Temporal Changes in the Infrared Spectra of Magellanic Carbon Stars

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I would like to commend the authors for their comprehensive study on carbon stars.

 This is a very interesting article  in which the comparison between  the  Infrared Spectra of Three Magellanic Carbon stars taken with MRS@JWST  and  IRS@Spitzer shows a temporal changes.

I have only  some minor requests:

To improve the text’s clarity, I think it would be worthwhile to mention in the introduction  what the authors will analyse.

For example, they could move the lines 77—80 from Sec. 3 to the Introduction.

 

 

In Section 2:

1. The legend and the caption in Fig. 1 are not completely  clear.

I understand that the magenta points represent the three targets observed prior to 2024 June, as  indicated in the text, but not in the caption or in the figure legend.  In particular,  I believe the word “ observed”  is too generic, it does not allow to get the meaning to the reader.  I suggest revising the label and enhancing the caption.

 

2)  Since the lines 40-42  refer only to the three sources observed prior to 2023 June, to enhance the clarity of the text the authors could move them  after the sentence “ Figure 1 Shows the full sample of nine carbon stars in our program and the three stars observed with MRS between 2023 November and 2023”. 

 

3) there is a typo in Tab.1 (yr, not y)

4) there is a typo at line 55. There is a point before the reference 9.

Author Response

First, a list of changes not motivated directly by the reviewers, but by 
issues the coauthors caught as they responsed to the comments by the reviewers.

Author list - Added an affiliation for J. Cami

Sec. 1 P3 S1 - Added "on the Mid-Infrared Instrument (MIRI)", with a reference
Sec. 1 P3 S2 - Moved parenthetical element to the end of the sentence
Sec. 1 P3 S3 - "were" --> "are"
Sec. 2 P4 last S - Hyphenated "dust-production rates"
Sec. 4 P3 lastg S - "for" --> "to"
Sec. 4 last P S5 - Changed "for previous efforts" to "previously"
Sec. 4 last P S6 - Moved citation later in the sentence to avoid the awkward
  construct of starting a sentence with the bracketed reference.
Sec. 5.1 P1 last S - Changed "[5.8]-[8] color" to "measured [5.8]-[8]"
Sec. 5.1 P3 S5 - Changed "A SiC dust emission feature" to "A dust emission
  feature from SiC"
Acknowledgements - Added an acknowledgement for J. Cami, to the reviewers,
  and to the COST program.

Review 1

Comment 1:

To improve the text’s clarity, I think it would be worthwhile to mention in
the introduction  what the authors will analyse.

For example, they could move the lines 77—80 from Sec. 3 to the Introduction.

Response 1:

Done.

Comment 2:

In Section 2:

1)  The legend and the caption in Fig. 1 are not completely  clear.

I understand that the magenta points represent the three targets observed
prior to 2024 June, as  indicated in the text, but not in the caption or in
the figure legend.  In particular,  I believe the word “observed”  is too
generic, it does not allow to get the meaning to the reader.  I suggest
revising the label and enhancing the caption.

Response 2:

We changed "observed" to "observed early" in the figure, and we modified
the last sentence of the caption to add the word "early" there, too.

Comment 3:

2)  Since the lines 40-42  refer only to the three sources observed prior to
2023 June, to enhance the clarity of the text the authors could move them
after the sentence “ Figure 1 Shows the full sample of nine carbon stars in
our program and the three stars observed with MRS between 2023 November and
2023”.

Response 3:

We have combined the paragraph in lines 40-42 with the preceding paragraph,
with some editing for continuity.

Comment 4:

3) there is a typo in Tab.1 (yr, not y)

Response 4:

Done

Comment 5:

4) there is a typo at line 55. There is a point before the reference 9.

Response 5:

Actually, that is not a typo.  It's a result of starting a sentence with
a citation, so that the first character of the new sentence is "[".  We
rewrote the sentence in passive voice to move the subject away from the
front end of the sentence.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents MRS/JWST spectra of three carbon stars in the LMC. The aim of the paper was to measure the resolved line structure to identify which molecules were responsible for the absorption at different wavelengths and to model the temperature and density of the absorbing gas.

Two of the three stellar spectra presented in the paper have changed significantly since they were observed with the IRS on Spitzer 16–19 years earlier. Pulsation periods were derived for the stars, and the absorption lines have been associated with carbon molecular lines.

This proceedings paper presents a (small?) part of a project that will hopefully include more stars. Presenting three different stars gives a good taste of the sample. The authors made an extensive effort to locate earlier observations and use them for the present paper.

In my opinion, this is an interesting result, with potentially valuable observations for further asteroseismology modeling. The peculiarity of WBP 29 as a transition star from SRV to Mira may also be important for stellar evolution modeling.

I recommend publication.


Suggestions:

* Citing some general/review papers on carbon stars in the introduction.

* I suggest adding periodograms and some uncertainty estimates for the derived periods. Especially for WBP 29, from the lightcurve alone, the derived period does not look so convincing (Figure 3). It might be contaminated by pulsations, but is there a way to tell from the way the observations are presented?

 
* The conclusions section can be extended.

Author Response

Comment 1:

* Citing some general/review papers on carbon stars in the introduction.

Response 1:

We added references the 1996 review by Harm Habing and the more recent
Annual Review article by Susanne Hoefner and Hans Olofsson to the first
sentence of the introduction.

Comment 2:

* I suggest adding periodograms and some uncertainty estimates for the 
derived periods. Especially for WBP 29, from the lightcurve alone, the 
derived period does not look so convincing (Figure 3). It might be 
contaminated by pulsations, but is there a way to tell from the way the 
observations are presented?

Response 2:

We agree with the reviewer that the phase information derived from fitting 
the given periods for J050629 is not reliable, and for WBP 29, it is 
questionable.  We are already planning to include phased light curves in 
the next paper.  For this paper, we have added or modified text in the
following places:

Sec. 4 P4:

As explained in the next section, the estimated phases for the 
MRS and IRS epochs are not reliable for J050629 and are uncertain for 
WBP 29.

Caption for Figure 3:

Section 5 discusses the reliability of the estimated phases for the
MRS and IRS observing epochs for the three targets.

The text as written for J050629 and MSX LMC 736 is left unchanged.  For
WBP 29, we added the following sentence to the last paragraph of Sec. 5.3:

However, the light curve fitted to the data for WBP~29 in Figure 3 is not 
conclusive, and further analysis is needed for any certainty in phases of 
the IRS and MRS observations. 

We have rephrased the last sentence of this paragraph for continuity.

Comment 3:

* The conclusions section can be extended.

Response 3:

We have treated this manuscript as a conference proceedings and have
tried not to say too much until our initial analysis of the spectra is
complete.

 

Reviewer 3 Report

Comments and Suggestions for Authors

When comparing data from different runs of the same instrument or totally different instruments, it is critical to get the calibration/sensitivity correct, so apples-to-apples comparison can be made.


[1] Is the factor 1.7 in figure caption 2 just due to the different sensitivities of the two instruments? If not, the reader would need to know where this "magic number" came from.

[2] Because the stars being imaged are variable, their magnitudes (Fluxes) vary depending on the phase in their cycle. This means that the comparisons of flux values from different instruments in Figure 2 needs to be at the same phase of the star's pulsation. The uncertainty in the determination of the periods means that Figure 2 should be plotted with error bars associated with this uncertainty in phase.

Author Response

Comment 1:

When comparing data from different runs of the same instrument or totally 
different instruments, it is critical to get the calibration/sensitivity 
correct, so apples-to-apples comparison can be made.

Response 1:

The reviewer raises a valid concern that we should address.  We have 
added a sentence to the end of Sec. 3 P1 referencing the paper on the MRS 
spectrophotometric calibration just placed on the arXiv this week and 
stating that the calibration of the MRS and IRS are good to within about 
2%.  That should alleviate some of the concerns about comparing spectra 
between the instruments and at different phases.  The first author of this 
paper will present further work comparing other spectroscopic targets in 
common between the MRS and IRS at the calibration workshop to be held at 
STScI in late October further confirming the consistency of their 
calibrations.

Comment 2:

[1] Is the factor 1.7 in figure caption 2 just due to the different 
sensitivities of the two instruments? If not, the reader would need to know 
where this "magic number" came from.

Response 2:

The Figure 2 caption now clarifies that the factor of 1.7 normalizes the
IRS spectrum to the MRS at 17 um.  The text in Sec. 5.2 (P1 S1) already
states that the MRS is 70% brighter than the IRS at 17 um.

Comment 3:

[2] Because the stars being imaged are variable, their magnitudes (Fluxes) 
vary depending on the phase in their cycle. This means that the comparisons 
of flux values from different instruments in Figure 2 needs to be at the 
same phase of the star's pulsation. The uncertainty in the determination of 
the periods means that Figure 2 should be plotted with error bars associated 
with this uncertainty in phase.

Response 3:

We are plotting the spectra as they were obtained and calibrated.  The 
additional text noted above on the spectrophotometric accuracy of the
MRS and IRS should comfort a reader concerned about comparing the spectra.
The referee's interest in obtaining spectra at matched phases is an 
excellent idea for WBP 29, as the question remains open currently if the 
changes in the spectrum are due to its pulsation cycle or evolution.