Observational white dwarf seismology

Kepler, S. O.

doi:10.1553/cia150s221

“…Mukadam et al (2004) found the DAV instability strip to be impure, when they used SDSS data as a homogeneous source of atmospheric parameters and found non-pulsators inside instability strip. Kepler (2007) later attributed this to the inaccuracy of those data, and said "The question of the purity of the ZZ Ceti instability strip also depends on the accuracy of determination of the effective temperatures and gravities, as the instability strip ranges only around 1200 K in T eff and depends on gravity." Detailed investigation in highresolution spectroscopic studies (Bergeron et al 1995(Bergeron et al , 2004Gianninas, Bergeron & Fontaine 2005 showed the DAV instability strip to be pure, i.e.…”

Section: Discussionmentioning

confidence: 99%

A search for non-pulsating, chemically normal stars in the δ Scuti instability strip using Kepler data

Murphy

¹

,

Bedding

²

,

Niemczura

³

et al. 2015

Monthly Notices of the Royal Astronomical Society

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We identify stars in the δ Sct instability strip that do not pulsate in p modes at the 50-µmag limit, using Kepler data. Spectral classification and abundance analyses from high-resolution spectroscopy allow us to identify chemically peculiar stars, in which the absence of pulsations is not surprising. The remaining stars are chemically normal, yet they are not δ Sct stars. Their lack of observed p modes cannot be explained through any known mechanism. However, they are mostly distributed around the edges of the δ Sct instability strip, which allows for the possibility that they actually lie outside the strip once the uncertainties are taken into account. We investigated the possibility that the non-pulsators inside the instability strip could be unresolved binary systems, having components that both lie outside the instability strip. If misinterpreted as single stars, we found that such binaries could generate temperature discrepancies of ∼300 K -larger than the spectroscopic uncertainties, and fully consistent with the observations. After these considerations, there remains one chemically normal nonpulsator that lies in the middle of the instability strip. This star is a challenge to pulsation theory. However, its existence as the only known star of its kind indicates that such stars are rare. We conclude that the δ Sct instability strip is pure, unless pulsation is shut down by diffusion or another mechanism, which could be interaction with a binary companion.

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“…White dwarfs constrain the age of the Galaxy and preserve an imprint of galactic history. For a short review of white dwarf seismology, see Kepler (2007); for extensive reviews see Fontaine & Brassard (2008) and Winget & Kepler (2008).…”

Section: Introductionmentioning

confidence: 99%

A search for a new class of pulsating DA white dwarf stars in the DB gap

Kurtz

¹

,

Shibahashi

²

,

Dhillon

³

et al. 2008

Monthly Notices of the Royal Astronomical Society

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While white dwarf stars are classified into many subgroups based on the appearance of hydrogen, helium, carbon, oxygen and other spectral lines -or even pure continuum with no lines in the case of the DC stars -the vast majority fall into two major subgroups: those with hydrogen atmospheres (the DA white dwarfs), and those with helium atmospheres (the DO and DB white dwarfs). Remarkably, in the range 45 000 ≥ T eff ≥ 30 000 K there are only a few white dwarfs with helium atmospheres to be found -the vast majority are DAs in this temperature range -although white dwarfs with helium atmospheres are found at both hotter (DO) and cooler (DB) effective temperatures. This dearth of helium atmosphere white dwarfs in this temperature range is known as the 'DB gap' and is understood in terms convective mixing of the outer atmospheres at the hot and cool ends of the gap, while radiative stability allows the lighter hydrogen to float to the top in the DB gap, so the stars are seen to be DA hydrogen atmosphere white dwarfs. Asteroseismology is an important tool for probing stellar interiors, and white dwarf stars are the most successfully studied group using this technique. In a stability analysis of the stars in the DB gap, Shibahashi has recently predicted the existence of a new class of pulsating white dwarf stars. He finds from models that DA white dwarfs near the red edge of the DB gap have convectively stable outer atmospheres because of a steep mean molecular weight gradient, yet nevertheless have a superadiabatic layer that renders them pulsationally unstable due to radiative heat exchange. There have been very few observational tests for pulsation among stars of this type. We have initiated a survey to search for the predicted pulsators and report here our first observations of five stars with the South African Astronomical Observatory 1.9-m telescope and University of Cape Town CCD photometer, and two stars with the William Herschel Telescope 4.2-m telescope and the ULTRACAM photometer. We have two detections at formal significance levels greater then 4σ ; the rest are null results with upper limits of about 6-8 mmag with the 1.9-m telescope and about 3 mmag with the 4.2-m telescope. The two formally significant detections need confirmation, but the cases for them are good. Should they be confirmed, a new class of pulsating white dwarfs will become available for asteroseismic investigation, providing new insight into white dwarfs in general and into the DB gap in particular.

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“…Mukadam et al (2004) found the DAV instability strip to be impure, when they used SDSS data as a homogeneous source of atmospheric parameters and found non-pulsators inside instability strip. Kepler (2007) later attributed this to the inaccuracy of those data, and said "The question of the purity of the ZZ Ceti instability strip also depends on the accuracy of determination of the effective temperatures and gravities, as the instability strip ranges only around 1200 K in T eff and depends on gravity." Detailed investigation in highresolution spectroscopic studies (Bergeron et al 1995(Bergeron et al , 2004Gianninas, Bergeron & Fontaine 2005 showed the DAV instability strip be pure, all the strip and outside do The of in δ instability without modes us the conclusion these the Sct strip pure, pulsation shut by or mechanism, could interactions a star.…”

Section: Discussionmentioning

confidence: 99%

A search for non-pulsating, chemically normal stars in the $δ$ Scuti instability strip using Kepler data

Murphy,

Bedding,

Niemczura

et al. 2014

Preprint

0

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We identify stars in the δ Sct instability strip that do not pulsate in p modes at the 50-µmag limit, using Kepler data. Spectral classification and abundance analyses from high-resolution spectroscopy allow us to identify chemically peculiar stars, in which the absence of pulsations is not surprising. The remaining stars are chemically normal, yet they are not δ Sct stars. Their lack of observed p modes cannot be explained through any known mechanism. However, they are mostly distributed around the edges of the δ Sct instability strip, which allows for the possibility that they actually lie outside the strip once the uncertainties are taken into account. We investigated the possibility that the non-pulsators inside the instability strip could be unresolved binary systems, having components that both lie outside the instability strip. If misinterpreted as single stars, we found that such binaries could generate temperature discrepancies of ∼300 K -larger than the spectroscopic uncertainties, and fully consistent with the observations. After these considerations, there remains one chemically normal nonpulsator that lies in the middle of the instability strip. This star is a challenge to pulsation theory. However, its existence as the only known star of its kind indicates that such stars are rare. We conclude that the δ Sct instability strip is pure, unless pulsation is shut down by diffusion or another mechanism, which could be interaction with a binary companion.

show abstract

Observational white dwarf seismology

Cited by 15 publications

References 80 publications

A search for non-pulsating, chemically normal stars in the δ Scuti instability strip using Kepler data

A search for non-pulsating, chemically normal stars in the δ Scuti instability strip using Kepler data

A search for a new class of pulsating DA white dwarf stars in the DB gap

A search for non-pulsating, chemically normal stars in the $δ$ Scuti instability strip using Kepler data

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