CrA-9周围的一个暗淡的伴星：原行星还是模糊的双星？
A faint companion around CrA-9: protoplanet or obscured binary?
摘要：
了解巨行星是如何形成的需要直接成像的原行星的观测输入。我们使用VLT/NACO和VLT/SPHERE在2 MASS J19005804-3645048（以下简称CrA-9）的过渡盘中寻找伴星，这是一颗M0.75的吸积矮星，估计年龄为1- 200万年。我们发现在$\sim$与CrA-9的0.7“分离处（$\sim$ 108 au投影分离）有一个微弱的点源。我们的3历元天体测量拒绝了一个固定的背景星与$5σ$的意义。该天体的近红外绝对星等指向一颗行星质量的伴星。然而，我们对从伴星中提取的1.0-3.8 $μ$ m光谱的分析表明，它是一个年轻的M5.5矮星，这是基于1.13- $μ$ m Na指数和与蒙特利尔光谱库模板的比较。观测到的光谱与高有效温度（$3057^{+119}_{-36}$ K）BT-DUSTY和BT-SETTL模型的重现性最好，但相应的光度半径需要匹配的测量流量只有$0.60^{+0.01}_{-0.04}$木星半径。我们讨论了可能的解释，以调和我们的测量，包括M-矮星同伴被边缘上的圆二次盘或吸积原行星的光球层的冲击加热部分所掩盖。为了区分这两种情况，需要进行覆盖更大波长范围和/或更精细光谱分辨率的后续观测。
英文摘要：
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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