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Robin Leadbeater
Meister
Joined: 25 Oct 2006
Posts: 236
Location: Cumbria England
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 Posted: 29. January 2010, 13:52:04 PM Post subject: velocity profiles in circumstellar discs |
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I am tracking eps Aur absorption line profiles as the 3.8AU radius rotating circumstellar disc surrounding the B star moves in front of the F star. I am seeing increasing contribution from higher radial velocity components as the eclipse progresses. More details here.
http://www.threehillsobservatory.co.uk/astro/spectra_40c.htm
I am interpreting this as due to the inner parts of the disc rotating faster than the outer parts (Keplerian motion as the mass is concentrated in the star) This applies to planets of course but my question is:
Does this also apply to a disc of gas or does for example drag within the material modify this?
Thanks
Robin
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Otmar Stahl
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Joined: 17 Dec 2006
Posts: 513
Location: Heidelberg
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| Posted: 29. January 2010, 14:39:31 PM Post subject: velocity profiles in circumstellar discs |
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Hi Robin,
Robin Leadbeater wrote:
| Quote: |
I am tracking eps Aur absorption line profiles as the 3.8AU radius
rotating circumstellar disc surrounding the B star moves in front of the
F star. I am seeing increasing contribution from higher radial velocity
components as the eclipse progresses. More details here.
http://www.threehillsobservatory.co.uk/astro/spectra_40c.htm
I am interpreting this as due to the inner parts of the disc rotating
faster than the outer parts (Keplerian motion as the mass is
concentrated in the star) This applies to planets of course but my
question is:
Does this also apply to a disc of gas or does for example drag within
the material modify this?
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Viscosity of the disc will not modify the motion. This force is far too
small. Other possible forces such as magnetic fields are - at these
large distances from the star - probably also too small to modify the
motion. I would say that Keplerian motion is a very safe assumption
here. Do the velocities you require match the expected Keplerian velocities?
There may be other effects, however: I am no export on eps Aur, but the
above should be true, if the disk is in stable rotational equilibrium.
How is the disc of eps Aur formed? If the disc is e.g. formed by
material expanding from the F-supergiant, and continuously replenished,
the assumption of purely rotational velocities may not be justified.
This is similar to the case of Be stars, where the disc are also
(probably) formed by ejection from the stars. However, even the Be-star
discs are rotating Keplerian, at least in the outer parts of the discs.
Best regards,
Otmar
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Robin Leadbeater
Meister
Joined: 25 Oct 2006
Posts: 236
Location: Cumbria England
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| Posted: 29. January 2010, 15:25:27 PM Post subject: Re: velocity profiles in circumstellar discs |
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| Otmar Stahl wrote: | Viscosity of the disc will not modify the motion. This force is far too
small. Other possible forces such as magnetic fields are - at these
large distances from the star - probably also too small to modify the
motion. I would say that Keplerian motion is a very safe assumption
here.
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Exellent, thank you Otmar. I had expected this but it is nice to have agreement from someone with more knowledge 
| Quote: |
Do the velocities you require match the expected Keplerian velocities?
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Well they are the right order of magnitude at least  . I am trying to improve my calculations. The estimate of the maximum RV in the line is difficult because there are contributions from lower RV components (and parts of the disc seen tangentially). Also the spectrum resolution is a significant proportion of the line width. Also the radius of the disc is not certain. Until a few weeks ago the favoured model was a supergiant F star and a disc radius of 10AU. Since the recent Hoard et al announcement at the AAS meeting , the F star is now thought to be post AGB and everything is smaller/lighter and the disc radius becomes 3.8AU. Another factor is the disc may not be circular (The RV line and intensity changes seen last eclipse were not symetric about the centre of the eclipse)
| Quote: |
There may be other effects, however: I am no export on eps Aur, but the
above should be true, if the disk is in stable rotational equilibrium.
How is the disc of eps Aur formed? If the disc is e.g. formed by
material expanding from the F-supergiant, and continuously replenished,
the assumption of purely rotational velocities may not be justified.
This is similar to the case of Be stars, where the disc are also
(probably) formed by ejection from the stars. However, even the Be-star
discs are rotating Keplerian, at least in the outer parts of the discs.
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Even eps Aur experts do not know the answer to this differences in the light curve (timing of the contact points) from eclipse to eclipse suggest the disc may be evolving but I am not convinced. (The photometry is influenced by quasi-periodic variations even outside eclipse which may distort the light curve) To add to the picture, the Ha line also shows variable Be star like emission so there may be a circumstellar disc round the F star too (and transfer between primary and secondary?) Also if the F star is post AGB there must be a lot of ejected material around. A fascinating (and complex) system
Best Wishes
Robin
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Robin Leadbeater
Meister
Joined: 25 Oct 2006
Posts: 236
Location: Cumbria England
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| Posted: 30. January 2010, 23:08:49 PM Post subject: |
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I have compared the maximum RV in my K I 7699 line profile with a
calculation of the orbital velocity for a circular Keplerian disc with
parameters from the new Hoard et al 2010 model, assuming the disc is in in equilibrium (attached). I found it difficult to estimate the maxium RV from the line profile with any great accuracy (the spectrum resolution is ~10 km/s) but the observed values so far are consistently about 15km/sec lower than the model. This could be connectd with the asymmetry (20km difference) between the mean ingress and egress RV seen last eclipse.
Robin
| Description: |
| eps Aur comparison of RV and Keplerian model |
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