jrasc june 1998 final - The Royal Astronomical Society of Canada

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jrasc june 1998 final - The Royal Astronomical Society of Canada

A SEARCH FOR THE PARENT CLUSTER OF THE CEPHEID SU CYGNIby David G. Turner 1,2 , Mansur A. Ibrahimov 3 ,Georgi I. Mandushev 4 , Leonid N. Berdnikov 5 , and Andrew J. Horsford 22 Saint Mary’s University, 3 Tashkent Astronomical Institute,4 University of British Columbia, 5 Sternberg Astronomical Institute(Received November 19, 1997; revised April 8, 1998)Abstract. We present photoelectric UBVR photometry, supplemented by photographic UBV photometry and spectroscopicobservations for the brighter objects, of a sample of 33 stars in an anonymous field located 17 arcminutes west of the classicalCepheid SU Cygni. The survey represents an attempt to identify members of the parent cluster from which the Cepheid originated.Photographic photometry is also presented for 14 stars that lie within about 15 arcminutes of the Cepheid and are known to be ofB or A spectral type. There is no evidence in our data to suggest that the survey field west of SU Cyg contains a previously-overlookedopen cluster. On the other hand, there is a group of 14 stars from our survey and that of Turner et al. (1997) that does appear torepresent the scattered remains of the parent cluster for SU Cyg. If SU Cyg is a member of the group, its luminosity as a “cluster”member is 3.19 0.07.Résumé. Nous présentons des données de photométrie UBVR photoélectrique, ainsi que des observations de photométrie UBVphotographique et des observations spectroscopiques d’objets les plus brillants d’un échantillon de 33 étoiles dans un champanonyme situé à 17 minutes de l’arc à l’ouest de SU Cygni, une céphéïde classique. L’enquête représente une tentative d’établirl’amas supposé duquel provient la céphéïde. Des données de photométrie photographique sont aussi présentées pour 14 étoilessituées à quelque 15 minutes de l’arc de la céphéïde, lesquelles sont de type spectrale B et A. Il n’y a aucune indication dans nosdonnées suggérant que le champ à l’ouest de SU Cygni contient un amas ouvert qui n’a pas encore été decouvert. Toutefois, il y aun groupe de 14 étoiles dans notre échantillon et dans celui de Turner et al. (1997) qui semble représenter les restants de l’amasoriginaire de SU Cygni. Si SU Cygni est bien un membre de ce groupe d’étoiles, sa luminosité en temps que membre de l’amas est 3.19 0.07. SEM1. IntroductionIn recent discussions pertaining to the Hubble constant and thedistance scale of the universe, it has sometimes been overlookedthat the fundamental standard candles used for such research,classical Cepheid variables, are calibrated in various ways, one ofthe most important being membership in open clusters. The presenceof a cluster of stars surrounding or adjacent to a Cepheid is of greatpotential value since the confirmation of a physical associationbetween the two, usually by means of photometric, radial velocity,proper motion and star count data, allows one to conclude thatthe distance, reddening and age of the Cepheid are identical to thesame values derived for cluster stars.The region along the Vulpecula-Cygnus border has beenexamined previously for coincidences between Cepheid variablesand open clusters or associations. The 68 d .5 Cepheid S Vulpeculae,for example, was studied by Turner (1980) as a likely member ofthe association Vulpecula OB2, about 4 kpc distant, and the variablewas subsequently discovered (Turner 1985) to be coincident witha sparse, uncatalogued, open cluster that is probably unrelated tothe Cepheid (Turner et al. 1986). The spatially-adjacent 45 d CepheidSV Vulpeculae is suspected by Turner (1984) to belong to an oldersubgroup of the association Vulpecula OB1, about 2 kpc distant.Fig. 1 — A visual image from the Palomar Observatory Sky Survey redplate containing the field of the Cepheid SU Cygni (left) and the putativecluster identified by Mandushev (right). The field measures 30 arcminuteson a side. [Copyright the Digitized Sky Survey (DSS).]1Guest Investigator, Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council of Canada.June/juin 1998 Journal of the Royal Astronomical Society of Canada, 92:145–152, 1998 June145

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