Resolving the inner regions of the HD 97048 circumstellar disk with VLT/NACO polarimetric differential imaging^⋆

¹ Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
e-mail: quanz@astro.phys.ethz.ch
² ESA/ESTEC, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands
³ Department of Astronomy/Department of Planetary Sciences, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁴ University of Kiel, Institute of Theoretical Physics and Astrophysics, Leibnizstrasse 15, 24098 Kiel, Germany
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 11 October 2011
Accepted: 7 December 2011

Abstract

Context. Circumstellar disks are the cradles of planetary systems and their physical and chemical properties directly influence the planet formation process. Because most planets supposedly form in the inner disk regions, i.e., within a few tens of AU, it is crucial to study circumstellar disks on these scales to constrain the conditions for planet formation.

Aims. Our aims are to characterize the inner regions of the circumstellar disk around the young Herbig Ae/Be star HD 97048 in polarized light.

Methods. We used VLT/NACO to observe HD 97048 in polarimetric differential imaging (PDI) mode in the H and K_s band. This technique offers high-contrast capabilities at very small inner working angles and probes the dust grains on the surface layer of the disk that act as the scattering surface.

Results. We spatially resolve the disk around HD 97048 in polarized flux in both filters on scales between  ~0.1″–1.0″ corresponding to the inner  ~16–160 AU. Fitting isophots to the flux calibrated H-band image between 13–14 mag/arcsec² and 14–15 mag/arcsec², we derive an apparent disk inclination angle of 34° ± 5° and 47° ± 2°, respectively. The disk position angle in both brightness regimes is almost identical and roughly 80°. Along the disk major axis the surface brightness of the polarized flux drops from  ~11 mag/arcsec² at  ~0.1″ (~16 AU) to  ~15.3 mag/arcsec² at  ~ 1.0″ (~160 AU). The brightness profiles along the major axis are fitted with power-laws falling off as  ∝ r ^{− 1.78 ± 0.02} in H and  ∝ r ^{− 2.34 ± 0.04} in K_s. Because the surface brightness decreases more rapidly in K_s compared to H, the disks becomes relatively bluer at larger separations, possibly indicating changing dust grain properties as a function of radius.

Conclusions. We imaged for the first time the inner  ~0.1″–1.0″ (~16–160 AU) of the surface layer of the HD 97048 circumstellar disk in scattered light, which demonstrates the power of ground-based imaging polarimetry. Our data fill an important gap in a large collection of existing data that include resolved thermal dust and polycyclic aromatic hydrocarbon (PAH) emission and also resolved gas emission lines. HD 97048 therefore is an ideal test case for sophisticated models of circumstellar disks and a prime target for future high-contrast imaging observations.