The blue-edge problem of the V1093 Herculis instability strip revisited using evolutionary models with atomic diffusion

We propose a methodological framework to perform forward asteroseismic modeling of stars with a convective core, based on gravity-mode oscillations. These probe the near-core region in the deep stellar interior. The modeling relies on a set of observed high-precision oscillation frequencies of lowdegree coherent gravity modes with long lifetimes and their observational uncertainties. Identification of the mode degree and azimuthal order is assumed to be achieved from rotational splitting and/or from period spacing patterns. This paper has two major outcomes. The first is a comprehensive list and discussion of the major uncertainties of theoretically predicted gravity-mode oscillation frequencies based on linear pulsation theory, caused by fixing choices of the input physics for evolutionary models. Guided by a hierarchy among these uncertainties of theoretical frequencies, we subsequently provide a global methodological scheme to achieve forward asteroseismic modeling. We properly take into account correlations amongst the free parameters included in stellar models. Aside from the stellar mass, metalicity and age, the major parameters to be estimated are the near-core rotation rate, the amount of convective core overshooting, and the level of chemical mixing in the radiative zones. This modeling scheme allows for maximum likelihood estimation of the stellar parameters for fixed input physics of the equilibrium models, followed by stellar model selection considering various choices of the input physics. Our approach uses the Mahalanobis distance instead of the often used χ 2 statistic and includes heteroscedasticity. It provides estimation of the unknown variance of the theoretically predicted oscillation frequencies.

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“…when estimating oscillation frequencies for hot subdwarfs in core helium burning (e.g., Charpinet et al 2011;Bloemen et al 2014).…”

Section: Microscopic Atomic Diffusionmentioning

confidence: 99%

Forward Asteroseismic Modeling of Stars with a Convective Core from Gravity-mode Oscillations: Parameter Estimation and Stellar Model Selection

Aerts

Molenberghs

Michielsen

et al. 2018

ApJS

Self Cite

164

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“…Schuh et al 2006). The instability regions are well defined observationally and theoretically (Charpinet et al 2001;Bloemen et al 2014) although the boundary between the two types becomes more fuzzy when viewed with space-borne sensitivity. Both classes of pulsating hot subdwarfs can be successfully modeled in terms of the same driving mechanism: a classical opacity (κ) mechanism associated with a local overabundance of iron and nickel in the driving region (Charpinet et al 1997).…”

Section: Introductionmentioning

confidence: 99%

Discovery of a second pulsating intermediate helium-enriched sdOB star

Latour

Green

Fontaine

2019

A&A

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We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. Up until now only one other He-enriched sdOB star (LS IV−14 • 116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294 s to 3400 s. We fitted our lowresolution, high signal-to-noise optical spectrum of the star with our grid of non-LTE model atmospheres and derived the following atmospheric parameters: T eff = 36120 ± 230 K, log g = 5.93 ± 0.04 and log N(He)/N(H) = −0.32 ± 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV−14 • 116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV−14 • 116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear but a possible scenario involves the -mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.

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“…These diffusive processes naturally produce the Fe and Ni enhancements. Bloemen et al [46] finally solved the blue-edge problem by calculating the pulsation properties for stellar models that included the necessary diffusion physics [55].…”

Section: The Importance Of Physical Diffusion Processesmentioning

confidence: 99%

Asteroseismic Constraints on the Models of Hot B Subdwarfs: Convective Helium-Burning Cores

2017

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Abstract.Asteroseismology of non-radial pulsations in Hot B Subdwarfs (sdB stars) offers a unique view into the interior of core-helium-burning stars. Ground-based and space-borne high precision light curves allow for the analysis of pressure and gravity mode pulsations to probe the structure of sdB stars deep into the convective core. As such asteroseismological analysis provides an excellent opportunity to test our understanding of stellar evolution. In light of the newest constraints from asteroseismology of sdB and red clump stars, standard approaches of convective mixing in 1D stellar evolution models are called into question. The problem lies in the current treatment of overshooting and the entrainment at the convective boundary. Unfortunately no consistent algorithm of convective mixing exists to solve the problem, introducing uncertainties to the estimates of stellar ages. Three dimensional simulations of stellar convection show the natural development of an overshooting region and a boundary layer. In search for a consistent prescription of convection in one dimensional stellar evolution models, guidance from three dimensional simulations and asteroseismological results is indispensable. Hot Subdwarf B starsSubdwarf B (sdB) stars are a class of hot (T eff = 20, 000−40, 000 K) and compact (log g = 5.0−6.2) stars with very thin hydrogen envelopes (M H < 0.01 M ) [1,2]. They form the so-called extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram, where most of them quietly burn helium in their cores for ∼10 8 years. A violent process stripped them of most of their hydrogen envelope before they underwent the helium flash near the tip of the red giant branch (RGB). As a consequence they will not climb up the asymptotic giant branch (AGB) once their core helium is exhausted, but rather evolve directly to become white dwarfs.Although some sdB stars appear to be single and others occur in wide binaries, surveys have concluded that the majority are in close binaries with white dwarf or lowmass main sequence companions [3][4][5]. A small fraction of the latter exhibit eclipses, offering insight into the orbital parameters and masses of the systems. These HW Virginis stars, named after the discovery system, play a crucial role in determining the mass distribution of sdB stars. The median mass of sdB stars estimated from these binaries is 0.469 M [6].The formation scenarios for sdB stars have to explain their existence in both single and binary systems as well as introduce a process by which the hydrogen envelope of the progenitor stars can be lost. Common envelope ejection (CE) or stable Roche lobe overflow (RLOF) during binary evolution result in systems with close and wide e-mail: jtschindler@email.arizona.edu separations, respectively, [7]. Population synthesis models can well explain the occurrence of binary systems and the mass distribution of the primary and secondary components [8]. To explain the existence of single sdB stars, extreme mass loss on the RGB [9-11] as well as white dwarf mergers...

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The blue-edge problem of the V1093 Herculis instability strip revisited using evolutionary models with atomic diffusion

Cited by 31 publications

References 43 publications

Forward Asteroseismic Modeling of Stars with a Convective Core from Gravity-mode Oscillations: Parameter Estimation and Stellar Model Selection

Forward Asteroseismic Modeling of Stars with a Convective Core from Gravity-mode Oscillations: Parameter Estimation and Stellar Model Selection

Discovery of a second pulsating intermediate helium-enriched sdOB star

Asteroseismic Constraints on the Models of Hot B Subdwarfs: Convective Helium-Burning Cores

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