revised spectral types for 64 b-supergiants in the small...

Astron. Astrophys. 317, 871–882 (1997) ASTRONOMYAND

ASTROPHYSICS

Revised spectral types for 64 B-supergiantsin the Small Magellanic Cloud: metallicity effects?

D.J. Lennon1,2

1 Universitats-Sternwarte Munchen, Scheinerstrasse 1, D-81679 Munchen, Germany ([email protected])2 Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching bei Munchen, Germany

Received 24 April 1996 / Accepted 9 May 1996

Abstract. The problem of the classification of metal poor stars,such as occur in the Small Magellanic Cloud (SMC), is dis-cussed with reference to the applicability of the MK system insuch an environment. An alternative method is presented hereand applied to B-type supergiants in the SMC. A local refer-ence system is first devised and then a transformation to MKspectral types is determined by comparing the trends of metalline strengths in these two systems. For the determination ofthe luminosity class, we emphasize the need to use the hydro-gen Balmer line strengths independently of metal line-strengthconsiderations. This method is used to determine new spectraltypes for 64 supergiants in the SMC, 75% of the sample requir-ing classifications different from previous findings. These newtypes result in much improved line strength – spectral type cor-relations for He, C, N, O, Mg and Si. Corresponding changesin the distribution of these stars in the Hertzsprung-Russell dia-gram of the SMC reveal more clearly than before the existenceof a ridge which may be the SMC analogue of a similar fea-ture found for the LMC by Fitzpatrick & Garmany (1990). Thegroup of very luminous supergiants lying above this ridge in-cludes the LBV AV415 (R40), a property which this object hasin common with LBVs in the Large Magellanic Cloud. Also, forthe first time, clear examples of BN/BC supergiants are foundin the SMC.

Key words: stars: fundamental parameters – supergiants – Mag-ellanic Clouds

1. Introduction

The great potential that blue supergiant stars have as diagnos-tics of metallicity, extinction and distance, has recently beendiscussed by Lennon et al (1994) and Kudritzki et al (1995).

Send offprint requests to: D.J. Lennon? Based on observations taken at the European Southern Observatory,La Silla, Chile

In particular the use of the wind-momentum – luminosity rela-tionship (WLR) promises to be an important advance in the useof these stars as standard candles. These ideas are already be-ing tested in galaxies as distant as M33 (McCarthy et al 1996),the ultimate aim being to extend this work to resolved galax-ies as distant as the Virgo cluster. However, from observationsof O-stars in the Galaxy and the Magellanic Clouds, Puls et al(1996) demonstrated that the WLR is metallicity dependent andtherefore needs to be calibrated in terms of this parameter. Notethat the technique proposed for applying the WLR (see refer-ences above) is a spectroscopic method and one can derive thechemical composition of supergiants directly from their spec-tra. The Small Magellanic Cloud (SMC) is the obvious place tocheck this metallicity dependence, which was the primary moti-vation for the spectroscopic investigation of B-type supergiantspresented below. However it became clear that their publishedspectral types were surprisingly inaccurate, necessitating a re-assessment of the classification system for these objects.

It might be expected that the spectral types for blue super-giants in the SMC, since they are visually the brightest stars, arewell determined but there are two complicating factors, both ofwhich may be attributed to this galaxy’s low metallicity. Firstly,the metal lines, which are important classification criteria forA- and B-type supergiants, are weak and difficult to detect intypical intermediate dispersion and low signal-to-noise classifi-cation spectrograms. Secondly, the MK classification system isdefined in terms of galactic standards which have significantlyhigher metallicity than comparable stars in the SMC. The firstproblem is solved in the present work since we have obtainedhigh quality spectra (discussed in the next section). We addressthe second problem (see Sect. 3) by not relying on the MKsystem but instead we define a new system for the SMC. Thecorrespondence between these two systems is determined suchthat stars with similar physical properties, metallicity aside, areclassified the same. This is an important issue, since we wishultimately to compare Hertzsprung-Russell (HR) diagrams fordifferent galaxies in the hope of providing information on theevolution of massive stars in regimes of different metallicities.

872 D.J. Lennon: Revised spectral types for B-type supergiants in the SMC

The evolution of massive stars is still poorly understood.For example, Langer & Maeder (1995) recently highlighted thelong-standing problem of the inability of stellar evolution cal-culations to reproduce the ratio of blue to red supergiants in astellar population as a function of metallicity. There is the ques-tion of the significance of the BN and BC stars (see Walborn1976) and the possibly related uncertainty over whether or notstars in this mass range undergo blue loops on the HR-diagram.In fact, until the present work there were no known BN/BC su-pergiants in the SMC, which may be attributable to the intrinsicweakness of the metal lines. Even the distribution of stars in theupper part of the HR-diagram is an important property to deter-mine for the SMC. We know that Fitzpatrick & Garmany (1990)uncovered a ledge of stars in the HR diagram of the LMC; couldsuch a feature also exist for the SMC? Garmany & Fitzpatrick(1989) attempted to answer this question but the results wereinconclusive. The connection with the problem of classificationmentioned in the preceeding paragraph is clear. Finally, in or-der to investigate the luminous star population of more distantgalaxies, which might have a range of metallicities (or indeedmetallicity gradients), it is desirable to have sets of standardstars for various metallicities. This last point is illustrated bythe discussion of the abundance gradient in M33 using spectraof supergiants by Monteverde et al (1996; see also Herrero et al1994).

2. Observational data

As target stars we have selected a subset of the brightest B0-B9supergiants in the SMC, taken from the compilation of Gar-many et al (1987), with particular emphasis on the earlier spec-tral types to investigate the behaviour of the important CNOions. The spectrograms were obtained at La Silla using the ESONTT and the EMMI instrument operated by remote control fromGarching in Germany. The data were acquired over two observ-ing runs, on October 7th 1993 and October 22nd – 24th 1994, us-ing gratings #3 and #6 in the blue and red arms respectively. Forboth observing runs the ESO CCD #31 (Tektronix TK 1024 AB)was used in the blue arm, while during the second run the ESOCCD #36 (Tektronix TK 2048) was used in the red arm. Dueto problems with the focus of the red arm during the first ob-serving run, the relevant data were discarded. Both CCDs havea pixel size of 24µm which resulted in dispersions of approx-imately 0.45A per pixel in the blue and 0.32A per pixel in thered. For a 1′′ slit (used throughout) this gave FWHM resolutionsof approximately 1.18A 1.27A and 1.21A at Hα, Hγ and Hδrespectively, a resolution element being equivalent to 3.7 pixelsin the red and 2.7 pixels in the blue. Each star was observedin three wavelength regions; in one exposure the dichroic wasused to simultaneously obtain data in the blue and red arms cov-ering the approximate wavelength regions 3925A – 4375A and6190A – 6830A, and in another exposure in the blue arm onlycovering the region 4300A – 4750A. It was necessary to carryout this latter exposure without the dichroic due to the poor in-strumental response in the relevant wavelength range with thedichroic.

Table 1. Classification criteria used for B-type supergiants in the SMC.All line wavelengths are in A.

Spectral bin Criteria DerivedSpectral type

1 He ii 4686 and 4542 present B0

2 He ii 4686 weak or absent B1Si iv 4088/4116 present

3 Si iv 4116 absent B1.5Si iv 4088 < O ii

4 Si iv, Si ii absent B2Si iii 4553 > Mg ii 4481

5 Si ii 4128/4132 < He i 4121 B2.5Si iii 4553 ∼Mg ii 4481

6 Si ii 4128/4132 < He i 4121 B3Si iii 4553 < Mg ii 4481

7 Si ii 4128/4132 < He i 4121 B5Si iii absent

8 He i 4121 < Si ii < He i 4143 B8Mg ii 4481 < He i 4471

9 Mg ii 4481 ∼ He i 4471 B8

10 Mg ii 4481 > He i 4471 B9Fe ii 4233 < Si ii 4128/4132

Note that the wavelength coverage provided by these threeranges contains the features necessary to adequately classify andanalyse B-type supergiants (see Lennon et al 1992, 1993) as wellas the important wind diagnostic line, Hα. Exposure times de-pended on conditions and the visual magnitude of the target butwere typically 10 to 15 minutes giving a S/N normally in excessof 70. The initial data reduction, such as bias/flat-field correc-tion and spectrum extraction was carried out within the midasenvironment, with subsequent wavelength calibration, rectifica-tion, merging of blue spectrograms and equivalent width mea-surement being performed using idl and dipso (Howarth &Murray 1988). He-Ar spectra were used for the wavelength cal-ibration, with at least 12 lines per wavelength region being usedto perform a low order (2nd or 3rd) polynomial fit to the linecentroids. This gave mean residuals between fitted and observedline positions of typically 0.1 pixels in the blue and 0.3 pixelsin the red.

3. Method of classification

The problem of spectral classification of metal poor stars hasbeen discussed previously in the literature, in particular by Fitz-patrick (1991) who re-classified a large number of B-type super-

D.J. Lennon: Revised spectral types for B-type supergiants in the SMC 873

Fig. 1. Comparison of equivalent width trends using the previous spec-tral types (top) taken from Garmany et al (1987), with the new spectralbins (bottom) for the Mg ii 4481A doublet. Equivalent widths are inmA.

giants in the Large Magellanic Cloud (LMC). In this case how-ever the rather moderate metal deficiency in the LMC allowedthe MK system to be transferred to the supergiants in that galaxywith relative ease. The SMC is a more difficult case howeversince the weakness of the metal lines is much more pronounced(Osmer 1973b, Nandy et al 1990). Nevertheless in much of theearly work on spectral classification in the SMC, the question ofthe applicability of the MK system to stars which were probablymetal deficient compared to standards was largely discounted(Ardeberg & Maurice 1977, Humphreys 1983, Azzopardi &Vigneau 1975). There were some attempts to disentangle theeffects of metallicity from those of temperature (spectral class)and gravity (luminosity class). For example, Dubois et al (1977)resorted to the use of the Balmer decrement and the He i linesfor spectral classes and the strengths of the hydrogen lines forluminosity classes. Walborn (1983, 1977) discussed this prob-lem further by considering the classification of O-type and B0supergiants in the SMC. Walborn was able to classify thesestars within the He i/He ii reference frame (and considering thewidths of the hydrogen lines), which is independent of metal-

licity considerations. He thus showed that these supergiants hadSi lines with strengths comparable to MK standard dwarfs orgiants and proposed a classification notation such that a super-giant classified as B0 Iaw(IV) referred to a metal weak (w) B0 Iasupergiant with metal lines comparable in strength to a galacticluminosity class IV star. Crampton & Greasley (1982) came tosimilar conclusions, also from consideration of O and B0 starsin the SMC. Note that a common thread in the luminosity clas-sifications just cited is the use of the hydrogen lines. In factAzzopardi (1987) used the Hγ equivalent width to derive lumi-nosity classes for 195 bright O9-F8 stars in the SMC, althoughhe adopted the MK spectral classes derived from objective prismdata by Azzopardi & Vigneau (1975).

This raises an important point concerning the work ofMassey and co-workers who have made extensive and impor-tant contributions to our knowledge of spectral types in the SMC(and indeed other galaxies in the Local Group). Their work, sum-marized by Massey et al (1995a), makes use of the MK systemfor classification except that it does not make use of the luminos-ity information inherent to the hydrogen lines. Instead, they usethe strength of the metal lines as the sole indicator of luminosity.Thus we see that SMC stars classified as supergiants on the basisof their Hγ equivalent widths by Azzopardi (1987) are classi-fied as dwarfs or giants according to their metal line strengths byMassey et al (see their Table 5). This problem was subsequentlyacknowledged by Massey et al (1995b) where they classified B-type supergiants in the metal poor galaxy NGC 6822 as havingluminosity classes III or V, again based upon the strengths ofthe metal lines, but found that their absolute visual magnitudeswere comparable to those supergiants. Adhering strictly to theMK system for such objects of course leads to contradictions be-tween the hydrogen and the metal lines. However luminosity isa measure of the surface gravity and the hydrogen lines are sen-sitive to this parameter irrespective of the metallicity question.It is therefore more consistent with MK classifications at solarmetallicity if the hydrogen lines are used for the determinationof the luminosity class.

In the present paper we follow the example of Azzopardi(1987) and use the equivalent widths of the Hγ line in the deter-mination of the luminosity classes. The determination of spec-tral types for stars cooler than B1 is problematic however sincethe He ii lines become weak or absent while reliance on theHe i lines alone leads only to very crude classifications (seeDubois et al 1977). In the classification of normal B-type spec-tra, heavy reliance is placed upon the strengths of the siliconand magnesium lines, often in comparison to helium lines (seeWalborn 1971, Walborn & Fitzpatrick 1990 and Lennon et al1992). Clearly this is not simply transferrable to the SMC. In-stead, since the data discussed here are of such high quality thatwe can reliably measure the weak metal line equivalent widths,we reclassify the spectra such that the trends of line strengthversus spectral type, as has been found for Galactic supergiants(see Lennon et al 1993), are reproduced. Implicit in this pro-cess is the assumption that the sample of stars is reasonablychemically homogeneous, and abundance studies of A- and F-type supergiants in the SMC support this assumption (Russell &


Dopita 1992, Venn 1996) and, as we shall demonstrate below,this is also supported by the present work. Below we discussthe spectral types and luminosity classes separately although inpractice one must consider both simultaneously or iteratively.

3.1. Luminosity classes

Since the distance to the SMC is reasonably well known wecan in principle make use of absolute visual magnitude (MV)calibrations. Indeed for the range of absolute visual magnitudesrepresented by the current sample of stars we can be sure thatalmost all are luminosity class I supergiants. However we preferto rely upon calibrations of MV in terms of the equivalent widthof the Hγ line since this is independent of extinction correc-tions (although these are small). [In fact, given the large amountof work invested in attempts to calibrate MV in terms of Hγ(see for example Balona & Crampton 1974 and Walker & Mill-ward 1985), these two methods should lead to identical results.]Therefore equivalent widths were determined for the Hγ linein all stars and we used the calibrations of Azzopardi (1987)and Balona & Crampton (1974) for the estimation of luminos-ity classes. This is in some sense a formality for most of theobjects considered since, as has been pointed out, they are sobright that a Ia subclass is mandatory. In fact for stars later thanapproximately B1, the strength of Hγ for a given subclass israther insensitive to the spectral type, and luminosity subclasschanges tend only to occur if a star is re-classified from one ofthese later spectral types to a B0 or earlier type.

One difficulty with the calibrations mentioned above is thatneither adequately samples the hypergiants; Balona & Cramp-ton have no hypergiants in their sample and Azzopardi has onlytwo B-type hypergiants, one of which (HD157038) is betterclassified as B3 Iap (Walborn 1976). Obviously the Hγ equiva-lent width calibration is very uncertain for this class of object.We have therefore made the rather arbitrary decision to clas-sify as hypergiants only those stars with the strongest Hα netemission (we conservatively adopt −4A as the discriminatoryequivalent width).

3.2. Spectral classes

Initially the spectra were ordered roughly according to tempera-ture, from the hottest, exhibiting lines of He ii and Si iv, throughto the coolest, which have strong Si ii and strengthening Fe iilines. Obvious O-stars were classified immediately using theirHe i and He ii lines, (AV6, AV479 and AV490), while the restof the stars were grouped into various spectral bins according tothe criteria summarized in Table 1. Each of these bins is simplyidentified by number, as indicated in the table. The goal of theclassification procedure is to map these numbers to MK spectraltypes. Equivalent widths of representative spectral lines werethen measured to examine the trends of ionization with the asyet undetermined spectral types. Equivalent widths were mea-sured for the following strategic lines; He iλλ4471, 4387, 4143,4026, 4009, C ii λ4267, N ii λ3995, O ii λ4661, Si ii λλ 4128,4132, Si iii λλ 4553, 4564, 4573, Si iv λλ 4088, 4116 and Mg ii

Fig. 2. Comparison of equivalent width trends using the previous (toppanel) and present (middle panel) spectral types for the Si iii 4553Aline. In the bottom figure is the analagous relationship for the Galaxy(from Lennon et al 1993). Notice the similar trends in the two lowerfigures but also the difference between SMC and Galactic equivalentwidths.


Fig. 3. This figure shows the differences in nitrogen and carbon line strengths between AV215 (BN0 Ia) and AV487 (BC0 Ia). The lines indicatedabove are N ii 3995A, N iii 4097A (blended with the blue wing of Hδ), N iii 4634/4640A, Si iv 4088A, He ii 4542A, Si iii 4553A and the C iii4650A blend. These spectrograms are the first clear examples of BN/BC supergiant morphology in the SMC.

λ4481 (all wavelengths in A). Plots of equivalent width versusspectral bin were then constructed and some iteration of theclassification process was performed to reduce the scatter of theresulting trends for Mg ii and silicon ions since the classificationprocess depends heavily on these species. This procedure alsoresulted in improved correlations for other species, consistentwith the idea that the stellar sample is reasonably homogeneous.An example of this is illustrated in Fig. 1 for the Mg ii 4481Aline where we can see how the new spectral bins lead to muchless scatter in the equivalent width trend.

By comparing these kinds of plots with analogous resultsfor Galactic supergiants (see Lennon et al 1993) it is possibleto derive a transformation from SMC spectral bin number toMK spectral class. For example, for B-type supergiants, C ii,N ii and O ii peak at spectral types of around B3, B2 and B1respectively which correspond to the SMC bin numbers 6, 4, and2 approximately. [It is probable that reducing the metallicity ata given effective temperature may result in different ionizationfractions for a particular ion, but this effect is most likely smallcompared to the uncertainties and qualitative nature of spectralclassification and so is ignored here.] For stars in bin 1 we areable to use the He i/ii reference frame and given the good qualityof the present data assign stars the interpolated spectral typesB0.5 and B0.7, also using the relative strengths of their siliconlines. For spectral types later than B5 the allocation of spectraltypes is problematic given the lack of distinct maxima. In thiscase we use the equivalent widths of the He i lines, although wedo not use the interpolated spectral types B6, B7 (or B4) andmerge bins 8 and 9 to represent spectral type B8. The successof this procedure is illustrated in Fig.2 where we compare thetrends of the Si iii 4553A line in the SMC using old and new

spectral types with the Galactic data; it can be seen that theagreement between the latter two results is excellent. Similaragreement is found for all other species.

4. Peculiar objects: LBVs, BN/BC supergiants

We observed the supergiant AV415 (aliases; HD6880, R40,Sk130) which was the first luminous blue variable (LBV) foundin the SMC (Szeifert et al 1993). The spectrum observed hereon Oct. 22 1994 appears to be characterized by a slightly coolereffective temperature than that of the Jan. 30 1993 spectrumdiscussed by Szeifert et al 1993. For example, for the equiva-lent widths of the He i 6678A Mg ii 4481A and Mg i 3838Alines, we obtain 50mA, 495mA and 85mA respectively com-pared to their values of 165mA, 540A and 70mA. Since we donot have an adequate sample of A-type supergiants here to de-fine the spectral sequence, the spectral type of AV415 must beconsidered uncertain. The extent to which spectral variabilitymight account for some of the uncertainty in the spectral typesfor the most luminous supergiants in the SMC is an importantquestion. This cannot be answered by the present study; long-term photometric and spectroscopic monitoring campaigns ofthe kind described by Szeifert et al are necessary. For one otherobject, AV78 (aliases HD5045, R9, Sk40), a Caspec spectro-gram (resolving power∼ 20000) has been published by Imhoff(1994). This spectrogram would be classified as B9 Ia underthe present system, compared with B1.5 Ia+ derived from ourspectrogram. However we suggest that the object discussed byImhoff was mis-identified, the actual target being the nearbyB9 Ia supergiant AV76.


Fig. 4. Hα and He ii 4686A line profiles of the supergiant AV490,the optical counterpart of the X-ray eclipsing binary system SMC X-1.Both the He ii 4686A and Hα profiles display emission, the formeris clearly double-peaked while the latter appears as a P-Cygni profilewith additional blueward emission. The absorption line redward ofHe ii 4686 is the He i 4713A line.

We also find the first clear SMC examples of the BN/BCdichotomy among B-type supergiants described by Walborn(1976). These are the stars AV215 (BN0 Ia) and AV487(BC0 Ia), and in Fig.3 we show their spectrograms which il-lustrate the difference in strength between their N ii and C iiifeatures. Note particularly the presence of N ii 3995A in AV215and the strength of N iii 4097A line in the blue wing of Hδ.Other stars with anomalous nitrogen or carbon line strengthsare indicated in Table 2.

AV490 (Sk160) is the optical counterpart of the X-ray sourceSMC X-1, an eclipsing X-ray binary system consisting of anearly type supergiant and a neutron star companion havingmasses of approximately 15.2 and 1.2 solar masses respectively,the spectral type of the primary normally being taken as B0 I(see van Kerkwijk et al 1995 for details). AV490 is reclassifiedhere as O9.5 II, but since the spectrum is variable (see Hutchingset al 1977, Reynolds et al 1993) this is probably not significant.Both the He ii 4686A and Hα profiles display emission, theformer is clearly double-peaked while the latter appears as aP-Cygni profile with additional blueward emission (see Fig.4).Hutchings (1977) discusses the He ii 4686A emission featurein detail but he did not detect the presence of a P-Cygni profileat Hα, this line being detected as a pure emission feature in hisobservations. Note that the 3-D hydrodynamic simulation of thestellar wind by Blondin & Woo (1995) predicts the presence ofa wind-compressed disk in the X-ray shadow of the primary (thesystem will also contain an accretion disk), and further spectro-

scopic monitoring of the primary over an orbital period (about3.9 days) is surely worthwhile.

5. Comparison with previous spectral types and the HRD ofthe SMC

The new spectral types are listed in Table 2 along with previousresults. To give some idea of the morphological differences be-tween SMC and Galactic supergiant spectra, we also plot spec-tral sequences for these two galaxies in Figs. 5 and 6, where theGalactic data are taken from Lennon et al (1992). These figuresillustrate the startling differences between supergiants in thesetwo galaxies resulting from the difference in their metallicities.This is the kind of three-dimensional sequence (temperature,luminosity and metallicity), which can be used to classify starsin situations where the metallicity is not a priori known (e.g.Monteverde et al 1996). The spectral sequence for the SMC(figure 6) is composed from a sample of stars which may beregarded as standards at the metallicity of the SMC. One cau-tionary note on the use of these standards is that the later spectraltypes are not as well sampled at lower luminosity as the earliertypes (see Fig.7), hence the break at B8 with the inclusion of thehypergiant AV65 (note the weakness of Hγ). Further refinementof the classification criteria for late-B supergaints in the SMCusing a larger sample is clearly needed.

We can gain some idea of the improvement the present workrepresents by comparing equivalent width – spectral type trendsfor some representative lines using both old and new classifica-tions. Since the compilation and spectral types of Garmany et al(1987) are often adopted in more recent papers, we concentrateon comparing with this source. Using the Mg ii 4481A and theSi iii 4553A lines, Figs. 1 and 2 demonstrate convincingly thatthe new spectral types lead to much better correlations than theearlier types. Similar comparisons can be shown for lines of theother species, in all cases the new spectral types leading to sig-nificant improvements. Typically this reduction in the scatter isconsistent with an improvement in classification accuracy fromabout 2 or 3 spectral classes to one class.

We have already commented on the problem of determin-ing luminosity classes and, since we use Azzopardi’s calibration(see Sect. 3.1) of Hγ, we of course obtain very good agreementwith his estimates, except for those cases where there is a signif-icant shift in the spectral class (see AV490). Turning to the workof Massey et al (1995a) we see that for the two stars in common,AV268 and AV271, we get good agreement with their spectralclasses, B2.5 and B1.5, but their luminosity classes are V and IIIcompared with Ib and Iab derived here and by Azzopardi (1987).In the notation of Walborn (1983), these two stars would beclassified as B2.5 Ibw(V) and B1.5 Iabw(III) respectively. Thisis not simply a question of notation, it has important implica-tions for estimates of extinction and stellar luminosities usingspectral types and assumed intrinsic color calibrations for B-stars. So for example, a supergiant mistakenly classified as adwarf would appear to be too luminous for its spectral type as-suming an absolute visual magnitude based upon the distancemodulus. Further, the intrinsic (B − V )o of a B2 dwarf/giant is


Table 2. In this table we compare the spectral types obtained here (thirdcolumn) with Azzopardi & Vigneau (1987; AZO87). Star identifyingnumbers refer to the catalogues of Azzopardi & Vigneau (1982; AV)and Sanduleak (1968; Sk). Other sources of spectral types are Feastet al (1960; FTW), Ardberg & Maurice (1977; ARM), Garmany etal (1987; GCM), Humphreys (1983; H), Dubois et al (1977; DJJ),Walborn (1983; W), Buscombe & Kennedy (1962; BK).

AV Sk Present AZO87 Others Comment2 3 B8 Ia+ – B6 Ie (FTW); B6 Iae+ (ARM)6 – O9 III B0 Ia Bpec+neb (GCM)10 7 B2.5 Ia B3 Ia B5 I (H)18 13 B2 Ia B1 Ia+ –22 15 B5 Ia B2 Ia B5 I (H)23 17 B3 Ia B3 Ia –48 27 B5 Ia B3 Ia+ B3 Ia (ARM, BK); B5-A1-B8 Ia (DJJ)56 31 B2.5 Ia B5 Ia B2 Ia (ARM, H)65 33 B8 Ia+ B6 Ia+ B6 Ia (ARM); B5 Ia (BK)76 39 B9 Ia+ A0 Ia+ A0 Ia+ (ARM); A0 Ia (FTW, BK); B5-A1 Ia (DJJ)78 40 B1.5 Ia+ – B3 Ia (ARM, FTW)86 42 B1 Ia – B2 I (H)96 46 B1.5 Ia B1 Ia B2 I (H)97 – B2 Iab B2 Iab –98 45 A0 Ia B9 Ia B9 Ia (ARM); B8-A0 Ia (DJJ)99 – B2.5 Iab B2 Ia-Iab –101 47 B9 Ia B5 Ia B5-B6 Ia (DJJ)103 – B0.7 Ib B1 Iab –104 – B0.5 Ia B1 Ia B0 I (GCM)125 52 B3 Ia B3 Ia B1-B3 Ia (DJJ)137 53 B2 Iab B4 Ia B3 I (H)151 57 B2.5 Ia B5 Ia B6 Ia (ARM); B0: I (GCM)173 62 B1.5 Ia B3 Ia B2 Ia (H)187 68 B3 Ia B2 Ia B2.5 Ia (ARM)200 69 B8 Ia (3) B8 I (H)210 73 B1.5 Ia B3 Ia B3 I (H)215 76 BN0 Ia B1 Ia B3 I (H) N strong (BN)242 85 B1 Ia B1 Ia B1.5 Ia (ARM); B1-B8 Ia (DJJ); B0.7 Iaw (W)252 87 B2.5 Iab B2 Ia –257 91 B2.5 Iab B3 Iab B3 I (H)260 92 B1.5 Iab B1 Iab –263 93 B9 Ia B6 Ib B8 Iab (H)264 94 B1 Ia B0 Ia-Ia+ B2 I (H)268 96 B2.5 Iab B0 Ia –270 98 A0 Ia B9 Ia A0 Ia (ARM); B7-A0 Ia (DJJ); A0 Ia (FTW)271 – B1.5 Iab B0 Iab –297 – B8 Ia B7 Ia A1 I (FTW)303 – B1.5 Iab B1 Iab – N weak314 – B5 Iab B2 Iab –315 106 A0 Ia B8 Ia-Ia+ B9 Ia (ARM, FTW); B8: Ia+ (DJJ)320 – B3 Iab B2 Ia –337 – B2 Iab B1 Ia –340 – B1 Ia B2 Ia –342 – B2.5 Iab B1 Iab B1 I: (FTW), B1 I (GCM)362 114 B3 Ia B3 Ia+ B3 Ia (ARM, FTW)367 117 B9 Ia+ B6 Ia+ B7 Ia (ARM); B4-A1 Ia (DJJ); B6 Ia (FTW)373 119 B2 Ia B2 Ia B2-B3 I (H) C weak374 – B2 Ib B1 Iab – N weak382 121 A0 Ia B9 Ia-Ia+ B8 Ia (ARM); B8-A1-B9 Ia+ (DJJ)404 128 B2.5 Iab B2 Ia-Iab B5 I (H)415 130 A2 Ia: – B8 Ie (FTW); B9 Iae+ (ARM) LBV420 131 B0.5 Ia B2 Iab B1 I (GCM)443 137 B2.5 Ia B2 Ia+ B3 Ia+ (ARM); B2 Ia (BK); B3 Ia (DJJ)445 138 B5 Iab B3 Iab –462 145 B1.5 Ia B1 Ia –472 150 B2 Ia B2 Ia – C weak479 155 O9 Ib B1 Ia B0-B5 Ia (DJJ); O9 Iw(III) (W)487 158 BC0 Ia B0 Ia – C strong (BC)490 160 O9.5 II B1 Iab B0 I (ARM); B0 Iwp(V) (W); B2 I pec (GCM) SMC X-1504 168 B9 Ia A0 Ia-Iab B9 Ia (ARM)– 56 B8 Ia+ – B8 Ia (ARM)– 191 B1.5 Ia – B1.5 Ia (ARM)– 196 B8 Ia – B8 Ia (ARM)– 202 B5 Iab – B5 I (H)

bluer by 0.06 magnitudes than that of a B2 supergiant. O-starsof course present no such problems since their intrinsic colorsare almost independent of luminosity (and temperature), whichis why distances to the Magellanic Clouds derived by the spec-troscopic parallax method (and extinctions) concentrate on theearliest spectral types (O-type – B0). Continuing with the exam-ple begun above, the effective temperature of a B2 dwarf/giantis approximately 3000 K hotter than the supergiant, while thebolometric correction is larger (more negative) by about 0.25magnitudes. Such a star would thus appear at too hot and tooluminous a position on the HR diagram if classified as a dwarfinstead of a supergiant; for AV268 and AV271 the error in MBOL

amounts to almost a magnitude.

This problem is reminiscent of the group of peculiar A-type supergiants with anomalously strong Balmer lines for theirspectral types, found in the SMC by Humphreys (1983). It wassubsequently argued (Humphreys et al 1991) that these objectsconstituted a post-red supergiant population of massive starswith enhanced helium abundances (resulting in stronger Balmerlines). A puzzling aspect of this picture is that the anomaloussupergiants tended to be less luminous than the normal ones,contrary to what one would expect from stellar evolution the-ory. An alternative explanation is namely, that these anomaloussupergiants are in fact less luminous stars of later spectral type,the lower metallicity causing them to be classified too early. Sup-port for this scenario comes from a model atmophere abundanceanalysis of the “anomalous” supergiant AV478 (Sk154) by Vennet al (1996). This star, classified as A2–A3I by Humphreys etal (1991), is found to have an effective temperature and surfacegravity consistent with an MK spectral type of A5–A7Ib.

Recalling the problems that stellar evolution calculationshave in reproducing the observed distribution of blue super-giants in HR diagram (see Langer 1991 for example), and inparticular the role that uncertainties in effective temperatureand luminosity might play in confusing this picture, it is im-portant to investigate the effect of the new spectral types on theappearance of the HR diagram of the SMC. Stellar luminosi-ties have therefore been computed using intrinsic colours andbolometric corrections calibrated against spectral types, plus theobservedUBV magnitudes. In order to compare with the resultsof Garmany et al (1987) we have adopted their UBV magni-tudes, which in turn are taken from the catalogue of Azzopardi& Vigneau (1982) or references therein.

We adopt the intrinsic color – spectral type calibration ofFitzpatrick (1988) and Fitzpatrick & Garmany (1990), whichwas derived for the LMC. It is probably inappropriate to applythis calibration to SMC supergiants, but given the lack of a suit-able SMC calibration we have no alternative. In any case sincewe are currently investigating the effect of the new spectral typeson the inferred HR diagram, the neglection of metallicity effectson the intrinsic colors is probably not important. Similarly, thechoice of the effective temperature scale is probably not crucialfor such a comparison, and we follow Lennon et al (1993) andadopt the calibration of Lamers (1981). [The use of this temper-ature scale is also consistent with the plots of equivalent widthversus spectral type discussed above.] Lamers (1981) is also thesource of the bolometric corrections, although these are similarto the values used by Fitzpatrick & Garmany (1990). A distancemodulus to the SMC of 19.1 is adopted, consistent with the re-cent value of 19.1±0.3 found by Massey et al (1995a) using themethod of spectroscopic parallax. We use a value for the ratioof total to selective absorption (RV ) of 3.1.

Reddening was typically small, the mean and sample stan-dard deviations of E(B − V ) being 0.058±0.008 for all stars.In some cases (13 stars) the computed value of E(B − V ) wasnegative or less than the expected foreground extinction, whichwe took to be 0.05 inE(B−V ). We therefore set a lower cut-offto the reddening of E(B − V ) = 0.05 which resulted in a meanE(B − V ) = 0.078 ± 0.008 for the remaining 51 objects. In


Fig. 5. MK spectral types for the B-supergiant sequence in the Milky Way. Data are taken from Lennon et al (1992) where line identificationscan be found. Contrast the strength of the metal lines with those in the SMC (Fig.6).


Fig. 6. A sequence of typical B-type supergiants in the SMC, note the weakness of the metal lines relative to the Galactic stars (Fig.5).


Fig. 7. Comparison of HR diagramsusing new (top) and old (bottom)spectral types. For reference onlywe have included the evolutionarytracks of Schaller et al (1993) whichwere computed for a metallicity ofZ = 0.001. Each track is labelled byits ZAMS mass in solar units.

practice however, the correction for reddening is not crucial forthe sample of stars considered here, since it is small. The resultsare shown in Fig.5, where we again compare with the resultsobtained using the spectral types as adopted by Garmany et al(1987). It is necesssary to preface the discussion of this compari-son with some qualifying remarks concerning selection effects.The survey of B-supergiants is obviously not complete. It isalso biased towards the earlier spectral types to investigate the

CNO lines, with the result that the later spectral types are notwell sampled at lower luminosities. Nevertheless, a significantfraction of supergiants between B0 and B5, outside of compactclusters, have been observed and therefore the picture for thesestars should be relatively secure.

The general impression from this comparison is that theolder spectral types tend to cluster in spectral classes B1, B2 andB3, which is where the metal line strengths in B-stars have their


maxima. The data considered here result in a more even distribu-tion of stars, allowing us to discriminate more finely in effectivetemperature and to correct many erroneous classifications, forexample, the number of B3 supergiants decreases from 11 to 5.In fact only a total of 15 stars have unaltered spectral classes,while the mean change in effective temperature expressed as apercentage of the old values is 13% . One feature of the newdistribution of stars is the presence of a group of very luminoussupergiants lying approximately along a (log(Teff ), log(L/L�))strip running from (4.3,5.9) to (4.0,5.1). This feature is remi-niscent of the S Doradus instability strip, as discussed by Wolf(1989), and indeed contains the LBV AV415 discussed above.Immediately below this strip there appears to be a gap in lu-minosity before one encounters the main bulk of supergiantswhich come in almost one bolometric magnitude fainter.

This kind of distribution, namely a sudden change in thedensity of stars along a locus in the HR diagram, is similar tothe “ledge” of stars found by Fitzpatrick & Garmany (1990)in the LMC. Garmany & Fitzpatrick (1989) also pointed outthat there was a suggestion of such a ledge in the SMC data,albeit poorly defined. One difficulty they mentioned with theSMC data was simply the smaller number of stars comparedto the LMC, however here we can now see that a compound-ing problem was the inaccuracy of the spectral types availableto them. Interestingly, a distribution similar to that found herewas also obtained by Osmer (1973a), who constructed the HRdiagram for Sanduleak’s catalogue of SMC stars (Sanduleak1968) using ubvy photometry to determine effective temper-atures. This method should be reasonably accurate for B-typestars but breaks down for O-stars due to the growing degeneracyof the [u− b] index. Certainly it is now clear that effective tem-peratures derived for B-type supergiants from uvby photometryshould be more reliable than those obtained using the previousuncertain spectral types.

6. Conclusions

This work shows that in order to classify Galactic and SMC B-type supergiants with comparable accuracy, higher s/n and res-olution data are required for the latter, due to their much weakermetal lines. With such data, we have devised a new classificationsystem for B-type supergiants in the SMC which takes into ac-count this galaxy’s low metallicity. Using this system 64 of thebrightest B-type supergiants have been re-classified, resultingin different spectral types for approximately 75% of the sam-ple. The marked improvement of the equivalent width – spectraltype trends for lines of many chemical species strongly supportsour contention that these new classifications represent a clearimprovement over earlier studies. They result in a much cleanerdelineation of the ledge structure, that was hinted at in previ-ous work, in the HR diagram of the SMC. It is quite likely thatthose bright B-type (and A-type) supergiants not included in thepresent investigation also suffer from the same deficiencies inclassification discussed above. It is therefore important that ob-servations of comparable quality be obtained for these objectsin order to further investigate the distribution of luminous stars

in the HR diagram. An additional improvement in progress, isthe determination of stellar parameters for each star in the sam-ple using stellar atmosphere techniques. This will improve thestill unsatisfactory situation where the effective temperaturesare assigned by spectral type, independently of luminosity ormetallicity effects.

We also show, for the first time, clear evidence of nitrogenand carbon peculiarities in the spectra of some B-type super-giants in the SMC. Clearly, the mechanism that is responsiblefor this phenomenon also operates under the lower metallicityconditions of the SMC. A quantitative analysis of BN/BC super-giants in the Milky Way, LMC and SMC is currently underway.

Acknowledgements. The assistance of Evelyn Clemens in data reduc-tion and preparation is gratefully acknowledged. I would also like tothank Ed Fitzpatrick and Nolan Walborn for many discussions on thesubject matter of this paper, and Ed for sharing his own findings con-cerning the anomalous A-type supergiants. Valuable comments on thispaper from Kim Venn, Norbert Langer and Rolf Kudritzki were alsomuch appreciated, and I thank Phil Massey for useful discussions con-cerning his work in the SMC and NGC 6822. The observations werecarried out under remote control from Garching and facilitated by theassistance of ESO staff and in particular the NTT Team. I am alsograteful for financial support from the Bundesminister fur Forschungund Technologie under grant 010 R 90080.

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