A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations

The observation of the strengths and profiles of the hydrogen Balmer absorption series is an established technique for determining the effective temperature and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman series lines should be equally useful but, lying in the far-ultraviolet (FUV), are only accessible from space. Nevertheless, there are situations (for example, where the optical white dwarf spectrum is highly contaminated by the presence of a companion) in which use of the Lyman series may be essential. Therefore, it is important to establish whether or not the Lyman lines provide an equally valid means of measurement. We have already made a first attempt to study this problem, comparing Lyman line measurements from a variety of FUV instruments with ground-based Balmer line studies. Within the measurement uncertainties, we found the results from each line series to be broadly in agreement. However, we noted a number of potential systematic effects that could bias either measurement. With the availability of the Far Ultraviolet Spectroscopic Explorer (FUSE) data archive and observations from our own Guest Observer programmes, we now have an opportunity to examine the use of the Lyman series in more detail from observations of 16 DA white dwarfs. Here we have data produced by a single instrument and processed with a uniform data reduction pipeline, eliminating some of the possible systematic differences between observations of the same or different stars. We have also examined the scatter in values derived from multiple observations of the same star, which is significant. The new results partially reproduce the earlier study, showing that Balmer and Lyman line determined temperatures are in good agreement up to similar to50 000 K. However, above this value there is an increasing systematic difference between the Lyman and Balmer line results, the former yielding the higher temperature. At the moment, there is no clear explanation of this effect but we think that it is most likely associated with deficiencies in the detailed physics incorporated into the stellar model atmosphere calculations. Even so, the data do demonstrate that, for temperatures below 50 000 K, the Lyman lines give reliable results. Furthermore, for the hotter stars, a useful empirical calibration of the relationship between the Lyman and Balmer measurements has been obtained, which can be applied to other FUSE observations.