Allan J. Willis

Publications (5)8.51 Total impact

• Source

  Available from: arxiv.org

  Article: Quantitative analysis of WC stars: Constraints on neon abundances from ISO/SWS spectroscopy

  Luc Dessart, Paul A. Crowther, D. John Hillier, Allan J. Willis, Patrick W. Morris, Karel A. van der Hucht
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  ABSTRACT: Neon abundances are derived in four Galactic WC stars -- gamma Vel (WR11, WC8+O7.5III), HD156385 (WR90, WC7), HD192103 (WR135, WC8), and WR146 (WC5+O8) - using mid-infrared fine structure lines obtained with ISO/SWS. Stellar parameters for each star are derived using a non-LTE model atmospheric code (Hillier & Miller 1998) together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of gamma Vel, we adopt results from De Marco et al. (2000), who followed an identical approach. ISO/SWS datasets reveal the [NeIII] 15.5um line in each of our targets, while [NeII] 12.8um, [SIV] 10.5um and [SIII] 18.7um are observed solely in gamma Vel. Using a method updated from Barlow et al. (1988) to account for clumped winds, we derive Ne/He=3-4x10^-3 by number, plus S/He=6x10^-5 for gamma Vel. Neon is highly enriched, such that Ne/S in gamma Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of two times 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--71kK), with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann (1995). Stellar luminosities range from 100,000 to 500,000 Lo, while 10^-5.1 < Mdot/(Mo/yr) < 10^-4.5, adopting clumped winds, in which volume filling factors are 10%. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0.08 < C/He < 0.25 by number, while oxygen abundances remain poorly constrained. Comment: 16 pages,7 figures accepted for MNRAS
  01/2000;
• Source

  Available from: ArXiv

  Article: Properties of hot stars in the Wolf‐‐Rayet galaxy NGC 5253 from ISO‐SWS spectroscopy

  Paul A. Crowther, S. C. Beck, Allan J. Willis, Peter S. Conti, Patrick W. Morris, Ralph S. Sutherland
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  ABSTRACT: ABSTRACTISO-SWS spectroscopy of the Wolf--Rayet galaxy NGC 5253 is presented, and analysed to provide estimates of its hot young star population. Our approach differs from previous investigations in that we are able to distinguish between the regions in which different infrared fine-structure lines form, using complementary ground-based observations. The high-excitation nebular [S IV] emission is formed in a very compact region, which we attribute to the central super-star nucleus, and lower excitation [Ne III] nebular emission originates in the galactic core. We use photoionization modelling coupled with the latest theoretical O-star flux distributions to derive effective stellar temperatures and ionization parameters of Teff≥ 38 kK, log Q ∼ 8.25 for the compact nucleus, with Teff∼ 35 kK, log Q ≤ 8 for the larger core. Results are supported by more sophisticated calculations using evolutionary synthesis models. We assess the contribution that Wolf--Rayet stars may make to highly ionized nebular lines (e.g. [O IV]).From our Brα flux, the 2-arcsec nucleus contains the equivalent of approximately 1000 O7 V star equivalents and the starburst there is 2--3 Myr old; the 20-arcsec core contains about 2500 O7 V star equivalents, with a representative age of ∼5 Myr. The Lyman ionizing flux of the nucleus is equivalent to that of the 30 Doradus region. These quantities are in good agreement with the observed mid-infrared dust luminosity of 7.8 × 108 L⊙. Since this structure of hot clusters embedded in cooler emission may be common in dwarf starbursts, observing a galaxy solely with a large aperture may result in confusion. Neglecting the spatial distribution of nebular emission in NGC 5253 implies `global' stellar temperatures (or ages) of 36 kK (4.8 Myr) and 39 kK (2.9 or 4.4 Myr) from the observed [Ne III/ II] and [S IV/ III] line ratios, assuming logQ=8.
  Monthly Notices of the Royal Astronomical Society 04/1999; 304(3):654 - 668. · 4.90 Impact Factor
• Article: The Neon Abundance in WC stars II. ISO/SWS Spectroscopy of HD 156385 (WR90)

  Luc Dessart, Allan J. Willis, Paul A. Crowther, D. John Hillier
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  ABSTRACT: Introduction In general, observationally derived wind compositions of WR stars are in reasonable agreement with predictions from stellar evolution models for massive stars. However, Barlow et al. (1988) identified a major discrepancy for neon in fl Vel (WC8+O) using ground-based observations. The advent of the ESA Infra-red Space Observatory (ISO) has allowed the study of neon in many more WC stars, using mid-IR fine structure lines ([NeII] 12.81m, [NeIII] 15.55m and [NeV]14.32m). Willis et al. (1998) used ISO observations of MR 112 (WR146, WC5+O) to derive a neon abundance that was within the range expected theoretically. Here we undertake a study of HD 156385 (WR90), the only (apparently) single WC7 star in our Galaxy, using ISO/SWS spectroscopy. The only spectroscopic neon feature in the mid-IR of WR90 is [NeIII] 15.55m, in addition to numerous C iv and He ii tran
  01/1999;
• Source

  Available from: psu.edu

  Article: The stellar content of the WR galaxy NGC5253 from ISO/SWS spectroscopy

  Paul A. Crowther, Sara C. Beck, Allan J. Willis, Peter S. Conti, Patrick W. Morris, Ralph S. Sutherland
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  ABSTRACT: previous investigations in that we are able to distinguish between the regions in which different infrared fine-structure lines form, us1 ing complementary ground-based observations from Beck et al. (1996). The high excitation nebular [S iv] emission is formed in the central super-star-nucleus, and lower excitation [Ne ii] nebular emission originates from the clusters located in much larger galactic core. From our Brff flux, the 2 00 nucleus contains the equivalent of approximately 1 000 O7V star equivalents and the starburst there is 2--3 Myr old; the older 20 00 core contains about 2 500 O7V star equivalents, in agreement with the previous determination by Martin & Kennicutt (1995) from Hff. The Lyman ionizing flux of the nucleus is equivalent to the 30 Doradus region. These quantities are in good agreement with the observed mid-IR dust luminosity of 7.8Theta10
  01/1999;
• Article: Observations of the atmospheres and winds of O-stars, LBVs and Wolf-Rayet stars

  Paul A. Crowther, Allan J. Willis
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  ABSTRACT: 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.
  Space Science Reviews 01/1993; 66(1):85-103. · 3.61 Impact Factor