First Detection of Micro-Structures in O Star Winds (The Last Run with the Reticon)
T. Eversberg, S. Lépine and A.F.J. Moffat, Département de Physique, Université de Montréal.
Background and Observations
Blobs are well established in the winds of Wolf-Rayet stars (Moffat & Robert, 1992) and are believed to represent supersonic, compressible turbulence (Henriksen 1994) driven by radiative instabilities (Owocki 1994). In O star winds only indirect evidence has been found so far, e.g., via Einstein X-ray observations (Chlebowski et al. 1989). The apparently brightest O-star in the sky, Puppis, happens also to be Of, i.e. it has a relatively strong wind. It is an obvious first target for search of blobs in the winds of OB-stars, if they should exist. We observed Pup at the f/8.2 Coudé focus during the two nights of 10/11 and 12/13 December 1995. Using the red Coudé train and image slicer, the 1800 l/mm holographic grating and the Reticon 1872 array as a detector, we obtained a S/N 1000/0.03 Å pixel in 10 minutes on a total range of 60 Å centered on HeII 4686 Å. The Reticon 1872 detector has been decomissioned after this run. The data reduction was carried out using IRAF with a Reticon reduction package developed by D. Bohlender and G. Hill, which includes the baseline reduction, flatfielding, heliocentric correction and wavelength calibration with a Thorium-Argon comparison spectrum. The FWHM of the Th-Ar lines covers ~2 pixels and the wavelength shift over the night is negligibly small.
The wind of Puppis shows spectral substructures (Fig. 1, and Eversberg, Lépine, Moffat 1996, henceforth ELM) similar to those seen in the winds of Wolf-Rayet stars, which are the likely descendents of Of stars. These substructures are likely the consequence of excess emission from clumps in the wind. We note the following:
- As the structures accelerate to the blue or red wing of the line, they tend to smear out. Their velocity width is larger when looking along the line-of-sight, both as observed in WR stars.
- The variation profile across the line, (), follows the line profile itself, with some increase in () on the blue side as seen in WR lines (ELM, Fig. 2). This is compatible with the fact that we detect a similar number of blobs on the 4686 line in Pup as on lines of smaller width in WR stars (e.g., CIII 5696 in WR 103: Robert 1992).
- The near central absorption component varies from one night to the next, much like that seen previously for Pup in H (Moffat & Michaud, 1981)(ELM, Fig. 3). Because of the relatively short coverage (~ 5 hours on each of two nights) we are not able to establish any periodicity in the variation of this absorption component, although sinusodial behavior cannot be excluded. In the case of the near-central absorption in H Moffat & Michaud (1981) found a 5.2-day periodicity, which led them to the model of an oblique rotator for Pup.
- Using a standard `` velocity law for Pup with an adopted stellar radius of 18 R* (Kudritzki, Simon & Hamann, 1983) and a terminal velocity of 2250 km/s (Puls et al., 1995), all blobs in HeII 4686 appear near the star's surface and disappear below 2R*. The best `fit' gives = 1.3 (ELM, Fig. 4), although a `' law may not necessarily be strictly valid.The commonly adopted value = 0.8 for OB winds does not appear compatible with our data. As a first approximation we have traced the blobs by straight lines; however, slight non-linear acceleration may also be possible.
We have no reason to expect that Pup is a peculiar O-star, other than its being the apparently brightest O-star in the sky. Thus, it may well be that all OB-star winds show micro-structures as seen in Pup. Of course this must be tested by looking at other bright O stars.
The authors would like to thank Dr. Grant Hill for making available the Reticon Reduction Package. T.E. is grateful for full financial aid from the Ev. Studienwerk/Germany which is supported by the German Government. S.L. acknowledges support from NSERC (Canada) for a doctoral scholarship. A.F.J.M. thanks NSERC and FCAR (Quebec) for financial assistance.
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