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T.R. Ayres, A. Brown (CASA), J.L. Linsky (JILA)
The broad coverage, high sensitivity, and precise wavelength calibration of the Space Telescope Imaging Spectrograph's medium-resolution echelle mode, coupled with the growing collection of GO and GTO E140M exposures, are ideal for surveys of specific spectral diagnostics across a diversity of stellar types, luminosities, and activity levels. Of great current interest are the weak coronal forbidden lines that appear in the far-UV, which are well known from solar flare work. Measuring coronal lines with STIS in the 1150--1700~Å\ band has significant advantages over using, say, Chandra~HETGS or XMM-Newton~RGS in the 1~keV range, because the STIS velocity resolution is 40\times, or more, higher; STIS has an absolute wavelength calibration established by an onboard emission lamp; and the large effective area of the HST\/ telescope compensates for the faintness of the forbidden lines. Here, we report a survey of Fe~{\small XXI} \lambda1354 in a sample of ~25 stars. The forbidden iron feature forms at a temperature of about 107~K, characteristic of very active or flaring coronal conditions. Clear detections of the coronal iron line are made in active M dwarfs (AU~Mic, AD~Leo), active giants (\alpha~Aur, \beta~Cet, \iota~Cap, 24~UMa, HR\,9024), short-period RS~CVn binaries (e.g., HR\,1099), and possibly in active solar-type dwarfs (\zeta~Dor, \chi1~Ori). We describe our semi-empirical method for removing the C~{\small I} blend that partially corrupts the Fe~{\small XXI} profile, and our measurements of coronal line widths and Doppler shifts. Although \alpha~Aur displays clear variability between Fe~{\small XXI} profiles obtained at the same orbital phase, but four years apart; the hyperactive HR\,1099 system showed virtually no change in its coronal iron feature during a sequence of 14 spectra taken over a 7~hr period in 1999~September, despite the occurrence of two large flares in far-UV lines such as Si~{\small IV} and C~{\small IV}.
This work was supported by grant GO-08280.01-97A from STScI. Observations were from the NASA/ESA HST,\/ collected at the STScI, operated by AURA, under contract NAS5-26555. We thank the other collaborators in HST\/ program 8280 for their help in carrying out the STIS stellar survey upon which the present study is based.
The author(s) of this abstract have provided an email address for comments about the abstract: ayres@casa.colorado.edu