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J/A+AS/111/57      Synthetic absorption in star forming regions (Olofsson, 1995)
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Synthetic absorption line strengths in star forming regions, and the effects
on the helium abundance determination
       Olofsson K.
      <Astron. Astrophys. Suppl. Ser. 111, 57 (1995)>
      =1995A&AS..111...57O      (SIMBAD/NED Reference)
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ADC_Keywords: Galaxies, optical; Stars, early-type; Extinction
Keywords: galaxies: evolution - galaxies: stellar content -
          interstellar medium: HII-regions - lines: profile -
          stars: mass function

Description:
    Spectral evolutionary calculations have been employed in order to
    follow the behaviour of absorption lines in single-burst star forming
    regions. The model calculations are based on theoretical evolutionary
    tracks and model atmosphere stellar spectra at 5A resolution and they
    include the pre-main sequence phase. Three different metallicities
    have been considered; Z=0.001, 0.008 and 0.020, this range covers
    easily the one occupied by blue compact dwarf galaxies. This analysis
    focuses on the post-active burst phase and follows the evolution up to
    15 million years after the episode of star formation has ceased, using
    an instantaneous burst of star formation. The absorption lines
    included are hydrogen, neutral and once ionized helium. Non-LTE
    absorption line profiles have been implemented to these lines, also at
    a 5A resolution. The equivalent widths and line profiles are studied
    as a function of lower and upper mass limits and slope of the mass
    function as well as the age. It is shown that the equivalent widths of
    the Balmer lines are increasing during the course of evolution, while
    the strength of the neutral and ionized helium lines decreases
    monotonically. Both the hydrogen and helium lines show high
    sensitivity to variations in the initial mass function (IMF). The
    Balmer line equivalent widths range from about 1.5 to 6.5A, depending
    on the IMF parameters used. As anticipated, these numbers are rather
    insensitive to the metallicity. Contrary to what has been claimed, the
    Balmer lines are prominent even if the lower mass limit is as high as
    10M_{sun}_. It has been proposed that the weak hydrogen lines
    sometimes seen in starburst galaxies, often interpreted as evidence
    for a deficiency in low mass stars, could be due to the finite
    lifetimes of pre-main sequence stars. This analysis shows that it is
    very much dependent on the slope of the mass function. The equivalent
    width of the HeI-4471A line never exceeds 0.35A for any choice of the
    IMF parameters while the equivalent width of the HeII-4686A line
    reaches at the most 0.20A using the most top-heavy IMF. As expected,
    these numbers are only slightly dependent on the metallicity in the
    optical spectral region.


File Summary:
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  FileName      Lrecl  Records   Explanations
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ReadMe             80        .   This file
table6             66     2040   Line fluxes for hydrogen lines
table7             65      360   Line fluxes for neutral helium lines
table8             65      270   Line fluxes for once ionized helium lines
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Byte-by-byte Description of file: table6 table7 table8
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   Bytes Format  Units   Label    Explanations
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   1-  3  I3     ---     Z        Metallicity
   5-  6  I2     ---     Mod      Model number
   8- 15  A8     ---     Line     Line considered
  17- 21  F5.1   0.1nm   dl       Continuum (1)
  23- 27  F5.3   0.1nm   F1Myr    Line flux for Age=1Myr (2)
  29- 33  F5.3   0.1nm   F2Myr    Line flux for Age=2Myr (2)
  35- 40  F6.3   0.1nm   F4Myr    Line flux for Age=4Myr (2)
  42- 46  F5.3   0.1nm   F6Myr    Line flux for Age=6Myr (2)
  48- 52  F5.3   0.1nm   F8Myr    Line flux for Age=8Myr (2)
  54- 58  F5.3   0.1nm   F10Myr   Line flux for Age=10Myr (2)
  60- 65  F6.3   0.1nm   F15Myr   Line flux for Age=15Myr (2)
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Note (1): The wavelength-range is 40A on either side of the centre of the
           line dl and normalized to 1.000 on the leftmost wavelength point
           at the age of 1 Myr
Note (2): The fluxes are presented with a resolution of 5A
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(End)                                           Patricia Bauer [CDS] 16-Jan-1995