A low optical depth region in the inner disk of the Herbig Ae star HR 5999^⋆,⋆⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Max Planck Institut für Astronomie, Konigstühl 17, 69117 Heidelberg, Germany
e-mail: benisty@mpia.de
³ Laboratoire d’Astrophysique de Grenoble, CNRS-UJF UMR 5571, 414 rue de la Piscine, 38400 St Martin d’Hères, France
⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁵ Universidade do Porto, Faculdade de Engenharia, SIM Unidade FCT 4006, Rua Dr. Roberto Frias, s/n P-4200-465 Porto, Portugal
⁶ Caltech, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125, USA
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁸ Centre de Recherche Astrophysique de Lyon, CNRS-UCBL-ENSL UMR5574, 69561 St Genis Laval, France
⁹ Laboratoire A. H. Fizeau, UMR 6525, Université de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 02, France
¹⁰ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 6 November 2010
Accepted: 8 April 2011

Abstract

Context. Circumstellar disks surrounding young stars are known to be the birthplaces of planetary systems, and the innermost astronomical unit is of particular interest. Near-infrared interferometric studies have revealed a complex morphology for the close environment surrounding Herbig Ae stars.

Aims. We present new long-baseline spectro-interferometric observations of the Herbig Ae star, HR 5999, obtained in the H and K bands with the AMBER instrument at the VLTI, and aim to produce near-infrared images at the sub-AU spatial scale.

Methods. We spatially resolve the circumstellar material and reconstruct images in the H and K bands using the MiRA algorithm. In addition, we interpret the interferometric observations using models that assume that the near-infrared excess is dominated by the emission of a circumstellar disk. We compare the images reconstructed from the VLTI measurements to images obtained using simulated model data.

Results. The K-band image reveals three main elements: a ring-like feature located at  ~0.65 AU, a low surface brightness region inside 0.65 AU, and a central spot. At the maximum angular resolution of our observations (B/λ  ~  1.3 mas), the ring is resolved while the central spot is only marginally resolved, preventing us from revealing the exact morphology of the circumstellar environment. We suggest that the ring traces silicate condensation, i.e., an opacity change, in a circumstellar disk around HR 5999. We build a model that includes a ring at the silicate sublimation radius and an inner disk of low surface brightness responsible for a large amount of the near-infrared continuum emission. The model successfully fits the SED, visibilities, and closure phases in the H and K bands, and provides evidence of a low surface brightness region inside the silicate sublimation radius.

Conclusions. This study provides milli-arcsecond resolution images of the environment of HR 5999 and additional evidence that in Herbig Ae stars, there is material in a low surface brightness region, probably a low optical depth region, located inside the silicate sublimation radius and of unknown nature. The possibility that the formation of such a region in a thick disk is related to disk evolution should be investigated.