Seven White Dwarfs

Figure 1: The original seven dwarfs. Credit: Encyclopedia Britannica
Astronomers in Budapest recently submitted their findings on seven white dwarfs to Acta Astronomica. These seven white dwarfs are potential ZZ Ceti (a.k.a., DAV) stars. ZZ Ceti are variable stars with small changes in their magnitude (i.e., low-amplitude) and short periods (i.e., the time from cycle to cycle of the change in magnitude). These are variations are thought to be a result of non-radial gravity mode (g-mode) pulsations. ZZ Ceti stars reside in the DAV instability strip (see Figure 2). They are distinguished from other white dwarfs by the fact that they still retain some hydrogen in their atmosphere. The ionization and recombination of this hydrogen is thought to drive the variability of these objects.

Figure 2: Hertzsprung-Russell diagram depicting location of ZZ Ceti stars in the DAV instability strip. Credit: Swinburne University
Observations were 30 second exposures taken with a CCD camera on a 1-meter telescope at the Konkoly Observatory in Hungary. Because detecting a signal in faint objects with small variations is difficult, after creating the light curves a polynomial model was fit to the data to account for atmospheric and instrumental effects. Using these models, only one of the seven candidates (PM J22299) was found to vary (See Figure 3). However, the researchers believe that another one of the targets, TON 451, remains a promising candidate to detect variation in the future because it is fainter than the other objects and was observed when the weather was not ideal. The location of PM J22299 and TON 451 within the instability strip (see Figure 3) would appear to support this notion. The continued study of white dwarfs is important as aids astronomers in piecing together a more comprehensive understanding of the evolution of the Milky Way.

Figure 3: Dashed lines represent the edges of the DAV instability strip. Red dots are known variable stars. Green, blue, and purple dots are ZZ Ceti candidates. The blue and purple dots represent the targets of this study. The purple dot (PM J22299) was the only candidate found to vary. Credit: Bognar et al. 2019

References

Bognar, Z., Kalup, C., & Sodor, A., https://arxiv.org/pdf/1902.03165.pdf