Kurtz, D. W., Shibahashi, H., Dhillon, V. S., Marsh, T. R. and Littlefair, S. P.. (2008) A search for a new class of pulsating DA white dwarf stars in the DB gap. Monthly Notices of the Royal Astronomical Society, Vol.389 (No.4). pp. 1771-1779. ISSN 0035-8711

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Abstract

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 000K 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 4s; 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.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science > Physics
Journal or Publication Title:	Monthly Notices of the Royal Astronomical Society
Publisher:	Wiley-Blackwell Publishing, Inc
ISSN:	0035-8711
Date:	1 October 2008
Volume:	Vol.389
Number:	No.4
Number of Pages:	9
Page Range:	pp. 1771-1779
Identification Number:	10.1111/j.1365-2966.2008.13664.x
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	PPARC
Grant number:	PP/D002370/1, PP/E001777/1
URI:	http://wrap.warwick.ac.uk/id/eprint/29363

Data sourced from Thomson Reuters' Web of Knowledge

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