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

Abstract

The Isaac Newton Telescope (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 H ii region IC 1396 in Cepheus OB2 and identify 158 pre‐main‐sequence candidates with masses between 0.2 and 2.0 M⊙ and accretion rates between 10−9.2 and 10−7.0 M⊙ 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 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 ∼1 Myr.

Classical T Tauri stars with VPHAS+ – I. H α andu-band accretion rates in the Lagoon Nebula M8

“…This strong dependence is well documented in a few star forming regions over a wide range of masses (0.1-3 M ; see Muzerolle et al 2003;Natta et al 2006). The median α ∼ 1.8-2.2 , although much steeper (Fang et al 2009) or shallower values (Barentsen et al 2011) have also been found.…”

“…The four confirmed O-type members plus some sixty B-type stars (Tothill et al 2008) represent a step toward increasing the massive star content compared to similar works using IPHAS in the Northern Galactic plane (Barentsen et al 2011(Barentsen et al , 2013.…”

“…This enables a uniformṀacc data-set in a variety of notable star-forming regions, including the Carina Nebula, NGC 6611, and NGC 3603. Such work has already begun by Vink et al (2008) in the Cygnus OB 2 massive star forming region, Barentsen et al (2011) in the IC 1396 star-forming region, and Barentsen et al (2013) in the open cluster NGC 2264 using data from the counterpart northern Galactic plane survey IPHAS .…”

“…Observations also reveal thatṀacc scales steeply to the square of the stellar mass M * (Muzerolle et al 2003;Natta, Testi & Randich 2006;Sicilia-Aguilar et al 2006;Rigliaco et al 2011;Manara et al 2012;Kalari & Vink 2015) in a variety of environments, however no such dependence is actually predicted by current theory ). ObservationalṀacc estimates can provide an empirical base E-mail: : vek@arm.ac.uk against which disc evolution models can be tested, and star formation histories of individual star-forming regions can be de-constructed (Barentsen et al 2011;Venuti et al 2014). Selection effects (Clarke & Pringle 2006) and intrinsic variability (Scholz et al 2005;Costigan et al 2012Costigan et al , 2014) may however affect the observed relations, and for these reasons large, uniform statistical samples are essential to constrain theory.…”

“…EmpiricalṀacc values have been derived from intermediate-to-high resolution spectra (Muzerolle et al 2003;Natta et al 2006;Sicilia-Aguilar et al 2006), U -band (Rigliaco et al 2011) or Hα photometry (Barentsen et al 2011;De Marchi et al 2010;Spezzi et al 2012;Manara et al 2012). Statistical studies ofṀacc have generally been restricted to relatively nearby star-forming regions, such that the effects of either massive stars or the variation of the star-forming environment on PMS evolution remain poorly constrained.…”