In recent years cool white dwarfs have been studied for various aspects ( see e.g.Winget et al.,1987 Winget and van Horn, 1987, Koester, 1987, Llebert, 1980) and much effort has been Invested in attempts to interpret the energy distributions of these stars ( Greenstein, 1984, Zeldler-K.T. et al, 1986, Llebert et al., 1987, and others). However, it seems that in spite of these efforts the spectra in particular of the very cool objects with effective temperatures below about 6000 K are not yet fully under-stood, since they are extremely diverse and each objects needs special consideration. In addition, the analyses are extremely difficult because the principal constiuents of the atmospheres ( H, He ) and elements, which may donate the majority of electrons, are essentially invisible. Since usually only one ionlsatlon stage of an element is present, this implies that the gas pressure Pg is high ( compared e.g. to the solar photosphere ), the accurate value of Pg, however, cannot be determined reliably.
Infrared photometry of cool white dwarfs