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We present a deep imaging and spectroscopic survey of the Local Group starburst galaxy IC10 using Gemini North/GMOS to unveil the global Wolf-Rayet population. It has previously been suggested that for IC10 to follow the WC/WN versus metallicity dependence seen in other Local Group galaxies, a large WN population must remain undiscovered. Our search revealed 3 new WN stars, and 5 candidates awaiting confirmation, providing little evidence to support this claim. We also compute an updated nebular derived metallicity of log(O/H)+12=8.40 ± 0.04 for the galaxy using the direct method. Inspection of IC10 WR average line luminosities show these stars are more similar to their LMC, rather than SMC counterparts.
The blue compact dwarf galaxy NGC 5253 hosts a very young starburst containing twin nuclear star clusters. Calzetti et al. (2015) find that the two clusters have an age of 1 Myr, in contradiction to the age of 3–5 Myr inferred from the presence of Wolf-Rayet (W-R) spectral features. We use Hubble Space Telescope (HST) far-ultraviolet (FUV) and ground-based optical spectra to show that the cluster stellar features arise from very massive stars (VMS), with masses greater than 100 M⊙, at an age of 1–2 Myr. We discuss the implications of this and show that the very high ionizing flux can only be explained by VMS. We further discuss our findings in the context of VMS contributing to He ii λ1640 emission in high redshift galaxies, and emphasize that population synthesis models with upper mass cut-offs greater than 100 M⊙ are crucial for future studies of young massive clusters.
We present VLT/MUSE observations of NGC 2070, the dominant ionizing nebula of 30 Doradus in the LMC, plus HST/STIS spectroscopy of its central star cluster R136. Integral Field Spectroscopy (MUSE) and pseudo IFS (STIS) together provides a complete census of all massive stars within the central 30×30 parsec2 of the Tarantula. We discuss the integrated far-UV spectrum of R136, of particular interest for UV studies of young extragalactic star clusters. Strong He iiλ1640 emission at very early ages (1–2 Myr) from very massive stars cannot be reproduced by current population synthesis models, even those incorporating binary evolution and very massive stars. A nebular analysis of the integrated MUSE dataset implies an age of ~4.5 Myr for NGC 2070. Wolf-Rayet features provide alternative age diagnostics, with the primary contribution to the integrated Wolf-Rayet bumps arising from R140 rather than the more numerous H-rich WN stars in R136. Caution should be used when interpreting spatially extended observations of extragalactic star-forming regions.
IAU Commission 29 - Stellar Spectra has been one of the IAU commissions from the onset, until its dissolution at the most recent IAU General Assembly in Honolulu in 2015. This commission belonged to IAU Division G (“Stars and Stellar Physics”), the latter committed with fostering research in stellar astrophysics. Within the general field of stellar astrophysics, stellar spectroscopy plays a key role, as stellar spectra are a powerful tool providing a view into the detailed physical properties of stars and the physical processes occuring within them.
Our Working Group (WG) studies massive, luminous stars, both individually and in resolved and unresolved populations, with historical focus on early-type (OB) stars, A-supergiants, and Wolf-Rayet stars. Our group also studies lower mass stars (e.g., central stars of planetary nebulae and their winds) which display features similar or related to those present in massive stars, and thus may improve our understanding of the physical processes occurring in massive stars. In recent years, massive red supergiants that evolve from hot stars have been included into our activities as well. We emphasize the role of massive stars in other branches of astrophysics, particularly regarding the First Stars, long duration Gamma-Ray bursts, formation of massive stars and their feedback on star formation in general, pulsations of massive stars, and starburst galaxies.
The locations of massive stars (≥ 8M⊙) within their host galaxies is reviewed. These range from distributed OB associations to dense star clusters within giant Hii regions. A comparison between massive stars and the environments of core-collapse supernovae and long duration Gamma Ray Bursts is made, both at low and high redshift. We also address the question of the upper stellar mass limit, since very massive stars (VMS, Minit ≫ 100M⊙) may produce exceptionally bright core-collapse supernovae or pair instability supernovae.
As recent observations have shown, luminous, hydrogen-rich WN5-7h stars (and their somewhat less extreme cousins, O3f/WN6 stars) are the most massive main-sequence stars known. However, not nearly enough very massive stars have been reliably weighed to yield a clear picture of the upper initial-mass function (IMF). We therefore have carried out repeated high-quality spectroscopy of four new O3f/WN6 and WN5-7h binaries in R136 in the LMC with GMOS at Gemini-South, to derive Keplerian orbits for both components, respectively, and thus to directly determine their masses. We also monitored binary candidates and other, previously unsurveyed stars, to increase the number of very massive stars that can be directly weighed.
The Tarantula Survey is an ESO Large Programme which has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. The assembled consortium will exploit these data to address a range of fundamental questions in both stellar and cluster evolution.
Observational and theoretical evidence in support of metallicity-dependent winds for Wolf-Rayet stars is considered. Well-known differences in Wolf-Rayet subtype distributions in the Milky Way, LMC and SMC may be attributed to the sensitivity of subtypes to wind density. Implications for Wolf-Rayet stars at low metallicity include a hardening of ionizing flux distributions, an increased WR population due to reduced optical line fluxes, plus support for the role of single WR stars as gamma-ray burst progenitors.
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).
Our Working Group studies massive, luminous stars, with historical focus on early-type (OB) stars, but extending in recent years to include massive red supergiants that evolve from hot stars. There is also emphasis on the role of massive stars in other branches of astrophysics, particularly regarding starburst galaxies, the first stars, core-collapse gamma-ray bursts, and formation of massive stars.
Less than a few hundred thousand years after the Big Bang, the temperature was high enough that cosmic gas consisted of protons, free electrons and light nuclei. Once the Universe cooled to about 3000 K, the electrons and protons were moving sufficiently slowly that they combined to form hydrogen atoms. With scattering of photons much reduced, they were able to move in straight lines indefinitely, and may be seen redshifted into the microwave part of the spectrum as the 2.7K CMB. So began the era of recombination, or so-called “dark ages” when the IGM became mostly neutral. Within the current cold Dark Matter model for the hierarchical formation of structure, mini-halos of mass ∼106M⊙ (Couchman & Rees 1986) provided the gravitational seeds for the first stars at z ≈ 20–30, ending the “dark ages” through re-ionization of the IGM. A comprehensive review of the astrophysical role of dark matter is provided by Jungman, Kamionkowski, & Griest (1996).
Galaxies formed as baryonic gas cooled in the centers of dark matter structures, from which galaxy mass built up via mergers of halos and proto-galaxies (White & Rees 1978; Davis et al. 1985). Since most present-day galaxies are relatively old, it follows that they formed at z ≥2. The timescale over which galaxies assembled remains unclear, particularly the bulges and disks which are the main components of present-day galaxies.
A detailed discussion of stellar atmospheres is beyond the scope of this book. Nevertheless, our means of studying the properties of hot massive stars relies upon our ability to properly interpret the stellar continuum and line information typically formed in the thin boundary layer between the unseen interior and effectively vacuum interstellar medium. An excellent monograph on the topic of stellar photospheres is provided by Gray (2005), whilst more advanced techniques are introduced by Mihalas (1978).
With respect to normal stars, our interpretation of hot, luminous stars is hindered by two effects. Firstly, the routine assumption of LTE breaks down for high-temperature stars, and particularly for supergiants, due to the intense radiation field, such that the solution of the statistical rate equations (non-LTE) is necessary. Secondly, the simplifying assumption of plane-parallel geometry is no longer valid for blue and red supergiants, so the scale heights of their atmospheres are no longer negligible with respect to their stellar radii. It is the combination of requiring non-LTE plus spherical geometry that has prevented the routine study of OB star atmospheres until recently.
Effective temperatures of early-type stars, essential for subsequent determinations of radii and luminosities, are derived from a comparison between observed photometry and/or spectroscopy and models. Surface gravities also require comparison between observed line profiles and models.
LTE model atmospheres developed by Robert Kurucz during the 1970s and 1980s account very thoroughly for metal line blanketing and are widely employed for both early- and late-type stars.
Massive stars are born in interstellar clouds made up of molecular gas and dusty material. Most of these stars originate from GMCs with typically ∼105M⊙. Upon collapse, these lead to massive star clusters. Some massive stars are born separate from these massive concentrations of gas and dust in smaller clouds and end up in more compact star clusters. Truly isolated massive stars seem to be rare in our Galaxy. Indeed, de Wit et al. (2005) argue that only a few percent of massive stars are born away from clusters. All these massive stars are found highly concentrated towards the Galactic plane where current star formation is still proceeding, albeit at a relatively restrained rate at present.
Consider first an individual massive star. It is formed when gravitational instability overwhelms a cloudlet of gas and dust which then begins a process of collapse. The collapse brings more and more material to the central object in a process of heating and rapid accretion. This phase is very rapid and will only be observed at far-IR wavelengths as the central material aggregates, begins to heat up, and emits radiation or molecular emission at radio wavelengths. Very quickly sufficient material accumulates such that an individual object can be identified. As this material continues to heat from the continuous contraction and accretion of more gas and dust it takes on the characteristic of what is called a “hot core”, radiating also now at mid-IR wavelengths.
Several textbooks discuss the interstellar medium in depth, notably The Physics of the Interstellar Medium (Dyson & Williams 1997) and The Physics and Chemistry of the Interstellar Medium (Tielens 2005). This chapter focuses on aspects relevant to massive stars, namely the properties of interstellar dust, ionized nebulae surrounding individual O stars (giant HII regions are discussed later on), wind blown bubbles and ejecta nebulae around LBVs and W-R stars.
Diffuse gas in the ISM may be in a neutral or ionized form, of which 90% is in the form of hydrogen, either in an atomic, molecular, or ionized state. Cold (∼100 K), atomic hydrogen can be traced via the 21 cm (1420 MHz) hyperfine line, first predicted by van de Hulst (Bakker & van de Hulst 1945) and observed by Ewen & Purcell (1951). This provided the key means of mapping out the structure of the Milky Way and external galaxies. The Lyman series of neutral hydrogen can also be observed in the UV against a suitably hot background source.
Most of the cold (∼10 K) molecular ISM is in the form of H2. This molecule, however, does not emit at radio wavelengths. Since H2 is well correlated with carbon monoxide (CO), the CO emission at 1.3 and 2.6 mm is used as a proxy for H2.
Stellar winds are ubiquitous amongst massive stars, although the physical processes involved depend upon the location of the star within the H-R diagram. Mass-loss crucially affects the evolution and fate of a massive star (Chapter 5), while the momentum and energy expelled contribute to the dynamics and energetics of the ISM (Chapter 8). The interested reader is referred to the monograph by Lamers & Cassinelli (1999) on the topic of stellar winds, or Kudritzki & Puls (2000) for a more detailed discussion of mass-loss from OB and related stars.
The existence of winds in massive stars was first proposed by Beals (1929) to explain the emission line spectra of Wolf–Rayet stars. This gained observational support in the 1960s when the first rocket UV missions revealed the characteristic P Cygni signatures of massloss from CIV λ1550, SilV λ1400, and NV λ1240 in O stars (Morton 1967). A theoretical framework for mass-loss in hot stars was initially developed by Lucy & Solomon (1970) involving radiation pressure from lines, and refined by Castor, Abbott, & Klein (1975), thereafter known as CAK theory. The observational characteristics of stellar winds are velocity and density. The former can be directly observed, whilst the latter relies on a varying complexity of theoretical interpretation.
When a photon is absorbed or scattered by matter, it imparts its energy, hv, and momentum, hv/c, where h is Planck's constant and c is the velocity of light. Consequently, radiation is a very inefficient carrier of momentum.