An X-ray survey of low-mass stars in Trumpler 16 with Chandra^*

¹ Centro Universitario Regional Zona Atlántica – Univ. COMAHUE, Monseñor Esandi y Ayacucho (CP 8500), Viedma, Argentina e-mail: donfaca@gmail.com
² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ AF Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138, USA

Received: 17 April 2008
Accepted: 23 July 2008

Abstract

Aims. We identify and characterize low-mass stars in the ~3 Myr old Trumpler 16 region by means of a deep Chandra X-ray observation, and study their optical and near-IR properties. We compare the X-ray activity of Trumpler 16 stars with the known characteristics of Orion and Cygnus OB2 stars.

Methods. We analyzed a 88.4 ks Chandra ACIS-I observation pointed at the center of Trumpler 16. Because of diffuse X-ray emission, source detection was performed using the PWDetect code for two different energy ranges: 0.5-8.0 keV and 0.9-8.0 keV. Results were merged into a single final list. We positionally correlated X-ray sources with optical and 2MASS catalogs. Source events were extracted with the IDL-based routine ACIS-Extract. X-ray variability was characterized using the Kolmogorov-Smirnov test and spectra were fitted by using XSPEC. The X-ray spectra of early-type, massive stars were analyzed individually.

Results. Our list of X-ray sources consists of 1035 entries, 660 of which have near-IR counterparts and are probably associated with Trumpler 16 members. From near-IR, color-color, and color-magnitude diagrams we compute individual masses of stars and their A_v values. The cluster median extinction is A_v = 3.6 mag, while OB-type stars appear less absorbed, having A_v = 2.0 mag. About 15% of the near-IR counterparts show disk-induced excesses. X-ray variability is found in 77 sources, and typical X-ray spectral parameters are 5.37  10²¹ cm^-2 and 1.95 keV. The OB stars appear, softer with a median 0.65 keV. The median X-ray luminosity is 6.3  10³⁰ , while variable sources show a larger median L_x value of 13  10³⁰ . OB-stars have an even higher median L_x of 80  10³⁰ , about 10 times that of the low-mass stars.

Conclusions. The Trumpler 16 region has a very rich population of low-mass X-ray emitting stars. A large fraction of its circumstellar disks have survived the intense radiation field of its massive stars. Stars with masses 1.5-2.5  display X-ray activity similar to the Cyg OB2 stars, but much less intense than observed for Orion nebula cluster members.