Radial Velocity Studies of Galactic Cepheids

Nancy Evans, Center for Astrophysics, Harvard University


September 1999

This seemed like an appropriate time to summarize the results of the Cepheid radial velocity studies which I have been carrying out at DDO for nearly 30 years now. The main thrust of the program has been radial velocity measurements of Cepheids, particularly binaries. This has been combined with satellite (IUE and HST) velocity work to measure Cepheid masses. These are still the best fiducial for testing evolutionary tracks of evolved objects, as well as quantitative measurements of primary distance indicators. In addition the program has produced a definitive distribution of mass ratios in intermediate mass binaries, important information on star formation. Finally the combined ground-based and satellite studies leave us poised to make use of newly resolved binary systems. The one program which I am currently working on is to try to measure the velocities of the bright companions of 3 Cepheids near 4000 A. This is particularly well-suited to DDO, since it requires spectral subtraction. If we (Evans, Vinko, Kiss, and Beattie) succeed - and I did many years ago with SU Cyg--three systems will be strong candidates for both resolution and HST velocity work. The program started in the 1970's and is still continuing. In 1989, a new mode of service observing has been started when I was associated with the Institute for Space and Terrestrial Science in Toronto. The first papers from this mode started appearing in 1994; since then we have had 6 papers in refereed journals and 9 papers in conference proceedings. The following persons contibuted to the program: Tom Bolton, John Percy, Ben Sugars, Jozsef Vinko, Ron Lyons, Jim Thomson, Irina Dashevsky and Andrzej Udalski. Below I include a list of publications, those either containing DDO data, or very directly following from DDO results such as orbits. (A couple of Southern stars are thrown in because they complete the Cepheid information.) DDO has been a wonderful facility to work on the multi-year orbits of Cepheids, and without the orbits, the breakthrough satellite results (double-lined spectroscopic binaries) would not have been possible. Nancy