Understanding High-Energy (UV and X-ray) Emission from AGB Stars—Episodic Accretion in Binary Systems

Round 1

Reviewer 1 Report

The manuscript contributed by R. Sahai et al. is devoted to the analysis of observations and chromosphere modeling of AGV stars to reveal the binarity and accretion activity based on UV and X-ray emission spectral features. Namely, FUV and NUV band AGV star fluxes are analyzed together with searches for their X-ray emission.

A major revision of the paper is suggested before it can be published.

The following points require to be clarified

1. Subsection 2.1 contains UV observations used to search for the binarity and accretion activity and the mention of the model for these observations.
2. The model is not discussed.
3. Reproduction of part of Fig 1 and Fig 2 from [17] Sahai et al.(2018 ) is presented in Fig 2 to support the binarity\accretion activity hypothesis and model results.
4. The text in 96 – 117 is chaotic; please fix it.
5. The typos in citing figures which suppose to demonstrate the long and sort time variations of X-ray emission found in the text with X-ray searches for accretion activity (lines 99 – 107). Why is the variability mentioned? Please, add details

Fig. 4 is a reproduction of Fig. 1 from R. Sahai et al 2015 ApJ 810 77

4. There is no description of Fig 4 in the text. How are the fits done?
5. There is no APEC model description and reference given before reporting its results in Fig. 3. How are the resulting fit parameters used then?

Fig. 3 is a reproduction of Fig. 1 from R. Sahai et al 2015 ApJ 810 77

6. What is a sample of UV-emitting AGB stars? Please, add details.
7. No figures support the statement about the dependence of existing of X-ray emission in fuvAGB stars with ratio Rfuv/nuv>17. As well as, there is no evidence presented for the indication of X-ray emission in the stars with chosen Rfuv/nuv parameter.

Please specify in the text or in the caption which points in Fig. 6  are model and observation data?

Author Response

We thank the referee for his/her thorough and detailed review. We have revised the ms to take into account all of the referee's points. Our responses below are preceded by ">>>"

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The following points require to be clarified

    Subsection 2.1 contains UV observations used to search for the binarity and accretion activity and the mention of the model for these observations.
    The model is not discussed.
    >>> We have provided relevant details of the geometric model (lines 83, 89, 93-107)
    
    Reproduction of part of Fig 1 and Fig 2 from [17] Sahai et al.(2018 ) is presented in Fig 2 to support the binarity\accretion activity hypothesis and model results.
    >>> We cited Sahai et al.(2018) in the caption.
    
    The text in 96 – 117 is chaotic; please fix it.
    >>> We have re-arranged the text for it to be more coherent (lines 126-157).
    
    The typos in citing figures which suppose to demonstrate the long and sort time variations of X-ray emission found in the text with X-ray searches for accretion activity (lines 99 – 107).
    >>> We have fixed these typos.
    
    Why is the variability mentioned? Please, add details
    >>> We have added text related to the significance of the variability (lines 136-138).

Fig. 4 is a reproduction of Fig. 1 from R. Sahai et al 2015 ApJ 810 77

    There is no description of Fig 4 in the text. How are the fits done?
    >>> We now refer to Fig. 4 in the text as evidence of short-term variability. The fits are done using non-linear least-squares fitting (now mentioned in caption).
    
    There is no APEC model description and reference given before reporting its results in Fig. 3. How are the resulting fit parameters used then?
    >>> We have now provided brief details of the APEC modeling and what the resulting fit parameters imply.

Fig. 3 is a reproduction of Fig. 1 from R. Sahai et al 2015 ApJ 810 77
>>> We have cited Sahai et al.(2015) in the caption.

    What is a sample of UV-emitting AGB stars? Please, add details.
    >>> details have been added
    
    No figures support the statement about the dependence of existing of X-ray emission in fuvAGB stars with ratio Rfuv/nuv>17. As well as, there is no evidence presented for the indication of X-ray emission in the stars with chosen Rfuv/nuv parameter.
    >>> (We assume referee means Rfuv/nuv>0.17) We have added reference to a figure (Figure 4b in Sahai et al. 2015) that supports this statement and shows the dependence of the X-ray emission on FUV/NUV.

Please specify in the text or in the caption which points in Fig. 6  are model and observation data?
>>> Fig. 6 caption has been modified to specify this

Reviewer 2 Report

Dear Raghvendra, Jorge, Martin, Roberto & Carmen,

This is a very interesting study, as both binarity and UV/X-ray emission are important aspects of AGB star evolution yet not receiving the attention it deserves.

I am happy for this to be published once my comments below are addressed satisfactorily.

My main reservation is with the assertion that UV variability, or UV/X-ray emission, is an indication of accretion. As a case in point, the Sun varies in UV and X-rays (much more than it does in bolometric luminosity). It may indicate that the AGB star has a companion.

In that light (unless refuted on the basis of scientific evidence) I would suggest to adjust the title.

Line 18: add a citation for the summary of mass loss from AGB stars.

Line 26: “at end of” -> “at the end of”

Line 35: you only explain what is meant by F25/F12 two lines further; please do it here.

Line 49: “inhomogenieties” -> “inhomogeneities”

Line 63: I wonder whether you could quote the long secondary periods of many early-AGB stars as evidence that many have close companions? (You might in fact want to target such systems in your UV/X-ray strategy.)

Line 73: make the dash indicating a range in wavelength longer (as you do elsewhere).

End of caption of figure 2: “Setal18” -> “Sahai et al. (2018)”

Line 81: “flickering and, and thus” -> “flickering, and thus”

Line 85: “velocity The” -> “velocity. The”

Line 100: you refer to figure 3 for analysis of five sources but figure 3 only shows two it for sources.

Line 105: do not capitalise “ONLY”, just write “only”, or at must italicise it to show emphasis.

Line 109: the reference to the figure is broken (showing a double question mark).

Figure 4: panel a2 can (and must) be enlarged.

Figure 4: state the statistical significance of these periodicities in the data.

Line 121: would the chromospheric activity come from a companion, or could it come from the AGB star?

Line 125: what do you mean by “statistical sample” – simply that the sample is plentiful? Where was the sample selected from? Are these all Galactic? Has this been published somewhere?

Line 135: 17 outliers were rejected, out of how many was this? What is your explanation for these outliers? (Likewise for the 14 outliers in a high-SNR sample, where they could not be explained due to noise.)

Caption of figure 6: “for AGB stars, that” -> “for AGB stars that”

Line 150: you say n(H) and DeltaR are not independent, but what are the typical values you find for FUV/K~0.2-0.3? Are these realistic, for AGB stars?

Lines 156 & 157: the Angstrom symbol is not typeset correctly.

Lines 159 & 160: “AU” -. “au” (as per the IAU convention)

Line 163: “X-rays Emission” -> “X-ray Emission”

Line 166: typeset “eps” as the Greek letter epsilon.

Lines 166 & 167: you say that eps Oct has a lower value than TW Hor but the value for eps Oct is 0.057 and for TW Hor it is 0.044 so the value for eps Oct is higher not lower than that for TW Hor.

Figure 7: enlarge the axis labels and panel titles.

Line 173: why do you imply accretion? Why can’t the companion have a chromosphere and corona without being accreting? The Sun is not accreting and yet it has a chromosphere and a corona.

Line 177: for the X-ray spectrum of RR Umi, refer to figure 7.

Line 182: “is now” -> “are now”

Line 186: “galactic” -> “Galactic”

Line 200: “R.S’s” -> “R.S.’s”

Line 203: typeset “eps” as the Greek letter epsilon.

Author Response

We thank the referee for his/her thorough and detailed review. We have revised the ms to take into account all of the referee's points. Our responses below are preceded by ">>>"

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My main reservation is with the assertion that UV variability, or UV/X-ray emission, is an indication of accretion. As a case in point, the Sun varies in UV and X-rays (much more than it does in bolometric luminosity). It may indicate that the AGB star has a companion.

In that light (unless refuted on the basis of scientific evidence) I would suggest to adjust the title.
>>> We had already considered and rejected, in Sahai et al. (2015), that the X-ray emission for the sample of AGB stars in this study, could come from a MS companion (based on the relatively high values of Lx), this is now mentioned. And we add arguments for why the observed FUV emission cannot arise in a MS companion. We have therefore retained the title as is.

Line 18: add a citation for the summary of mass loss from AGB stars.
>>> done (line 19)
 
Line 26: “at end of” -> “at the end of”
>>> done (line 26)

Line 35: you only explain what is meant by F25/F12 two lines further; please do it here.
>>> done (line 35)
 
Line 49: “inhomogenieties” -> “inhomogeneities”
>>> done (line 49)
 
Line 63: I wonder whether you could quote the long secondary periods of many early-AGB stars as evidence that many have close companions? (You might in fact want to target such systems in your UV/X-ray strategy.)
>>> text related to LSP's has been added (Lines 65-67)

Line 73: make the dash indicating a range in wavelength longer (as you do elsewhere).
>>> done (line 75)
 
End of caption of figure 2: “Setal18” -> “Sahai et al. (2018)”
>>> done
 
Line 81: “flickering and, and thus” -> “flickering, and thus”
>>> done (line 84)
 
Line 85: “velocity The” -> “velocity. The”
>>> done (line 88)
 
Line 100: you refer to figure 3 for analysis of five sources but figure 3 only shows two it for sources.
>>>yes, we only show 2 of the 5 sources, this is now noted (line 140-141)
 
Line 105: do not capitalise “ONLY”, just write “only”, or at must italicise it to show emphasis.
>>> done (line 150)
 
Line 109: the reference to the figure is broken (showing a double question mark).
>>> the text has been modified here (lines 148-152))

Figure 4: panel a2 can (and must) be enlarged.
>>> done

Figure 4: state the statistical significance of these periodicities in the data.
>>> done (uncertainties added in caption)
 
Line 121: would the chromospheric activity come from a companion, or could it come from the AGB star?
>>> we have added text showing that a MS companion's chromosphere is unlikely to produce the observed FUV fluxes; and we have referenced Sahai et al. (2015) conclusion that a MS companion's corona cannot produce the observed X-ray emission if it exceeds 0.001 Lsun (lines 164-170).
 
Line 125: what do you mean by “statistical sample” – simply that the sample is plentiful? Where was the sample selected from? Are these all Galactic? Has this been published somewhere?
>>> modified "statistical" to "large". We have described the process for constructing our sample. It has not been published elsewhere (lines 171-179).
 
Line 135: 17 outliers were rejected, out of how many was this? What is your explanation for these outliers? (Likewise for the 14 outliers in a high-SNR sample, where they could not be explained due to noise.)
>>> The total number was already provided (316 objects, line 131 of original ms). The total number for the high-SNR sample (184) was given in line 136 or original ms. We now provide an explanation for the outliers (lines 192-195).
 
Caption of figure 6: “for AGB stars, that” -> “for AGB stars that”
>>> done

Line 150: you say n(H) and DeltaR are not independent, but what are the typical values you find for FUV/K~0.2-0.3? Are these realistic, for AGB stars?
>>> The typical values of n(H) and DeltaR for (FUV/K)*1e6~0.2-0.3, where the majority of the AGB stars are located, are 10^(~8.5) cm^-3 and ~0.5 au for an AGB star with radius 1.5 au. This has now been included in the text (lines 210-212).
 
Lines 156 & 157: the Angstrom symbol is not typeset correctly.
>>> fixed
 
Lines 159 & 160: “AU” -. “au” (as per the IAU convention)
>>> fixed (lines 215-216)
 
Line 163: “X-rays Emission” -> “X-ray Emission”
>>> fixed (line 222)
 
Line 166: typeset “eps” as the Greek letter epsilon.
>>> done (line 224)

Lines 166 & 167: you say that eps Oct has a lower value than TW Hor but the value for eps Oct is 0.057 and for TW Hor it is 0.044 so the value for eps Oct is higher not lower than that for TW Hor.
>>> fixed, this was a typo, eps Oct has a higher value of FUV/NUV than TW Hor (line 226)
 
Figure 7: enlarge the axis labels and panel titles.
>>> done
 
Line 173: why do you imply accretion? Why can’t the companion have a chromosphere and corona without being accreting? The Sun is not accreting and yet it has a chromosphere and a corona.
>>> we have removed the text "with active, but variable accretion" (line 232-233).
 
Line 177: for the X-ray spectrum of RR Umi, refer to figure 7.
>>> done
 
Line 182: “is now” -> “are now”
>>> done
 
Line 186: “galactic” -> “Galactic”
>>> done

Line 200: “R.S’s” -> “R.S.’s”
>>> done
 
Line 203: typeset “eps” as the Greek letter epsilon.
>>> done

Round 2

Reviewer 1 Report

The authors revised the manuscript and make some changes to the text of the manuscript.

But some points require to be fixed, namely:

Lines 106, 134, 145, 152, 164:  

The citing form of Sahai et al. 2015 would be more useful than the brief one Setal15.

Line 216: It seems, there is a typo in “CII] UV0.01”.

Line 226: there is a typo “eps Oct”.

Line 228: I suppose, it should be the reference to Fig. 7a instead of the Fig. 4a.

The same as for Line 228, but with respect to Fig. 7b.

Author Response

Dear Referee/Editors,

 my response is below, please look for >>>

1) Lines 106, 134, 145, 152, 164:  

The citing form of Sahai et al. 2015 would be more useful than the brief one Setal15.

>>> fixed

2) Line 216: It seems, there is a typo in “CII] UV0.01”.

>>> No, this is not a typo, this is the correct way  to label this transition (its semi-forbidden, hence a single bracket)

3) Line 226: there is a typo “eps Oct”.

>>> fixed

4) Line 228: I suppose, it should be the reference to Fig. 7a instead of the Fig. 4a.

>>> yes, it should be 7a, fixed

The same as for Line 228, but with respect to Fig. 7b.

>>> >>> yes, it should be 7b, fixed