The Secret Lives of Cepheids: Evolutionary Changes and Pulsation-Induced Shock Heating in the Prototype Classical Cepheid {\delta} Cep

摘要

Over the past decade, the Secret Lives of Cepheids (SLiC) program has beencarried out at Villanova University to study aspects and behaviors of classicalCepheids that are still not well-understood. In this, the first of severalplanned papers on program Cepheids, we report the current results for deltaCep, the Cepheid prototype. Ongoing photometry has been obtained to search forchanges in the pulsation period, light curve morphology and amplitude.Combining our photometry with the times of maximum light compilation byBerdnikov 2000 returns a small period change of dP/dt ~ -0.1006 +/- 0.0002 secyr^-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 werecarried out to investigate the high-temperature plasmas present above theCepheid photospheres. In total, from the five visits (eight exposures) withXMM-Newton, delta Cep is found to be a soft X-ray source (Lx [0.3--2 keV] ~4.5-13x10^28 erg sec^-1) with peak flux at kT = 0.6-0.9 keV. The X-ray activityis found to vary, possibly in phase with the stellar pulsations. From2010-2013, nine observations of delta Cep were carried out with HST-COS. The UVemissions are also variable, and well-phased with the stellar pulsations.Maximum UV line emissions occur near, or slightly before, maximum opticallight, varying by as much as twenty times. This variability shows thatpulsation-induced shock-heating plays a significant role in Cepheidatmospheres, possibly in addition to a quiescent, magnetic heating. The resultsof this study show Cepheid atmospheres to be rather complex and dynamic.