatic declines of 90–99% in the Indian Subcontinent, as a consequence of poisoning by veterinary use of the drug diclofenac, and are at a high risk of extinction. Cambodia supports one of the only populations of three species (White-rumped Vulture Gyps bengalensis, Slender-billed Vulture G. tenuirostris and Red-headed Vulture Sarcogyps calvus) outside of South Asia where diclofenac use is not widespread. Conservation of the Cambodian sub-populations is therefore a global priority. This study analyses the results of a long-term research programme into Cambodian vultures that was initiated in 2004. Population sizes of each species are estimated at 50–200+ individuals, ranging across an area of approximately 300 km by 250 km, including adjacent areas in Laos and Vietnam. The principal causes of vulture mortality were poisoning (73%), probably as an accidental consequence of local hunting and fishing practices, and hunting or capture for traditional medicine (15%). This represents a significant loss from such a small population of long-lived, slow breeding, species such as vultures. Cambodian vultures are severely food limited and are primarily dependent on domestic ungulate carcasses, as wild ungulate populations have been severely depleted over the past 20 years. Local people across the vulture range still follow traditional animal husbandry practices, including releasing livestock into the open deciduous dipterocarp forest areas when they are not needed for work, providing the food source. Reducing threats through limiting the use of poisons (which are also harmful for human health) and supplementary food provisioning in the short to medium-term through ‘vulture restaurants’ is critical if Cambodian vultures are to be conserved.

We simulated an isolated quiescent Milky Way-type galaxy with a maximum effective resolution of 7.8 pc. Clouds formed in the interstellar medium through gravitational fragmentation and became the sites for star formation. We tracked the evolution of the clouds through 300 Myr in the presence of star formation, photoelectric heating and feedback from Type II supernovae. The cloud mass distribution agreed well with observational results. Feedback suppressed star formation but did not destroy the surrounding cloud. Collisions between clouds were found to be sufficiently frequent to be a significant factor in determining the star formation rate.

I discuss current theoretical expectations of how primordial, Pop III.1 stars form. Lack of direct observational constraints makes this a challenging task. In particular predicting the mass of these stars requires solving a series of problems, which all affect, perhaps drastically, the final outcome. While there is general agreement on the initial conditions, H2-cooled gas at the center of dark matter minihalos, the subsequent evolution is more uncertain. In particular, I describe the potential effects of dark matter annihilation heating, fragmentation within the minihalo, magnetic field amplification, and protostellar ionizing feedback. After these considerations, one expects that the first stars are massive ≳100M⊙, with dark matter annihilation heating having the potential to raise this scale by large factors. Higher accretion rates in later-forming minihalos may cause the Pop III.1 initial mass function to evolve to higher masses.

I review the status of massive star formation theories: accretion from collapsing, massive, turbulent cores; competitive accretion; and stellar collisions. I conclude the observational and theoretical evidence favors the first of these models. I then discuss: the initial conditions of star cluster formation as traced by infrared dark clouds; the cluster formation timescale; and comparison of the initial cluster mass function in different galactic environments.

I review some important questions in the field of massive star formation: What are the initial conditions for proto star clusters and how do they arise? What are the initial conditions for individual massive star formation within star clusters? How do massive protostars accumulate their mass? I compare the Turbulent Core Model (McKee & Tan 2003) to several nearby regions, including Orion KL. Here I also discuss the origin of BN's high proper motion.