Spatially resolved spectroscopy of the debris disk HD 32297

Further evidence of small dust grains^★,★★

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: Trisha.Bhowmik@obspm.fr
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³ CNRS, IPAG, Université Grenoble Alpes, 38000 Grenoble, France
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Avenue Gran Bretaña 1111, Valparaíso, Chile
⁶ Núcleo Milenio Formación Planetaria â NPF, Universidad de Valparaíso, Avenue Gran Bretaña 1111, Valparaíso, Chile
⁷ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
⁸ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
⁹ Laboratoire AIM, CEA/DRF – CNRS – Université Paris Diderot, IRFU/SAp, UMR 7158, 91191 Gif-sur-Yvette, France
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 12 June 2019
Accepted: 13 August 2019

Abstract

Context. Spectro-photometry of debris disks in total intensity and polarimetry can provide new insight into the properties of the dust grains therein (size distribution and optical properties).

Aims. We aim to constrain the morphology of the highly inclined debris disk HD 32297. We also intend to obtain spectroscopic and polarimetric measurements to retrieve information on the particle size distribution within the disk for certain grain compositions.

Methods. We observed HD 32297 with SPHERE in Y, J, and H bands in total intensity and in J band in polarimetry. The observations are compared to synthetic models of debris disks and we developed methods to extract the photometry in total intensity overcoming the data-reduction artifacts, namely the self-subtraction. The spectro-photometric measurements averaged along the disk mid-plane are then compared to model spectra of various grain compositions.

Results. These new images reveal the very inner part of the system as close as 0.15″. The disk image is mostly dominated by the forward scattering making one side (half-ellipse) of the disk more visible, but observations in total intensity are deep enough to also detect the back side for the very first time. The images as well as the surface brightness profiles of the disk rule out the presence of a gap as previously proposed. We do not detect any significant asymmetry between the northeast and southwest sides of the disk. The spectral reflectance features a “gray to blue” color which is interpreted as the presence of grains far below the blowout size.

Conclusions. The presence of sub-micron grains in the disk is suspected to be the result of gas drag and/or “avalanche mechanisms”. The blue color of the disk could be further investigated with additional total intensity and polarimetric observations in K and H bands respectively to confirm the spectral slope and the fraction of polarization.