scholarly journals Asteroseismology of evolved stars to constrain the internal transport of angular momentum

2020 ◽  
Vol 634 ◽  
pp. L16 ◽  
Author(s):  
J. W. den Hartogh ◽  
P. Eggenberger ◽  
S. Deheuvels
Keyword(s):  
Angular Momentum ◽  
Stellar Evolution ◽  
White Dwarf ◽  
Momentum Transport ◽  
Intermediate Mass ◽  
Angular Momentum Transport ◽  
Rotation Rates ◽  
The Core ◽  
Intermediate Mass Stars ◽  
Core Rotation

Context. The internal characteristics of stars, such as their core rotation rates, are obtained via asteroseismic observations. A comparison of core rotation rates found in this way with core rotation rates as predicted by stellar evolution models demonstrate a large discrepancy. This means that there must be a process of angular momentum transport missing in the current theory of stellar evolution. A new formalism was recently proposed to fill in for this missing process, which has the Tayler instability as its starting point (by Fuller et al. 2019, MNRAS, 485, 3661, hereafter referred to as “Fuller-formalism”). Aims. We investigate the effect of the Fuller-formalism on the internal rotation of stellar models with an initial mass of 2.5 M⊙. Methods. Stellar evolution models, including the Fuller-formalism, of intermediate-mass stars were calculated to make a comparison between asteroseismically obtained core rotation rates in the core He burning phase and in the white dwarf phase. Results. Our main results show that models including the Fuller-formalism can match the core rotation rates obtained for the core He burning phases. However, these models are unable to match the rotation rates obtained for white dwarfs. When we exclude the Fuller-formalism at the end of the core He burning phase, the white dwarf rotation rates of the models match the observed rates. Conclusions. We conclude that in the present form, the Fuller-formalism cannot be the sole solution for the missing process of angular momentum transport in intermediate-mass stars.

scholarly journals Constraining transport of angular momentum in stars

2019 ◽  
Vol 622 ◽  
pp. A187 ◽  
Author(s):  
J. W. den Hartogh ◽  
P. Eggenberger ◽  
R. Hirschi
Keyword(s):  
Angular Momentum ◽  
Stellar Evolution ◽  
White Dwarf ◽  
Artificial Viscosity ◽  
Momentum Transport ◽  
Initial Mass ◽  
Helium Burning ◽  
Angular Momentum Transport ◽  
Rotation Rates ◽  
The Core

Context. Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars. Aims. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M⊙ stars. Methods. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M⊙. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates. Results. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity νadd around 107 cm2 s−1 provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the νadd during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.

scholarly journals γ Doradus stars as a test of angular momentum transport models

2019 ◽  
Vol 626 ◽  
pp. A121 ◽  
Author(s):  
R.-M. Ouazzani ◽  
J. P. Marques ◽  
M.-J. Goupil ◽  
S. Christophe ◽  
V. Antoci ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Main Sequence ◽  
Evolutionary Models ◽  
Intermediate Mass ◽  
Angular Momentum Transport ◽  
Rotation Rates ◽  
Giant Stars ◽  
Intermediate Mass Stars ◽  
Red Giant ◽  
Core Rotation

Helioseismology and asteroseismology of red giant stars have shown that distribution of angular momentum in stellar interiors, and the evolution of this distribution with time remains an open issue in stellar physics. Owing to the unprecedented quality and long baseline of Kepler photometry, we are able to seismically infer internal rotation rates in γ Doradus stars, which provide the main-sequence counterpart to the red-giants puzzle. Here, we confront these internal rotation rates to stellar evolution models which account for rotationally induced transport of angular momentum, in order to test angular momentum transport mechanisms. On the one hand, we used a stellar model-independent method developed by our team in order to obtain accurate, seismically inferred, buoyancy radii and near-core rotation for 37 γ Doradus stars observed by Kepler. We show that the stellar buoyancy radius can be used as a reliable evolution indicator for field stars on the main sequence. On the other hand, we computed rotating evolutionary models of intermediate-mass stars including internal transport of angular momentum in radiative zones, following the formalism developed in the series of papers started by Zahn (1992, A&A, 265, 115), with the CESTAM code. This code calculates the rotational history of stars from the birth line to the tip of the RGB. The initial angular momentum content has to be set initially, which is done here by fitting rotation periods in young stellar clusters. We show a clear disagreement between the near-core rotation rates measured in the sample and the rotation rates obtained from the evolutionary models including rotationally induced transport of angular momentum following Zahn’s prescriptions. These results show a disagreement similar to that of the Sun and red giant stars in the considered mass range. This suggests the existence of missing mechanisms responsible for the braking of the core before and along the main sequence. The efficiency of the missing mechanisms is investigated. The transport of angular momentum as formalized by Zahn and Maeder cannot explain the measurements of near-core rotation in main-sequence intermediate-mass stars we have at hand.

scholarly journals Asteroseismology of red giants to constrain angular momentum transport

2014 ◽  
Vol 9 (S307) ◽  
pp. 165-170
Author(s):  
P. Eggenberger
Keyword(s):  
Angular Momentum ◽  
Rotational Frequency ◽  
Momentum Transport ◽  
Red Giants ◽  
Angular Momentum Transport ◽  
Rotation Rates ◽  
Giant Stars ◽  
Red Giant ◽  
Core Rotation

AbstractAsteroseismic data obtained by theKeplerspacecraft have led to the recent detection and characterization of rotational frequency splittings of mixed modes in red-giant stars. This has opened the way to the determination of the core rotation rates for these stars, which is of prime importance to progress in our understanding of internal angular momentum transport. In this contribution, we discuss which constraints can be brought by these asteroseismic measurements on the modelling of angular momentum transport in stellar radiative zones.

scholarly journals The effect of tides on near-core rotation: analysis of 35 Kepler γ Doradus stars in eclipsing and spectroscopic binaries

2020 ◽  
Vol 497 (4) ◽  
pp. 4363-4375
Author(s):  
Gang Li ◽  
Zhao Guo ◽  
Jim Fuller ◽  
Timothy R Bedding ◽  
Simon J Murphy ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Eclipsing Binaries ◽  
Momentum Transport ◽  
Spectroscopic Binaries ◽  
Angular Momentum Transport ◽  
Rotation Rates ◽  
Rotation Periods ◽  
Spectroscopic Binary ◽  
Orbital Periods ◽  
Core Rotation

ABSTRACT We systematically searched for gravity- and Rossby-mode period spacing patterns in Kepler eclipsing binaries with γ Doradus pulsators. These stars provide an excellent opportunity to test the theory of tidal synchronization and angular momentum transport in F- and A-type stars. We discovered 35 systems that show clear patterns, including the spectroscopic binary KIC 10080943. Combined with 45 non-eclipsing binaries with γ Dor components that have been found using pulsation timing, we measured their near-core rotation rates and asymptotic period spacings. We find that many stars are tidally locked if the orbital periods are shorter than 10 d, in which the near-core rotation periods given by the traditional approximation of rotation are consistent with the orbital period. Compared to the single stars, γ Dor stars in binaries tend to have slower near-core rotation rates, likely a consequence of tidal spin-down. We also find three stars that have extremely slow near-core rotation rates. To explain these, we hypothesize that unstable tidally excited oscillations can transfer angular momentum from the star to the orbit, and slow the star below synchronism, a process we refer to as ‘inverse tides’.

scholarly journals Rotation rate of the solar core as a key constraint to magnetic angular momentum transport in stellar interiors

2019 ◽  
Vol 626 ◽  
pp. L1 ◽  
Author(s):  
P. Eggenberger ◽  
G. Buldgen ◽  
S. J. A. J. Salmon
Keyword(s):  
Angular Momentum ◽  
Rotation Rate ◽  
Open Clusters ◽  
Momentum Transport ◽  
The Sun ◽  
Angular Momentum Transport ◽  
Chemical Gradients ◽  
The Core ◽  
Stellar Interiors ◽  
Core Rotation

Context. The internal rotation of the Sun constitutes a fundamental constraint when modelling angular momentum transport in stellar interiors. In addition to the more external regions of the solar radiative zone probed by pressure modes, measurements of rotational splittings of gravity modes would offer an invaluable constraint on the rotation of the solar core. Aims. We study the constraints that a measurement of the core rotation rate of the Sun could bring on magnetic angular momentum transport in stellar radiative zones. Methods. Solar models accounting for angular momentum transport by hydrodynamic and magnetic instabilities were computed for different initial velocities and disc lifetimes on the pre-main sequence to reproduce the surface rotation velocities observed for solar-type stars in open clusters. The internal rotation of these solar models was then compared to helioseismic measurements. Results. We first show that models computed with angular momentum transport by magnetic instabilities and a recent prescription for the braking of the stellar surface by magnetized winds can reproduce the observations of surface velocities of stars in open clusters. These solar models predict both a flat rotation profile in the external part of the solar radiative zone probed by pressure modes and an increase in the rotation rate in the solar core, where the stabilizing effect of chemical gradients plays a key role. A rapid rotation of the core of the Sun, as suggested by reported detections of gravity modes, is thus found to be compatible with angular momentum transport by magnetic instabilities. Moreover, we show that the efficiency of magnetic angular momentum transport in regions of strong chemical gradients can be calibrated by the solar core rotation rate independently from the unknown rotational history of the Sun. In particular, we find that a recent revised prescription for the transport of angular momentum by the Tayler instability can be easily distinguished from the original Tayler–Spruit dynamo, with a faster rotating solar core supporting the original prescription. Conclusions. By calibrating the efficiency of magnetic angular momentum transport in regions of strong chemical gradients, a determination of the solar core rotation rate through gravity modes is of prime relevance not only for the Sun, but for stars in general, since radial differential rotation precisely develops in these regions during the more advanced stages of evolution.

Angular momentum transport in pre-main-sequence stars of intermediate mass

Solar Physics ◽  
1990 ◽  
Vol 128 (1) ◽  
pp. 287-298 ◽  
Author(s):  
C. Vigneron ◽  
A. Mangeney ◽  
C. Catala ◽  
E. Schatzman
Keyword(s):  
Angular Momentum ◽  
Main Sequence ◽  
Momentum Transport ◽  
Main Sequence Stars ◽  
Intermediate Mass ◽  
Angular Momentum Transport

scholarly journals Testing models of rotating stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 73-78
Author(s):  
Adrian T. Potter ◽  
Christopher A. Tout
Keyword(s):  
Angular Momentum ◽  
Stellar Evolution ◽  
Momentum Transport ◽  
Rotating Stars ◽  
Stellar Rotation ◽  
Rapid Rotation ◽  
Angular Momentum Transport ◽  
Free Parameters ◽  
Stellar Models ◽  
The Many

AbstractThe effects of rapid rotation on stellar evolution can be profound but we are only now starting to gather the data necessary to adequately determine the validity of the many proposed models of rotating stars. Some aspects of stellar rotation, particularly the treatment of angular momentum transport within convective zones, still remain very poorly explored. Distinguishing between different models is made difficult by the typically large number of free parameters in models compared with the amount of available data. This also makes it difficult to determine whether increasing the complexity of a model actually results in a better reflection of reality. We present a new code to straightforwardly compare different rotating stellar models using otherwise identical input physics. We use it to compare several models with different treatments for the transport of angular momentum within convective zones.

scholarly journals Angular Momentum Transport in Stellar Interiors

2019 ◽  
Vol 57 (1) ◽  
pp. 35-78 ◽  
Author(s):  
Conny Aerts ◽  
Stéphane Mathis ◽  
Tamara M. Rogers
Keyword(s):  
Angular Momentum ◽  
Current Theory ◽  
Momentum Transport ◽  
High Mass ◽  
Helium Burning ◽  
Angular Momentum Transport ◽  
The Core ◽  
Data Driven Approach ◽  
Red Giant ◽  
Hydrodynamical Simulations

Stars lose a significant amount of angular momentum between birth and death, implying that efficient processes transporting it from the core to the surface are active. Space asteroseismology delivered the interior rotation rates of more than a thousand low- and intermediate-mass stars, revealing the following: ▪ Single stars rotate nearly uniformly during the core-hydrogen and core-helium burning phases. ▪ Stellar cores spin up to a factor of 10 faster than the envelope during the red giant phase. ▪ The angular momentum of the helium-burning core of stars is in agreement with the angular momentum of white dwarfs. Observations reveal a strong decrease of core angular momentum when stars have a convective core. Current theory of angular momentum transport fails to explain this. We propose improving the theory with a data-driven approach, whereby angular momentum prescriptions derived frommultidimensional (magneto)hydrodynamical simulations and theoretical considerations are continuously tested against modern observations. The TESS and PLATO space missions have the potential to derive the interior rotation of large samples of stars, including high-mass and metal-poor stars in binaries and clusters. This will provide the powerful observational constraints needed to improve theory and simulations.

scholarly journals Can plume-induced internal gravity waves regulate the core rotation of subgiant stars?

2017 ◽  
Vol 605 ◽  
pp. A31 ◽  
Author(s):  
C. Pinçon ◽  
K. Belkacem ◽  
M. J. Goupil ◽  
J. P. Marques
Keyword(s):  
Angular Momentum ◽  
Differential Rotation ◽  
Internal Gravity Waves ◽  
Low Mass Stars ◽  
Rotation Rates ◽  
The Core ◽  
Low Mass ◽  
Red Giant ◽  
Core Rotation ◽  
Subgiant Stars

Context. The seismic data provided by the space-borne missions CoRoT and Kepler enabled us to probe the internal rotation of thousands of evolved low-mass stars. Subsequently, several studies showed that current stellar evolution codes are unable to reproduce the low core rotation rates observed in these stars. These results indicate that an additional angular momentum transport process is necessary to counteract the spin up due to the core contraction during the post-main sequence evolution. For several candidates, the transport induced by internal gravity waves (IGW) could play a non-negligible role. Aims. We aim to investigate the effect of IGW generated by penetrative convection on the internal rotation of low-mass stars from the subgiant branch to the beginning of the red giant branch. Methods. A semi-analytical excitation model was used to estimate the angular momentum wave flux. The characteristic timescale associated with the angular momentum transport by IGW was computed and compared to the contraction timescale throughout the radiative region of stellar models at different evolutionary stages. Results. We show that IGW can efficiently counteract the contraction-driven spin up of the core of subgiant stars if the amplitude of the radial-differential rotation (between the center of the star and the top of the radiative zone) is higher than a threshold value. This threshold depends on the evolutionary stage and is comparable to the differential rotation rates inferred for a sample of subgiant stars observed by the satellite Kepler. Such an agreement can therefore be interpreted as the consequence of a regulation mechanism driven by IGW. This result is obtained under the assumption of a smooth rotation profile in the radiative region and holds true even if a wide range of values is considered for the parameters of the generation model. In contrast, on the red giant branch, we find that IGW remain insufficient, on their own, to explain the observations because of an excessive radiative damping. Conclusions. IGW generated by penetrative convection are able to efficiently extract angular momentum from the core of stars on the subgiant branch and accordingly have to be taken into account. Moreover, agreements with the observations reinforce the idea that their effect is essential to regulate the amplitude of the radial-differential rotation in subgiant stars. On the red giant branch, another transport mechanism must likely be invoked.

scholarly journals Impact of convection and resistivity on angular momentum transport in dwarf novae

2018 ◽  
Vol 609 ◽  
pp. A77 ◽  
Author(s):  
N. Scepi ◽  
G. Lesur ◽  
G. Dubus ◽  
M. Flock
Keyword(s):  
Angular Momentum ◽  
Accretion Disk ◽  
White Dwarf ◽  
Momentum Transport ◽  
Simple Relationship ◽  
Dwarf Novae ◽  
Angular Momentum Transport ◽  
Viscous Instability ◽  
S Curve ◽  
The Impact

The eruptive cycles of dwarf novae are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to imply enhanced angular momentum transport in the accretion disk during outburst. This is measured by the stress to pressure ratio α, with α ≈ 0.1 required in outburst compared to α ≈ 0.01 in quiescence. Such an enhancement in α has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO magnetohydrodynamic (MHD) simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to dwarf novae. The results are robust against the choice of vertical boundary conditions. The thermal equilibrium solutions found by the simulations trace the well-known S-curve in the density-temperature plane that constitutes the core of the disk thermal-viscous instability model. We confirm that the high values of α ≈ 0.1 occur near the tip of the hot branch of the S-curve, where convection is active. However, we also present thermally stable simulations at lower temperatures that have standard values of α ≈ 0.03 despite the presence of vigorous convection. We find no simple relationship between α and the strength of the convection, as measured by the ratio of convective to radiative flux. The cold branch is only very weakly ionized so, in the second part of this work, we studied the impact of non-ideal MHD effects on transport. Ohmic dissipation is the dominant effect in the conditions of quiescent dwarf novae. We include resistivity in the simulations and find that the MRI-driven transport is quenched (α ≈ 0) below the critical density at which the magnetic Reynolds number Rm ≤ 104. This is problematic because the X-ray emission observed in quiescent systems requires ongoing accretion onto the white dwarf. We verify that these X-rays cannot self-sustain MRI-driven turbulence by photo-ionizing the disk and discuss possible solutions to the issue of accretion in quiescence.

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