Post-AGB stars

Heinz,Proto-Planetary Nebulae, or PPNs, are thought to be formed by AGB stars, but an AGB star isn't quite synonymous with PPNs. Several posters at the AAS meeting in DC discussed their connection and the current interpretation is that as an AGB star evolves, its wind deposits the material the eventually shows up as a PPN. If you would like to know more, you might check out this website which offers a little more detail.One interesting thing is that most AGB stars show some evidence for dust around them as has been observed by SPITZER and AKARI, but eps is too far (625 pc or ~2100 light years) away for either of these instrument to resolve any dust in images. Furthermore, the spectra from this region (using SPITZER) shows no evidence for silicates (i.e. dust). Eps continues to offer challenges to astronomers, and an interesting thesis topic for me.Brian

Hi all: So-if we have a post-AGB star... Some questions spring to mind 1. Is radiation pressure from the B5 star clearing out a central hole and preventing further mass transfer? 2. Does this imply that mass that would have gone into a PPN instead went to increase the mass of the B5 star?
3. Or is envelope ejection just beginning and the B star had its mass kicked upwards by mass loss from the primaries' first trip up the RGB branch. That would provide both a dust source and explain an evolved lower mass star involved with an un-evolved higher mass star. I don't think mass ejection and the immediate post-AGB stages are all that well understood anyway-and binarity complicates that!. Another curious thing. All the models I have seen show the dust disk and secondary star passing in front of the F star. But if the F-star is the less massive component isn't this actually a case of the F star passing behind the dust disk/B star? Do the spectral lines radial velocity profiles support that? I agree with Arne. I don't think the mystery here is near being solved. We may have answered one question that triggered several new ones! Dave M.

Hi Dave "Another curious thing. All the models I have seen show the dust disk and secondary star passing in front of the F star. But if the F-star is the less massive component isn't this actually a case of the F star passing behind the dust disk/B star? Do the spectral lines radial velocity profiles support that?" I can have a go at answering this part. We have pretty good information about the radial velocity of the F star throughout the orbit but the RV of the secondary is not well established, which is the reason why the two alternative mass scenarios could not be distinguished. The absorption spectrum of the eclipsing body is only seen in the visible region during eclipse and during that time the change in radial velocityof the two components will be small as the motion is almost tangential to our line of site. Added to that, the spectrum comes from the rotating disc which has its own radial velocity component due to rotation. The upshot is there is not a good measure of the eclipsing object RV. With the detection of the B star spectrum in the UV however perhaps it could now be possible to measure the B star RV around the next 27 year orbit and get a good handle on the orbital parameters which could be used to confim the low mass scenario. Robin www.threehillsobservatory.co.uk

Thanks for the explanation of the RV profiles Robin. It cleared up some confusion I've had since I read about the F star being a less massive than had been thought. Clarifying the other questions and adding a few more:

Assuming the F star as a Post-AGB object holds up. Recognizing that our understanding of post-AGB stars and the accompaning mass ejection is not that well understood Recognizing that binarity complicates everything I have a few more questions that come to mind.

1. The F star should be in the process of ejecting the envelope. Is there evidence for a circumstellar dust shell or is the dust going to feed the occulting disk instead of forming a PPN?

2. Is this occulting disk an accreting disk for the B5V (assuming that spectral class holds up!)?

3. If there is a central hole where the B5V is- is this component clearing out a central hole via radiation pressure stopping accretion?

5. If so will the radiation pressure from this star destroy the occulting disk?

6. Is the occulting disk from the post-AGB ejection or a fossil from the primary’s first ascent up the RGB branch?

7. Or did the secondary amass enough to become a B5V from mass ejected during the first RGB phase and the new occulting disk formed from the post-AGB envelope ejection phase.

8. If the second case do we have an instance where we are seeing the initial phases of post-AGB envelop ejection?

Dave M.

Dave, Very, very good questions. So good, as a matter of fact, that they are, for the most part, open areas of reserach. I've tried to answer your questions below, but some of them just don't have answers at the present time. I agree with your skepticism that the F-star is indeed a masquerading AGB star. At the moment, I am not convinced. Although I agree with the low-mass conclusion reached in recent works, Iwould not claim the F-star is an AGB as the evidence to support that claim has not been presented in detail. My biggest question is "what makes an AGB star, an AGB star?" I'll be reviewing some literature on this subject in the coming months so perhaps I'll be able to answer that sometime soon. In either case, the AGBscenario would be an interesting Team project, not to mention a great chapter for my thesis! From your first post:

1. Is radiation pressure from the B5 star clearing out a central hole and preventing further mass transfer? In terms of the radiation pressure from the B5V clearing out the central hole, probably in conjunction with the UV radiation from the star. As for mass transfer, the material surrounding the B5V (i.e. the disk) would still be experiencing mass transfer as the B5 star's radiation pressure wouldn't penetrate the disk (otherwise we wouldn't see the disk). In terms of mass transfer, the current party line is that there is no active mass transfer in the system (the data for this claim are unpublished).

2. Does this imply that mass that would have gone into a PPN instead went to increase the mass of the B5 star? Don Hoard's recent paper discusses this in some detail. If mass transfer did indeed occur (notice I'm being cautious here, as the origin of the disk is still unknown), then some of the mass would have gone through the inner Lagrangian point, L1, and more of the mass would have gone through the outer Lagrangian point, L2. This material may be present as a circum-system ring, but no published observations indicate the presence of this material. 3. Or is envelope ejection just beginning and the B star had its mass kicked upwards by mass loss from the primaries' first trip up the RGB branch. That would provide both a dust source and explain an evolved lower mass star involved with an un-evolved higher mass star. I would doubt the ejection is just beginning as the presence of the material around the B-star implies the ejection has already happened. Furthermore, historical/ancient star catalogs show no large change in brightness of eps Aur. that would be expected if an ejection had recently occurred (i.e. in the last few thousand years). Another curious thing. All the models I have seen show the dust disk and secondary star passing in front of the F star. But if the F-star is the less massive component isn't this actually a case of the F star passing behind the dust disk/B star? Do the spectral lines radial velocity profiles support that? This is entirely a matter of perspective and it actually happens both ways. During primary eclipse we say that the disk passes in front of the F-star because that is what we observe. Secondary eclipse happens when the F-star passes in front of the disk, but this has yet to be observed/found in the data. Also, as Robin mentioned above, very little spectrosopic information is available about the disk. We could, presumably, track the radial velocity of the B5V star, but we would need time on space-based UV spectrometers. There are some historical data from IUE in the archives that could be used for this purpose, but caution must be taken as the F-star dominates all but the shortest wavelengths. Now in reply to your most recent post:

1. The F star should be in the process of ejecting the envelope. Is there evidence for a circumstellar dust shell or is the dust going to feed the occulting disk instead of forming a PPN?

If the F-star is presently ejecting material, it would be very close in. The presence of the disk further complicates the issue because the standard methods of identifying circumstellar material (i.e. looking for an infrared excess) find the disk. As far as I am aware, no circumstellar materials have been observed. Again, there could be material there (lost through L2 instead of L1), but we just can't see it.

2. Is this occulting disk an accretion disk for the B5V (assuming that spectral class holds up!)? I would say no based upon some unpublished results that indicate there is no "hot spot" in the system. These data need to be re-analysed, and this may be a chapter in my thesis or a good Team project if the right people come on board.

3. If there is a central hole where the B5V is- is this component clearing out a central hole via radiation pressure stopping accretion? See reply to the similar question above.

5. If so will the radiation pressure from this star destroy the occulting disk? In the long run, the lifetimes of disks are short in comparision to their hosts stars, so more than likely yes.

6. Is the occulting disk from the post-AGB ejection or a fossil from the primary’s first ascent up the RGB branch? Good question! The evolutionary status of the system is in doubt. For the longest time the disk was theorized to be a proto-planetary disk (akin to a YSO's evolutionary track), but if this system is evolved(as would be the case with an AGB), what is a protoplanetary disk doing there?

7. Or did the secondary amass enough to become a B5V from mass ejected during the first RGB phase and the new occulting disk formed from the post-AGB envelope ejection phase. This is an open area for research.

8. If the second case do we have an instance where we are seeing the initial phases of post-AGB envelop ejection? As mentioned above, no.

Brian: Thanks for the responses. One of the reason's for asking that last question is that the F star appears to be too massive and cool for envelope ejection to be far along. Certainly neither parameter seems to put it near the PNN stage. Of course I'm not all that certain where the envelope ejection actually begins or even whether there is a demarcation between the strong stellar wind during the TP-AGB phase that increases to the point of envelope ejection Post-AGP. But of course-if we already knew the answers... For what its worth: Eps Aur has been shown at our local club's monthly public star parties but I don't think we've tried to get anyone to try their hand at estimating the magnitude. Our monthly public star party is this Sat and I was thinking I might see if anyone's interested. One of our members has a great motorized binocular chair that's always a favorite anyway