Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a very interesting paper, with high quality ALMA data, on an
important source and, as such, is certainly worth publishing. The ALMA
data has many different aspects to it, that are touched on in this
paper, and the data (and the associated analysis) certainly merits a
much longer paper (or series of papers) to be published elsewhere.

In general, the paper is well written and I have a few minor comments below. The paper could do with a light editing for some English language issues.

Issues to be addressed by the author:

1) Could you define the spectral index - there is always the issue of the spectral index being defined as a function of wavelength or frequency. I think it is good practice to define what version you use (though it is often obvious).

2) Spectral index of 1.7 at lower frequency. Is this important - the ionized stellar winds of massive stars have a rather different slope and I'm wondering if this steeper slope tells us something important. Could you comment please.

3) p.6 It is unfortunate that one of your main results - velocity gradient - which is mentioned in abstract and conclusions - is based on the PV diagram which is not shown. Is there a way to include this?

4) p.7 Fig. 5d - a left-over "beam size" seems to be on this figure and should be removed.

5) Small point - p.9 Sect. 4 - "last results" - presumably you mean latest results (last would imply the final results).

Comments on the Quality of English Language

As mentioned, the paper could do with a small bit of editing for language. Nothing major, just some small changes.

Author Response

Dear editor and referee,

We appreciate the referee effort to improve our manuscript. Their comments were accurate and we tried to address them all.

Please, find below our reply.

Best wishes.

--------------------------------------

0) The paper could do with a light editing for some English language issues.

- We have reviewed the English language and rephrased certain sentences throughout the text to improve the quality.

1) Could you define the spectral index - there is always the issue of the spectral index being defined as a function of wavelength or frequency. I think it is good practice to define what version you use (though it is often obvious).

- Done. It has been defined in the abstract and in Section 3.1

2) Spectral index of 1.7 at lower frequency. Is this important - the ionized stellar winds of massive stars have a rather different slope and I'm wondering if this steeper slope tells us something important. Could you comment please.

- Theoretical work was done in the past to determine the spectral index below the turnover frequency from the properties of the emitting region (e.g., Reynolds 1986). Many scenarios can imply a spectral index of 1.7 and the problem is degenerated so it is necessary to model the brightness distribution in detail to obtain information about the emitting structure.

We have informed the reader about this by citing the work of Reynolds (1986) and informing them about the typical spectral index in the optically thick regime for a spherically symmetric ionized wind, which explains the spectral index of massive stars.

3) p.6 It is unfortunate that one of your main results - velocity gradient - which is mentioned in abstract and conclusions - is based on the PV diagram which is not shown. Is there a way to include this?

- Done. We have added the PV diagrams in the E-W and N-S directions in Figure 6 (new version of the manuscript).

4) p.7 Fig. 5d - a left-over "beam size" seems to be on this figure and should be removed.

- Done.

5) Small point - p.9 Sect. 4 - "last results" - presumably you mean latest results (last would imply the final results).

- Corrected.

 

Reviewer 2 Report

Comments and Suggestions for Authors

Reviewer Report 

I have carefully read the manuscript entitled "Exploring the ionized core of the pPN CRL 618 and its vicinity with ALMA" by José Pablo Fonfría et al..

This manuscript has presented the new results based on the preliminary analysis of  high angular resolution ALMA observations of the late pPN/yPN CRL 618 at 1.35 mm. This work illustrates that the joint analysis of the free-free continuum emission and recombination lines allow to trace the gas structures, kinematics, and the nature of continuum emission inside the ionized core of post-AGB stars. Furthermore, complementary molecular observations trace the structures and kinematics of the neutral envelope outside  the ionized core. As a consequence, a combined analysis of both data sets provides us with a complete picture of the gas evolution close to the central stellar system in post-AGBs. As such, the topic is appropriate and worthy of consideration for publication in the Galaxies Journal.

The manuscript is almost ready for publication,  I would like to recommend the manuscript for the publication after the author address several minor comments as follows.

1. Line 187: the authors mentioned "The disagreement between the observed and modeled H30a emission is higher than for the continuum if the scenario in Figure 5a is adopted". I suggest that the authors present a brief discussion on this issue.

2. Figure 6 is very confused. I suggest that that authors mark some simply names for those molecular emission lines. Other detailed information can be mentioned in the figure caption.

3. Figure 5: "The model of CRL 618 done with MORELI [22] is the solid curve in magenta and the model of Co3RaL is in green". However, from the legends these two colors should be blue and magenta, respecitvely.

4. Figure 7: "Moments 0 and 1 for the HC3N(24-23) and SiO(5-4) lines."->"Moments 0 and 1 for the HC3N(24-23) and SiO(5-4) lines, respectively."

Comments on the Quality of English Language

No.

Author Response

Dear editor and referee,

We appreciate the referee effort to improve our manuscript. Their comments were accurate and we tried to address them all.

Please, find below our reply.

Best wishes.

--------------------------------------

1. Line 187: the authors mentioned "The disagreement between the observed and modeled H30a emission is higher than for the continuum if the scenario in Figure 5a is adopted". I suggest that the authors present a brief discussion on this issue.

- We have briefly discussed the problem from the point of view of the radiation transfer and modeling.

2. Figure 6 is very confused. I suggest that that authors mark some simply names for those molecular emission lines. Other detailed information can be mentioned in the figure caption.

- Done. We have modified Fig. 3 accordingly. Actually, detailed line identifications are not very important in the current paper.

3. Figure 5: "The model of CRL 618 done with MORELI [22] is the solid curve in magenta and the model of Co3RaL is in green". However, from the legends these two colors should be blue and magenta, respecitvely.

- We have checked the colors and concluded that MORELI's curve is plotted in blue, as the referee indicates, but Co3RaL's curve is in orange (at least in the electronic version of the manuscript). The caption of the old Figure 5 (Figure 7 in the new version of the manuscript) has been updated. Hopefully, this choice will properly describe the colors in the figure.

4. Figure 7: "Moments 0 and 1 for the HC3N(24-23) and SiO(5-4) lines." -> "Moments 0 and 1 for the HC3N(24-23) and SiO(5-4) lines, respectively."

-  We think that the use of the word "respectively" would mean that we present the moment 0 for the HC3N line and the moment 1 for the SiO one but the moments 0 and 1 are included for both lines. Hence, we prefer not to include this word in the text in order to prevent misunderstandings.

 

Reviewer 3 Report

Comments and Suggestions for Authors

The topic is very interesting, and indeed, a complex study of the emission-line morphologies and kinematics in the very young planetary nebulae is quite necessary to reveal its inner structure and to explore its evolution. The ALMA high-resolution data for CRL 618 in the mm-range are unique and invaluable to analyse the origin and the future fate of this object. The data presentation is impressive as concerns the mom-0 and mom-1 maps; however some more figures would be useful to give additional arguments to the authors' model for CRL 618 - in particular velocity-channel maps and  PV diagrams mentioned in the text but marked as 'not shown'. I hope that 'Galaxies' does not restrict the paper size so the figures that are discussed in the text but 'are not shown' can be added to the paper. I would stress the necessity of the PV-diagrams in two directions - in the west-east AND in the NORTH-SOUTH, - because the equatorial plane may have some signatures of ionized gas rotation, not only expansion. If the rotation is present, it must be included into the model. Perhaps, in the frame of a more refined model the velocities of the outermost parts of the nebula (Fig. 7b) would be explained; now their treatment as 'expanding neutral matter in the equatorial torus' (lines 218-221) cannot be accepted because the equatorial torus lies in the north-south direction, and the gradient of the outermost velocities is observed in the east-west direction, and it is contrary to the inner outflow. A minor comment: in the caption to Fig.5 the statement is that 'the model of Co3RaL is in green' while in the Fig.5 it is indeed in yellow. 

Author Response

Dear editor and referee,

We appreciate the referee effort to improve our manuscript. Their comments were accurate and we tried to address them all.

Please, find below our reply.

Best wishes.

--------------------------------------

1) The data presentation is impressive as concerns the mom-0 and mom-1 maps; however some more figures would be useful to give additional arguments to the authors' model for CRL 618 - in particular velocity-channel maps and PV diagrams mentioned in the text but marked as 'not shown'. I hope that 'Galaxies' does not restrict the paper size so the figures that are discussed in the text but 'are not shown' can be added to the paper. I would stress the necessity of the PV-diagrams in two directions - in the west-east AND in the NORTH-SOUTH, - because the equatorial plane may have some signatures of ionized gas rotation, not only expansion. If the rotation is present, it must be included into the model. Perhaps, in the frame of a more refined model the velocities of the outermost parts of the nebula (Fig. 7b) would be explained; now their treatment as 'expanding neutral matter in the equatorial torus' (lines 218-221) cannot be accepted because the equatorial torus lies in the north-south direction, and the gradient of the outermost velocities is observed in the east-west direction, and it is contrary to the inner outflow.

- Two PV diagrams along the N-S and E-W directions and the some velocity channel maps for the H30a line have been added as requested. Regarding the inclusion of other figures, we remark that our study is still on-going and part of the results are preliminary, in particular those related to the spectral index map and the molecular emission. Hence, we would rather prefer not to publish more figures until we are completely sure about our results. They will be presented elsewhere.

It is unlikely that gas rotation exists in the outermost part of the equatorial plane since the velocities of the appendices seen in Fig. 9b (new version of the manuscript) grow with the distance from the center of the nebula, contrary to what it is expected for a Keplerian rotation. Moreover, the only way of having a Keplerian rotation at distances as high as 1.5"-2" and rotation velocities of 15-20km/s would need a mass of more than 450M_sun at the center of the orbit. Therefore, these appendices are more compatible with radial ejections.

Gas rotation in the inner layers of the ionized core is possible but difficult to justify due to the high velocity dispersion seen in the PV diagrams for H30a. We are working on this topic and plan to publish our results based on solid reasonings and detailed models in the near future.

The referee is right about the impact of a possible rotation on the model but we cannot currently publish risky statements about gas rotation at this point of the research. Hopefully, we will be able to find clear evidence of gas rotation (or a way to reject its existence) during the months to come. Concerning the paper under review, we have only included a mention to gas rotation in Section 3.2, at the end par. 3, to let the reader know that we consider this possibility.

 

2) A minor comment: in the caption to Fig.5 the statement is that 'the model of Co3RaL is in green' while in the Fig.5 it is indeed in yellow.

- We have checked the colors and concluded that MORELI's curve is plotted in blue, as the referee indicates, but Co3RaL's curve is in orange (at least in the electronic version of the manuscript). The caption of the old Figure 5 (Figure 7 in the new version of the manuscript) has been updated. Hopefully, this choice will properly describe the colors in the figure.