{
  "id": "1206.4282",
  "version": "v1",
  "published": "2012-06-19T18:03:12.000Z",
  "updated": "2012-06-19T18:03:12.000Z",
  "title": "X-Ray, UV and Optical Observations of Classical Cepheids: New Insights into Cepheid Evolution, and the Heating and Dynamics of Their Atmospheres",
  "authors": [
    "Scott G. Engle",
    "Edward F. Guinan"
  ],
  "comment": "11 pages, 6 figures, Published in Journal of Astronomy and Space Sciences (JASS), vol. 29, no. 2, pp 181-189, June, 2012",
  "journal": "JASS, vol. 29, no. 2, pp.181-189, June, 2012",
  "doi": "10.5140/JASS.2012.29.2.181",
  "categories": [
    "astro-ph.SR"
  ],
  "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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2012-06-19T18:03:12.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "classical cepheid",
      "cepheid evolution",
      "optical observations",
      "emission lines",
      "fuv line emissions peak"
    ],
    "tags": [
      "journal article"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 11,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1118717,
      "adsabs": "2012JASS...29..181E"
    }
  }
}