The ROLE OF TURBULENT PRESSURE AS a COHERENT PULSATIONAL DRIVING MECHANISM: THE CASE OF THE Δ SCUTI STAR HD 187547

Antoci, V.; Cunha, M. S.; Houdek, G.; Kjeldsen, H.; Trampedach, Regner; Handler, G.; Lüftinger, T.; Arentoft, T.; Murphy, Simon J.

doi:10.1088/0004-637x/796/2/118

Cited by 88 publications

(88 citation statements)

References 41 publications

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“…The sequence with M = 0.2390 M (not included in the figure) exhibits unstable models with periods longer than ∼13 000 s. More massive model sequences do not show unstable modes for the range of effective temperatures explored (T eff > ∼ 6000 K). It is important to mention that, at effective temperatures higher than ∼6000 K, there exist pulsating stars with modes that appear to be excited either stochastically or coherently by near surface convection, an example being the δ Scuti star HD 187547 (T eff ∼ 7500 K, log g ∼ 3.90; Antoci et al 2011Antoci et al , 2014. So, it is not inconceivable that some pulsation modes detected in low-mass pre-WD stars can be driven not by the κ − γ mechanism, but by stochastic excitation instead.…”

Section: Instability Domainsmentioning

confidence: 99%

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Córsico

Althaus

Serenelli

et al. 2016

A&A

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Context. Many low-mass (M /M < ∼ 0.45) and extremely low-mass (ELM, M /M < ∼ 0.18−0.20) white-dwarf stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period gravity-mode (g-mode) pulsations, and constitute the class of pulsating white dwarfs called ELMV stars. In addition, two low-mass pre-white dwarfs, which could be precursors of ELM white dwarfs, have been observed to show multiperiodic photometric variations. They could constitute a new class of pulsating low-mass pre-white dwarf stars. Aims. Motivated by this finding, we present a detailed nonadiabatic pulsation study of such stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. Methods. Our pulsation stability analysis is based on a set of low-mass He-core pre-white dwarf models with masses ranging from 0.1554 to 0.2724 M , which were derived by computing the nonconservative evolution of a binary system consisting of an initially 1 M ZAMS star and a 1.4 M neutron star companion. We have considered models in which element diffusion is accounted for and also models in which it is neglected. Results. We confirm and explore in detail a new instability strip in the domain of low gravities and low effective temperatures of the T eff − log g diagram, where low-mass pre-white dwarfs are currently found. The destabilized modes are radial and nonradial p and g modes excited by the κ − γ mechanism acting mainly at the zone of the second partial ionization of He, with non-negligible contributions from the region of the first partial ionization of He and the partial ionization of H. The computations with element diffusion are unable to explain the pulsations observed in the two known pulsating pre-white dwarfs, suggesting that element diffusion might be inhibited at these stages of the pre-white dwarf evolution. Our nonadiabatic models without diffusion, on the other hand, naturally explain the existence and range of periods of the pulsating pre-white dwarf star WASP J1628+10B, although they fail to explain the pulsations of WASP J0247−25B, the other known member of the class, indicating that the He abundance in the driving region of this star might be substantially higher than predicted by our models. Conclusions. Discoveries of additional members of this new class of pulsating stars and their analysis in the context of the theoretical background presented in this paper will shed new light on the evolutionary history of their progenitor stars.

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Section: Instability Domainsmentioning

confidence: 99%

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Córsico

Althaus

Serenelli

et al. 2016

A&A

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“…Rigorous studies of the Reynolds stress tensor in connection with time-dependent convection Antoci et al 2014) and eventually finite amplitude perturbations are required to investigate the physical origin of the possible connection between high-order g-modes, υ mac , and sub-surface turbulent pressure fluctuations.…”

Section: The γ Dor Instability Stripmentioning

confidence: 99%

“…Moreover, a recent study by Antoci et al (2014) showed that turbulent pressure in the hydrogen ionization zone is likely to explain the high-frequency, coherent pulsation modes observed in the δ Scuti pulsator HD 187547. Hydrogen recombines at a temperature on the order of 10 000 K, leading to high values of the Rosseland opacity and to a high Eddington factor in the corresponding layers (Sanyal et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Relating turbulent pressure and macroturbulence across the HR diagram with a possible link toγDoradus stars

Grassitelli

Fossati

Langer

et al. 2015

A&A

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A significant fraction of the envelope of low-and intermediate-mass stars is unstable to convection, leading to sub-surface turbulent motion. Here, we consider and include the effects of turbulence pressure in our stellar evolution calculations. In search of an observational signature, we compare the fractional contribution of turbulent pressure to the observed macroturbulent velocities in stars at different evolutionary stages. We find a strong correlation between the two quantities, similar to what was previously found for massive OB stars. We therefore argue that turbulent pressure fluctuations of finite amplitude may excite high-order, high-angular degree stellar oscillations, which manifest themselves at the surface an additional broadening of the spectral lines, i.e., macroturbulence, across most of the HR diagram. When considering the locations in the HR diagram where we expect high-order oscillations to be excited by stochastic turbulent pressure fluctuations, we find a close match with the observational γ Doradus instability strip, which indeed contains high-order, non-radial pulsators. We suggest that turbulent pressure fluctuations on a percentual level may contribute to the γ Dor phenomenon, calling for more detailed theoretical modeling in this direction.

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“…A similar conclusion has been reached by Cunha et al (2013) when considering the oscillations observed in the roAp star α Cir. As suggested at the time, and also by Antoci et al (2014) when considering the higher frequency pulsations observed in the δ Scuti star HD 187547, a possible alternative would be that these oscillations are still excited in the hydrogen ionization region, but by the perturbation of the turbulent pressure, not by the opacity. Testing this possibility will require addressing the complex dynamics associated with highfrequency oscillations in the strongly magnetic atmospheres of roAp stars.…”

Section: Resultsmentioning

confidence: 88%

The fundamental parameters of the roAp star HD 24712

Perraut

Brandão

Cunha

et al. 2016

A&A

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Context. There is still a debate about the nature of the mechanism that causes the pulsation excitation of the rapidly oscillating Ap stars that oscillate above the highest theoretically acoustic frequency. HD 24712 is a good test case for such a study because it is bright, its parallax accurately determined, and its frequency spectrum is well known. Aims. Visible long-baseline interferometry is a unique technique for measuring accurate angular diameters of targets as small as the brightest roAp stars, and thus estimating accurate radii by a method as independent as possible of atmosphere models. Methods. We used the visible spectrograph VEGA at the CHARA long-baseline optical array to observe HD 24712, and we derived its limb-darkened diameter. We also estimated its bolometric flux from spectroscopic data in the literature and determined its radius, luminosity, and effective temperature. Results. We determined a limb-darkened angular diameter of 0.335 ± 0.009 mas for HD 24712 and derived a radius of R = 1.772 ± 0.057 R , a luminosity of L = 7.2 ± 1.8 L , and an effective temperature of T eff = 7235 ± 280 K, which is in very close agreement with the values provided by the self-consistent stratified model developed for this star. We used these fundamental parameters to set HD 24712 in the Hertzsprung-Russell diagram. Its position is marginally consistent with the region where high radial order modes are predicted to be excited by the κ-mechanism. Conclusions. We conclude that oscillations in this star are most likely not driven by the κ-mechanism.

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The ROLE OF TURBULENT PRESSURE AS a COHERENT PULSATIONAL DRIVING MECHANISM: THE CASE OF THE Δ SCUTI STAR HD 187547

Cited by 88 publications

References 41 publications

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Relating turbulent pressure and macroturbulence across the HR diagram with a possible link toγDoradus stars

The fundamental parameters of the roAp star HD 24712

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