A search for accreting young companions embedded in circumstellar disks

High-contrast Hα imaging with VLT/SPHERE^★,★★

¹ ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
e-mail: gabriele.cugno@phys.ethz.ch
² National Center of Competence in Research “PlanetS”, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ LESIA, CNRS, Observatoire de Paris, Université Paris Diderot, UPMC, 5 place J. Janssen, 92190 Meudon, France
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Unidad Mixta International Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departemento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁸ Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹¹ Space Telescope Science Institute, Baltimore 21218, MD, USA
¹² Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France
¹³ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
¹⁴ Centre de Recherche Astrophysique de Lyon, CNRS/ENSL Université Lyon 1, 9 av. Ch. André, 69561 Saint-Genis-Laval, France
¹⁵ CNRS, IPAG, 38000 Grenoble, France
¹⁶ The University of Michigan, Ann Arbor, MI 48109, USA
¹⁷ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
¹⁸ Physikalisches Institut, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁹ Monash Centre for Astrophysics (MoCa) and School of Physics and Astronomy, Monash University, Clayton Vic 3800, Australia
²⁰ NOVA Optical Infrared Instrumentation Group at ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
²¹ Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
²² Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²³ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile

Received: 31 August 2018
Accepted: 13 December 2018

Abstract

Context. In recent years, our understanding of giant planet formation progressed substantially. There have even been detections of a few young protoplanet candidates still embedded in the circumstellar disks of their host stars. The exact physics that describes the accretion of material from the circumstellar disk onto the suspected circumplanetary disk and eventually onto the young, forming planet is still an open question.

Aims. We seek to detect and quantify observables related to accretion processes occurring locally in circumstellar disks, which could be attributed to young forming planets. We focus on objects known to host protoplanet candidates and/or disk structures thought to be the result of interactions with planets.

Methods. We analyzed observations of six young stars (age 3.5–10 Myr) and their surrounding environments with the SPHERE/ZIMPOL instrument on the Very Large Telescope (VLT) in the Hα filter (656 nm) and a nearby continuum filter (644.9 nm). We applied several point spread function (PSF) subtraction techniques to reach the highest possible contrast near the primary star, specifically investigating regions where forming companions were claimed or have been suggested based on observed disk morphology.

Results. We redetect the known accreting M-star companion HD142527 B with the highest published signal to noise to date in both Hα and the continuum. We derive new astrometry (r=62.8_−2.7^+2.1 mas and PA=(98.7±1.8)°) and photometry (ΔN_Ha = 6.3_−0.3^+0.2 mag, ΔB_Ha = 6.7 ± 0.2 mag and ΔCnt_Ha = 7.3_−0.2^+0.3 mag) for the companion in agreement with previous studies, and estimate its mass accretion rate (Ṁ ≈ 1−2 × 10⁻¹⁰ M_⊙yr⁻¹). A faint point-like source around HD135344 B (SAO206462) is also investigated, but a second deeper observation is required to reveal its nature. No other companions are detected. In the framework of our assumptions we estimate detection limits at the locations of companion candidates around HD100546, HD169142, and MWC 758 and calculate that processes involving Hα fluxes larger than ~ 8 × 10⁻¹⁴–10⁻¹⁵ erg s⁻¹ cm⁻² (Ṁ > 10⁻¹⁰−10⁻¹² M_⊙yr⁻¹) can be excluded. Furthermore, flux upper limits of ~10⁻¹⁴−10⁻¹⁵ erg s⁻¹ cm⁻² (Ṁ < 10⁻¹¹–10⁻¹² M_⊙yr⁻¹) are estimated within the gaps identified in the disks surrounding HD135344 B and TW Hya. The derived luminosity limits exclude Hα signatures at levels similar to those previously detected for the accreting planet candidate LkCa15 b.