XMM-Newton X-Ray Observations of LkCa 15: A T Tauri Star with a Formative Planetary System

Abstract

High-resolution ground-based images of the T Tauri star LkCa 15 have revealed multiple companions that are thought to comprise a formative planetary system. The candidate protoplanets orbit at distances of ∼15-20 au within the dust-depleted inner region of the circumstellar disk. Because of its young age (∼1-4 Myr), LkCa 15 provides a benchmark system for testing planet-formation models. We detected LkCa 15 as a bright X-ray source in a short 10 ks Chandra observation in 2009. We report here new results obtained from a deeper 37 ks XMM-Newton observation in 2014. The new data provide better sampling in the time domain and improved sensitivity at low energies below 1 keV. Spectral fits with thermal emission models require at least two temperature components at kT _cool ≈ 0.4 keV and kT _hot ≈ 2.2 keV. The value of kT _hot is about a factor of two less than inferred from Chandra, suggesting that the hot-component temperature is variable. The best-fit absorption column density is in good agreement with that expected from optical extinction estimates {A}_{{V}} ≈ 1.3-1.7 mag. The intrinsic X-ray luminosity is L _x (0.2-10 keV) = 3 × 10³⁰ erg s^-1. Estimates of the X-ray heating rate of the inner disk and protoplanets are sensitive to the assumed disk gas surface density for which recent ALMA observations give estimates {{{Σ }}}_0,{gas} ∼10² g cm^-2 at 1 au from the star. At such densities, X-ray heating is confined mainly to the upper disk layers and X-ray penetration through the disk midplane to the protoplanets at r ≈ 15-20 au is negligible.