Attack of the vampire sun! Astronomer...

Attack of the vampire sun! Astronomers spot binary system where one...

There are 1 comment on the Mail on Sunday story from Jul 25, 2012, titled Attack of the vampire sun! Astronomers spot binary system where one.... In it, Mail on Sunday reports that:

Now an international team has used the Very Large Telescope in Chile to study what are known as O-type stars, which have very high temperature, mass and brightness.

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“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Jul 25, 2012
Nice article.

I have been looking though relatively nearby O stars in the Hipparcos data and find that many of these are also in binaries (Delta Orionis, and Tau Canis Majoris for example).

The result appears the same there but surprisingly the mass of the O stars appears lower and the luminosity MUCH lower than the original MK classification states.

Apparently many of the O stars start off as very dense and low luminosity H burners via the C-N-O catalytic chain and only become the luminous monster O stars once the core switches to He or C burning at which point the star develops a Gamma cassiopeiae signature and the star inflates while maintaining a relatively similar temperature.

This is the cause of the mass transfer situation in binaries where the over luminous primary dumps onto a secondary which may accrete more then double its original mass while remaining under 1/10 of the luminosity.

Also of note: most of the close binary O and B stars seen are members of multiple systems and the central pair are being tidally driven to a merger by a more distant companion. This has implications for the maximum masses of stars produced. ie. in our Galaxy the maximum condensation mass as an H burner before the star blows its accretion disc away by its own radiation is ~60 solar masses. A merger of a close binary pair while still in the nebula can produce a ~120 solar mass star right there and then; (nebula at that point goes pop rather quickly as then will the merger remnant). Eta Carinae appears to be one such example and has been listed as a mass transfer binary implying that its initial 3rd companion is now being dumped on by the merger remnant (which is now either an O or an Ne burner).

The Globular cluster R136A in the LMC appears to be another case of ~60 solar mass O stars undergoing mergers, in this case the close proximity of the stars may result in direct collision. The largest star (mass) there is approx 230 solar masses indicating at least a 3 way, and possibly a 4 way merger.

This particular space is rather interesting to watch. I have also found in many cases that the expanding shell (and nebula) around the Hipparcos viewed O type stars results in a negative parallax as the 1st point dimming of the occulting bar registered on a luminous expanding shell resulting in a negative parallax result. As a result a list of distances via a VLT where the * rather then the expanding shell is viewed for parallax would come in very handy, as most of the interesting ones such as Theta 1 orionis can only be roughly worked out as being more distant than the foreground B stars in the same general direction.

My rough guess therefore is that Theta 1 orionis (Spectrum O5e and ~45 solar masses) has an absolute magnitude of about or just under -5. and is probably already a He burner.

Have a nice day: Ag

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