arXiv:astro-ph/0404551v1 28 Apr 2004 ⋆⋆⋆ rsoi uvyo apeo Rbnr addts n of One candidates. binary WR of sample a spec of snapshot survey - a OB- troscopic spectrum performed we an composite - of a multiplicity confirm for light hence search continuum and to the order by In dilution companion. the type cou of such that result as lines the emission classified be weak were comparatively candidates their of binary because these of Most ries. otfnaetlise steiprac fms rnfri O transfer massive of of the evolution importance Among the the objects. is these issues of fundamental evolution most ques- the open about many remain O-, tions massive (WR) of Wolf-Rayet descendants that evolved accepted are generally is it Although Introduction 1. is a e uh 20)poie ito 0rte faint rather 30 WR of potential are list that a objects bin provides studied confirmed (2001) poorly these and Hucht to der addition van re- In have ries, them solutions. of orbital 32 only liable and binaries on spectroscopic are there known 2001), 52 Hucht der (van Stars Wolf-Rayet Galactic udmna rpriso niiulmsiebnr syste com t binary investigate as massive to individual achieve and of possible to properties as interest fundamental binaries WR prime of of census a therefore plete is It 1998). al. bevtr L il,Chile) Silla, (La Observatory orsodneto Correspondence edo Send ⋆⋆ ⋆ later) hand by inserted be will Astrophysics (DOI: & Astronomy R2a asv onrtn iaysse opiigtwo comprising system binary cornerstone massive a 20a: WR mn h 2 bet itdi h It aaou of Catalogue VIIth the in listed objects 227 the Among ae nosrain olce tteErpa Southern European the at collected observations on Based eerhFlo NS(Belgium) FNRS Fellow Research (Belgium) FNRS Associate Research ff n 68 and e words. Key O3If 3 2 4 5 ihams rbbeobtlpro of period orbital probable most a with Abstract. eevddate Received 1 rn eussto requests print RNNtoa nttt o pc eerh obnean2 Sorbonnelaan Research, Space for Institute National SRON eateto hsc srnm,Uiest fShe of University Astronomy, & Physics of Department srnmclIsiueAtnPneok nvriyo Ams of University Pannekoek, Anton Institute Astronomical nttt o srnm,Uiest fEibrh oa Ob Royal Edinburgh, of University Astronomy, for Institute ntttdAtohsqee eG´eophysique, Universit´e de et d’Astrophysique Institut .Rauw G. ∗ / Nh pcrltp.Teobtlslto o eido 3 of period a for solution orbital The type. spectral WN6ha . 8 ± 3 [email protected] : eaayesetocpcosrain fW 0 eeln t revealing 20a WR of observations spectroscopic analyse We . M 8 iais pcrsoi tr:erytp tr:funda stars: – early-type stars: – spectroscopic binaries: 1 / ,⋆⋆ cetddate Accepted ⊙ .Rauw G. : o h w tr.Teepoete aeW 0 cornerstone a 20a WR make properties These stars. two the for .D Becker De M. , + aucitn.Gc251 no. manuscript iais(..Vneee et Vanbeveren (e.g. binaries O ..vndrHucht der van K.A. xrm al-yestars early-type extreme 1 .Naz´e Y. , ∼ 3 . 7 as u pcr niaeta ohcmoet r most are components both that indicate spectra Our days. 675 + 1 ,⋆⋆⋆ Bbina- OB 3 , 4 .M Vreux J.-M. , ..Crowther P.A. , ms. eL`g,Ale u6Aˆt ˆtBc 00Li`ege, Belgium 4000 Aoˆut, Bˆat B5c, 6 Li`ege, All´ee du de ffi he ld a- ly l,HcsBidn,HusedR. She Rd., Hounsfield Building, Hicks eld, n s - - evtr,BakodHl,Eibrh H H,UK 3HJ, EH9 Edinburgh, Hill, Blackford servatory, edm rila 0,19 JAsedm h Netherlands The Amsterdam, SJ 1098 403, Kruislaan terdam, 54C teh,TeNetherlands The Utrecht, CA 3584 , . rmo h tradpooe N pcrltp,although type, He spectral a WN7 of a lack the proposed noted and they sp star Å) 7 the of of resolution of pixel trum course (2 low-resolution the autho a latter in obtained The Way. galactic also (1991) Milky for southern al. survey the et in photometric stars Shara broadband Wolf-Rayet by and narrow star deep WR a a be to ered he(96 na H an in (1966) Th´e h otitrsigojcsta mre rmorsre wa survey our from 20a. emerged WR that objects interesting most the 19)sgetdta R2ahdarte ihH high rather a had 20a WR that suggested (1991) ac ai.Ltr h betwsr-lsie sWN7:h as re-classified was object the Later, ratio. dance h on pncutrWseln ntecr fteH the of of member core a the (Mo be 49 in RCW to 2 gion Westerlund believed cluster is open star young The the (1999). al. et Shara by eghcvrg f40Åprstu.Tecnrlwavelength central The set-up. per Å mm 460 lines of (1200 coverage 3 length # grating spectroscopic dispersion instru- New with medium EMMI BLMD mode m 3.5 the The in ESO’s Silla. used was La on ment at mounted (NTT) Telescope instrument Technology 2002 EMMI March in the obtained were with 20a WR of spectra snapshot Two Observations 2. 4. Sect. in discu results briefly our we of Finally, implications with velocities. deals radial 3.2 the Sect. o whilst of nature 3.1, analysis the Sect. as in revisited well are as 20a features WR spectral The 2). (Sect. 20a WR 7 asyed xrml ag iiu asso 70 of minimum large extremely yields days 675 etlprmtr tr:idvda:W 20a WR individual: stars: – parameters mental R2a( 20a WR nti etr efis rsn u blue our present first we letter, this In 1 n ..Williams P.M. and , a hssa samsieerytp iaysystem binary early-type massive a is star this hat 2 .Gosset E. , = ytmfrtesuyo asv trevolution. star massive of study the for system H 35- THA ff te l 1991). al. et at α ⋆ 1 msinln uvy n rtdiscov- first and survey, line emission ,⋆⋆  -036 .Sana H. ,  5 λ 41eiso ie hr tal. et Shara line. emission 5411 = ffi MP)wsctlge by catalogued was SMSP2) l,S R,UK 7RH, S3 eld, 1 ieyo Nh or WN6ha of likely ,⋆⋆⋆ − / 1 iltosrain of observations violet rvdn wave- a providing ) , uut2,2018 29, August . 7 ± / 4 eabun- He . 0 sthe ss  / WC ec- re- an rs s f 2 G. Rauw et al.: WR 20a: a massive cornerstone binary system comprising two extreme early-type stars

Fig. 1. Spectrum of WR20a as observed on HJD2453034.715 (violet setting, top panel) and 2453034.741 (blue setting, lower panel). Note the double N  λ 4058 emission line as well as the double N  λ 4604 and N  λ 4620 absorptions. The stellar lines are labelled above the spectrum, whereas the interstellar lines and diffuse interstellar bands (DIBs) are indicated below. was set to 4080Å (violet setting) or 4530Å (blue setting). The (see e.g. Fig.1) indicating that the componentsof WR20a must exposure times were 30min for each setting and the corre- have identical spectral types and a visible brightness ratio of sponding S/N was about 100. The spectral resolution as de- ∼ 1. The spectrum of WR20a displays many absorption lines termined from the FWHM of the lines of the Th-Ar comparison of H , He  and N  unlike what we would expect if WR20a spectra was about 1.25Å. The data were reduced in the stan- were indeed a ‘classical’ WN7 star (see however below). Only dard way using the long context of the  package. The a few (rather narrow) emission lines are seen in the blue/violet violet spectrum taken on HJD2452355.60 revealed double ab- spectrum: N  λ 4058, He  λ 4686 as well as a broad complex sorption lines of hydrogen and helium as well as a moderately probably due to N  λλ 4634-41, C  λλ 4647-50 and possi- strong double-peaked N λ 4058 emission. On the other hand, bly C  λ 4658. The Balmer absorption lines also display some the blue spectrum obtained on HJD2452354.56 showed single broad underlying emission wings on several spectra. The emis- absorption lines of hydrogen, He  and N . These properties sions seen around the Hδ absorption in Fig.1 could actually be suggested that WR20a could be a short period binary consist- due to Si  λλ 4089, 4116, but the spectra obtained on other ing of two very early extreme Of-type stars (Driesens 2003). nights do not allow us to unambiguously confirm this identifi- We thus organized a follow-up observing campaign to confirm cation. In addition to the strong diffuse interstellar bands, we the binarity of WR20a. The star was monitored over 12 con- note the presence of the interstellar He  λ 3889 absorption line. secutive nights in January-February 2004 with the same instru- This latter feature was also observed in the spectra of several mentation as in March 2002.The weather conditionswere good early-type stars of the (Walborn & Hesser 1975). except for the last two nights where the seeing and the trans- This line is not usually seen in interstellar sightlines (cf. ζ Oph, parency were poor. The data were reduced in the same way as Shulman et al. 1974) and may be formed locally as in the case in 2002 and the complete journal of the useable observations is of the Carina stars. given in Table 1. No stellar He  lines are clearly seen in the spectrum. Together with the presence of rather strong N  absorption lines, this would suggest a very early O-type classification 3. Results for the components of WR20a. The presence of He  λ 4686 3.1. The spectrum of WR 20a in strong emission further suggests a supergiant class. Comparing our spectra to the classification scheme of Our data reveal that WR20a is indeed a short-period binary Walborn et al. (2002, see also Walborn 2001), we would pro- system (see below). All prominent spectral features appear pose an O3If∗ spectral type for both stars. However, distin- double with nearly identical intensities on most of our spectra guishing an extreme Of star from a weak-lined WNL star G. Rauw et al.: WR 20a: a massive cornerstone binary system comprising two extreme early-type stars 3 is a very tricky issue. In this context, Crowther & Dessart (1998) proposed EW(He  λ 4686) = 12Å as a boundary be- tween O3If∗/WN and WN5-6 stars. For WR20a, we determine EW(He  λ 4686) = 13.1Å with a 1-σ dispersion of 1.3Å, right at the borderline proposed by Crowther & Dessart. Since the He  λ 4686 emission could contain a contribution from a wind interaction process, we consider this an upper limit on the actual strength of the line. Comparing the spectrum of WR20a obtained near conjunction with the spectra of several extreme Of and weak-lined WN stars, we find that the components of WR20a are indeed intermediate between Mk42 (O3If∗/WN) and HD97950C (= WR 47c, WN6ha). Finally, the spectrum of WR20a presented by Shara et al. (1991) indicates that the Hβ line is in weak emission. The latter feature favours a WN6ha spectral type. We shall thus adopt a WN6ha classification for both components of WR20a, though we cannot fully exclude an O3If∗/WN6ha type. The Shara et al. (1991) spectrum of WR20a suggested the presence of a C  λ 5808 emission with a normalized intensity stronger relative to He  λ 5876 than in normal WN stars, thus leading to the WN/WC classification proposed by Shara et al. (1999). Note that our blue/violet spec- tra do not provide support for such a hybrid type. Fig. 2. Top panel: power spectrum of our |RV1 − RV2| time se- ries of WR20a. The bottom panel provides the corresponding power spectral window. 3.2. Orbital solution We have measured the radial velocities (RVs) of the compo- nents of WR20a by fitting Gaussians. The typical uncertainties and Hδ lines respectively. Finally, we averaged the RVs of the on individual RV measurements are of order 10 – 15kms−1 for different lines listed above after subtracting their systemic ve- those phases when the components are resolved1. locities. The results are listed in Table 1. A major issue in determining the arises from Due to the ambiguity about the right alias in the peri- the fact that the spectral types of both stars are identical. In odogram, we caution that the orbital phases as well as the pri- fact, the spectra do not allow us to distinguish the components mary/secondary identifications in Table 1 are only valid if the a priori. Therefore, we performed a period search on the abso- orbital period is indeed 3.675days. Adopting this period, we lute values of the radial velocity differences |RV1 − RV2|. In this have derived the orbital solution of WR 20a using the technique way, we expect to find the highest peak in the periodogram at described by Sana et al. (2003). Since the RVs show no indica- ν1 = 2/Porb. Using the Fourier technique of Heck et al. (1985, tion of a significant , we assumed a circular see also Gosset et al. 2001), we find that the most likely value . The results are providedin Table2 and the RV curve is il- −1 of ν1 is 0.544±0.009day . However, there is a strong ambigu- lustrated in Fig. 3. We emphasize that orbital solutions derived −1 ity with the 1 − ν1 alias at 0.453 ± 0.009day which produces from the RVs of individual spectral lines overlap within the er- only a slightly smaller (formal) peak in the periodogram (see rors with the results in Table 2. Fig.2). Our preferred value for the orbital period is therefore The most important results are the large minimum masses 3.675 ± 0.030days, but we cannot discard the alternative value of 68.8 ± 3.8 and 70.7 ± 4.0M⊙ derived for the secondary and of 4.419 ± 0.044 days. Alternatively, if the (slightly smaller) primary component respectively. It should be emphasized that −1 peak at the 1 + ν1 alias (1.547day ) were to be selected, the these values strongly depend on the adopted orbital period. In orbital period would be ∼ 1.29days, i.e. even shorter than the fact, the alternative orbital period (4.419days) would lead to 1.64day period of CQCep, the shortest period WN binary sys- even larger minimum masses of 80.8 ± 5.1 and 84.0 ± 5.3M⊙ tem known to date (see e.g. van der Hucht 2001). respectively. If, on the contrary, the 1 + ν1 alias corresponding As a next step, we determined the apparent systemic veloc- to an orbital period of 1.293days were selected, the minimum ity of each spectral line from an orbital solution obtainedfor the masses would be much lower (24.9 ± 1.4 and 25.9 ± 1.4M⊙). RVs of the specific line. In this way, we found apparent sys- We note however, that in the latter case, the orbital solutionisof temic velocities of −75, −18, −77, −42, −51 and −26kms−1 significantly lower quality than the one obtained for an orbital for the Hγ, He  λ 4542, N  λ 4604, N  λ 4620, N  λ 4058 period of 3.675days.

1 Note that we also determined the heliocentric radial velocities of the sharp interstellar features: we obtain RVs of −16.6 ± 15.0, −7.8 ± 4. Discussion and future perspectives 10.5 and −10.2 ± 14.0 km s−1 for the He  λ 3889, Ca  λ 3934 and ∗ Ca  λ 3968 lines. The 1σ dispersions listed here provide some rough The WN6ha or O3If /WN6ha spectral type of the components indication of the accuracy of our wavelength calibration. of WR 20a is significantly earlier than those of the earliest 4 G. Rauw et al.: WR 20a: a massive cornerstone binary system comprising two extreme early-type stars

Table 1. Journal of our spectroscopic observations of WR 20a. Table 2. Orbital solution for WR 20a assuming a circular orbit The first and second columns provide the heliocentric Julian and an orbital period of 3.675days. T0 refers to the time of date (in the format HJD−2450000) and the orbital phase as- conjunction with the primary being behind. γ, K and a sin i suming a period of 3.675days and adopting phase φ = 0.0 as denote respectively the systemic velocity, the amplitude of the given in Table2. The third column indicates whether the blue radial velocity curve and the projected separation between the (B) or violet (V) spectral range was observed. The recentred centre of the star and the centre of mass of the binary system. and averaged primary and secondary radial velocities are given RRL stands for the radius of a sphere with a volume equal to in columns 4 and 5 respectively (see text). Finally, the last col- that of the computed according to the formula of umn yields the weight that we assigned to the various RV data Eggleton (1983). All error bars indicate 1-σ uncertainties. in the orbital solution. Primary Secondary

Date φ Set-up RV1 RV2 weight T0(HJD−2 450 000) 3043.480 ± 0.014 (km s−1) (km s−1) γ (km s−1) 18.8 ± 5.6 −3.5 ± 5.7 2354.560 0.542 B −11.7 −11.7 0.1 K (km s−1) 353.1 ± 8.6 362.6 ± 8.8 2355.600 0.825 V 290.7 −319.3 0.7 a sin i (R⊙) 25.6 ± 0.2 26.3 ± 0.6 3032.716 0.074 B −131.9 182.9 0.2 q = m1/m2 1.03 ± 0.03 3 3032.742 0.081 V 9.1 9.1 0.1 m sin i (M⊙) 70.7 ± 4.0 68.8 ± 3.8 3033.715 0.346 V −288.4 277.4 0.7 RRL sin i (R⊙) 19.8 ± 0.2 19.5 ± 0.2 3033.739 0.353 B −285.1 292.6 1.0 3034.715 0.618 V 235.8 −219.1 0.7 3034.741 0.625 B 287.6 −265.5 1.0 3035.719 0.892 V 269.3 −222.7 0.7 3035.746 0.899 B 245.2 −219.4 1.0 3036.719 0.164 V −259.7 287.0 0.7 3036.743 0.170 B −302.6 309.5 1.0 3037.738 0.441 V −78.3 −78.3 0.1 3037.762 0.447 B −74.0 −74.0 0.1 3038.723 0.709 V 363.5 −354.0 0.7 3038.746 0.715 B 374.6 −375.9 1.0 3039.718 0.980 V 12.4 12.4 0.1 3039.752 0.989 B −28.0 −28.0 0.1 3040.717 0.252 V −309.3 368.4 0.7 3040.741 0.258 B −336.1 394.0 1.0 3041.722 0.525 V 4.3 4.3 0.1 3041.747 0.532 B 22.9 22.9 0.1 3042.714 0.795 B 375.4 −374.4 0.2 members (O6:V – O7:V) of Westerlund2 analysed by Moffat Fig. 3. Radial velocity curve of WR20a for an orbital period et al. (1991). of 3.675days. The filled (resp. open) symbols stand for the Several authors have estimated the distance of primary (resp. secondary) RVs. The sizes of the symbols re- +1.2 Westerlund2: 7.9−1.0 kpc (Moffat et al. 1991); 5.7 ± 0.3kpc flect the relative weight assigned to the various points in the (Piatti et al. 1998); 6.4 ± 0.4kpc (Carraro & Munari 2004). orbital solution (see Table1). The solid line and the dashed line Adopting a visual brightness ratio of 1 together with the yield our best fit orbital solution assuming a circular orbit (see photometry of Moffat et al. (1991; V = 13.58, B − V = 1.51) Table 2). the various distance estimates yield absolute magnitudes MV in the range −5.0 to −6.1. This is fainter than the average of 2 three WN6ha stars in NGC3603, namely −6.9 to −7.7mag , angular proximity of WR20a to the cluster strongly suggest such that only the largest published distance of membership) or the photometry of Moffat et al. could be af- would be in marginal agreement with the photometry of fected by photometric . Concerning the last possibility, WR20a, allowing for a cosmic scatter of 0.5 – 0.7mag we note that given the extremely large minimum masses of on MV. This problem would be less severe if the absolute the components in WR20a, it appears very likely that the magnitudes of the components of WR20a were more typical system could display eclipses. A photometric monitoring of of O3I stars (MV = −5.9 according to Crowther & Dessart this binary is therefore of the utmost importance in order to 1998). Alternatively, WR20a could actually be unrelated to help derive its , to get rid of the aliasing Westerlund2 (though the rarity of early-type stars and the problem of the RV data and to constrain the orbital inclination. 2 We intend to organize such a campaign in the near future. Moffat et al. (1994) derived an average MV of −7.7 for the three H-rich WN stars in NGC 3603, whilst Crowther & Dessart (1998) An important issue in the context of the formation and the re-evaluated their distance and reddening and obtained an average of evolution of very massive stars is that of the mass of the most −6.9 mag. massive ones. Masses up to ∼ 200M⊙ have been inferred for G. Rauw et al.: WR 20a: a massive cornerstone binary system comprising two extreme early-type stars 5 the most luminous O2 stars from a comparison with evolution- ary tracks (Walborn et al. 2002). However, to date, none of the O2 stars is known to belong to a binary system that would allow to determine its mass in a less model dependent way. The most massive main-sequence stars in binary systems known so far were found in R136-38 (O3V + O6V) and R136-42 (O3V + O3V). Massey et al. (2002) determined masses of 57 and 40M⊙ respectively for the O3V primary stars of these systems. Another extremely massive object is the WN7ha primary in the 80-day period binary WR22 (Rauw et al. 1996; Schweickhardt et al.1999) with aminimummass of55M⊙. WR20a thusprob- ably consists of two of the most massive stars with a direct mass determination known so far. Therefore, WR20a is cer- tainly a cornerstone system for future investigations of massive star evolution. Finally, we note that the emission lines in the spectrum of WR 20a display strong phase-locked profile variability. This as- pect will be discussed along with an analysis of the spectrum with a model atmosphere code in a forthcoming paper.

Acknowledgements. The Li`ege team is greatly indebted to the Fonds National de la Recherche Scientifique (Belgium) for multiple as- sistance. This research is also supported in part by contract P5/36 “Pˆole d’Attraction Interuniversitaire” (Belgian Federal Science Policy Office) and through the PRODEX XMM-OM and INTEGRAL projects. The SIMBAD database has been consulted for the bibliog- raphy.

References Carraro, G., & Munari, U. 2004, MNRAS, 347, 625 Crowther, P.A., & Dessart, L. 1998, MNRAS, 296, 622 Driesens, L. 2003, Master thesis, University of Li`ege Eggleton, P.P. 1983, ApJ, 268, 368 Gosset, E., Royer, P., Rauw, G., Manfroid, J., & Vreux, J.-M. 2001, MNRAS, 327, 435 Heck, A., Manfroid, J., & Mersch, G. 1985, A&AS, 59, 63 Massey, P., Penny, L.R., & Vukovich, J. 2002, ApJ, 565, 982 Moffat, A.F.J., Shara, M.M., & Potter, M. 1991, AJ, 102, 642 Moffat, A.F.J., Drissen, L., & Shara, M.M. 1994, ApJ, 436, 183 Piatti, A.E., Bica, E., & Clari´a, J.J. 1998, A&AS, 127, 423 Rauw, G., Vreux, J.-M., Gosset, E., Hutsem´ekers, D., Magain, P., & Rochowicz, K. 1996, A&A, 306, 771 Sana, H., Hensberge, H., Rauw, G., & Gosset, E. 2003, A&A, 405, 1063 Schweickhardt, J., Schmutz, W., Stahl, O., Szeifert, T., & Wolf, B. 1999, A&A, 347, 127 Shara, M.M., Moffat, A.F.J., Smith, L.F., & Potter, M. 1991, AJ, 102, 716 Shara, M.M., Moffat, A.F.J., Smith, L.F., Niemela, V.S., Potter, M., & Lamontagne, R. 1999, AJ, 118, 390 Shulman, S., Bortolot, V.J., & Thaddeus, P. 1974, ApJ, 193, 97 Th´e, P.S. 1966, Contributions Bosscha Observatory, 35, 1 Vanbeveren, D., de Loore, C., & van Rensbergen, W. 1998, A&ARv, 9, 63 van der Hucht, K.A. 2001, New Astronomy Reviews, 45, 135 Walborn, N.R. 2001, in and Other Mysterious Stars, ASP Conf. Ser., 242, eds. T. Gull, S. Johansson, & K. Davidson, 217 Walborn, N.R., & Hesser, J.E. 1975, ApJ, 199, 535 Walborn, N.R., Howarth, I.D., Lennon, D.J., et al. 2002, AJ, 123, 2754