Discovery and asteroseismological analysis of a new pulsating DB white dwarf star, PG 2246+121

Abstract

We report 36.6 h of time-resolved CCD photometry of the DB white dwarf star PG 2246+121 and the discovery that it is a new pulsating variable. Analysis of our compact single-site data set allowed the detection of three mode multiplets, two triplets at 256 and 329 s, respectively, and one doublet at 286 s. The frequency splitting within those structures is exactly the same within the length and accuracy of our data set. We argue that these multiplets are the result of non-radial g-mode pulsations, most probably of spherical degree ℓ = 1, which then yields a formal stellar rotation period of 2.00 ± 0.12 d. We suggest that the excited modes are three consecutive radial overtones of order 3-7, most likely k = 4,5,6. This discovery's impact on the understanding of pulsating DB white dwarfs is discussed.