Hot Stars, Young Stellar Populations and Dust with Swift/UVOT
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis manuscript provides an excellent review of the contribution of UVOT to studies of (nearby) resolved and (distant) unresolved stellar populations, plus dust attenuation laws. No major concerns identified, but the authors are encouraged to consider the following minor points
General: Large field-of-view (17x17 arcmin^2) and modest spatial resolution (2.3 arcsec) of UVOT are mentioned on numerous occasions so please add this information somewhere in the review!
Intro, page 1, line 31+: Although the focus of the present work is on UV extinction law, the continuously increasing slope from IR to UV is modified by IR (e.g. 9.7 micron) as well as the cited UV (0.22 micron) feature (e.g. Rieke & Lebofsky 1985)
S2 page 3, line 98+: SEDs come from theoretical *atmospheric* models, not models of stellar evolution in isolation. Atmospheric models can be combined with evolutionary models *and* initial mass functions (+ sometimes binarity) to produce integrated SEDs of stellar populations.
S2 page 4, line 120: Need to note that open clusters surveyed have ages of 0.1-1 Gyr, since brightest members of younger (modestly reddened) clusters will be too bright for UVOT. NGC2360 shown in Fig 3 is an open cluster, not a globular.
S3, page 7, para 1. Refer to Fig 1 when discussing differences in extinction laws for members of Local Group.
S3, page 9, line 253: add "for one star forming region in the SMC" after "..synthetic model spectra." Add "within the SMC" after s.f. region 165 in caption of Fig 6
S3, page 12, lines 295-298: In addition to SFH of Mag Clouds, an important omission from the review is the recent identification of candidate stripped stars courtesy of UVOT (Drout et al. 2023, Science 382 1287, their Fig 1)
S3, page 15, lines 334-338: It is true that the upcoming UVEX does not have a high spatial resolution, but Castor https://www.castormission.org/ would have this capability: 0.15 arcsec FWHM plus 0.25 sq. deg FoV (if funded)
S4, page 15, lines 340-343: I fully recognise the focus of this section is on UVOT beyond the Local Group, but there probably ought to be some mention of higher spatial resolution studies of nearby s.f. galaxies in the UV with HST (notably LEGUS and GULP https://ui.adsabs.harvard.edu/abs/2020hst..prop16316S/abstract)
Comments on the Quality of English LanguageA myriad of acronyms in review (OIR, PAGB, AGB-M, BHB, EHB, FIR...). Please define all, including AGB manque -> AGB Manqu\'{e} (AGB-M) on liine 173-174
Avoid starting sentence with "And" (lines 69, 208)
S3, page 7, line 211: "As with studies of nearby stars and the Milky Way.." replace "and" with "within"?
S3, page 7, line 229: "It also benefits.." Rephrase sentence (poorly constructed)
Replace AV and RV with A_V and R_V throughout
S3, page 14, line 317. "the" is repeated
Figs 15 and 16 include extraneous text (upper left).
S4, page 17, line 374: missing reference "?"
S4, page 19, line 394: Angstrom symbol runs into bump -> "\AA~", and on page 21, line 459 Angstrom symbol is written as AA.
Reference 38 is Pei 1992 not Pei et al. 1992
Author Response
Comment: General: Large field-of-view (17x17 arcmin^2) and modest spatial resolution (2.3 arcsec) of UVOT are mentioned on numerous occasions so please add this information somewhere in the review!
Response: We have added a paragraph with details of the UVOT instrument
Comment: Intro, page 1, line 31+: Although the focus of the present work is on UV extinction law, the continuously increasing slope from IR to UV is modified by IR (e.g. 9.7 micron) as well as the cited UV (0.22 micron) feature (e.g. Rieke & Lebofsky 1985)
Response: We have added this reference.
Comment: S2 page 3, line 98+: SEDs come from theoretical *atmospheric* models, not models of stellar evolution in isolation. Atmospheric models can be combined with evolutionary models *and* initial mass functions (+ sometimes binarity) to produce integrated SEDs of stellar populations.
Response; We have clarified this in the text.
Comment: S2 page 4, line 120: Need to note that open clusters surveyed have ages of 0.1-1 Gyr, since brightest members of younger (modestly reddened) clusters will be too bright for UVOT. NGC2360 shown in Fig 3 is an open cluster, not a globular.
Response: We have added this and fixed the caption
Comment: S3, page 7, para 1. Refer to Fig 1 when discussing differences in extinction laws for members of Local Group.
Response: We have added this
Comment: S3, page 9, line 253: add "for one star forming region in the SMC" after "..synthetic model spectra." Add "within the SMC" after s.f. region 165 in caption of Fig 6
Response: We have fixed the text.
Comment: S3, page 12, lines 295-298: In addition to SFH of Mag Clouds, an important omission from the review is the recent identification of candidate stripped stars courtesy of UVOT (Drout et al. 2023, Science 382 1287, their Fig 1)
Response: We have added this reference.
Comment: S3, page 15, lines 334-338: It is true that the upcoming UVEX does not have a high spatial resolution, but Castor https://www.castormission.org/ would have this capability: 0.15 arcsec FWHM plus 0.25 sq. deg FoV (if funded)
Response: We have added this.
Comment: S4, page 15, lines 340-343: I fully recognise the focus of this section is on UVOT beyond the Local Group, but there probably ought to be some mention of higher spatial resolution studies of nearby s.f. galaxies in the UV with HST (notably LEGUS and GULP https://ui.adsabs.harvard.edu/abs/2020hst..prop16316S/abstract)
Response: We have added references to this.
Comment: A myriad of acronyms in review (OIR, PAGB, AGB-M, BHB, EHB, FIR...). Please define all, including AGB manque -> AGB Manqu\'{e} (AGB-M) on liine 173-174
Response: We have fixed this.
Comment: Avoid starting sentence with "And" (lines 69, 208)
Response: "And" removed.
Comment: S3, page 7, line 211: "As with studies of nearby stars and the Milky Way.." replace "and" with "within"?
Response: Fixed.
Comment: S3, page 7, line 229: "It also benefits.." Rephrase sentence (poorly constructed)
Response: We have rephrased the sentence.
Comment: Replace AV and RV with A_V and R_V throughout
Response: We have fixed this.
Comment: S3, page 14, line 317. "the" is repeated
Response: Fixed.
Comment: Figs 15 and 16 include extraneous text (upper left).
Response: We have replaced the figures to remove those captions
Comment: S4, page 17, line 374: missing reference "?"
Response: We have fixed the reference
Comment: S4, page 19, line 394: Angstrom symbol runs into bump -> "\AA~", and on page 21, line 459 Angstrom symbol is written as AA.
Response: We have fixed the latex.
Comment: Reference 38 is Pei 1992 not Pei et al. 1992
Response: We have fixed this.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis is very nice overview of different ways to use the Swift UV observations to constrain the UV absorption law. The paper is well structured and helps the reader to gain insight into the different measurements. This referee would have like to see a bit more of a discussion of the variations in the UV bump for the different sight lines and what this implies for other generic measurements. Very often generic corrections for the UV absorption are made in luminosity studies, using either the Milky Way or the LMC reddening laws to infer, e.g., galaxy luminosity functions. The Swift observations clearly demonstrate that no universal absorption law exists and even individual sight lines to resolved galaxies show significant differences in the absorption law. The conclusions could be more explicit in stating this fact and describe the corresponding uncertainties for studies applying simplified or absorption laws.
There are a few minor comments concerning the text, which are listed below.
1. The authors should read the paper carefully for grammar and some misspellings/typos. One example is the use of 'constraint' in places, where is is a verb and should be 'constrain' instead. There are other examples, where a simple spell checker will point out such inconsistencies.
2. Fig. 3: It would be very useful to draw a reddening vector in these diagrams to show in which direction the measurements are moved for a given reddening. This would help the interpretation.
3. Line 137 ff.: The red leak is a bit disconcerting. This could severely affect the interpretation. It would be good to give a reference for this red leak and either refer to a study of this red leak or give a brief description how it could alter the analyses. As it is written, this referee was confused whether the red leak could invalidate the results!
4. Line 165: This should probably read '... becomes a planetary nebula.'
5. Line 200: 'far-reaching impact' on the distance scale are mention here, but no explanation how and why this would be the case. The reader is left with no help on what this statement is based. A quick example might be helpful.
6. Figs. 7 and 8: One assumption of these maps is that the Milky Way foreground absorption has already been removed. If this is the case, it should be stated clearly in the text and also what value has been assumed for this subtraction. If the foreground is not subtracted then this adds a base value to the maps, which would be interesting to know.
7. Lines 281-283 and Fig. 9: This statement is puzzling. The increased uncertainty appears not reflected in the error bars. While they to increase towards larger ages, the size is never enough to be compared to other measurements. Should data outside the applicable area be plotted at all?
8. Fig. 10, caption: The lines in this figure show the evolution for different ages. It would be very helpful to use different colors for different ages to make the figure easier to understand.
9. Line 318: The argument here is not understandable. How does correlation between the PAH maps and the bump strength relate to the bump? Is there a physical connection?
10. Figs. 15 and 16 appear to have been lifted from a differen paper and their origins captions are still visible (as Fig. 2 and Fig. 13, respectively). This is very confusing and should be removed.
11. Line 374: It appears that a reference has been lost here.
Comments on the Quality of English LanguageSome minor improvements could be implemented.
Author Response
Comment: 1. The authors should read the paper carefully for grammar and some misspellings/typos. One example is the use of 'constraint' in places, where is is a verb and should be 'constrain' instead. There are other examples, where a simple spell checker will point out such inconsistencies.
Response: we have gone over the text to fix these issues.
Comment: 2. Fig. 3: It would be very useful to draw a reddening vector in these diagrams to show in which direction the measurements are moved for a given reddening. This would help the interpretation.
Response: The problem with adding a reddening vector is that the reddening is non-linear, with the vector changing depending on temperature of the individual stars. This is why we have overlaid isochrones at different reddening values. This has been clarified in the text.
COMMENT 3. Line 137 ff.: The red leak is a bit disconcerting. This could severely affect the interpretation. It would be good to give a reference for this red leak and either refer to a study of this red leak or give a brief description how it could alter the analyses. As it is written, this referee was confused whether the red leak could invalidate the results!
RESPONSE: We have added a paragraph in the introduction noting the red leak (which does not occur the uvm2 filter). We also have added notes in the next that modeling includes the red leak so this should not be a concern for the results.
COMMENT: 4. Line 165: This should probably read '... becomes a planetary nebula.'
RESPONSE: Fixed.
COMMENT: 5. Line 200: 'far-reaching impact' on the distance scale are mention here, but no explanation how and why this would be the case. The reader is left with no help on what this statement is based. A quick example might be helpful.
RESPONSE: This is perhaps an overstatement on the utility of PAGB stars for the distance ladder. We have removed it.
COMMENT: 6. Figs. 7 and 8: One assumption of these maps is that the Milky Way foreground absorption has already been removed. If this is the case, it should be stated clearly in the text and also what value has been assumed for this subtraction. If the foreground is not subtracted then this adds a base value to the maps, which would be interesting to know.
RESPONSE: The foreground dust is corrected for. We have now noted this in the text.
COMMENT: 7. Lines 281-283 and Fig. 9: This statement is puzzling. The increased uncertainty appears not reflected in the error bars. While they to increase towards larger ages, the size is never enough to be compared to other measurements. Should data outside the applicable area be plotted at all?
RESPONSE: The error bars represent formal errors the Markhov chain fit. But there is remaining bias in not having access to the stars of older populations that we wish to reflect in the figure. The text has been clarified.
COMMENT: 8. Fig. 10, caption: The lines in this figure show the evolution for different ages. It would be very helpful to use different colors for different ages to make the figure easier to understand.
RESPONSE: We have added colors for clarification.
COMMENT: 9. Line 318: The argument here is not understandable. How does correlation between the PAH maps and the bump strength relate to the bump? Is there a physical connection?
RESPONSE: The theory is that the bump is produced by PAHs. However, this is not supported by the data. Our data do show a weak connection to the spiral arms, which may suggest a different mechanism. This has been clarified in the text.
COMMENT: 10. Figs. 15 and 16 appear to have been lifted from a differen paper and their origins captions are still visible (as Fig. 2 and Fig. 13, respectively). This is very confusing and should be removed.
RESPONSE: We have replaced the figures
COMMENT: 11. Line 374: It appears that a reference has been lost here.
RESPONSE: Fixed
