AAS 197, January 2001
Session 44. Observations and Analysis of Stellar Atmospheres
Display, Tuesday, January 9, 2001, 9:30am-7:00pm, Exhibit Hall

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[44.15] The Chromospheres of G-type Ib Supergiants

A. Lobel (SAO), A. K. Dupree (SAO)

This preliminary study of the detailed chromospheric conditions of G-type Ib-supergiants provides a foundation for atmospheric models, complementing {\em STIS} and {\em FUSE} surveys of cool stars.

We compare optical and near-UV high-resolution Utrecht Echelle Spectrograph (William Herschel Telescope), and {\em SOFIN} (Nordic Optical Telescope) echelle spectra of \alpha~Aqr (G2~Ib), 9~Peg (G5~Ib), and \epsilon~Gem (G8~Ib) with the Ia-supergiant HD~179821 (G2-G5). The latter has recently been proposed as a post-AGB star (with a detached dust shell), instead of a population I massive supergiant, for which we do not detect chromospheric emission in Ca {\sc ii}~H~& K.

The three Ib-supergiants reveal much stronger H\alpha absorption lines than HD~179821, while the core intensities of H\beta and H\delta (showing broad damping line wings) in \alpha~Aqr, \epsilon~Gem, and HD~179821 are similar. The H\alpha profile of HD~179821 has a weak absorption core with satellite emission, typically formed in a supersonic expanding wind. This is regularly observed in very luminous cool pulsating super- and hypergiants of the \rho~Cas-type (F2-G~\rm Ia+), which lack H~& K emission.

Our spectral synthesis of these observations shows that the H\alpha absorption in G~Ib-supergiants is strongly enhanced due to chromospheric excitation, whereas the higher Balmer lines are mainly of photospheric origin. This finding is parallel to the chromospheric nature of H\alpha absorption in \alpha~Ori (M2~Iab), recently discussed by Lobel & Dupree (ApJ, 545, Dec. 2000).

This research is supported in part by STScI grant GO-08280.02-97A to the SAO.

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