T Tauri candidates and accretion rates using IPHAS: method and application to IC 1396

Geert Barentsen, Jorick S. Vink, J. E. Drew, R. Greimel, N. J. Wright, J. J. Drake, E. L. Martin, L. Valdivielso, R. L. M. Corradi

Figure 1. Hα-mosaic of IPHAS observations towards IC1396 at the edge of Cepheus OB2. The white dashed line shows the region studied in this paper, with a radius of 1.5^° centered on the massive star system HD 206267 (O6.5V), which is the main ionizing source of the region. Grey areas near the top indicate the upper edge of the IPHAS survey.

ABSTRACT
The INT Photometric H-Alpha Survey (IPHAS) is a 1800 deg² 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 deg² towards the Hii region IC1396 in Cepheus OB2 and identify 158 pre-main sequence candidates with masses between 0.2 and 2.0 M_☉ and accretion rates between 10^òÈÒ9.2 and 10^òÈÒ7.0 M_☉yr^òÈÒ1. 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 M_☉). Many candidates are clustered in front of three bright-rimmed molecular clouds, which are being ionized by the massive star HD 206267 (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 HD 206267 during the last òÈ®1 Myr.

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Last Revised: 2011 March 14th