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The dispersal of planet-forming discs: theory confronts
observations
Barbara Ercolano and Ilaria Pascucci
Article citation details
R. Soc. open sci. 4: 170114.
http://dx.doi.org/10.1098/rsos.170114
Review timeline
Original submission:
Revised submission:
Final acceptance:
7 February 2017
28 March 2017
29 March 2017
Note: Reports are unedited and appear as
submitted by the referee. The review history
appears in chronological order.
Review History
RSOS-170114.R0 (Original submission)
Review form: Reviewer 1 (Cornelis Dullemond)
Is the manuscript scientifically sound in its present form?
Yes
Are the interpretations and conclusions justified by the results?
Yes
Is the language acceptable?
Yes
Is it clear how to access all supporting data?
Not Applicable
Do you have any ethical concerns with this paper?
No
Have you any concerns about statistical analyses in this paper?
No
© 2017 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use,
provided the original author and source are credited
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
2
Recommendation?
Accept with minor revision (please list in comments)
Comments to the Author(s)
I have read the review paper by Ercolano and Pascucci on "The Dispersal of
Planet-forming discs: Theory confronts Observations" to the Royal Society
Open Science journal. This paper gives a comprehensive overview of this
topic, primarily focussed on disk dispersal through photoevaporative winds
and magnetocentrifugal winds, and how they may explain the existence of
"transition disks". Theory is compared to observation throughout this review
paper, and the effect of the disk dissipation process on the formation of
planets plays a central role in this manuscript.
Scientifically I think that the review is very good, and can be considered an
excellent overview of the status of the field. I therefore recommend this
review for publication, albeit with some small revision.
My main concern is the readability.
* This journal apparently uses "[123]"-style citations instead "Xu et
al."-style citations. This is very uncommon in the astrophysical
literature. And when used in the standard astronomy way of citing ("Xu et
al. recently studied this and that. They found that blabla is not
correct." ----> "[123] recently studied this and that. They found that
blabla is not correct.") this may look a bit odd and does not read
pleasantly.
But [123]-style can also be of advantage, if used in the same way as
footnote-style citations are used in books such as biographies. With
footnote-style citations it is impossible to write "[123] said this". It
forces you to write the content of the citation in your own words and
position yourself (i.e. do you agree with that statement?). In the above
example this could then become "An investigation of this and that shows,
however, that blabla is incorrect [123].". To still keep fairness to other
opinions, one could write "An investigation of this and that shows,
however, that blabla is incorrect [123], although this view is not
uncontroversial [161]."
Long story short: The [123]-style of citation should (in my view) not
be used in the same way as the astronomy-style of citation. It should
be used in the same way as footnote-style citations used in humanities.
That would improve the readability of the manuscript considerably, and
as a bonus give the reader more "guidance" of what the authors think is
the real story.
* Some sentences are long and can/should be cut in half at the location of
the comma. Instances I found are:
- Page 6 "...in transition discs, it traces gas..."
- Page 7 "...in the protostellar phase, they also probe..."
- Page 8 "...active area of research, an attempt at..."
- Page 9 "...homogeneously draining disc, current data..."
* The figure quality should be checked. It appears that they are screen-shots
made in jpg format with too strong compression. For graphs with lines this
produces bad "dirt" around the lines. This is particularly bad in Fig. 4 in
the line profiles. In my experience .png format does not produce such bad
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3
behavior.
Some more detailed comments:
- Page 3: "The advent of infrared observatories such as the Spitzer Space
Telescope have enabled" --> replace "have" with "has" (the advent has).
- Page 3: "Disc fraction estimates cannot be extended at far-infrared..."
--> replace "at" to "to" (extend to).
- Same sentence: Why can't these estimates be extended to far-IR? I do not
understand the argument with the stellar photosphere. Why is the stellar
photosphere needed? One can of course argue that one needs the stellar
photosphere to define an "excess". But we are not interested in the relative
emission, but instead to the absolute amount of emission from the disk.
The stellar photosphere would only make that more difficult. In that sense
it should become *easier* to define disk fractions at long wavelengths,
as long as one defines a flux level below which the disk is considered too
tenuous to be considered existing.
- Page 5, Fig. 1: Please (!) use nu*F_nu instead of F_nu. I know that F_nu
is closer to what the observers measure. But scientifically nu_*F_nu is
much more meaningful in terms of energy conservation considerations, as it
shows the actual "energy distribution". These "super-broadband-spectra"
are called "Spectral Energy Distribution" for a reason...
- Page 6, top: "..., implying so little gas to even circularize the orbits of
terrestrial planets." --> What is meant? Could it be that the word "so"
should be replaced by "too"?
- Page 6 "As transitional objects have a dearth of dust grains in their
inner disc and might be, on average, lower accretors (...), it should be
demonstrated that reduced dust and/or accretion heating cannot be
responsible for these observables." ---> I could not follow this logic.
- Page 6 "scale-down" --> "scaled-down"
- Page 8 "Global MHD discs simulations have until now only been possible in
the idea limit." --> I disagree. See Flock et al and Dzyurkevich et al.,
and maybe some more recent stuff.
- Page 8 "driving mechanisms beyond disc winds" ---> What does this mean?
Beyond disc winds?
- Page 9 on the conditions of MRI: "...the disc is threaded by a magnetic
field..." ---> I would say: "...the disc is weakly, but non-negligibly,
magnetized...". Because (1) "threading" implies a vertical B field (which
is not required; it can also be a radial component of a field) and (2) the
field must be existent but weak (a strong field would in fact suppress the
MRI).
- Same sentence "...and that the angular velocity decreases with radius." -->
The term "angular velocity" is sometimes a bit ambiguous. What is really
meant: the azimuthal velocity v_phi or the angular frequency Omega_phi?
- Page 9 "...driven by photoevaporation from the central star" ---> replace
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4
"from" by "by".
- Page 11 "The wind profile, i.e. the region of the disc..." ---> I am
confused: how can a wind profile be a region of the disc?
- Page 12 "turbolence" --> "turbulence"
- Page 12 "...key to predict..." --> "...key to predicting..."
- Page 15 & Fig 6: why are stars of spectral type G marked?
- Page 15 "...snapshots of EUV- plus X-ray-driven photoevaporating discs
from [96]" ---> "...snapshots of models of EUV- plus X-ray-driven
photoevaporating discs from [96]"
- Page 16 & Fig 6: "Twenty nine out of 72 discs..." ---> how I can see this
from Fig 6? The "grey area" is not so clearly marked (only grey models are
shown; is that what you mean?).
- Same page: "Fourteen out of the 72 discs..." --> again, how can I see
this? I counted the symbols marked with a downward arrow (upper limits)
and counted only 10.
- Fig 6: what is the meaning of the two vertical dashed grey lines?
- Page 18: "...while on the other hand large reductions (~$%^&*)"
- Page 18: "...run-away convergence of radial drift to form particle clouds
leading to the streaming instability." ---> What do you mean with runaway
convergence of radial drift?
- Fig. 7: "ovarlain" --> "overlain"
- Fig. 7: I could not find the yellow contour...
- Page 19 "photoeavporation"
- Page 19 "This means that building a density bump or vortensity minimum in
photoevaporating discs may require mass to return to the disk..." --->
I do not follow this argument...
- Page 19 "embrios" ---> "embryos"
- Page 19 "...reproduce many of the exoplanets trends" --> Which trends?
Be more specific.
- Page 21 "truely" --> "truly"
Greetings,
Kees Dullemond
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5
Review form: Reviewer 2
Is the manuscript scientifically sound in its present form?
Yes
Are the interpretations and conclusions justified by the results?
Yes
Is the language acceptable?
Yes
Is it clear how to access all supporting data?
No
Do you have any ethical concerns with this paper?
No
Have you any concerns about statistical analyses in this paper?
No
Recommendation?
Accept with minor revision (please list in comments)
Comments to the Author(s)
The reviewed manuscript provides a comprehensive review of the most recent observational
and theoretical results in the field of the dispersal of protoplanetary disks.
The text is clear and concise in describing recent achievements and future challenges.
Overall, I do not have any major comment. However, I have a few suggestions that
the authors might want to take into consideration in revising the manuscript.
My main comment is related to the pros and cons of SED modeling in probing the
physical structure of protoplanetary disks, and, in particular, in identifying disks in the act of
dispersing. Although it is undoubtedly true that Infrared photometry (Spitzer observations in
particular)
have been pivotal in identifying a large population of transition disks, it should be recognized
that these
models suffer from two, maybe three, severe issues. First, they assume azimuthally symmetric
distribution of dust,
such that the lack, or presence, of infrared excess is a binary indicator of the lack, or presence,
of dust close to the central star. The diagnostic power of IR SED might drastically decrease if the
dust distribution is
azimuthally asymmetric, e.g., if the innermost disk material is concentrated in narrow but dense
streamers of dust or
gas. Second, due to the large IR opacity of dust grains, the IR SED is a very poor indicator of the
overall dust column
density. For example, millimeter observations are revealing that many IR SED "classical" disks are
actually characterized by
large dust cavities (e.g., AB Aur, Pietu et al. 2005; MWC 758, Isella et al. 2010; RY Tau, Isella et al.
2010; SR24S and DoAr 44Andrews et al. 2011).
This clearly shows the limit of SED modeling in properly identifying the disk structure.
Third, IR variability might also significantly complicate the disk classification and, ultimately,
increase the uncertainty in the
fraction of disks in transition derived from the observations.
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6
I believe that these caveats in the identification of transitional disks from SED modeling should
be presented
early on in the text, perhaps in page 3 where the dust diagnostics are discussed. The effects on the
measurement of
the fraction of transition disks (e.g., around line 12 of page 4) should also be discussed.
Related to this topic, you might also want to mention the work of Furlan et al. (2009), who found
that the
equivalent feature of the 10um silicate spectral feature is a good indicator of the presence of large
dust
cavities, and allows to identify some of the transition disks missed by SED modeling.
Other minor comments are listed below:
- Page 6, line 22-23. Please provide references for the discussed models.
- Page 6, line 24-26, "it is also worth noting that the main CO isotopologue may not be a good
tracer for the gas in the inner disc ... etc".
I do not understand this statement. 12CO line becomes optically thick at relatively low gas
density. This molecule is therefore a good tracer
of rarefied gas (as small dust grains are a good tracer of rarefied dust). Could you please clarify?
- Page 8, line 16-17, "Of all mechanism ... accretion and disc wind.". I partially disagree with this
statement because I think that the formation
of giant planets might have a strong influence in the early evolution and dispersal of disks.
Though, planets themselves do not disperse the
circumstellar material, their gravitational perturbation might have a large effect in removing the
innermost disk regions, particularly if such planets form very early on in the disk evolution,
before photoevaporation could play an important role.
- Page 8, line 29-37. A caveat of these reasoning is that the interpretation of the claimed fast disc
dispersal might be more complicated if the mass accretion rate throughout the disk is not radially
constant as assumed by most models (see, e.g. Chiang & Murray-Clay, 2007, for a model of
inside-out disk clearing created by MRI turbulence).
- Page 9, line 34-47. You might want to mention that a prediction of viscous disk evolution is that
the disk radius should increase with time. There is some observational evidence that this might
indeed be the case in at least a sample of disk in Taurus (Guilloteau et al. 2011, Isella et al. 2009)
- Figure 3. I find this figure a bit confusing. Does NH=0 indicate the non-attenuated model? You
might want to consider plotting the ratio between non attenuated and attenuated models instead
of the model flux.
- Page 11, line 19-20, " ... The same scatter will be reflected in the mass-loss-rate and ... expected
lifetime of their discs". Young low mass stars are characterized by short time X-ray variability
which might explain the large observed scatter. However, such variability would not necessarily
affect the disk lifetime, because the X-ray luminosity averaged on a long time interval might be
rather constant.
- Figure 4, line 50, "magenta solid line". The line in my version of the paper is green, not magenta.
Also, could you please explain where is the emission coming from in the case of 90deg
inclination? Shouldn't the emission along the line of sight be absorbed by the disk?
- Page 14, line 30. Could you please comment on the relevance of free-free emission from a stellar
wind compared to that of a disk wind? Also, what kind of observations would be necessary to tell
them apart?
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7
- Figure 6. In discussing this figure, you might want to mention that the large scatter in Macc
might also be due to variability.
- Page 17, line 43 and following. Could you comment on the possibility that the formation of
planetesimals and giant planets might happen before photoevaporation became important?
Current models indicate that the assembly of rocky planets requires several Myr. However, the
formation of planetesimals might occur in the first few Myr.
- Figure 7. The figure indicates that the dust-to-gas ratio at 2 Myr is around 10^-4. This is much
lower than the canonical value of 10^-2. Why is this value so low? Also, I do not see any yellow
line in my version of the figure.
Decision letter (RSOS-170114)
09-Mar-2017
Dear Professor Dr Ercolano
I am pleased to inform you that your Manuscript RSOS-170114 entitled "The Dispersal of Planetforming discs: Theory confronts Observations" has been accepted for publication in Royal Society
Open Science subject to minor revision in accordance with the referee suggestions. Please find the
referees' comments at the end of this email.
The reviewers and handling editors have recommended publication, but also suggest some minor
revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your
manuscript.
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9
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Associate Editor Comments to Author:
Comments to the Author:
Please consider the detailed comments by the two referees in preparing the revised version of
your review paper.
One of the referees noted (in a comment to the editor):
"Concerning the supplementary data: All data is published data. So in principle this data does not
need to be put as supplementary material. On the other hand: it would be nice to have the data
used in some of the figures available."
Given the nature of this journal, I strongly favour open access to the data used in the papers.
Please include pointers to the data (observations or models) that are already publicly available
and let us know whether you can provide the rest for publication.
Reviewer comments to Author:
Reviewer: 1
Comments to the Author(s)
I have read the review paper by Ercolano and Pascucci on "The Dispersal of
Planet-forming discs: Theory confronts Observations" to the Royal Society
Open Science journal. This paper gives a comprehensive overview of this
topic, primarily focussed on disk dispersal through photoevaporative winds
and magnetocentrifugal winds, and how they may explain the existence of
"transition disks". Theory is compared to observation throughout this review
paper, and the effect of the disk dissipation process on the formation of
planets plays a central role in this manuscript.
Scientifically I think that the review is very good, and can be considered an
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
10
excellent overview of the status of the field. I therefore recommend this
review for publication, albeit with some small revision.
My main concern is the readability.
* This journal apparently uses "[123]"-style citations instead "Xu et
al."-style citations. This is very uncommon in the astrophysical
literature. And when used in the standard astronomy way of citing ("Xu et
al. recently studied this and that. They found that blabla is not
correct." ----> "[123] recently studied this and that. They found that
blabla is not correct.") this may look a bit odd and does not read
pleasantly.
But [123]-style can also be of advantage, if used in the same way as
footnote-style citations are used in books such as biographies. With
footnote-style citations it is impossible to write "[123] said this". It
forces you to write the content of the citation in your own words and
position yourself (i.e. do you agree with that statement?). In the above
example this could then become "An investigation of this and that shows,
however, that blabla is incorrect [123].". To still keep fairness to other
opinions, one could write "An investigation of this and that shows,
however, that blabla is incorrect [123], although this view is not
uncontroversial [161]."
Long story short: The [123]-style of citation should (in my view) not
be used in the same way as the astronomy-style of citation. It should
be used in the same way as footnote-style citations used in humanities.
That would improve the readability of the manuscript considerably, and
as a bonus give the reader more "guidance" of what the authors think is
the real story.
* Some sentences are long and can/should be cut in half at the location of
the comma. Instances I found are:
- Page 6 "...in transition discs, it traces gas..."
- Page 7 "...in the protostellar phase, they also probe..."
- Page 8 "...active area of research, an attempt at..."
- Page 9 "...homogeneously draining disc, current data..."
* The figure quality should be checked. It appears that they are screen-shots
made in jpg format with too strong compression. For graphs with lines this
produces bad "dirt" around the lines. This is particularly bad in Fig. 4 in
the line profiles. In my experience .png format does not produce such bad
behavior.
Some more detailed comments:
- Page 3: "The advent of infrared observatories such as the Spitzer Space
Telescope have enabled" --> replace "have" with "has" (the advent has).
- Page 3: "Disc fraction estimates cannot be extended at far-infrared..."
--> replace "at" to "to" (extend to).
- Same sentence: Why can't these estimates be extended to far-IR? I do not
understand the argument with the stellar photosphere. Why is the stellar
photosphere needed? One can of course argue that one needs the stellar
photosphere to define an "excess". But we are not interested in the relative
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
11
emission, but instead to the absolute amount of emission from the disk.
The stellar photosphere would only make that more difficult. In that sense
it should become *easier* to define disk fractions at long wavelengths,
as long as one defines a flux level below which the disk is considered too
tenuous to be considered existing.
- Page 5, Fig. 1: Please (!) use nu*F_nu instead of F_nu. I know that F_nu
is closer to what the observers measure. But scientifically nu_*F_nu is
much more meaningful in terms of energy conservation considerations, as it
shows the actual "energy distribution". These "super-broadband-spectra"
are called "Spectral Energy Distribution" for a reason...
- Page 6, top: "..., implying so little gas to even circularize the orbits of
terrestrial planets." --> What is meant? Could it be that the word "so"
should be replaced by "too"?
- Page 6 "As transitional objects have a dearth of dust grains in their
inner disc and might be, on average, lower accretors (...), it should be
demonstrated that reduced dust and/or accretion heating cannot be
responsible for these observables." ---> I could not follow this logic.
- Page 6 "scale-down" --> "scaled-down"
- Page 8 "Global MHD discs simulations have until now only been possible in
the idea limit." --> I disagree. See Flock et al and Dzyurkevich et al.,
and maybe some more recent stuff.
- Page 8 "driving mechanisms beyond disc winds" ---> What does this mean?
Beyond disc winds?
- Page 9 on the conditions of MRI: "...the disc is threaded by a magnetic
field..." ---> I would say: "...the disc is weakly, but non-negligibly,
magnetized...". Because (1) "threading" implies a vertical B field (which
is not required; it can also be a radial component of a field) and (2) the
field must be existent but weak (a strong field would in fact suppress the
MRI).
- Same sentence "...and that the angular velocity decreases with radius." -->
The term "angular velocity" is sometimes a bit ambiguous. What is really
meant: the azimuthal velocity v_phi or the angular frequency Omega_phi?
- Page 9 "...driven by photoevaporation from the central star" ---> replace
"from" by "by".
- Page 11 "The wind profile, i.e. the region of the disc..." ---> I am
confused: how can a wind profile be a region of the disc?
- Page 12 "turbolence" --> "turbulence"
- Page 12 "...key to predict..." --> "...key to predicting..."
- Page 15 & Fig 6: why are stars of spectral type G marked?
- Page 15 "...snapshots of EUV- plus X-ray-driven photoevaporating discs
from [96]" ---> "...snapshots of models of EUV- plus X-ray-driven
photoevaporating discs from [96]"
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12
- Page 16 & Fig 6: "Twenty nine out of 72 discs..." ---> how I can see this
from Fig 6? The "grey area" is not so clearly marked (only grey models are
shown; is that what you mean?).
- Same page: "Fourteen out of the 72 discs..." --> again, how can I see
this? I counted the symbols marked with a downward arrow (upper limits)
and counted only 10.
- Fig 6: what is the meaning of the two vertical dashed grey lines?
- Page 18: "...while on the other hand large reductions (~$%^&*)"
- Page 18: "...run-away convergence of radial drift to form particle clouds
leading to the streaming instability." ---> What do you mean with runaway
convergence of radial drift?
- Fig. 7: "ovarlain" --> "overlain"
- Fig. 7: I could not find the yellow contour...
- Page 19 "photoeavporation"
- Page 19 "This means that building a density bump or vortensity minimum in
photoevaporating discs may require mass to return to the disk..." --->
I do not follow this argument...
- Page 19 "embrios" ---> "embryos"
- Page 19 "...reproduce many of the exoplanets trends" --> Which trends?
Be more specific.
- Page 21 "truely" --> "truly"
Greetings,
Kees Dullemond
Reviewer: 2
Comments to the Author(s)
The reviewed manuscript provides a comprehensive review of the most recent observational
and theoretical results in the field of the dispersal of protoplanetary disks.
The text is clear and concise in describing recent achievements and future challenges.
Overall, I do not have any major comment. However, I have a few suggestions that
the authors might want to take into consideration in revising the manuscript.
My main comment is related to the pros and cons of SED modeling in probing the
physical structure of protoplanetary disks, and, in particular, in identifying disks in the act of
dispersing. Although it is undoubtedly true that Infrared photometry (Spitzer observations in
particular)
have been pivotal in identifying a large population of transition disks, it should be recognized
that these
models suffer from two, maybe three, severe issues. First, they assume azimuthally symmetric
distribution of dust,
such that the lack, or presence, of infrared excess is a binary indicator of the lack, or presence,
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
13
of dust close to the central star. The diagnostic power of IR SED might drastically decrease if the
dust distribution is
azimuthally asymmetric, e.g., if the innermost disk material is concentrated in narrow but dense
streamers of dust or
gas. Second, due to the large IR opacity of dust grains, the IR SED is a very poor indicator of the
overall dust column
density. For example, millimeter observations are revealing that many IR SED "classical" disks are
actually characterized by
large dust cavities (e.g., AB Aur, Pietu et al. 2005; MWC 758, Isella et al. 2010; RY Tau, Isella et al.
2010; SR24S and DoAr 44Andrews et al. 2011).
This clearly shows the limit of SED modeling in properly identifying the disk structure.
Third, IR variability might also significantly complicate the disk classification and, ultimately,
increase the uncertainty in the
fraction of disks in transition derived from the observations.
I believe that these caveats in the identification of transitional disks from SED modeling should
be presented
early on in the text, perhaps in page 3 where the dust diagnostics are discussed. The effects on the
measurement of
the fraction of transition disks (e.g., around line 12 of page 4) should also be discussed.
Related to this topic, you might also want to mention the work of Furlan et al. (2009), who found
that the
equivalent feature of the 10um silicate spectral feature is a good indicator of the presence of large
dust
cavities, and allows to identify some of the transition disks missed by SED modeling.
Other minor comments are listed below:
- Page 6, line 22-23. Please provide references for the discussed models.
- Page 6, line 24-26, "it is also worth noting that the main CO isotopologue may not be a good
tracer for the gas in the inner disc ... etc".
I do not understand this statement. 12CO line becomes optically thick at relatively low gas
density. This molecule is therefore a good tracer
of rarefied gas (as small dust grains are a good tracer of rarefied dust). Could you please clarify?
- Page 8, line 16-17, "Of all mechanism ... accretion and disc wind.". I partially disagree with this
statement because I think that the formation
of giant planets might have a strong influence in the early evolution and dispersal of disks.
Though, planets themselves do not disperse the
circumstellar material, their gravitational perturbation might have a large effect in removing the
innermost disk regions, particularly if such planets form very early on in the disk evolution,
before photoevaporation could play an important role.
- Page 8, line 29-37. A caveat of these reasoning is that the interpretation of the claimed fast disc
dispersal might be more complicated if the mass accretion rate throughout the disk is not radially
constant as assumed by most models (see, e.g. Chiang & Murray-Clay, 2007, for a model of
inside-out disk clearing created by MRI turbulence).
- Page 9, line 34-47. You might want to mention that a prediction of viscous disk evolution is that
the disk radius should increase with time. There is some observational evidence that this might
indeed be the case in at least a sample of disk in Taurus (Guilloteau et al. 2011, Isella et al. 2009)
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
14
- Figure 3. I find this figure a bit confusing. Does NH=0 indicate the non-attenuated model? You
might want to consider plotting the ratio between non attenuated and attenuated models instead
of the model flux.
- Page 11, line 19-20, " ... The same scatter will be reflected in the mass-loss-rate and ... expected
lifetime of their discs". Young low mass stars are characterized by short time X-ray variability
which might explain the large observed scatter. However, such variability would not necessarily
affect the disk lifetime, because the X-ray luminosity averaged on a long time interval might be
rather constant.
- Figure 4, line 50, "magenta solid line". The line in my version of the paper is green, not magenta.
Also, could you please explain where is the emission coming from in the case of 90deg
inclination? Shouldn't the emission along the line of sight be absorbed by the disk?
- Page 14, line 30. Could you please comment on the relevance of free-free emission from a stellar
wind compared to that of a disk wind? Also, what kind of observations would be necessary to tell
them apart?
- Figure 6. In discussing this figure, you might want to mention that the large scatter in Macc
might also be due to variability.
- Page 17, line 43 and following. Could you comment on the possibility that the formation of
planetesimals and giant planets might happen before photoevaporation became important?
Current models indicate that the assembly of rocky planets requires several Myr. However, the
formation of planetesimals might occur in the first few Myr.
- Figure 7. The figure indicates that the dust-to-gas ratio at 2 Myr is around 10^-4. This is much
lower than the canonical value of 10^-2. Why is this value so low? Also, I do not see any yellow
line in my version of the figure.
Author's Response to Decision Letter for (RSOS-170114)
See Appendix A.
Decision letter (RSOS-170114.R1)
29-Mar-2017
Dear Professor Ercolano,
I am pleased to inform you that your manuscript entitled "The Dispersal of Planet-forming discs:
Theory confronts Observations" is now accepted for publication in Royal Society Open Science.
You can expect to receive a proof of your article in the near future. Please contact the editorial
office ([email protected] and [email protected]) to let us know if
you are likely to be away from e-mail contact. Due to rapid publication and an extremely tight
schedule, if comments are not received, your paper may experience a delay in publication.
Royal Society Open Science operates under a continuous publication model
(http://bit.ly/cpFAQ). Your article will be published straight into the next open issue and this
will be the final version of the paper. As such, it can be cited immediately by other researchers.
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
15
As the issue version of your paper will be the only version to be published I would advise you to
check your proofs thoroughly as changes cannot be made once the paper is published.
In order to raise the profile of your paper once it is published, we can send through a PDF of your
paper to selected colleagues. If you wish to take advantage of this, please reply to this email with
the name and email addresses of up to 10 people who you feel would wish to read your article.
On behalf of the Editors of Royal Society Open Science, we look forward to your continued
contributions to the Journal.
Best wishes,
Alice Power
Editorial Coordinator
Royal Society Open Science
[email protected]
Appendix A
Dear Editor,
Downloaded from http://rsos.royalsocietypublishing.org/ on June 17, 2017
Many thanks for forwarding the reviewers' comments to our Manuscript. We thank the reviewers for a
careful read of our work, which has helped us improve it.
We have addressed all of the points raised by the two reviewers as detailed below. Significant changes
to the manuscript are highlighted in red colour. For the sake of clarity, changes consisting of small
rephrasing of some text (mainly to do with the references) have not been highlighted.
With best regards,
Barbara Ercolano
Ref Report status
Reviewer comments to Author:
Reviewer: 1
Comments to the Author(s)
I have read the review paper by Ercolano and Pascucci on "The Dispersal of
Planet-forming discs: Theory confronts Observations" to the Royal Society
Open Science journal. This paper gives a comprehensive overview of this
topic, primarily focussed on disk dispersal through photoevaporative winds
and magnetocentrifugal winds, and how they may explain the existence of
"transition disks". Theory is compared to observation throughout this review
paper, and the effect of the disk dissipation process on the formation of
planets plays a central role in this manuscript.
Scientifically I think that the review is very good, and can be considered an
excellent overview of the status of the field. I therefore recommend this
review for publication, albeit with some small revision.
My main concern is the readability.
* This journal apparently uses "[123]"-style citations instead "Xu et
al."-style citations. This is very uncommon in the astrophysical
literature. And when used in the standard astronomy way of citing ("Xu et
al. recently studied this and that. They found that blabla is not
correct." ----> "[123] recently studied this and that. They found that
blabla is not correct.") this may look a bit odd and does not read
pleasantly.
But [123]-style can also be of advantage, if used in the same way as
footnote-style citations are used in books such as biographies. With
footnote-style citations it is impossible to write "[123] said this". It
forces you to write the content of the citation in your own words and
position yourself (i.e. do you agree with that statement?). In the above
example this could then become "An investigation of this and that shows,
however, that blabla is incorrect [123].". To still keep fairness to other
opinions, one could write "An investigation of this and that shows,
however, that blabla is incorrect [123], although this view is not
uncontroversial [161]."
Long story short: The [123]-style of citation should (in my view) not
be used in the same way as the astronomy-style of citation. It should
be used in the same way as footnote-style citations used in humanities.
That would improve the readability of the manuscript considerably, and
as a bonus give the reader more "guidance" of what the authors think is
the real story.
ANSWER: We agree with referee. We have changed the text according to his
suggestion and we hope this has improved readability. Note that for the sake
of clarity we have not highlighted these changes in the text.
* Some sentences are long and can/should be cut in half at the location of
the comma. Downloaded
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- Page 6 "...in transition discs, it traces gas..."
- Page 7 "...in the protostellar phase, they also probe..."
- Page 8 "...active area of research, an attempt at..."
- Page 9 "...homogeneously draining disc, current data…”
ANSWER: We have changed the sentences above and some others. As these are
only minor stylistic changes they are not highlighted in the new version.
* The figure quality should be checked. It appears that they are screen-shots
made in jpg format with too strong compression. For graphs with lines this
produces bad "dirt" around the lines. This is particularly bad in Fig. 4 in
the line profiles. In my experience .png format does not produce such bad
behavior.
ANSWER: This is strange. The quality in our file is good - perhaps something
has gone wrong with the referee’s version? We hope that the editorial staff
can help with this.
Some more detailed comments:
- Page 3: "The advent of infrared observatories such as the Spitzer Space
Telescope have enabled" --> replace "have" with "has" (the advent has).
ANSWER:Done
- Page 3: "Disc fraction estimates cannot be extended at far-infrared..."
--> replace "at" to "to" (extend to).
ANSWER:Done
- Same sentence: Why can't these estimates be extended to far-IR? I do not
understand the argument with the stellar photosphere. Why is the stellar
photosphere needed? One can of course argue that one needs the stellar
photosphere to define an "excess". But we are not interested in the relative
emission, but instead to the absolute amount of emission from the disk.
The stellar photosphere would only make that more difficult. In that sense
it should become *easier* to define disk fractions at long wavelengths,
as long as one defines a flux level below which the disk is considered too
tenuous to be considered existing.
ANSWER: We have changed the sentence to clarify that disk fraction estimates
are not carried out, though in principle they could as the referee points out.
One issue in the computation of an absolute amount of disk emission is that it
will depend strongly on disk and dust properties that are not well constrained.
A second issue that we do not mention in the text is that at long wavelengths
one could also pick up emission from debris disks which do not trace disk dispersal.
- Page 5, Fig. 1: Please (!) use nu*F_nu instead of F_nu. I know that F_nu
is closer to what the observers measure. But scientifically nu_*F_nu is
much more meaningful in terms of energy conservation considerations, as it
shows the actual "energy distribution". These "super-broadband-spectra"
are called "Spectral Energy Distribution" for a reason…
ANSWER: We have changed the y-axis as suggested by the referee.
- Page 6, top: "..., implying so little gas to even circularize the orbits of
terrestrial planets." --> What is meant? Could it be that the word "so"
should be replaced by "too”?
ANSWER: Done
- Page 6 "As transitional objects have a dearth of dust grains in their
inner disc and
might be,
onhttp://rsos.royalsocietypublishing.org/
average, lower accretors (...), on
it should
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2017
demonstrated that reduced dust and/or accretion heating cannot be
responsible for these observables." ---> I could not follow this logic.
ANSWER: We have clarified the text: the dearth of small grains and/or accretion
heating could reduce the gas scale height, hence lead to diminished gas emission
in the inner disk.
- Page 6 "scale-down" --> "scaled-down”
ANSWER: Done
- Page 8 "Global MHD discs simulations have until now only been possible in
the idea limit." --> I disagree. See Flock et al and Dzyurkevich et al.,
and maybe some more recent stuff.
ANSWER: The referee is correct this is an unaccuracy on our part. We have now changed
the text to read as follows:
Global MHD discs simulations have until very recently only been possible in the ideal
limit \cite{2014ApJ...784..121S,2015ApJ...801...84G}, or including only Ohmic diffusion
\cite{2013ApJ...765..114D, 2017ApJ...835..230F}. Generally the Ohmic term is sub-dominant
to the Hall term in the inner disk (inside ~10 AU), and sub-dominant to ambipolar diffusion
further out.
The most recent simulations in the ideal limit \cite{2017arXiv170104627Z}, which improve on
resolution and convergence compared to previous work, do not show winds that significantly
contribute to the evolution of the surface density of the disc under the assumption made.
At the time of writing only one set of global simulations of protoplanetary discs including
all three non-ideal MHD effects (ohmic and ambipolar diffusions, and the Hall drift) has been
performed \cite{2016arXiv161200883B}. Discs are found to accrete (in which case a disc wind
is also present) only for given configurations of the large-scale magnetic field, which thus
remains an important uncertainty.
- Page 8 "driving mechanisms beyond disc winds" ---> What does this mean?
Beyond disc winds?
ANSWER: We meant behind - text changed.
- Page 9 on the conditions of MRI: "...the disc is threaded by a magnetic
field..." ---> I would say: "...the disc is weakly, but non-negligibly,
magnetized...". Because (1) "threading" implies a vertical B field (which
is not required; it can also be a radial component of a field) and (2) the
field must be existent but weak (a strong field would in fact suppress the
MRI).
ANSWER: Done
- Same sentence "...and that the angular velocity decreases with radius." -->
The term "angular velocity" is sometimes a bit ambiguous. What is really
meant: the azimuthal velocity v_phi or the angular frequency Omega_phi?
ANSWER: Done
- Page 9 "...driven by photoevaporation from the central star" ---> replace
"from" by "by”.
ANSWER: Done
- Page 11 "The wind profile, i.e. the region of the disc..." ---> I am
confused: how can a wind profile be a region of the disc?
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ANSWER: Text changed to: "The wind profile, which determines the region of the disc that
is most affected by photoevaporation"
- Page 12 "turbolence" --> “turbulence"
ANSWER: Done
- Page 12 "...key to predict..." --> "...key to predicting…
ANSWER: Done
- Page 15 & Fig 6: why are stars of spectral type G marked?
ANSWER: We highlighted early SpTy stars because they appear to be mostly high accretors
and have large dust holes, this is now explicitly stated in the text.
- Page 15 "...snapshots of EUV- plus X-ray-driven photoevaporating discs
from [96]" ---> "...snapshots of models of EUV- plus X-ray-driven
photoevaporating discs from [96]”
ANSWER: Done
- Page 16 & Fig 6: "Twenty nine out of 72 discs..." ---> how I can see this
from Fig 6? The "grey area" is not so clearly marked (only grey models are
shown; is that what you mean?).
ANSWER: The referee is correct we mean the grey models: grey squares and grey
solid lines. We have corrected the text to clarify this aspect. In addition,
we added a number next to a symbol in case there would be more sources at that
specific location. We have also expanded footnote #4 and mention which sources
overlap in Fig. 6.
- Same page: "Fourteen out of the 72 discs..." --> again, how can I see
this? I counted the symbols marked with a downward arrow (upper limits)
and counted only 10.
ANSWER: There are 11 Macc upper limits that can be counted in Fig. 6 + 2 other
upper limits at (2,10^(-11)), now clearly marked in the figure, which leads to 13 WTTs.
We have corrected the text (we previously counted 14 because at (2,-10.2) there are
two sources but only one of them is not accreting, see expanded footnote #4).
Summing up the three sources
- Fig 6: what is the meaning of the two vertical dashed grey lines?
ANSWER: We discuss this in the review "The vertical dashed lines corresponds to models where the
planet is assumed to trap all grains in the outer disc, such that as soon as gap forms, the disc is
observed to have a dust cavity of size equal to the radius at which the gap was opened by the planet"
- Page 18: "...while on the other hand large reductions (~$%^&*)”
ANSWER: Done
- Page 18: "...run-away convergence of radial drift to form particle clouds
leading to the streaming instability." ---> What do you mean with runaway
convergence of radial drift?
ANSWER: Removed run-away
- Fig. 7: "ovarlain" --> “o
verlain”
ANSWER: Done
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- Fig. 7: I could not find the yellow contour…
ANSWER: Those were for different models which we do not show any more.. corrected.
- Page 19 “p
hotoeavporation"
ANSWER: Done
- Page 19 "This means that building a density bump or vortensity minimum in
photoevaporating discs may require mass to return to the disk..." --->
I do not follow this argument…
ANSWER: The sentence is confusing and it has been now removed
- Page 19 "embrios" ---> “embryos"
ANSWER: Done
- Page 19 "...reproduce many of the exoplanets trends" --> Which trends?
Be more specific.
ANSWER: we have added references and a sentence: ", including the observed giant exoplanet –
stellar metallicity correlation and the lack of such correlation for super-Earths."
- Page 21 "truely" --> “truly"
ANSWER: Done
=== new REF report
Reviewer: 2
Comments to the Author(s)
The reviewed manuscript provides a comprehensive review of the most recent observational
and theoretical results in the field of the dispersal of protoplanetary disks.
The text is clear and concise in describing recent achievements and future challenges.
Overall, I do not have any major comment. However, I have a few suggestions that
the authors might want to take into consideration in revising the manuscript.
My main comment is related to the pros and cons of SED modeling in probing the
physical structure of protoplanetary disks, and, in particular, in identifying disks in the act of
dispersing. Although it is undoubtedly true that Infrared photometry (Spitzer observations in
particular)
have been pivotal in identifying a large population of transition disks, it should be recognized that
these
models suffer from two, maybe three, severe issues. First, they assume azimuthally symmetric
distribution of dust,
such that the lack, or presence, of infrared excess is a binary indicator of the lack, or presence,
of dust close to the central star. The diagnostic power of IR SED might drastically decrease if the dust
distribution is
azimuthally asymmetric, e.g., if the innermost disk material is concentrated in narrow but dense
streamers of dust or
gas. Second, due to the large IR opacity of dust grains, the IR SED is a very poor indicator of the
overall dust column
density. For example, millimeter observations are revealing that many IR SED "classical" disks are
actually characterized by
large dust cavities (e.g., AB Aur, Pietu et al. 2005; MWC 758, Isella et al. 2010; RY Tau, Isella et al.
2010; SR24S and DoAr 44Andrews et al. 2011).
This clearly shows the limit of SED modeling in properly identifying the disk structure.
Third, IR variability might also significantly complicate the disk classification and, ultimately, increase
the uncertainty in the
fraction of disks
in transition
derived from the observations.
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I believe that these caveats in the identification of transitional disks from SED modeling should be
presented
early on in the text, perhaps in page 3 where the dust diagnostics are discussed. The effects on the
measurement of
the fraction of transition disks (e.g., around line 12 of page 4) should also be discussed.
Related to this topic, you might also want to mention the work of Furlan et al. (2009), who found that
the
equivalent feature of the 10um silicate spectral feature is a good indicator of the presence of large
dust
cavities, and allows to identify some of the transition disks missed by SED modeling.
ANSWER: This is a good point, we have added a short sentence as well as two references to raise the
issue. We feel that a longer discussion of this issue may distract the reader from the main point, which
is to illustrate that dust disperses and we have diagnostics to trace its evolution
Other minor comments are listed below:
- Page 6, line 22-23. Please provide references for the discussed models.
ANSWER: done
- Page 6, line 24-26, "it is also worth noting that the main CO isotopologue may not be a good tracer
for the gas in the inner disc ... etc".
I do not understand this statement. 12CO line becomes optically thick at relatively low gas density.
This molecule is therefore a good tracer
of rarefied gas (as small dust grains are a good tracer of rarefied dust). Could you please clarify?
ANSWER: Text has been changed to clarify the statement
- Page 8, line 16-17, "Of all mechanism ... accretion and disc wind.". I partially disagree with this
statement because I think that the formation
of giant planets might have a strong influence in the early evolution and dispersal of disks. Though,
planets themselves do not disperse the
circumstellar material, their gravitational perturbation might have a large effect in removing the
innermost disk regions, particularly if such planets form very early on in the disk evolution, before
photoevaporation could play an important role.
ANSWER: Text changed to "true global disc dispersal mechanism"
- Page 8, line 29-37. A caveat of these reasoning is that the interpretation of the claimed fast disc
dispersal might be more complicated if the mass accretion rate throughout the disk is not radially
constant as assumed by most models (see, e.g. Chiang & Murray-Clay, 2007, for a model of inside-out
disk clearing created by MRI turbulence).
ANSWER: This is a good point. We have added the following sentence: “ This scenario is based on the
assumption that the mass accretion rate is radially constant throughout. "
- Page 9, line 34-47. You might want to mention that a prediction of viscous disk evolution is that the
disk radius should increase with time. There is some observational evidence that this might indeed be
the case in at least a sample of disk in Taurus (Guilloteau et al. 2011, Isella et al. 2009)
ANSWER: The referee is correct that a zeroth order prediction of viscous disk models is that gas disk
radii increase with time. However, the two references cited here report measurements of dust disk
radii which have been shown not to trace the gaseous disk component and most likely be affected by
inward radial drift rather than viscous evolution (e.g. de Gregorio-Monsalvo et al. 2013).
We have modified the sentence to read:
“one expects the surface density to decrease with time and the gas outer radius to increase, as a
result of viscous draining and spreading”.
- Figure 3. I find this figure a bit confusing. Does NH=0 indicate the non-attenuated model? You might
want to consider
plotting
the
ratio between non attenuatedonand
attenuated
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June
17, 2017 models instead of the
model flux.
ANSWER: We prefer to keep the figure as it is. This figure was taken from Ercolano et al. 2009 and it
is known plotted this way. We have now specified that N_H = 0 is the unattenuated model in the
caption to the figure.
- Page 11, line 19-20, " ... The same scatter will be reflected in the mass-loss-rate and ... expected
lifetime of their discs". Young low mass stars are characterized by short time X-ray variability which
might explain the large observed scatter. However, such variability would not necessarily affect the
disk lifetime, because the X-ray luminosity averaged on a long time interval might be rather constant.
ANSWER: We agree with the referee that some fraction of the scatter in X-ray luminosity will certainly
come from random flaring events during the X-ray observation.
However, the two orders of magnitude we refer to in the paper is still present in the COUP survey.
Given the typical duration of Chandra and XMM observations of 50-70 ksec and the typical flare
timescales (one to a few hours), the observed "mean" countrate is already an average value over
several flares. In most cases, the effect of individual flares on the determined X-ray luminosity is thus
nor very large and not the dominant factor for the observed scatter.
Exceptions are those cases, where a very weak X-ray source is only detected during a single huge
flare, but such cases arerather rare.
In the COUP project, with its exceptional long observing time, a quantitative analysis was carried out
about how strongly flares affect the inferred X-ray luminosity.
This is described by Preibisch et al. (2005) "ORIGIN OF T TAURI X-RAY EMISSION” The Astrophysical
Journal Supplement Series, 160:401–422, 2005 October
page 403.
Here a "characteristic X-ray luminosity” was defined by "cutting out” the flares from the lightcurves,
the authors found that this is not very different from the mean X-ray luminosity (i.e. the full time
average).
Here is what they wrote:
"
The difference between the average and characteristic X-ray
luminosities is generally not large: the median value of the correction
factor is 0.78; for only 13% of the TTSs this factor is <1/2,
and for only 4% of the TTSs it is <1/3.We will show below that
the choice of either the average or the characteristic X-ray luminosities
has generally very little effect on the observed relations.
We will therefore mainly use the temporally averaged X-ray luminosities
and consider the characteristic luminosities only in a
few cases.
"
Of course, in some rare cases, and especially in observations of shorter durations, a single random
flare can lead to a substantial increase in the derived X-ray luminosity; but this does not happen very
often.
The largest part of the scatter of X-ray luminosity for a given stellar mass is related to other factors,
such as different rotational velocities or differences in the stellar interior structure (since even the
stars in an almost co-eval cluster do not have exactly the same age).
We have added the following sentence to the paper:
" The contribution of short term variability to the observed scatter is minor
\cite{2005ApJS..160..390P}, with intrinsic differences in stellar rotational velocities or internal
structure being instead the dominant factor.”
- Figure 4, line 50, "magenta solid line". The line in my version of the paper is green, not magenta.
Also, could you please explain where is the emission coming from in the case of 90deg inclination?
Shouldn't the emission along the line of sight be absorbed by the disk?
ANSWER: We have corrected magenta for yellow. The emission is coming from the diffuse disc
atmosphere in our flared discs and it is thus not absorbed out by the disc.
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- Page 14, line 30. Could you please comment on the relevance of free-free emission from a stellar
wind compared to that of a disk wind? Also, what kind of observations would be necessary to tell them
apart?
ANSWER: We have re-written the sentence to clarify that the excess cm emission (which includes freefree from the disk and any other process producing emission at those wavelengths, including stellar
winds or dust thermal emission from large cm grains) already excludes that there are enough incident
EUV photons reaching the disk. High spatial resolution cm observations could distinguish between
these different processes, such observations are not feasible yet, but we felt it is important to
highlight that even spatially unresolved emission can place useful constraints to photoevaporation
models.
- Figure 6. In discussing this figure, you might want to mention that the large scatter in Macc might
also be due to variability.
ANSWER: Measured mass accretion rate are known to vary on timescales of hours to years with a
typical magnitude of 0.4 dex, a factor of ~3 (Costigan et al. 2012, 2014; Venuti et al. 2014). While
this is not negligible it cannot account for the over 4 orders of magnitude range of Macc in Fig. 6.
- Page 17, line 43 and following. Could you comment on the possibility that the formation of
planetesimals and giant planets might happen before photoevaporation became important? Current
models indicate that the assembly of rocky planets requires several Myr. However, the formation of
planetesimals might occur in the first few Myr.
ANSWER: We agree, we did not mean to suggest that this is the ONLY route to planetesimal
formation. We have now changed the sentence to read as follows:
"While recent observations of ringed structures in young protoplanetary discs suggest that the planet
formation process may have already begun, well before photoevaporation becomes dominant, this
process remains relevant for the formation of terrestrial planets or debris disc
\cite{2008ARA&A..46..339W}. "
- Figure 7. The figure indicates that the dust-to-gas ratio at 2 Myr is around 10^-4. This is much lower
than the canonical value of 10^-2. Why is this value so low? Also, I do not see any yellow line in my
version of the figure.
ANSWER: The yellow contour refered to a previous version of the figure including more case studies.
We have now removed the sentence about the yellow contours.