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Kinematical Structure of Wolf-Rayet Winds. II. Internal Velocity Scatter in WN Stars

Niedzielski A., Nugis T., Skórzyński W.

Acta Astronomica

2004

Vol. 54, No. 4

405--427

The shortward edge of the absorption core velocities - vblack as determined from low resolution archived IUE spectra from the INES database are presented for three P Cyg profiles of NV 1240, HeII 1640 and NIV 1720 for 51 Galactic and 64 LMC Wolf-Rayet stars of the WN subtype. These data, together with vblack of CIV 1550 line presented in Niedzielski and Skórzyński (2002) are discussed. Evidences are presented that vblack of CIV 1550 rarely displays the largest wind velocity among the four lines studied in detail and therefore its application as an estimator of the terminal wind velocity in WN stars is questioned. An average vblack of several lines is suggested instead but it is pointed out that vblack of HeII 1640 usually reveals the highest observable wind velocity in Galactic and LMC WN stars. It is shown that the stratification strength decreases from WNL to WNE stars and that for WNL stars there exists a positive relation between vblack and the Ionization Potential. The velocity scatter between vblack obtained from different UV lines is found to correlate well with the X-ray luminosity of single WN stars (correlation coefficient R=0.82 for the data obtained from the high resolution IUE spectra) and therefore two clumpy wind models of single WN stars are presented that allow the velocity scatter to persist up to very large distances from the stellar surface (r≈500-1000R*). These models are used to explain the specific features of single WN stars like broad absorption troughs of strong lines having different vblack, X-ray fluxes, IR/radio continua and stratification relations.

Kinematical Structure of Wolf-Rayet Winds. I. Terminal Wind Velocity

Niedzielski A., Skórzyński W.

Acta Astronomica

2002

Vol. 52, No. 1

81--104

New terminal wind velocities for 164 Wolf-Rayet stars (from the Galaxy and LMC) based on P Cyg profiles of λ1550 CIV resonance line were derived from the archive high and low resolution IUE spectra available form the INES database. The high resolution data on 59 WR stars (39 from the Galaxy and 20 from LMC) were used to calibrate the empirical relation (λminAbs-λpeakEmis) vs. terminal wind velocity, which was then used for determinations of the terminal wind velocities from the low resolution IUE data. We almost doubled the previous most extended sample of such measurements. Our new measurements, based on high resolution data, are precise within 5-7%. Measurements, based on the low resolution spectra have the formal errors of ≈40-60%. A comparison of the present results with other determinations suggests higher precision of ≈20%. We found that the terminal wind velocities for the Galactic WC and WN stars correlate with the WR spectral subtype. We also found that the LMC WN stars have winds slower than their Galactic counterparts, up to two times in the case of the WNE stars. No influence of binarity on terminal wind velocities was found. Our extended set of measurements allowed us to test application of the radiation driven wind theory to the WR stars. We found that, contrary to OB stars, terminal wind velocities of the WR stars correlate only weakly with stellar temperature. We also note that the terminal to escape velocity ratio for the WR stars is relatively low: 2.55±1.14 for the Galactic WN stars and 1.78±0.70 for the Galactic WCs. This ratio decreases with temperature of WR stars, contrary to what is observed in the case of OB stars. The presented results show complex influence of chemical composition on the WR winds driving mechanism efficiency. Our kinematical data on WR winds suggest evolutionary sequence: WNL→WNE→WCE→WCL.