Mathematics – Logic
Scientific paper
Jul 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009hst..prop12302g&link_type=abstract
HST Proposal ID #12302. Cycle 18
Mathematics
Logic
Scientific paper
Classical Cepheids, although well studied in terms of their cosmologically crucial Period-Luminosity Law, are proving themselves to be increasingly complex and astrophysically intriguing objects. The major aim of this proposal is to probe their stellar atmospheres and try to understand the mechanisms by which they are heated. Archival IUE UV spectra revealed the presence and variability of 10^4 - 10^5 K plasma emission lines, correlated to the pulsation period of the studied Cepheids, indicating that a pulsation-driven heating mechanism may be at work. However, the spectra were of low resolution and also contaminated by scattered light, precluding a more thorough analysis. We propose multiple observations of two selected Cepheids {beta Dor and delta Cep - the prototype of all Classical Cepheids} with HST-COS through the G130M and G160M gratings. The COS spectra will provide comprehensive and detailed diagnostics of the atmospheric plasmas around these Cepheids, and will give us the best look yet at how large-scale, radial pulsations affect the upper atmospheres of supergiants. The phase constraints placed on some of the visits will also allow possible phase-lags between the emission lines to be detailed, giving important additional information into the heating mechanism. Numerous emission lines are covered by the G130M and G160M wavelength range { 1150-1750A}, including N V 1240, O I 1305, C IV 1550 and He II 1640. When combined with our approved and ongoing X-ray observations of Cepheids, the HST-COS data will allow us to construct an understanding of Cepheid atmospheric plasmas with temperature from tens of thousands up to millions of degrees - the most thorough atmospheric study to date for this important class of pulsating stars.;
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