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

TL;DR

This study reports the detection of a faint companion near CrA-9, with spectral analysis suggesting it could be either a young M-dwarf star or a protoplanet, highlighting the challenges in distinguishing between these possibilities.

Contribution

The paper presents multi-epoch high-contrast imaging and spectral analysis of a faint companion, proposing two possible interpretations and emphasizing the need for further observations.

Findings

Detected a faint point source at 0.7'' from CrA-9.

Spectral analysis indicates the companion could be a young M5.5 dwarf or a protoplanet.

Further observations are needed to determine the true nature of the companion.

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 $5\sigma$ 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-$\mu$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.