Evolutionary status of Polaris

Fadeyev, Yu. A.

doi:10.1093/mnras/stv412

Cited by 23 publications

(26 citation statements)

References 36 publications

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“…In the present study the equations of radiation hydrodynamics for nonlinear stellar pulsations are solved together with transport equations for time-dependent convection in sphericallysymmetric geometry (Kuhfuß, 1986). The system of differential equations and the adopted parameters of the convection model are given in our previous paper (Fadeyev, 2015). The solution of the equations of hydrodynamics is represented by physical quantities as a function of mass coordinate M r and time t. Time t = 0 corresponds to the initial state of hydrostatic equilibrium.…”

Section: Calculations Of Stellar Evolutionmentioning

confidence: 99%

Nonlinear pulsations of stars with initial mass 3 $${M_ \odot }$$ M ⊙ on the asymptotic giant branch

Fadeyev

2016

Astron. Lett.

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-Pulsation period changes in Mira type variables are investigated using the stellar evolution and nonlinear stellar pulsation calculations. We considered the evolutionary sequence of stellar models with initial mass M ZAMS = 3M ⊙ and population I composition.Pulsations of stars in the early stage of the asymptotic giant branch are shown to be due to instability of the fundamental mode. In the later stage of evolution when the helium shell source becomes thermally unstable the stellar oscillations occur in either the fundamental mode (for the stellar luminosuty L < 5.4 × 10 3 L ⊙ ) or the first overtone (L > 7 × 10 3 L ⊙ ). Excitation of pulsations is due to the κ-mechanism in the hydrogen ionization zone. Stars with intermediate3 L ⊙ were found to be stable against radial oscillations.The pulsation period was determined as a function of evolutionary time and period change ratesΠ were evaluated for the first ten helium flashes. The period change rate becomes the largest in absolute value (Π/Π ≈ −10 −2 yr −1 ) between the helium flash and the maximum of the stellar luminosity. Period changes with rate |Π/Π| 10 −3 yr −1 take place during ≈ 500 yr, that is nearly one hundredth of the interval between helium flashes.

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Section: Calculations Of Stellar Evolutionmentioning

confidence: 99%

Nonlinear pulsations of stars with initial mass 3 $${M_ \odot }$$ M ⊙ on the asymptotic giant branch

Fadeyev

2016

Astron. Lett.

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“…Constraints have been obtained from the ratios of small separations of radial and dipole modes to the large separation for solar-like oscillators on the MS (e.g. Silva Aguirre et al 2011;Liu et al 2014;Yang et al 2015;Deheuvels et al 2016), the period spacings of mixed modes in RGB- (Deheuvels & Michel 2011;Montalbán et al 2013;Arentoft et al 2017), retired A-type (Hjørringgaard et al 2017) and δ Sct stars (Lenz et al 2010), rate of period change of Cepheids (Fadeyev 2015) and seismic modelling of β Cep stars (Aerts et al 2003;Pamyatnykh et al 2004;Walczak & Handler 2015). However, just like the case of isochrone fitting and binary stars, none of these asteroseismics modelling efforts have been able to constrain the shape of convective core overshooting.…”

Section: Introductionmentioning

confidence: 99%

The shape of convective core overshooting from gravity-mode period spacings

Pedersen

Aerts

Pápics

et al. 2018

A&A

111

123

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Context. The evolution of stars born with a convective core is highly dependent on the efficiency and extent of near core mixing processes, which effectively increases both the core mass and main-sequence lifetime. These mixing processes remain poorly constrained and therefore result in large uncertainties in the stellar structure and evolution models of such stars. Aims. We investigate to what extent gravity-mode period spacings in slowly pulsating B-type stars observed by the Kepler mission can be used to constrain both the shape and extent of convective core overshoot and additional mixing in the radiative envelope. Methods. We compute grids of 1D stellar structure and evolution models for two different shapes of convective core overshooting and three shapes of radiative envelope mixing. The models in these grids are compared to a set of benchmark models to evaluate their capability of mimicking the dipole prograde g-modes of the benchmark models.Results. Through our model comparisons we find that at a central hydrogen content of X c = 0.5, dipole prograde g-modes in the period range 0.8-3 d are capable of differentiating between step and exponential diffusive overshooting. This ability disappears towards the terminal age main-sequence at X c = 0.1. Furthermore, the g-modes behave the same for the three different shapes of radiative envelope mixing considered. However, a constant envelope mixing requires a diffusion coefficient near the convective core five times higher than chemical mixing from internal gravity waves to obtain a surface nitrogen excess of ∼ 0.5 dex within the main-sequence lifetime. Conclusions. Within estimated frequency errors of the Kepler mission, the ability of g-modes to distinguish between step and exponential diffusive overshooting depends on the evolutionary stage. Combining information from the average period spacing and observed surface abundances, notably nitrogen, could potentially be used to constrain the shape of mixing in the radiative envelope of massive stars.

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“…This would require it to be more luminous, its distance to be at least 118 pc (parallax less than ∼8.5 mas), and it to be pulsating in the first overtone. However, Fadeyev (2015), based on hydrodynamic pulsation models, reached the opposite conclusion: Polaris is crossing the instability strip for the first time and is a fundamental-mode pulsator.…”

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Hubble Space Telescope Trigonometric Parallax of Polaris B, Companion of the Nearest Cepheid*

Bond

Nelan

Evans

et al. 2018

ApJ

207

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Polaris, the nearest and brightest Cepheid, is a potential anchor point for the Leavitt period-luminosity relation. However, its distance is a matter of contention, with recent advocacy for a parallax of ∼10 mas, in contrast with the Hipparcos measurement of 7.54 ± 0.11 mas. We report an independent trigonometric parallax determination, using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope. Polaris itself is too bright for FGS, so we measured its 8th-magnitude companion Polaris B, relative to a network of background reference stars. We converted the FGS relative parallax to absolute, using estimated distances to the reference stars from ground-based photometry and spectral classification. Our result, 6.26 ± 0.24 mas, is even smaller than found by Hipparcos.We note other objects for which Hipparcos appears to have overestimated parallaxes, including the well-established case of the Pleiades. We consider possible sources of systematic error in the FGS parallax, but find no evidence they are significant. If our "long" distance is correct, the high luminosity of Polaris indicates that it is pulsating in the second overtone of its fundamental mode. Our results raise several puzzles, including a long pulsation period for Polaris compared to second-overtone pulsators in the Magellanic Clouds, and a conflict between the isochrone age of Polaris B (∼2.1 Gyr) and the much younger age of Polaris A.We discuss possibilities that B is not a physical companion of A, in spite of the strong evidence that it is, or that one of the stars is a merger remnant. These issues may be resolved when Gaia provides parallaxes for both stars.

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Evolutionary status of Polaris

Cited by 23 publications

References 36 publications

Nonlinear pulsations of stars with initial mass 3 $${M_ \odot }$$ M ⊙ on the asymptotic giant branch

Nonlinear pulsations of stars with initial mass 3 $${M_ \odot }$$ M ⊙ on the asymptotic giant branch

The shape of convective core overshooting from gravity-mode period spacings

Hubble Space Telescope Trigonometric Parallax of Polaris B, Companion of the Nearest Cepheid*

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