6. Future work

The proposed classification of 1.5 Å spectra of hot subdwarfs has obvious merits for selecting targets for more detailed study, for providing a precise nomenclature for hot subluminous stars and for making statistical studies of the hot subdwarf population. A number of deficiencies still need to be addressed.

• A large number of subluminous stars with a non- sdB4 designation will be included to check the proposed classifications and to provide standards for currently empty boxes.
• The density dimension will be calibrated, including a spectral definition of the difference between sdB and HBB stars.
• The behaviour of the metal absorption lines must be quantified. Are there metal-poor and metal-rich subdwarfs, or C-rich and N-rich sequences?
• Spectra showing Balmer lines and no helium lines are degenerate - all are classified sdB:He0. Are there secondary indicators of effective temperature?
• For spectra dominated by HeII, the scheme becomes degenerate as HeI4471Å disappears. An extension to higher temperatures will require alternative diagnostics (cf. Werner 1992).

It should be emphasized that the proposed scheme does not replace the 10 Å classifications (e.g. Green et al. 1986). There will be circumstances such as studies of subdwarfs in distant clusters or further survey work where one will be forced to work at the lower resolution.