Introduction and motivations

Despite theoretical progress and rapid advances in the discovery of high-redshift galaxies, many questions about the first stars remain open: What was their IMF? How long did the epoch of metal-free stars last? Did they contribute much to reionization? Do their remnants include compact objects, and if so, where are they? The highly interdisciplinary “first stars” field is just beginning to confront observations in the form of reionization tests with the Gunn-Peterson effect and the Wilkinson Microwave Anisotropy Probe (WMAP), and chemical abundances from extremely metal-poor Galactic stars. These indicators have yielded some indirect constraints on the nature of the first stars. I believe that neither high-z galaxies nor the “second stars” will tell the whole story on their own.

Introduction

Wolf-Rayet (WR) stars represent the final phase in the evolution of very massive stars prior to core collapse, in which the H-rich envelope has been stripped away via either stellar winds or close binary evolution, revealing products of H-burning (WN sequence) or He-burning (WC sequence) at their surfaces, i.e., He, N or C, O (Crowther 2007).

WR stellar winds are significantly denser than O stars, as illustrated in Figure 1, so their visual spectra are dominated by broad emission lines, notably He II λ4686 (WN stars) and C III λ4647–51, CIII λ5696, C IV λ5801–12 (WC stars). The spectro-scopic signature of WR stars may be seen individually in Local Group galaxies (e.g., Massey & Johnson 1998), within knots in local star-forming galaxies (e.g., Hadfield & Crowther 2006) and in the average rest frame UV spectrum of Lyman Break galaxies (Shapley et al. 2003).

Introduction

The very first stars to form in the universe, commonly called Population III stars, are now thought to have been very massive stars forming out of primordial metal-free gas at redshifts exceeding z ∼ 10 (see review by Bromm & Larson 2004). Assuming that the density field responsible for structure formation is given by the ∧ Cold Dark Matter (CDM) model, the first collapsing haloes hosting such stars may be too faint to be observed with present telescopes. Their studies may, however, be possible via the cumulative radiation emitted by the first luminous objects, most of which has by now been shifted into the near-IR wavelengths of ∼1–10 μm.

Introduction

Eta Carinae (η Car) has intrigued astronomers for well over a century, beginning with its brightening to —1 magnitude in the late 1830s, rivaling Sirius as the brightest star in the sky for nearly two decades, then fading below naked-eye sensitivity. Observers in the Southern Hemisphere have pointed their telescopes in its direction since the 1820s; some navigational records exist even back to the late sixteenth century with visual magnitudes noted between 2nd and 4th magnitude (Frew 2004). Characterization of this peculiar star has been a challenge; D. Frew (private communication, 2003) noted that η Car was monitored by observers at Sydney Observatory in the nineteenth century in the suspicion that it was a binary system. Yet still today not all are convinced as direct evidence of the secondary star is not in hand.

η Car is of great interest, as at least one of the companions is at the end of its hydrogenburning phase.

The Space Telescope Science Institute Symposium on Massive Stars: From Pop III and GRBs to the Milky Way took place during May 8–11, 2006.

These proceedings represent only a part of the invited talks that were presented at the symposium. We thank the contributing authors for preparing their manuscripts.

Traditionally, massive stars played the important roles of being responsible for supernova explosions, of being the progenitors of stellar-mass black holes, and of producing heavy elements. In recent years, massive stars have gained additional importance in our understanding of cosmic history. Very massive stars (Population III) are now recognized as constituting the first population of stars in the universe, and massive stars have been identified as being the progenitors of the long-duration Gamma-ray Bursters. In addition, very massive stars may produce supernova explosions by a new mechanism—pair instability—that has been anticipated theoretically, but has never been unambiguously detected (the recent SN 2006gy may have been such an event).

The ST ScI symposium on Massive Stars attempted to capture all the aspects involved in the astrophysics of massive stars.

We thank Sharon Toolan of ST ScI for her help in preparing this volume for publication.

Introduction

It is now well accepted that most star formation occurs in clustered environments, such as associations, groups and clusters (e.g., Lada & Lada 2003). In addition, it is clear that star formation is greatly enhanced in merging galaxies, making them an excellent place to study the formation of large numbers of young, massive stars—albeit with the disadvantage of having to work with stars at larger distances than the nearby groups and clusters in our own galaxy. In keeping with their galactic counterparts, most of the stars in merging galaxies also form in clusters, the brightest and most compact of which have been dubbed “super star clusters.” Hence, understanding what triggers the formation of star clusters in mergers may be an important clue for understanding the formation of stars in general.

When an asteroid or a comet has just been discovered, its orbit is weakly constrained by the available astrometric observations and it might be the case that an impact on the Earth in the near future (within the next 100 years) cannot be excluded. If additional observations are obtained, the uncertainty of the orbit decreases and the impact may become incompatible with the available information. Thus, if we are aware that an impact is possible, it is enough to spread this information to the astronomers to convince them to follow up the object. On the contrary if this piece of information is not available, or is made available when the asteroid has been lost, the impact risk will remain until the same asteroid is accidentally recovered. This might occur too late for any mitigation action.

This problem can be solved if all the asteroids/comets, immediately after being discovered and before they can be lost, are “scanned” for possible impacts in the near future. If impacts are possible, this information has to be broadcast to the astronomers. This is the goal of impact monitoring.

It is somewhat surprising that this was not really possible until late 1999, when the first impact monitoring system, the CLOMON software robot of the University of Pisa, became operational.

Introduction

It is often stated that zero-age main-sequence (ZAMS) O stars should not be and are not observed. This view arises from at least three sources: star-formation theory, which suggests that the embedded accretion (merger?) phases constitute a significant fraction of the main-sequence lifetimes of massive stars (2.5 Myr for the most massive); statistical studies of UCHII and IR objects relative to optically observed ones; and detailed physical analyses of optical O-star samples that find very few on the ZAMS.