At the OHP I was able to get some nice spectra of VV Cep. Having an image at hand, probably helps to follow the discussion about spectral profiles of VV Cep, beta Lyr (shelyak), etc. So, I created a simple mount of certain raw stellar spectra, that I’ve taken at the OHP with my Vixen 8“ Cassegrain, Alpy 600 and modified Canon EOS 60D DSLR with Astronomik UV/IR L-Filter. Although, wavelength range is uncalibrated, Balmer lines of alpha Lyr will yield good watermarks for typical wavelengths of spectral properties.
This is an invite to discuss (and correct me, if I'm wrong, of course)!
From comparison of these stars, some of them Be stars, like Sheliak, VV Cep yields very confusing aspects from having a first look onto stellar spectrum obtained:
- Prominent emission lines
- No He emission
- Very low UV excess at Balmer lines below H-gamma (434 nm)
See: Hohle, Neuhäuser, Schulz, 2010: Astron. Nachr. / AN 331, No. 4, 349 – 360 (2010)
Any additions or comments on this remark?
From other papers, it is believed, the companion should have spectral type of a late B or A type. Taking into account given mass ratio of the dominant supergiant (64 solar masses) and the hot companion (35 solar masses), the hot companion of the VV Cep binary system also doesn't really seem to be a "light-weight" main sequence star.
I found a nice animation, how stars like the two components of VV Cep may evolve over time:
http://spiff.rit.edu/classes/phys230/le ... vol_hr.swf
I tried the animation to compare two stars with 30 and 70 solar masses. Luminosity and color of both stars will yield different results. The heavy 70 solar mass star will yield a huge diameter and red color, while the 35 solar mass companion should end up with a hot surface and lower overall brightness compared to the more heavy super giant. This could be an indication, that the companion will not directly be visible in the composite stellar spectrum of the binary system. Indeed, there is a publication, where authors tried to evaluate the two composite spectra. The authors reported, that there was no success to find a match for the companion (N. Ginestet and J. M. Carquillat, 2002. The Astrophysical Journal Supplement Series, Volume 143, Issue 2, pp. 513-537).
Simulated luminosities from the animation expressed in solar luminosities L_sun yield about 1.4 Mio L_sun for the red supergiant and 0.33 L_sun for the hot companion at similar time scales. However, this may indicate, brightnesses aren't as different, as one might expect. Thus, in a later state, like that we observe now for the M-type supergiant, the difference in brightness of both binary components should be about 1.5 magnitude difference, which is not really a big difference of apparent magnitude in the night sky.
From my spectrum, however, it is very impressive, how faint the blue and UV part of the spectrum (below 434 nm) really is. In this wavelength range one should expect the maximum of light of the hot companion. Please consider, the image intensities are already enhanced by amplifying low image intensities using a square-root like intensity correction. Thus, taking into account a difference in luminosity (apparent magnitude) the companion should show up a little brighter, like CH Cyg. Of course, this is not absolutely correct, when talking about "luminosity" or "bolometric magnitude" vs. absolute (or apparent) magnitudes of the two binary components of VV Cep. But, taking into account, that a hot star shows a more blue compressed spectrum, the blue part from the hot companion of type B should show up a little brighter compared to the broad M spectrum. To evaluate this, look how different spectra of an A type spectrum of alpha Lyr look compared to VV Cep or CH Cyg. Nevertheless, even after amplification of the spectral intensities the blue part of the spectrum of VV Cep remains "dark". Please correct me, if I'm wrong here.
Questions:
What might be reasons, why the two spectral components could not be obtained from matching synthetic stellar spectra?
Is there a professional here in the group, able to provide more precise estimates to the luminosities or absolute magnitudes of the two binary components?
What might be the reason not to find more obvious hints from a binary companion, whose magnitude difference shouldn't be as low, as seen from my spectrum?
What do you think?
Cheers,
Thilo