Andrew Davis and Dr. Eric Hintz, Physics and Astronomy
The old open cluster NGC 188 is unique because of its large number of W UMa type contact binaries. This allows astronomers to use these stars to refine models of the origin and evolution of these systems. Also because of its age, NGC 188 may display a wide range of contact binary systems, from the W UMa binaries to the highly evolved FK Comae type star in which the two stellar cores have coalesced. It was our goal to find and classify new variable stars in this cluster.
NGC 188 was observed on 5 nights during the summer of 2004. Two nights of data taken in 1994 by Dr. Hintz and Prof. Michael Joner were also used. Nine stars were observed to be variables, eight of which were previously identified in the literature as variables. All except V372 Cep are W UMa type variables; V372 is probably an RS CVn variable. Of these eight, we were able to obtain newly determined periods for three stars: ER Cep, ES Cep, and V13. New periods for all nine variable stars are presented in Table 1.
One new variable star was discovered, temporarily labeled as Var 1. This star is probably a δ Scuti variable star, based on its V-R color index, its location as a blue straggler above the turnoff point of the cluster on the HR diagram, and the shape of its light curve. The period found was 0.111721 days. It has a V-R index of 0.1137, making it a late A-type star, which lies in the instability strip. More observations of this star are necessary to obtain better color indices and to obtain its absolute magnitude. Also, spectral data should be taken to identify its spectral class, and to determine if it is a cluster member. A phase plot of this new variable star is shown in Figure 1.
Zhang et al. (2002) identified V13 as a W UMa binary system, but they also mentioned that it could be a pulsating variable due to its sharp minima and maxima. I have phased the data using their published period of 0.3606 days, as well as a period of 0.129233 days, which was obtained using Period98 and phases the data much better. From the phased data using the new period, it is possible that this star is a pulsating variable with multiple periods. However, on the HR diagram the star is located below the turnoff point, and hence below the instability strip. Platais et al. (2003) give a 98 % probability of cluster membership for this star. This star should be observed further to better determine the period.
For ER Cep, the two nights in 1994 phased well with each other using the published period of 0.28574 days. However, data taken on July 3, 2004 and August 10, 2004 showed that the star is obviously variable, but did not phase with the old data. The data were phased with the new period of 0.2857377 days, which show a better fit of the data. ES Cep also experienced the same problem, and was phased with a new period of 0.3424571 days. Due to the longer baseline of this survey, we were able to obtain a more precise period for these two systems. For the other previously known variable stars, no new periods were able to be determined.
I would like to thank the ORCA office for funding this work, as well as support from BYU’s Research Experience for Undergraduates. Special thanks to Elizabeth Jefferies for her help in data analysis; Jen Pollack, Tabitha Bush, and Kathleen Moncreiff for their help in observing during long summer nights.
References:
- Kafka, S., and Honeycutt, R. K., 2003, Astrophysical Journal, 126, 276
- Platais, I., et al., 2003, Astrophysical Journal, 126, 2922
- Zhang, X. B., et al. 2002, Astrophysical Journal, 123, 1548