Spectro-astrometry of LkCa 15 with X-Shooter: Searching for emission from LkCa 15b^⋆

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, 72076 Tübingen, Germany
e-mail: emma.whelan@astro.uni-tuebingen.de
² Centro de Astrobiología (INTA-CSIC); ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ INAF-Osservatorio Astronomico di Capodimonte, via Moiariello, 16, 80131 Napoli, Italy
⁴ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ European Space Astronomy Center (ESAC), PO box 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 23 December 2014
Accepted: 17 April 2015

Abstract

Planet formation is one explanation for the partial clearing of dust observed in the disks of some T Tauri stars. Indeed studies using state-of-the-art high angular resolution techniques have very recently begun to observe planetary companions in these so-called transitional disks. The goal of this work is to use spectra of the transitional disk object LkCa 15 obtained with X-Shooter on the Very Large Telescope to investigate the possibility of using spectro-astrometry to detect planetary companions to T Tauri stars. It is argued that an accreting planet should contribute to the total emission of accretion tracers such as Hα and therefore planetary companions could be detected with spectro-astrometry in the same way as it has been used to detect stellar companions to young stars. A probable planetary-mass companion was recently detected in the disk of LkCa 15. Therefore, it is an ideal target for this pilot study. We studied several key accretion lines in the wavelength range 300 nm to 2.2 μm with spectro-astrometry. While no spectro-astrometric signal is measured for any emission lines, the accuracy achieved in the technique is used to place an upper limit on the contribution of the planet to the flux of the Hα, Paγ, and Paβ lines. The derived upper limits on the flux allow an upper limit of the mass accretion rate, log (Ṁ_acc) = −8.9 to −9.3 for the mass of the companion between 6 M_Jup and 15 M_Jup, respectively, to be estimated (with some assumptions).