Investigating point sources in MWC 758 with SPHERE

A Boccaletti,D Mesa,C Ginski,C Lazzoni,R Galicher,V D’Orazi,A Zurlo,G Rousset,G Chauvin,A Vigan,A.-M Lagrange,F Cantalloube,J Ramos,J Hagelberg,M Benisty,Y Magnard,R Gratton,M Bonavita,E Pantin,M Langlois,J.-F Sauvage,M Feldt,F Ménard,M Bonnefoy,C Perrot,S Desidera,Michael Meyer ,S B Brown-Sevilla ,Beth Biller

doi:10.1051/0004-6361/202141177

Abstract

Context. Spiral arms in protoplanetary disks could be shown to be the manifestation of density waves launched by protoplanets and propagating in the gaseous component of the disk. At least two point sources have been identified in the L band in the MWC 758 system as planetary mass object candidates. Aims. We used VLT/SPHERE to search for counterparts of these candidates in the H and K bands, and to characterize the morphology of the spiral arms. Methods. The data were processed with now-standard techniques in high-contrast imaging to determine the limits of detection, and to compare them to the luminosity derived from L band observations. Results. In considering the evolutionary, atmospheric, and opacity models we were not able to confirm the two former detections of point sources performed in the L band. In addition, the analysis of the spiral arms from a dynamical point of view does not support the hypothesis that these candidates comprise the origin of the spirals. Conclusions. Deeper observations and longer timescales will be required to identify the actual source of the spiral arms in MWC 758.

Highlights

While studies of the demographics of exoplanets are well underway, the processes leading to planet formation are still poorly constrained
Opacity effects are critical and scale inversely with the wavelength, given that long wavelengths in the thermal regime are more appropriate for reaching the midplane of a disk, whereas short wavelengths in the scattered light regime trace the surface of flared protoplanetary disks
Many structures are revealed along the spirals, some of which could be confused with the usual signatures of point sources in angular differential imaging (ADI)

Summary

Introduction

While studies of the demographics of exoplanets are well underway, the processes leading to planet formation are still poorly constrained. One obvious case is the presence of spiral arms, which – in the case of disks that are not especially massive – could be attributed to density waves launched by planets (Goldreich & Tremaine 1980), and potentially by other types of perturbations (e.g. vortices). In this context, MWC 758 is of particular interest as a protoplanetay disk with two prominent spiral arms observed in scattered light (Grady et al 2013; Benisty et al 2015). The purpose of this Letter is to explore the detection limits provided by SPHERE (Beuzit et al 2019), the high-contrast imaging instrument at the Very Large Telescope, based on two data sets obtained in 2016 and 2018 in the H and K s bands

Observations and data reduction

Global description

Limits of detection to point sources

Extinction by the disk

Spirals as tracers of point sources

Conclusion