UV Spectroscopy of Massive Stars

Round 1

Reviewer 1 Report

This is a very interesting article that I enjoyed reading and I learnt a lot. I think the article is almost publication ready and have just a few comments to add some references to the text:

Paragraph 4 – reference for these numbers please

Section 2 – is quite short for a separate section. Can I suggest adding this into the introduction rather than it's own section.

Section 3 – is there a reference for Nolan Walborn’s comment? I know he did say this a lot but a pointer to an example in print would be good.

Section 6.3 – it would be good to explicitly state what a superion is, rather than just get reads to infer that it’s an ion that should not exist given the effective stellar temperature of the star. Possibly as a footnote when it's first introduced earlier in the article. This is a small change to improve accessibility to a non-expert in stellar atmospheres reader.

Page 11 – "In some P Cygni stars (such as P Cygni itself) …" this looked odd and I suggest the addition of an itself will help.

Section 7.3 – the comment of binarity needs to be clarified, as a binary can be observed as a single star after a merger or if we’re looking at the companion star where the primary has already exploded. It will probably be best to talk about “interacting binaries” here rather than just “binarity” as it’s the fact that the stars interact (either in the past or today) that will be important here not the fact they’re in a binary or not. Also it might be worth noting that LBV behaviour might be a complicated if it's a result of both rapid rotation & binary interactions!

Section 8.2 – since most modern stellar evolution models use Vink et al. (2001) mass-loss rates for O stars, can a sentence be added to the end of this section to say whether this is still suitable or not

Author Response

Please see the attachment.

There pdf file showing the changes is included with the paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

General comments.

This paper is a comprehensive but abridged review of what is currently known about UV spectroscopy of massive stars, with a focus on single stars. It provides an excellent overview of the current status and summarizes some of the intriguing problems that still need to be resolved.

The paper is well written in general and I recommend its publication with, however, a revision along the lines detailed below.

The one important omission in the paper concerns binary stars, an omission that is understanable given the complexities of dealing with atmosphere and stellar wind structures that are distorted from spherical symmetry and requires computational tools that are only now becoming available. My suggestion is to clarify this point early in the review, perhaps adding a few references on the 2D radiative transfer models (if they exist and are able to provide theoretical UV spectra, and if not, state so).

I have separated my comments into two sections: A. Specific comments/suggestions on the text of the paper; and B. Simple typo's.

A. Specific comments/suggestions

Introduction: lines 10-14 are sentences that should follow some broader introductory ideas. Section 2 is very short paragraph that does not merit a whole section to itself and it could serve this purpose.

Section 3.1: explicit mention is made of Copernicus, FUSE and HST, but no IUE. There must be an Atlas of O-stars based on IUE spectra, No?

line 133: perhaps add: "Where rho is the local mass density and the brackets indicate averages"

lines 142-152: the clear and succint description of the terms porosity and vorosity is appreciated by this referee.

lines 179-181: isn't "clumping" simply a generic term that includes porosity and vorosity? The sentence here is confusing, given what is said in lines 140-141 and line 187.

line 182: the sentence in the footnote should probably be incorporated into the text. Also, at this point, the reader becomes aware that colliding wing binary stars (actually all interacting binaries) are being excluded in this review.

lines 208 to 209: "there is no consistency of the fit parameters with the star's radiation field." This statement is probably too obscure for anyone other than radiation transfer theorists. A few clarifying sentences would be appreciated.

line 231: individual species ---> individual atomic/ionic species ?

line 232: "However, there are advantages" is a confusing start of the sentence. There is no explicit reference to disadvantages stated previously, unless this referee missed something.

lines 249-260: this paragraph talks about porosity and clumping and seems to be out of place here. I assume it is here because it is related to the particular models being discussed, but the connection is not clear. The results discussing clumping in Section 4 also are derived from model fits, though not explicitly stated.

lines 275-279: The reader is left wondering how well the ionization flux derived by this method compares with that predicted by the models that are fitted to spectral regions lambda > 900 A.

lines 304-306: If this were true, wouldn't you expect a correlation with the X-ray luminosity?

line 320: "(arising from shocks)" : since the origin of the X-rays is addressed in the next
paragraph, this parenthesis may be unnecessary.
line 336: "A major thrust is" ---> "A major thrust for detailed modeling of UV spectra is"

lines 340-343: Not clear whether the sentences below line 340 refer to IC 1613 and WM (What is "WM"?) or to what galaxies the text is referring to.

line 377: "Both of these" --- which "both"? The previous sentence mentions 3 phenomena.

line 392-393: it could be convenient to add a sentence stating what is meant by "red-shifted electron scattering wings" and why clumping affects this, and/or a reference for follow up.

line 404: Add Eaton et al. (1985; https://ui.adsabs.harvard.edu/abs/1985ApJ...297..266E/abstract) in addition to the references [135, 136]

line 416: any hypothesis as to how these stars came to exist or what they are? If not, then maybe add a sentence remarking that they are puzzling.

Eqs. 1-5: the various symbols should be defined

line 486: "to deduce a low terminal velocity" --- where? What object? (see my next comment)

line 488: it seems that the author is referring to HD 50806 in line 486. I suggest moving this sentence (line 488) to line 485, right before "As part of", and start the sentence with "For example, V_edge in HD 50896 is ....."

line 489: "Microturbulence" ? The effect is described in Fig. 7, where one can see that microturbulence refers to the intrinsic line profile. Perhaps rephrase the sentence: "It is shown by [60, 162, 163] that the microturbulence that is assumed for the intrinsic line profile in spectral modeling leads to a redshift of the emission line"

Figure 7 (inset): Vtrub ---> Vturb

line 507: Could a sentence be added here to tell the reader what needs to be known about clumping in order to reduce its impact on the uncertainty in the velocity law?

Figure 8 (caption): We are told that the blue profiles are not saturated. This leads the reader to assume that the red profiles are saturated. However, neither set (blue or red) has a P Cyg absorption that goes to zero flux. Hence, it is not clear what is meant by "saturated" or not "saturated".

lines 530-534: This is a very confusing set of sentences. It is first stated that assuming a smooth wind, the derived mass-loss rates are an upper limit. Then it is stated that they are a lower limit ("underestimate") if porosity/vorosity (=clumping?) is assumed. However, the derived mass-loss rates from the spectral diagnostics are usually stated in terms of dot{M}/sqrt(f), and clumping implies f<1. They would be underestimated only if the value of f is underestimated, Right? Or what am I missing?

B. Simple Typo's

line-10 for star

lines 120-122: C IV is listed twice, and the sentence is somewhat awkward.

line 149: in some case ---> in some cases

line 207: assumptions.fitting---> assumptions, fitting

line 221: analysis of UV wind profiles in UV stars ---> analysis of UV wind profiles in O stars ??

line 228: of wind lines the UV ---> of wind lines, the UV

line 308: it was discovered the UV of O stars ---> it was discovered that the UV spectra of O stars

line 381: ---> Other classes of

line 386: ---> models and observations are (delete the "commas")

line 406: "youngish" --- not sure this word will be admitted in the text

line 479: ---> gas that arises from

Caption Fig. 6: ---> kindly supplied to the author

line 498: ---> such as H-alpha) (need to close parenthesis)

Author Response

Please see the attachment.

There pdf file showing the changes is included with the paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Very good review.  All comments from my previous review were satisfactorily addressed.