What is the more common way that spectral types of subdwarf stars are written?
Celestia uses a suffixed VI, but the sd prefix seems quite frequent.
Unfortunately SIMBAD query search doesn't seem to be able to search for the sd prefix, but the subdwarfs I have found (sometimes having to use display MK measurements option) seem to be written with the sd prefix rather than the VI suffix.
Display of subdwarf spectral types
I'm not sure it matters, so long as you're consistent and/or use whichever luminosity class notation was used in the catalog you're referencing. For example, the following online lecture notes both mention that both suffixes and prefixes are used and make no comment as to preference:
http://www.astro.virginia.edu/class/maj ... lec2b.html
http://www-astro.physics.ox.ac.uk/~podsi/lec_b3_1_c.pdf
At least some publications even use both simultaneously. (The full text of the example I found is readable only with a subscription, although google found it. *grump*)
Costa et al., The Solar Neighborhood. XVI.
It probably would be appropriate for Celestia to be able to handle both.
http://www.astro.virginia.edu/class/maj ... lec2b.html
http://www-astro.physics.ox.ac.uk/~podsi/lec_b3_1_c.pdf
At least some publications even use both simultaneously. (The full text of the example I found is readable only with a subscription, although google found it. *grump*)
Costa et al., The Solar Neighborhood. XVI.
It probably would be appropriate for Celestia to be able to handle both.
Selden