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Pulsating or not? A search for hidden pulsations below the red edge of the ZZ Ceti instability strip

Kotak, Rubina LU ; van Kerkwijk, M. H.; Clemens, J. C. and Bida, T. A. (2002) In Astronomy & Astrophysics 391(3). p.1005-1012
Abstract
The location of the red edge of the ZZ Ceti instability strip is definedobservationally as being the lowest temperature for which a white dwarfwith a H-rich atmosphere (DA) is known to exhibit periodic brightnessvariations. Whether this cut-off in flux variations is actually due to acessation of pulsation or merely due to the attenuation of anyvariations by the convection zone, rendering them invisible, is notclear. The latter is a theoretical possibility because with decreasingeffective temperature, the emergent flux variations become an eversmaller fraction of the amplitude of the flux variations in theinterior. In contrast to the flux variations, the visibility of thevelocity variations associated with the pulsations is not thought to... (More)
The location of the red edge of the ZZ Ceti instability strip is definedobservationally as being the lowest temperature for which a white dwarfwith a H-rich atmosphere (DA) is known to exhibit periodic brightnessvariations. Whether this cut-off in flux variations is actually due to acessation of pulsation or merely due to the attenuation of anyvariations by the convection zone, rendering them invisible, is notclear. The latter is a theoretical possibility because with decreasingeffective temperature, the emergent flux variations become an eversmaller fraction of the amplitude of the flux variations in theinterior. In contrast to the flux variations, the visibility of thevelocity variations associated with the pulsations is not thought to besimilarly affected. Thus, models imply that were it still pulsating, awhite dwarf just below the observed red edge should show velocityvariations. In order to test this possibility, we used time-resolvedspectra of three DA white dwarfs that do not show photometricvariability, but which have derived temperatures only slightly lowerthan the coolest ZZ Ceti variables. We find that none of our threetargets show significant periodic velocity variations, and set 95%confidence limits on amplitudes of 3.0, 5.2, and 8.8 km s<SUP>-1</SUP>.Thus, for two out of our three objects, we can rule out velocityvariations as large as 5.4 km s<SUP>-1</SUP> observed for the strongestmode in the cool white dwarf pulsator <ASTROBJ>ZZ Psc</ASTROBJ>. Inorder to verify our procedures, we also examined similar data of a knownZZ Ceti, <ASTROBJ>HL Tau 76</ASTROBJ>. Applying external informationfrom the light curve, we detect significant velocity variations for thisobject with amplitudes of up to 4 km s<SUP>-1</SUP>. Our results suggestthat substantial numbers of pulsators having large velocity amplitudesdo not exist below the observed photometric red edge and that the latterprobably reflects a real termination of pulsations. The data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
stars: white dwarfs, stars: oscillations, convection
in
Astronomy & Astrophysics
volume
391
issue
3
pages
1005 - 1012
publisher
EDP Sciences
external identifiers
  • wos:000177590500019
  • scopus:0036728620
ISSN
0004-6361
DOI
10.1051/0004-6361:20020846
language
English
LU publication?
yes
id
a0802b97-d36b-4ac5-9a0c-f802b52ca4c8 (old id 130327)
date added to LUP
2007-07-13 13:42:49
date last changed
2017-04-09 04:18:56
@article{a0802b97-d36b-4ac5-9a0c-f802b52ca4c8,
  abstract     = {The location of the red edge of the ZZ Ceti instability strip is definedobservationally as being the lowest temperature for which a white dwarfwith a H-rich atmosphere (DA) is known to exhibit periodic brightnessvariations. Whether this cut-off in flux variations is actually due to acessation of pulsation or merely due to the attenuation of anyvariations by the convection zone, rendering them invisible, is notclear. The latter is a theoretical possibility because with decreasingeffective temperature, the emergent flux variations become an eversmaller fraction of the amplitude of the flux variations in theinterior. In contrast to the flux variations, the visibility of thevelocity variations associated with the pulsations is not thought to besimilarly affected. Thus, models imply that were it still pulsating, awhite dwarf just below the observed red edge should show velocityvariations. In order to test this possibility, we used time-resolvedspectra of three DA white dwarfs that do not show photometricvariability, but which have derived temperatures only slightly lowerthan the coolest ZZ Ceti variables. We find that none of our threetargets show significant periodic velocity variations, and set 95%confidence limits on amplitudes of 3.0, 5.2, and 8.8 km s&lt;SUP&gt;-1&lt;/SUP&gt;.Thus, for two out of our three objects, we can rule out velocityvariations as large as 5.4 km s&lt;SUP&gt;-1&lt;/SUP&gt; observed for the strongestmode in the cool white dwarf pulsator &lt;ASTROBJ&gt;ZZ Psc&lt;/ASTROBJ&gt;. Inorder to verify our procedures, we also examined similar data of a knownZZ Ceti, &lt;ASTROBJ&gt;HL Tau 76&lt;/ASTROBJ&gt;. Applying external informationfrom the light curve, we detect significant velocity variations for thisobject with amplitudes of up to 4 km s&lt;SUP&gt;-1&lt;/SUP&gt;. Our results suggestthat substantial numbers of pulsators having large velocity amplitudesdo not exist below the observed photometric red edge and that the latterprobably reflects a real termination of pulsations. The data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation.},
  author       = {Kotak, Rubina and van Kerkwijk, M. H. and Clemens, J. C. and Bida, T. A.},
  issn         = {0004-6361},
  keyword      = {stars: white dwarfs,stars: oscillations,convection},
  language     = {eng},
  number       = {3},
  pages        = {1005--1012},
  publisher    = {EDP Sciences},
  series       = {Astronomy & Astrophysics},
  title        = {Pulsating or not? A search for hidden pulsations below the red edge of the ZZ Ceti instability strip},
  url          = {http://dx.doi.org/10.1051/0004-6361:20020846},
  volume       = {391},
  year         = {2002},
}