Interacting OB star binaries: LZ Cep, SZ Cam and IU Aur

Abstract

New spectroscopic observations of three O+O binary stars (LZ Cep, SZ Cam and IU Aur), obtained over a number of years at the Dominion Astrophysical Observatory, are analysed using state-of-the-art cross-correlation techniques to produce radial velocity curves. These results are combined with new (LZ Cep) and published (SZ Cam and IU Aur) photometry to yield masses, temperatures, radii and luminosities. We find that LZ Cep is a semi-detached system, in agreement with the results of Howarth et al., with the secondary star filling its Roche lobe. We find a distance modulus of 9.6 ± 0.2, which is slightly smaller than the distance modulus to Cep OB2 (= 9.9).The reddening of LZ Cep is also at the lower end of the cluster range, suggesting that the binary is located on the near side of the cluster. SZ Cam is a triple system, and we are able to measure the radial velocities of the three components that are observed in the absorption lines. A light-curve solution employing our new mass ratio shows that the short-period binary is detached. The third body emits about 40 per cent of the radiation from the system, and is probably a binary itself. The distance modulus for SZ Cam (= 9.88) is in good agreement with that of its parent cluster NGC 1502 (= 9.71). IU Aur is another triple system, although we are unable to find spectroscopic evidence for the tertiary. The close binary is semi-detached, with the secondary filling its Roche lobe. The distance modulus of the system is 11.0, which is in approximate agreement with that of the Aur OB1 association (= 10.6 ± 0.2). We note that the current models for interacting massive binaries all assume case B mass transfer, in which the mass-loser becomes a Wolf—Rayet star. Case A mass-transfer models (in which the mass-loser fills its Roche lobe while still on the main sequence) are required in order to see whether or not they can explain the observed properties of these interacting systems.