Up: The classification of
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.
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