Early-A stars from IPHAS, and their distribution in and around the Cyg OB2 association

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 ~10⁴M_solar.