[Paper Review] A protoplanet candidate in the transition disk around the Herbig Ae star MWC 758

Maddalena Reggiani, Valentin Christiaens+21 more|arXiv (Cornell University)|Oct 31, 2017

Astrophysics and Star Formation Studies2 references7 citations

TL;DR

The study presents a protoplanet candidate in the transition disk around the Herbig Ae star MWC 758, detected via high-contrast L-band imaging with Keck/NIRC2's vector vortex coronagraph. At 20 au from the star, a bright point-like source (ΔL = 7.0 ± 0.3 mag) is observed, potentially indicating an embedded, accreting protoplanet, while three spiral arms—including a newly discovered southwest arm—are also resolved, though their link to the candidate remains uncertain.

ABSTRACT

Transition disks offer the extraordinary opportunity to look for newly born planets and investigate the early stages of planet formation. In this context we observed the Herbig A5 star MWC 758 with the L band vector vortex coronagraph installed in the near-infrared 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. Our high-contrast imaging observations show a bright (delta L=7.0+/-0.3 mag) point-like emission, south of MWC 758 at a deprojected separation of about 20 au (r=0.111+/- 0. 004 arcsec) 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 arcsec from the star. 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.

Motivation & Objective

• To investigate the nature of a spiral structure in the transition disk around the Herbig Ae star MWC 758.
• To determine whether the spiral structure is driven by planetary companions or other disk instabilities.
• To search for embedded planetary companions using high-contrast imaging at near-infrared wavelengths.
• To assess whether a bright point-like source at 20 au is a protoplanet or a disk feature.
• To constrain the mass and orbital parameters of potential companions beyond 0.6 arcsec.

Proposed method

• High-contrast imaging was performed using the L-band vector vortex coronagraph on the NIRC2 instrument at the Keck II telescope.
• The observations targeted the Herbig Ae star MWC 758 to resolve faint structures in the circumstellar disk at high angular resolution.
• The data were processed to suppress the bright stellar halo and enhance detection of faint point sources and extended features.
• The deprojected separation of the point source was calculated as 20 au, based on the measured angular separation and system distance.
• The brightness of the point source was measured in the L band, yielding a magnitude difference of ΔL = 7.0 ± 0.3 mag.
• The detection limit for companions was set at 5 Jupiter masses beyond 0.6 arcsec from the star.

Experimental results

Research questions

• RQ1 Is the bright point-like source at 20 au in the MWC 758 disk a protoplanet or a disk asymmetry?
• RQ2 What is the origin of the three spiral arms observed in the disk, particularly the newly detected southwest arm?
• RQ3 Could the spiral structure be driven by a planetary companion, and if so, where is it located relative to the spiral pattern?
• RQ4 Is the point-like source physically associated with the spiral arms, or are they unrelated features?
• RQ5 What is the mass and orbital configuration of any unseen planetary companion that might be responsible for the disk structures?

Key findings

• A bright point-like source was detected at a deprojected separation of 20 au (0.111 ± 0.004 arcsec) from MWC 758, with an L-band magnitude difference of ΔL = 7.0 ± 0.3 mag.
• The source is located south of the star and is consistent with an embedded, accreting protoplanet, though a disk feature cannot be ruled out.
• Three spiral arms were resolved: two previously known (southeast and northwest) and one newly discovered in the southwest quadrant.
• No additional companions were detected down to 5 Jupiter masses beyond 0.6 arcsec from the star.
• The spiral structure is likely not directly related to the protoplanet candidate unless the planet is on an inclined and eccentric orbit.
• The true nature of the point-like source and its potential link to the spiral arms remain uncertain and require further observations and simulations.