Letter Regarding Planetary Nebula
and Evolution Theory

This appeared in “Neat Southern Planetaries“ and
Being an Endnote of Neat Southern Planetaries 5
(NSP 05)

On 11th July 1997, Dr.Jessica Chapman of the AAO addressed the Society on ‘The Life and Death of Stars’. Within this talk, she discussed the Asymptotic Giant Branch Phase (AGB) and the Formation of Planetary Nebulae. Unfortunately due to time constrictions in the old venue, Dr. Chapman did not have enough time to discuss some ideas within her talk. As we have been talking about planetaries in this series, some important points were raised in a personal communication about some glaring discrepancies with information often published in popular books and magazines. I thought that the following could be of some interest to members of the Society. If you attended my own lecture to the Society (Sept.1996) on ‘Planetary Nebulae’, some of the statements made during this lecture have had to be updated.

A major question that was bugging me for more than three years; - Will the Sun form a Planetary?

Most common books and articles, (including my presentation and text), as recent as August 1997 eiter stae nothing or will still state this incorrectly - the Sun will form a planetary.

The Letter to Dr.Chapman was as follows;

Thank you for your address at the Astronomical Society of NSW last Friday night on Asymtopic Giant Stars. It was certainly an enjoyable lecture, and also obtained a good general response. I am sorry that the circumstances of occupying the hall made everything so rushed, and had little time to talk to you.

In summary:- I have been an adult education teacher for the last ten years, and am interested in the general topic of stellar evolution. I have found that most topics within this subject are usually easily to explain. This is made using the H-R Diagrams and a series of slides - from the birth of stars to their deaths as either supernovae (SN’s), neutron stars and white dwarfs. ( N.B. The level that I am teaching is similar to many portions of your lecture.)

In regards, planetaries I have been explaining (with suitable given text) that the progenitors generally are between 4.0 and 8.0 Mυ, producing WD’s 0.7 Msmsun to 1.3 M. This data is based primarily (and elsewhere) on Iben, I. Jr. article in the QJRAS Vol.26, entitled “Life and Times of an Intermediate Mass Stars- in Isolation/in a Close Binary.” This I was recommended by other sources.

In the literature I have searched (till Jan 1997), this range of the original progenitors is as low as 1.5 M, with the upper limit around 10 M. The only general text I know about that discusses mass ranges in any detail is Ken Croswell “The Alchemy of the Heavens” (1996).

I posed the following questions and below are her replies;
1. What is the mass range of the progenitors of planetaries in current theory prior to the AGB (Asymptotic Giant Branch) stage?

The mass range is not well determined but the minimum mass is below one solar mass while the upper mass is currently thought to be between 6 and 8 solar masses. The lower and upper limits are difficult to pin down precisely as the evolutionary models depend critically on things like the stellar metallicies and on the rate and duration of mass-loss in the AGB phase.
2. As the mass increases within this range, is their any known correlation between the structures observed in the PN to the amount of gas ejected in the AGB phase as seen in the illuminated circumstellar envelope?

Good question - I wish I knew the answer to this. About half of planetary nebulae show non-spherical shapes. I don’t think there is a known correlation though between shape and progenitor mass. In some cases the asymmetries (especially bipolarity) may be caused by binary companions. In other cases its thought to be related to an enhancement of a small density variation in the AGB phase which gets `amplified' in the post-AGB phase. The stellar magnetic fields may also play a role - though no-one has yet proved this.
3. What is the minimum mass suggested for the mass of the PNN - that does not have enough UV energy to illuminate the circumstellar envelope?

As above - I’m not sure exactly but well below 1 solar mass.
4. In current theory, does Sun produce a (proto-) planetary nebula?

5. Could you also suggest other authors that may assist in this subject (Other than Iben.)

Here’s a list of some related research papers. Most of these are heavy going but have useful tables or diagrams.

Referred References

1. Chapman J.M. & Cohen R.J., “The Circumstellar Envelope of VX Sgr.”, MNRAS, 220, 513 (1986)
- A detailed radio maser study of mine
2. Bedijn P.J.; “Pulsation, Mass-Loss and Evolution of AGB Stars.” A&A., 205, 105 (1988)
- Bedijn’s work produced many of the current ideas on AGB evolution
3. Vassiliadas and Wood, “Evolution of Low and Intermediate Mass Stars to the End of the AGB.” AJ., 413, 641 (1993)
- Probably best current paper on this topic
4. Vassiliadas and Wood; “Post-AGB Evolution of Low and Intermediate Mass Stars.”, AJ.Sup.Ser., 92, 125 (1994)
- On evolution from AGB to PN and white dwarfs.
5. Habing H.J. “Circumstellar Envelopes and AGB Stars.” A&A. Review, 7, 97 (1996)
- Good review paper with hundreds of references

Second Letter Regarding Planetary Nebula and Evolution Theory

It is interesting to note that the text published in so many general sources are wrong in their presentation on the formation and evolution on PN’s. If I may suggest, this information should be directly available for educational purposes. One of the most important aspects in teaching in evolution theory in astronomy is the relationship of the Sun to the other stars. Most of the text that I am aware of never actually state that the Sun will produce a planetary nebula, and if it does, the detail explaining this is often minor.

At present, a popularized article is well in order. An article in a magazine like Sky and Telescope maybe a suitable place for an overview. If you know of someone in the AAO staff who could right such an article, this maybe a good idea to publish - besides, it would make good publicity for the AAO!

If I should be so bold, a final suggestion for your ‘popular’ presentations of your Research is the relationship to the discards of the AGB phase of stars to the formation of elements essential to life and planetary formation. (Carl Sagan’s ‘Star Stuff’.) In education, this gives a ‘personal’ relationship to the evolution of stars. More importantly, if the Sun does produce a planetary nebula - this gives a visual and understandable relationship to the spectacular images of PN’s, like those of David Malin.

Again, thank you for you very much for your time, and best wishes for the Research projects that you follow in the future.

Regards, Andrew James

Dr.Chapman Replies;

I should perhaps have told you that I did in fact write up a popular article on just this topic - for the Australian Sky and Space magazine. I think the issue was December 95. I’ve also seen other good popular articles on stellar evolution in the American Sky and Telescope magazine.

The whole field of mass-loss from stars is very recent. The importance of mass-loss has only been recognised within the last 20 years and many of the details (i.e. structures/mass-ranges) are not yet properly understood. It is irritating that most of the popular text books present very out of date ideas on this topic but it does take time for front-line research to become properly known in the popular literature. Personally I try to give occasional popular talks on my own research as do many of my colleagues. Probably the best source for more recent information is the magazines, especially Scientific American and I think you can send them enquiries for past issues on selected topics.

Best wishes, Jessica


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