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arXiv · 2102.10288

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

V. Christiaens·M. -G. Ubeira-Gabellini·H. Cánovas·P. Delorme·B. Pairet·O. Absil·S. Casassus·J. H. Girard·A. Zurlo·Y. Aoyama·G-D. Marleau·L. Spina·N. van der Marel·L. Cieza·G. Lodato·S. Pérez·C. Pinte·D. J. Price·M. Reggiani
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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 $\sim$0.7'' separation from CrA-9 ($\sim$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$μ$m spectrum extracted for the companion suggests it is a young M5.5 dwarf, based on both the 1.13-$μ$m 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.

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V. Christiaens, M. -G. Ubeira-Gabellini, H. Cánovas, P. Delorme, B. Pairet, O. Absil, S. Casassus, J. H. Girard, A. Zurlo, Y. Aoyama, G-D. Marleau, L. Spina, N. van der Marel, L. Cieza, G. Lodato, S. Pérez, C. Pinte, D. J. Price, M. Reggiani. 2021-02-20. A faint companion around CrA-9: protoplanet or obscured binary?. https://doi.org/10.1093/mnras%2Fstab480

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