IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery

We report on the discovery of over 50 strong Halpha emitting objects towards the large OB association Cyg OB2 and the HII region DR 15 on its southern periphery. This was achieved using the INT Photometric Halpha Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r, i and Halpha photometry from IPHAS, and near-infrared J, H, and K photometry from 2MASS. The position of the objects in the (J - H) versus (H - K) diagram strongly suggests most of them are young. Many show CaII IR triplet emission indicating that they are in a pre-main sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the HII region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable (LBV). The emission line objects toward Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.

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

Stellar photometry derived from the INT/WFC Photometric H$alpha$ Survey of the Northern Galactic Plane (IPHAS) can be used to identify large, reliable samples of A0-A5 dwarfs. For every A 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 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. 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 to 12.5. These are consistent with a ~7 Myr old stellar population at DM = 10.8, or with an age of ~5 Myr at DM = 11.2. The number of A stars found in this clustering alone is consistent with a lower limit to the cluster mass of ~10000 M-sun.