Asteroseismology of Compact Stars

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript "Asteroseismology of compact stars" deals with the relevant issue of containing properties of the equation of state (EOS) of dense matter with (possible) astronomical observations like, e.g., gravitational wave emission from oscillation modes of neutron stars. The manuscript is reasonably clear but publication can be considered only after a major revision of the work, as detailed below.

1) The biggest problem with the present paper is that it is unclear whether it is a review article or a research one. If it is a review, then the level of detail and bibliography is not sufficient, not even for an introductory review. On the other hand, if this is an original research paper, then it is not clear at all where the novelty is, or the message (the message "we may constrain aspects of neutron star composition and structure" is way too generic for a research paper). The Authors should revise their presentation to make this very clear from the start.

2) Related to the point above: it is not clear what the authors use to compute the oscillation modes. They "revise" the standard way of computing oscillation modes (both radial and spheroidal) but they give no details on things like boundary conditions or the numerical method employed, so the reader can't repeat any calculation. My feeling is that the Authors never solve any of those systems of differential equations: they always use approximate formulae. If this is the case, then why do they spend time recalling the equations? They should follow a direct and clear approach, stating that they do use approximate solutions (providing clear references).

3) Section 2 is quite poor regarding the EOS in the core. One key question addressed in the paper is if it is possible to distinguish between quark stars and "canonical" neutron stars, so the Authors should provide a brief overview of the models they consider (both nucleonic and quark). Which of those models gives rise to a solid core and how is this relevant for the study?

4) Radial modes: the citation to the seminal book of Shapiro is OK, but maybe there is also some more specific paper to be addressed. Do the authors solve the system of differential equations? If not this should be honestly and clearly stated. If yes, details on boundary conditions and numerical methods must be provided.

5) The same issue in the above point applies to the spheroidal modes. The seminal paper of Sotani (see https://arxiv.org/abs/1012.4042), where the system of equations is derived, is not even cited.

6) The Authors also comment on pulsar glitches in the Introduction, saying that the starquake model can explain the observed rotational instabilities of pulsars. I strongly suggest referring to the recent review https://arxiv.org/abs/2301.12769 for three reasons: (1) The role of starquakes is revised and discussed. (2) The role of glitches in constraining the EOS (or composition) of neutron stars is also discussed: maybe this could be used in tandem with constraints from asteroseismology. (3) The current wording "the crust breaking causes pulsar glitches in the starquake model of Ruderman [16] and another model [17]." makes little sense, since it is very reductive: many studies considered starquakes as causes for glitches, so it would be better to refer to a comprehensive review. 

7) In general, I would like to stress that, given the current manuscript, it is not clear what the Authors do and what is the novel contribution to the field. I guess that somehow "revise" (rather than "review") some aspects of the asteroseismology of quark and neutron stars, but this is not sufficient for a research paper: What is the specific message of the paper? Where is the novelty? Many details are given (like in a "review" paper), but not all of them are useful to reproduce the results (while important details that are fundamental to reproducing the results are not explicitly discussed). Therefore, a systematic revision is needed to make the paper more direct, clear and honest on what has actually been done.

Comments on the Quality of English Language

The quality of the English language is reasonable, so no other actions other than the standard language revision are needed.

Author Response

Dear referee,

We are writing to inform you that we have revised the manuscript entitled "Asteroseismology of compact stars" according to the valuable feedback provided by you. We sincerely appreciate the time and effort you invested in reviewing our work.

We hope the revised version addresses the query of the referee and meets the requirement for publication in Universe.

Best regards,

Hong-Bo Li

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors In the present work the authors review some recent studies concerning
constrains of bulk properties of compact stars via astroseismology. The study focus mainly to the case of neutron stars,
quark stars and strangeon. The role of the crust has been included
and analyzed. This review paper is complete and very
useful mainly for instructive reasons. In addition, the paper
is well written and the bibliography is quite complete.
Thus, in accordance with the above,
I recommend publication in the present form.

Author Response

We thank for the referee being positive about the paper and publication in Universe.

Reviewer 3 Report

Comments and Suggestions for Authors

Astroseismology is a very popular topic nowadays within the astrophysics of compact stars. This article is designed to be a review of the present status of the topic. The review is not very extensive, actually the emphasis is on the authors' earlier work. Sections 2-4 contain a discussion of some models of compact stars, the equation of state, in particular. One mentions the bag model and the strangeon (strangelet) model.. Applications to helioseismology are presented in Section 5. Clearly oscillations and their stability is the main issue, magnetic field effects are also very strong in compact stars.

I think this article is publishable in the present form. However, I find the list of models of compact star interiors included here somewhat limited, perhaps old fashioned. It would be very interesting to know how nstars with a deconfined interior following the laws of QCD (as given by the Lagrangian, not simple bag models) oscillate. An example of this type of interior dynamics is 2303.11356, there are others, also. Maybe it is too much to demand the inclusion of this in the present work, but it is at least a suggestion for the future

Author Response

We thank the referee for suggesting new references and cite the papers in the Introduction. We hope the revised version addresses the query of the referee and meets the requirement for publication in Universe.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I have revised the final version of the manuscript, and I think that the paper can be published in its current form (i.e. in the form where the Authors have addressed the points raised in my previous report).

Comments on the Quality of English Language

Moderate English revision is suggested.

Author Response

We thank for the referee being positive about the paper and publication in Universe.