2019MNRAS.483.5291W

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 logg >= 7.0) have effective temperatures around T_eff = 140 000 K, followed by only five objects in the range 104 000-120 000 K. They are by far outnumbered by 40 non-DAs with T_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 ± 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_eff~105 000 K. We discuss the possibility that the paucity of very hot DAs is a consequence of their fast evolutionary rate.