Kinematical Structure of Wolf-Rayet Winds. I. Terminal Wind Velocity. A. N i e d z i e l s k i and W. S k ó r z y ń s k i

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1 ACTA ASTRONOMICA Vol. 52 (2002) pp Kinematical Structure of Wolf-Rayet Winds. I. Terminal Wind Velocity by A. N i e d z i e l s k i and W. S k ó r z y ń s k i Toruń Centre for Astronomy, N. Copernicus University, ul. Gagarina 11, Toruń, Poland (aniedzi,red)@astri.uni.torun.pl Received October 22, 2001 ABSTRACT 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 λ Abs min λemis peak µ 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: for the Galactic WN stars and 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. Key words: Stars: Wolf-Rayet Stars: winds, outflows Ultraviolet: stars 1. Introduction Wolf-Rayet stars are luminous objects with characteristic spectra which make them easy to detect even in distant galaxies. They are composed mainly of strong Based on INES data from the IUE satellite.

2 82 A. A. and broad emission lines of helium, carbon and nitrogen in various ionization stages. No trace of hydrogen is usually observed. Wolf-Rayet (WR) stars with strong nitrogen lines are called WN, while those with spectra rich in carbon lines WC. The strange elemental abundance of the atmospheres of WR stars, suggested by their spectra, is supported by more detailed studies (see for example Nugis and Niedzielski 1995). Their distribution in the Galaxy as well as identification of their binary companions allows to connect them to the OB stars. The analysis of known binary systems containing WR components shows that WR stars are very massive, with masses of M (van der Hucht 2001a). WN stars appear to be more massive, with average masses of M, while WC stars average mass is 12 3 M. This difference is in qualitative agreement with present models of the massive star evolution which leads to the WR phase. According to them WN stars are evolutionally less advanced. The masses of WR stars decrease during their evolution due to extreme mass loss: Ṁ M /year. The mass-loss is currently recognized as the most important factor influencing massive star evolution. However, in the case of WR stars, determination of its rate is not simple. To obtain correct result one has to bear in mind the wind s nature and geometry, its terminal velocity (v ) and chemical composition. Depending on the method adopted for mass-loss determinations the obtained results are proportional either tov or tov 2. Intensities of mass-loss rate in the case of WR stars give rise to the so called momentum problem: Ṁv ηl c, the mechanical momentum carried by the wind is η times larger than the radiative momentum produced by the star. In the case of WR stars η may be of the order of However, the wind momentum, as we determine it, is proportional to the second or third power of the terminal wind velocity again. Terminal wind velocity appears to be one of the most important parameters required in quantitative analyses of WR stars. It is also one of very few quantitative parameters of WR stars that can be accurately measured with virtually no modeldependent assumptions. We decided therefore to determine them in a uniform way for as many WR stars as possible. Such measurements can be confronted with predictions of the line driven wind theory. 2. Terminal Wind Velocity Determination Terminal wind velocity is defined as the velocity that the wind (the outflowing matter) reaches at large distance from the central star, where it is not accelerated anymore by the wind driving force but its deceleration due o interaction with the ISM is negligible. There are several methods (proposed by different authors) to determine terminal wind velocities in hot stars. It can be obtained by measuring the width of the widest spectral line in the optical spectrum (Torres et al. 1986, Hamann et al. 1998, 2000). Also an analysis of profiles of the spectral lines, present in the IR spectral range,

3 Vol which develop in the outmost parts of the wind may lead to correct estimates of terminal wind velocity (Eenens and Williams 1994). Recently Ignace et al. (2001) used the IR forbidden lines of [CaIV] at µm and [NeIII] at µm, seen in the spectra obtained by the Infrared Space Observatory (ISO), to determine realistic wind velocity estimates. v may also be obtained through the analysis of the Narrow Absorption Components (NAC) that appear in O stars winds (Howarth and Prinja 1989). Most frequently, however, P Cyg profiles of UV resonance lines are used to obtain terminal wind velocities by measuring thev black lowest wavelength of the flat P Cyg absorption component (see Fig. 1). This method is the most precise and gives results accurate to 10%. All working definitions lead to slightly different values of the terminal wind velocity and lead to their own technical difficulties. A proper interpretation of the wind velocity determined from NACs, for example, requires a long period of observations. The optical line widths vary with ionization potential (Kuhi 1973) and show line optical depth effects (Niedzielski 1994). The IR emission lines show important influence from an additional turbulence present in the wind (Ignace et al. 2001). All these effects result in observational uncertainties which lead to nonuniformity of existing measurements of terminal wind velocities. To avoid the problems with heterogeneity of the existingv determinations we decided to obtain terminal wind velocity for as many WR stars as possible with one method, based on analysis of P Cyg profiles of the UV resonance line of CIV λ1550. We used all high and low resolution IUE spectra of the WR stars with measurable CIV P Cyg profile, available from the INES database. Since only for a limited sample of the Galactic and LMC WR stars the high resolution UV spectra are available, we used also the low resolution data after empirical calibration similar to that of Prinja (1994). Therefore, for most of our program stars, the determination ofv is a two-step procedure. First we determined terminal wind velocities for the WR stars observed by IUE with high resolution. Then we calibrated those terminal wind velocities with λ λ Abs min λ Emis peak µ as measured from low resolution spectra and used the empirical calibration to obtain terminal wind velocities for all WR stars having low resolution short wavelength IUE spectra High Resolution Data High resolution IUE spectra of 59 WR stars, 39 from the Galaxy and 20 from the Large Magellanic Cloud, are available from the INES database. We used all spectra available for every star except for those few stars for which many spectra were available due to intensive monitoring done in search for their possible spectral variability. In such cases arbitrarily selected subsets of about 20 good quality spectra were used instead. All velocity components of P Cyg profile defined in Fig. 1 were measured for 59 WR stars of our sample. The profile of CIV λ1550 is best suited for terminal

4 84 A. A. Fig. 1. Definitions of different P Cyg profile velocity components. The profile shown is CIV λ1550 line of HD in high (upper panel) and low (lower panel) resolution. Location of different velocity parameters as defined by different authors is indicated:v max is defined by Abbott and Conti (1987) and is also calledv a in Willis (1982) orv edge in Prinja et al. (1990); v black is defined by Prinja et al. (1990) and is also calledv in Abbott and Conti (1987);v min is defined by Abbott and Conti (1987);v phot is defined by Marchenko and Moffat (1999);v abs andv em are defined by Prinja (1994);v red is a long wavelength extent of the emission profile. wind velocity determinations because it is saturated in almost all WR stars (Prinja et al. 1990). We also measured the same profile components in three more lines: NV λ1240, HeII λ1640 and NIV λ1720. Those lines are not always saturated and usually are not used forv determinations. Results forv black are presented in Table 1 (Galactic stars) and Table 2 (LMC stars). In each case we show the obtained mean value ofv black, the error (if applicable) and the number of spectra used (in

5 Vol T a b l e 1 v black from the high resolution IUE spectra Ident. Sp v black NV λ1240 CIV λ1550 HeII λ1640 NIV λ WN (16) (17) (17) (16) 8 WN7/WCE+? 1621 (1) 1488 (1) 1584 (1) 10 WN5ha(+A2V) 1575 (1) 1346 (1) 1758 (1) 1300 (1) 16 WN8h 1009 (1) 604 (1) 649 (1) 1289 (1) 22 WN7h+O9III-V (10) (11) (11) (11) 24 WN6ha (20) (20) (20) (20) 25 WN6h+O4f (3) (9) (9) (9) 31b WN11h 778 (2) (3) 40 WN8h (20) (20) (20) (20) 46 WN3p+OB? 2611 (2) 2431 (2) 71 WN6+OB? 1488 (1) 1570 (1) 1701 (1) 1547 (1) 78 WN7h (8) (8) (8) (8) 79a WN9ha (11) (12) (12) (12) 85 WN6h+OB? 1553 (2) 1574 (2) 1587 (2) 128 WN4(h)+OB? 2151 (2) 2234 (2) 1914 (2) 1784 (2) 133 WN5+O9I (5) (5) (5) (5) 134 WN (18) (18) (18) (18) 136 WN6(h) (18) (18) (18) (18) 138 WN5+B? (21) (21) (20) (20) 139 WN5+O6III-V (3) (3) (4) (4) 153 WN6/WCE+O6I (5) (5) (5) (5) 155 WN6+O9II-Ib (8) (13) (13) (13) 11 WC8+O7.5III-V (23) (20) 14 WC7+? (4) (4) 23 WC (4) (4) 42 WC7+O7V (10) (10) 48 WC6(+O9.5/B0Iab) (17) (15) 52 WC (1) 2694 (1) 57 WC (3) (3) 69 WC9d+OB 1309 (1) 959 (1) 79 WC7+O (21) (20) 90 WC (4) (4) 92 WC9 966 (1) 886 (1) 103 WC9d+? 1044 (2) 856 (2) 111 WC (2) 2007 (2) 113 WC8d+O8-9IV 1945 (2) 2082 (2) 135 WC (3) (3) 137 WC7pd+OB (10) (10) 140 WC7pd+O (7) (7) parenthesis). In the following we will assume thatv black CIV λ1550µ v. The wind velocities observed in NV, NIV and HeII P Cyg profiles as well as other than terminal wind velocity kinematical properties of our program WR stars obtained from the high resolution spectra will be discussed elsewhere. The uncertainties associated with our terminal wind velocity determinations were calculated from the observed scatter and assuming the Student-t distribution. It is noticeable that the uncertainties of terminal wind velocity determinations,

6 86 A. A. T a b l e 2 v black from the high resolution IUE spectra for LMC WR stars Ident. Sp v black BAT99 Brey NV λ1240 CIV λ1550 HeII λ1640 NIV λ O6-7n-nn+WN5-6A 1246 (1) 1263 (1) 1851 (1) 1256 (1) WN8h 738 (1) 689 (1) 742 (1) 886 (1) WN9h 997 (1) WN5?b+B1Ia 1723 (1) 784 (1) 2155 (1) WN6h 1021 (1) 563 (1) 663 (1) 1244 (1) WN6(h) 1604 (1) 1539 (1) 1332 (1) 984 (1) WN5?b+(B3I) (3) 1747 (2) (4) 55 WN11h 474 (2) 463 (2) 800 (1) WN (1) 83 Ofpe/WN (1) 955 (1) WN6+B1Ia 1429 (1) 1552 (2) WN5h (5) (8) (7) (8) WN6h (4) (4) (4) (4) WN6(h) (3) (3) (3) (3) 133 WN11h 527 (1) 947 (1) 10 9 WC (3) 1870 (1) WC (1) 2593 (1) WC4(+O?)+O8I: (5) 2305 (2) WC4+O6V-III 3243 (1) 2730 (1) WC5(+O?) 2054 (2) 2737 (2) based on the high resolution spectra, are in most cases lower than 10%. The average error in this group of data is 5% in the Milky Way and 7% in the LMC. The determinations of terminal wind velocities of the Galactic WC stars are the most precise. They are accurate up to 4% on average. The least precise are those for LMC WN stars, accurate to within 7% on average. In generalv determinations for the WN stars are less precise than for the WC objects Low Resolution Data Terminal wind velocities, as determined from the high resolution IUE spectra, can now be used to calibrate the empirical relationsv black λ λ Abs min λ Emis peak ) similar to that introduced by Prinja (1994) for all four P Cyg profiles under consideration. To minimize the uncertainty in the determination of coefficients in the linear fit we divided our sample of 59 WR stars into 4 subgroups: Galactic WN stars, Galactic WC stars, LMC WN stars and LMC WC stars. This allowed us to obtain the best defined relations betweenv black from the high resolution and λ λ Abs min λ Emis peak ) from the low resolution spectra. Correlation coefficients for the four groups in CIV line are very high: r 0 953, 0.937, and 0.957, respectively. The relations for the WN stars are steeper than for the WC ones. The v black λ relations for the Galactic and LMC WN stars are not identical. The relation for the Galactic WN stars is steeper than for the LMC ones. For the WC

7 Vol stars in Galaxy and LMC the relations are very similar. For each of the four groups in our sample of 59 WR stars a linear relation: v black a λ b 1µ was found for every considered spectral line (CIV, NIV, HeII, NV) separately. Details of the resulting fits for the CIV line are presented in Table 3 and in Fig. 2. T a b l e 3 Values of correlation coefficients r and coefficients of Eq. (1) for the line CIV λ1550 Group r N a S a b S b Gal. WN WC LMC WN WC With the empirical Eq. (1) we were able to obtain v black estimates for all WR stars for which the low resolution IUE spectra are available from the INES database. After measuring λ λ Abs min λ Emis peak ) in all IUE low resolution spectra of WR stars we could determine v black for CIV λ1550, NIV λ1240, HeII λ1640 and NV λ1720. Again, as in the case of high resolution data we assume thatv black CIV λ1550µ v. This way we obtained terminal wind velocities for 164 Galactic and LMC WR stars. The sample of 82 Galactic WR stars is composed of 50 WN and 32 WC stars. For 5 WN and 2 WC stars our determinations are the first published. The sample of 82 LMC WR stars is composed of 64 WN and 18 WC stars. For 21 LMC WN and 6 LMC WC stars our terminal wind velocities are the first published. Altogether we obtained 34 new terminal wind velocity determinations. Our results obtained from the low resolution spectra are presented in Tables 4 7. The uncertainties connected with terminal wind velocities determined from the low resolution spectra were calculated from the observed scatter assuming the Student-t distribution. Progression of uncertainties through the empirical relations introduced in Section 2.2 was taken into account (the empirical relation introduces most of the uncertainty in the determination of terminal wind velocity from the low resolution data). The uncertainties of low resolution measurements are of course much larger than those obtained from the high resolution data. The estimated uncertainties show that the method presented here allows to estimate terminal wind velocities of WR stars from the low resolution IUE spectra with average precision of about 40% in the Milky Way, and about 60% in the LMC. The

8 88 A. A. Fig. 2. Four empirical relations between λ λ Abs min λemisµ peak andv black for the CIV line λ1550 ( v ). Note that relations for the WN stars are steeper. See parameters of fits in Table 3. determinations for the Galactic WC stars appear to be the most precise (32%) while the largest uncertainty exists for the LMC WN stars (61%). Like in the case of the high resolution data the results for the WN stars appear less precise both in the Galaxy and in LMC. Formal errors of the low resolution terminal wind velocity determinations appear to be much larger than the actual deviations from the high resolution data presented in Section 2.1. A comparison between high and low resolution data obtained in the presented paper shows that the largest scatter of 17% appears in the case of LMC WN stars. In the case of Galactic WR stars the scatter is less than 11%. This suggests higher than formal precision of the results based on low resolution spectra.

9 Vol Comparison with other Determinations The precision of ourv determinations can also be estimated by comparison with results of other studies. In the case of high resolution data we limited ourselves to comparison with Prinja et al. (1990). In the case of the low resolution data much richer set of published data was used High Resolution Data Our values ofv of the Galactic WR stars can be compared to those of Prinja et al. (1990). Although terminal wind velocity determinations differ by as much as 200 km/s for individual stars, the agreement is very good in general. Our terminal wind velocities for the sample of 34 stars in common with Prinja et al. (1990) are on average a bit smaller ( ) than theirs. They agree, however, within 1σ. The correlation coefficient between our and Prinja et al. (1990) data on CIV is In the case of velocities as determined from the HeII and NIV lines our measurements are on average and times larger, respectively but again agree within 1σ Low Resolution Data Our results for the Galactic WN stars (Table 4) can be compared with those of Hamann and Koesterke (1998, HK98) since we have 34 stars in common. Our values are times bigger on average. The correlation coefficient between our and Hamann and Koesterke (1998) data is only r We compared our data also to Rochowicz and Niedzielski (1995, RN95) based on 31 stars in common. The values presented here are times larger on average (r 0 85). We also have 18 Galactic WN stars in common with Prinja et al. (1990, P90). Our data as compared to theirs give lower values, on average (r 0 888). Finally for 13 Galactic WN stars in common with Eenens and Williams (1994, EW94) our values are slightly higher, times on average and agree within 1σ (r 0 920). For the Galactic WC stars (Table 5) we may perform comparison with Torres (1986, T86). We have 27 stars in common and our values ofv are times lower. The correlation, however, is good r For 20 stars in common with Rochowicz and Niedzielski (1995) we found that the ratio of the above presented to their data is (r 0 942). For 16 stars in common with Prinja et al. (1990) our values coincide on average with theirs with the correlation r Comparing our data for the Galactic WC stars with those of Hamann and Koesterke (1998) we found that our data are times larger on average with r only. Finally we compared also with Eenens and Williams (1994) using 14 stars in common. We found our data to be times bigger and correlate well, r Our determinations of terminal wind velocity for the LMC WN stars (Table 6) can be compared with three extended works. For 29 stars in common with Rochow-

10 90 A. A. T a b l e 4 Terminal wind velocityv for 50 Galactic WN stars obtained from low resolution spectra Ident. Sp v This work σ N HK98 EW94 P90 RN95 1 WN WN3+O WN WN WN7/WCE+? WN5ha9+A2V) WN8h+? WN8h WN WN5+O WN7h+O9III-V WN6ha WN6h+O4f WN4+O8V WN8h WN6ha WN4+OB? WN3p+OB? WN6+O5V WN WN4/WCE WN WN6+OB? WN WN7h WN6h+OB? WN9h+OB WN WN WN8h WN3+O9.5V WN4(h)+OB? WN5+O9I WN WN6(h) WN5+B? WN5+O6III-V WN5+O5V-III WN8h+B3IV/BH WN4+O5V WN3(h) WN6/WCE+O6I WN6+O9II-Ib WN8h+OB? WN5(+B1II) WN7h+Be? a WN11h b WN11h a WN9ha b WN9ha

11 Vol T a b l e 5 Terminal wind velocityv for 32 Galactic WC stars determined from low resolution spectra Ident. Sp v This work σ N KH95 T86 EW94 P90 RN95 4 WC5+? WC WC5+O WC8+O7.5III-V WC7+? WC WC WC WC6+O WC WC7+O7V WC6(+O9.5/B0Iab) WC7+OB WC WC8d WC WC WC9d+OB WC9vd+B0I WC7+O WC7(+B0III-I) WC WC WC9d WC9d+? WC9d WC WC8d+O8-9IV WC WC7pd+OB WC7pd+O WC icz and Niedzielski (1995) we found weak correlation of r 0 708, our values are on average of theirs. We have also 17 stars in common with Hamann and Koesterke (2000, HK00) and with Crowther and Smith (1997, CS97). Correlation of our data with those of Hamann and Koesterke (2000) does not exist (r 0 353) our data are on average of theirs. Agreement is a bit better when comparing our data to those of Crowther and Smith (1997). Our data are times larger on average but the correlation coefficient is r Values ofv presented here for the LMC WC stars (Table 7) can be compared to those of Rochowicz and Niedzielski (1995). On average these data for 6 stars in common are practically identical, of ours (r 0 934). While com-

12 92 A. A. T a b l e 6 Terminal wind velocityv for 63 Large Magellanic Cloud WN stars obtained from low resolution spectra Ident. Sp v BAT99 Brey This work σ N HK00 CS97 RN WN3b WN WN4b O6-7n-nn+WN5-6A WN4b a O3If/WN6-A WN WN4+OB? WN4b WN8h WN4o WN4(h) WN4b+OB? WN9h WN4b WN4b WN5?b+B1Ia WN4b WN6h WN3b WN6(h) Ofpe/WN WN3o WN4b/WCE WN4b WN5?b+(B3I) WN4+OB WN8h WN10h WN4o WN4b a WN3+O WN11h WN4b WN4b WN6h WN4o?+B WN4h+abs WN5o?+OB WN WN3o pec WN WN9h WN WN7h+OB WN3b Ofpe/WN

13 Vol T a b l e 6 Concluded Ident. Sp v BAT99 Brey This work σ N HK00 CS97 RN WN7h WN6+B1Ia WN4b WN7h O3If/WN6-A WN5h WN5+B? WN6h WN6(h) WN9h WN5(h) WN11h WN WN4b WN11h WN4b T a b l e 7 Terminal wind velocityv for 17 Large Magellanic Cloud WC stars and one WO star obtained form low resolution spectra Ident. Sp v BAT99 Brey This work σ N T86 G98 RN WC WC WC WC WC6+O5-6V-III WC4+OB WC4(+O?)+O8I: WC4+O6V-III WC WC4+O WC WC WC5+OB WC4+OB WC4+OB? WC5(+O?) WC5+O WO

14 94 A. A. pared to Gräfener et al. (1998, G98) for 5 stars in common our determinations are on average times larger but correlation does not exist ( r 0 320). For 10 stars in common with Torres (1986) our values are times smaller with the correlation coefficient of only r Fig. 3. Comparison of the terminal wind velocities presented here with those of van der Hucht (2001a). The general agreement is good. See text for details. Ourv determinations can also be compared to the compilation of van der Hucht (2001a). For the sample of 73 Galactic WR stars in common for we found a good correlation of and our values are only times larger on average. This generally good agreement is noticeable in Fig. 3. We note, however, that our data for all WC stars and WN3 7 stars agree much better than on average with those of van der Hucht (2001a). We conclude that our determinations of terminal wind velocities, obtained from the low resolution UV data, when compared to other studies show higher accu-

15 Vol racy ( 20%) than that appearing from calculation of formal errors. Surprisingly enough they compare best to the high resolution and high precision studies of Prinja et al. (1990) and Eenens and Williams (1994) or compilation of van der Hucht (2001a). They do not correlate well with the terminal wind velocity determinations based on the equivalent width measurements of optical lines. The differences between Rochowicz and Niedzielski (1995) and the present study are due to different empirical calibrations. Fig. 4. Relation between the terminal wind velocity as determined from P Cyg profiles of CIV λ1550 and FWHM of the most prominent lines present in the optical spectra of WR stars: HeII λ4686 in WN (Torres et al. 1986) and CIII λ4650 in WC (Conti and Massey 1989). We note the lack of correlation between these data, especially for the LMC. Since terminal wind velocities are sometimes estimated from the FWHM of strong emission lines present in the optical spectra of WR stars we searched for a correlation between our determinations ofv and the widths (FWHM) of the strong

16 96 A. A. lines published in the literature. We decided to check the most easily available data on the strongest optical lines HeII λ4686 in WN and CIII λ4650 in WC stars. The results are presented in Fig. 4. We found that the FWHM of the lines under consideration are not well correlated with the terminal wind velocities and usually indicate much larger velocities. Only in the case of the Galactic WN stars a weak correlation and a linear relation exists. The FWHM of the strong optical emission lines gives therefore poor estimate of the terminal wind velocity. 4. Correlation betweenv and the WR Spectral Subtype The correlation between the WR spectral type andv for the Galactic WC stars was previously investigated by several authors (Abbott and Conti 1987, Koesterke et al. 1991, Howarth and Schmutz 1992, Hamann et al. 1993, Eenens and Williams 1994, Rochowicz and Niedzielski 1995). In the paper of Rochowicz and Niedzielski (1995) the existence of a strong correlation between the WR spectral subtype and the terminal wind velocity for WC stars was confirmed. Existence of similar correlation for WN stars was questioned, however, contrary to the previous finding of Hamann et al. (1993). With many more stars included in the present study we can re-discuss this issue again. As illustrated in Fig. 5 we found a very good correlation between the terminal wind velocity and the WR spectral subtype (from van der Hucht 2001a) for the Galactic WC stars. Similar but weaker correlation exists for the Galactic WN stars. A weak correlation betweenv and WR spectral subtype exists also for the LMC WN stars. Due to a small population of the LMC WC stars we cannot search for similar correlation for them. Correlation between terminal wind velocity and spectral subtype in the case of Galactic WC stars reaches r In this case our data from both high and low resolution spectra show a smooth decrease ofv with spectral type. In the case of the Galactic WN stars the correlation of terminal wind velocity with the WR spectral subtype is r We can see in Fig. 5 that the scatter of v within given subclass in WN stars is larger than in WC stars but the decrease of terminal wind velocity with the WR spectral subtype is clear. We note that among the the Galactic WR stars the largest scatter exists within the WN 6 subtype where a stellar wind may reach a terminal velocity of km/s (WR 85) or km/s (WR 43) as well. In the Large Magellanic Cloud the search for correlation between terminal wind velocity and the WR spectral subtype (from Breysacher et al. 1999) is more complicated due to lower statistics. Terminal wind velocities of the LMC WC stars as determined here are generally in good agreement with those of the Galactic counterparts. Like in the Galaxy they reach higher velocities than the neighboring WN stars. Since only a few WC stars

17 Vol Fig. 5. Terminal wind velocity vs. spectral type for the Galactic (upper panels) and LMC (lower panels) WR stars of the WC (left panels) and WN (right panels) type. Dots represent the determinations ofv based on low resolution data, circlesv black of CIV λ1550 line as measured on the high resolution IUE spectra. are present in our LMC sample nothing can be said on the correlation with spectral type. The formal correlation between wind terminal velocities and spectral subtype for the LMC WN stars is not as strong as in the Galaxy but it is still significant (r 0 878). The LMC data show that the large scatter in terminal wind velocities is evidently intrinsic to WN stars. This is best illustrated by WN 4 stars in the LMC. The mean value of 1525 km/s based on 23 stars shows scatter of 353 km/s. The mean terminal wind velocities in all WR spectral subtypes are presented in Table 8. We found no evidence of influence of the binarity on terminal wind velocities of WR stars.

18 98 A. A. T a b l e 8 The mean values of terminal wind velocities within spectral subtypes of WR stars in the Galactic and LMC. Both high resolution and low resolution results are presented. The standard deviations of the mean and the number of averaged data points (in parenthesis) are presented. Sp v Milky Way LMC High resolution Low resolution High resolution Low resolution WN (1) WN (4) (7) WN (2) (9) (23) WN (4) (9) (4) (8) WN (8) (11) (5) (8) WN (3) (5) 1379 (1) (4) WN (2) (7) 689 (1) (2) WN (1) (3) (2) (5) WN (2) WN (1) (2) (2) (3) WC (1) 2498 (1) (3) (12) WC (1) (5) (2) (4) WC (2) (6) 2916 (1) WC (6) (9) WC (4) (5) WC (3) (6) WO (1) The WN winds in the LMC are slower than in the Galaxy. This is evident in both high and low resolution data. Table 8 shows the mean values of terminal wind velocity in spectral subtypes of WR stars in the Galaxy and LMC together with the observed scatter. We can see that in the case of WN stars the winds in our Galaxy are faster in all subtypes except for WN 11. The Galactic WN winds are only 1 6% faster for the late WN stars but up to 26 93% faster in the case of the early WN stars. In the case of WC stars such a comparison is not possible since in the LMC we observe only WC4 6 stars. Our data suggest, however, that the WCE winds in the LMC are approximately of the same velocity as in the Galaxy or faster. The difference of wind velocities between the Galactic and LMC stars was discussed previously in Smith and Willis (1983), Abbott and Conti (1987), Koesterke et al. (1991), Rochowicz and Niedzielski (1995) and in Crowther and Smith (1997). Rochowicz and Niedzielski (1995) found the LMC WN stars to have slower winds while Crowther and Smith (1997) found no significant difference between Galaxy and LMC WN stars winds. The results presented here support earlier findings of Rochowicz and Niedzielski (1995). They agree also with the results of Garmany

19 Vol and Conti (1985), Haser et al. (1994) and Walborn et al. (1995) who found in the LMC O stars lower terminal velocities than in their Galactic counterparts. 5. Correlation betweenv and Stellar Temperature According to the radiation driven wind theory, the terminal wind velocities depend on the effective temperatures of wind driving stars. Prinja et al. (1990) found that for OB stars the correlation between v and effective temperature is even stronger than betweenv andv esc. One problem that immediately appears while trying to check such a correlation for WR stars is that the effective temperature is not a well defined parameter in the case of these objects. Since continuum radiation is in part produced by expanding envelope and the line spectrum develops in varying physical conditions all temperature estimates differ very much depending on adopted assumptions. To study the terminal wind velocity dependence on temperature of the wind driving stars we decided to use the T data of the Hamann group (Koesterke and Hamann 1995, Hamann and Koesterke 1998). These data correlate well with the presented here terminal wind velocities, contrary to all other temperature estimates published in the literature. In the case of the Galactic WN stars we obtained correlation of r for a group of 31 stars from Hamann and Koesterke (1988). In the case of LMC WN stars, r for a group of 28 stars from Hamann and Koesterke (2000). Both values of the correlation coefficient are low and the resulting correlation can only be considered as marginal. In Fig. 6 we plotted the terminal wind velocities as a function of effective temperature of OBA and Central Stars of Planetary Nebulae (CSPN) stars, and T of WR stars and WR type CSPN. One can note that the Galactic and LMC WNL stars occupy the position of O supergiants and dwarfs. WNE and WC stars fall far from the relation defined by the OBA stars since they are too hot. WR stars generally follow, although with large scatter, the relation defined by CSPN. 6. Correlation ofv v esc with Stellar Temperature According to the radiation driven wind theory (Castor et al. 1975, Abbott 1978, Abbott 1982) the terminal wind velocity is related to the effective escape velocity at the stellar surfacev esc through the relation: v v esc α 1 αµµ 0 5 where α is a parameter which depends on the optical depths of the wind driving lines and on the stellar temperature. The effective escape velocity is the gravitation escape velocity corrected for the radiation pressure due to electron scattering. v esc Ö 2GMeff R where M eff M 1 Γ e µ and Γ e σ el 4πcGM

20 100 A. A. Fig. 6. Terminal wind velocityv vs. effective temperature of the OBA, CSPN, WR and [WR] stars. In the case of WR and [WR] stars T is used. All groups are plotted with different symbols: Galactic WR stars (WNL small circles, WNE big circles, WC skewed crosses), LMC WR stars (WNL small squares, WNE big squares, WC crosses), OBA type stars (dots), CSPN (black asterisks), [WR] stars (white asterisks). The OB stars terminal velocities come from Howarth et al. (1997) and temperatures from Humphreys and McElroy (1984). The A type supergiants terminal velocities and temperatures are taken from Lamers et al. (1995), for CSPN from Kudritzki et al. (1997), Modigliani et al. (1993), Pauldrach et al. (1988) and Kaler et al. (1985). The data for [WR] stars come form Leuenhagen, Koesterke and Hamann (1993), Koesterke and Hamann (1997), Leuenhagen, Hamann and Jeffery (1996). According to Nugis and Lamers (2000): σ e ³ X Y 2 Z 4µ cm 2 where X Y Zµ define the chemical composition of the star. Numerous previous analyses of OB stars winds show thatv v esc indeed depends on the temperature of underlying star. It is equal to for T eff K, 1 3 for T eff K and for T eff K (Lamers et al. 1995). Using the data on Planetary Nebulae Nuclei form Perinotto (1993) and Kudritzki et al. (1997), Lamers and Cassinelli (1999) estimatedv v esc 4 4. To obtain the escape velocities for our sample of WR stars we adopted the data on masses, luminosities and radii form Hamann and Koesterke (1998), Koesterke and Hamann (1995), Hamann and Koesterke (2000) and Gräfner et al. (1998). Re-

21 Vol Fig. 7. Ratiov v esc vs. temperature for: Galactic WR stars (WNL small circles, WNE big circles, WC skewed crosses), LMC WR stars (WNL small squares, WNE big squares, WC crosses), OBA type giants and supergiants (dots); O type dwarfs (filled circles) and CSPN (asterisks). For O type stars temperatures, v esc are taken from Howarth and Prinja (1989) andv black from Howarth et al. (1997). For B supergiantsv black,v esc are taken from Prinja et al. (1990) and temperatures from Humphreys and McElroy (1984). For A type supergiants all data are taken from Lamers et al. (1995) for CSPN from Malkov (2000), Kudritzki et al. (1997), Modigliani et al. (1993), Patriarchi et al. (1989), Pauldrach et al. (1988) and Kaler et al. (1985). sults are presented in Fig. 7 where thev v esc ratio for Wolf-Rayet stars, OBA stars and PNNs is plotted against stellar temperature. Generally there exists no correlation between temperature (T ) of WR star and the terminal to escape velocity ratio. One can easily note, however, that both the Galactic and LMC WN late stars are placed around the location occupied by O stars. WNE and WC stars fall very much out of normal OBA stars location in Fig. 7. For WNE and WC stars we actually observe a kind of reverse relation between temperature of the underlying star and the terminal to escape wind ratio. We found that thev v esc ratio for WR stars is relatively low. In the case of the Galactic WN stars we foundv v esc on average ( for WNE and for WNL). In the case of the Galactic WC starsv v esc on average ( for WCE and for WCL). The v v esc ratio is generally lower for the LMC stars. In the case of WN stars it is

22 102 A. A on average ( for WNE and for WNL). For the LMC WCE stars it is The observed low terminal to escape wind velocity ratios suggest much lower than estimated temperatures of WCE and WNE stars if the radiation driven wind mechanism, similar to that of OB stars, applies. On the contrary, if we accept the present (high) temperatures of WR stars, it appears that another wind driving engine, less efficient than that working in OB stars, should be proposed. It is interesting to note that in the Galaxy the WR wind driving force as measured by thev v esc ratio decreases from WNL stars through the WNE stars down to WC stars. This suggests that in the case of the Galactic WR stars the wind driving force decreases with increasing (in the course of stellar evolution) metallicity of the wind. 7. Summary and Conclusions Using the archived IUE data from the INES database we obtained terminal wind velocities for 59 WR stars using the high resolution data. We estimated the precision of these determinations as 5 7% for the Galaxy and LMC, respectively. Using the empirical relation between terminal wind velocity determined from the high resolution spectra and λ measured in low resolution spectra we obtained terminal wind velocities for 164 Galactic and LMC stars, 34 of them being the first determinations published. Our terminal wind velocity determinations were used in search for correlation ofv with WR spectral subtype. We found strong correlation for the Galactic WC stars and a weaker one for the Galactic WN stars. Similar but weaker correlation was found also for the LMC WN stars. The existence of correlation between WR spectral subtype andv in the Galaxy suggests that if (1) the stellar evolution within the WR phase is a smooth process and (2) if the WN stars evolve from WNL to WNE then the WNE stars should evolve towards WCE (which have similar terminal wind velocities) and then towards WCL. Such a scenario requires, however, an explanation of the wind breaking mechanism in the WC star winds. First part of this finding is supported by van der Hucht (2001b) analysis of the Galactic distribution of WR stars. Alternatively the WNE should evolve towards WCE and WNL stars towards WCL as postulated by van der Hucht (2001b). This scenario requires, however, that only stars earlier than WN8 9 evolve towards WCL. WN10 11 stars should evolve towards WN8 9 first as suggest their very slow winds. Van der Hucht (2001b) scenario fails in the LMC where we see many WNL stars but no WCL stars. Terminal wind velocities for the LMC WNL stars are comparable to their Galactic counterparts. The winds of the LMC WNE stars are, however, up to almost two times slower than in similar stars in the Galaxy. The LMC WC stars show winds with terminal velocities similar to the Galactic ones. The difference between

23 Vol terminal wind velocities of the Galactic and LMC WNE stars suggests important influence of metallicity on stellar winds, in agreement with radiation driven wind theory. However, according to Crowther and Smith (1997) the LMC WNL stars do show lower metallicities than the Galactic ones. It is interesting therefore why metallicity does not influence WNL winds? Terminal wind velocities in both the Galactic and LMC WR stars correlate very weakly with stellar temperatures, contrary to the Galactic OB stars where this relation is more evident than the relation between the terminal to escape velocity ratio and the effective temperature (Prinja et al. 1990). Both the effective temperatures and escape velocities are not precisely known for WR stars but it is interesting to note that lowering the existing effective temperatures of WR stars inside reasonable limits does not help to reach relation defined by the Galactic OBA stars. Reasonable variation of stellar parameters, necessary to calculate the escape velocities (M L R), allows to push the WR stars towards the O stars onv vs. stellar temperature diagrams, however. The terminal to escape velocity ratio for WNL stars from both the Galaxy and LMC is similar to that of the Galactic O stars. Its value for WNE stars is lower than that. It is even lower for the Galactic WC stars. Low terminal to escape velocity ratio for the WNL stars, comparable to O stars, is reasonable, since WNL stars are evolutionally strongly connected to O stars. The decrease of the terminal to escape velocity ratio with stellar temperature and evolutional stage of WR stars (its chemical composition) is surprising. It is not clear why the wind driving force (measured by terminal to escape velocity ratio) weakens with stellar temperature and metallicity of WR stars. The role of metallicity in the WR winds seems to be especially complex. The existing data on WR stars do not allow to reject the radiation driven wind theory. They substantially question, however, its application to WR stars. Acknowledgements. This work was partly supported by the Polish KBN grant 5 P03D to AN. REFERENCES Abbott, D.C. 1978, Astrophys. J., 225, 893. Abbott, D.C. 1982, Astrophys. J., 259, 282. Abbott, D.C., and Conti, P.S. 1987, Ann. Rev. Astron. Astrophys., 25, 113. Breysacher, J., Azzopardi, M., and Testor, G. 1999, Astron. Astrophys. Suppl. Ser., 137, 117. Castor, J.I., Abbott D.C., and Klein R.I. 1975, Astrophys. J., 195, 157. Conti, P.S., and Massey, P. 1989, Astrophys. J., 337, 251. Crowther, P.A., Smith, L.J., and Hillier, D.J. 1995, Astron. Astrophys., 302, 457. Crowther, P.A., Smith, L.J., Hillier, D.J., and Schmutz, W. 1995, Astron. Astrophys., 293, 427. Crowther, P.A., and Smith, L.J. 1997, Astron. Astrophys., 320, 500. Eenens, P.R.J., and Williams, P.M. 1994, MNRAS, 269, Garmany, C.D., and Conti, P.S. 1985, Astrophys. J., 293, 407. Gräfener, G., Hamann, W.-R., Hillier, D.J., and Koesterke, L. 1998, Astron. Astrophys., 329, 190.

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