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A response to P.S. Conti's comment on “A three-dimensional classification for WN stars”
New Astronomy 5 (2000) 423–425 www.elsevier.nl / locate / newast
A response to P.S. Conti’s comment on ‘‘A three-dimensional classification for WN stars’’ Lindsey F. Smith a , Michael Shara b , Anthony Moffat c b
a University of Sydney, Sydney, NSW 2006, Australia American Museum of Natural History, Central Park West at 79 th Street, New York, NY 10024 -5192, USA c Universite´ de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, QC, Canada H3 C 3 J7
Received 10 August 2000; accepted 30 October 2000 Communicated by G.F. Gilmore
Abstract Conti (1999, NewA 4, 489) has recently commented on the three-dimensional spectrographic classification system for WN Wolf-Rayet stars proposed by Smith, Shara and Moffat (1996, MNRAS 281, 163). We present here a response which counters his arguments and reinforces the utility of the Smith et al. system. 2000 Elsevier Science B.V. All rights reserved. PACS: 97.10.Ex; 97.10.Me; 97.10.Ri Keywords: Stars: Wolf-Rayet; Stars: fundamental parameters
1. Introduction Conti (1999) in a recent paper in New Astronomy has commented on the three-dimensional classification system proposed by us (Smith, Shara & Moffat, 1996) (5 SSM). Specifically, he challenges the appropriateness and usefulness of the line-width / strength dimension with the ‘b’ designating spectra with broad / strong lines, and the ‘o’ sub-category within the narrow / weak-line spectra designating spectra with no detectable hydrogen (using only visual inspection of the Pickering decrement). His principal argument (his section 1) is that ‘‘... the newly introduced subscript ‘b’ (for broad or strong E-mail addresses: [email protected] (L.F. Smith), [email protected] (M. Shara), [email protected] (A. Moffat).
emission) is not clearly delineated and should not be used. It then follows that the ‘o’, for the apparent absence of hydrogen in narrow lined WN stars, is unnecessary.’’. The point of the present paper is to refute both the above statements and to re-emphasize the importance and utility of the subscripts ‘b’ and ‘o’ as well as ‘h’ and ‘(h)’. We address his specific points in the order he makes them.
2. Refutation of Conti’s criticisms Conti (section 2) suggests that SSM’s ‘‘assignment of numerical subtypes is based upon a slight refinement of previous line ratio methods ...’’. All previous subtypes were based on qualitative comparisons of line ratios (e.g. line A is stronger than, equal to, or weaker than line B). The lines used were predomi-
1384-1076 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1384-1076( 00 )00043-9
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L.F. Smith et al. / New Astronomy 5 (2000) 423 – 425
nantly those of N, often leading to discordant results. SSM selected, as the primary criterion, a single He-line ratio that is easily observed and quantified: e.g. for a WN7 classification, the HeII(5411) / HeI(5876) ratio lies between 0.65 and 1.25. Changing the primary classification criterion and quantifying it for the first time is more than a ‘‘slight refinement’’. Care was taken to minimize changes to the existing classifications. Conti (section 2) concedes that ‘‘It is probably useful to have such a designation [‘h’] in the classification ...’’ and we agree with this statement. However, he then adds the clause ‘‘... even though it does not fit ‘MK’ principles ...’’. Strict MK principles also do not apply to the accepted designations of other peculiar spectra, such as Of, Be and Ap, but that does not diminish the utility of those classifications or of this one. Conti (section 2) criticizes the subscript ‘a’ because ‘‘... intrinsic absorption features are always found in the upper Balmer lines ...’’. However, SSM make clear that such cases are excluded. SSM state that ‘‘... the ‘a’ designation is applied only to the type of spectra in which violetshifted absorption edges belonging to the WR spectrum can be confused with absorption lines from a companion. Specifically, the Balmer and Pickering emissions are weak, and the EW’s of the absorption edges are comparable to the EW’s of the emission.’’. Conti is also incorrect in stating that SSM claim that ‘‘stars with absorption lines due to a companion are labeled ‘1abs’ or ...’’. ‘1abs’ is reserved by SSM for cases where the origin is unknown. In criticizing the ‘b’ (broad or strong) designation of SSM, Conti (section 3) states that ‘‘For objects with values of the FWHM near the peak, small errors in determining the FWHM would shunt a star from one group to the other. There is no physical basis ˚ FWHM division, nor is there a underlying the 30 A clear separation for stars with median values.’’. However, Conti concedes that there are two different groups, weak and strong line spectra, as others have before (Conti, Leep & Perry, 1983; Hamann, Koesterke & Wessolowski, 1995; Hiltner & Schild, 1966). ˚ He himself confirms that ‘‘the WNb criterion at 40 A [EW] could discriminate between narrow and broad stars’’. The change suggested by SSM addresses the problem – acknowledged by Conti – that, because of binaries and overlapping images, the EW alone is not
a good criterion. SSM therefore use the FWHM as the primary criterion while still retaining the EW criterion suggested by Hamann, Koesterke & Wessolowski (1995). With regard to the observational uncertainty, SSM clearly state ‘‘Measured line width is affected by spectral resolution and this is taken into account when assigning ‘b’ type; however, the matter is critical only when the spectrum is composite and the line strength is unknown.’’. It is however true (as with all classifications) that, as better observations become available, the classification of spectra on the boundaries between subtypes may be revised. Since most spectral properties vary smoothly between classes, there are always stars near a boundary. We disagree with Conti’s conclusion in section 3 ‘‘I doubt the utility of the SSM subscript ‘b’.’’. WNb stars have been shown to differ in several ways: consistent absence of hydrogen; difference in effective temperature and other atmospheric parameters (Hamann, Koesterke & Wessolowski, 1995). However, we emphasize (cf. Smith, Shara & Moffat, 1995) that the purpose of classification is to sort spectra into categories such that all spectra with the same label look very similar. Broad & narrow line spectra look radically different. The boundary between ‘b’ and not-‘b’ was guided by the consistent physical differences that were already known. The dividing line may, in future, be refined but is, for the moment, definitely useful. Conti (section 4) maintains that ‘‘A good case can be made that if a subscript ‘b’ is not needed, then the ‘o’ isn’t either, since it only indicates not ‘h’.’’. First, we have just demonstrated that the subscript ‘b’ is useful and needed. Just as important, ‘o’ is the lower end of the hydrogen abundance dimension. We classify stars as ‘h’, ‘(h)’ and ‘o’, not just as ‘h’ and ‘o’. Further, Conti is incomplete in stating that ‘o’ implies ‘‘the absence of hydrogen’’ because he leaves out the critical qualification ‘detectable by visual inspection’ for which the reliable limit is H1 / He11 5 0.5 by number. There will certainly be stars with hydrogen that remain undetected by this method. It is not yet known whether the ‘o’ and ‘b’ stars differ in (range of) hydrogen abundance as well as in line strength / width. Hydrogen abundance is an independent dimension and not assumed to have a one-to-one correspondence with another dimension.
L.F. Smith et al. / New Astronomy 5 (2000) 423 – 425
Conti (section 5) suggests ‘‘Thus most of the future newly identified Galactic WN stars will not be able to be classified within the context of narrow or broad subgroups.’’. His reasoning is that detection and classification will need to be done in the IR and current resolution is insufficient. We disagree and point out that instrumentalists have increased the sensitivity of IR detectors by orders of magnitude in the past decade alone. We are certain that similar advances in sensitivity and spectral resolution will ˚ at 1 micrometer continue. With FWHM . 60 A needed for ‘b’ classification, the distinction should be one of the easiest to make. Against the hydrogen designation (section 5), Conti argues ‘‘It is unclear that those with and without hydrogen could be distinguished given the paucity of Bracket and near IR HeII lines ...’’. This may prove true, but is not reason to delete a useful distinction from the classification of those spectra where it can be made. Conti’s final point concerns difficulties of measurement of the FWHM and the EW for WR stars in galaxies more distant than the LMC. He asks ‘‘For these very faint stars, sufficient spectral resolution might well be missing for some time. What purpose would a comparison to the stars in SSM with ‘b’ and ‘o’ subscripts serve?’’. In response, we point out that highly reddened spectra of galactic WN stars down to v 5 18 mag have been easily classified by Shara et al. (1999) using nothing larger than a 2.1 m telescope. Spectra with 10 m class telescopes of WR stars in M31, M33 and other Local Group members
425
can, even today with modest exposure times ( | 2 hours), distinguish between ‘b’ and ‘o’ subscripts. Next Generation Space Telescope spectra will allow us to push out to at least M81 and NGC 2403. This further emphasizes the utility of the ‘o’ and ‘b’ subscripts.
3. Summary Most astronomers have adopted the full SSM classification scheme. We maintain that the entire SSM scheme is robust and valuable, and that this is why the majority of the massive star community has accepted and is using it.
References Conti, P.S., 1999, NewA, 4, 489. Conti, P.S., Leep, E.M. & Perry, D.N., 1983, ApJ, 268, 228. Hamann, W.-R., Koesterke, L. & Wessolowski, U., 1995, A&A, 299, 151. Hiltner, W.A. & Schild, R.E., 1966, ApJ, 143, 770. Shara, M.M., Moffat, A.F.J., Smith, L.F., Niemela, V.S., Potter, M., & Lamontagne, R., 1999, AJ, 118, 390. Smith, L.F., Shara, M.M. & Moffat, A.F.J., 1995, in: van der Hucht, K.A., Williams, P., (Eds.), Proc. IAU Symp. 163, Wolf-Rayet stars: binaries, colliding winds, evolution, Reidel, Dordrecht, p. 48. Smith, L.F., Shara, M.M. & Moffat, A.F.J., 1996, MNRAS, 281, 163.
A response to P.S. Conti’s comment on ‘‘A three-dimensional classification for WN stars’’ Lindsey F. Smith a , Michael Shara b , Anthony Moffat c b
a University of Sydney, Sydney, NSW 2006, Australia American Museum of Natural History, Central Park West at 79 th Street, New York, NY 10024 -5192, USA c Universite´ de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, QC, Canada H3 C 3 J7
Received 10 August 2000; accepted 30 October 2000 Communicated by G.F. Gilmore
Abstract Conti (1999, NewA 4, 489) has recently commented on the three-dimensional spectrographic classification system for WN Wolf-Rayet stars proposed by Smith, Shara and Moffat (1996, MNRAS 281, 163). We present here a response which counters his arguments and reinforces the utility of the Smith et al. system. 2000 Elsevier Science B.V. All rights reserved. PACS: 97.10.Ex; 97.10.Me; 97.10.Ri Keywords: Stars: Wolf-Rayet; Stars: fundamental parameters
1. Introduction Conti (1999) in a recent paper in New Astronomy has commented on the three-dimensional classification system proposed by us (Smith, Shara & Moffat, 1996) (5 SSM). Specifically, he challenges the appropriateness and usefulness of the line-width / strength dimension with the ‘b’ designating spectra with broad / strong lines, and the ‘o’ sub-category within the narrow / weak-line spectra designating spectra with no detectable hydrogen (using only visual inspection of the Pickering decrement). His principal argument (his section 1) is that ‘‘... the newly introduced subscript ‘b’ (for broad or strong E-mail addresses: [email protected] (L.F. Smith), [email protected] (M. Shara), [email protected] (A. Moffat).
emission) is not clearly delineated and should not be used. It then follows that the ‘o’, for the apparent absence of hydrogen in narrow lined WN stars, is unnecessary.’’. The point of the present paper is to refute both the above statements and to re-emphasize the importance and utility of the subscripts ‘b’ and ‘o’ as well as ‘h’ and ‘(h)’. We address his specific points in the order he makes them.
2. Refutation of Conti’s criticisms Conti (section 2) suggests that SSM’s ‘‘assignment of numerical subtypes is based upon a slight refinement of previous line ratio methods ...’’. All previous subtypes were based on qualitative comparisons of line ratios (e.g. line A is stronger than, equal to, or weaker than line B). The lines used were predomi-
1384-1076 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1384-1076( 00 )00043-9
424
L.F. Smith et al. / New Astronomy 5 (2000) 423 – 425
nantly those of N, often leading to discordant results. SSM selected, as the primary criterion, a single He-line ratio that is easily observed and quantified: e.g. for a WN7 classification, the HeII(5411) / HeI(5876) ratio lies between 0.65 and 1.25. Changing the primary classification criterion and quantifying it for the first time is more than a ‘‘slight refinement’’. Care was taken to minimize changes to the existing classifications. Conti (section 2) concedes that ‘‘It is probably useful to have such a designation [‘h’] in the classification ...’’ and we agree with this statement. However, he then adds the clause ‘‘... even though it does not fit ‘MK’ principles ...’’. Strict MK principles also do not apply to the accepted designations of other peculiar spectra, such as Of, Be and Ap, but that does not diminish the utility of those classifications or of this one. Conti (section 2) criticizes the subscript ‘a’ because ‘‘... intrinsic absorption features are always found in the upper Balmer lines ...’’. However, SSM make clear that such cases are excluded. SSM state that ‘‘... the ‘a’ designation is applied only to the type of spectra in which violetshifted absorption edges belonging to the WR spectrum can be confused with absorption lines from a companion. Specifically, the Balmer and Pickering emissions are weak, and the EW’s of the absorption edges are comparable to the EW’s of the emission.’’. Conti is also incorrect in stating that SSM claim that ‘‘stars with absorption lines due to a companion are labeled ‘1abs’ or ...’’. ‘1abs’ is reserved by SSM for cases where the origin is unknown. In criticizing the ‘b’ (broad or strong) designation of SSM, Conti (section 3) states that ‘‘For objects with values of the FWHM near the peak, small errors in determining the FWHM would shunt a star from one group to the other. There is no physical basis ˚ FWHM division, nor is there a underlying the 30 A clear separation for stars with median values.’’. However, Conti concedes that there are two different groups, weak and strong line spectra, as others have before (Conti, Leep & Perry, 1983; Hamann, Koesterke & Wessolowski, 1995; Hiltner & Schild, 1966). ˚ He himself confirms that ‘‘the WNb criterion at 40 A [EW] could discriminate between narrow and broad stars’’. The change suggested by SSM addresses the problem – acknowledged by Conti – that, because of binaries and overlapping images, the EW alone is not
a good criterion. SSM therefore use the FWHM as the primary criterion while still retaining the EW criterion suggested by Hamann, Koesterke & Wessolowski (1995). With regard to the observational uncertainty, SSM clearly state ‘‘Measured line width is affected by spectral resolution and this is taken into account when assigning ‘b’ type; however, the matter is critical only when the spectrum is composite and the line strength is unknown.’’. It is however true (as with all classifications) that, as better observations become available, the classification of spectra on the boundaries between subtypes may be revised. Since most spectral properties vary smoothly between classes, there are always stars near a boundary. We disagree with Conti’s conclusion in section 3 ‘‘I doubt the utility of the SSM subscript ‘b’.’’. WNb stars have been shown to differ in several ways: consistent absence of hydrogen; difference in effective temperature and other atmospheric parameters (Hamann, Koesterke & Wessolowski, 1995). However, we emphasize (cf. Smith, Shara & Moffat, 1995) that the purpose of classification is to sort spectra into categories such that all spectra with the same label look very similar. Broad & narrow line spectra look radically different. The boundary between ‘b’ and not-‘b’ was guided by the consistent physical differences that were already known. The dividing line may, in future, be refined but is, for the moment, definitely useful. Conti (section 4) maintains that ‘‘A good case can be made that if a subscript ‘b’ is not needed, then the ‘o’ isn’t either, since it only indicates not ‘h’.’’. First, we have just demonstrated that the subscript ‘b’ is useful and needed. Just as important, ‘o’ is the lower end of the hydrogen abundance dimension. We classify stars as ‘h’, ‘(h)’ and ‘o’, not just as ‘h’ and ‘o’. Further, Conti is incomplete in stating that ‘o’ implies ‘‘the absence of hydrogen’’ because he leaves out the critical qualification ‘detectable by visual inspection’ for which the reliable limit is H1 / He11 5 0.5 by number. There will certainly be stars with hydrogen that remain undetected by this method. It is not yet known whether the ‘o’ and ‘b’ stars differ in (range of) hydrogen abundance as well as in line strength / width. Hydrogen abundance is an independent dimension and not assumed to have a one-to-one correspondence with another dimension.
L.F. Smith et al. / New Astronomy 5 (2000) 423 – 425
Conti (section 5) suggests ‘‘Thus most of the future newly identified Galactic WN stars will not be able to be classified within the context of narrow or broad subgroups.’’. His reasoning is that detection and classification will need to be done in the IR and current resolution is insufficient. We disagree and point out that instrumentalists have increased the sensitivity of IR detectors by orders of magnitude in the past decade alone. We are certain that similar advances in sensitivity and spectral resolution will ˚ at 1 micrometer continue. With FWHM . 60 A needed for ‘b’ classification, the distinction should be one of the easiest to make. Against the hydrogen designation (section 5), Conti argues ‘‘It is unclear that those with and without hydrogen could be distinguished given the paucity of Bracket and near IR HeII lines ...’’. This may prove true, but is not reason to delete a useful distinction from the classification of those spectra where it can be made. Conti’s final point concerns difficulties of measurement of the FWHM and the EW for WR stars in galaxies more distant than the LMC. He asks ‘‘For these very faint stars, sufficient spectral resolution might well be missing for some time. What purpose would a comparison to the stars in SSM with ‘b’ and ‘o’ subscripts serve?’’. In response, we point out that highly reddened spectra of galactic WN stars down to v 5 18 mag have been easily classified by Shara et al. (1999) using nothing larger than a 2.1 m telescope. Spectra with 10 m class telescopes of WR stars in M31, M33 and other Local Group members
425
can, even today with modest exposure times ( | 2 hours), distinguish between ‘b’ and ‘o’ subscripts. Next Generation Space Telescope spectra will allow us to push out to at least M81 and NGC 2403. This further emphasizes the utility of the ‘o’ and ‘b’ subscripts.
3. Summary Most astronomers have adopted the full SSM classification scheme. We maintain that the entire SSM scheme is robust and valuable, and that this is why the majority of the massive star community has accepted and is using it.
References Conti, P.S., 1999, NewA, 4, 489. Conti, P.S., Leep, E.M. & Perry, D.N., 1983, ApJ, 268, 228. Hamann, W.-R., Koesterke, L. & Wessolowski, U., 1995, A&A, 299, 151. Hiltner, W.A. & Schild, R.E., 1966, ApJ, 143, 770. Shara, M.M., Moffat, A.F.J., Smith, L.F., Niemela, V.S., Potter, M., & Lamontagne, R., 1999, AJ, 118, 390. Smith, L.F., Shara, M.M. & Moffat, A.F.J., 1995, in: van der Hucht, K.A., Williams, P., (Eds.), Proc. IAU Symp. 163, Wolf-Rayet stars: binaries, colliding winds, evolution, Reidel, Dordrecht, p. 48. Smith, L.F., Shara, M.M. & Moffat, A.F.J., 1996, MNRAS, 281, 163.