The Cygnus OB2 Radial Velocity Survey: Solutions of Four More Systems

Over the summer of 2013, six undergraduate students from around the country, including myself, participated in an REU at the University of Wyoming, where we contributed to the orbital solutions of 22 binary systems in the Cyg OB2 Association. This is part of the larger Cygnus OB2 Radial Velocity Survey, which has been conducted intermittently by Henry Kobulnicky, the project head, for the past fourteen years. So far it has uncovered 47 binary systems out of the 143 known massive star systems in the association, and is the most complete survey of a massive star association yet done. Radial velocity measurements were done over the summer on the He 5876 line using a spectrograph at the 2.3 m telescope at WIRO, the Wyoming InfraRed Observatory. The four systems I solved, MT202, MT295, MT455, and MT555, are all low-amplitude, with MT202 having the highest velocity semi-amplitude of 21 km/s, and the others having semi-amplitudes under 10 km/s. Three of the systems are short-period, with periods of 2.60077, 2.0900, and 4.1797 days for MT202, 295, and 455 respectively, while MT555 has a much longer period of 279.467 days. MT202 and MT295 are both B2V stars, and MT455 and MT555 are O8V stars. There is difficulty in constraining the masses and spectral types of the secondary stars due to the lack of information about the inclination of the systems, so lower limits for the masses of the secondaries are typically around 0.5 solar masses, whereas upper limits for the mass are the masses of the primaries. This work is supported by the National Science Foundation under REU grant AST 1063146.