Resolving faint structures in the debris disk around TWA 7^★

Tentative detections of an outer belt, a spiral arm, and a dusty cloud

¹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: johan.olofsson@uv.cl
² Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
³ Núcleo Milenio Formación Planetaria–NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
⁶ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
⁷ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Dipartimento di Fisica a Astronomia “G. Galilei”, Universita’ di Padova, Via Marzolo, 8, 35121 Padova, Italy
⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ European Space Astronomy Centre (ESA), Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
¹¹ Departamento de Ciencias Fisicas, Facultad de Ciencias Exactas, Universidad Andres Bello. Av. Fernandez Concha 700, Las Condes, Santiago, Chile
¹² Université Grenoble-Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Monash Centre for Astrophysics (MoCA) and School of Physics and Astronomy, Monash University, Clayton Vic 3800, Australia
¹⁴ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36, Santiago, Chile
¹⁵ CRAL, UMR 5574, CNRS/ENS-L/Université Lyon 1, 9 Av. Ch. André, 69561 Saint-Genis-Laval, France
¹⁶ Aix-Marseille Université, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
¹⁷ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
¹⁸ Exoplanets and Stellar Astrophysics Laboratory, Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20770, USA
¹⁹ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²⁰ Astrophysics Research Centre, Queen’s University Belfast, Belfast, Northern Ireland, UK
²¹ Eureka Scientific, 2452 Delmer, Suite 100, Oakland, CA 96002, USA
²² INAF–Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
²³ Department of Astrophysics, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91101, USA
²⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²⁵ INCT, Universidad De Atacama, Calle Copayapu 485, Copiapó, Atacama, Chile
²⁶ Département d’Astronomie, Université de Genève, 51 Chemin des Maillettes, 1290, Versoix, Switzerland
²⁷ Université Côte-d’Azur, OCA, CNRS, Lagrange, France

Received: 3 January 2018
Accepted: 4 April 2018

Abstract

Context. Debris disks are the intrinsic by-products of the star and planet formation processes. Most likely due to instrumental limitations and their natural faintness, little is known about debris disks around low mass stars, especially when it comes to spatially resolved observations.

Aims. We present new VLT/SPHERE IRDIS dual-polarization imaging (DPI) observations in which we detect the dust ring around the M2 spectral type star TWA 7. Combined with additional angular differential imaging observations we aim at a fine characterization of the debris disk and setting constraints on the presence of low-mass planets.

Methods. We modeled the SPHERE DPI observations and constrain the location of the small dust grains, as well as the spectral energy distribution of the debris disk, using the results inferred from the observations, and performed simple N-body simulations.

Results. We find that the dust density distribution peaks at ~0.72′′ (25 au), with a very shallow outer power-law slope, and that the disk has an inclination of ~13° with a position angle of ~91° east of north. We also report low signal-to-noise ratio detections of an outer belt at a distance of ~1.5′′ (~52 au) from the star, of a spiral arm in the southern side of the star, and of a possible dusty clump at 0.11′′. These findings seem to persist over timescales of at least a year. Using the intensity images, we do not detect any planets in the close vicinity of the star, but the sensitivity reaches Jovian planet mass upper limits. We find that the SED is best reproduced with an inner disk at ~0.2′′ (~7 au) and another belt at 0.72′′ (25 au).

Conclusions. We report the detections of several unexpected features in the disk around TWA 7. A yet undetected 100M_⊕ planet with a semi-major axis at 20−30 au could possibly explain the outer belt as well as the spiral arm. We conclude that stellar winds are unlikely to be responsible for the spiral arm.