Early-A stars from IPHAS, and their distribution in and around the Cyg OB2 association |
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| Authors: | Janet E Drew R Greimel M J Irwin S E Sale |
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| Institution: | Physics Department, Imperial College London, Exhibition Road, London SW7 2AZ;Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield AL10 9AB;Institut für Physik, Karl-Franzens Universität Graz, Universitätsplatz 5, 8010 Graz, Austria;Isaac Newton Group of Telescopes, Apartado de correos 321, E-38700 Santa Cruz de la Palma, Tenerife, Spain;Institute of Astronomy, Madingley Road, Cambridge CB3 0HA |
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| Abstract: | Stellar photometry derived from the INT/WFC Photometric Hα Survey (IPHAS) of the Northern Galactic plane can be used to identify large, reliable samples of A0–A5 stars. For every A-type star, so identified, it is also possible to derive individual reddening and distance estimates, under the assumption that most selected objects are on or near the main sequence, at a mean absolute r ' magnitude of 1.5–1.6. This study presents the method for obtaining such samples and shows that the known reddenings and distances to the open clusters NGC 7510 and NGC 7790 are successfully recovered. A sample of over 1000 A-type stars is then obtained from IPHAS data in the magnitude range 13.5 < r ' < 20 from the region of sky including the massive northern OB association Cyg OB2. An analysis of these data reveals a concentration of ～200 A stars over an area about a degree across, offset mainly to the south of the known 1–3 Myr old OB stars in Cyg OB2: their dereddened r ' magnitudes fall in the range 11.8–12.5. These are consistent with a ～7 Myr old stellar population at distance modulus DM = 10.8, or with an age of ～5 Myr at DM = 11.2. The number of A-type stars found in this clustering alone is consistent with a lower limit to the cluster mass of ～104 M⊙ . |
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| Keywords: | surveys stars: distances stars: early-type open clusters and associations: individual: Cyg OB2 open clusters and associations: individual: NGC 7510 open clusters and associations: individual: NGC 7790 |
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