A New Instability Strip for Hot Degenerates

Sumner G. Starrfield, Arthur N. Cox, Stephen W. Hodson
1980 International Astronomical Union Colloquium  
A completely new kind of variable star has recently been discovered (McGraw, et. al. 1979). Designated as PG1159-035 (hereafter PG) this star is distinguished not only by the complete lack of hydrogen in its spectrum but also by an effective temperature that exceeds 8 × 104K (McGraw, private communication). The photometric data show that this star is pulsating with two periods - 539 seconds and 460 seconds and the light curve is very reminiscent of that of a ZZ Ceti variable star (DA white
more » ... star (DA white dwarfs pulsating in non-radial modes). However, its spectral characteristics show that it cannot be included in this class since analysis of both optical and IUE spectra show that the major atmospheric constituents are probably helium and carbon and that its surface gravity is considerably lower than 108cm sec−2characteristic of a DA white dwarf. Its energy distribution suggests a small amount of reddening and since it is far out of the plane it must be at a distance at least 1 kpc. This estimate is supported by a null proper motion over a 13 year baseline (Luyten 1979, private communication to J Liebert). These data suggest that its luminosity exceeds 10 L⊙. In any case, it would need a luminosity as large as 10 L⊙to fall on or above the white dwarf cooling curve at Te= 8 × 104K (Lamb and Van Horn 1975). All of these facts suggest strongly that this star is unique and a new kind of pulsating variable.
doi:10.1017/s0252921100082142 fatcat:4ynp3xicozamtgl7te3riocnyi