J/A+A/595/A35 Low-mass helium white dwarfs evolutionary models (Istrate+, 2016)
Models of low-mass helium white dwarfs including gravitational settling,
thermal and chemical diffusion, and rotational mixing.
Istrate A., Marchant P., Tauris T.M., Langer N., Stancliffe R.J.,
Grassitelli L.
<Astron. Astrophys. 595, A35 (2016)>
=2016A&A...595A..35I 2016A&A...595A..35I (SIMBAD/NED BibCode)
ADC_Keywords: Models, evolutionary ; Stars, ages ; Stars, white dwarf ;
Binaries, X-ray
Keywords: white dwarfs - binaries: general - stars: low-mass -
pulsars: general - binaries: close
Abstract:
A large number of extremely low-mass helium white dwarfs (ELM WDs)
have been discovered in recent years. The majority of them are found
in close binary systems suggesting they are formed either through a
common-envelope phase or via stable mass transfer in a low-mass X-ray
binary (LMXB) or a cataclysmic variable (CV) system. Here, we
investigate the formation of these objects through the LMXB channel
with emphasis on the proto-WD evolution in environments with different
metallicities. We study for the first time the combined ects of
rotational mixing and element diffusion (e.g. gravitational settling,
thermal and chemical diffusion) on the evolution of proto-WDs and on
the cooling properties of the resulting WDs. We present
state-of-the-art binary stellar evolution models computed with MESA
for metallicities of Z=0.02, 0.01, 0.001 and 0.0002, producing WDs
with masses between ∼0.16-0.45M☉. Our results confirm that
element diffusion plays a significant role in the evolution of
proto-WDs that experience hydrogen shell flashes. The occurrence of
these flashes produces a clear dichotomy in the cooling timescales of
ELM WDs, which has important consequences e.g. for the age
determination of binary millisecond pulsars. In addition, we confirm
that the threshold mass at which this dichotomy occurs depends on
metallicity. Rotational mixing is found to counteract the effect of
gravitational settling in the surface layers of young, bloated ELM
proto-WDs and therefore plays a key role in determining their surface
chemical abundances, i.e. the observed presence of metals in their
atmospheres. We predict that these proto-WDs have helium-rich
envelopes through a significant part of their lifetime. This is of
great importance as helium is a crucial ingredient in the driving of
the κ-mechanism suggested for the newly observed ELM proto-WD
pulsators. However, we find that the number of hydrogen shell flashes
and, as a result, the hydrogen envelope mass at the beginning of the
cooling track, are not influenced significantly by rotational mixing.
In addition to being dependent on proto-WD mass and metallicity, the
hydrogen envelope mass of the newly formed proto-WDs depends on
whether or not the donor star experiences a temporary contraction when
the H-burning shell crosses the hydrogen discontinuity left behind by
the convective envelope. The hydrogen envelope at detachment, although
small compared to the total mass of the WD, contains enough angular
momentum such that the spin frequency of the resulting WD on the
cooling track is well above the orbital frequency.
Description:
Evolutionary models of low-mass helium white dwarfs including element
diffusion and rotational mixing. The WDs are produced considering
binary evolution through the LMXB channel, with final WDs masses
between ∼0.16-∼0.44. The models are computed using MESA, for different
metallicities: Z=0.02, 0.01, 0.001 and 0.0002.
For each metallicity, the models are divided in three categories:
(1) basic (no diffusion nor rotation are considered)
(2) diffusion (element diffusion is considered)
(3) rotation+diffusion (both element diffusion and rotational mixing
are considered)
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 53 266 List of files
basic/* . 104 Individual files for basic models
(no diffusion nor rotation are considered)
diff/* . 80 Individual files for diffusion models
(element diffusion is considered)
rot/* . 82 Individual files for rotation+diffusion models
(both element diffusion and rotational mixing
are considered)
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Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 30 A30 --- FileName Name of the file (dir/filename)
33- 38 F6.4 --- Z Initial Z abundance (0.0002, 0.001, 0.01, 0.02)
40- 42 F3.1 Msun M1 Initial mass of the primary (1.0, 1.4)
44- 46 F3.1 Msun M2 Initial mass of the secondary (1.2, 1.4)
48- 53 F6.3 d Prob Initial orbital period (1.73-70.0)
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Byte-by-byte Description of file: basic/*
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
18- 22 I5 -- Model Model number counting from the start of the
run (model_number)
51- 72 E22.17 yr AgeSt Time since the start of the run
(star_age) (G1)
92-113 E22.17 Msun MassSt Mass of the proto-WD (star_mass)
132-154 E23.17 [Msun/yr] logdM/dt log10 of mass transfer rate
(masstransferrate) (G2)
174-195 E22.17 d Porb Orbital period (orbital_period)
215-236 E22.17 Msun MassNS Mass of the neutron star acretor
(neutronstarmass)
256-277 E22.17 [K] logTeff log10 effective temperature
296-318 E23.17 [Lsun] logL log10 luminosity
337-359 E23.17 [Rsun] logR log10 radius
378-400 E23.17 [cm/s2] logg log10 surface gravity
419-441 E23.17 Msun MHec Mass of the helium core (Hecoremass)
460-482 E23.17 Msun Menv Envelope mass (star_mass - hecoremass)
(envelope_mass)
501-523 E23.17 [Lsun] logLnuc Total thermal power given by nuclear
burning, excluding neutrinos
542-564 E23.17 [Lsun] logLpp log10 total luminosity for pp burning
583-605 E23.17 [Lsun] logLcno log10 total luminosity for CNO burning
625-646 E22.17 -- surfH1 Mass fractions of H1 near surface
(surface_H1)
666-687 E22.17 -- surfHe4 Mass fractions of He4 near surface
(surface_He4)
707-728 E22.17 -- surfC12 Mass fractions of C12 near surface
(surface_c12)
748-769 E22.17 -- surfO16 Mass fractions of O16 near surface
(surface_o16)
789-810 E22.17 Msun MtotH1 Mass of hydrogen for entire star
(totalmassH1)
830-851 E22.17 Msun MtotHe4 Mass of helium for entire star
(totalmassHe4)
871-892 E22.17 [K] logTc log10 temperature near center
912-933 E22.17 [g/cm3] logrhoc log10 density near center
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Byte-by-byte Description of file: diff/* rot/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
18- 22 I5 --- Model Model number counting from the start of
the run (model_number)
51- 72 E22.17 yr AgeSt Time since the start of the run
(star_age) (G1)
92- 113 E22.17 Msun MassSt Mass of the proto-WD (star_mass)
132- 154 E23.17 [Msun/yr] logdM/dt log10 of mass transfer rate
(masstransferrate) (G2)
174- 195 E22.17 d Porb Orbital period (orbital_period)
215- 236 E22.17 Msun MassNS Mass of the neutron star acretor
(neutronstarmass)
256- 277 E22.17 --- Domega Angular velocity at equator divided by
the critical angular velocity at
equator (surfavgomega/omega_crit)
296- 318 E23.17 km/s vrot Rotational velocity at equator
337- 359 E23.17 --- Prot/Porb1 Rotational over orbital period for
proto-WD
379- 400 E22.17 [K] logTeff log10 effective temperature
419- 441 E23.17 [Lsun] logL log10 luminosity
460- 482 E23.17 [Rsun] logR log10 radius
501- 523 E23.17 [cm/s2] logg log10 surface gravity
542- 564 E23.17 Msun MHec Mass of the helium core (Hecoremass)
583- 605 E23.17 Msun Menv Envelope mass (star_mass - hecoremass)
(envelope_mass)
624- 646 E23.17 [Lsun] logLnuc Total thermal power given by nuclear
burning, excluding neutrinos
665- 687 E23.17 [Lsun] logLpp log10 total luminosity for pp burning
706- 728 E23.17 [Lsun] logLcno log10 total luminosity for CNO burning
748- 769 E22.17 -- surfH1 Mass fractions of H1 near surface
(surface_H1)
789- 810 E22.17 -- surfHe4 Mass fractions of He4 near surface
(surface_He4)
830- 851 E22.17 -- surfC12 Mass fractions of C12 near surface
(surface_c12)
871- 892 E22.17 -- surfO16 Mass fractions of O16 near surface
(surface_o16)
912- 933 E22.17 -- surfCa40 Mass fractions of Ca40 near surface
(surface_ca40)
953- 974 E22.17 Msun MtotH1 Mass of hydrogen for entire star
(totalmasshydrogen)
994-1015 E22.17 Msun MtotHe4 Mass of helium for entire star
(totalmasstotalmasshelium)
1035-1056 E22.17 [K] logTc log10 temperature near center
1076-1097 E22.17 [g/cm2] logrhoc log10 density near center
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Global notes:
Note (G1): the evolutionary tracks are give from the end of the mass transfer
(end of the LMXB phase) until the WD reaches the age of 14Gyr.
Note (G2): we consider detachment from RLO when the mass transfer rate drops
below -13.5.
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Acknowledgements:
Alina Istrate, istrate(at)uwm.edu
(End) Patricia Vannier [CDS] 19-Jul-2016