X-ray view of IC 348 in the light of an updated cluster census^⋆

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: stelzer@astropa.inaf.it
² Universitäts-Sternwarte München, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany

Received: 17 September 2011
Accepted: 8 November 2011

Abstract

Context. IC 348 is a nearby ( ≈ 310 pc), young (~2–3 Myr) open cluster with  >300  members identified from optical and infrared observations. It comprises young stellar objects in various evolutionary phases from protostars over disk-bearing to diskless pre-main sequence stars. This gives us the opportunity to study evolutionary effects in the high-energy emissions in a homogeneous environment.

Aims. We study the properties of the coronae of the young low-mass stars in IC 348 combining X-ray and optical/infrared data. In particular, we intend to shed light on the dependence of X-ray luminosity and spectral hardness on evolutionary stage and on stellar parameters such as mass, effective temperature, and bolometric luminosity.

Methods. The four existing Chandra observations of IC 348 were merged, thus providing a deeper and spatially more complete X-ray view than previous X-ray studies of the cluster. We have compiled a comprehensive catalog of IC 348 members taking into account recent updates to the cluster census. Our data collection comprises fundamental stellar parameters, infrared excess indicating the presence of disks, Hα emission as a tracer of chromospheric emission or accretion, and mass accretion rates.

Results. We have detected 290 X-ray sources in four merged Chandra exposures, of which 187 are associated with known cluster members, which corresponds to a detection rate of  ~60% for the cluster members of IC 348 that are identified in optical/infrared studies. According to the most recent spectral classification of IC 348 members, only four of the X-ray sources are brown dwarfs (spectral type M6 and later). The detection rate is highest for diskless Class III stars and increases with stellar mass. This may be explained with higher X-ray luminosities for higher mass and later evolutionary stage that is evident in the X-ray luminosity functions. In particular, we find that for the lowest examined masses (0.1−0.25   M_⊙) there is a difference between the X-ray luminosity functions of accreting and non-accreting stars (classified on the basis of their Hα emission strength) as well as those of disk-bearing and diskless stars (classified on the basis of the slope of the spectral energy distribution). These differences disappear for higher masses. This is related to our finding that the L_x/L_bol ratio is non-constant across the mass/luminosity sequence of IC 348 with a decrease toward lower luminosity stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster suggests that the decline of L_x/L_bol for young stars at the low-mass end of the stellar sequence is likely universal.