Discovery of a point-like source and a third spiral arm in the transition disk around the Herbig Ae star MWC 758

Abstract

Context. Transition disks offer the extraordinary opportunity to look for newly born planets and investigate the early stages of planet formation. Aims. In this context we observed the Herbig A5 star MWC 758 with the L'-band vector vortex coronagraph installed in the nearinfrared camera and spectrograph NIRC2 at the Keck II telescope, with the aim of unveiling the nature of the spiral structure by constraining the presence of planetary companions in the system. Methods. Our high-contrast imaging observations show a bright (∆L = 7.0 ± 0.3 mag) point-like emission, south of MWC 758 at a deprojected separation of ∼20 au (r = 0 . 111 ± 0 . 004) from the central star. We also recover the two spiral arms (south-east and north-west), already imaged by previous studies in polarized light, and discover a third one to the south-west of the star. No additional companions were detected in the system down to 5 Jupiter masses beyond 0 . 6 from the star. Results. We propose that the bright L'-band emission could be caused by the presence of an embedded and accreting protoplanet, although the possibility of it being an asymmetric disk feature cannot be excluded. The spiral structure is probably not related to the protoplanet candidate, unless on an inclined and eccentric orbit, and it could be due to one (or more) yet undetected planetary companions at the edge of or outside the spiral pattern. Future observations and additional simulations will be needed to shed light on the true nature of the point-like source and its link with the spiral arms.

Detectability of embedded protoplanets from hydrodynamical simulations

“…For the last two decades a large number of exoplanet detections have remarkably expanded and shaped the prevailing planet formation theories. Most of these discoveries have been accomplished using indirect techniques, although in a few cases detections were achieved using direct imaging (Quanz et al 2015;Reggiani et al 2018;Keppler et al 2018). Direct observations are only possible for systems close enough to us, with planets far from their host stars (Rameau et al 2015).…”

“…While apparently convincing, both detections have been disputed and further confirmation is still pending. More recently, an additional candidate, still requiring confirmation, has been detected by Reggiani et al (2018) in the spiral arm of MWC 758 in L-band. To date, the most convincing direct detection of a young planet in a protoplanetary disc, is the multi-wavelength detection of PDS 70b, confirmed using multiple epoch data (Keppler et al 2018); the system also includes an additional planet, PDS 70c (Haffert et al 2019).…”

Subwavelength diamond gratings for vortex coronagraphy: towards an annular groove phase mask for shorter wavelengths and topological charge 4 designs

“…A final etch time was then set, the twin was once again glued to the carrier wafer and the etching continued. Following the final etch step, both substrates were stripped of their remaining mask material and re-deposited Al 2 GVC4s were etc the profile pa es but no sever , see Fig. 4.…”

“…Even so, progress is being made in the fields of direct imaging of exoplanets [1] and characterization of proto-star systems [2,3]. This progress is likely to accelerate due to a new generation of 30 m-class telescopes, such as the European ELT, already under construction.…”

“…A method for increasing or decreasing the depth of the grating for the L-band has been developed [7], which allows optimization of the phase mask rejection ratio. The phase masks have been installed in infrared coronagraphic instruments (N-and L-band) on several 10-m class telescopes (VLT/NACO [8], VLT/VISIR [9], LBT/LMIRCam [10] and Keck/NIRC2 [1][2][3]) around the world. Following the success of these infrared coronagraphs, AGPMs for the NIR (1-3 μm) and ultimately for VIS (400-700 nm) spectral bands have become attractive, in particular for current high contrast instruments like VLT/SPHERE.…”