A faint companion around CrA-9: protoplanet or obscured binary?

• Published in: Monthly notices of the Royal Astronomical Society Vol. 502; no. 4; pp. 6117 - 6139
• Main Authors: Christiaens, V, Ubeira-Gabellini, M-G, Cánovas, H, Delorme, P, Pairet, B, Absil, O, Casassus, S, Girard, J H, Zurlo, A, Aoyama, Y, Marleau, G-D, Spina, L, van der Marel, N, Cieza, L, Lodato, G, Pérez, S, Pinte, C, Price, D J, Reggiani, M
• Format: Journal Article Web Resource
• Language: English
• Published: Oxford University Press 01.04.2021; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: Aérospatiale, astronomie & astrophysique; Physical, chemical, mathematical & earth Sciences; Physique, chimie, mathématiques & sciences de la terre; planet-disc interactions; planets and satellites: formation; protoplanetary discs; Sciences of the Universe; Space science, astronomy & astrophysics; techniques:imageprocessing; protoplanetary discs; planet–disc interactions; techniques: image processing; planets and satellites: formation; planet-disc interactions; Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Solar and Stellar Astrophysics
• ISSN: 0035-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/stab480

ABSTRACT Understanding how giant planets form requires observational input from directly imaged protoplanets. We used VLT/NACO and VLT/SPHERE to search for companions in the transition disc of 2MASS J19005804-3645048 (hereafter CrA-9), an accreting M0.75 dwarf with an estimated age of 1–2 Myr. We found a faint point source at ∼0.7-arcsec separation from CrA-9 (∼108 au projected separation). Our 3-epoch astrometry rejects a fixed background star with a 5σ significance. The near-IR absolute magnitudes of the object point towards a planetary-mass companion. However, our analysis of the 1.0–3.8$\,\mu$m spectrum extracted for the companion suggests it is a young M5.5 dwarf, based on both the 1.13-μm Na index and comparison with templates of the Montreal Spectral Library. The observed spectrum is best reproduced with high effective temperature ($3057^{+119}_{-36}$K) BT-DUSTY and BT-SETTL models, but the corresponding photometric radius required to match the measured flux is only $0.60^{+0.01}_{-0.04}$ Jovian radius. We discuss possible explanations to reconcile our measurements, including an M-dwarf companion obscured by an edge-on circum-secondary disc or the shock-heated part of the photosphere of an accreting protoplanet. Follow-up observations covering a larger wavelength range and/or at finer spectral resolution are required to discriminate these two scenarios.