jrasc june 1998 final - The Royal Astronomical Society of Canada

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jrasc june 1998 final - The Royal Astronomical Society of Canada

Table IIPhotographic ObservationsHDE X Y V B–V U–B SpectralType332535 –6.04 +17.32 11.08 0.17 0.16 A2332536 +4.06 +14.01 10.55 0.44 0.10 A5 V332537 +7.51 +16.80 10.95 0.31 0.19 A3332538 +8.28 +13.06 11.25 0.17 0.02 B9 V332539 +15.00 +2.50 11.36: 0.15: 0.06 B9 Vn332542 +13.80 –5.89 9.49 0.02 0.04 B9332543* +9.11 –3.99 10.56 0.20 0.05 B9.5 Vnn332552 –11.66 –14.76 10.77 0.34 0.08 A7332553 –5.63 –11.80 10.43 0.55 0.21 A2 III332557 +6.44 –9.71 10.38 0.21 –0.19 B8 V332558 +13.02 –10.91 11.00: 0.40: 0.04 A1332559 +12.69 –13.13 11.11: 0.16: 0.14 A0332563 +8.65 –16.92 10.70 0.28 –0.45 B2332565 –2.73 –16.64 11.14 0.10 –0.32: B8Notes: * Star 201 of Turner et al. (1997).Fig. 3 —Colour-colour diagram for stars in the field of the putative cluster(filled circles) as well as B and A-type stars near SU Cyg (filled squares).The intrinsic relation for main-sequence stars (solid curve) is depicted fora reddening of E B–V = 0.16 (dotted curve) using a reddening slope of E U–B /E B–V= 0.74 (shown as a dashed line originating at spectral type A2).the length and shape of their spectral images, which, as describedin similar work by Morgan et al. (1954), depend critically upon thetemperature types of the stars and the wavelength sensitivity of theemulsion used — in the present case Panchromatic-X film. Indeed,the sparse group identified by Mandushev (1994) seems to containa number of stars that are “of early type” — namely of spectral typesB, A or F — according to their appearance on our low-dispersionobjective-prism spectrum of the field. Classifications derived insuch fashion are of relatively low accuracy, but are also listed inTable I for stars whose spectra are present on our objective-prismplate.3. Interstellar ReddeningBecause several fields of the Milky Way containing or adjacent toSU Cyg have been investigated previously (Neckel & Klare 1980;Turner 1980; Turner 1986; Turner et al. 1986; Turner et al. 1997),the run of extinction with distance towards the Cepheid is knownwith some confidence. As noted by Turner et al. (1997), there isessentially no extinction along the first 200–300 pc of space in thedirection of the Cepheid. Beyond that, dust complexes lying atdistances of up to perhaps 500–600 pc give rise to minimumreddenings for more distant stars of roughly E B–V 0.15 to 0.6,depending upon spatial location. The UBV reddening relation forthe dust in the field has also been established by Turner (1980,1989), and is described by E U–B /E B–V = 0.740 + 0.026 E B–V . We adoptedthat dependence here to correct the observations of program starsfor the effects of interstellar reddening.The observed U–B and B–V colour indices for all stars in TablesI and II are plotted in figure 3. Shown there are the intrinsic relationfor unreddened dwarf stars (solid curved line), the intrinsic relationcorresponding to an interstellar reddening of E B–V = 0.15 (dottedline), and the reddening law for early-type stars described above(dashed line originating at spectral type A2 — coincident with theleftmost inflection point in the intrinsic relation). The diagram hasmany similarities to the colour-colour diagram for stars lying inthe field immediately surrounding SU Cyg itself (cf. Turner et al.1997). There are many cool stars in the sample (lower right in figure3) possessing colours similar to those of unreddened G and K-typegiants and dwarfs, as well as a small population of stars (left centralsection of figure 3) that have the colours of A and F-type stars oflittle or no reddening. Although a number of reddened B and A-type stars can be identified in figure 3 on the basis of their colours,there is otherwise no evidence for the presence in our programsample of a large population of stars of similar reddening and spectraltype. On that basis it seems rather unlikely that the field of figure2 contains a well-populated open cluster. Most of the stars appearto represent the general population seen towards SU Cyg.Figure 4 is a plot of the apparent distance moduli, V–M V , forearly-type stars in Tables I and II as derived from the adoption ofzero-age main-sequence (ZAMS) or spectroscopic luminosities foreach star as a function of their inferred reddenings, E B–V . Such a plotis referred to as a variable-extinction diagram. The effect ofunderestimating a star’s luminosity by adopting a ZAMS luminosityfor it is to produce a systematic scatter upwards in the diagram.The non-linear response of the UBV filters to the continua of starsof different temperature can also result in slightly different colour148JRASC June/juin 1998

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