J/ApJ/419/596 Pop I UV radiation models (Dorman+, 1993)
Ultraviolet radiation from evolved stellar populations I. Models
DORMAN B., ROOD R.T., O'CONNELL R.W.
<Astrophys. J. 419, 596 (1993)>
=1993ApJ...419..596D 1993ApJ...419..596D (SIMBAD/NED Reference)
ADC_Keywords: Ultraviolet; Abundances; Abundances, [Fe/H]
Keywords: galaxies: stellar content - stars: AGB and post-AGB -
stars: evolution - stars: Population II - ultraviolet: galaxies
Abstract:
This series of papers comprises a systematic exploration of the
hypothesis that the far-ultraviolet radiation from star clusters and
elliptical galaxies originates from extremely hot horizontal-branch
(HB) stars and their post-HB progeny. This first paper presents an
extensive grid of calculations of stellar models from the zero-age
horizontal branch (ZAHB) through to a point late in post-HB evolution
or a point on the white dwarf cooling track. The grid will be used to
produce synthesized UV fluxes for the interpretation of existing and
future short-wavelength (900-3000A) observations. Our sequences have
been computed for a range of masses which concentrates on models that
begin their HB evolution very close to the hot end of the ZAHB. We
have calculated tracks for three metal-poor compositions
([Fe/H]=-2.26, -1.48, -0.47 with [O/Fe]>0), for use with globular
cluster observations. We have also chosen three metal rich
compositions (Z=0.017=Z☉, Z=0.04, 0.06) for use in the study
of elliptical galaxy populations. For each of the two super-metal-rich
compositions, for which the helium abundance is unconstrained by
observation, we have computed two sets of sequences: one assuming
no additional helium, and a second with a large enhancement
(Y(HB)=0.29 and 0.36 for Z=0.04), and (Y(HB)=0.29 and 0.46 for Z=0.06).
For each set of sequences, our lowest ZAHB envelope masses (M0env)
are in the range 0.002M☉<M0env<0.006M☉.
We use the term extreme horizontal branch (EHB) to refer to HB
sequences of constant mass that do not reach the thermally pulsing
stage on the AGB. These models evolve after core helium exhaustion
into post-early asymptotic giant branch (AGB) stars, which leave the
AGB before thermal pulsing, and AGB-manque stars, which never reach the
AGB. We describe various features of the evolution of post-HB stars,
discussing the correspondence between slow phases of evolution at high
temperature and the early-AGB evolution. We note that the relationship
between core mass and luminosity for stars on the upper AGB is not
straightforward, because stars arrive on the ZAHB with a range of
masses and subsequently burn different amounts of fuel. We determine
from our models an upper bound to the masses of EHB stars, finding that
it varies little for [Fe/H]<0, but that it is sensitive to the helium
abundance. We show that for each composition there is a range of
M0env (at least a few hundredths M_☉) in which the models have a
slow phase of evolution at high temperature. The duration of this phase
is found to increase with the metallicity, but its luminosity is lower,
so that total UV energy output is not significantly different from
metal-poor sequences. The properties of very metal rich stars are,
however, made uncertain by our lack of knowledge of the helium
abundance for [Fe/H]>0; the range of stellar masses in which high
temperatures are attained for significant periods of time increases
with Y. There is no intrinsic composition dependence of the peak UV
output from evolved stars; the output from a stellar population depends
most directly on the mass distribution of stars arriving on the ZAHB.
This is determined mainly by the mass loss that occurs on the red giant
branch.
Addresses:
Dorman B.
Rood R.T.
O'Connell R.W.
Department of Astronomy, University of Virginia, P.O. Box 3818,
University Station, Charlottesville VA 22903-0818
Description:
Tables are given for all of the evolutionary tracks in the paper, which
represent the evolution from the Zero-Age Horizontal Branch to a point
either late in the AGB evolution, or in many cases to a point on the
white dwarf cooling sequences.
There are sequences for 8 different compositions. The original grid of
tracks comprises several tens of thousands of models. In order to make
this grid easy to use while not seriously compromising the
representation of the evolution, much shorter tables have been derived
for the general user. The data given here have been derived from these
original calculations by linear and quadratic interpolation routines.
For the HB phase, the evolution is represented at a fixed set of core
helium values. After core exhaustion, the table points are at fixed
intervals along the evolutionary path.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table 105 10756 Model results
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Byte-by-byte Description of file: table
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Bytes Format Units Label Explanations
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1- 5 F5.3 solMass M Mass
7- 11 F5.2 [Sun] [Fe/H] Metallicity
13- 16 F4.2 [Sun] [O/Fe] Oxygen abundance
18- 22 F5.3 --- Y Helium abundance
25- 33 F9.5 Myr Age Age since the Zero-Age Horizontal Branch
(ZAHB) point
37- 42 F6.4 --- Yc Central helium fraction
46- 51 F6.4 [K] log(Teff) Effective temperature
54- 60 F7.4 [solLum] log(L) Surface luminosity
63- 69 F7.4 [cm/s2] logg Surface gravity
72- 78 F7.4 [cm] log(Rad) Radius
82- 87 F6.4 solMass Msh Mass at peak energy production rate of
hydrogen burning shell
91- 96 F6.4 [K] log(Tc) Central temperature
100-105 F6.4 [g/cm3] log(rhoc) Central density
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Origin: AAS CD-ROM series, Volume 2, 1994
(End) Lee Brotzman [ADS] 09-May-1994, Patricia Bauer [CDS] 25-Oct-1994