Letter to the Editor

Searching for H_α emitting sources around MWC 758

SPHERE/ZIMPOL high-contrast imaging^★,★★

¹ Centro de Astrobiología (CSIC-INTA), Camino bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: nhuelamo@cab.inta-csic.es
² Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ ETH Zurich, Institute of Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁵ Maynooth University Department of Experimental Physics, National University of Ireland, Maynooth Co. Kildare, Ireland
⁶ European Southern Observatory, Alonso de Cordova 3107, Casilla 19, Vitacura, Santiago, Chile
⁷ INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁸ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹⁰ ESAC Science Data Centre, ESA-ESAC, Madrid, Spain
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² Universidad Autónoma de Madrid, Dpto. Física Teórica, Madrid, Spain
¹³ Laboratoire AIM, CEA/DRF, CNRS, Université Paris Diderot, IRFU/DAS, 91191 Gif-sur-Yvette, France

Received: 21 February 2017
Accepted: 22 March 2018

Abstract

Context. MWC 758 is a young star surrounded by a transitional disk. The disk shows an inner cavity and spiral arms that could be caused by the presence of protoplanets. Recently, a protoplanet candidate has been detected around MWC 758 through high-resolution L′-band observations. The candidate is located inside the disk cavity at a separation of ~111 mas from the central star, and at an average position angle of ~165.5°.

Aims. We aim at detecting accreting protoplanet candidates within the disk of MWC 758 through angular spectral differential imaging (ASDI) observations in the optical regime. In particular, we explore the emission at the position of the detected planet candidate.

Methods. We have performed simultaneous adaptive optics observations in the H_α line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VLT).

Results. The data analysis does not reveal any H_α signal around the target. The derived contrast curve in the B_Ha filter allows us to derive a 5σ upper limit of ~7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H_α line luminosity of L_Hα ≲ 5×10⁻⁵ L_⊙ at 111 mas. Assuming that L_Hα scales with L_acc as in classical T Tauri stars (CTTSs) as a first approximation, we can estimate an accretion luminosity of L_acc < 3.7 × 10⁻⁴ L_⊙ for the protoplanet candidate. For the predicted mass range of MWC 758b, 0.5–5 M_Jup, this implies accretion rates smaller than Ṁ < 3.4 × (10⁻⁸−10⁻⁹)M_⊙ yr⁻¹, for an average planet radius of 1.1 R_Jup. Therefore, our estimates are consistent with the predictions of accreting circumplanetary accretion models for R_in = 1R_Jup. The ZIMPOL line luminosity is consistent with the H_α upper limit predicted by these models for truncation radii ≲3.2 R_Jup.

Conclusions. The non-detection of any H_α emitting source in the ZIMPOL images does not allow us to unveil the nature of the L′ detected source. Either it is a protoplanet candidate or a disk asymmetry.