A Chandra and Spitzer census of the young star cluster in the reflection nebula NGC 7129^*

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: [B.Stelzer;stelzer]@astropa.unipa.it
² SUPA, School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY 16 9SS, UK

Received: 10 April 2009
Accepted: 20 July 2009

Abstract

Context. The reflection nebula NGC 7129 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.

Aims. This study represents the next step towards a systematic assessment of the pre-main sequence population in NGC 7129. 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 ~3 Myr, NGC 7129 is in the critical range where disks around young stars disappear.

Methods. We make use of X-ray and IR imaging observations to identify the pre-main sequence stars in NGC 7129. We define a sample of young stellar objects based on color–color diagrams composed from IR photometry between 1.6 and 8 μm, from 2 MASS and Spitzer, and based on X-ray detected sources from a Chandra observation.

Results. This sample is composed of 26 Class II and 25 Class III 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 ~ 0.5 . The most restricted and least biased subsample of pre-main sequence stars is composed of lightly absorbed (A_V < 5 mag) stars in the cluster core. This sample comprises 7 Class II and 14 Class III sources and has a disk fraction of % and a median X-ray luminosity of log [erg/s] = 30.3.

Conclusions. 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 ∼3 Myr for the NGC 7129 cluster. The derived disk fraction is similar to that of σ Orionis, smaller than for Cha I (∼2 Myr), and larger than for Upper Sco (5 Myr). The X-ray luminosity function is similar to that of NGC 2264 (2 Myr) but fainter than for the Orion Nebula Cluster (1 Myr). The pre-main sequence census should be further refined and extended with optical photometric and spectroscopic searches for cluster members.