The Secret Lives of Cepheids: A Multi-Wavelength Study of the Atmospheres and Real-Time Evolution of Classical Cepheids

The primary goal of this study is to observe how complex the behaviors of Cepheids can be, and to show how the continued monitoring of Cepheids at multiple wavelengths can begin to reveal their "secret lives." We aim to achieve this through optical photometry, UV spectroscopy and X-ray imaging. Through Villanova's guaranteed access to ground-based telescopes, we have secured well-covered light curves as regularly as possible. Amplitudes and times of max brightness were obtained and compared to previous literature results. At UV wavelengths, we have secured hi-res spectra of 2 nearby Cepheids - delta Cep and beta Dor - with HST-COS. Also, we have obtained X-ray images of 5 Cepheids with XMM-Newton and the Chandra X-ray Observatory, and further observations with both satellites have been proposed for (XMM) and approved (Chandra). Optical photometry has shown that 8 of the 10 observed Cepheids have amplitude variability, or hints thereof, and all 10 show period variability (recent, long-term or possibly periodic). The UV spectra reveal emission lines from heated atmospheric plasmas of 10^4 - 10^5 K that vary in phase with the Cepheid pulsations. The X-ray images have detected the three nearest Cepheids observed (Polaris, delta Cep and beta Dor), while the distances of the other two place their fluxes likely at or below detector background levels. The X-ray fluxes for delta Cep show possible phased variability, but anti-correlated with the UV emission lines (i.e. high X-ray flux during low UV flux, and vice versa). Further data are required to ultimately confirm Blazhko-like cycles in Cepheids, X-ray variability with phase and the particulars of the high-energy variability such as phase-lags between atmospheric plasma emissions of different temperature and the exact contributions of the possible heating mechanism.