The Secret Lives of Cepheids: Evolutionary Changes and Pulsation-Induced Shock Heating in the Prototype Classical Cepheid {\delta} Cep
Scott G. Engle, Edward F. Guinan, Graham M. Harper, Hilding R., Neilson, Nancy Remage Evans

TL;DR
This study investigates the complex atmospheric behaviors of the prototype Cepheid delta Cep, revealing pulsation-induced shock heating, gradual amplitude increase, and variable high-energy emissions, advancing understanding of Cepheid evolution and atmospheres.
Contribution
First comprehensive analysis combining photometry, UV, and X-ray data to reveal dynamic atmospheric processes and evolutionary changes in delta Cep.
Findings
Detected a small period decrease of ~0.1 sec per year.
Observed a gradual increase in light amplitude over 50 years.
Found delta Cep to be a variable X-ray and UV source with pulsation phase correlation.
Abstract
Over the past decade, the Secret Lives of Cepheids (SLiC) program has been carried out at Villanova University to study aspects and behaviors of classical Cepheids that are still not well-understood. In this, the first of several planned papers on program Cepheids, we report the current results for delta Cep, the Cepheid prototype. Ongoing photometry has been obtained to search for changes in the pulsation period, light curve morphology and amplitude. Combining our photometry with the times of maximum light compilation by Berdnikov 2000 returns a small period change of dP/dt ~ -0.1006 +/- 0.0002 sec yr^-1. There is also evidence for a gradual light amplitude increase of ~0.011-mag (V-band) and ~0.012-mag (B-band) per decade over the last ~50 years. In addition, HST-COS UV spectrophotometry and XMM-Newton X-ray data were carried out to investigate the high-temperature plasmas present…
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