by Thilo Bauer » Wed Sep 02, 2015 6:13 pm
Paolo,
this is very interesting to see the UV part in more detail. Having the plots "normalized" to equal width of the uploaded images probably would it make easier to compare spectral portions and wavelength identification of lines of alpha Lyr and VV Cep.
Ginestet and Carquillat (2002) so far noted the following about spectral classification in this wavelength range (3800-4800 A):
"Nevertheless, the various spectral classifications published for a given star are often very different, for both the hot and the cool components, and, for the latter, the determination of the luminosity class was a further difficulty. Moreover, in the conventional spectral domain of the MK classification (3800–4800 A ̊ ), the spectra of the two com- ponents appear inextricably tangled. Consequently, the spectral classifications of these binaries carried out in this spectral domain will inevitably be erroneous: the hot components will be classified as too late and the cool com- ponents as too early." - Ginestet and Carquillat, 2002. The Astrophysical Journal Supplement Series, 143:513–537
I think "different" spectral classification found for the VV Cep system is due to its variability during orbital phases and probably the stars and the shell (disk?) themselves.
The published work so far tends to recommend to observe both, the near UV (~3800-4800 A) and IR (~8000–8900 A) part of the composite spectrum.
Finally the authors noted for VV Cep:
"HD 208816.—VV Cep. No appropriate standard for subtraction."
Ernst, what do you think? Seems not that easy.
BTW: The UV part for me is included with Alpy, while the IR (~8000–8900 A) will be excluded due to geometry of optics and mean wavelength of the grism. Maybe the IR part is better for observers using Lisa with IR pack.
Thilo