There is a striking paucity of hydrogen-rich ( DA ) white dwarfs ( WDs ) relative to their hydrogen-deficient ( non-DA ) counterparts at the very hot end of the WD cooling sequence .
The three hottest known DAs ( surface gravity \log g \geq 7.0 ) have effective temperatures around T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 140 000 K , followed by only five objects in the range 104 000 – 120 000 K. They are by far outnumbered by forty non-DAs with T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 100 000 – 250 000 K , giving a DA/non-DA ratio of 0.2 .
In contrast , this ratio is the inverse of that for the cooler WDs .
One reason for this discrepancy could be uncertainties in the temperature determination of hot DAs using Balmer-line spectroscopy .
Recent investigations involving metal-ionization balances in ultraviolet ( UV ) spectra indeed showed that the temperatures of some DAs were underestimated , but the paucity of extremely hot DAs prevailed .
Here we present the results of a UV spectral analysis of one of the three hottest DAs , PG 0948+534 .
We find that its temperature was strongly overestimated by recent Balmer line analyses .
We correct it downward to 105 000 \pm 5000 K , aggravating the hot-DA paucity .
The Balmer-line problem encountered previously is not resolved by our non-LTE line-blanketed model atmospheres .
We speculate that it might be related to the possible presence of a magnetosphere .
This is supported by the V-band variability that shows a period of P = 3.45 d ( amplitude 0.19 mag ) , which we interpret as the star ’ s rotation period .
The metal abundances in PG 0948+534 are affected by atomic diffusion and we conclude that the onset of diffusion in hot DAs occurs when they cool below T \mathrm { \hskip { -1.72 pt } { } _ { eff } } \approx 105 000 K. We discuss the possibility that the paucity of very hot DAs is a consequence of their fast evolutionary rate .