Photometric Monitoring of the First Eclipsing Binary Be Star: V658 Car

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

It is somewhat difficult to assess a manuscript that relies heavily on unpublished work "in prep" (l. 40, and several other places). This manuscript is basically an application of that model to observables other than reported in that primary work. Not knowing what observables are covered in Paper I makes it difficult to judge on the work reported here. While this could be fine as such, a bigger problem is that if that original work does not hold, or has to be substantially modified during its own refereeing process, this manuscript will probably become invalid, as far as its precise numerical values are concerned; a fact the authors indirectly acknowledge with the last sentence of their manuscript (l.191). So, while I find the manuscript itself to be fine, with only some rather minor modifications requested, I would suggest the authors consider waiting with a go-ahead for a final publication of this work until Paper I has at least completed the refereeing process.

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General: For the second attenuation, the observations are described, but it is not explained by the model, not even conceptually. This limitation should be made very clear, probably already even in the abstract, that this work explains/models only the first attenuation, and merely describes the second.

General: Unless it is already part of Paper I, consider a schematic image, or series of such images, showing the view onto the system at various key phases, like beginning of first attenuation, first stellar occultation, end of first attenuation,  beginning of second attenuation, second stellar occultation, end of second attenuation, to demonstrate the respective positions of the two stellar components and the disk vs. each other at those key moments. It is much more difficult to form such an image from only the textual description. If such a figure exists in Paper I, reference to it.

l. 31: "Dra" or "Draconis", but not "Drac"

l. 38/39: Is this already referring to Paper I, introduced in the next paragraph? If yes, consider joining the paragraphs to make that clear. If not, add reference.

Fig. 1: The use of opaque symbols is somewhat unfortunate, as the green dots cover the blue ones. Consider using open symbols (circles), symbols made of lines only (crosses), or chose colors with transparency to show the otherwise hidden blue points.

l. 84: reference to Fig. 2: As it is formulated, one might expect Fig. 2 to be the binned version of Fig.1. Maybe reformulate instead that "Fig. 2 uses the binned data".

l. 135: The amplitude in the B and V data seem different than in the TESS (0.4 vs. 0.3 for the primary eclipse, but 0.12 vs. 0.18 for the secondary). Some comment on this would be in order. Since I do not have access to Paper I, and thus do not know how the system parameters were derived (presumably from the eclipsing light-curve, but which one?), I cannot judge how much that may impact the results.

Table 1: indicate which of those are taken from another work (most are presumably from Paper I?), which were fixed here (and motivate why and how in the text), and which were derived. At the moment I find a discussion only for the values of m and beta.

Table 1: With the given parameters, it might be somewhat misleading to speak of an sdOB companion (abstract and elsewhere). Checking Table 7 of Klement et al (2022, reference [7] in the manuscript) one finds typical values for an sdB vs. a Be star, namely a luminosity ration of 1:20, rather than 1:2 as reported here. Again, this presumably comes from Paper I, but considering the values given in the table at face value, a still contracting nature in a post-interaction phase seems more likely than a true sdB star (and at the given temperature certainly not sdO).

Fig. 2, upper row: Add error bars for V-magnitude. Also, for better comparability, use the same y-range in both rows: right no upper row is 0.5mag, lower row is about 0.42 mag. This is close enough to each other to be confusing.

Fig. 2, left column: For the first attenuation (around phase 0), it is clear why there is a distinction between ingress, stellar eclipse, and egress, but not why the egress itself is subdivided into green and purple. I also cannot find an explanation in the text. I can probably see why (symmetric vs. non-symmetric to ingress), but if that is so either explicitly explain and motivate that in the main text, or remove the subdivision if you do not want to discuss it.

Fig. 2, lower right panel: Why is the red part defined so wide? Should it not just be the stellar part of the eclipse, instead of covering part of the second attenuation? In particular since the second attenuation is not modeled at all? In the same spirit, why does the cyan part here cover a part of the second attenuation, which it did not for the first? That this it is not modeled is not really a justification, observationally, there should be a clear distinction between unocculted continuum in the quadratures (cyan), stellar eclipses (red) and attenuations (blue/green/purple) for both attenuation phases, not just the first.

l.170: Make clear here (and elsewhere, where applicable) which of the two attenuations you are discussing. Here it is the first, I suppose.

 

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

I read your manuscript "Photometric monitoring of the first eclipsing binary Be star: V 658 Car" with great interest. I sorted my comments into Main comments and Minor comments, see below.

Main comments:
- my main comment is the following: this paper relies heavily on paper I (e.g., L40, ...), it uses the model developed in paper I and applies it to a (from what I understand) additional dataset to that used in paper I. This manuscript also does not provide any extra conclusions or findings compared to paper I (at least those are not made clear, but also I did not see paper 1). So I am wondering what the argument is for this to be an extra paper, and not one chapter in paper 1.
Several sentences in the paper underline my statement, for example:
  + L40 and following: "... (hereafter Paper I) based on a more comprehensive dataset and improved modeling". Also the rest of the paragraph, and the next one, leave the question how this work adds to new findings about this systems that are not (or cannot be) incorporated into paper I, especially given that paper I is apparently in prep (and not submitted).
  + L86 and following: this paragraph describes the model in paper I, but several aspects remain unclear. For example, how were the best-fit parameters found, and which of the solutions given in Table 1 actually correspond to those, there are two solutions in Table 1.
  + L162 and following: the authors state that "similarly to Paper I, where the model successfully reproduced some observational features but failed to match others, the same pattern is evident in Figure 2.". So what are the new findings of this work that were not already shown (or could not easily and logically incorporated) in paper I? Given the lack of a clear conclusion of this paper, I would argue that it can be made part of paper I.

Additionally I cannot comment on several aspects of the paper (i.e., the model) as the main information is provided in a (so far unpublished and not accessible) work that I do not have access to.

So as a summary, unless there is a clear reason I am missing, I suggest to incorporate this manuscript into paper 1.

- L125: do the TLUSTY models really cover the parameter space in which subdwarfs are found (both in terms of Teff and logg?). I did not think this was the case.
- Figure 2 and corresponding text: it remains unclear why the lightcurve was cut in the way it was cut. Why are there two different parts after the primary eclipse, but not before the eclipse? For an untrained eye, this blue part looks very similar. Also, in the secondary eclipse, why was the red fraction selected to be so wide and why does it not only include the eclipse itself?
- Figure 2: the captions mentions that "the m=2 and m=1.5 models are represented by black and colored lines". What are those lines? why do they evolve and what exactly do they cover? the orbital evolution? this is probably stated in paper I, but without paper I it remains unclear.


Minor comments:
- L2: out of curiosity: can you also estimate disk masses from your model?
- L66: can you please clarify the sentence about the cadence - I do not understand the meaning of it.
- L124: given the rest of the reference formatting, I think "Coelho" should be removed here and only the reference number should remain.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Report on the paper "Photometric monitoring of the first eclipsing 
binary Be star: V658 Car" by de Amorim et al. 

This is a welcome paper on a remarkable binary system V658 Car. 
This binary consists of a spun-up Be star and a stripped subdwarf 
companion.  It is an eclipsing binary that has been observed by 
TESS (one broad band) and by the COPA collaboration (B and V-bands). 
The authors present physically based models for the photometric 
and color variations related to the orbit, and these are 
successful in fitting the main photometric properties. 
I look forward to reading the full paper (in preparation). 

My primary comments relate to several issues that are not 
discussed in the paper, but I think readers would appreciate 
learning the authors' opinions even at this early stage. 

The definition of phase zero is the epoch of the deepest eclipse. 
I think this must be the epoch when the sdO component is 
occulted (sdO star superior conjunction).  I think this is 
the assumption made in the discussion section, but it would be 
helpful to state this earlier in the paper where phase is defined. 

The observations and models show a curious brightening just before 
and after the deeper eclipse (Fig. 1 and 2).  The authors do not 
comment on this feature.  Does this result from the smaller scale 
height H of the disk near the Be star, so that the line of sight
to the sdO star has lower obscuration at those phases? 
Is the surprising bluer color at these before and after phases 
the result of the relatively greater sdO flux contribution seen then? 

The models are less successful in matching the very broad fading 
around orbital phase 0.5.  The phase extent is too large to be 
the result of the sdO blocking the outer disk flux if the disk 
resides within the Roche lobe of the Be star.  Might the outer 
rim of the Be disk facing the sdO star be slightly cooler 
because of the Roche distortion and greater distance from the
Be star? 

The authors may want want to mention the earlier period from
Otero et al. (2005IBVS.5586....1O). 

The masses and radial velocity curve are not discussed here, 
and the only velocity measurements I found are from 
Gieseking (1981A&AS...43...33G).  Although the orbital period 
is incorrect in that paper, the velocities presented (presumably
for the narrow-lined sdO component) have a smaller amplitude 
than predicted for the masses adopted in this paper. 

Minor issues:

The variable star designation is usually presented without a 
space, e.g., "V658 Car" instead of "V 658 Car".   The authors
might also give the Henry Draper Catalog designation "HD92406".

line 84.  I was confused by the reference to "binned data in 
Figure 2" that I presumed would be the light curve, not the 
color-magnitude diagram.  Revise the text to point out the 
difference. 

line 100.  Is n_0 the H number density or something else? 

Footnote 2  "TESS data have "

Table 1  It would be helpful to list the out-of-eclipse flux 
fractions in the V-band for each of the Be star, disk, and 
sdO star that are used in the model. 

Figure 2  Are the model colors reddened for the color excess? 

Figure 2 caption.  The color relations are helpful but might
be explained better.  The dots and solid lines in the upper 
panels correspond to the B,V photometry and models, respectively, 
while the points in the lower panels are the TESS observations. 
Perhaps change "identical phases" in line 3 to "corresponding 
phases". 

line 170. "during the first attenuation phase". 

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

thanks for the elaborate reply with the extra explanation. I better understand the point of submitting this analysis as an extra publication now, your arguments make sense to me. I also appreciate the fact that paper I is now available on arxiv. These facts combined alleviate some of my concerns with the paper, and I am happy to accept the current manuscript for publication after the following comments are taken into account:

General: 

- I now better understand why this analysis was split up from the one in the main paper. However, what I already mentioned in report 1, the conclusions of this work, and what can be learned in addition to the conclusions from paper 1, remains unclear to me. The main conclusion as stated in the current draft is that future modeling efforts are required that take disk opacity and binary dynamics into account (lines ~199 and following). However, in lines 58-59, the conclusion of paper 1 is summarized as exactly the same, that "more sophisticated modeling is needed". Are there any additional conclusion that can be derived from the here presented work? What does the multi-color analysis show that was not already presented in paper 1? Is there more we can learn about what causes the attenuation by using different colors, and the CMD positions? Are there specific parts of he model that need to be changed based on the work here, that was not clear yet from paper 1? 

- Fig. 2 is the main Figure of the paper. While it already became much clearer with the current iteration, I would like to ask the authors to work some more on its clarity. For example, please zoom into the relevant parameter space to make the data points and model more visible. Please also consider if it's possible to include the temporal information in the figure (for example by adding numbers, or by adding symbols, or by using different color gradients instead of red, green and cyan). That would make it much easier to follow the discussion in the paper. Also, why is there no model of the flat portion of the spectrum? 

Minor: 
- Abstract: multicolor vs. multi-color => please stick to one way of writing
- Abstract: COPA is mentioned, but the acronym is not introduced. Please consider putting the footnote already here. 
- line 38: classified as => classified it as
- line 40: 0.7 kK), this => 0.7 kK). This
- line 46: the current phrasing is first very strong about the star being a Be+sdOB ("fully consistent), but the next half sentence weakens this by saying it could also be a still contracting stripped star. Please consider rephrasing it to make the entire sentence less strong.
- line 60: I think it would be good to mention more details about how this analysis is complementary (=> two-color analysis)
- line 65: remove "very"
- Fig.1: please consider changing "B+0.1 - Model" and "V - Model" to "B+0.1: Model" and "V: Model" to not give the impression that you're subtracting Model and Observations here.
- Tab. 1: please refer to the table in the text when appropriate, and add a formal reference to paper 1
- line 153: "More importantly, they are" => what is meant by "they"? The first attenuation?
- line 164: "difficult to account for in the models" => why? can you be more specific here which assumption is not valid, or whether we can learn something from this about the disk, or the model, or both? 
- line 171: "as the Be star would be further attenuated" .. why? and by what? is this not concerning the second eclipse where the sdOB star is in front of the Be star, which should be the dominating effect?
- line 188: "this scenario is comparable, as disk attenuation resembles the build-up process" => is this the general interpretation of the attenuation visible in the light curve? If yes, I think this should be stated more broadly, and earlier on in the paper.
- lines 192-196: I feel like this paragraph should be moved to section 4 where Figure 2 is described. 
- line 193: the models gets dimmer => the models get dimmer
- line 203: during the second. => during the second eclipse?

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

N/A