Aircraft inteference

[Jack Martin]

jsandse,

I followed the link to BeSS thank you and saw the absorption feature, whats going on in Gamma Cas is fascanating.

Regards,

Jack

[jsandse]

Whilst waiting for your other "expert" to come back I checked the literature myself about Be stars and gamma cas in particular and found some answers...

The reason for the double peak was explained by Struve in 1931

Reference here:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1931ApJ....73...94S&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

To summarise: Gamma Cass is the archetypal Be star.

It is a fast rotator and has a thin disk of gas round it which is escaping from the star - this is what is called a decretion disk. The shape of this disk has been calculated in some detail using interferometry and the disk is inclined at around 40 degrees to us.

Anyway the Halpha emission is caused by recombination of Hydrogen on the disk.

The double emission line is caused hydrogen recombination on the disk with the dip in the middle caused by the star and the part of the disk occluded by the star and the absorbing part of the disk in front of the star. (a picture would probably have helped here)

When I observed gamma cass the two emission lines are of the same height - in the past they haven't been though and this was due to a part of the disk having an increased density. (One-armed oscillatations actually - reference here http://www.aanda.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/aa/pdf/2006/07/aa2727-05.pdf )

And yes there is a lot more to gamma cass than this - there is the X-ray source to explain and its also a binary star.

Also there does not appear to be an agreed position on how these disks round Be stars form.

And the variability of gamma cass can be quite extreme - in the past the emission of the Halpha line has disappeared altogether! and there have been short periods of dramatically increasing emission in the Halpha line

So plenty to see and find out about this star.

[Jack Martin]

jsandse,

Checking the literature yourself is a good idea it provides a thorough answer/explanation to your question this makes fascanating reading the more I read and speak to people like you who understand spectroscopy the more motivated I become. Here is the answer from another professional;

'The absorption feature is very weak. The H-alpha emission line is

so strong that I think there is no possibility of seeing anything

of the absorption line that is expected to arise in the photosphere

of gamma Cas. A more likely explanation would be "self-absorption",

arising from cool plasma that lies just above and around the region(s)

from which H-alpha emission arises.'

Regards,

Jack

[gdheib0430]

And to think I have trouble taking pictures or obvserving lol. Great work this looks really interesting just not my intellectual cup of tea at the moment. Wish there was a post to explain all of these posts haha.

[jsandse]

Thanks for the reply Jack.Some cooler gas in front of the disk radially at rest to us could explain an absorption feature using Kirchoff's laws but I don't think this explains the changes in the feature over time.

The double peak is always weak for Gamma Cas - sometimes though stronger than I have observed and sometimes it isn't there at all!

The Shelyak site has a diagram below of what I was trying to describe previously

Gamma Cass would correspond to viewing position B which is approximately the viewing angle we are looking at the star at.

However a recent paper 2010 by Silaj

H\alpha PROFILES OF Be STARS

does a pretty comprehensive analysis of the Halpha line in Be stars - they use a model and try to get it to fit to the Halpha line profiles seen in Be stars. One of the main findings from this paper is that the angle that the disk is orientated to the observer does not always result in a spectra like the spectra described in the diagram above!

So I suppose that leaves me still confused

I'll need to do some more reading unless anyone out there has some good answers

[Jack Martin]

gdheib0430,

The explanations you are are looking for are in the posts you have to work at Spectroscopy I assure you that the results are very rewarding.

Regards,

Jack

[Jack Martin]

jsandse,

I think we need more than Kirchoff's laws to explain whats going on. The double peak is changing with time. The paper infers the spectrum is due to a line of sight effect which sounds plausible to me. You should do some more reading as I doubt very much if anyone has good answers apart from the authors.

Regards,

Jack

[durr]

Jack, can you please explain what a shell star is. Very interesting stuff and some real science for a change. Like Merlin said this is very interesting stuff and so much can be learned about a star and how it works from this equipment. Steve

[Jack Martin]

Steve,

B-emission (Be) stars are rapidly rotating stars of class B that are encircled by equatorial disks that radiate hydrogen emission lines. They constitute some 20% of B stars. The best known are Gamma Cas and Zeta Tauri. The spectrum of a Be star depends on the thickness of its disk, and on the disks inclination to the line of sight. If the disk which is about 20 times larger than the star, is viewed more or less pole-on, the emissions are single. If the disk is tilted more into the line of sight, the lines become split by the Doppler effect. The component from the receding portion of the disk is shifted to the red of centre, that from the approaching portion to the blue. If the disk lies close enough to the line of sight, metallic absorptions from the disk are superimposed on the stellar spectrum, and the Be star is called a shell star.

Regards,

Jack

[durr]

Thanks for the information Jack very helpful and concise. Some very interesting stuff being done on this part of the forum, very impressive. Steve

[durr]

To summarise: Gamma Cass is the archetypal Be star.

It is a fast rotator and has a thin disk of gas round it which is escaping from the star - this is what is called a decretion disk. The shape of this disk has been calculated in some detail using interferometry and the disk is inclined at around 40 degrees to us.

Jsandse or Jack, any ideas how this disc that surrounds the star gets there in the first place? What is the mechanism? Steve

[jsandse]

Thats a very good question you ask there Steve....and I don't think there is a definitive answer to it. Remembering that Be stars are all fast rotators - reckoned to be spinning at about 80% of break up speed then there needs to be some additional mechanism to cause mass to flow to create the disk. Best candidates are:

- where the Be star is part of a close binary then a disk could be formed by mass passing from one star to another (eg Roche overflow)

- for single Be stars outbursts or pulsations (non-radial pulsations in particular) likely cause the disk to be formed. However the mechanism(s) at work to produce the outbursts is not yet known