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T Tauri candidates and accretion rates using IPHAS: method and application to IC1396

The INT Photometric H-Alpha Survey (IPHAS) is a 1800 deg^2 survey of the Northern Galactic Plane, reaching down to r'~21. We demonstrate how the survey can be used to (1) reliably select classical T Tauri star candidates and (2) constrain the mass accretion rates with an estimated relative uncertainty of 0.6 dex. IPHAS is a necessary addition to spectroscopic surveys because it allows large and uniform samples of accretion rates to be obtained with a precise handle on the selection effects. We apply the method on a region of 7 deg^2 towards the HII region IC1396 in Cepheus OB2 and identify 158 pre-main sequence candidates with masses between 0.2 and 2.0 Msun and accretion rates between 10^-9.2 and 10^-7.0 Msun yr^-1. We find a power-law dependency between the stellar mass and the accretion rates with a slope of α= 1.1 +/- 0.2, which is less steep than indicated by previous studies. We discuss the influence of method-dependent systematic effects on the determination of this relationship. The majority of our sample consists of faint, previously unknown, low-mass T Tauri candidates (56 per cent between 0.2 and 0.5 Msun). Many candidates are clustered in front of three bright-rimmed molecular clouds, which are being ionized by the massive star HD206267 (O6.5V). We discover a spatio-temporal gradient of increasing accretion rates, increasing Spitzer infrared excess, and younger ages away from the ionizing star, providing a strong indication that the formation of these clusters has been sequentially triggered by HD206267 during the last ~1 Myr.