J/A+A/625/A132 Models for massive stars in the SMC (Schootemeijer+, 2019)
Constraining mixing in massive stars in the Small Magellanic Cloud.
Schootemeijer A., Langer N., Grin N.J., Wang C.
<Astron. Astrophys. 625, A132 (2019)>
=2019A&A...625A.132S 2019A&A...625A.132S (SIMBAD/NED BibCode)
ADC_Keywords: Models, evolutionary ; Magellanic Clouds ; Stars, OB ;
Stars, early-type; Stars, supergiant; Stars, Wolf-Rayet
Keywords: stars: massive - stars: early-type - stars: Wolf-Rayet -
stars: interiors - stars: rotation - stars: evolution
Abstract:
The evolution of massive stars is strongly influenced by internal
mixing processes such as semiconvection, convective core overshooting,
and rotationally induced mixing. None of these processes are currently
well constrained.
We investigate models for massive stars in the Small Magellanic Cloud
(SMC), for which stellar-wind mass loss is less important than for
their metal-rich counterparts. We aim to constrain the various mixing
efficiencies by comparing model results to observations.
For this purpose, we use the stellar-evolution code MESA to compute
more than 60 grids of detailed evolutionary models for stars with
initial masses of 9...100M☉, assuming different combinations of
mixing efficiencies of the various processes in each grid. Our models
evolve through core hydrogen and helium burning, such that they can be
compared with the massive main sequence and supergiant population of
the SMC.
We find that for most of the combinations of the mixing efficiencies,
models in a wide mass range spend core-helium burning either only as
blue supergiants, or only as red supergiants. The latter case
corresponds to models that maintain a shallow slope of the
hydrogen/helium (H/He) gradient separating the core and the envelope
of the models. Only a small part of the mixing parameter space leads
to models that produce a significant number of blue and red
supergiants, which are both in abundance in the SMC. Some of our grids
also predict a cut-o in the number of red supergiants above
logL/L☉=5...5.5. Interestingly, these models contain steep H/He
gradients, as is required to understand the hot, hydrogen-rich
Wolf-Rayet stars in the SMC. We find that unless it is very fast,
rotation has a limited effect on the H/He profiles in our models.
While we use specific implementations of the considered mixing
processes, they comprehensively probe the two firstorder structural
parameters, the core mass and the H/He gradient in the core-envelope
interface. Our results imply that in massive stars, mixing during the
main-sequence evolution leads to a moderate increase in the helium
core masses, and also that the H/He gradients above the helium cores
become very steep. Our model grids can be used to further refine the
various mixing efficiencies with the help of future observational surveys
of the massive stars in the SMC, and thereby help to considerably
reduce the uncertainties in models of massive star evolution.
Description:
Stellar evolution models for stars with a chemical composition
appropriate for the Small Magellanic Cloud. The initial masses cover
the range from 9 to 100 Solar masses. For stars in this mass range, we
cover 60 combinations of semiconvection (0.01≤alphasc≤300) and
overshooting (0≤alphaov≤0.55). In the same mass range, we provide
the evolutionary models for the different initial rotational velocity
and alpha_ov combinations that were used to create figures 8, 11, and
A2.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 61 741 *List of all evolutionary sequence files
data/*/* . 741 Evolutionary sequence with a certain initial mass
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Note on list.dat: The name of its host folder specifies the mixing efficiency
that was adopted in the evolutionary model.
-In the asc<alphasc>aov<alpha_ov> folders, we have nonrotating models with
various efficiencies of semiconvection + overshooting.
Total: 60*11=660 evolutionary sequences.
-In the vrot<xxx>asc<alphasc>aov<alpha_ov> folders there are models that
have an initial rotation velocity of kilometer/sec.
Total: 12 (Fig.8) + 31 (Fig.11) + 40 (extra for Fig.A2) =
83 evolutionary sequences
Apart from 'massinnermix', all variables have the same value as in
the MESA output.
More information about MESA and references to the instrument paper can be found
on: http://mesa.sourceforge.net/
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- vrot [075/375]? Initial rotational velocity
5- 10 F6.2 --- asc [0.01/300] Semiconvection alphasc
12- 15 F4.2 --- aov [0/0.55] Overshooting alphaov
17- 21 F5.1 Msun Mass [9/100] Initial mass
24- 61 A38 ---- FileName Name of the file with evolutionary sequence
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Byte-by-byte Description of file: data/*/*
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Bytes Format Units Label Explanations
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1- 17 E17.9 yr Age Star age (star_age)
18- 34 E17.5 Msun Mass Star mass (star_mass)
35- 51 E17.5 [K] logTeff Effective temperature (lot_Teff)
52- 68 E17.5 [Lsun] logL Luminosity (log_L)
69- 85 E17.5 [K] logTc Center temperature (logcenterT)
86-102 E17.5 g/cm3 logRhoc Center density (logcenterRho)
103-119 E17.5 --- H1c Center mass fraction H1 (center_h1)
120-136 E17.5 --- He4c Center mass fraction He4 (center_he4)
137-153 E17.5 --- C12c Center mass fraction C12 (center_c12)
154-170 E17.5 --- H1s Surface mass fraction H1 (surface_h1)
171-187 E17.5 --- He4s Surface mass fraction He4 (surface_he4)
188-204 E17.5 --- N14s Surface mass fraction N14 (surface_n14)
205-221 E17.5 Msun Masscc Mass coordinate of convective border
(massconvcore)
222-238 E17.5 Msun Massim This is the product of the MESA variables
'Mass' and the relative mass coordinate of
the top of the innermost mixing region,
i.e., 'mx1_top' or 'mx2_top'
(massinnermix) (1)
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Note (1): If the bottom of the innermost mixing region is not in the center
(further out than relative mass coordinate q=0.02), we set 'massinnermix'
to zero. In case of rotation, the border of the inner mixing region is not
well defined. Therefore, we set 'massinnermix' to zero for all models
that are rotating.
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Acknowledgements:
Abel Schootemeijer, aschoot(at)astro.uni-bonn.de
(End) Patricia Vannier [CDS] 23-Apr-2019