X-Ray, UV and Optical Observations of Classical Cepheids: New Insights into Cepheid Evolution, and the Heating and Dynamics of Their Atmospheres

TL;DR

This study uses X-ray and UV observations to reveal the complex, dynamic atmospheres of classical Cepheids, showing phase-dependent emissions and suggesting shock heating as a key process in their atmospheric activity.

Contribution

The paper provides new, detailed X-ray and UV spectra of Cepheids, demonstrating their complex atmospheres and phase-dependent emissions, advancing understanding of their atmospheric heating mechanisms.

Findings

Cepheids exhibit rich, complex UV emission lines indicating multi-temperature plasmas.

X-ray luminosities of Cepheids range from log Lx ~ 28.5 to 29.1 ergs/sec.

Enhanced emissions peak near phase 0.8-1.0, supporting shock heating hypothesis.

Abstract

To broaden the understanding of classical Cepheid structure, evolution and atmospheres, we have extended our continuing secret lives of Cepheids program by obtaining XMM/Chandra X-ray observations, and Hubble space telescope (HST) / cosmic origins spectrograph (COS) FUV-UV spectra of the bright, nearby Cepheids Polaris, {\delta} Cep and {\beta} Dor. Previous studies made with the international ultraviolet explorer (IUE) showed a limited number of UV emission lines in Cepheids. The well-known problem presented by scattered light contamination in IUE spectra for bright stars, along with the excellent sensitivity & resolution combination offered by HST/COS, motivated this study, and the spectra obtained were much more rich and complex than we had ever anticipated. Numerous emission lines, indicating 10^4 K up to ~3 x 10^5 K plasmas, have been observed, showing Cepheids to have complex, dynamic outer atmospheres that also vary with the photospheric pulsation period. The FUV line emissions peak in the phase range {\phi} ~ 0.8-1.0 and vary by factors as large as 10x. A more complete picture of Cepheid outer atmospheres is accomplished when the HST/COS results are combined with X-ray observations that we have obtained of the same stars with XMM-Newton & Chandra. The Cepheids detected to date have X-ray luminosities of log Lx ~ 28.5-29.1 ergs/sec, and plasma temperatures in the 2-8 x 10^6 K range. Given the phase-timing of the enhanced emissions, the most plausible explanation is the formation of a pulsation-induced shocks that excite (and heat) the atmospheric plasmas surrounding the photosphere. A pulsation-driven {\alpha}^2 equivalent dynamo mechanism is also a viable and interesting alternative. However, the tight phase-space of enhanced emission (peaking near 0.8-1.0 {\phi}) favor the shock heating mechanism hypothesis.