Helen Sawyer Hogg

VARIABLE STARS IN GALACTIC GLOBULAR CLUSTERS

Based on a search of the literature up to 2001 May, the number of known variable stars in Galactic globular clusters is approximately 3000. Of these, more than 2200 have known periods and the majority (approximately 1800) are of the RR Lyrae type. In addition to the RR Lyrae population, there are approximately 100 eclipsing binaries, 120 SX Phoenicis variables, 60 Cepheids (including Population II Cepheids, anomalous Cepheids and RV Tauri), and 120 SR/red variables. The mean period of the fundamental mode RR Lyrae variables is 0.585 days, for the overtone variables it is 0.342 days (0.349 days for the first-overtone pulsators and 0.296 days for the second-overtone pulsators) and approximately 30% are overtone pulsators. These numbers indicate that about 65% of RR Lyrae variables in Galactic globular clusters belong to Oosterhoff type I systems. The mean period of the RR Lyrae variables in the Oosterhoff type I clusters seems to be correlated with metal abundance in the sense that the periods are longer in the more metal poor clusters. Such a correlation does not exist for the Oosterhoff type II clusters. Most of the Cepheids are in clusters with blue horizontal branches.

Long-term behavior of the population II Cepheid V1 in the globular cluster Messier 12

Observations made over an interval of 73 yr have been used to study the period changes of the W Virginis star in M12. They show that the period has undergone a series of abrupt changes, both increasing and decreasing, instead of a smooth change as predicted by evolutionary theory. This result is compared with period changes of W Virginis and RV Tauri stars in other galactic globular clusters, and it is found that the long-term behavior of the periods of most of these stars is similar to that of the variable in M12. However, two stars, V42 in M5 and V1 in Omega Centauri, may be in their final blueward evolutionary phase. 45 references.

First optical candidate for a recovered classical nova in a globular cluster - Nova 1938 in M14

U, B, V, R, and H-alpha CCD frames of the field of the nova which appeared in the globular cluster M14 in 1938 have been compared with the nova discovery images. On the basis of positional coincidence, brightness, and blue color, a candidate nova was identified and its right ascension and declination to within 1 arcsec each. Confirmation of the candidate and detailed study of the quiescent nova will probably require Hubble Space Telescope observations.

The LongTerm Behaviour of Two Pulsating Variables in M56

Of twelve variables known in Messier 56 in Lyra, two, V1 and V6 are the subject of this paper. VI was discovered by Shapley (1920) and first determined by Sawyer Hogg (1942) to be a Cepheid with a period of 1.5 days. V6 was discovered by Sawyer Hogg (1940) from her early plates at the David Dunlap Observatory. Later Sawyer Hogg (1949) showed it to be an RV Tauri type with a period of 90.02 days, one of the first such to be identified in a globular cluster. A.H.Joy (1949) determined spectral type and radial velocity for both of these variables.

Variables in Globular Clusters, the Common and the Rare

The title of this talk is really just a different phrasing from one I have used at several IAU meetings on the subject of numbers and kinds of variables in globular clusters. To furnish this material, I have finished the Third Catalogue of Variables in Globular Clusters. Since many of you are coming to this Colloquium with new information, the Catalogue is in draft form with a request that corrections and additions be given me by October 2, after which the draft will go to the printer.