Red Supergiant Mass Loss and Mass-Loss Rates

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is a very interesting review about the mass loss of red supergiant stars. It begins with the author's view on the underlying physics governing the mass loss in those stars. This part brings interesting and physically well-motivated insights on this process. The text reads well and conveys an interesting personal view on the topic. This, in my view, makes the text much more interesting than a simple list of processes with references. May be, however, a few additional references at a few important places in sections 2 and 3 would be useful, especially for those who will use this paper as an introduction to the topic.

 

A well-documented section 4 is devoted to the different approaches for measuring the mass loss rates of red supergiants and what can be deduced from them. The author remains careful in making too strong conclusions and warns the readers about the complexities of the processes involved. In the last section, the author indicates how future developments aided by artificial intelligence may help open new windows on this process.

 

On the whole, I find this review very interesting and I recommend the editor to accept it, with maybe some additional references in sections 2 and 3, in its present form.

 

There are two points that came to my mind reading this review and on which the authors might give his point of view if he judges them relevant or useful (may be thes epoints might be added in this review or in further papers).

 

The first one is the possibility to use the distributions of red supergiants in luminosity (the luminosity function) to constrain, not the individual instantaneous mass loss rate during the red supergiant phase, but a time-averaged mass loss rates (typically see for instance the paper Dorn-Wallerstein et al. (2023) ApJ 959, 64).

 

The second concern the question of providing an accurate birth mass to a red supergiant. Maybe the author could mention. the fact that from photometry alone this is not possible since many different internal structures may correspond to identical values of the effective temperature and luminosity. Spectroscopy is needed to obtain surface gravity to obtain a more accurate estimate (see for instance https://ui.adsabs.harvard.edu/abs/2022PhDT........16F/abstract

the thesis of Eoin Farrell.)

Author Response

Comments 1:
The paper is a very interesting review about the mass loss of red supergiant stars. It begins with the author's view on the underlying physics governing the mass loss in those stars. This part brings interesting and physically well-motivated insights on this process. The text reads well and conveys an interesting personal view on the topic. This, in my view, makes the text much more interesting than a simple list of processes with references. May be, however, a few additional references at a few important places in sections 2 and 3 would be useful, especially for those who will use this paper as an introduction to the topic.
 
A well-documented section 4 is devoted to the different approaches for measuring the mass loss rates of red supergiants and what can be deduced from them. The author remains careful in making too strong conclusions and warns the readers about the complexities of the processes involved. In the last section, the author indicates how future developments aided by artificial intelligence may help open new windows on this process.
 
On the whole, I find this review very interesting and I recommend the editor to accept it, with maybe some additional references in sections 2 and 3, in its present form.
 
Response 1:
I agree. Initially I did not want to interrupt the train of thought in that preliminary part but was indeed planning on including some during the reviewing stage. Without going over the top, I've added a few pertinent citations such as the kappa mechanism, Parker solar wind model and the Castor, Abbott & Klein hot star wind model and a bunch of others.

Comments 2:
There are two points that came to my mind reading this review and on which the authors might give his point of view if he judges them relevant or useful (may be thes epoints might be added in this review or in further papers).
 
The first one is the possibility to use the distributions of red supergiants in luminosity (the luminosity function) to constrain, not the individual instantaneous mass loss rate during the red supergiant phase, but a time-averaged mass loss rates (typically see for instance the paper Dorn-Wallerstein et al. (2023) ApJ 959, 64).

Response 2:
This is indeed one of the accessible contextual constraints, thank you for pointing it out. I've included this example at the end of the penultimate paragraph of the concluding section, citing that impressive work.

Comments 3: 
The second concern the question of providing an accurate birth mass to a red supergiant. Maybe the author could mention. the fact that from photometry alone this is not possible since many different internal structures may correspond to identical values of the effective temperature and luminosity. Spectroscopy is needed to obtain surface gravity to obtain a more accurate estimate (see for instance https://ui.adsabs.harvard.edu/abs/2022PhDT........16F/abstract
the thesis of Eoin Farrell.)

Response 3:
Absolutely. I have added this reference work when discussing masses in star clusters, as well as their work on the final masses of supernova progenitors (in the final section).

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a comprehensive and well-written review of mass loss in red supergiants. He discuss in detail of both observational evidence and theoretical models. The paper offers an almost complete overview highlighting the complexity of the topic, the state of the art of current researches with the main results and points of contrast between them. A good description of the historical background is also included.
The author well identifies and discusses the limitations of current mass-loss prescriptions, including the often-misrepresented role of metallicity.
The review is well-structured and logically organized, with dedicated sections addressing physical mechanisms, observational methods, and evolutionary implications.
While the review is excellent overall, I suggest the following minor revisions to further strengthen the manuscript:
I suggest to stress the definition and differences among the terms winds, mass-loss events, and eruptive episodes. A clearer distinction would help the reader, as these represent different physical processes, in particular for such kind of text that could be a useful reading for students.
I would also suggest to  add either a small summary diagram reporting the different mass-loss regimes as a function of effective temperature, mass, and metallicity  and/or a very short paragraph summarizing  the conclusions in bullet point list, this will make it easy to assume the conclusions of a very long article.

Author Response

Comments 1:
The manuscript presents a comprehensive and well-written review of mass loss in red supergiants. He discuss in detail of both observational evidence and theoretical models. The paper offers an almost complete overview highlighting the complexity of the topic, the state of the art of current researches with the main results and points of contrast between them. A good description of the historical background is also included.
The author well identifies and discusses the limitations of current mass-loss prescriptions, including the often-misrepresented role of metallicity.
The review is well-structured and logically organized, with dedicated sections addressing physical mechanisms, observational methods, and evolutionary implications.
While the review is excellent overall, I suggest the following minor revisions to further strengthen the manuscript:
I suggest to stress the definition and differences among the terms winds, mass-loss events, and eruptive episodes. A clearer distinction would help the reader, as these represent different physical processes, in particular for such kind of text that could be a useful reading for students.

Response 1:
This is a great idea. I had been thinking of a way to encapsulate these various concepts and thank the reviewer for pushing me to come up with a solution in the form of a table. This table can be found in section 2.

Comments 2:
I would also suggest to  add either a small summary diagram reporting the different mass-loss regimes as a function of effective temperature, mass, and metallicity  and/or a very short paragraph summarizing  the conclusions in bullet point list, this will make it easy to assume the conclusions of a very long article.

Response 2:
This is another great idea. I had been pondering about sketching a diagram and this encouraged me to do it. I have chosen to represent the regimes in log(g)-Teff space as this charts the boundary layer conditions best when limiting to only two dimensions. Sharp borders cannot be drawn, and metallicity effects are indirect. But it combines the spatial and evolutionary aspects of the mass loss processes, in a beautifully novel way.