Mid-Infrared diagnostics of metal-rich HII regions from VLT and Spitzer Spectroscopy of Young Massive Stars in W31

We present near-IR VLT/ISAAC and mid-IR Spitzer/IRS spectroscopy of the young massive cluster in the W31 star-forming region. H-band spectroscopy provides refined classifications for four cluster members O stars with respect to Blum et al. In addition, photospheric features are detected in the massive Young Stellar Object (mYSO) #26. Spectroscopy permits estimates of stellar temperatures and masses, from which a cluster age of ~0.6 Myr and distance of 3.3 kpc are obtained, in excellent agreement with Blum et al. IRS spectroscopy reveals mid-infrared fine structure line fluxes of [Ne II-III] and [S III-IV] for four O stars and five mYSOs. In common with previous studies, stellar temperatures of individual stars are severely underestimated from the observed ratios of fine-structure lines, despite the use of contemporary stellar atmosphere and photoionization models. We construct empirical temperature calibrations based upon the W31 cluster stars of known spectral type, supplemented by two inner Milky Way ultracompact (UC) HII regions whose ionizing star properties are established. Calibrations involving [NeIII] 15.5um/[NeII] 12.8um, [SIV] 10.5um/[NeII] 12.8um or [ArIII] 9.0um/[NeII] 12.8um have application in deducing the spectral types of early- to mid- O stars for other inner Milky Way compact and UCHII regions. Finally, evolutionary phases and timescales for the massive stellar content in W31 are discussed, due to the presence of numerous young massive stars at different formation phases in a `coeval' cluster.Comment: 16 pages, 13 figures, accepted for MNRA

SwSt 1: an O-rich planetary nebula around a C-rich central star

The hydrogen-deficient [WCL] type central star HD 167362 and its planetary nebula (PN) SwSt 1 are investigated. The central star has a carbon-rich emission-line spectrum, and yet the nebula exhibits a 10-μm emission feature from warm silicate dust, perhaps indicating a recent origin for the carbon-rich stellar spectrum. Its stellar and nebular properties might therefore provide further understanding as to the origin of the [WCL] central star class. The central star optical and UV spectra are modelled with state-of-the art non-LTE codes for expanding atmospheres, from which the stellar parameters are determined. Using the Sobolev approximation code ISA-Wind, we find graphic, graphic, graphic (for a distance of 2.0 kpc), and graphic. The abundance mass fractions for helium, carbon and oxygen are determined to be 37, 51 and 12 per cent, respectively. From this we derive graphic (by mass), confirming that the star suffered efficient third dredge-up. The nitrogen abundance is close to zero, while an upper limit of <10 per cent by mass is established for H. The model uses a composite beta velocity law which allows us to reproduce the optical line profiles. The overall shape of the dereddened spectrum agrees with the V-scaled [graphic, graphic] model atmosphere, showing the nebular-derived reddening to be consistent with the reddening indicated by the stellar analysis. We confirm our model results by using the comoving frame code CMFGEN, although a few differences remain. The PN has a high electron density graphic and a small ionized radius (0.65 arcsec – measured from the HST-WF/PC Hβ images), indicating a young object. Its nebular abundances are not peculiar. The nebular C/O ratio is close to solar, confirming the PN as an O-rich nebula. The nebular N/O ratio of 0.08 is not indicative of a Type-I PN, although the high stellar luminosity points to a relatively high stellar mass. Near-IR spectroscopy is presented and fitted together with IRAS fluxes by using two blackbody curves with temperatures of 1200 and 230 K, indicating the presence of hot dust. We also report the first detection of H2 in this young and compact PN. All of the published spectroscopy since the discovery of SwSt 1 in 1895 has been re-examined, and it is concluded that no clear spectral variability is seen, in contrast to claims in some previously published studies. If an event occurred that has turned it into a hydrogen-deficient central star, it did not happen in the last 100 years

Physical Properties of Wolf-Rayet Stars

The striking broad emission line spectroscopic appearance of Wolf-Rayet (WR) stars has long defied analysis, due to the extreme physical conditions within their line and continuum forming regions. Recently, model atmosphere studies have advanced sufficiently to enable the determination of stellar temperatures, luminosities, abundances, ionizing fluxes and wind properties. The observed distributions of nitrogen (WN) and carbon (WC) sequence WR stars in the Milky Way and in nearby star forming galaxies are discussed; these imply lower limits to progenitor masses of ~25, 40, 75 Msun for hydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars, respectively. WR stars in massive star binaries permit studies of wind-wind interactions and dust formation in WC systems. They also show that WR stars have typical masses of 10-25 Msun, extending up to 80 Msun for H-rich WN stars. Theoretical and observational evidence that WR winds depend on metallicity is presented, with implications for evolutionary models, ionizing fluxes, and the role of WR stars within the context of core-collapse supernovae and long-duration gamma ray bursts.Comment: 76 pages, 8 figures. Minor revision to "Annual Review of Astronomy & Astrophysics" review article Volume 45 (2007) following editors comments. Version with full resolution figures is available from ftp://astro1.shef.ac.uk/pub/pac/AnnRev_revised.pd

Stellar winds from Massive Stars

We review the various techniques through which wind properties of massive stars - O stars, AB supergiants, Luminous Blue Variables (LBVs), Wolf-Rayet (WR) stars and cool supergiants - are derived. The wind momentum-luminosity relation (e.g. Kudritzki et al. 1999) provides a method of predicting mass-loss rates of O stars and blue supergiants which is superior to previous parameterizations. Assuming the theoretical sqrt(Z) metallicity dependence, Magellanic Cloud O star mass-loss rates are typically matched to within a factor of two for various calibrations. Stellar winds from LBVs are typically denser and slower than equivalent B supergiants, with exceptional mass-loss rates during giant eruptions Mdot=10^-3 .. 10^-1 Mo/yr (Drissen et al. 2001). Recent mass-loss rates for Galactic WR stars indicate a downward revision of 2-4 relative to previous calibrations due to clumping (e.g. Schmutz 1997), although evidence for a metallicity dependence remains inconclusive (Crowther 2000). Mass-loss properties of luminous (> 10^5 Lo) yellow and red supergiants from alternative techniques remain highly contradictory. Recent Galactic and LMC results for RSG reveal a large scatter such that typical mass-loss rates lie in the range 10^-6 .. 10^-4 Mo/yr, with a few cases exhibiting 10^-3 Mo/yr.Comment: 16 pages, 2 figures, Review paper to appear in Proc `The influence of binaries on stellar population studies', Brussels, Aug 2000 (D. Vanbeveren ed.), Kluwe

A dusty pinwheel nebula around the massive star WR 104

Wolf-Rayet (WR) stars are luminous massive blue stars thought to be immediate precursors to the supernova terminating their brief lives. The existence of dust shells around such stars has been enigmatic since their discovery some 30 years ago; the intense radiation field from the star should be inimical to dust survival. Although dust-creation models, including those involving interacting stellar winds from a companion star, have been put forward, high-resolution observations are required to understand this phenomena. Here we present resolved images of the dust outflow around Wolf-Rayet WR 104, obtained with novel imaging techniques, revealing detail on scales corresponding to about 40 AU at the star. Our maps show that the dust forms a spatially confined stream following precisely a linear (or Archimedian) spiral trajectory. Images taken at two separate epochs show a clear rotation with a period of 220 +/- 30 days. Taken together, these findings prove that a binary star is responsible for the creation of the circumstellar dust, while the spiral plume makes WR 104 the prototype of a new class of circumstellar nebulae unique to interacting wind systems.Comment: 7 pages, 2 figures, Appearing in Nature (1999 April 08

Supernovae from rotating stars

The present paper discusses the main physical effects produced by stellar rotation on presupernovae, as well as observations which confirm these effects and their consequences for presupernova models. Rotation critically influences the mass of the exploding cores, the mass and chemical composition of the envelopes and the types of supernovae, as well as the properties of the remnants and the chemical yields. In the formation of gamma-ray bursts, rotation and the properties of rotating stars appear as the key factor. In binaries, the interaction between axial rotation and tidal effects often leads to interesting and unexpected results. Rotation plays a key role in shaping the evolution and nucleosynthesis in massive stars with very low metallicities (metallicity below about the Small Magellanic Cloud metallicity down to Population III stars). At solar and higher metallicities, the effects of rotation compete with those of stellar winds. In close binaries, the synchronisation process can lock the star at a high rotation rate despite strong mass loss and thus both effects, rotation and stellar winds, have a strong impact. In conclusion, rotation is a key physical ingredient of the stellar models and of presupernova stages, and the evolution both of single stars and close binaries. Moreover, important effects are expected along the whole cosmic history.Comment: 36 pages, 15 figures, published in Handbook of Supernovae, A.W. Alsabti and P. Murdin (eds), Springe

The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II lambda 1640 in young star clusters

We introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec × 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than mF555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from C IVλλ1548–51, including 16 O2–3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1–a3), two O supergiants (a5–a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung–Russell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5 +0.3−0.7 −0.7+0.3 Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent He II λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100 M⊙. The presence of strong He II λ1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100 M⊙. We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations