Karel A. van der Hucht

Quantitative analysis of WC stars: constraints on neon abundances from ISO-SWS spectroscopy

Neon abundances are derived in four Galactic WC stars - gamma(2) Vel (WR 11, WC8+O7.5III), HD 156385 (WR 90, WC7), HD 192103 (WR 135, WC8) and WR 146 (WC5+O8) - using mid-infrared fine-structure lines obtained with ISO-SWS. Stellar parameters for each star are derived using the non-local thermodynamic equilibrium model atmospheric code of Hillier & Miller, together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of gamma(2) Vel, we adopt very recent results from De Marco et al., who followed an identical approach.ISO-SWS data sets reveal the [Ne III] 15.5-mu m line in each of our targets, while [Ne II] 12.8 mu m, [S IV] 10.5 mu m and [S III] 18.7 mu m are observed solely in gamma(2) Vel. Using a method updated from Barlow et al. to account for clumped winds, we derive Ne/He = (3-4) x 10(-3) by number, plus S/He = 6 x 10(-5) for gamma(2) Vel. Neon is highly enriched, such that Ne/S in gamma(2) Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of 2 lower than predictions of current evolutionary models of massive stars. An imprecise mass loss and distance were responsible for the much greater discrepancy in neon content identified by Barlow et al.Our sample of WC5-8 stars span a narrow range in T-* (=55-71 kK), with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann. Stellar luminosities range from 100 000 to 500 000 L., while 10(-5.1) less than or equal to M/(M. yr(-1)) less than or equal to 10(-4.5) adopting clumped winds, in which volume filling factors are 10 per cent. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0.08 less than or equal to C/He less than or equal to 0.25 by number, while oxygen abundances remain poorly constrained.

Recurrent dust formation by WR 48a on a 30‐year time‐scale★

We present infrared photometry of the WC8 Wolf–Rayet system WR 48a observed with telescopes at the European Southern Observatory, the South African Astronomical Observatory and the Anglo Australian Telescope between 1982 and 2011, which show a slow decline in dust emission from the previously reported outburst in 1978–79 until about 1997, when significant dust emission was still evident. This was followed by a slow rise, accelerating to reach and overtake the first (1978) photometry, demonstrating that the outburst observed in 1978–79 was not an isolated event, but that they recur at intervals of 32+ years. This suggests that WR 48a is a long-period dust maker and colliding-wind binary. The locus of WR 48a in the (H − L), K colour–magnitude diagram implies that the rate of dust formation fell between 1979 and about 1997 and then increased steadily until 2011. Superimposed on the long-term variation are secondary (‘mini’) eruptions in (at least) 1990, 1994, 1997, 1999 and 2004, characteristic of relatively brief episodes of additional dust formation. Spectra show evidence for an Oe or Be companion to the WC8 star, supporting the suggestion that WR 48a is a binary system and indicating a system luminosity consistent with the association of WR 48a and the young star clusters Danks 1 and Danks 2. The range of dust formation suggests that these stars are in an elliptical orbit having e ∼ 0.6. The size of the orbit implied by the minimum period, together with the WC wind velocity and likely mass-loss rate, implies that the post-shock WC wind is adiabatic throughout the orbit – at odds with the observed dust formation. A similar conflict is observed in the ‘pinwheel’ dust-maker WR 112.

A Westerbork Synthesis Radio Telescope 1400 and 350 MHz Continuum Survey of the Cygnus OB2 Association, in Search of Hot Massive Stars*

We present a radio continuum survey at 1400 and 350 MHz of a region of 2degrees x 2degrees centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13 and 55. The resulting 5 sigma flux-density limits of, respectively, similar to2 mJy and similar to10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9theta(beam) (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9theta(beam)) still having well-defined peak intensities. The observed spectral index alpha(350)(1400) distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2. We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (alpha greater than or similar to +0.6), or nonthermal emission (-1 less than or similar to alpha less than or similar to +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., similar to2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin. We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly. at or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.

Observational properties of population I Wolf-Rayet stars

The classification, numbers, physical parameters, energy distributions, and ring nebulae of Pop. I Wolf-Rayet (WR) stars are reviewed. The numbers of known WR stars in the Galaxy, in the LMC and in the SMC are 195, 114, and 9, respectively.

The Iron Curtain of the WC 9 Star HD 164270

The emission lines in the optical and UV spectrum of the WC 9 star HD 164270 indicate a terminal velocity of its wind of v∞ ≈ 1400 km s-1. In the UV spectrum Fe III absorption lines appear from transitions with metastable lower levels. Because they are displayed over only 830 km s-1, it is suggested that they are formed in the decelerating part of the wind. A radius of 1.8 x 104 R⊙ is found for the circumstellar Fe III shell. This value is within the range of radii calculated by Cohen et al. (1975) for the dust shells of WC 9 stars.

The ultraviolet spectra of four binaries observed with the S59 spectrometer

Ultraviolet spectra ofo And, α CrB, η Ori A and α Vir, which were obtained with the S59 spectrometer at a resolution of 1.7 Å in three 100 Å wide regions centered at 2110, 2454 and 2825 Å, have been studied for the presence or the absence of the effects due to their binary nature. As may have been anticipated from their orbital and other characteristics, no indication of strong binary interactions were seen in these observations. However, there are certain spectral peculiarities suggesting the possibility of modifications of spectral classifications for some of these objects. A rather unusual spectral behavior in α Vir is also noted. In addition, based primarily on the reviewing of the available literature, we draw new attention to a remarkable property of the third component in η Ori A.

Near-ultraviolet observation of four bright Mn stars

Near-ultraviolet spectra, obtained with the ESRO TD-1A/S59 experiment, of the four Mn stars β Tau, γ Crv, α And, and μ Lep and six normal stars are compared. The normal stars show good agreement with synthetic spectra. The Mn stars have spectra which become hotter at shorter wavelengths. They also show strong Mnii lines and weak Mgii lines. A time-variation for Tiii λ 2828 was noted in γ Crv and α And.

Variable dust formation by the double-line spectroscopic binary WR 70 (HD 137603, WC9vd+B0I)

Infrared photometry of the WC9vd+B0I spectroscopic binary WR 70 over two decades shows variable dust emission. The variations are relatively slow and show evidence for an underlying process with a period P = 1045 ± 60 d, together with modulation on shorter and longer time-scales.

WSRT 21-cm continuum field in Cygnus OB2 : in search of more Wolf-Rayet stars

We present a deep look into the Cygnus OB2 region using the WSRT. A 2degrees x 2degrees map of the optically highly-obscured region was obtained at 1400 MHz, with a noise level down to 0.2 mJy and an angular resolution of 1. We will compare the resulting radio point source list with optical and near-IR catalogues, in order to identify stellar wind sources like WR and OB stars.

Physical Properties of Wolf-Rayet Winds

We review aspects of Population I Wolf-Rayet stars, like classification, inventory, chemical composition, temperature, stellar wind, mass loss rate, ring nebulae, and galactic enrichment in specific isotopes.