J/A+AS/111/57 Synthetic absorption in star forming regions (Olofsson, 1995)
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 1995A&AS..111...57O (SIMBAD/NED Reference)
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☉. 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