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The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| Photometric studies of stars in the young open cluster NGC 6823
CCD and photoelectric UBV photometry are presented for 32 stars in thenuclear and coronal regions of NGC 6823 and for 13 stars in its centraltrapezium system. The stars in the inner and outer regions of thecluster exhibit a wide range in their derived color excesses, while thecomputed reddenings for the trapezium stars are found to be verysimilar. Photometric data for 36 field stars toward the clusterindicates that about 0.6 mag of absorption is due to foregroundinterstellar material. From near-IR photometry, the reddening law isfound to be uniform across the cluster and is characterized by R(V) =3.2. Using the ZAMS fitting procedure and by assuming a distance of theHyades of 3.3 mag, a mean reddening corrected distance to NGC 6823 of2.1 +/-0.1 kpc is obtained. An age range between 2 and 11 x 10 exp 6 yris found for the cluster stars. The trapezium stars are the youngest,the nuclear objects have intermediate ages, and the stars in the coronaare the oldest.
| Confirmation among visual multiples of an increase of AP stars with age
Open clusters with ages below certain threshold values contain no Apstars and those with greater ages contain numbers of Ap stars thatapparently increase with age. But in view of the few young clustersstudied, the data could also be interpreted in terms of randomdifferences in the frequencies of Ap stars between individual clusters,rather than an age effect. Data on 77 field visual multiple systems(that originated from many different clusters and associations) in whichthe primaries are O5-A1 stars and the secondaries occur in the absolutemagnitude range of the Ap stars were, therefore, obtained. Againspectral classification shows no Ap stars in systems with ages notgreater than 1,000,000 yr and a steady increase in Ap stars thereafter.The numerical agreement with the cluster data is good, confirming thatthe cluster data are exhibiting a real age effect.
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