2009A&A...507..227S

The reflection nebula NGC7129 has long been known to be a site of recent star formation as shown, e.g., by the presence of deeply embedded protostars and HH objects. However, studies of the stellar population produced in the star formation process have remained rudimentary. A major step forward has been made with recent Spitzer imaging of the region. This study represents the next step towards a systematic assessment of the pre-main sequence population in NGC7129. Completeness of the pre-main sequence sample is necessary for studying key features that allow the star-forming process to be understood, such as disk evolution, dynamical evolution, and mass function. At a presumed age of ∼3Myr, NGC7129 is in the critical range where disks around young stars disappear. We make use of X-ray and IR imaging observations to identify the pre-main sequence stars in NGC7129. We define a sample of young stellar objects based on color-color diagrams composed from IR photometry between 1.6 and 8µm, from 2MASS and Spitzer, and based on X-ray detected sources from a Chandra observation. This sample is composed of 26 ClassII and 25ClassIII candidates. It has been selected from infrared sources in the Chandra field (287 objects with photometry in all four Spitzer/IRAC bands, 811 objects with near-IR photometry) and the 59 X-ray sources detected with Chandra. The sample is estimated to be complete down to ∼ 0.5M_☉. The most restricted and least biased subsample of pre-main sequence stars is composed of lightly absorbed (A_V<5mag) stars in the cluster core. This sample comprises 7 ClassII and 14 ClassIII sources and has a disk fraction of 33⁺²⁴_–19% and a median X-ray luminosity of logL_x[erg/s]=30.3. Despite the various uncertainties related to the sample selection, absorption, mass distribution, distance, and, consequently in the computation of disk fraction and X-ray luminosities, the data yield consistent results. In particular, we confirm the age of ∼3Myr for the NGC7129 cluster. The derived disk fraction is similar to that of σOrionis, smaller than for ChaI (∼2Myr), and larger than for Upper Sco (5Myr). The X-ray luminosity function is similar to that of NGC2264 (2Myr) but fainter than for the Orion Nebula Cluster (1Myr). The pre-main sequence census should be further refined and extended with optical photometric and spectroscopic searches for cluster members.