Stark Broadening of Co II Lines in Stellar Atmospheres

Round 1

Reviewer 1 Report

The article is devoted to the analysis of competition between two broadening mechanisms in the formation of several spectral lines of Co II, emitted by stars. Doppler and impact Stark broadening are considered using the synthetic models of stellar atmospheres. The necessary data for the Stark half width are calculated within the framework of the Modified Semiempirical Method (MSE), proposed and developed mainly in the works of M. Dimitrijevic and N. Konjevic. In fact, this approach is based on the semiempirical method previously proposed by Hans Griem [8], duly cited in the manuscript. The manuscript is of particular interest to specialists in the field of astrophysical spectroscopy.

However, the following shortcomings in the manuscript must be taken into account when improving the manuscript to make it suitable for publication.

1. Here we will not discuss the details of these calculations since this is the topic of an already published article [3] of the same authors, but only recall that from the point of view of general consideration the accuracy of these data is in the range from 50% to 100%. In this regard, a question arises for the authors: how do they assess the overall accuracy of their analysis since the accuracy of synthetic models of stellar atmospheres also cannot be high. Thus, a detailed description of the accuracy of the final results is needed.

2. The large values of Stark widths in Figures 5 and 6 have to be explained in detail. In particular, the applicability of impact approximation at large values of the frequency detuning and the possible contribution of the static mechanism of Stark broadening in this spectral range should be discussed.

3. The two above comments can be extended to a request for discussion/confirmation of the accuracy of the results by directly comparing theoretical results with experimental data not only for spectral line widths, but also for spectral line shape (at least, for few examples).

4. Figures 1-4 show dependence of the half-width of spectral lines on the optical depth. The authors should explain the meaning of this dependence to a wide audience. Is this a solution to the problem of radiative transfer for given spatial profiles of temperature and densities, and how is this problem solved? How can the same data be represented as a function of local characteristics (temperature) and essentially non-local ones (optical depth which is an integral over the spatial coordinate)?

5. The list of misprints.

Line 36:  beacuse

Line 38:  nad

Line 83: Furier

Line 205: caqlculated

The article, in our opinion, needs major revision.

Author Response

We are very grateful to reviewer for his careful reading
of our manuscript and for the valuable comments and remarks
which improved our work.
Our answers to remarks and comments are:

1. ...the accuracy of these data is in the range from 50% to 100%.
In this regard, a question arises for the authors: how do they assess
the overall accuracy of their analysis since the accuracy of synthetic
models of stellar atmospheres also cannot be high. Thus, a detailed
description of the accuracy of the final results is needed.

In order to answer this remark we added in the manuscript the following text and references:

The predicted accuracy of MSE method is around ±50 per cent, but even in the cases of  emitters with complex spectrum, as for example Xe II and Kr II, this method gives often better agreement with experiments, with relative error less than ±30 per cent [30, 31]. Of course the used model also has some error bars but our qualitative conclusions are confirmed with calculations used three different papers with model atmospheres for DA and DB white dwarfs and for A type stras. A high precision can not be achieved since we, use the published models and include Stark broadening of spectral lines a posteriori. However,
the presence of Stark broadening influences on electron density and temperature, and consequently, on parameters of the model of atmosphere and for the best precision the Stark broadening data should be introduced a priori, during the calculation of model atmosphere.
30. Popović L. Č.; Dimitrijević M. S. Stark broadening of Xe II lines, Astron. Astrophys. Suppl. Ser. 1996, 116, 359-365.
31. Popović L. Č.; Dimitrijević M. S. Stark broadening parameters for Kr II lines from 5s − 5p transitions, Astron. Astrophys. Suppl. Ser. 1998, 127, 295-297.

2. The large values of Stark widths in Figures 5 and 6 have to be explained in detail. In particular, the applicability of impact approximation at large values of the frequency detuning and the possible contribution of the static mechanism of Stark broadening in this spectral range should be discussed.

Thank you very much for drawing our attention to this point. Indeed in the deepest parts of the considered stellar atmospheres electrons densities are so high that impact approximation is not valid. So we we made new figures 5 an 6 where we excluded parts of the atmosphere where conditions for impact approximation are not satisfied.

3. The two above comments can be extended to a request for discussion/confirmation of the accuracy of the results by directly comparing theoretical results with experimental data not only for spectral line widths, but also for spectral line shape (at least, for few examples).

In Conclusions We added:
There is no other experimental or theoretical data for Stark broadening of Co II spectral lines analyzed here. As follows from our work, measurements of Stark broadening of Co II spectral lines will be of interest not only for comparison with the results obtained here but also for analyzis and synthezis of stellar Co II spectral lines.

4. Figures 1-4 show dependence of the half-width of spectral lines on the optical depth. The authors should explain the meaning of this dependence to a wide audience. Is this a solution to the problem of radiative transfer for given spatial profiles of temperature and densities, and how is this problem solved? How can the same data be represented as a function of local characteristics (temperature) and essentially non-local ones (optical depth which is an integral over the spatial coordinate)?

In astrophysics, optical depth is a measure of the extinction coefficient or absorptivity, integrated from zero towards deeper layers up to a specific depth in stellar atmosphere. So it is local caracteristic as electron temperature and it increases from zero towards deeper layers. Because it varies with wavelength, it is usually given for a standard wavelength of 5150 Å or as Rosseland mean optical depth averaged over frequencies. Since we use published models of stellar atmospheres we use optical depth as provided by authors of the models.

5.The list of misprints.

Line 36: beacuse

Corrected, thanks.

Line 38: nad

Corrected, thanks

Line 83: Furier

Corected, thanks

Line 205: caqlculated

Corrected, thanks

Reviewer 2 Report

I recommend the article for publication, but I have some suggestions for changes in some expressions that are not well understood, such as:

1. Line 51, "for example, in plasma diagnostics, e.g determination of plasma density or plasma temperature". I would delete the "e.g" and change it to "for the determination of the density and temperature of the plasma"
2. Line 58, Here is a typo on "apsorption"
3.  Line 85, I think it is better to change it to something like "and, therefore, it further contributed to the completion of the knowledge of these complex spectra."

On the other hand, the effective temperature in some cases appears like "Teff" instead of "T_eff"(line 101 and at the bottom of figure 3)

Finally, in the figures 1a, 2a and 4 the optical depth (τ₅₁₅₀) always appears but in line 122 of the text it is mentioned that this optical depth is in the atmosphere at 1550Å. Is there an error?

Author Response

We are very grateful to reviewer for his careful reading of our manuscript and for the valuable comments.
Our answers to remarks and comments are:

1. Line 51, "for example, in plasma diagnostics, e.g determination of plasma density or plasma temperature". I would delete the "e.g" and change it to "for the determination of the density and temperature of the plasma"

Changed, thanks.

2. Line 58, Here is a typo on "apsorption"

Corrected, thanks.

3. Line 85, I think it is better to change it to something like "and, therefore, it further contributed to the completion of the knowledge of these complex spectra."

Changed, thanks.

4. On the other hand, the effective temperature in some cases appears like "Teff" instead of "Teff"(line 101 and at the bottom of figure 3)

Corrected, thanks.

5. Finally, in the figures 1a, 2a and 4 the optical depth (τ5150) always appears but in line 122 of the text it is mentioned that this optical depth is in the atmosphere at 1550Å. Is there an error?

This error is corrected. Thank you very much

Round 2

Reviewer 1 Report

The authors have improved the manuscript, and it can be recommended for publication. We ask the authors to correct few typos in the newly entered text (e.g., line 143, caracteristic).