The Astrophysical Journal, 611:L33–L36, 2004 August 10 ൴ ᭧ 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A. WR 20a IS AN ECLIPSING BINARY: ACCURATE DETERMINATION OF PARAMETERS FOR AN EXTREMELY MASSIVE WOLF-RAYET SYSTEM1 A. Z. Bonanos and K. Z. Stanek Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; [email protected], [email protected] and A. Udalski, L. Wyrzykowski,2 K. Z˙ ebrun´ , M. Kubiak, M. K. Szyman´ ski, O. Szewczyk, G. Pietrzyn´ ski,3 and I. Soszyn´ ski Warsaw University Observatory, Al. Ujazdowskie 4, PL-00-478 Warsaw, Poland; [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] Received 2004 May 18; accepted 2004 June 24; published 2004 July 8 ABSTRACT We present a high-precision I-band light curve for the Wolf-Rayet binary WR 20a, obtained as a subproject of the Optical Gravitational Lensing Experiment. Rauw et al. have recently presented spectroscopy for this system, M, for the component stars 3.8 ע and 68.8 4.0 ע strongly suggesting extremely large minimum masses of70.7 of the system, with the exact values depending strongly on the period of the system. We detect deep eclipses of about 0.4 mag in the light curve of WR 20a, confirming and refining the suspected period ofP p 3.686 days and 2Њ.0 . Using these photometric data and the radial velocity data of Rauw ע deriving an inclination angle ofi p 74Њ.5 ,M, . Therefore 5.0 ע and 82.0 5.0 ע et al., we derive the masses for the two components of WR 20a to be83.0 WR 20a is confirmed to consist of two extremely massive stars and to be the most massive binary known with an accurate mass determination. Subject headings: binaries: eclipsing — binaries: spectroscopic — stars: fundamental parameters — stars: individual (WR 20a) — stars: Wolf-Rayet Online material: machine-readable table 1. INTRODUCTION 7.3 M, (Rauw et al. 1996; Schweickhardt et al. 1999), and Plaskett’s star with a minimum primary mass of 51 M, (Bag- Measuring accurate masses for the most massive stars in our nuolo et al. 1992). However, WR 20a, a Wolf-Rayet (W-R) Galaxy and beyond is important for constraining star formation binary in the compact cluster Westerlund 2, seems to be the and stellar evolution theories, which have indirect implications new record holder. Rauw et al. (2004) obtained spectroscopy for many objects that are not well understood, such as super- for WR 20a and measured extremely large minimum masses ע ע novae, gamma-ray bursts, and Population III stars. The most of70.7 4.0 and 68.8 3.8 M, for the components. The massive candidates in the Milky Way are LBV 1806Ϫ20 (Ei- final masses strongly depend on both the period and the in- kenberry et al. 2004) and the Pistol Star (Figer et al. 1998), clination of the binary, which can only be measured from the light curve. They derived a period of 3.675 days from the which have inferred masses up to ∼200 M,. HDE 269810 in absolute values of the radial velocity differences and assumed the LMC is another massive candidate ∼150 M, (Walborn et al. 2004). Finally, h Carinae, one of the most studied massive a circular orbit to derive the masses of the components. In their discussion of WR 20a, Rauw et al. (2004) stressed the impor- stars, has an estimated mass of ∼120 M,, yet it remains un- known whether it is a rapid rotator or a close binary system tance and necessity of photometric monitoring of this binary. (see review by Davidson & Humphreys 1997). However, all Shara et al. (1991) were the first to make the discovery that these masses are only indirect estimates and thus have large WR 20a was a W-R star, by obtaining spectroscopy of the uncertainties associated with them. The only direct way of system. Moffat et al. (1991) obtained UBV photometry; how- measuring accurate masses of distant stars is through double- ever, as Rauw et al. (2004) suggest, it is likely affected by lined spectroscopic binary systems with eclipses present in their photometric eclipses. WR 20a was classified as a candidate light curves. The limited number of existing mass measure- binary, along with 30 other W-R stars in the seventh catalogue ments for massive stars (120 M,) can be explained by their of galactic Wolf-Rayet stars (van der Hucht 2001), because of intrinsic rarity and the observationally demanding process of its comparatively weak emission lines, which were thought to discovering them in eclipsing binary systems. be diluted by the continuum light of an OB-type companion. Until recently, the most massive stars ever weighed in bi- Rauw et al. (2004) spectroscopically surveyed these candidate naries were R136-38 (O3 VϩO6 V) in the LMC with a primary W-R binaries, and WR 20a clearly emerged as an interesting M, (Massey et al. 2002), WR 22 object, which they followed up. Due to its favorable location 0.6 ע mass of 56.9 in the sky, this object was included in the target list covered ע WN7ϩabsϩO) with a minimum primary mass of 55.3) by the Optical Gravitational Lensing Experiment (OGLE) 1 Based on observations obtained with the 1.3 m Warsaw telescope at Las photometric survey. We have quickly obtained a precise light Campanas Observatory, which is operated by the Carnegie Institute of curve of WR 20a and thus confirm and refine, by measuring Washington. the period and inclination, the remarkable masses of the WR 2 School of Physics and Astronomy and Wise Observatory, Tel Aviv Uni- versity, Tel Aviv 69978, Israel. 20a components. 3 Universidad de Concepcio´n, Departamento de Fı´sica, Casilla 160-C, Con- In this Letter, we present photometry for WR 20a, which cepcio´n, Chile. clearly shows the presence of eclipses. In§2wedescribe the L33 L34 BONANOS ET AL. Vol. 611 TABLE 1 TABLE 2 I-Band Photometry of WR 20a Light-Curve Parameters for WR 20a UT HJD I jI Parameter Value days 0.01 ע May 1 ...... 2,453,126.57440 10.932 0.01 Period, P .......................... 3.686 2004 2,453,126.57581 10.931 0.01 Primary eclipse, Tprim ............. 2453124.569 2Њ.0 ע Inclination, i ...................... 74Њ.5 0.01 10.920 2,453,126.58207 2,453,126.58350 10.925 0.01 Eccentricity, e .................... 0 (fixed) 2004 May 2 ...... 2,453,127.57488 10.657 0.01 Effective temperature, Teff1 ....... 42,000 K (fixed) ע 2,453,127.57883 10.658 0.01 Effective temperature, Teff2 ....... 40,300 1000 K ע p Surface potential (Q12Q ) ...... 3.92 0.03 ע -Note.—Table 1 is published in its entirety in the elec Radius, rpole ....................... 18.7 0.3 R, ע tronic edition of the Astrophysical Journal. A portion is Radius, rpoint ....................... 22.0 0.3 R, ע .shown here for guidance regarding its form and content Radius, rside ....................... 19.3 0.3 R, ע Radius, rback ....................... 20.4 0.3 R, observations, in§3wepresent the light curve and the analysis, and in § 4 we discuss the results. thus confirming and refining the orbital period of WR 20a and 2. OGLE OBSERVATIONS the remarkable masses of its components. WR 20a (pSMSP 2 p THA 35-II-036) is located in the In addition, we can derive an inclination angle i for the constellation Carina ata p 10hms 23 58.0 , d p Ϫ57Њ45 49 system from our well-sampled light curve. To first order, the (J2000.0). Observations were carried out with the 1.3 m Warsaw value of the inclination angle is given by the depth of the ∼ telescope at Las Campanas Observatory, Chile, which is operated eclipses ( 0.4 mag), and we find that the exact value of i only by the Carnegie Institute of Washington. The telescope is weakly depends on the details of the model fit or even on which equipped with a mosaic CCD camera with8192 # 8192 pixels. eclipsing binary model we use to fit. To demonstrate this, we WR 20a is very bright,I ∼ 11.0 , so very short exposures of first fitted the light curve with a simple model of two spherical 10 s each were used to avoid saturating the CCD. The obser- stars with limb darkening (J. Devor 2004, private communi- vations presented here started on 2004 May 1 and consist of 83 cation), with the more complex Eclipsing Binary Orbit Program measurements obtained on 17 consecutive nights. (EBOP) model (Nelson & Davis 1972; Popper & Etzel 1981), The absolute calibration of our photometry was obtained on and finally with the Wilson-Devinney (WD) code (Wilson & one photometric night with a couple of standards from Landolt Devinney 1971; Wilson 1979; van Hamme & Wilson 2003) (1992). The uncertainty in the zero points of the photometry for modeling distorted stars. We find that indeed the inclination is about 0.03 mag. angle is insensitive to the model, and the best-fit value with Њ ע p Њ The data were reduced using the standard OGLE-III data the WD code isi 74.5 2.0 . pipeline, as described by Udalski (2003). The photometry was We ran the WD code in the overcontact mode (mode 3), fixing p derived using difference imaging analysis. This method is the Teff1 42,000 K (since WR 20a is intermediate between Mrk current state of the art for photometric accuracy in crowded 42 and WR 47c; see Rauw et al.