HST/GHRS and EUVE Spectra of the Active Clump Giant $\beta$~Ceti (K0~III)

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Session 45 -- Stellar Activity I
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[45.10] HST/GHRS and EUVE Spectra of the Active Clump Giant $\beta$~Ceti (K0~III)

T.R.Ayres, A.Brown (CASA/U.Colorado), S.Drake (GSFC), T.Simon (IfA/U.Hawaii), R.A.Stern (LPARL), B.E.Wood (JILA/U.Colorado)

The nearby red giant $\beta$~Ceti (HD~4128: K0~III; $d= 16$~pc) is in the postflash core helium burning phase. It is a member of a class of coronally active ``Clump'' giants that includes the G8 primary of Capella ($\alpha$~Aur: G8~III + G1~III) and the Hyad $\theta^1$~Tauri (K0~III). Prior to the first ascent of the giant branch, such stars very likely were hyperactive Hertzsprung-gap giants, like the G0 secondary of Capella and 31~Comae (G0~III).

HST/GHRS obtained spectra of $\beta$~Ceti on 1994 June 2, in five intervals using both the SSA and LSA. Exposure times ranged from 5.4~minutes (ECH-B/2800\AA) to 43.5~minutes (G160M/1400\AA). Strong emissions of H~I, Mg~II, Si~III, Si~IV, C~IV, and N~V were recorded with high S/N. Fainter lines of N~I, C~I, Si~I, O~IV], and O~V] also were seen. After correction for systematic effects, we find that emissions which form below about $3\times10^4$~K are close to the rest frame of the star, but higher excitation species are systematically redshifted \/ (as had been suggested previously on the basis of lower S/N IUE echelle spectra). The redshifts are 10$\pm$2~~km~s$^{-1}$ at N~V/O~IV temperatures ($2\times10^5$~K); 16$\pm$1 at C~IV ($1\times10^5$~K); and 7$\pm$1 at Si~IV ($6\times10^4$~K). The strong high-excitation emissions have nearly uniform FWHM's ($\approx 90$~km~s$^{-1}$), but show evidence for ``broad components'' (FWHM~$\approx 150$~km~s$^{-1}$) at the bases of their profiles which are slightly more redshifted than the parent lines. We see no evidence for wind absorptions in the very high S/N ($>50:1$) profiles of Mg~II~k \/ or H~I Ly$\alpha$, although the former displays a clear chromospheric central reversal outside of the sharp ISM feature.

We will compare the HST/GHRS spectra with a 140~ks pointing on $\beta$~Ceti by the EUVE, conducted during the six day period beginning 1994 September 30.

This work was supported by NASA grants GO-5323.01-93A (HST) and NAG5-2274 (EUVE).

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