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Spectral synthesis in the ultraviolet. IV - A library of mean stellar groups
A library of mean UV stellar energy distributions is derived from IUEspectrophotometry of 218 stars. The spectra cover 1230-3200 A with aspectral resolution of about 6 A. They have been corrected forinterstellar extinction and converted to a common flux and wavelengthscale. Individual stars were combined into standard groups according totheir continuum colors, observed UV spectral morphology, MK luminosityclass, and metal abundance. The library consists of 56 groups: 21dwarf(V), 8 subgiant(IV), 16 giant(III), and supergiant(I + II) groups,covering O3-M4 spectral types. A metal-poor sequence is included,containing four dwarf and two giant groups, as is a metal-enhancedsequence with a single dwarf, subgiant, and giant group. Spectralindices characterizing the continuum and several strong absorptionfeatures are examined as temperature, luminosity, and abundancediagnostics. The library is intended to serve as a basis forinterpreting the composite UV spectra of a wide variety of stellarsystems, e.g., elliptical galaxies, starburst systems, and high-redshiftgalaxies.
| On the utility of low resolution IUE spectroscopy of the 2800 A Mg II lines as a stellar chromosphere indicator
Low resolution IUE spectroscopy of the 2800-A Mg II h and k lines isshown to provide a useful means for documenting chromospheric activityamong relatively young dwarf stars. An index I(Mg II) has been definedwhich measures the integrated flux in the region 2784-2814 A relative tothe flux interpolated from nearby comparison regions. Values of thisindex have been derived from low resolution IUE spectra for a sample offield dwarfs for which Ca II H and K line indices have been published aspart of the Mount Wilson HK program. The large range in chromosphericactivity among field dwarfs that is exhibited by the Mount Wilson Ca IIS index is found to also be reflected by the lower resolution I(Mg II)index. Using an age calibration of Ca II emission line strengths derivedby Barry, it is found that the value of I(Mg II) can be used todistinguish between dwarfs younger and older than 3 Gyr. The lowresolution nature of the I(Mg II) index means that it holds potentialfor use as an age diagnostic for stellar population studies. Amongdwarfs of age greater than 3 Gyr there is some evidence that this Mg IIindex is affected by line blanketing.
| Spectral synthesis in the ultraviolet. III - The spectral morphology of normal stars in the mid-ultraviolet
The morphology of 218 mid-UV spectra of stars ranging from O through Kin spectral type is examined. Several new line and continuum indices aredefined and their usefulness as temperature, luminosity, and metallicitydiscriminants is discussed. Mid-UV stellar continua are found to bemarkedly affected by abundance. A UV excess, delta(2600-V), is computedwhich is more sensitive by a factor of 10 to (Fe/H) than is delta(U-B).The relative strength of spectral lines in the mid-UV is not as stronglyaffected by abundance. Mid-UV spectra are much more sensitive to thetemperature of the stellar population than to either metallicity or thedwarf/giant ratio. Mg II 2800 shows unexpected behavior, displaying nosensitivity to abundance for cool stars and a reversed sensitivity in FGdwarfs such that metal-poor stars have stronger Mg II strengths at thesame temperature than more metal-rich stars.
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