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

Classical Cepheids are variable, yellow supergiants that undergo radial pulsations primarily arising from opacity variations in their stellar interiors. Over a century ago, the discovery of a reliable relationship between the period of a Cepheid's pulsations and its luminosity made them "standard candles" and, as such, interest in studying Cepheids boomed. The generally held belief that their pulsations are essentially static over human timescales has sadly led to a narrowing in the field of Cepheid studies. This is in addition to the widespread adoption of high-sensitivity CCD instruments that can quickly saturate when observing nearby Cepheids. The result is that many of the brightest Cepheids, with the longest observational histories, have recently stopped being systematically observed. The primary overall 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 goal through optical photometry, UV spectroscopy and X-ray imaging. Through Villanova University's guaranteed access to ground-based photometric telescopes, we have endeavored to secure well-covered light curves of a 10 Cepheid selection as regularly as possible. Amplitudes and times of maximum brightness were obtained from these lightcurves, and compared to previous literature results. At UV wavelengths, we have been very fortunate to secure numerous high-resolution spectra of two nearby Cepheids – δ Cep and β Dor – with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and additional future spectra have recently been approved. Finally, at X-ray wavelengths, we have (again, very fortunately) thus far obtained images and X-ray (0.3 – 5 keV) fluxes and luminosities of five Cepheids with XMM-Newton and the Chandra X-ray Observatory, and further observations with both satellites have been proposed for (XMM) and approved (Chandra). 
 
Our analysis of optical photometry has shown that 8 of the 10 observed Cepheids have amplitude variability, or hints thereof, and all 10 Cepheids show evidence of period variability (recent, long-term or even possibly periodic). The UV spectra reveal a wealth of emission lines from heated atmospheric plasmas of 10⁴ – 10⁵ K that vary in phase with the Cepheid pulsation periods. The X-ray images have detected the three nearest Cepheids observed (Polaris, δ Cep and β Dor), while the distances of the farther two Cepheids place their fluxes likely at or below detector background levels. The X-ray fluxes for δ Cep show possible phased variability, but possibly anti-correlated with the UV emission line fluxes (i.e. high X-ray flux at phases of low UV flux, and vice versa). 
 
In conclusion, the optical studies have shown that Cepheids may likely undergo period and amplitude variations akin to the Blazhko Effect observed in RR Lyr stars, but on longer timescales. The heating mechanism(s) of their atmospheres appears to be a combination of magnetic/acoustic activity, common in many cool stars, along with pulsation-related effects (shock propagation and possibly convective strength variability). 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 mechanisms.