Update on WASP-19

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer Report 

I have carefully read the manuscript entitled "Update on WASP-19" by Judith Korth, and Hannu Parviainen.

Analyzing Transiting Exoplanet Survey Satellite (TESS) data for the ultra-short period gas giant WASP-19, this manuscript aims to measure orbital period variations and constrain the stellar tidal quality parameter. It is an interesting topic in exoplanetary systems field. As such, the topic is appropriate and worthy of consideration for publication in Universe. 

The manuscript is almost ready for publication, I only have few small comments/suggestions for the author before recommending the manuscript.

1.Line 32-34, the orbit of the planet shrinks, its orbital angular momentum decreases. The stellar rotational period increases, its spin angular momentum also decreases. The total angular momentum is nonconservative?

2.In my understanding, a_s should be 4a/R_* (is not a/R_*) in Eq. (6).

3. Lines 102 and 182, what is the impact parameter?

4. Lines 160-162, as you mentioned "Mp/M_*, is obtained from RV mass measurements, the scaled semi-major axis, a/R_*, from transit light curve modeling", you should provide these two parameters.

5. Line 175, the tidal quality factor should be ~1.0d5 (not 1.0d-5) if Pdot is ~1.0d-10 s s^-1.

***end***

Author Response

We thank the reviewer for their helpful comments and careful review. We have addressed the comments, marked the changes in bold in the manuscript, and reply to the comments here. 

1. We have changed "increase" to "decrease".
2. The equation you refer to is from e.g. Charbonneau et al., 2005 (eq. 2) where f lies in the interval from 1 to 2. We use another version of the equation where f goes from 1/4 to 2/3 by López-Morales & Seager 2007. We added the reference in the paper. 
3. We added the definition of the impact parameter in the text. 
4. We added the exact values and their references to the text. 
5. We have fixed the typo. 

Reviewer 2 Report

Comments and Suggestions for Authors

Review of "Update on WASP-19" by Korth and Parviainen

 

This is a brief and interesting paper about newly measured transit times for WASP-19b, a very-short-period hot Jupiter. The authors find evidence that the transit period is shrinking. Below are some comments.  I recommend that the editor withhold judgment on accepting the paper until the authors have been given the chance to respond to this review, and in particular the comments marked with (*).

 

In Equation 3 there is an "s" in the denominator on the right side. Should this be "s_i"?

 

Fig. 1 should highlight the newly measured transit times, in particular those from LCOGT.

 

(*) It is impossible for a reader to assess independently the validity of the newly measured TESS transit times and uncertainties. This could be fixed with a figure that displays the light curves and residuals (perhaps phase folded) or by tabulating/plotting some goodness-of-fit statistics such as the traditional chi-square statistic.

 

It should be noted that Equation (4) is derived in a rather specific framework: I think it is valid only in the oversimplified model in which the only effect of friction is to displace the tidal equilibrium bulge by a constant lag angle, and dynamical tides are neglected. This model dates back at least to Goldreich & Soter (1964) but is probably even older, maybe even to the 19th century.

 

(*) The comparison with Ref. 55 is insufficient. I would like to know the level of agreement between the two teams' measurements of TESS transit times for Sectors 62 and 63.  Also, to substantiate the claim that the difference in results is due to the treatment of outliers, can the current authors reproduce the null result of Ref. 55 if they use the same assumptions?

 

(*) A change in the transit period can be caused by orbital decay, but also by a steady acceleration of the star-planet system toward the Sun, due to the presence of a distant companion.  Can this possibility be ruled out?

 

A less likely possibility is apsidal precession, if the hot Jupiter's orbital eccentricity is maintained at the level of ~0.001 due to perturbations from a nearby companion; the authors might choose to comment on this possiblity.

 

Comments on the Quality of English Language

no comment

Author Response

We thank the reviewer for their careful reading and thoughtful comments. We have addressed the comments in the manuscript as indicated below and marked our changes in bold. 

In Equation 3 there is an "s" in the denominator on the right side. Should this be "s_i"?

Yes, we have fixed the typo. 

Fig. 1 should highlight the newly measured transit times, in particular those from LCOGT.

We have now highlighted the newly estimated transit center times from TESS and LCO with different symbols in Fig. 1. 

It is impossible for a reader to assess independently the validity of the newly measured TESS transit times and uncertainties. This could be fixed with a figure that displays the light curves and residuals (perhaps phase folded) or by tabulating/plotting some goodness-of-fit statistics such as the traditional chi-square statistic.

We modified Fig. 4 to show the phase-folded TESS Sectors and their residuals to help the reader assess the quality of our transit fit. 

It should be noted that Equation (4) is derived in a rather specific framework: I think it is valid only in the oversimplified model in which the only effect of friction is to displace the tidal equilibrium bulge by a constant lag angle, and dynamical tides are neglected. This model dates back at least to Goldreich & Soter (1964) but is probably even older, maybe even to the 19th century.

Yes, we have now explicitly stated that we use the simplified model and refer also to Goldreich & Soter (1966). 

The comparison with Ref. 55 is insufficient. I would like to know the level of agreement between the two teams' measurements of TESS transit times for Sectors 62 and 63.  Also, to substantiate the claim that the difference in results is due to the treatment of outliers, can the current authors reproduce the null result of Ref. 55 if they use the same assumptions?

The comparison is unfortunately not possible at the current state since the authors have not made their TESS transit center time estimates publicly available. We cannot reproduce their results since we use a different method and their times are not publicly available. However, we found that our results agree qualitatively with theirs if we remove the same data points as they did. We want to note that ref 55. did not claim a null result but they called their result marginal because it failed the leave-one-out test. We expanded the discussion on this in the manuscript. 

(*) A change in the transit period can be caused by orbital decay, but also by a steady acceleration of the star-planet system toward the Sun, due to the presence of a distant companion.  Can this possibility be ruled out?

No, we cannot rule out a distant companion to be responsible for the TTVs. We have added a statement to the discussion. 

A less likely possibility is apsidal precession, if the hot Jupiter's orbital eccentricity is maintained at the level of ~0.001 due to perturbations from a nearby companion; the authors might choose to comment on this possiblity.

We have expanded the discussion about possible effects that can cause similar TTVs. 

Reviewer 3 Report

Comments and Suggestions for Authors

Referee's report on the paper  universe-2793138 «Update on WASP-19» by Judith Korth and Hannu Parviainen

 

In this paper the authors report results of their recent analysis of the transit timing of the exoplanet WASP-19 b. In comparison with the previous studies they use some new data and an improved data analysis. They find that the orbital period changes at the rate ~-3.7ms/yr, which is in agreement with some previous studies.The subject matter of this paper is very interesting since such studies provide a direct link between observations and theory. As I am going to stress below, the result obtained by the authors is also in a qualitative agreement with some theoretical expectations.

 

Before making my comments, I'd like to admit that I am a theoretician and, therefore, I can't comment much on the details of observations and data analysis.

 

I have the following comments:

 

1) There is an obvious misprint in the value of tidal quality factor, clearly, it should be ~ 7*10^5 and not 7*10^-5, as in many places in the text starting from the abstract.

2) If I am correct the observed period change implies a characteristic orbital evolution time order of ~ 10^7 yrs. It seems that this is in a qualitative agreement with the theory, provided tides are determined by excitation of internal gravity waves and operate in the so-called regime of «moderately large viscosity», see e.g. https://ui.adsabs.harvard.edu/abs/2013MNRAS.432.2339I/abstract

Fig.18, which gives the evolution time approximately equal to 10^7 for a one Jupiter mass planet orbiting a solar type star with 1 day period. For a quantitative analysis one should know basic characteristics of the host star (its mass, radius, age, metallicity, an upper limit on its rotational frequency, if possible), the orbit (its period and an upper limit on eccentricity, if possible) and the mass of the planet. This information should, in my opinion, be provided in one or several tables (see, e.g. tables 1-4 of

https://ui.adsabs.harvard.edu/abs/2017MNRAS.470.2054C/abstract for an example) and I would like to ask the authors to provide such tables. Of course, such information can be found somewhere else, but it is much more convenient to have it all in one place!

3) Just a comment. I am personally rather sceptical about the use of tidal Q. As seen e.g. from the first cited paper (its Fig. 15 and eqns. (139) and (140)) this quantity strongly depends on orbital period, it does not characterize a tidally perturbed body alone. Therefore, it seems to me rather useless.

I conclude that, in my opinion, the paper can be published in the journal provided the authors address the points 1 and 2 raised above.

 

Author Response

We thank the referee for their helpful comments and provided literature. We have addressed the comments in the manuscript and marked the changes in bold. Here are our answers to the comments: 

1) Yes, we have corrected the misprint. 

2) We added a table with the stellar and planet parameters. 

3) We thank the referee for the comment. We are using the modified tidal quality factor Q'* because it is widely used in tidal interaction studies within exoplanet systems. We will look into other possible quantities to report in the future.