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

TL;DR

This study uses Chandra X-ray and Spitzer IR data to identify and analyze young stars in NGC 7129, confirming its age and assessing disk presence and X-ray properties of its stellar population.

Contribution

It provides a comprehensive census of pre-main sequence stars in NGC 7129 using combined X-ray and IR observations, improving understanding of star formation at ~3 Myr.

Findings

Disk fraction of 33% in the cluster core.

Age of the cluster confirmed at ~3 Myr.

X-ray luminosity similar to 2 Myr clusters.

Abstract

The reflection nebula NGC 7129 has long been known to be a site of recent star formation as evidenced, 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. At a presumed age of ~3 Myr, NGC7129 is in the critical range where disks around young stars disappear. We make use of Chandra X-ray and Spitzer and 2MASS 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 8 mu, from 2MASS and Spitzer, and based on X-ray detected sources from a Chandra observation. This sample is composed of 26 Class II and 25 Class III candidates. The sample is estimated to be complete down to ~ 0.5 solar masses. The most restricted and least biased sub-sample 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, it has a disk fraction of 33^{+24}_{-19} %, and a median X-ray luminosity of log (L_x) [erg/s] = 30.3. Despite the various uncertainties related to the sample selection, absorption, mass distribution, distance and, consequently, 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 NGC7129 cluster. The derived disk fraction is similar to that of sigma Orionis, smaller than that of Cha I (~2 Myr), and larger than that of Upper Sco (5 Myr). The X-ray luminosity function is similar to that of NGC 2264 (2 Myr) but fainter than that of the Orion Nebula Cluster (1 Myr).