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Richard J. Stancliffe - One of the best experts on this subject based on the ideXlab platform.
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models of low mass helium white dwarfs including Gravitational Settling thermal and chemical diffusion and rotational mixing
Astronomy and Astrophysics, 2016Co-Authors: Richard J. Stancliffe, Alina G. Istrate, Pablo Marchant, Thomas M. Tauris, Norbert Langer, L. GrassitelliAbstract: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 effects 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 ~ 0.16−0.45 M ⊙ . 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.
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Models of low-mass helium white dwarfs including Gravitational Settling, thermal and chemical diffusion, and rotational mixing
Astronomy & Astrophysics, 2016Co-Authors: Alina G. Istrate, Richard J. Stancliffe, Pablo Marchant, Thomas M. Tauris, Norbert Langer, L. GrassitelliAbstract: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 effects 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 between Z=0.0002 and Z=0.02, producing WDs with masses between 0.16-0.45 M$_{\odot}$. 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. 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. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime, a crucial ingredient for understanding the newly observed ELM proto-WD pulsators. 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.
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Gravitational Settling in pulsating subdwarf B stars and their progenitors
Astronomy and Astrophysics, 2010Co-Authors: Evert Glebbeek, Richard J. Stancliffe, Anne Thoul, Marcantoine Dupret, G Nelemans, C AertsAbstract:Context. Di usion of atoms can be important during quiescent phases of stellar evolution. Particularly in the very thin inert envelopes of subdwarf B stars, di usive movements will considerably change the envelope structure and the surface abundances on a short timescale. Also, the subdwarfs will inherit the e ects of di usion in their direct progenitors, namely giants near the tip of the red giant branch. This will influence the global evolution and the pulsational properties of subdwarf B stars. Aims. We investigate the impact of Gravitational Settling, thermal di usion and concentration di usion on the evolution and pulsations of subdwarf B stars. Although radiative levitation is not explicitly calculated, we evaluate its e ect by approximating the resulting iron accumulation in the driving region. This allows us to study the excitation of the pulsation modes, albeit in a parametric fashion. Our di usive stellar models are compared with models evolved without di usion. Methods. We use a detailed stellar evolution code to solve simultaneously the equations of stellar structure and evolution, including the composition changes due to di usion. The di usion calculations are performed for a multicomponent fluid using di usion coe cients derived from a screened Coulomb potential. We constructed subdwarf B models with a mass of 0.465 M from a 1 M and 3 M zero-age main sequence progenitor. The low mass star ignited helium in an energetic flash, while the intermediate mass star started helium fusion gently. For each progenitor type we computed series with and without atomic di usion. Results. Atomic di usion in red giants causes the helium core mass at the onset of helium ignition to be larger. We find an increase of 0.0015 M for the 1 M model and 0.0036 M for the 3 M model. The e ects on the red giant surface abundances are small after the first dredge up. The evolutionary tracks of the di usive subdwarf B models are shifted to lower surface gravities and e ective temperatures due to outward di usion of hydrogen. This a ects both the frequencies of the excited modes and the overall frequency spectrum. Especially the structure and pulsations of the post-non-degenerate sdB star are drastically altered, proving that atomic di usion cannot be ignored in these stars. Sinking of metals could to some extent increase the gravities and temperatures due to the associated decrease in the stellar opacity. However, this e ect should be limited as it is counteracted by radiative levitation.
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Gravitational Settling in pulsating subdwarf b stars and their progenitors
arXiv: Solar and Stellar Astrophysics, 2009Co-Authors: Evert Glebbeek, Richard J. Stancliffe, Anne Thoul, Marcantoine Dupret, G Nelemans, C AertsAbstract:Diffusion of atoms can be important during quiescent phases of stellar evolution. Particularly in the very thin inert envelopes of subdwarf B stars, diffusive movements will considerably change the envelope structure and the surface abundances on a short timescale. Also, the subdwarfs will inherit the effects of diffusion in their direct progenitors, namely giants near the tip of the red giant branch. This will influence the global evolution and the pulsational properties of subdwarf B stars. We investigate the impact of Gravitational Settling, thermal diffusion and concentration diffusion on the evolution and pulsations of subdwarf B stars. Our diffusive stellar models are compared with models evolved without diffusion. We constructed subdwarf B models with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low mass star ignited helium in an energetic flash, while the intermediate mass star started helium fusion gently. For each progenitor type we computed series with and without atomic diffusion. Atomic diffusion in red giants causes the helium core mass at the onset of helium ignition to be larger. We find an increase of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun model. The effects on the red giant surface abundances are small after the first dredge up. The evolutionary tracks of the diffusive subdwarf B models are shifted to lower surface gravities and effective temperatures due to outward diffusion of hydrogen. This affects both the frequencies of the excited modes and the overall frequency spectrum. Especially the structure and pulsations of the post-non-degenerate sdB star are drastically altered, proving that atomic diffusion cannot be ignored in these stars.
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Thermohaline mixing and Gravitational Settling in carbon-enhanced metal-poor stars
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Richard J. Stancliffe, Evert GlebbeekAbstract:We investigate the formation of carbon-enhanced metal-poor (CEMP) stars via the scenario of mass transfer from a carbon-rich asymptotic giant branch primary to a low-mass companion in a binary system. We explore the extent to which material accreted from a companion star mixes with that of the recipient, focusing on the effects of thermohaline mixing and Gravitational Settling. We have created a new set of asymptotic giant branch models to determine what the composition of material being accreted in these systems will be. We then model a range of CEMP systems by evolving a grid of models of low-mass stars, varying the amount of material accreted by the star (to mimic systems with different separations), and also the composition of the accreted material (to mimic accretion from primaries of different mass). We find that with thermohaline mixing alone, the accreted material can mix with 16–88 per cent of the pristine stellar material of the accretor, depending on the mass accreted and the composition of the material. If we include the effects of Gravitational Settling, we find that thermohaline mixing can be inhibited and, in the case that only a small quantity of material is accreted, can be suppressed almost completely.
Georges Michaud - One of the best experts on this subject based on the ideXlab platform.
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Models for Solar Abundance Stars with Gravitational Settling and Radiative Accelerations: Application to M67 and NGC188
The Astrophysical Journal, 2004Co-Authors: Georges Michaud, Jacques Richer, O Richard, Don A VandenbergAbstract:Evolutionary models taking into account radiative accelerations, thermal diffusion, and Gravitational Settling for 28 elements, including all those contributing to OPAL stellar opacities, have been calculated for solar metallicity stars of 0.5 to 1.4 solar masses. The Sun has been used to calibrate the models. Isochrones are fitted to the observed color-magnitude diagrams (CMDs) of M67 and NGC188, and ages of 3.7 and 6.4 Gyr are respectively determined. Convective core overshooting is not required to match the turnoff morphology of either cluster, including the luminosity of the gap in M67, because central convective cores are larger when diffusive processes are treated. This is due mainly to the enhanced helium and metal abundances in the central regions of such models. The observation of solar metallicity open clusters with ages in the range 4.8--5.7Gyr would further test the calculations of atomic diffusion in central stellar regions: according to non-diffusive isochrones, clusters should not have gaps near their main-sequence turnoffs if they are older than ~4.8Gyr, whereas diffusive isochrones predict that gaps should persist up to ages of ~5.7Gyr. Surface abundance isochrones are also calculated. In the case of M67 and NGC188, surface abundance variations are expected to be small. Abundance differences between stars of very similar Teff are expected close to the turnoff, especially for elements between P and Ca. Moreover, in comparison with the results obtained for giants, small generalized underabundances are expected in main-sequence stars. The lithium to beryllium ratio is discussed briefly and compared to observations.
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Models of Metal-poor Stars with Gravitational Settling and Radiative Accelerations. II. The Age of the Oldest Stars
The Astrophysical Journal, 2002Co-Authors: Don A Vandenberg, Georges Michaud, O. Richard, Jacques RicherAbstract:Isochrones for ages between 12 and 18 Gyr have been derived from the evolutionary tracks presented in Paper I (Richard et al.) for masses from 0.5 to 1.0 Mand initial chemical abundances corresponding to (1) Y ¼ 0:2352, Z ¼ 1:69 � 10 � 4 ð½Fe=H �¼� 2:31; ½�= Fe �¼ 0:3Þ and (2) Y ¼ 0:2370, Z ¼ 1:69 � 10 � 3 ð½Fe=H ¼� 1:31; ½�= Fe �¼ 0:3Þ. These are the first models for Population II stars in which both Gravitational Settling and radiative accelerations have been taken into account. Allowance for these diffusive processes leads to a 10%-12% reduction in age at a given turnoff luminosity. However, in order for the diffusive models to satisfy the constraints from Li and Fe abundance data (see Paper I) and to reproduce the observed mor- phologies of globular cluster (GC) color-magnitude diagrams (CMDs) in a straightforward way, extra mix- ing just below the boundary of the convective envelope seems to be necessary. Indeed, when additional turbulent mixing is invoked, the resultant models are able to satisfy all of these constraints, as well as those provided by the CMDs of local subdwarfs, rather well. Moreover, they imply an age near 13.5 Gyr for M92, which is one of the most metal-deficient (and presumably one of the oldest) of the Galaxy's GCs, if the field subgiant HD 140283 is used to derive the cluster distance. Comparisons of field subdwarfs and subgiants with a recently published fiducial for M5 suggests that the cluster has ½Fe=Hd � 1:4, in conflict with some esti- mates based on high-resolution spectroscopy, if the metallicities of the field stars are to be trusted. In addi- tion, an age of � 11.5 Gyr is found for M5, irrespective of whether diffusive or nondiffusive isochrones are employed in the analysis. The implications of our results for the extragalactic distance scale and for the Hub- ble constant are briefly discussed in the context of the presently favoredM � 0:35, � � � 0:65 cosmological model.
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Models of metal poor stars with Gravitational Settling and radiative accelerations: I. evolution and abundance anomalies
The Astrophysical Journal, 2002Co-Authors: O Richard, Georges Michaud, Jacques Richer, S Turcotte, S. Turck-chieze, Don A VandenbergAbstract:Evolutionary models have been calculated for Population II stars of 0.5-1.0 M? from the pre-main sequence to the lower part of the giant branch. Rosseland opacities and radiative accelerations were calculated taking into account the concentration variations of 28 chemical species, including all species contributing to Rosseland opacities in the OPAL tables. The effects of radiative accelerations, thermal diffusion, and Gravitational Settling are included. While models were calculated for both Z = 0.00017 and 0.0017, we concentrate on models with Z = 0.00017 in this paper. These are the first Population II models calculated taking radiative acceleration into account. It is shown that, at least in a 0.8 M? star, it is a better approximation not to let Fe diffuse than to calculate its Gravitational Settling without including the effects of grad(Fe). In the absence of any turbulence outside of convection zones, the effects of atomic diffusion are large mainly for stars more massive than 0.7 M?. Overabundances are expected in some stars with Teff ? 6000 K. Most chemical species heavier than CNO are affected. At 12 Gyr, overabundance factors may reach 10 in some cases (e.g., for Al or Ni), while others are limited to 3 (e.g., for Fe). The calculated surface abundances are compared to recent observations of abundances in globular clusters as well as to observations of Li in halo stars. It is shown that, as in the case of Population I stars, additional turbulence appears to be present. Series of models with different assumptions about the strength of turbulence were then calculated. One series minimizes the spread on the Li plateau, while another was chosen with turbulence similar to that present in AmFm stars of Population I. Even when turbulence is adjusted to minimize the reduction of Li abundance, there remains a reduction by a factor of at least 1.6 from the original Li abundance. Independent of the degree of turbulence in the outer regions, Gravitational Settling of He in the central region reduces the lifetime of Population II stars by 4%-7% depending on the criterion used. The effect on the age of the oldest clusters is discussed in a forthcoming paper (Paper II). Just as in Population I stars where only a fraction of stars, such as AmFm stars, have abundance anomalies, one should look for the possibility of abundance anomalies of metals in some Population II turnoff stars but not necessarily in all. Expected abundance anomalies are calculated for 28 species and compared to observations of M92 as well as to Li observations in halo field stars.
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Models of Metal-poor Stars with Gravitational Settling and Radiative Accelerations. III. Metallicity Dependence
The Astrophysical Journal, 2002Co-Authors: O Richard, Georges Michaud, Jacques RicherAbstract:Evolutionary models have been calculated for Population II stars of 0.5-1.2 Mfrom the pre-main sequence to the lower part of the giant branch. Models were calculated for Z ¼ 0:017 � 10 � 4 to 0.0068 (½Fe=H �¼� 4:31 to � 0.71) to determine the effect of metallicity on the size of abundance anomalies to be expected from Gravitational Settling, thermal diffusion, and radiative accelerations. Rosseland opacities and radiative accelerations were calculated taking into account the concentration variations of 28 chemical spe- cies, including all species contributing to Rosseland opacities in the OPAL tables. It is shown that while radi- ative accelerations and Gravitational Settling may lead to abundance anomalies by factors of 2-10 in turnoff stars of metal-poor clusters such as M92, much smaller abundance anomalies are expected in relatively metal-rich globular clusters such as M5, M71, or 47 Tuc. Even in NGC 6397, which is only a factor of 2 more metal-rich than M92, atomic diffusion is expected to lead to smaller anomalies than in M92. In field stars with Teff � 6000 K and ½Fe=H� < � 2:3, the abundance anomalies might be even larger than in M92. Reduction of metallicity beyond ½Fe=H �¼� 3:31 is shown not to cause further structural changes to models. Below that metallicity, all metals may be treated as trace elements. Comparisons are made to abundance observations in a number of clusters to determine if hydrodynamic processes competing with atomic diffusion are required by observations. For most metals the situation remains ambiguous: observations, taking into account the error bars, do not yet require additional processes. Monte Carlo simulations are used to show that the Spite plateau for Li in low-metallicity field stars remains the strongest argument for the presence of a process com- peting with atomic diffusion. Subject headings: convection — diffusion — stars: abundances — stars: evolution — stars: interiors — stars: Population II — turbulence On-line material: color figures
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Helium Gravitational Settling in the Envelopes of Evolving Main-Sequence A and F Stars
The Astrophysical Journal Supplement Series, 1992Co-Authors: Jacques Richer, Georges Michaud, Charles R. ProffittAbstract:A modeling method has been developed to determine the envelope structure of A and F stars in presence of the Gravitational Settling of helium. This covers the mass range where Gravitational Settling and evolutionary effects have similar time scales and where the effects of radiative acceleration on diffusion become progressively more important, as T eff increases. Treating the competition between radiative acceleration and gravity requires, during evolution, a more detailed description of the envelope than standard evolutionary methods allow
David Hall - One of the best experts on this subject based on the ideXlab platform.
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Gravitational Settling of 22Ne in Liquid White Dwarf Interiors
The Astrophysical Journal, 2001Co-Authors: Lars Bildsten, David HallAbstract:The nuclear reactions that occur in the stellar progenitors of white dwarfs (WDs) lead to an internal composition of 12C, 16O, and a ``contaminant'' nucleus, 22Ne. The 22Ne is produced by helium captures on 14N left from hydrogen burning via the CNO cycle. By virtue of its two excess neutrons (relative to the predominant A=2Z nuclei), a downward force of 2m_pg is exerted on 22Ne in the WD interior. This biases its diffusive equilibrium, forcing 22Ne to settle towards the center of the WD. We discuss the physics of the Gravitational Settling when the WD is in the liquid state and the luminosity generated by it. This modifies the cooling of WD's with masses in excess of a solar mass. The current uncertainties in the microphysics even allow for solutions where a 1.2M_\odot WD remains mostly liquid for a few Gyrs due to the internal heating from 22Ne sedimentation. This highlights the need for an accurate calculation of the inter-diffusion coefficient, especially in the quantum liquid regime relevant for high mass WD's. There is also time in old, liquid WD's (such as those found in cataclysmic variables and possibly in accreting Type Ia progenitors) for partial Settling.
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Gravitational Settling of 22ne in liquid white dwarf interiors
The Astrophysical Journal, 2001Co-Authors: Lars Bildsten, David HallAbstract:The nuclear reactions that occur in the stellar progenitors of white dwarfs (WDs) lead to an internal composition of 12 C, 16 O, and a “contaminant” nucleus, 22 Ne . The 22 Ne is produced by helium captures on 14 N left from hydrogen burning via the CNO cycle. By virtue of its two excess neutrons (relative to the predominant A = 2Z nuclei), a downward force of ≈ 2mpg is exerted on 22 Ne in the WD interior. This biases its diffusive equilibrium, forcing 22 Ne to settle towards the center of the WD. We discuss the physics of the Gravitational Settling when the WD is in the liquid state and the luminosity generated by it. This modifies the cooling of WD’s with masses in excess of M⊙. The current uncertainties in the microphysics even allow for solutions where a 1.2M⊙ WD remains mostly liquid for a few Gyrs due to the internal heating from 22 Ne sedimentation. This highlights the need for an accurate calculation of the inter-diffusion coefficient , especially in the quantum liquid regime relevant for high mass WD’s. There is also time in old, liquid WD’s (such as thos e found in cataclysmic variables and possibly in accreting Type Ia progenitors) for partial Settling. Subject headings: diffusion ‐ novae, cataclysmic variables ‐ stars: abundances, interiors ‐ supernovae: general ‐ white dwarfs
L. Grassitelli - One of the best experts on this subject based on the ideXlab platform.
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models of low mass helium white dwarfs including Gravitational Settling thermal and chemical diffusion and rotational mixing
Astronomy and Astrophysics, 2016Co-Authors: Richard J. Stancliffe, Alina G. Istrate, Pablo Marchant, Thomas M. Tauris, Norbert Langer, L. GrassitelliAbstract: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 effects 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 ~ 0.16−0.45 M ⊙ . 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.
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Models of low-mass helium white dwarfs including Gravitational Settling, thermal and chemical diffusion, and rotational mixing
Astronomy & Astrophysics, 2016Co-Authors: Alina G. Istrate, Richard J. Stancliffe, Pablo Marchant, Thomas M. Tauris, Norbert Langer, L. GrassitelliAbstract: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 effects 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 between Z=0.0002 and Z=0.02, producing WDs with masses between 0.16-0.45 M$_{\odot}$. 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. 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. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime, a crucial ingredient for understanding the newly observed ELM proto-WD pulsators. 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.
Alina G. Istrate - One of the best experts on this subject based on the ideXlab platform.
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models of low mass helium white dwarfs including Gravitational Settling thermal and chemical diffusion and rotational mixing
Astronomy and Astrophysics, 2016Co-Authors: Richard J. Stancliffe, Alina G. Istrate, Pablo Marchant, Thomas M. Tauris, Norbert Langer, L. GrassitelliAbstract: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 effects 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 ~ 0.16−0.45 M ⊙ . 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.
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Models of low-mass helium white dwarfs including Gravitational Settling, thermal and chemical diffusion, and rotational mixing
Astronomy & Astrophysics, 2016Co-Authors: Alina G. Istrate, Richard J. Stancliffe, Pablo Marchant, Thomas M. Tauris, Norbert Langer, L. GrassitelliAbstract: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 effects 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 between Z=0.0002 and Z=0.02, producing WDs with masses between 0.16-0.45 M$_{\odot}$. 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. 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. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime, a crucial ingredient for understanding the newly observed ELM proto-WD pulsators. 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.
