A Comparative Study of Optical and Ultraviolet Effective Temperatures for DA White Dwarfs from the IUE Archive

TL;DR

This study compares optical and UV effective temperatures of 140 DA white dwarfs from the IUE archive, finding good agreement below 40,000 K and within 75 pc, but significant discrepancies at higher temperatures or distances, mainly due to wavelength-dependent extinction.

Contribution

It provides a comprehensive comparison of optical and UV temperature measurements for DA white dwarfs and investigates the causes of discrepancies, highlighting interstellar reddening as a key factor.

Findings

Good agreement between optical and UV temperatures below 40,000 K within 75 pc.

Discrepancies at higher temperatures or larger distances are mainly due to interstellar reddening.

Simulations help identify binary candidates and understand temperature differences.

Abstract

We present a comparative study of effective temperatures determined from the hydrogen Balmer lines and from the UV energy distribution for 140 DA white dwarfs drawn from the IUE archive. Our results indicate that the optical and UV temperatures of the majority of stars below T~40,000 K and within ~75 pc are in fairly good agreement given the uncertainties. At higher temperatures and/or larger distances, however, significant discrepancies are observed. Several mechanisms are investigated to account for these discrepancies including the effect of interstellar reddening, the presence of metals in the photosphere, and the existence of unresolved binary white dwarfs. The results of our analysis reveal that wavelength-dependent extinction is the most natural explanation for the observed temperature differences. We also attempt to predict the differences in optical and UV temperatures expected from unresolved degenerate binaries by performing an exhaustive simulation of composite model spectra. In light of these simulations, we then discuss some known double degenerates and identify new binary candidates by restricting our analysis to stars located within 75 pc where the effect of interstellar reddening is significantly reduced.