Spectral synthesis in the ultraviolet. III. The spectral morphology of normal stars in the mid-ultraviolet

Michael N. Fanelli, Robert W. O'Connell, David Burstein, Chi Chao Wu

Research output: Contribution to journalArticlepeer-review

38 Scopus citations

Abstract

A data set of 218 I U E spectra covering stars of spectral types O through K is used to examine the morphology of mid-ultraviolet λλ1900-3200 spectra with respect to basic physical properties. Several new line and continuum indices are defined and their usefulness as temperature, luminosity, and metallicity discriminants is discussed. These include (2600 - V), (3000 - V), Fe II 2402 + 2609, Fe I 3000, Mg II 2800, and Mg I 2852. The mid-UV provides excellent temperature discrimination: ultraviolet spectral indices have large amplitudes, and in general there are good correlations of line indices with (V-K) and (B-V). We find modest luminosity discrimination among dwarfs, subgiants, and giants; supergiants form a more distinct sequence in most of the mid-UV indices. Mid-UV stellar continua are markedly affected by abundance. We have computed an ultraviolet excess, δ(2600 -V), which is a factor of 10 more sensitive to [Fe/H] than δ(U-B). Mid-UV photometry opens new horizons on the investigation of metal-poor stellar populations. Conversely, the relative strength of spectral lines in the mid-UV is not as strongly affected by abundance. Variations in line strength appear to be diluted by corresponding variations in the blanketing of the adjacent continuum. The result is that mid-UV spectra are much more sensitive to the temperature of the stellar population than to either metallicity or the dwarf/ giant ratio. Mg II 2800 exhibits unexpected behavior, displaying no sensitivity to abundance for cool stars and a reversed sensitivity in FG dwarfs such that metal-poor stars have stronger Mg II strengths at the same temperature than more metal-rich stars. We attribute this to chromospheric emission. The results indicate that net Mg II equivlaent widths will depend significantly on age. Mid-UV spectral lines in star-forming galaxies are indicative of the intermediate-temperature AF stars. These can be used to determine the stellar luminosity function for intermediate masses and address questions such as the duration and strength of star formation episodes with negligible contamination from the ubiquitous cool stars present in all galaxies. In old stellar systems, the sensitivity of the mid-UV continuum to the warmest population, combined with the rich spectral morphology found between 2600 and 3100 Å, permits the turnoff population to be much more readily studied than with optical spectra.

Original languageEnglish
Pages (from-to)272-294
Number of pages23
JournalAstrophysical Journal
Volume364
Issue number1
DOIs
StatePublished - 20 Nov 1990
Externally publishedYes

Keywords

  • Spectrophotometry
  • Ultraviolet : spectra

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