Villanova University Astronomers pursue the strange case of Hubble’s 19th Variable Star in M33: A Cepheid that inexplicably stopped pulsating

Cepheid Variable stars play one of the most crucial roles in Astronomy – measuring the scale of the Universe. They do this by way of radial pulsations – where the entire star expands and contracts with a specific period – and once this period is determined, it relates to the luminosity of the Cepheid which then gives the Cepheid’s distance. This Period-Luminosity Law, recently renamed the Leavitt Law in honor of Henrietta Leavitt who discovered the Law over a century ago, has long constituted the backbone of the Cosmic Distance Ladder.

In his 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. Observations revealed V19 to have a 54.7-day pulsation period and B-band (converted from photographic blue magnitudes) light amplitude of 1.1-mag. Its mean B-magnitude was 19.59±0.23 and its 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-mag brightness had increased by 1.6x to B =19.08 ±0.05 and its amplitude had fallen to < 0.1-mag (< 10%). The Macri study thoroughly checked for possible misclassifications of the variable or contamination by nearby objects, and found none. For all intents and purposes, V19 was no longer a Classical Cepheid, or at least it was varying below the detectable levels of the photometry.

Now I am, admittedly, a ‘Cepheid person’…” begins Engle, “but, in my opinion, for a member of a well-known class of variable stars – whose long-term pulsational stability is a crucial element in one of Astronomy’s most fundamental laws – to undergo such a drastic change on a human timescale is an amazing  thing.

The only other well-documented instance of Cepheid pulsations declining over time is in the case of Polaris - whose V-band (visual brightness) amplitude fell from just over 0.1-mag to below 0.03-mag during the course of a century (Engle et al. 2004). Also, a study of Polaris’ visual magnitudes over the past two millennia has shown a possible increase in brightness of 1-mag over the past 1000 years. The amplitude changes present in V19 are obviously on a much more dramatic scale.

A very interesting aspect of the study” adds Guinan, “is how this Cepheid in another galaxy relates to Polaris, which is the nearest Cepheid to us. Both stars are experiencing unexpectedly fast and large changes in their pulsation properties and brightness that are not yet explained  by theory. Our observations indicate that there is still a lot to learn about this important class of pusating variable stars”

We report on our continuing efforts to monitor the behavior and properties of Hubble’s V19 in M33. Photometry reported here has been carried out with the WIYN 3.5-m telescope and during 2010/11 by the 1.3-m (50-inch) RCT (Robotically Controlled Telescope). 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. Consortium members are Western Kentucky University, the Planetary Science Institute, South Carolina State University, and Villanova University. The telescope, originally called the Remotely Controlled Telescope, has been renamed the Robotically Controlled Telescope to reflect the change in operational control and mode of use. Our continued photometry with the RCT has confirmed the earlier findings that the Cepheid’s amplitude has indeed fallen to below 0.1-mag, and has possibly even ceased.  However, there is evidence of low-amplitude variability in V19 and, as always, more data is needed and it is our hope that future high-precision observations may solve the mystery of V19 and its unprecedented evolutionary behavior.

V19 remains a strange case. We have found some hints that the Cepheid pulsations may still persist, albeit at a very low amplitude, but the faintness of the star and the resulting photometric errors in the observations are preventing conclusive evidence from being found. If the star continues to pulsate, but with an amplitude similar to that of Polaris, our current observations would not be precise enough to observe it. Our current plans for the future are:

-          Continue to carry out photometry

-          Increase the precision of the measures. (increased exposure times, larger instruments)

-          Try to obtain spectroscopy of V19 to confirm its stellar properties

We gratefully acknowledge support from NASA/HST and NSF/RUI grants.

References

-          Engle, S. G., Guinan, E. F. & Koch, R. H. (2004), BAAS, 36, 744

-          Macri, L. M., Stanek, K. Z., Sasselov, D. D., Krockenberger, M. & Kaluzny, J. (2001), AJ, 121, 870

-          Macri, L. M., Sasselov, D. D. & Stanek, K. Z. (2001), ApJ, 550, L159

-          Hubble, E. P. (1926), ApJ, 63, 236