A High Angular Resolution Survey of Massive Stars in Cygnus OB2:
Results from the Hubble Space Telescope Fine Guidance Sensors
S. M. Caballero-Nieves (1,10), E. P. Nelan (2), D. R. Gies (1), D. J. Wallace (3), K. DeGioia-Eastwood (4), A. Herrero (5,6), W.-C. Jao (1), B. D. Mason (7), P. Massey (8), A. F. J. Moffat (9), and N. R. Walborn (2)
(1) Center for High Angular Resolution Astronomy, Department of Physics and Astronomy, Georgia State University, P.O. Box 4106, Atlanta, GA 30302-4106, USA
(2) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
(3) Department of Natural Sciences, University of South Carolina Beaufort, 1 University Boulevard, Bluffton, SC 29909, USA
(4) Department of Physics and Astronomy, Northern Arizona University, P.O. Box 6010, Flagstaff, AZ 86011-6010, USA
(5) Instituto de Astrofísica de Canarias, C/ Via Lactea s/n, E-38280 La Laguna, Spain
(6) Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, 2, E-38205 La Laguna, Spain
(7) U. S. Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420, USA
(8) Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
(9) Département de physique, Université de Montréal, CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada
(10) Current Address: Department of Physics and Astronomy University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK.
We present results of a high angular resolution survey of massive OB stars in the Cygnus OB2 association that we conducted with the fine guidance sensor 1R (FGS1r) on the Hubble Space Telescope. FGS1r is able to resolve binary systems with a magnitude difference delta-V < 4 down to separations as small as 0.''01. The sample includes 58 of the brighter members of Cyg OB2, one of the closest examples of an environment containing a large number of very young and massive stars. We resolved binary companions for 12 targets and confirmed the triple nature of one other target, and we offer evidence of marginally resolved companions for two additional stars. We confirm the binary nature of 11 of these systems from complementary adaptive optics imaging observations. The overall binary frequency in our study is 22% to 26% corresponding to orbital periods ranging from 20 to 20,000 yr. When combined with the known short-period spectroscopic binaries, the results support the hypothesis that the binary fraction among massive stars is >60%. One of the new discoveries is a companion to the hyper giant star MT 304 = Cyg OB2-12, and future measurements of orbital motion should provide mass estimates for this very luminous star.
Reference: AJ, 147, 40
Status: Manuscript has been accepted