957 Publications of the Astronomical Society of the Pacific, 113:957–963, 2001 August  2001. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. The Orbit and Spectral Line Variations of the Massive Binary HD 163181 (V453 Scorpii) T. S. Josephs, D. R. Gies, 1,2 W. G. Bagnuolo, Jr., 1 and M. A. Shure Center for High Angular Resolution Astronomy, Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303; tammy@chara.gsu.edu, gies@chara.gsu.edu, bagnuolo@chara.gsu.edu, shure@chara.gsu.edu L. R. Penny College of Charleston, Physics Department, Charleston, SC 29424; pennyl@cofc.edu and Z. Wang Center for Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Building 37, Cambridge, MA 02139; wangzx@space.mit.edu Received 2001 February 16; accepted 2001 April 4 ABSTRACT. We present revised orbital elements for the massive interacting binary HD 163181, and we offer the first evidence of apsidal motion in the orbit (apsidal period yr). We searched for but found no U p 510  100 obvious evidence of the secondary’s spectral lines in our optical spectra. We do observe, however, systematic profile variations with orbital phase that probably result from a gas stream from the Roche lobe–filling supergiant to the faint secondary. The relatively small projected rotational velocity of the primary ( km s -1 ), the V sin i p 88  3 faintness of the secondary, the reported lack of orbital motion in the Ha emission, and the presence of an IR excess all suggest that the system is surrounded by an extensive circumbinary disk (as is found in a similar binary, RY Scuti). 1. INTRODUCTION The system HD 163181 (V453 Sco, , B- V p 6.52 V p ) is a massive binary in an advanced stage of evolution. 0.50 Walborn (1972) first pointed out the excessive strength of the nitrogen lines in the optical spectrum of the primary (classified as BN0.5 Iae), and Kane, McKeith, & Dufton (1981) showed that the enhanced abundance of nitrogen and reduced abun- dances of oxygen and carbon are compatible with those of CNO-processed material. Kane et al. (1981) suggest that some 50% of the primary’s original mass was removed by mass loss or mass transfer to reveal this nuclear processed gas. This gas was probably lost during a prior contact binary phase from which the system has now emerged (Vanbeveren, Van Rens- bergen, & De Loore 1998). Sahade & Friebose-Conde (1965) presented a seminal paper on the orbital elements ( days), and they argued P p 12.0058 that the binary has a small but nonzero eccentricity (e p ). They also observed a weak, narrow component of Ha 0.08 emission that showed antiphase Doppler shifts, and they (as well as earlier authors cited) used this feature to estimate the orbital motion of the companion. This identification led to mass estimates of approximately 27 and for the primary (de- 30 M , fined as the more luminous, larger star) and secondary, respec- 1 Guest Observer, International Ultraviolet Explorer Observatory. 2 Guest Observer, Mount Stromlo and Siding Springs Observatories. tively. A first identification of absorption lines of the secondary was made by Hutchings (1975), who found weak extensions in the wings of several lines at the quadrature phases. Hutchings (1975) used radial velocity measurements of these secondary features to determine a mass ratio and in- q p M /M p 1.7 s p dividual masses of and . This mass M /M p 13 M /M p 22 p , s , ratio was confirmed by Woodward & Koch (1975) in a study of the b Lyrae–like light curve (due mainly to the tidal dis- tortion of the primary; see also Madore 1975). Speckle inter- ferometric observations by Hartkopf et al. (2000) show no evidence of other close astrometric components in the range 35–1000 mas that might confound spectroscopic and photo- metric analyses. The orbital period, light curve, evidence of circumstellar gas, and faintness of the mass gainer all suggest that HD 163181 is related to the W Serpentis class of massive interacting binaries (Tarasov 2000). The system has received little attention from observers since the work of Hutchings (1975), which is unfortunate because this binary is probably representative of the poorly understood stage of active mass transfer in massive binaries. Here we present a new investigation based on radial velocity measure- ments from the International Ultraviolet Explorer (IUE) and from optical spectra obtained at Mount Stromlo Observatory (MSO) that provide the first evidence of apsidal motion in this binary (potentially an important probe of the interior of the mass donor; Monet 1980). We also report on an unsuccessful search to detect the secondary’s lines in the optical spectra that