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FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER SPECTROSCOPY of the O Vi RESONANCE DOUBLET in SAND 2 (WO) Paul A

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FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER SPECTROSCOPY of the O Vi RESONANCE DOUBLET in SAND 2 (WO) Paul A The Astrophysical Journal, 538:L51±L55, 2000 July 20 q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER SPECTROSCOPY OF THE O vi RESONANCE DOUBLET IN SAND 2 (WO) Paul A. Crowther,1 A. W. Fullerton,2,3 D. J. Hillier,4 K. Brownsberger,5 L. Dessart,1,6 A. J. Willis,1 O. De Marco,1 M. J. Barlow,1 J. B. Hutchings,7 D. L. Massa,8 D. C. Morton,7 and G. Sonneborn9 Received 2000 March 17; accepted 2000 June 8; published 2000 July 14 ABSTRACT We present Far Ultraviolet Spectroscopic Explorer spectroscopy of Sand 2, an LMC WO-type Wolf-Rayet star, revealing the O vi resonance P Cygni doublet at 1032±1038 AÊ . These data are combined with Hubble Space Telescope Faint Object Spectrograph ultraviolet and Mount Stromlo 2.3 m optical spectroscopy and analyzed using a spherical, non-LTE, line-blanketed code. Our study reveals exceptional stellar parameters: T¤ » 21 Ç 25 21 150,000 K,v` = 4100 km s , log (L/L,,) = 5.3 , andM=1 # 10 M yr , if we adopt a volume ®lling factor » 5 » 10.10 of 10%. Elemental abundances ofC/He 0.7 0.2 and O/He 0.1520.05 by number qualitatively support pre- vious recombination line studies. We con®rm that Sand 2 is more chemically enriched in carbon than LMC WC stars and that it is expected to undergo a supernova explosion within the next5 # 10 4 yr. Subject headings: stars: evolution Ð stars: individual (Sand 2) Ð stars: Wolf-Rayet 1. INTRODUCTION (Sanduleak 1971), Sk 2687145 = Brey 93 = BAT 992123 (Breysacher, Azzopardi, & Testor 1999), together with Hubble Wolf-Rayet (WR) stars provide keys to our understanding Space Telescope (HST) and ground-based data sets. of massive stellar evolution, nucleosynthesis processes, and chemical enrichment of the interstellar medium (ISM). Among 2. OBSERVATIONS WR stars, the oxygen sequence (WO) introduced by Barlow & Hummer (1982) is by far the rarest. Their high-excitation Previously unpublished far-UV, UV, and optical/near-IR spec- oxygen emission lines are widely interpreted as revealing the troscopy of Sand 2 has been obtained with FUSE, HST, and the late core helium-burning or possibly carbon-burning stage Mount Stromlo and Siding Spring Observatory (MSSSO) 2.3 m (Smith & Maeder 1991), which is important for constraining telescope, respectively. the controversial 12C(a, g)16O reaction rate. In spite of searches in all Local Group galaxies, only six 2.1. Far-UV Spectroscopy massive WO stars are known to date, namely Sand 1 (Sk 188) Sand 2 was observed by FUSE as part of the Early Release 2 in the SMC, Sand 2 (BAT 99 123) in the LMC, Sand 4 Observation program X018 on 1999 October 31. A 8134 s (WR 102), Sand 5 (WR 142), and MS4 (WR 30a) in our Galaxy exposure of Sand 2 with the30 00# 30 00 large aperture (LWRS) and DR1 in IC 1613. Since O vi ll3811±3834 is a primary provided data atR » 12,000 , with the two lithium ¯uoride (LiF) WO classi®cation diagnostic, with an equivalent width of up to channels, covering 979±1187 AÊ , having been obtained in time- Ê 1700 A (Kingsburgh, Barlow, & Storey 1995, hereafter KBS), tag (TTAG) mode. At this epoch, the two silicon carbide (SiC) observations of O vi ll1032±1038 are keenly sought. However, channels, covering 905±1104 AÊ , were badly aligned so that its location in the far-UV has to date ruled out such observations. SiC data were of poor quality. This situation has changed following the successful launch of Sand 2 data were processed through the pipeline data reduc- the Far Ultraviolet Spectroscopic Explorer (FUSE; Moos et al. tion, CALFUSE (version 1.6.8), and are shown in Figure 1. The 2000), which permits routine high-dispersion far-UV spectros- pipeline extracted the one-dimensional spectrum, removed the copy of massive stars in the Magellanic Clouds. background, and corrected for grating wobble and detector drifts. In this Letter, we analyze FUSE spectroscopy of Sand 2 No corrections for astigmatism or ¯at-®elding have been applied. From Figure 1, the far-UV continuum ¯ux of Sand 2 is low 1 Department of Physics and Astronomy, University College London, Gower (¼5 # 10214 ergs s21 cm22 AÊ 21), with two principal stellar fea- Street, London, WC1E 6BT, England, UK. 2 Department of Physics and Astronomy, University of Victoria, P.O. Box tures, O vi ll1032±1038 and C iv l1168. The latter is blended 3055, Victoria, BC V8W 3P6, Canada. with C iii l1175, while C iv l1108 and O vi l1125 are present, 3 Department of Physics and Astronomy, Johns Hopkins University, 3400 though weak. The FUSE spectrum is affected by a multitude of North Charles Street, Baltimore, MD 21218. interstellar absorption features, principally H i and H , plus air- 4 2 Department of Physics and Astronomy, University of Pittsburgh, 3941 glow emission features due to Ly ,[Oi], and [N i]. O'Hara Street, Pittsburgh, PA 15260. b 5 Center for Astrophysics and Space Astronomy, University of Colorado, Campus Box 389, Boulder, CO 80309. 2.2. Near-UV Spectroscopy 6 Present address: DeÂpartement de Physique, Universite Laval and Obser- vatoire du Mont MeÂgantic, QueÂbec, QC G1K 7P4, Canada. Sand 2 was observed with the HST Faint Object Spectrograph 7 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, (FOS) instrument during 1996 March (PI: D. J. Hillier, Program National Research Council of Canada, 5071 West Saanich Road, Victoria, BC ID 5460). Exposures totaling 5980, 3780, and 1090 s were ob- V8X 4M6, Canada. tained with the G130H, G190H, and G270H gratings, respec- 8 Raytheon Information Technology and Scienti®c Services, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771. tively. Many important spectral features are identi®ed in the 9 Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight HST/FOS data set (see also KBS), principally C iv l1550, C iii Center, Code 681, Greenbelt, MD 20771. l2297, He ii l1640, O iv l1340, O v l1371, and ll2781±2787. L51 L52 EVOLUTIONARY STATUS OF SAND 2 Vol. 538 line blanketing, both of which have a signi®cant effect on the physical properties of WC stars (e.g., Hillier & Miller 1999). Through the use of ªsuperlevels,º extremely complex atoms can be included. For the present application, a total of 3552 levels (combined into 796 superlevels), 60 depth points, and 63,622 spectral lines of He i±He ii,Cii±iv,Oii±vi,Neii± Ne iv,Siiv,Siv±Si vi,Ariii±Ar v, and Fe iv±Fe viii are considered simultaneously (see Dessart et al. 2000 for the source of atomic data used). We adopt a form for the velocity law (eq. [8] from Hillier & Miller 1999) such that two exponents are considered (b1 = 21 21 1,b2 =50 ,vext= 2900 km s ,kmsv turb = 100 ), with the result that acceleration is modest at small radii but continues to large distance (0.9v` is reached at 100R ). * Our spectroscopic analysis derivesMÇÇ/ÎfM , rather than and f individually, since line blending is severe in Sand 2. A series of models were calculated in which stellar parameters T , Ç Î * log (L/L,),M/ f , and C/He and O/He were adjusted until the observed line strengths and spectroscopic ¯uxes were repro- Fig. 1.ÐObserved FUSE (CALFUSE 1.6.8) spectroscopy of Sand 2, re- binned to 15 km s21, revealing the P Cygni O vi resonance doublet at duced. We adopt 0.4 Z, abundances for Ne, Si, S, Ar, and Fe. 1032±1038 AÊ . The far-UV spectrum is affected by interstellar atomic and The distance to the LMC was assumed to be 51.2 kpc (Panagia molecular hydrogen lines, plus airglow from [O i], [N i], and Lyb. et al. 1991). The wind ionization balance is ideally deduced using isolated Following Prinja, Barlow, & Howarth (1990), we ®nd that spectral lines from adjacent ionization stages of carbon (C iii 21 v` = 4100 km s from C iv ll1548±1551 (KBS derived 4500 l2297 and C iv ll5801±5812) or oxygen (O iv l3404±3414, km s21 from IUE spectroscopy). O v l3144, and O vi l5290). In practice this was dif®cult to achieve for Sand 2 because of the severe line blending. 2.3. Optical Spectroscopy We have used the double-beam spectrograph (DBS) at the 3.2. Stellar Parameters 2.3 m MSSSO telescope to observe Sand 2 on 1997 December Our initial parameter study of Sand 2 revealed a fairly similar 24±27. Use of a dichroic and the 300B, 600B, and 316R grat- spectral appearance spanning120,000 K · T¤ · 170,000 K , ings permitted simultaneous spectroscopy covering 3620±6085 Ç 21 withlog (L/L,,) = 5.28 and log (M/M yr ) = 24.94 ®xed. Ê and 6410±8770 A for 1200 s, plus 3240±4480 and 8640±11010 The principal differences between these models are that (1) the Ê 0 # A for 2500 s. A 2 slit and the1752 532 pixel SITe CCDs observed strength of the doublet O vi ll3811±3834, plus lines provided a 2 pixel spectral resolution of »5AÊ . A standard data in the red such as C iv l7700, favor a highT¤ and (2) the reduction was carried out, including absolute ¯ux calibration, weakness of the doublet O vi ll1032±1038, as revealed by using wide-slit spectrophotometry of HD 60753 (B3 IV) and FUSE, favors a lowerT¤ . Since other parameters, in particular m Col (O9 V), plus atmospheric correction using HR 2221 abundances, are largely unaffected by these discrepancies, we (B8 V).
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