Production of Lithium and Heavy Elements in AGB Stars Experiencing PIEs

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article reads well and presents important results on the effects of PIEs on the abundances and yields of AGB stars.

I have only minor comments/language corrections and suggestions on the consequences of the PIEs 12C/13C ratio in AGB and post-AGB stars.

In line 3, I will change "was also" to "is also" or "have also been". Using the past simple may suggest that the statement is no longer valid.

In line 33, "If convection is active". I will rephrase to "when convection" or if not specify when/where convection is active.

In line 36, "that undergo HBB". Using "that" implicitly means that not all the stars in this range of masses undergo HBB. Is this true? In not use ", which" instead of "that". 

Line 39. Change to "efficient Li Burning". Regarding the temperature of the base of the envelope, is it the same as stated before "> 2.5 MK"?

Line 40. "lower mass AGB stars experiencing a PIE [3]."  It is not clear whether these stars produce Li in the same way explained before. 

Overall, I think section 1 is a bit confusing, at least for me. It would be better if you simply explain the only mechanism in which Li is produced in PIEs and then state in what stars and when this mechanism operates.

Lines 58-59. From the present writing, it is not clear if this new fitting of the formula for the electron capture reaction has been used in the paper or not. Please state clearly this.

Line 61. Change "were" to "has been" or "is" for the reason explained before.

Line 70. "can develop" is confusing. For me, a model either develops or does not develop PIE. "cand develop" sounds like sometimes it does, sometimes it does not.

Fig 2. Add yr after the number below arrows labelled a-f, also for consistency with Fig 3.

Fig 3. caption. Indicate that the grey areas are the same as in Fig 2.

Line 79. Change "details" to "detail". Same in line 104.

Lines 153-153. Here you state that AGB stars > 1.5 Msun develop further 3DUPs after PIE and so the 12C/13C ratio increases again. I do not understand how PIEs can explain the presence of J-type stars. For a star above 1.5 Msun to become C-rich, several 3TUDs are required. How does this confront the relationship between PIEs and C-rich stars with low 12C/13C ratios? Can a PIE occur anytime during the AGB evolution, in particular after a sufficient number of 3DUP events so the star has already become C-rich?

Lines 189-192. See the comment just before. In addition, you may also want to comment on post-AGB sources  (both PNe and pPNe) with very low 12C/13C ratios, which supposedly are the descendants of J-type stars. Are those objects the descendants of C-rich stars that left the AGB right after a PIE? See the cases of KJPN 8,  M 1-7, M 2-9, M 2-48, M 3-28, M 3-55, K3-5, and K4-47. 

Finally, it would be nice if you compare the 12C/13C ratio for yields you predict with those observed in J-type stars and possible descendants (pPNe and pPNe with reported low 12C/13C values

Comments on the Quality of English Language

Minor issues and a just suggestion for improvements.

Author Response

We thank the referee for her/his careful reading, relevant remarks and references. All changes in the paper are in bold.

Comments 1: In line 3, I will change "was also" to "is also" or "have also been". Using the past simple may suggest that the statement is no longer valid.
Response 1: we corrected

Comments 2: In line 33, "If convection is active". I will rephrase to "when convection" or if not specify when/where convection is active.
Response 2: we reorganized Sect 1 (cf. next comment)

Comments 3: In line 36, "that undergo HBB". Using "that" implicitly means that not all the stars in this range of masses undergo HBB. Is this true? In not use ", which" instead of "that". 
Response 3: indeed, we meant that all of these stars undergo HBB. However, we reorganized Sect 1 (cf. next comment)

Comments 4: Line 39. Change to "efficient Li Burning". Regarding the temperature of the base of the envelope, is it the same as stated before "> 2.5 MK"?
Response 4: we reorganized Sect 1 (cf. next comment)

Comments 5: Line 40. "lower mass AGB stars experiencing a PIE [3]."  It is not clear whether these stars produce Li in the same way explained before. 
Response 5: we reorganized Sect 1 (cf. next comment)

Comments 6: Overall, I think section 1 is a bit confusing, at least for me. It would be better if you simply explain the only mechanism in which Li is produced in PIEs and then state in what stars and when this mechanism operates.
Response 6: we reorganized Sect 1 and tried to make it clearer with a focus on PIE only 

Comments 7: Lines 58-59. From the present writing, it is not clear if this new fitting of the formula for the electron capture reaction has been used in the paper or not. Please state clearly this.
Response 7: we clarified

Comments 8: Line 61. Change "were" to "has been" or "is" for the reason explained before.
Response 8: we corrected

Comments 9: Line 70. "can develop" is confusing. For me, a model either develops or does not develop PIE. "cand develop" sounds like sometimes it does, sometimes it does not.
Response 9: indeed : depending on the amount of overshooting, a PIE can or cannot develop. For instance, without overshooting, the 3 M_sun models do not experience PIEs but if overshooting is included, they do

Comments 10: Fig 2. Add yr after the number below arrows labelled a-f, also for consistency with Fig 3.
Response 10: the figure was updated

Comments 11: Fig 3. caption. Indicate that the grey areas are the same as in Fig 2.
Response 11: the caption was changed

Comments 12: Line 79. Change "details" to "detail". Same in line 104.
Response 12: we corrected

Comments 13: Lines 153-153. Here you state that AGB stars > 1.5 Msun develop further 3DUPs after PIE and so the 12C/13C ratio increases again. I do not understand how PIEs can explain the presence of J-type stars. For a star above 1.5 Msun to become C-rich, several 3TUDs are required. How does this confront the relationship between PIEs and C-rich stars with low 12C/13C ratios? Can a PIE occur anytime during the AGB evolution, in particular after a sufficient number of 3DUP events so the star has already become C-rich?
Response 13: A PIE occur preferentially at the beginning of the TP-AGB phase (at least in our model). After a PIE, the envelope becomes highly enriched in C,N,O, heavy elements, … The surface C/O is variable but is > 1 . It is about 1 in a number of our models (which is similar to the C/O in J-type stars). Once enriched (after the PIE), the envelope opacity increases, as well as the (very uncertain) mass loss rate. This shorten the TP-AGB phase. If TP develop after the PIE, they will contribute to increase the C/O and 12C/13C ratio, in contradiction with J-type stars properties. However (1) the change in the C-O composition of the envelope after a PIE is so large that the impact of subsequent 3DUPs will  be rather small and (2) it might be that J-type stars are transient stellar population (just after PIE => J-type and later, after 3DUP episodes, the star change type and becomes e.g of N-type). The link between J-type stars and AGB with PIE stays hypothetical but is investigated in detail in the paper mentioned (in press). We added a sentence in the conclusion.

Comments 14: Lines 189-192. See the comment just before. In addition, you may also want to comment on post-AGB sources  (both PNe and pPNe) with very low 12C/13C ratios, which supposedly are the descendants of J-type stars. Are those objects the descendants of C-rich stars that left the AGB right after a PIE? See the cases of KJPN 8,  M 1-7, M 2-9, M 2-48, M 3-28, M 3-55, K3-5, and K4-47. 
Response 14: we thank the referee for pointing this interesting point. Indeed these post-AGB might be the descendants of AGB that experienced a PIE. And indeed, no (or few) further TP should have developed after the PIE so as to keep the 12C/13C ratio low.

Comments 15: Finally, it would be nice if you compare the 12C/13C ratio for yields you predict with those observed in J-type stars and possible descendants (pPNe and pPNe with reported low 12C/13C values
Response 15: we considered a sample of J-type stars (mostly from Abia et al.) and post-AGB sources listed in Table 2 of  Ziurys et al. 2020. These objects are now included in Fig. 7 and mentioned in the conclusions. More detailed comparisons are done in the paper mentioned (in press). 

Reviewer 2 Report

Comments and Suggestions for Authors

This is a theoretical paper that describes the
predictions on production of Li, and 13C,
due to Proton Ingestion Events (PIE), and also leading to the intermediate neutron capture process producing heavy elements.
The text is very clear and  straighforward, and the authors
are the main experts in the field.
The paper is acceptable as it is.

Minor corrections:

Introduction, 1st line: end life -->life end

2nd paragraph, 3rd line: abundnace --> correct spelling

2nd page, 1st line: temperature at the base of the envelope allow --> allows

Comments for author File: Comments.pdf

Comments on the Quality of English Language

none

Author Response

Comments 1: Introduction, 1st line: end life -->life end

Response 1 : we corrected

 

Comments 2: 2nd paragraph, 3rd line: abundnace --> correct spelling

Response 2 : we corrected

 

Comments 3: 2nd page, 1st line: temperature at the base of the envelope allow --> allows

Response 3 : we removed this part, as suggested by another referee

Reviewer 3 Report

Comments and Suggestions for Authors

This is a good, clear paper. There are only a few minor comments.

p.1 Sect 1 par. 2 line 3: 'abundnance'

near the bottom of page 1: the term A(Li) is introduced here but is not defined. Although in common use, it should be fully defined

p. 3  Fig 2. Clarify where does the TP occur in this diagram?

footnote 1: brackets around '1/Mtot' may help

p. 6 items 1 & 2: isn't the total production of Li always determined by the amount of 3He? If not all is used up initially, won't this happen eventually during the next PIE event? And if this is the second PIE event, won't the result depend on what happened during the previous one?

The term 'yield' is introduced here. The definition in footnote 2 has units of mass. However, in Fig 6 (p. 7) it is plotted as a unitless ratio.  Please clarify its definition and usage

 

 

Author Response

Comments 1: p.1 Sect 1 par. 2 line 3: 'abundnance'
Response 1: we corrected

Comments 2: near the bottom of page 1: the term A(Li) is introduced here but is not defined. Although in common use, it should be fully defined
Response 2: A(Li) is now defined the first time it appears in the text (footnote 1). We also defined the notation [X/Y] in another footnote

Comments 3: p. 3  Fig 2. Clarify where does the TP occur in this diagram?
Response 3: the thermal pulse has already developed at the beginning of the plot (i.e. at point a (or at t = 0 yr)). We clarified in the caption

Comments 4: footnote 1: brackets around '1/Mtot' may help
Response 4: we corrected

Comments 5: p. 6 items 1 & 2: isn't the total production of Li always determined by the amount of 3He? If not all is used up initially, won't this happen eventually during the next PIE event? And if this is the second PIE event, won't the result depend on what happened during the previous one?
Response 5: indeed, if a second PIE would develop, the unburnt He3 could be burn and produce Li. In our models however, we never obtain a second PIE : after a PIE, the AGB envelope becomes highly enriched in metals, increasing the opacity and mass loss. In stars with M_ini < 1.5 M_sun (with a not so big envelope), the AGB envelope is quickly lost, before any further thermal pulse can develop (e.g. Fig. 1 in paper https://ui.adsabs.harvard.edu/abs/2021A%26A...648A.119C/abstract). In these cases, no second PIE can develop. Actually, even if further thermal pulses develop, it is harder to obtain a second PIE : the envelope is enriched in metals and behaves like a higher metallicity star in which PIE are much less likely to develop (e.g. last paragraph of Sect 3.1.2 in paper https://ui.adsabs.harvard.edu/abs/2022A%26A...667A.155C/abstract).

Comments 6: The term 'yield' is introduced here. The definition in footnote 2 has units of mass. However, in Fig 6 (p. 7) it is plotted as a unitless ratio.  Please clarify its definition and usage
Response 6: we clarified in footnote 4