AAS 205th Meeting, 9-13 January 2005
Session 52 Stellar Atmospheres, Abundances and Opacities
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

Previous   |   Session 52   |   Next

[52.07] Radiative Transfer Modeling of Warm Transition Region Winds in F- and G-type Supergiants

A. Lobel, E. H. Avrett (Harvard-Smithsonian Center for Astrophysics, MA), J. P. Aufdenberg (National Optical Astronomy Observatory, AZ)

We present FUSE spectra of upper transition region emission lines of O VI in the dynamic atmosphere of the short-period classic Cepheid Beta Dor (F-G Ia). The far-UV O VI 1032 & 1037 Ålines indicate a heating mechanism in the outer atmospheres of strongly pulsating F- and G-type supergiants sustaining hot plasmas at kinetic gas temperatures between 100 kK and 300 kK. Our observation of prominent upper transition region emission lines in Beta Dor contrasts with the very low X-ray luminosities of Cepheid variables that signal only weak coronal plasmas.

On the other hand, FUSE and HST-STIS observations of the non-variable yellow (hybrid) supergiants Alpha Aqr (G2 Ib) and Beta Aqr (G0 Ib), having large X-ray fluxes, reveal supersonic warm wind velocities of 140 km/s and 90 km/s, respectively, in lower transition region emission lines of C III 977 Åand Si III 1206 Å. Our semi-empiric radiative transfer models show that these optically thick winds occur at kinetic gas temperatures well above 70 kK, much larger than assumed for the chromospheres of cool supergiants. Remarkably, these emission lines reveal peculiar shapes reminiscent of P-Cygni type line profiles observed in UV spectra of hot supergiants. Both hybrid supergiants lack the strongly oscillating photospheres of Cepheids, suggesting that their transition region wind acceleration and heating do not result from a pure mechanical driving mechanism due to atmospheric pulsations.

We present detailed semi-empiric radiative transfer models of the thermal and dynamic structures of the outer atmospheres of these luminous F- and G-type supergiants based on the FUSE and HST-STIS spectra. We investigate if warm accelerating winds observed in high ions of cool supergiants can (partly) be driven by radiation pressure.  

This research is based on data obtained with the NASA/ESA Hubble Space Telescope, collected at the STScI, operated by AURA Inc., under contract NAS5-26555. Financial support has been provided by STScI grant HST-GO-10212.01-A and NASA FUSE grant GI-D107.  

If you would like more information about this abstract, please follow the link to http://alobel.freeshell.org. This link was provided by the author. When you follow it, you will leave the Web site for this meeting; to return, you should use the Back comand on your browser.

The author(s) of this abstract have provided an email address for comments about the abstract: alobel@cfa.harvard.edu

Previous   |   Session 52   |   Next

Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.