The Gemini NICI planet-finding campaign: The offset ring of HR 4796 A^⋆,⋆⋆

¹ European Southern Observatory Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago Chile
² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu HI 96822, USA
³ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena, Chile
⁵ NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics Laboratory, Greenbelt MD 20771, USA
⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson AZ 85721, USA
⁷ Mauna Kea Infrared, LLC, 21 Pookela St., Hilo HI 96720, USA

Received: 25 December 2013
Accepted: 23 April 2014

Abstract

We present J,H, CH₄ short (1.578 μm), CH₄ long (1.652 μm) and K_s-band images of the dust ring around the 10 Myr old star HR 4796 A obtained using the Near Infrared Coronagraphic Imager (NICI) on the Gemini-South 8.1 m Telescope. Our images clearly show for the first time the position of the star relative to its circumstellar ring thanks to NICI’s translucent focal plane occulting mask. We employ a Bayesian Markov chain Monte Carlo method to constrain the offset vector between the two. The resulting probability distribution shows that the ring center is offset from the star by 16.7 ± 1.3 milliarcseconds along a position angle of 26 ± 3°, along the PA of the ring, 26.47 ± 0.04°. We find that the size of this offset is not large enough to explain the brightness asymmetry of the ring. The ring is measured to have mostly red reflectivity across the JHK_s filters, which seems to indicate micron-sized grains. Just like Neptune’s 3:2 and 2:1 mean-motion resonances delineate the inner and outer edges of the classical Kuiper belt, we find that the radial extent of the HR 4796 A and the Fomalhaut rings could correspond to the 3:2 and 2:1 mean-motion resonances of hypothetical planets at 54.7 AU and 97.7 AU in the two systems, respectively. A planet orbiting HR 4796 A at 54.7 AU would have to be less massive than 1.6 M_Jup so as not to widen the ring too much by stirring.