Abstract Details

Title: RCT photometry of the Hubble Classical Cepheid V19 in M33: Evidence for the Cessation of Pulsations - A Case of Stellar Evolution in Real Time

Authors: Scott G Engle, Edward F Guinan, Lucas Macri, Ann Pellerin

Institute/Affiliation: Villanova University and the RCT Consortium

Abstract Type: poster

Abstract Information:

We report on our continuing efforts to monitor the photometric behavior of Hubble\'s Variable Star V19 in the Triangulum Spiral Galaxy M33. B,V photometry has been carried out of this unusual 18th mag (previous) Cepheid with the 1.3-m RCT (Robotically Controlled Telescope) at KPNO. With time-series photometry, with a dedicated robotic telescope, we can hope to solve the mystery of V19 and its unprecedented evolutionary behavior.

In the influential work \"A Spiral Nebula as a Stellar System: Messier 33\" (Hubble 1926) Edwin Hubble determined the distance to M33 by using 35 Cepheids he discovered. One of those Cepheids was designated V19. At that time observations revealed V19 to have a 54.7-day period and B-band (converted from photographic magnitudes) light amplitude of 1.1-mag. Its mean B-magnitude was 19.6+/-0.2. V19 properties were consistent with the Period-Luminosity Law for M33 derived by Hubble at that time. Follow-up observations in 1996-1997 as part of the DIRECT Program (Macri et al. 2001), however, revealed large and surprising changes in the properties of V19. Its mean B-magnitude had risen to 19.05+/-0.05 and its amplitude had decreased to less than 0.1-mag. The DIRECT study thoroughly checked for possible misclassifications of the variable or contamination by nearby objects, and found none. For all intents and purposes, V19 is no longer a Classical Cepheid, or at least varying below the detectable levels of the photometry. The only other well-documented instance of Cepheid pulsations declining over time is in the case of Polaris - whose V-band amplitude decreased from just over 0.1-mag to below 0.03-mag over the course of a century (Engle et al 2004). Also, a study of the visual magnitudes of Polaris over the past two millennia has shown a possible increase in brightness of 1-mag over the past 1000 years. The changes observed for V19 are obviously on a much more dramatic scale. We discuss the properties of this unusual (former) Cepheid and discuss possible reasons for its apparent fast evolutionary changes. In this study we have combined the recent RCT CCD photometry with unpublished photometry secured a few years ago with the WIYN 3.5-m telescope as well as using the available photometry back to Hubble\'s original observations carried out during the mid-1920s. These data have been subjected to various period search routines to identify possible periods. We also discuss some possible scenarios to explain the strange behavior of the star.

We gratefully acknowledge support from a NASA/HST grant and NSF/RUI grant AST-1009903. The RCT is operated by the RCT Consortium - a group of universities and research institutions who have assumed control of the 1.3-m (50-inch) telescope on Kitt Peak, Arizona. The Consortium members are Western Kentucky University, the Planetary Science Institute, South Carolina State University, and Villanova University.