The fundamental parameters of the central stars of eight WR ring nebulae

We describe work that has recently been completed on deriving the fundamental parameters of eight WR stars through the photoionization modelling of their surrounding nebulae using non-LTE WR flux distributions. The resulting effective temperatures range from 57 000–71 000 K for the WN4-5 stars and <30 000–42 000 K for the WN6-8 stars. The derived stellar parameters are compared with those obtained from stellar emission line modelling. We find good agreement for the hot early WN stars, indicating that the non-LTE WR flux distributions have essentially the correct shape in the crucial far-UV region. We find lower temperatures for the four cooler late WN stars, particularly for the two WN6 stars. For the nebulae surrounding these stars, we find that the model flux distributions produce too much nebular ionization. We suggest that these discrepancies arise because of the lack of line-blanketing in the WR atmospheres. For the WO1 central star of G2.4+1.4, with strong nebular He II 4686 A emission, we derive a temperature of 105 000 K, somewhat less than previous estimates. The positions of our eight WR stars on the H-R diagram are compared with the evolutionary tracks of Maeder (1990) for solar metallicity. In common with previous workers, we find that our derived luminosities are too low, giving an initial mass range of 25–40 M, below that expected for the majority of WR stars.     

Improved bolometric corrections for WR stars

Evolutionary models allow an assignment of both a mass and a luminosity to a Wolf-Rayet (WR) star in a cluster, and hence allow a determination of the Bolometric Correction (B.C.). The B.C.'s derived for WN stars range from –4.0 to –6.0 with the expected trend of larger values (in absolute values) for stars with higher excitation spectra. For WC stars, there is little evidence for a similar trend; most observations presented here are consistent with B.C.=–4.5, as found by Smith and Maeder (1989). The convergence of B.C. values derived from evolutionary and atmospheric models is extremely satisfactory, giving increased confidence in both methods.     

Tailored analyses of 24 Galactic WN stars

The fundamental properties of 24 Galactic WN stars are determined from analyses of their optical, UV and IR spectra using sophisticated model atmosphere codes (Hillier, 1987, 1990). Terminal velocities, stellar luminosities, temperatures, mass loss rates and abundances of hydrogen, helium, carbon, nitrogen and oxygen are determined. Stellar parameters are derived using diagnostic lines and interstellar reddenings found from fitting theoretical continua to observed energy distributions.Our results confirm that the parameters of WN stars span a large range in temperature (T_*=30–90,000 K), luminosity (log L_*/L=4.8–5.9), mass loss (M=0.9–12×10^–5 M yr^–1) and terminal velocity (v =630–3300 km s^–1). Hydrogen abundances are determined, and found to be low in WNEw and WNEs stars (<15% by mass) and considerable in most WNL stars (1–50%). Metal abundances are also determined with the nitrogen content found to lie in the range N/He=1–5×10^–3 (by number) for all subtypes, and C/N 0.02 in broad agreement with the predictions of Maeder (1991). Enhanced O/N and O/C is found for HD 104994 (WN3p) suggesting a peculiar evolutionary history. Our results suggest that single WNL+abs stars may represent an evolutionary stage immediately after the Of phase. Since some WNE stars exist with non-negligible hydrogen contents (e.g. WR136) evolution may proceed directly from WNL+abs to WNE in some cases, circumventing the luminous blue variable (LBV) or red supergiant (RSG) stage.     

Mixing in Stars and the Evolution of the 3He Abundance

We first recall the observational and theoretical facts that constitute the so-called 3He problem. We then review the chemical anomalies that could be related to the destruction of 3He in red giants stars. We show how a simple consistent mechanism can lead to the destruction of 3He in low mass stars and simultaneously account for the low 12C/13C ratios and low lithium abundances observed in giant stars of different populations. This process should both naturally account for the recent measurements of 3He/H in galactic HII regions and allow for high values of 3He observed in some planetary nebulae. We propose a simple statistical estimation of the fraction of stars that may be affected by this process.     

OB Associations, Wolf–Rayet Stars, and the Origin of Galactic Cosmic Rays

We have measured the isotopic abundances of neon and a number of other species in the galactic cosmic rays (GCRs) using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the ACE spacecraft. Our data are compared to recent results from two-component (Wolf–Rayet material plus solar-like mixtures) Wolf–Rayet (WR) models. The three largest deviations of galactic cosmic ray isotope ratios from solar-system ratios predicted by these models, ¹²C/¹⁶O, ²²Ne/²⁰Ne, and ⁵⁸Fe/⁵⁶Fe, are very close to those observed. All of the isotopic ratios that we have measured are consistent with a GCR source consisting of ∼20% of WR material mixed with ∼80% material with solar-system composition. Since WR stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of our data with WR models suggests that OB associations within superbubbles are the likely source of at least a substantial fraction of GCRs. In previous work it has been shown that the primary ⁵⁹Ni (which decays only by electron-capture) in GCRs has decayed, indicating a time interval between nucleosynthesis and acceleration of >10⁵ y. It has been suggested that in the OB association environment, ejecta from supernovae might be accelerated by the high velocity WR winds on a time scale that is short compared to the half-life of ⁵⁹Ni. Thus the ⁵⁹Ni might not have time to decay and this would cast doubt upon the OB association origin of cosmic rays. In this paper we suggest a scenario that should allow much of the ⁵⁹Ni to decay in the OB association environment and conclude that the hypothesis of the OB association origin of cosmic rays appears to be viable.     

The Milky Way 3-Helium Abundance

We are making precise determinations of the abundance of the light isotope of helium, ³He. The ³He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of ³He is derived from measurements of the hyperfine transition of ³He⁺ which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar ³He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the ³He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass (M≤ 2M _⊙) to intermediate-mass (M∼2–5M _⊙) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths, our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature, excitation, etc.). The ³He abundances we derive have led to what has been called “The ³He Problem”.     

Helium-3: Status and Prospects

We report on our continuing efforts to determine 3He abundances in H II regions and planetary nebulae. Our detections of 3He in some PNe show that some stars produce large amounts of 3He. However the H II region abundances show no evidence for this production. From our sample of > 40 H II regions, the subsample which should yield the most reliable abundances has 3He/H abundances which scatter between 1-2 × 10-5. There is no trend with either galactocentric distance or metallicity. Even if we do not understand the underlying mechanisms, we see empirically that stars neither produce nor destroy 3He in a major way. We thus suggest that the level of the "3He Plateau" (3He/H = 1.5 _-0.5 ^+1.0 × 10^-5) is a reasonable estimate for the primordial 3He.     

Measurements of the 12C/13C Ratio in Planetary Nebulae and Implications for Stellar Evolution

We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebulae (PNe) by means of mm-wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition; the other group consists of 23 nebulae with rich molecular envelopes. We have determined the isotopic ratio in 14 objects and the results indicate a range of values between 9 and 23. In particular, three PNe have ratios well below the value predicted by standard evolutionary models ( 20), indicating that some extra-mixing process has occurred in these stars. We briefly discuss the implications of our results for standard and nonstandard stellar nucleosynthesis.     

Hydrodynamic atmosphere models for hot luminous stars II. Method and improvements over unified models

In a paper submitted to A&A we present the first line blanketed hydrodynamic models of spherically expanding atmospheres of hot stars. This paper is complementary to the submitted paper. Here, we emphasize the advantages and the weak points of our approach and we present additional technical aspects.The models are characterised by a simultaneous solution of the equation of motion, the non-LTE populations of H and He, and radiation transfer in a line blanketed atmosphere. The entire domain from the optically thick photosphere out to the terminal velocity of the wind is treated. The radiative forces are evaluated consistently with the depth-dependent radiation field, taking into account multiple scattering by metal lines and line overlap.     

Observations of the atmospheres and winds of O-stars,LBVs and Wolf-Rayet stars

This review summarises recent studies of O-stars, Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars, emphasising observations and analyses of their atmospheres and stellar winds yielding determinations of their physical and chemical properties. Studies of these stellar groups provide important tests of both stellar wind theory and stellar evolution models incorporating mass-loss effects. Quantitative analyses of O-star spectra reveal enhanced helium abundances in Of and many luminous O-supergiants, together with CNO anomalies in OBN and Ofpe/WN9 stars, indicative of evolved objects. Enhanced helium, and CNO-cycle products are observed in several LBVs, implying a highly evolved status, whilst for the WR stars there is strong evidence for the exposition of CNO-cycle products in WN stars, and helium-burning products in WC and WO stars. The observed wind properties and mass-loss rates derived for O-stars show, in general terms, good agreement with predictions from the latest radiation-driven wind models, although some discrepancies are apparent. Several LBVs show similar mass-loss rates at maximum and minimum states, contrary to previous expectations, with the mechanism responsible for the variability and outbursts remaining unclear. WR stars exhibit the most extreme levels of mass-loss and stellar wind momenta. Whilst alternative mass-loss mechanisms have been proposed, recent calculations indicate that radiation pressure alone may be sufficient, given the strong ionization stratification present in their winds.     

Lithium Abundance in Pop.II Stars A post-HIPPARCOS discussion

The relation between the lithium abundance observed in Population II stars and the primordial abundance, is still an open question (see Cayrel and Duncan, this meeting). A few recent results are discussed. HIPPARCOS data show that the standard model of stellar evolution can explain the 6Li detection in HD 84937, suggesting a negligible depletion of 7Li. A slope in the Li/Teff relation for Pop II dwarfs and a spread of their Li abundance have been advocated, and both used as arguments in favor of Li depletion. The slope is not confirmed when two other independent temperature scales are used. The Li scatter around the plateau is hardly larger than the scatter predicted from determination errors. Hints from a scatter of Li in subgiants of the globular cluster M92 are not completely conclusive. The determination of more accurate Li abundances in the Pop II stars is an urgent but difficult task, requiring better model atmosphere (better convection treatment) and the help of observational data about Pop II stars (such as long base interferometry).     

The evolution of massive stars to explosion

We review the possible evolutionary paths from massive stars to explosive endpoints as various types of supernovae associated with Population I and hence with massive stars: Type II-P, Type II-L, Type Ib, Type Ic, and the hybrid events SN 1987K and SN 1993J. We identify SN 1954A as another hybrid event from the evidence for both H and He in its spectrum with velocities nearly the same as SN 1983J. Evidence for ejected⁵⁶Ni mass of 0.07 M suggests that SN II-P underwent standard iron core collapse, not collapse of an O–Ne–Mg core nor thermonuclear explosion of a C–O core. Most SN II-P presumably arise in single stars or wide binaries of 10–20 M. There may be indirect evidence for duplicity in some cases in the form of strong Ba II lines, such as characterized SN 1987A. SN II-L are recognizably distinct from typical SN II-P and must undergo a significantly different evolution. Despite indications that SN II-L have small envelopes that may be helium enriched, they are also distinct from events like SN 1993J that must have yet again a different evolution. The SN II-L that share a common Luminosity seem to have ejected a small nickel mass and hence may come from stars with O–Ne–Mg cores. The amount of nickel ejected by the exceptionally bright events, SN 1980K and SN 1979C, remains controversial. SN Ib require the complete loss of the H envelope, either to a binary companion or to a wind. The few identified have relatively large ejecta masses. It is not clear what evolutionary processes distinguish SN Ib's evolving in binary systems from hybrid events that retain some H in the envelope. SN Ic events are both H and He deficient. Binary models that can account for transfer of an extended helium envelope from low mass helium cores, 2 to 4 M, imply C–O core masses that are roughly consistent with that deduced from the ejecta mass plus a neutron star, 2 to 3 M. It is possible that the hybrid events are the result of Roche lobe overflow and that the pure events, SN Ib or SN Ic, result from common envelope evolution.     

Galactic Evolution of D and 3He

The most recent chemical evolution models for D and 3He are reviewed and their results compared with the available data.Models in agreement with the major galactic observational constraints predict deuterium depletion from the Big Bang to the present epoch smaller than a factor of 3 and therefore do not allow for D/H primordial abundances larger than 5 × 10-5. Models predicting higher D consumption do not seem to be able to reproduce other observed features of our galaxy (e.g. SFR, abundances, abundance ratios and/or gradients of heavier elements, metallicity distribution of G-dwarfs).Observational and theoretical 3He abundances can be reconciled with each other if the majority of low mass stars experience in the red giant phase a deep mixing allowing the consumption of most of the 3He produced during core-hydrogen burning.     

Stellar flares

Radio and X-ray observations of stellar flares provide the most direct probes of energy relaase particle acceleration, and energy transport on stars other than the Sun. In this review, the observational basis for our understanding of the flare phenomenon on other stars is briefly described and outstanding interpretive and theoretical issues are discussed. I shall confine my attention to objects which are solar-like, to the extent that they possess deep convective envelopes and display activity which is presumed to be magnetic in origin. These include pre-main sequence objects, classical flare stars, and close binaries. Future directions are briefly discussed.     

Late Stages of Stellar Evolution

A large fraction of ISO observing time was used to study the late stages of stellar evolution. Many molecular and solid state features, including crystalline silicates and the rotational lines of water vapour, were detected for the first time in the spectra of (post-)Asymptotic Giant Branch (AGB) stars. Their analysis has greatly improved our knowledge of stellar atmospheres and circumstellar environments. A surprising number of objects, particularly young planetary nebulae with Wolf-Rayet (WR) central stars, were found to exhibit emission features in their ISO spectra that are characteristic of both oxygen-rich and carbon-rich dust species, while far-IR observations of the PDR around NGC 7027 led to the first detections of the rotational line spectra of CH and CH⁺. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

Isotopic Composition of H,HE and NE in the Protosolar Cloud

Observations and measurements in the solar wind, the Jovian atmosphere and the gases trapped in lunar surface material provide the main evidence from which the isotopic composition of H, He and Ne in the Protosolar Cloud (PSC) is derived. These measurements and observations are reviewed and the corrections are discussed that are needed for obtaining from them the PSC isotopic ratios. The D/H, ³He/⁴He (D+³He)/H, ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios adopted for the PSC are presented. Protosolar abundances provide the basis for the interpretation of isotopic ratios measured in the various solar system objects. In this article we discuss constraints derived from the PSC abundances on solar mixing, the origin of atmospheric neon, and the nature of the “SEP” component of neon trapped at the lunar surface. We also discuss constraints on the galactic evolution provided by the isotopic abundances of H and He in the PSC. This revised version was published online in August 2006 with corrections to the Cover Date.     

Abundances of Light Elements

This paper briefly reviews a few relevant features about the abundances of light elements (D, ⁴He, ⁶Li, ⁷Li, ⁹Be) in the Milky Way. It places special emphasis on metal-poor stars. Observational concerns are discussed. The use of ⁷Li and ⁶Li as cosmological probes and of ⁹Be as a chronometer for the early evolution of our Galaxy are discussed.     

Nucleosynthesis in massive stars

We discuss three aspects of the nucleosynthesis in massive and intermediate-mass stars during their early evolutionary phases. These are related to the CNO abundances in giant or supergiant stars, to the²⁶Al yield from massive stars via stellar wind, and to the production of the s-process nuclei in massive stars.     

O5+ in High Speed Solar Wind Streams: SWICS/Ulysses Results

Recent observations with UVCS on SOHO of high outflow velocities of O⁵⁺ at low coronal heights have spurred much discussion about the dynamics of solar wind acceleration. On the other hand, O⁶⁺ is the most abundant oxygen charge state in the solar wind, but is not observed by UVCS or by SUMER because this helium-like ion has no emission lines falling in the wave lengths observable by these instruments. Therefore, there is considerable interest in observing O⁵⁺ in situ in order to understand the relative importance of O⁵⁺ with respect to the much more abundant O⁶⁺. High speed streams are the prime candidates for the search for O⁵⁺ because all elements exhibit lower freezing-in temperatures in high speed streams than in the slow solar wind. The Ulysses spacecraft was exposed to long time periods of high speed streams during its passage over the polar regions of the Sun. The Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses is capable of resolving this rare oxygen charge state. We present the first measurement of O⁵⁺ in the solar wind and compare these data with those of the more abundant oxygen species O⁶⁺ and O⁷⁺. We find that our observations of the oxygen charge states can be fitted with a single coronal electron temperature in the range of 1.0 to 1.2 MK assuming collisional ionization/recombination equilibrium with an ambient Maxwellian electron gas. This revised version was published online in June 2006 with corrections to the Cover Date.     

Wolf-Rayet stars in starbursts

Let us suppose that it is possible observationally to determine the number ratio of WR to O stars in a starburst galaxy (cf. e. g. Vacca &; Conti 1992) and that one can also have some information on the way the different WR subtypes are distributed (number ratios as WN/WR, WNL/WR etc ...), the question is, what can we deduce from these values on the burst of star formation which gave birth to these WR stars? Is it possible for instance to constrain the age of the burst (i.e. the time elapsed since the beginning of the burst of star formation), its intensity (i.e. the ratio of the star formation rate during the burst to that before the burst) or the metallicity of the cloud from which the stars formed? We present here models of starbursts based on the most recent models for single stars computed by the Geneva group and show that the study of the WR population in a starburst provides very useful insights on the age of the burst and on the metallicity of the star forming zone.